Controlled charge-flow photocatalysts

Abstract

Disclosed herein are compositions of doped TiO2 ultra-nano particles and methods of making and using the doped TiO2 ultra-nano particles. The present disclosure provides a method for synthesizing dopedTiO2 ultra-nano particles, the method comprising (a) mixing a dopant solution comprising dopant metal atoms with an aqueous solution of TiO2 ultra-nano particles; and (b) incubating the mixture of dopant solution and aqueous solution of TiO2 ultra-nano particles for a time to form doped TiO2 ultra-nano particles.

Description

[0001] Ref: 166118.01470 CONTROLLED CHARGE-FLOW PHOTOCATALYSTS CROSS-REFERENCE TO RELATED APPLICATIONS This application claims benefit of priority to U.S. Provisional Application No.63 / 599,134, filed November 15, 2023. The contents of which is incorporated by reference in its entirety. BACKGROUND Anatase titania (TiO2) photocatalysts have been the focus of extensive research exploring their potential in applications such as water purification, energy production, and environmental remediation. Despite their promise, the photocatalytic efficiency of anatase titania remains relatively low, which has prompted researchers to explore various strategies to enhance its performance. One of the most widely studied approaches involves doping TiO2 with a range of elements, including metals and non-metals, to alter its electronic properties and improve light absorption, charge carrier dynamics, and overall catalytic activity. These doped photocatalysts are expected to address key limitations, such as poor visible light absorption and rapid recombination of charge carriers, which hinder the efficiency of pure TiO2. However, the results from studies on doped TiO2photocatalysts are often inconsistent or contradictory, with variations depending on factors such as catalyst size, morphology, and the specific dopants used. For instance, while some research suggests that certain dopants enhance photocatalytic efficiency, other studies report minimal or even negative effects. Similarly, the size and shape of TiO2nanoparticles can significantly influence their photocatalytic behavior, but the relationship between these properties and performance is not always clear. This complexity has led to ongoing debates in the literature about the optimal conditions for doping and the mechanisms that govern the enhanced photocatalytic activity. As a result, further research is needed to better understand the underlying factors that contribute to the discrepancies in outcomes and to develop more reliable methods for improving the photocatalytic performance of anatase TiO2. BRIEF SUMMARY OF THE INVENTION The present disclosure provides a method for synthesizing doped TiO2 ultra-nanoparticles. the method comprises (a) mixing a dopant solution comprising dopant metal atoms with a solution of TiO2ultra-nanoparticles; and (b) incubating the mixture of dopant solution and solution of TiO2ultra-nanoparticles for a time to form doped TiO2ultra-nano particles. 1 QB\166118.01470\93195440.1 Ref: 166118.01470 In some embodiments, the method further comprises preparing the solution of TiO2 ultra- nanoparticles. Preparing the solution of TiO2 ultra-nanoparticles may comprise hydrolyzing TiCl4 in an aqueous solution, adjusting the pH of the aqueous solution of hydrolyzed TiCl4, and incubating the pH-adjusted, aqueous solution of hydrolyzed TiCl4 for a time to form TiO2 ultra- nanoparticles. In some embodiments, the pH of the aqueous solution of hydrolyzed TiCl4 is adjusted to form (TiO2)159particles. In some embodiments, the dopant solution comprises a concentration of dopant metal atoms that when mixed with the aqueous solution of TiO2 ultra-nanoparticles results in formation of a plurality of doped TiO2ultra-nano particles comprising a single dopant metal atom. In some cases, the dopant solution is prepared from a chloride salt, nitrate salt, sulfate salt, acetate salt, carbonate salt, potassium salt, or a sodium salt comprising the dopant metal atoms or the dopant metal atoms comprise Cr, Fe, V, Mn, Co, Ni, Cu, or Zn. In some cases, the dopant solution is prepared from a chloride salt, nitrate salt, sulfate salt, acetate salt, carbonate salt, potassium salt, or a sodium salt comprising the dopant metal atoms and the dopant metal atoms comprise Cr, Fe, V, Mn, Co, Ni, Cu, or Zn. The present disclosure also provides a method for preparing a catalyst. The method comprises decorating a doped TiO2ultra-nanoparticle with a single catalytic metal atom. In some embodiments, decorating the catalyst comprises photo irradiating the doped TiO2 ultra-nanoparticles in the presence of catalytic metal atoms and oxygen. The method may further comprise mixing an acidic solution with the photo-irradiated doped TiO2ultra-nanoparticles and secondary metal atoms and re-irradiating the mixture, optionally wherein mixing the acidic solution with the photo-irradiated doped TiO2ultra-nanoparticles and secondary metal atoms and re-irradiating the mixture is repeated one or more times. In some embodiments, the acidic solution is a hypochlorous acid solution or the catalytic metal atoms comprise Pt, Au, Os, or Ir. In other embodiments, the acidic solution is a hypochlorous acid solution the catalytic metal atoms comprise Pt, Au, Os, or Ir. In some embodiments, decorating the photocatalyst comprises mixing doped TiO2 ultra- nanoparticles with a chloride salt solution or a sodium salt solution comprising the catalytic metal atoms. In further embodiments, the chloride salt solution is a chloride salt comprising V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W, Ru, Rh, or Pd. 2 QB\166118.01470\93195440.1 Ref: 166118.01470 In some embodiments, the method for preparing a catalyst further comprises synthesizing the doped TiO2 ultra-nanoparticles by (a) mixing a dopant solution comprising dopant metal atoms with a solution of TiO2ultra-nanoparticles; and (b) incubating the mixture of dopant solution and solution of TiO2 ultra-nanoparticles for a time to form doped TiO2 ultra-nanoparticles. In some cases, the method further comprises preparing the solution of TiO2 ultra-nanoparticles. In some embodiments, preparing the solution of TiO2ultra-nanoparticles comprises hydrolyzing TiCl4in an aqueous solution, adjusting the pH of the aqueous solution of hydrolyzed TiCl4, and incubating the pH-adjusted, aqueous solution of hydrolyzed TiCl4 for a time to form TiO2 ultra-nanoparticles. The pH of the aqueous solution of hydrolyzed TiCl4may be adjusted to form (TiO2)159particles. In some embodiments, the dopant solution comprises a concentration of dopant metal atoms that when mixed with the aqueous solution of TiO2 ultra-nano particles results in formation of doped TiO2 ultra-nano particles comprising a single dopant metal atom. In some embodiments, the dopant solution is prepared from a chloride salt, nitrate salt, sulfate salt, acetate salt, carbonate salt, potassium salt or a sodium salt comprising the dopant metal atom and / or the dopant metal atom is V, Cr, Mn, Fe, Co, Ni, Cu, or Zn. The present disclosure provides composition comprising a plurality of TiO2ultra- nanoparticles prepared by the methods described herein. In some embodiments, the plurality of TiO2 ultra-nanoparticles is prepared by the methods described herein. Another aspect of this invention provides a composition comprising a plurality of catalysts prepared by the methods described herein. In some embodiments, the plurality of catalysts is prepared by the methods described herein. In some embodiments, greater than 20% of the TiO2 ultra-nanoparticles are decorated with the single catalytic metal atom. This invention also discloses a composition comprising a plurality of catalysts, wherein the catalyst comprises doped TiO2 ultra-nanoparticles decorated with a single catalytic metal atom. In some embodiments, the single catalytic metal atom is Au, Ru, Os, Rh, Ir, Pd, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, W, or Mo. In some embodiments, greater than 20% of the TiO2ultra-nanoparticles are decorated with the single catalytic metal atom. The single catalytic metal atom may be selected from Au, Ru, Os, Rh, Ir, Pd, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, W, Mo, or Pt. In some embodiments, the doped TiO2ultra-nanoparticles comprise a single V, Cr, Mn, Fe, Co, Ni, Cu, Zn, or Mo dopant metal atom positioned as an engineered defect within the doped TiO2ultra-nanoparticle. In some 3 QB\166118.01470\93195440.1 Ref: 166118.01470 embodiments, the doped TiO2 ultra-nanoparticles comprise a Cu, V, Ni, Zn, Ag, or Mo dopant metal atom positioned as an engineered defect within the doped TiO2 ultra-nanoparticle. The present disclosure also provides a method for reducing CO2, the method comprising irradiating a catalyst comprising doped TiO2 ultra-nanoparticles decorated with a single catalytic metal atom in the presence of CO2. In some embodiments, the catalyst comprises doped TiO2 ultra- nanoparticles decorated with the single catalytic metal atom is the catalyst as described herein. Another aspect of the disclosure provides a method for oxidizing an organic substrate. The method comprises irradiating a doped TiO2 ultra-nanoparticles decorated with a single catalytic metal atom in the presence of an organic substrate. In some embodiments, the catalyst comprises doped TiO2ultra-nanoparticles decorated with the single catalytic metal atom is the catalyst as described herein. In further embodiments, the TiO2 ultra-nanoparticles decorated with a single catalytic metal atom are irradiated in the presence of oxygen. BRIEF DESCRIPTION OF THE DRAWINGS Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. Figure 1 illustrates a flowchart of the synthesis of the metal doped and decorated titania ultra-nanoparticles (TiUNP). Figure 2 shows TiUNP doped with fourth-period transition elements form highly colored, solid particles. Ti:dopant doping level is 1 dopant per particle (approximately the optimal oxidation efficiency doping level). Particles, shown under ambient air, are stable. Figure 3 shows a) MALDI-TOF spectrum and ideal isomorphic series structure of TiUNP. The parent peak at 12730 amu corresponds to TiUNP (TiO2)159as synthesized in this Examples. Figure 3 also shows b) TiUNP (TiO2)29 with 3x3 Ti(IV) equator; c) TiUNP (TiO2)78 with 4x4 Ti(IV) equator, and d) TiUNP (TiO2)159 with 5x5 Ti(IV) equator. Titanium lattice (light gray) and oxygen lattice (dark gray) are oriented similarly for comparison. Figure 4 shows Au(III)-Fe•TiUNP solution after irradiation. Left solution was saturated with nitrogen gas and generates gold nanoparticles as evidenced by the characteristic gold 4 QB\166118.01470\93195440.1 Ref: 166118.01470 nanoparticle color. Right solution was saturated with oxygen gas during Au decoration of Fe•TiUNP, suppressing generation of gold nanoparticles. Use of oxygen results in a nearly colorless solution, indicating minimal production of gold nanoparticles. Figure 5 shows organic degradation slope (efficiency) of Fe•TiUNP, Co-Fe•TiUNP, Ni- Fe•TiUNP, and W-Fe•TiUNP. Figure 6 shows chromatogram of Mo-Cr•TiUNP in carbon dioxide reduction, left peak for formaldehyde-PFBHA derivative and right peak for PFBHA. Mass spectrometry (MS) analysis of formaldehyde-PFBHA derivative (i.e., the left peak) confirms the existence of formaldehyde generation in carbon dioxide reduction process (MS data not shown). Figure 7 shows kinetic characterization and optical characterization of TiUNPs. a) Oxygen saturated solution. With O2 saturation, Fe-TiUNP shows a 3.42-fold enhancement while V-TiUNP, Cr-TiUNP and Cu-TiUNP show somewhat enhanced efficiency. b) Diffuse reflectance spectrum TiUNP, Cr-TiUNP, Cu-TiUNP, and Fe-TiUNP revealing the difference between band-gap and localized d-d transitions. Figure 8 shows the electronic structure of TiUNP illustrated with a standard reduction potential diagram. Anatase CB / VB, Cr(III) / Cr(II), Cu(II) / Cu(I), Fe(III) / Fe(II), and O2 / H2O illustrating relationship of couples to the homogeneous radical reaction. Conduction band electrons relax and localize on the Cu(II) / Fe(III) dopant site lose the potential to evolve hydrogen. Figure 9 shows diffuse reflectance spectra of TiUNPs: Mn•TiUNP, Co•TiUNP, Ni•TiUNP, Zn•TiUNP. Figure 10 shows electronic structure of TiUNP. Raman spectrum of 1% mol Fe•TiUNP and TiUNP showing interband fluorescence. Removing fluorescence, the Raman spectrum of Fe•TiUNP and TiUNP are identical. DETAILED DESCRIPTION OF THE INVENTION Disclosed herein are compositions of doped TiO2 ultra-nanoparticles (TiUNPs), catalysts prepared from the doped TiUNPs decorated with a single catalytic metal atom, and methods of making and using the doped TiUNPs and catalysts. The doping methodology exploits defect sites in combination with transition elements, allowing for precise modulation of photocatalytic reaction pathways. The formation of surface complexes between dopants and reactants shifts the balance between direct charge transfer and homogeneous radical reactions, thereby enhancing the overall efficacy of the photocatalytic processes. The reaction pathway can also be regulated by dopants, 5 QB\166118.01470\93195440.1 Ref: 166118.01470 with the distinct chemical properties of each dopant enabling the particles to perform differently in various scenarios. For instance, iron (III) dopants may be effective for organic pollutant degradation, while chromium (III) dopants may be effective for carbon dioxide reduction. Decorating doped TiUNP with a catalytic metal atom further enhances the utility of the disclosed synthetic methods. Integrating a second catalytic metal element enables fine-tuning of TiUNPs electronic states, which allows for optimizing the TiUNPs for various applications. This methodology represents a new technique for decorating doped TiUNP with different catalytic metal atoms, increasing the utilization rate of the catalytic metal atoms. As a result, the synthetic methods may result in a majority of the TiO2ultra-nanoparticles being decorated with the single catalytic metal atom. Figure 1 illustrates a synthetic method 10 for preparing catalysts comprising doped TiUNPs decorated with a single catalytic metal atom. The synthetic method 10 comprises synthesizing doped TiUNP. Synthetizing doped TiUNP comprises mixing a dopant solution comprising dopant metal atoms with an aqueous solution of TiO2 ultra-nano particles14 and incubating the mixture of dopant solution and aqueous solution of TiO2 ultra-nano particles for a time to form doped TiO2 ultra-nano particles 16. The doped TiUNPs may be decorated with a single catalytic metal atom 18. The synthetic method may optionally comprise preparing a solution of undoped TiUNPs 12. The synthetic method illustrated in Figure 1 may be performed together or in a series of stages over the course of days, weeks, or months. The TiUNPs and doped TiUNPs prepared in steps 12 and 16 may be prepared for storage prior to commencing a subsequent step. For example, the TiUNPs and doped TiUNPs may be formulated as a powder that that is stable for more than 1 year. Accordingly, steps may be performed alone or in combination with other steps. Preparing the solution of undoped TiUNPs 12 may involve hydrolysis of TiCl4in an aqueous solution. Preparing the aqueous solution of TiUNPs comprises hydrolyzing TiCl4 in an aqueous solution, adjusting the pH of the aqueous solution of hydrolyzed TiCl4, and incubating the pH adjusted, aqueous solution of hydrolyzed TiCl4 for a time to form TiO2ultra-nano particles. The pH of the aqueous solution of hydrolyzed TiCl4 may be adjusted to form (TiO2)29 particles, (TiO2)78 particles, (TiO2)159 particles, and combinations thereof. In some instances, the pH is adjusted to create a preponderance or majority of (TiO2)159particles. In some embodiments, the pH is adjusted to at least 1, at least 1.5, at least 2, at least 2.2, at least 2.5, at least 3, at least 4, at least 5, at least 6. In some embodiments the pH is adjusted to 2.2. In some embodiments, the 6 QB\166118.01470\93195440.1 Ref: 166118.01470 synthesis of TiUNP requires a dry atmosphere with humidity levels below 20%, below 10%, below 5%, or below 1%. In some embodiments, the aqueous solution is maintained at 0 ℃ for the duration of step 1, in some embodiments it is only maintained at 0 ℃ for a short time. In some embodiments, the aqueous solution is incubated for at least 1 hour, at least 2 hours, at least 3 hours, or at least 4 hours. In some embodiments, the TiCl4 is injected through a septum cap with a syringe. The TiUNPs solution may be dialyzed prior to doping. Doped TiUNP may be prepared with an engineered defect site having one dopant atom per TiUNP. The dopant may be introduced to the TiUNP solution by mixing a dopant solution with the TiUNP solution 14. The mixture of TiUNP solution and dopant solution is incubated to form the doped TiUNPs 16. The dopant solution may be prepared from a salt comprising the dopant metal atom. The dopant metal atom may be a fourth-row, fifth-row, or sixth-row transition metal. The salt can be selected from a chloride salt, nitrate salt, sulfate salt, acetate salt, halide salt, carbonate salt, or a basic salt, such as a sodium salt or a potassium salt. Chloride salts are of particular interest because they avoid introduction of other cations / anions as impurities. Exemplary salts include, but are not limited to, vanadium(III) chloride (VCl₃), sodium metavanadate (NaVO₃), chromium(III) chloride (CrCl₃), chromium(III) nitrate (Cr(NO₃)₃), manganese(II) chloride (MnCl₂), manganese(II) sulfate (MnSO₄), manganese(II) nitrate (Mn(NO₃)₂), potassium permanganate (KMnO₄), iron(III) chloride (FeCl₃), iron(III) sulfate (Fe₂(SO₄)₃), iron(III) nitrate (Fe(NO₃)₃), iron(III) acetylacetonate (Fe(acac)₃), cobalt(II) chloride (CoCl₂), cobalt(II) sulfate (CoSO₄), cobalt(II) nitrate (Co(NO₃)₂), nickel(II) chloride (NiCl₂), nickel(II) sulfate (NiSO₄), nickel(II) nitrate (Ni(NO₃)₂), copper(II) chloride (CuCl₂), copper(II) sulfate (CuSO₄), copper(II) nitrate (Cu(NO₃)₂), zinc chloride (ZnCl₂), zinc sulfate (ZnSO₄), zinc nitrate (Zn(NO₃)₂), sodium molybdate (Na₂MoO₄), molybdenum(V) chloride (MoCl₅), sodium tungstate (Na₂WO₄), tungsten(V) chloride (WCl₅), platinum(IV) chloride (PtCl₄), Hexachloroplatinic acid (H₂PtCl₆), gold(III) chloride (HAuCl₄), and silver nitrate (AgNO₃). In some embodiments, acid, such as HCl, is injected to neutralize the basic salt. In some embodiments, the acid is introduced to adjust the solution pH to at least 1, at least 1.5, at least 2, at least 2.2, at least 2.5, or at least 3. In some embodiments, the Ostwald ripening and self-assembly takes at least 48 hours, at least 72 hours, or at least 96 hours. In some embodiments, the Ostwald ripening and self-assembly occurs at room temperature. When a sodium salt or basic salt is used to introduce the dopant, dialysis should be performed. Dialysis of the mixture takes at least 1 hour, 7 QB\166118.01470\93195440.1 Ref: 166118.01470 at least 2 hours, at least 4 hours, at least 6 hours, or at least 8 hours. In some embodiments, the dialysis occurs at room temperature. Once the TiO2 ultra-nano particle is doped, evaporation of the ripened particles to dryness may occur. In some embodiments, the evaporation is rotary evaporation. In some embodiments, the temperature is controlled between 20 °C and 35 °C. The dry particles remain stable for at least several months. In some embodiments, the dopant solution comprises a concentration of dopant metal atoms that results in formation of doped TiO2ultra-nano particles comprising a single dopant metal atom when mixed with the aqueous solution of TiO2 ultra-nano particles. The dopant solution can result in formation of doped TiO2ultra-nano particles, wherein greater than 50%, 75%, 80%, 85%, 90%, 95%, or more than 95% of the TiO2ultra-nano particles are doped. The doped TiO2 ultra-nano particle is decorated with a catalytic metal atom. In some embodiments, the ultra-nano titanium dioxide particles are a dry powder or in solution. The secondary metal can be introduced as a metal chloride salt, a sodium salt, or a potassium salt. When the secondary metal is introduced via a salt, the salt can be pre-dissolved in an acidic solution, like a solution containing hydrochloric acid. In some embodiments, the salt is a fourth- row, fifth-row, or sixth-row transition metal chloride salt. Solutions of these fourth-row transition metal chloride salt are stable under ambient conditions. In some embodiments, the chloride salt is for a for sixth-row transition metals (e.g., Pt, Au, Ir, Os, or W), which are light sensitive. In some embodiments, the decoration occurs via irradiation with the Xenon lamp. In further embodiments, the irradiation occurs for at least 30 minutes, at least 1 hour, at least 2 hours, at least 3 hours, or at least 4 hours. In some embodiments, an acid solution, like hypochlorous acid solution, is added to the mixture, after which the mixture is re-irradiated with a xenon lamp to continue decoration until maximum yield is achieved. In some embodiments, the yield of decorated, doped TiUNPs is at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99%. Once the doped TiO2ultra-nano particle is decorated, dialysis can occur. In some embodiments, dialysis takes at least 1 hour, at least 2 hours, at least 4 hours, at least 6 hours, or at least 8 hours. In some embodiments, the dialysis occurs at room temperature. In some embodiments, the evaporation of the decorated particles to dryness may occur. In some embodiments, the evaporation is rotary evaporation. In some embodiments, the temperature is controlled between 20 °C and 35 °C. 8 QB\166118.01470\93195440.1 Ref: 166118.01470 The present disclosure also provides a composition comprising doped TiO2 ultra-nano particles prepared by the method described herein. The TiO2 ultra-nano particles may be further decorated with a single catalytic metal atom. Decorating the doped TiUNP may result in more than 20%, more than 30%, more than 50%, more than 75%, more than 90%, or at most 100% of the TiO2 ultra-nano particles being decorated with the single catalytic metal atom. Exemplary single catalytic metal atoms include, but are not limited to, forth-row, fifth-row, and sixth-row transition metals, e.g., Pt, Au, Pd, Ir, Ru, Rh, Os, Re, Ag, Ni, Zn, Ni, Cu, Co, and W, and exemplary dopant metal atoms include, but are not limited to, forth-row, fifth-row, and sixth-row transition metals, e.g., Cr, Mn, Fe, Co, Ni, Cu, and Zn). Another aspect of the present application provides a method of using doped TiO2ultra- nano particles to catalyze the oxidation of an organic substrate. Organic substrate refers to organic compounds comprising carbon and one or more of hydrogen, oxygen, nitrogen, and sulfur. For example, the present disclosure provides a method of using doped TiO2ultra-nano particles to oxidize methanol to formaldehyde, comprising contacting methanol with a doped TiO2 ultra-nano particle; and oxidizing the methanol to produce formaldehyde. Oxidation may occur by photo irradiating doped TiUNPs or decorated TiUNPs in the presence of the organic substrate. In some instances, photo irradiation occurs in the presence of oxygen. The presence of oxygen may act as an electron acceptor and promote a direct charge transfer pathway. In the absence of oxygen, the most favorable electron acceptor is likely protons or water, which can generate radicals in a homogenous pathway. In some instances, the solution containing the organic substrate is saturated with oxygen. Another aspect of the present application provides a method of using doped TiO2ultra- nano particles to catalyze the reduction of CO2.For example, the present disclosure provides a method of using doped TiO2 ultra-nano particles to reduce CO2 to formic acid, formaldehyde, methanol, methane, or combinations thereof comprising contacting CO2 with a doped TiO2 ultra- nano particle; and reducing the CO2. Oxidation may occur by photo irradiating doped TiUNPs or decorated TiUNPs in the presence of the CO2. As used herein, "ultra-nano" refers to particles that have a diameter of less than 2.5 nm and / or particles having fewer than 1000 formula units of TiO2. The ultra-nanoparticle has a diameter of between 0.8 nm and 2.5 nm. In some cases, the ultra-nanoparticle has a diameter of at least 0.8 nm, or at least 1.0 nm. In some cases, the ultra- 9 QB\166118.01470\93195440.1 Ref: 166118.01470 nanoparticle has a diameter of at most 2.5, at most 2.4, at most 2.3, at most 2.2, at most 2.1, at most 2.0 nm, at most 1.9 nm, at most 1.8 nm, at most 1.7 nm, at most 1.6 nm. at most 1.5 nm, at most 1.4 nm, at most 1.3 nm, at most 1.2 nm, at most 1.1 nm, or at most 1.0 nm. The ranges defined by combining each of the lower limits and each of the upper limits articulated in these lists are expressly contemplated. The ultra-nanoparticle contains between 20 formula units of TiO2and 1000 formula units of TiO2, including but not limited to, between 50 formula units and 200 formula units. In some cases, the ultra-nano-particle contains at least 20 formula units of TiO2, including but not limited to, at least 25 formula units, at least 30 formula units, at least 40 formula units, at least 50 formula units, at least 75 formula units, at least 90 formula units, at least 100 formula units, and at least 200 formula units of TiO2. In some cases, the ultra-nano-particle contains at most 500 formula units of TiO2, including but not limited to, at most 400 formula units, at most 350 formula units, at most 300 formula units, at most 275 formula units, at most 250 formula units, at most 225 formula units, at most 200 formula units, at most 190 formula units, at most 175 formula units, at most 150 formula units, at most 125 formula units, or at most 100 formula units of TiO2. The ranges defined by combining each of the lower limits and each of the upper limits articulated in these lists, with the exception of those where the lower limit is greater than the upper limit, are expressly contemplated. In some cases, the engineered defect is located in an energetically favorable portion of the particle, such as a

[0001] facet of the TiO2. Without wishing to be bound by any particular theory, the evidence suggests that the energy difference between the

[0001] face and the

[0101] face (the other dominant face) is large enough that the single metal atom is energetically driven to this face. However, it is also true that the ultra-nanoparticles described herein have facets that are less purely defined than facets in larger, more extensive crystalline structures. The evidence suggests that the single metal atom is located in substantially the same position in all particles and the phraseology of this paragraph is intended to identify that reproducible location. In some cases, when the particle is below 500 units, the TiO2 is anatase form. In cases where the particle is above 500 units, the TiO2 is rutile form or a mixed phase of anatase and rutile form. The photocatalyst can have any particle shape that allows the single metal catalyst atom and other properties described herein. In some cases, the photocatalyst substantially has a particle 10 QB\166118.01470\93195440.1 Ref: 166118.01470 shape that is a truncated square bipyramid. In other words, the photocatalyst can have a shape that is substantially a truncated square bipyramid. In some instances, the photo efficiency is characterized with conversion of methanol to formaldehyde, a 2-electron oxidation. In some instances, the photo efficiency is characterized with oxidation of formaldehyde to CO2 and H2O, a 4-electron oxidation. In other instances, oxidation is characterized by degradation of methylene blue, or methyl orange, or other hydrocarbons. In other instances, the photo efficiency is characterized with conversion of CO2to formaldehyde, i.e., CO2 reduction. From a practical standpoint, the particles described herein are far too small to be individually isolated and sorted to identify which particular particles contain the single metal catalyst atom and which do not, at least by current methods. Thus, in some cases where the photosynthetic reaction that deposits the single metal catalyst atom results in fewer than 100% of the particles containing the single metal catalyst atom, the resulting mixture of particles will have some proportion which have the single metal catalyst atom and some proportion which do not. In view of this reality, the yield of the reactions described herein is a lower limit and can be an important determining factor in the overall oxidative capabilities of the compositions produced. Similarly, the overall catalytic capabilities of a composition can be impacted by both the reactivity of the particles containing a single metal catalyst atom and their relative proportion. One exemplary storage method includes removing the aqueous solution via evaporation and storing the particles as a crystalline solid comprised of ultra-nano particles and counter ions. The present disclosure provides a powder including the catalysts and / or the composition disclosed herein. The present disclosure provides a film. The film includes the catalysts and / or the composition disclosed herein. The film can be made by drop-casting the catalysts and / or the composition onto a substrate. Specific structures, devices and methods relating to generating single-atom catalysts are disclosed. It should be apparent to those skilled in the art that many additional modifications beside those already described are possible without departing from the inventive concepts. In interpreting this disclosure, all terms should be interpreted in the broadest possible manner consistent with the context. 11 QB\166118.01470\93195440.1 Ref: 166118.01470 Unless otherwise specified or indicated by context, the terms “a”, “an”, and “the” mean “one or more.” For example, “a molecule” should be interpreted to mean “one or more molecules.” As used herein, “about”, “approximately,” “substantially,” and “significantly” will be understood by persons of ordinary skill in the art and will vary to some extent on the context in which they are used. If there are uses of the term which are not clear to persons of ordinary skill in the art given the context in which it is used, “about” and “approximately” will mean plus or minus ≤10% of the particular term and “substantially” and “significantly” will mean plus or minus >10% of the particular term. As used herein, the terms “include” and “including” have the same meaning as the terms “comprise” and “comprising.” The terms “comprise” and “comprising” should be interpreted as being “open” transitional terms that permit the inclusion of additional components further to those components recited in the claims. The terms “consist” and “consisting of” should be interpreted as being “closed” transitional terms that do not permit the inclusion of additional components other than the components recited in the claims. The term “consisting essentially of” should be interpreted to be partially closed and allowing the inclusion only of additional components that do not fundamentally alter the nature of the claimed subject matter. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. Preferred aspects of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred aspects may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect a person having ordinary skill in the art to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited 12 QB\166118.01470\93195440.1 Ref: 166118.01470 in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. EXAMPLES In the Examples, a versatile platform based on TiUNP, that minimizes defects, produces uniform morphology, and enables systematic investigation of dopant control of product formation is demonstrated. Charge-carrier destination is determined by the redox couple in the oxide matrix; a couple that is measured by diffuse reflectance spectroscopy. Versatility is demonstrated via doping with all fourth-period transition elements. For oxidation reactions, two distinct reaction pathways are identified; the balance between the pathways is regulated by the reduction potential of the dopant in the matrix. The prospect for directing charge transfer significantly expands potential applications for titania. Titanium dioxide (TiO2) is a wide-bandgap semiconductor. Its strong oxidation potential and hydrophilic properties render it an advantageous material for wastewater treatment applications. However, its wide bandgap restricts photocatalytic activity to the ultraviolet range, with minimal utilization of the more abundant visible range in the solar spectrum. As a result, photo activity is minimal. The disclosed Examples present a systematic methodology for doping TiUNP, such as doping with fourth-row transition metals, and decorating the doped TiUNP with single catalytic metal atoms. This methodology exploits defect sites in conjunction with secondary metals, enabling precise modulation of photocatalytic reaction pathways. Formation of surface complexes between dopants and reactants leads to a shift in the balance between direct charge transfer and homogeneous radical reactions, impacting overall efficacy of the photocatalytic processes. The ability to limit charge carrier recombination results increased in photo efficiency. Synthesis of transition metal ion doped TiUNP Synthesis of doped TiO2 ultra-nanoparticles (TiUNPs) commences with preparation of undoped TiUNP. In this method, the inorganic precursor, TiCl4, is chosen due to its minimal introduction of organic impurities from the titania precursor. In the inorganic method, 0.7 mL of titanium tetrachloride (TiCl4) is hydrolyzed in 200 mL 18 MΩ water maintained at 0 ℃ with an ice bath. TiCl4 is injected through a septum cap with a syringe. The mixture is incubated at room temperature for 2 hours to complete hydrolysis. The solution is filtered through a 0.2 μm nylon 13 QB\166118.01470\93195440.1 Ref: 166118.01470 filter. The concentration of titania is determined by Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES). The wet synthesis of TiUNP utilizes a dry atmosphere with humidity levels below 10%. If ambient conditions do not support such low humidity, synthesis should be conducted within a glove box with either dry air or N2. TiCl4 is added to an ice-bath dropwisely, e.g., at a rate of 2 drops per second, to ensure complete dispersion and hydrolysis of TiCl4in the aqueous phase. During hydrolysis, moisture is generated, which may lead to local hydrolysis of TiCl4at the needle tip. Therefore, it is important to avoid introducing white snowflake-like crystals into the solution, as they can act as nucleation seeds and cause unintended agglomeration. The solution is transferred into a cellulose dialysis membrane with approximately 1 nm pores (Fisher Scientific, Spectrum™, 12 kDa). The tubing is placed in a stainless-steel lobster pot containing 16 L of 18 MΩ water for dialysis. The pH of the solution is adjusted to 2.2, as measured accurately with a pH meter. The dialysis process typically takes 4 hours at room temperature. Doped TiUNP preparation After the TiUNP solution was prepared, the concentration of titania in the solution was measured. At pH 2.2, TiUNP forms clusters with a chemical formula of (TiO2)159. Each particle is doped with one dopant atom per particle. A dopant chloride salt was pre-dissolved in an HCl solution at pH 2.2, with its concentration determined by ICP-OES. An accurately measured volume of the dopant solution was then added to the TiUNP solution and mixed thoroughly. Dopants can also be introduced using a basic salt, such as sodium tungstate, sodium metavanadate, potassium permanganate, or sodium molybdate. A stoichiometric amount of salt is measured and dissolved in 10 mL of 18 MΩ water. The solution is stirred at the maximum rate without spillage, and a specific amount of concentrated HCl is immediately injected to neutralize the basic salt and adjust the solution pH to 2.2. The solution is then promptly poured into the TiUNP solution and mixed thoroughly. The mixture undergoes Ostwald ripening and self-assembly at room temperature for at least 48 hours. If starting with a chloride salt, a second dialysis is unnecessary. However, if using a basic salt, further dialysis is required. In this case, the solution is transferred into a dialysis membrane and dialyzed in a lobster pot containing 16 L of pH 2.2 HCl solution. The dialysis process takes approximately 4 hours to reduce inorganic ions, such as sodium or potassium, to undetectable levels as measured by ICP-OES. 14 QB\166118.01470\93195440.1 Ref: 166118.01470 Dry TiUNP is obtained by rotary evaporation of the ripened particles to dryness, with temperature controlled between 20 °C and 35 °C. The resulting doped TiUNPs exhibit distinctive colors, depending on the type of transition metal dopant used (Figure 2). The dry particles remain stable for at least several months. Decoration of doped TiUNP with a secondary metal The decoration of doped TiUNP can begin with either dry TiUNP powder or doped TiUNP solution described above. For dry TiUNP, approximately 125 mg of doped TiUNP is carefully measured, fully dissolved in 50 mL of HCl solution at pH 2.2, and ultrafiltered through a 0.2 μm nylon filter. For the concentrated TiUNP solution, 50 mL of the solution is directly measured for use. The titania concentration is determined by ICP-OES before adding the decorative metal salt, with each particle decorated at a ratio of one decorative metal atom per particle. Decorated metal salts begin as metal chloride salt solutions. The decorative chloride salt is pre-dissolved in an HCl solution at pH 2.2, with its concentration determined by ICP-OES. Fourth- row transition metal chloride salt solutions are stable under ambient conditions, so no additional protection is required after preparation. However, metal chloride salts for noble metals, such as platinum and gold, are light-sensitive. Once prepared, these solutions should be stored in amber bottles or covered with aluminum foil to protect them from light. An accurately measured volume of the decorative metal chloride salt solution was added to the doped TiUNP solution and mixed thoroughly. For fourth-row transition metals, the decorative metal can self-assemble at the dopant sites due to its natural affinity, requiring no additional action. In contrast, decoration with heavier noble metals requires irradiation to overcome the activation barrier. The 50 mL noble metal salt mixed doped TiUNP solution is then transferred to a clean 100 mL round-bottom flask for irradiation. A xenon arc lamp is used to provide a broad spectrum of light, with a water filter to absorb all IR light and reduce heat generated during the process. The solution is saturated with oxygen gas at a rate of 5 SCFH throughout the irradiation, which is carried out for a total of 2 hours. Synthesis of Mo-Cr•TiUNP and W-Fe•TiUNP The first step synthesis of Mo-Cr•TiUNP starts with a synthesis of 2 chromium (III) dopant per particle Cr•TiUNP following the same synthetic method to other doped particles. Particles were Ostwald ripened for at least 48 hours at pH 2.2 in an HCl solution to reach a desired 15 QB\166118.01470\93195440.1 Ref: 166118.01470 Cr•TiUNP platform for decoration. The solution can be kept for next step synthesis without rotary evaporation to dryness. An solution having an amount of 2 decorating Mo(VI) per particle was prepared from an equivalent amount of Na2MoO4 salt measured and dissolved in 20 mL of 18 MΩ water with continuous stirring. After the solid fully dissolves, a certain volume of concentrated HCl is injected into the stirring liquid to adjust pH to 2.2. The liquid is then filtered with 0.2 μm nylon filter. While continuously stirring the Cr•TiUNP solution, and the HCl acidified sodium molybdate solution is poured into the Cr•TiUNP container. Stirring of the solution continued for at least 30 minutes to reach a full decoration of the Cr•TiUNP particles. The synthesis of W-Fe•TiUNP has the same procedure to the synthesis of Mo-Cr•TiUNP, when starting with sodium tungstate. Alternatively, W-Fe•TiUNP may be prepared from WCl6 with photo-irradiation to push an elimination reaction. CO2reduction by Mo-Cr•TiUNP To reduce carbon dioxide into organic materials, a solution of Mo-Cr•TiUNP was used. Optionally, the particle concentration may be from 1000 ppm to 3000 ppm. The solution can be prepared by rotary evaporated dry powder, or directly use the solution in last section. 50 mL of Mo-Cr•TiUNP solution is transferred into a clean 100 mL round bottom flask and continuously purged with carbon dioxide gas flow at 1 SCFH. An ozone-free 1000 W xenon lamp (Oriel model 66921), with input power set to 750 watts, simulated natural sunlight. To mitigate heating, a 10 cm long deionized water IR filter was positioned in front of the round bottom flask, blocking IR wavelengths longer than 800 nm and UV wavelengths shorter than 280 nm. Each kinetic experiment was run for 60 minutes with aliquots collected at ten-minute intervals, and analyzed via GC-FID. Carbon dioxide is first reduced into formic acid as the primary product via a 2-electron reduction reaction. However, formic acid does not accumulate to a high concentration because it is further reduced into formaldehyde. With continuous irradiation, further reduced product, such as methanol and methane, or other carbon-carbon coupled product like acetic acid, acetaldehyde or ethanol are generated. Retrieval of noble metal precursor from agglomerated metal boulders During the photodeposition of noble metals onto doped TiUNP, saturating with oxygen gas suppresses reduction-nucleation formation of noble metal nanoparticles. Nonetheless, formation 16 QB\166118.01470\93195440.1 Ref: 166118.01470 of some noble metal particles is inevitable; metal nanoparticles reduce the yield of the desired Pt(IV)-Fe•TiUNP or Au(III)-Fe•TiUNP. After irradiation with the Xenon lamp for 2 hours, the flask is removed from the light source, and 1.67 mL of a 500 ppm hypochlorous acid solution is added to oxidize platinum or gold nanoparticles back to their H2PtCl6 or HAuCl4 precursors. The reaction is allowed to proceed for 10 minutes, after which the flask is re-irradiated with the xenon lamp to continue decoration until maximum yield is achieved. After reaching maximum yield, a small quantity of unreacted precursors (H2PtCl6 or HAuCl4) and metal nanoparticles (PtNP, AuNP) may still remain in the solution. These impurities are very small, with diameters of less than 1 nm. To remove them, the solution is transferred into a dialysis membrane and dialyzed in a lobster pot containing 16 L of pH 2.2 HCl solution for approximately 4 hours. Dry decorated, doped TiUNP can be obtained by rotary evaporation of the solution to dryness, with the temperature controlled between 20 °C and 35 °C. Materials in Synthesis TiCl4 from Sigma-Aldrich, ReagentPlus®, 99.9% trace metals basis. VCl3, CrCl3, MnCl2, FeCl3, CoCl2, NiCl2, CuCl2, ZnCl2from Sigma-Aldrich, ReagentPlus®, 99.9% trace metals basis. HCl from Sigma-Aldrich, reagent grade 37%. α-Cyano-4-hydroxycinnamic acid from Sigma- Aldrich, matrix-HPLC grade. O-(2,3,4,5,6-Pentafluorobenzyl)hydroxylamine hydrochloride from Sigma-Aldrich, LiChropur™, ≥99.0%. n-Hexane from Sigma-Aldrich, GC / HPLC grade. ICP-OES standard, (NH4)2TiF6, V(NO3)3, Cr(NO3)3, Mn(NO3)2, Fe(NO3)3, Co(NO3)2, Ni(NO3)2, Cu(NO3)2, Zn(NO3)2 from Sigma-Aldrich, TraceCERT®, 1g / L analyte ion in 2% HNO3. The dialysis tubing used was obtained from Fisher Scientific, with MWCO of 12 to 14 kDa and dimensions of 45 mm, constructed from regenerated cellulose. Nylon syringe filters with a pore size of 0.22 μm, hydrophilic host, Fisher Scientific. Syringes equipped with luer-lock tips and 4-inch 22-G needles, Fisher Scientific. Oxygen and Nitrogen are Ultra-high purity grade, from Airgas East. Polydispersity characterization UV-Vis spectrometry, based on Brus's (23,24) and Viswanatha’s (25) method, is employed for TiUNP size determination. TiUNP solid is dissolved in aqueous HCl at pH 2.2 to achieve a catalyst concentration of 10 ppm. Size is based on extrapolation of the linear UV absorbance. 17 QB\166118.01470\93195440.1 Ref: 166118.01470 Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI- TOFMS) provides a direct measure of particle size (26, 27). This method utilizes a soft ionization technique to partially ionize TiUNPs, thereby preserving their structure. A solution with 3 ppm TiUNP is combined with a saturated α-Cyano-4-Hydroxycinnamic Acid (Sigma-Aldrich, matrix- HPLC grade) solution in a 1:1 volume ratio. A 1 μL aliquot is deposited onto a grounded steel target and evaporatively dried for MALDI analysis (Figure 3a). Optical characterization of TiUNP The crystal structure of TiUNP was analyzed using Raman spectroscopy with a Jasco NRS- 3100 Laser Raman spectrophotometer (785 nm, 22.1 mW excitation). The spectra were collected using CCD (charge-coupled device) detection, which was air-cooled to -65 °C with a resolution of 1 cm-1 (1024 × 128 pixels). The instrument featured a notch filter, cutting off 100 cm-1 below excitation. Polypropylene served as the calibration standard, and all spectra were recorded at room temperature. Diffuse reflectance UV-Vis spectra (DRS) were used to investigate the electronic structure of dry TiUNP particles (28). DRS was performed on solid TiUNP particulates. The TiUNP particulates were meticulously ground to a fine powder using a corundum mortar. Any ungrounded, large particles were sieved out using a 200-mesh stainless steel sieve. The ground solid was then transferred onto a concave glass microscope slide, and sealed with a 1 mm thick, UV transparent fused-silica microscope slide. A white background spectrum was collected using 150-mesh standard grade aluminum oxide from Sigma-Aldrich following the same procedure, and normalizing reflectance at all wavelength as 100%. Spectra were acquired using an Ocean Optics USB 200 spectrometer, equipped with a PX- 2 pulsed xenon light source. Spectral data were collected from 350 nm to 800 nm, at a resolution of 0.3 nm, utilizing an integration time of 8 milliseconds. To enhance signal reliability, multiple spectral scans were collected and averaged. Data smoothing uses a 5-point boxcar average. Determination of TiUNP bandgap is based on the bulk anatase baseline at 387 nm (3.2 eV). This analytical step is crucial for identifying spectral features indicative of TiUNP or doped TiUNP electronic properties. TiUNP Characterization All kinetic characterizations are performed in aqueous solutions. To characterize photon utilization efficiency, catalyst and target loading are chosen to ensure photon-limited conditions. 18 QB\166118.01470\93195440.1 Ref: 166118.01470 Kinetic characterization used a catalyst loading of 1 g / L in aqueous HCl, pH 2.2, ensuring photon limited condition. Methanol, used as a standard hole acceptor, was added in a ten-fold excess relative to TiUNP to ensure complete saturation. An ozone-free 1000 W xenon lamp (Oriel model 66921), with input power set to 750 watts, simulated natural sunlight. To mitigate heating, a 10 cm long deionized water IR filter was positioned in front of the photolysis cell, blocking IR wavelengths longer than 800 nm and UV wavelengths shorter than 280 nm. Each kinetic experiment was run for 60 minutes with aliquots collected at ten-minute intervals. The photolysis apparatus comprised a Pyrex cylinder (5 cm diameter by 5 cm length) with four glass inlets on top. The cylinder capacity is 100 mL, although kinetic experiments were conducted with 40 mL samples to prevent spattering caused by gas bubbles. UV grade fused silica windows (JGS-2) sealed both sides of the cell. Light was actively monitored at the outlet of the cell by a calorimetric meter (Scientech 365 Power And Energy Meter 35-5010). Prior to methanol addition, TiUNP solutions were irradiated for 30 minutes with oxygen saturation at a rate of 5 SCFH to eliminate any potential organic residuals. Both oxygen-saturated and nitrogen-saturated conditions were used for characterizing TiUNPs, with nitrogen saturation including a purging step before methanol addition to remove dissolved oxygen. N2was purged at a rate of 5 SCFH for 15 minutes to remove dissolved O2without light irradiation. For characterization experiments, either O2 or N2 was purged at a rate of 5 SCFH. The direct methanol oxidation product, formaldehyde, was quantified following a modified EPA standard method (29). Approximately 0.7 mL aliquots were extracted, 0.5 mL precisely measured for analysis. The 0.5 mL aliquot was mixed with 2.5 mL of 0.2 g / L solution of (2,3,4,5,6- pentafluorobenzyl) hydroxylamine hydrochloride (PFBHA, LiChropur™, ≥99.0%) and allowed to react for at least 8 hours to form the formaldehyde-PFBHA derivative (30,31). This derivative was extracted with 2 mL hexane and analyzed using an Agilent 7890B GC-FID system. Analysis conditions included a splitless injection of 1 μL into a 30 m Agilent HP-5 GC column, with temperature programming from 75 ℃ to 150 ℃ at 20 ℃ / min, and helium as the carrier gas. The kinetic characterization is under photon limited conditions, formaldehyde production is linear in time. To correct lamp intensity fluctuations, efficiency is measured relative to an oxygen-saturated, undoped TiUNP solution. All reported measurements are done at pH 2.2; similar results are obtained for smaller or larger particles, hence lower or higher pH. 19 QB\166118.01470\93195440.1 Ref: 166118.01470 A typical kinetic characterization collects data for 60 minutes; there is no loss of activity over this period that corresponds to hundreds of turnovers per particle. UV-Vis band gap measurement to determination of particle size UV-visible spectroscopy in the UV range proves instrumental in determining the size of TiUNP. As semiconductor quantum dots diminish in size, there is a continuous blue-shift in the absorbance of band gap excitation, attributed to quantum confinement (18). Utilizing the empirical constants established for anatase nanocrystalline structures, the particle size can be inferred by analyzing the absorbance spectrum (34). UV-vis spectra were acquired at a concentration of 10 ppm. Extrapolating the linear range to zero absorbance allows for a direct measurement of the band gap. The diameter of TiUNPs was calculated using the empirical equation ΔEg≅ 100(18.1d2+ 41.4d − 0.8)-1, where ΔEg represents the shift in band gap from bulk anatase in electron volts and d denotes the nanoparticle diameter in nanometers. The band gap shift, ΔEg, is determined by the difference between the UV-vis spectra linear range extension intersection with the baseline and the band gap of bulk anatase. Photo efficiency The definition of the photo efficiency of a catalyst varies across different studies and can sometimes be vague due to variations in the light source used. Herein, photo efficiency is defined as: Photo efficiency ൌTotal transferred electrons Total absorbed photonsൈ 100% ^1^Where the number of transferred electrons is formaldehyde molecules produced per hour times two (two electron oxidation). Absorbed photons are calculated as follows. Light output from the Xenon lamp is quantified using a pyroelectricity meter (Scientech 365 Power And Energy Meter 35-5010). Absorption of UV light is determined as the measured difference in light intensity between the TiUNP-loaded photolysis cell and a control cell containing DI water. Absorbed photons are estimated using the band gap of anatase (387 nm) assuming all absorbed energy is 387 nm. This represents a lower bound for photons absorbed. 20 QB\166118.01470\93195440.1 Ref: 166118.01470 Example calculation: Formaldehyde generation rate for Fe•TiUNP is 31.458 ppm per hour, 1.258×10-3grams. The total number of transferred electrons are 5.044×1019electrons. Fe•TiUNP solution absorbs 0.183 W more light than the DI water control cell. The energy of a photon with a wavelength of 387 nm is 5.133×10-19joule, thus within 1-hour irradiation, a total 1.283×1021UV photons are absorbed. Combining (Equation 1) photo efficiency of Fe•TiUNP is 5.044×1019 / (1.283×1021)×100%=3.93% Experimental Results This method focuses on generating TiUNPs. Limiting defects enhances reproducibility and enables a systematic examination of dopant effects. The flexibility to dope TiUNP with any fourth- period transition element and to decorate it with any fourth-period transition element or noble metal broadens the potential applications of titania. In the previous synthesis methods, TiCl4was initially hydrolyzed in a dopant-containing solution to directly assemble the dopant into TiUNP during the Ostwald ripening process. However, it has been shown that the combined Ostwald ripening and dopant assembly requires at least 48 hours for complete reaction, making a 2-hour incubation insufficient for full dopant incorporation. During the dialysis step, free dopant ions are lost through the pores of the dialysis membrane. The presently disclosed methodology improves TiUNP doping by conducting TiCl4hydrolysis in water rather than in a dopant solution. An ice bath at 0 °C is used to slow the rapid hydrolysis of TiCl4 in water, preventing the formation of TiO2 aggregates. The solution is then incubated at room temperature for 2 hours to ensure complete hydrolysis of TiCl4. Following this, the solution is dialyzed to pH 2.2, the optimal pH for Ostwald ripened particles to form (TiO2)159clusters. After dialysis, the solution is ultrafiltered with a nylon filter to remove any potential TiO₂ aggregates and debris from the dialysis membrane. The titania concentration in the solution is then measured by ICP-OES. A specific amount of metal chloride is added to achieve a one dopant-per- particle ratio. This method allows precise control of the doping process, maximizes doping efficiency, and improves the reproducibility of particle synthesis. A distinctive color change, as shown in Figure 2, indicates successful doping. 21 QB\166118.01470\93195440.1 Ref: 166118.01470 With the current improved methodology, it is possible to decorate the majority or substantially all of the doped TiUNP, e.g., with Pt, Au, or other catalytic metal atoms, and extends the synthesis to any desired transition or noble metal element. Decorating TiUNP with fourth-row transition metals is straightforward due to the natural affinity between the decorative metal ions and the fourth-row transition metal dopant ions. Once the decorative metal salt solution is added to the TiUNP solution, no additional action is required; incubating the solution for 48 hours completes the decoration process. Decorating doped TiUNP with noble metals is more challenging due to the size mismatch between the decorative and dopant ions and interaction between the noble metal ion and its ligands. Light irradiation facilitates deposition by providing the additional energy needed to overcome the ligand-metal interaction activation barrier. Approximating the ratio of doped TiUNP and an approximate quantity of catalytic metal atoms introduces two variables in the synthesis that can result in lower reproducibility. Utilization of a measurement, such as a two-step ICP measurement described herein, allows for precise control of the quantities of both the doped TiUNP and the catalytic metal atoms, leading to more accurate particle synthesis. Previously disclosed methods of Pt(IV)-Fe•TiUNP yielded not more than 17%. This limitation is due to the self-reduction, nucleation of catalytic metal atoms, such as platinum and gold precursors, into nanoparticles, which competes with the decoration reaction. In the previous approach, it was thought that capture would be a reduction process. To prevent competition with reduction of dissolving molecular oxygen, nitrogen gas was used to displace molecular oxygen from solution. However, in the presently disclosed methods, oxygen gas replaces nitrogen to suppress self-reduction, thus increasing the yield of catalyst, such as Pt(IV)-Fe•TiUNP and Au(III)-Fe•TiUNP, as shown in Figure 4. Although saturating with oxygen gas can suppress formation of noble metal nanoparticles, it cannot entirely eliminate the side reaction. As a result, some metal nanoparticles still form during the photodeposition process, preventing the actual yield from reaching the theoretical maximum. This methodology also offers a solution for treating these noble metal nanoparticles, allowing the recovery of precursors from the aggregates. Noble metal nanoparticles are highly robust and are generally considered chemically inert in most scenarios. Only powerful oxidants, such as aqua regia, fluorine gas, or hot chlorine gas, 22 QB\166118.01470\93195440.1 Ref: 166118.01470 can oxidize them. In aqueous solutions, common oxidants like hydrogen peroxide or nitric acid are ineffective against noble metal nanoparticles. The current method employs hypochlorous acid to oxidize noble metal aggregates. For example, gold nanoparticles can be effectively oxidized by hypochlorous acid under low-concentration stoichiometric conditions. The advantage of hypochlorous acid is that it can fully oxidize metal nanoparticles, generating HCl as product, which does not introduce extra contaminants. Furthermore, if excess hypochlorous acid is added to the solution, irradiating with Xenon lamp can rapidly decompose excess hypochlorous acid into HCl and oxygen gas. The doped and decorated TiUNPs are highly versatile and suitable for multiple applications. Here, we highlight two examples: organic pollutant degradation and carbon dioxide reduction. Error! Reference source not found. 5 shows the photo efficiency of various decorated Fe•TiUNP catalysts compared to plain Fe•TiUNP in the photodegradation of methanol to formaldehyde under photon-limited conditions. This setup directly reflects the absorbed photon utilization efficiency of each photocatalyst. As shown in Figure 5, cobalt-, nickel-, and tungsten- decorated Fe•TiUNP demonstrate approximately 30% improvement in photoefficiency compared to plain Fe•TiUNP. It is worth noting that Fe•TiUNP already exhibits a 3.5-fold enhancement over undoped TiUNP, which itself shows an order-of-magnitude improvement compared to the Degussa P25 photocatalyst. Figure 6 illustrates the reduction of carbon dioxide to formaldehyde by Mo-Cr•TiUNP, a valuable product that serves as a foundation for organic synthesis. It is conjectured that the chromium (III) dopant provides high-reduction-potential electrons necessary for carbon dioxide reduction, while the molybdenum (VI) decoration provides strong CO2binding at the reaction site. Doping TiUNPs with 4thRow Transition Metals Successful dopant incorporation is evidenced by a distinctive color due to the dopant in the oxide matrix. Figure 2 depicts the outcomes of modifying particles with any fourth-period transition element (aside from Sc). The photos in Figure 2 were taken with an iPhone® under ambient light: to the naked eye, TiUNP, Mn•TiUNP, and Zn•TiUNP all have the same white appearance. Standard conditions consist of ripening at pH 2.2, yielding 2 nm particles and doping at one atom per particle (Figure 3d). Smaller (larger) particles produced at lower (higher) pH levels necessitate proportionately higher (lower) dopant concentrations to net the targeted single-atom 23 QB\166118.01470\93195440.1 Ref: 166118.01470 doping. The observed color is quantified by Diffuse Reflectance Spectroscopy (DRS) spectra (Figure 7b). The photocatalytic efficiency depends on dopant Kinetic characterization is key for understanding photocatalytic efficiency. In this study, methanol photooxidation to formaldehyde is chosen as a standard reaction, consistent with prior research. The formaldehyde product is quantified using gas chromatography, flame ionization detection, GC-FID, as described in the supplementary section. Photocatalytic efficiency is significantly different when the solution is saturated with molecular oxygen than when molecular oxygen is displaced by nitrogen. Due to high electronegativity, molecular oxygen acts as an electron acceptor – a direct charge transfer pathway. Absent molecular oxygen, the most favorable electron acceptor is either proton or water both generate radicals – a homogeneous pathway. Doping TiUNP with first-row transition metal ions modulates the oxidation pathway as summarized in Table 1. Among all SA catalysts, Cr•TiUNP, Cu•TiUNP, and Fe•TiUNP significantly increase efficiency under oxygen saturation - direct charge transfer - with enhancements of 12%, 61%, and 242% times, respectively, while the homogeneous radical pathway remains unaffected. V•TiUNP exhibits an enhanced (~30%) photocatalytic efficiency in both oxygen and nitrogen saturation likely due to the multiple stable oxidation states of V. Particle size and band-gap The band gap is measured using UV-Vis spectroscopy following the method of Anderson et al.(20) building upon Brus’ model (23). The TiUNP band gap blue-shifts with respect to the bulk anatase and is consistent at 322 nm with various dopants. Calculation suggests a diameter of approximately 2.0 nm for TiUNP particulates, which remains unchanged after dopant incorporation. Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry, MALDI- TOFMS, complements UV-Vis spectroscopy to determine particle mass. Mild laser ionization reveals the parent and fragmentation peaks, enabling precise mass measurement by an average of 100 ionization events. Figure 3 shows a distinct parent peak at 12.7 kDa, conversion to TiO2 monomer units suggests that TiUNP particles consist of 159 TiO2units. This particle mass is consistent with theoretical calculations of a 5x5 isomorphic series (32), indicating a particle diameter of approximately 2 nm, depicted in Figure 3. 24 QB\166118.01470\93195440.1 Ref: 166118.01470 Electronic structure relationship to photo-oxidation Previous studies (19) using Raman spectroscopy determines the crystalline form of TiUNP is anatase. Diffuse reflectance spectroscopy (Figure 7b) was used to investigate the optical properties of solid TiUNP particles. A rising edge beginning at 387 nm and ending at 415 nm is evident. In the visible range, light of varying wavelengths was uniformly reflected. Two distinct types of optical properties are observed: [1] A red-shifted rising edge is present in Fe•TiUNP and Cu•TiUNP spectra relative to undoped TiUNP. [2] Valleys of reflectance are present in doped TiUNP within the flat, highly reflective visible range of undoped TiUNP, with characteristic wavelengths corresponding to the dopant. Titanium dioxide is one of the most abundant minerals on Earth. As a wide band gap semiconductor, TiO2 is extensively investigated in photocatalysis reactions ranging from water splitting to organic degradation (8,12,33,34,35). This versatility has led to thousands of reports annually. Frequently the goal of these efforts is to enhance photo efficiency, prolong catalyst longevity, or extend light absorption from ultraviolet range into the visible range (13,36,37). Among the methods for synthesizing TiO2 nanoparticles, sol-gel synthesis followed by calcination at different temperatures is commonly employed (38,39). While this approach yields stable solid particles, it often results in uncontrollable particle agglomeration and formation of multiple grain boundaries. Thermal treatment can also induce a phase transition from anatase to rutile, leading to mixed phases and potential loss of dopants from the platform. Likely due to this substrate variability, reported results are frequently contradictory, providing unclear guidance for targeting and enhancing desired transformations. In contrast to most reports, this report focuses on generating ultrasmall, < 2nm, particles – dubbed Ti ultrananoparticle – TiUNP. At this small size, the particles are more like large molecules with limited defects. Limiting defects both enhances reproducibility and facilitates a systematic examination of the impact of dopants. The Conclusion section contains a guide for selecting dopant for selected reactions. The ability to dope with any fourth-period transition element directing charge transfer significantly expands potential applications for titania. As indicated above, to address the challenge of reproducibility, this study adopted a wet synthesis-doping method, that limits particle size via pH, producing stable bipyramidal particles in an isomorphic series (Figure 3). Lower pH values yield smaller particles, while higher pH values yield larger ones, up to a limit of pH 2.4. Uncontrolled agglomeration is observed when pH exceeds 25 QB\166118.01470\93195440.1 Ref: 166118.01470 2.4. Dopants are incorporated during the Ostwald ripening, self-assembly process. The controlled- size supports a stoichiometric relationship between the number of TiO2 formula units and dopant atoms incorporated in the particle. Results reported herein are from particles Ostwald ripened at pH 2.2, generating 5x5 equator particles, with a formula of (TiO2)159 (Figure 3). Characterizing the morphology of ultrasmall particles is challenging; x-ray diffraction peaks tend to broaden significantly with only two to four atomic layers. However, molecular techniques, including Raman spectroscopy, are highly effective. Raman scattering (Figure 10) indicates that both TiUNP and doped TiUNP maintain an anatase crystal structure. Furthermore, the band gap, measured by extrapolating the linear UV absorbance, remains unaltered by doping. As expected, the band gap is larger for TiUNP compared to bulk anatase; however, as discussed below, the efficiency of transferring photogenerated charge carriers to adsorbent is greater, resulting in enhanced photo oxidation efficiency for the ultra-nano particles. The square bipyramid anatase structure is truncated with a high-energy

[0001] facet, which serves as the catalytic site for undoped TiUNP. During Ostwald ripening, dopants self-assemble into this high-energy face through a thermodynamically driven process. The covalently bonded dopant atoms are robust, resistant to loss during dialysis, particle immobilization on substrates, and photo-catalysis, all verified through ICP analysis. In contrast, non-covalently bonded dopants are prone to loss during dialysis due to the small size of the free ions, allowing them to flow through the dialysis tubing. Previous studies on iron doped TiUNP, Fe•TiUNP (18,19), suggest that photo efficiency is maximized at a 1:1 dopant-to-particle ratio, further supporting the stoichiometric relationship between dopant and particle. XAFS data (20) indicate that the iron dopant is located in the high-energy

[0001] face. In contrast, randomly distributed doping or encapsulated dopants resulting from thermal treatment contribute minimally to catalytic efficiency (40,41). Although TiO2 has been extensively used as a photocatalyst for treating organic water pollution, its impact is constrained by low efficiency. Previous research (19) has identified two distinct reaction pathways for TiUNP: a homogeneous radical reaction (HRR) pathway and a direct charge transfer (DCT) pathway. HRR, which has been extensively studied, involves generation of hydroxyl radicals and water reduction (8,12). Free radicals are easily deactivated, so TiO2 is often used as a co-catalyst with iron salts to enhance radical production in processes such as Fenton or photo-Fenton chemistry (33,34). In contrast, TiUNP and doped TiUNP enhance efficiency by promoting DCT. This pathway offers greater efficiency for several reasons. Firstly, the structured 26 QB\166118.01470\93195440.1 Ref: 166118.01470 platform facilitates adsorption of organic molecules and molecular oxygen onto the particle surface, significantly increasing the local concentration of both reactants. Secondly, DCT reactions occur faster because limitations due to diffusion of active species in the aqueous phase are lifted, resulting in a higher turnover frequency. It is found that direct charge transfer is 2.5 times more efficient than the homogeneous radical pathway on the TiUNP platform (Table 1 and Figure 7a). Thirdly, DCT can be modulated and enhanced by dopants, while HRR remains insensitive to such dopant modulation. Photo efficiency is evaluated using a standard methanol oxidation reaction under pseudo- first order conditions: saturated particle and methanol loading ensuring that the reaction is photon flux limited. Molecular oxygen serves as the ultimate electron destination in DCT. Hence, DCT and HRR can be distinguished by oxygen concentration: either by saturating the solution with molecular oxygen or by displacing oxygen with molecular nitrogen. Kinetic characterization of all doped TiUNPs, (Figure 7a and Table 1), demonstrates a consistent trend towards higher photo efficiency in direct charge transfer (oxygen saturation) compared to the homogeneous radical (nitrogen saturation). Characterization with various dopants reveals that dopants with stable redox pairs, such as Cr(II / III), Fe(II / III), and Cu(I / II) significantly enhance DCT. Absorption of a UV photon generates an electron-hole pair. Dopants with a suitable redox couple introduce a temporary destination for the conduction band electron, enhancing charge separation, ultimately transferring the electron to adsorbed molecular oxygen. Redirecting electrons to molecular oxygen and holes toward adsorbed organic molecules extends charge-carrier lifetime hence increases efficiency. Enhancement of DCT is calculated as the ratio of the rate of oxidation for doped TiUNP to the rate for undoped TiUNP with oxygen saturation. Enhancement from Cr(II / III), Cu(I / II), and Fe(II / III) are measured at 12%, 61%, and 242% times, respectively. Differences in modulation are attributed to the reduction potential of the dopants (Figure 8) with Fe(III) being the most efficient electron capture agent, Cr(III) the least. Kinetic characterization results suggest a connection between reduction potential and enhanced efficiency. Dopant-platform interaction data is obtained via diffuse reflectance spectroscopy (Figure 7b and Figure 9). Undoped TiUNP exhibits low reflectance to the rising edge at 387 nm, beyond which reflectance is consistently high. This reflectance spectrum aligns with the optical properties of anatase, featuring a band gap at 387 nm and no absorbance in the visible 27 QB\166118.01470\93195440.1 Ref: 166118.01470 range, corresponding to the widely reported energy gap of 3.2 eV. Thus, although the bandgap of TiUNP blue shifts with diminishing size, its emission (reflectance) remains at the bulk band-edge energy suggesting nonradiative relaxation to the bulk conduction band energy. Electrons in anatase excited by UV photons temporarily reduce lattice Ti(IV) to Ti(III) (39). Without chemical reaction or other destination, the localized electron recombines with the valence band hole within nanoseconds. Both Cu(II) and Fe(III) have a more negative reduction potential than that of Ti(IV), as depicted schematically in Figure 8, leading to electron localization on these dopants. Consistently, the DRS rising edges for Cu•TiUNP and Fe•TiUNP are measured at 2.75 eV and 2.51 eV, respectively, reflecting the reduction potential of these ions in the lattice. Consequently, Cu(II) or Fe(III) incorporated into TiUNP serves as a temporary electron destination instead of Ti(III) in the lattice. These dopants ultimately transfer the electron to adsorbed molecular oxygen, hence the reaction is quenched in the absence of molecular oxygen. In the presence of organic substances, the valence band hole is transferred to the organic substance, to cycle the catalyst the conduction band electron must be consumed. Therefore, the fate of the conduction band electron directly affects photo efficiency. The TiUNP conduction band reduction potential is -0.18 V, a potential high enough to either reduce water generating hydrogen, HRR, or to reduce oxygen generating water, DCT. In the presence of adsorbed molecular oxygen, DCT is faster and thermodynamically favored over the homogeneous, radical reaction. Radical deactivation reduces charge utilization and lowers photo efficiency. These separate pathways explain both the boost in photo efficiency observed with Cu•TiUNP and Fe•TiUNP over that of undoped TiUNP under oxygen saturation, and the unaltered photo efficiency of these two relative to undoped TiUNP under nitrogen saturation (Table 1). Fe, Cu and Cr all shift the reduction potential of the electron so that it no longer generates hydrogen from water or proton, thus quenching competition for the electron. Molecular oxygen captures the electron, the ideal destination for an environmentally friendly oxidation process. While many studies focus on doping TiO2or thermally treating it to broaden absorbance into the visible range, it is important to recognize that photo efficiency relies on the net utilization of photons rather than the broadness of photon absorbance. Greater visible light absorbance can be achieved by introducing dopants to TiUNP, as depicted in Figure 9, verified by DRS. However, the absorbance of visible light does not necessarily lead to greater photo efficiency. Rather visible absorbance is attributed to the dopant d-d transition. While the d-d transition is forbidden in an 28 QB\166118.01470\93195440.1 Ref: 166118.01470 octahedral environment, at the surface the covalently bonded dopant has lower symmetry, lifting symmetry restriction, resulting in intense absorbance characteristic of the dopants. As a localized transition, the excited electron remains localized on the dopant lacking energy to drive photocatalysis or transfer charge to the conduction band. Thus, the absorbed visible light is trivially consumed via non-radiative charge recombination and does not contribute to enhancement of photo efficiency. The case of V doping is more complex: V has multiple, air-stable oxidation states. This is likely the source of enhancement under both aerobic and anerobic conditions. TiUNP provides a reproducible, molecule-like platform for understanding catalytic mechanisms and characterizing photo efficiency. TiUNP exhibits both HRR and DCT, with the charge-transfer path controlled by the dopant. Thus, fourth-period transition metal ions with favorable redox pairs enhance efficiency in the presence of molecular oxygen by localizing charge and subsequently transferring that charge to molecular oxygen. Among all dopants, those with redox pairs below HER enhance DCT. For example, Cu(I / II) and Fe(II / III) pairs show significant improvements in DCT due to a more negative reduction potential than that of the Ti(IV), allowing them to capture the electron directly transferring it to molecular oxygen. UV photons with sufficient energy efficiently trigger oxidation, while electrons generated via visible photons lack the potential to separate charge carrier pairs. Therefore, a promising approach to improving photo efficiency is to enhance DCT by doping with suitable dopants and focusing on improving the net utilization of absorbed UV light. Conclusion This work develops a tunable method for synthesizing a versatile TiO2 photocatalyst platform. The small molecule-like TiUNP platform exhibits reproducible results in size, morphology, optical properties, and photocatalytic efficiency. The thermodynamically driven dopant incorporation method offers a reproducible way to assemble all fourth-row transition metal ions onto the catalyst. TiUNP forms an isomorphic series via pH control during the synthesis-self-assembly process. Optimal doping is observed to be 1:1 dopant-to-particle ratio. The platform exhibits both a homogeneous radical reaction (HRR) and direct charge transfer (DCT) pathway, the balance is influenced by the presence of molecular oxygen. Doping TiUNP with transition metal ions selectively modulates the DCT pathway while leaving the HRR pathway unaffected. Notably, dopants with a more negative reduction potential, such as Cu(II) and Fe(III), show significant 29 QB\166118.01470\93195440.1 Ref: 166118.01470 enhancements to the DCT pathway. These dopants effectively trap conduction band electrons, efficiently utilizing molecular oxygen attached to the particle, thereby enhancing photo efficiency. Table 1 Oxidation efficiency for various single-atom doped TiUNP Efficiency (ppm H2CO / min) Relative to O2 saturated TiUNP O2 saturation N2 saturation O2 saturation N2 saturation TiUNP 0.1532 0.0608 1 0.40 V•TiUNP 0.2053 0.0831 1.34 0.54 Cr•TiUNP 0.1719 0.0535 1.12 0.35 Mn•TiUNP 0.1522 0.0606 0.99 0.39 Fe•TiUNP 0.5243 0.0500 3.42 0.33 Co•TiUNP 0.1469 0.0558 0.96 0.36 Ni•TiUNP 0.1612 0.0559 1.05 0.36 Cu•TiUNP 0.2466 0.0641 1.61 0.42 Zn•TiUNP 0.1580 0.0481 1.03 0.31 REFERENCES (1) Ni, M.; Leung, M. K.; Leung, D. Y.; Sumathy, K. A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renewable and Sustainable Energy Reviews 2007, 11 (3), 401-425. (2) Naldoni, A.; Altomare, M.; Zoppellaro, G.; Liu, N.; Kment, S.; Zboril, R.; Schmuki, P. Photocatalysis with reduced TiO2: from black TiO2 to cocatalyst-free hydrogen production. ACS catalysis 2018, 9 (1), 345-364. (3) Van Driel, B.; Kooyman, P.; Van den Berg, K.; Schmidt-Ott, A.; Dik, J. A quick assessment of the photocatalytic activity of TiO2 pigments—From lab to conservation studio! Microchem. J. 2016, 126, 162-171. (4) Chen, J.; Liu, M.; Zhang, L.; Zhang, J.; Jin, L. Application of nano TiO2 towards polluted water treatment combined with electro-photochemical method. Water Res.2003, 37 (16), 3815-3820. 30 QB\166118.01470\93195440.1 Ref: 166118.01470 (5) Kapoor, A.; Rajput, J. K. Solar light photocatalytic activity of CuO / TiO2 mixed oxide derived from conjugated azomethine metal complex for degradation of food colorants. J. Mol. Liq. 2022, 366, 120280. (6) Gesenhues, U. Al-doped TiO2 pigments: influence of doping on the photocatalytic degradation of alkyd resins. J. Photochem. Photobiol. A: Chem.2001, 139 (2-3), 243-251. (7) Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. Explaining the enhanced photocatalytic activity of Degussa P25 mixed-phase TiO2 using EPR. The Journal of Physical Chemistry B 2003, 107 (19), 4545-4549. (8) Odling, G.; Robertson, N. Why is anatase a better photocatalyst than rutile? The importance of free hydroxyl radicals. ChemSusChem 2015, 8 (11), 1838-1840. (9) Siddhapara, K.; Shah, D. Characterization of nanocrystalline cobalt doped TiO2 sol– gel material. J. Cryst. Growth 2012, 352 (1), 224-228. (10) Yoshida, T.; Niimi, S.; Yamamoto, M.; Nomoto, T.; Yagi, S. Effective nitrogen doping into TiO2 (N-TiO2) for visible light response photocatalysis. Journal of colloid and interface science 2015, 447, 278-281. (11) Park, Y.; Kim, W.; Park, H.; Tachikawa, T.; Majima, T.; Choi, W. Carbon-doped TiO2 photocatalyst synthesized without using an external carbon precursor and the visible light activity. Applied Catalysis B: Environmental 2009, 91 (1-2), 355-361. (12) Zhang, J.; Zhou, P.; Liu, J.; Yu, J. New understanding of the difference of photocatalytic activity among anatase, rutile and brookite TiO2. Physical Chemistry Chemical Physics 2014, 16 (38), 20382-20386. (13) Sahoo, S. S.; Mansingh, S.; Babu, P.; Parida, K. Black titania an emerging photocatalyst: review highlighting the synthesis techniques and photocatalytic activity for hydrogen generation. Nanoscale Advances 2021, 3 (19), 5487-5524. (14) Ren, J.; Tao, L.; Luo, Z.; Zeng, J.; Yin, D. Solvent-Free Synthesis of Cu / In Dual- Single-Atom-Doped TiO2 for Photocatalytic Reduction of Cr (VI) with High Efficiency. ACS Applied Engineering Materials 2024, 2 (4), 988-999. (15) Xu, C.; Liu, L.; Zhang, X.; Guo, L.; Zhang, X.; Huang, Z.; Wu, X.; Zhao, H.; Jing, G.; Shen, H. Enhanced catalytic oxidation of benzene though the synergistic Pt-Ni bimetallic single-atom catalyst. Chem. Eng. J.2024, 480, 148361. 31 QB\166118.01470\93195440.1 Ref: 166118.01470 (16) Jaimes Ramirez, R.; Pineda Arellano, C. A.; Alvarez Gallegos, A. A.; Jimenez Gonzalez, A. E.; Silva Martinez, S. H2O2-assisted TiO2 generation during the photoelectrocatalytic process to decompose the acid green. textile dye by Fenton reaction. JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 2015, 305, 51-59. (17) Zhang, A.-Y.; Lin, T.; He, Y.-Y.; Mou, Y.-X. Heterogeneous activation of H2O2 by defect-engineered TiO2− x single crystals for refractory pollutants degradation: A Fenton-like mechanism. J. Hazard. Mater.2016, 311, 81-90. (18) Wang, C.-y.; Bahnemann, D. W.; Dohrmann, J. K. A novel preparation of iron-doped TiO2 nanoparticles with enhanced photocatalytic activity. Chem. Commun.2000, (16), 1539-1540. (19) Dukes, F. M.; Iuppa, E.; Meyer, B.; Shultz, M. J. Differing photo-oxidation mechanisms: electron transfer in TiO2 versus iron-doped TiO2. Langmuir 2012, 28 (49), 16933- 16940. (20) Anderson, N. J.; Xu, T.; Ouyang, M.; Bisson, P. J.; Davies, R. G.; Marmolejos, J. M.; Shultz, M. J. Photosynthesis of a photocatalyst: single atom platinum captured and stabilized by an iron (III) engineered defect. The Journal of Physical Chemistry A 2020, 125 (1), 88-98. (21) Wang, C.-y.; Groenzin, H.; Shultz, M. J. Comparative study of acetic acid, methanol, and water adsorbed on anatase TiO2 probed by sum frequency generation spectroscopy. Journal of the American Chemical Society 2005, 127 (27), 9736-9744. (22) Wang, C.-y.; Groenzin, H.; Shultz, M. J. Direct observation of competitive adsorption between methanol and water on TiO2: an in situ sum-frequency generation study. Journal of the American Chemical Society 2004, 126 (26), 8094-8095. (23) Brus, L. Electronic wave functions in semiconductor clusters: experiment and theory. The Journal of Physical Chemistry 1986, 90 (12), 2555-2560. (24) Lin, H.; Huang, C.; Li, W.; Ni, C.; Shah, S. I.; Tseng, Y.-H. Size dependency of nanocrystalline TiO2 on its optical property and photocatalytic reactivity exemplified by 2- chlorophenol. Applied Catalysis B: Environmental 2006, 68 (1-2), 1-11. (25) Viswanatha, R.; Sapra, S.; Satpati, B.; Satyam, P.; Dev, B.; Sarma, D. Understanding the quantum size effects in ZnO nanocrystals. J. Mater. Chem.2004, 14 (4), 661-668. (26) Schnaible, V.; Michels, J.; Zeth, K.; Freigang, J.; Welte, W.; Bühler, S.; Glocker, M. O.; Przybylski, M. Approaches to the characterization of membrane channel proteins (porins) by 32 QB\166118.01470\93195440.1 Ref: 166118.01470 UV MALDI-MS. International journal of mass spectrometry and ion processes 1997, 169, 165- 177. (27) Yoo, H.-J.; Kim, D.-H.; Shin, D.; Oh, Y.; Lee, S.; Lee, J. Y.; Choi, Y.-J.; Lee, S. H.; Lee, K.-S.; Kim, Y. Recent developments in pre-treatment and analytical techniques for synthetic polymers by MALDI-TOF mass spectrometry. Analytical Methods 2020, 12 (48), 5767-5800. (28) Lin, Z.; Natoli, J. M.; Picuri, J. C.; Shaw, S. E.; Bowyer, W. J. Replication of the conversion of goethite to hematite to make pigments in both furnace and campfire. Journal of Archaeological Science: Reports 2021, 39, 103134. (29) To, N. D. K.; Theruvathu, J. A. Determination and Quantification of Acetaldehyde, Acetone, and Methanol in Hand Sanitizers Using Headspace GC / MS: Effect of Storage Time and Temperature. International Journal of Environmental Research and Public Health 2024, 21 (1), 74. (30) Ferreira, V.; Culleré, L.; Loscos, N.; Cacho, J. Critical aspects of the determination of pentafluorobenzyl derivatives of aldehydes by gas chromatography with electron-capture or mass spectrometric detection: Validation of an optimized strategy for the determination of oxygen- related odor-active aldehydes in wine. Journal of Chromatography A 2006, 1122 (1-2), 255-265. (31) Gabrio, T.; Bertsch, A. Determination of carbonyl compounds in pool water with O- (2, 3, 4, 5, 6-pentafluorobenzyl) hydroxyamine hydrochloride and gas chromatographic–tandem mass spectrometric analysis. Journal of Chromatography A 2004, 1046 (1-2), 293-296. (32) Asong, N.; Dukes, F.; Wang, C.-y.; Shultz, M. J. The effect of iron doping on the adsorption of methanol on TiO2 probed by sum frequency generation. Chem. Phys.2007, 339 (1- 3), 86-93. (33) Bauer, R.; Waldner, G.; Fallmann, H.; Hager, S.; Klare, M.; Krutzler, T.; Malato, S.; Maletzky, P. The photo-fenton reaction and the TiO2 / UV process for waste water treatment− novel developments. Catal. Today 1999, 53 (1), 131-144. (34) Malato, S.; Blanco, J.; Cáceres, J.; Fernández-Alba, A.; Agüera, A.; Rodrıguez, A. Photocatalytic treatment of water-soluble pesticides by photo-Fenton and TiO2 using solar energy. Catal. Today 2002, 76 (2-4), 209-220. (35) Reddy, K. M.; Reddy, C. G.; Manorama, S. Preparation, characterization, and spectral studies on nanocrystalline anatase TiO2. J. Solid State Chem.2001, 158 (2), 180-186. 33 QB\166118.01470\93195440.1 Ref: 166118.01470 (36) Basavarajappa, P. S.; Patil, S. B.; Ganganagappa, N.; Reddy, K. R.; Raghu, A. V.; Reddy, C. V. Recent progress in metal-doped TiO2, non-metal doped / codoped TiO2 and TiO2 nanostructured hybrids for enhanced photocatalysis. Int. J. Hydrogen Energy 2020, 45 (13), 7764- 7778. (37) Nah, Y. C.; Paramasivam, I.; Schmuki, P. Doped TiO2 and TiO2 nanotubes: synthesis and applications. Chemphyschem 2010, 11 (13), 2698-2713. (38) Bahnemann, D. W.; Hilgendorff, M.; Memming, R. Charge carrier dynamics at TiO2 particles: reactivity of free and trapped holes. The Journal of Physical Chemistry B 1997, 101 (21), 4265-4275. (39) Kuznetsov, A.; Kameneva, O.; Alexandrov, A.; Bityurin, N.; Marteau, P.; Chhor, K.; Sanchez, C.; Kanaev, A. Light-induced charge separation and storage in titanium oxide gels. Physical Review E 2005, 71 (2), 021403. Espino-Valencia, J.; Romero, R.; Rico-Cerda, J. L.; Arroyo- Mo doped TiO2: synthesis, characterization and photocatalytic activity. Fuel 2017, 198, 31-41. (41) Jiang, P.; Xiang, W.; Kuang, J.; Liu, W.; Cao, W. Effect of cobalt doping on the electronic, optical and photocatalytic properties of TiO2. Solid State Sciences 2015, 46, 27-32. 34 QB\166118.01470\93195440.1

Claims

Ref: 166118.01470 CLAIMS We claim:

1. A method for synthesizing doped TiO2ultra-nanoparticles, the method comprising (a) mixing a dopant solution comprising dopant metal atoms with a solution of TiO2 ultra- nanoparticles; and (b) incubating the mixture of dopant solution and solution of TiO2ultra-nanoparticles for a time to form doped TiO2ultra-nano particles.

2. The method of claim 1, further comprising preparing the solution of TiO2ultra- nanoparticles.

3. The method of claim 2, wherein preparing the solution of TiO2 ultra-nanoparticles comprises hydrolyzing TiCl4in an aqueous solution, adjusting the pH of the aqueous solution of hydrolyzed TiCl4, and incubating the pH-adjusted, aqueous solution of hydrolyzed TiCl4 for a time to form TiO2 ultra-nanoparticles.

4. The method of claim 3, wherein the pH of the aqueous solution of hydrolyzed TiCl4is adjusted to form (TiO2)159 particles.

5. The method of any of claims 1-4, wherein the dopant solution comprises a concentration of dopant metal atoms that when mixed with the aqueous solution of TiO2 ultra-nanoparticles results in formation of a plurality of doped TiO2ultra-nano particles comprising a single dopant metal atom.

6. The method of claim 5, wherein the dopant solution is prepared from a chloride salt, sodium salt, nitrate salt, sulfate salt, acetate salt, carbonate salt, or potassium salt comprising the dopant metal atoms or the dopant metal atoms comprise Cr, Fe, V, Mn, Co, Ni, Cu, or Zn.

7. The method of claim 5, wherein the dopant solution is prepared from a chloride salt, sodium salt, nitrate salt, sulfate salt, acetate salt, carbonate salt, or potassium salt comprising the dopant metal atoms and the dopant metal atoms comprise Cr, Fe, V, Mn, Co, Ni, Cu, or Zn. 35 QB\166118.01470\93195440.1Ref: 166118.01470 8. A method for preparing a catalyst, the method comprising decorating a doped TiO2 ultra- nanoparticle with a single catalytic metal atom.

9. The method of claim 8, wherein decorating the catalyst comprises photo irradiating the doped TiO2ultra-nanoparticles in the presence of catalytic metal atoms and oxygen.

10. The method of claim 9 further comprising mixing an acidic solution with the photo- irradiated doped TiO2ultra-nanoparticles and secondary metal atoms and re-irradiating the mixture, optionally wherein mixing the acidic solution with the photo-irradiated doped TiO2ultra- nanoparticles and secondary metal atoms and re-irradiating the mixture is repeated one or more times.

11. The method of claim 10, wherein the acidic solution is a hypochlorous acid solution or the catalytic metal atoms comprise Pt, Au, Os, orIr.

12. The method of claim 10, wherein the acidic solution is a hypochlorous acid solution the catalytic metal atoms comprise Pt, Au, W, Os, or Ir.

13. The method of claim 8, wherein decorating the photocatalyst comprises mixing doped TiO2ultra-nanoparticles with a chloride salt solution or a sodium salt solution comprising the catalytic metal atoms.

14. The method of claim 13, wherein the chloride salt solution or the sodium salt solution comprises V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W, Ru, Rh, or Pd.

15. The method of any one of claims 8-14 further comprising synthesizing the doped TiO2 ultra-nanoparticles by (a) mixing a dopant solution comprising dopant metal atoms with a solution of TiO2ultra- nanoparticles; and 36 QB\166118.01470\93195440.1Ref: 166118.01470 (b) incubating the mixture of dopant solution and solution of TiO2 ultra-nanoparticles for a time to form doped TiO2 ultra-nanoparticles.

16. The method of claim 15, further comprising preparing the solution of TiO2 ultra- nanoparticles.

17. The method of claim 16, wherein preparing the solution of TiO2ultra-nanoparticles comprises hydrolyzing TiCl4 in an aqueous solution, adjusting the pH of the aqueous solution of hydrolyzed TiCl4, and incubating the pH-adjusted, aqueous solution of hydrolyzed TiCl4for a time to form TiO2ultra-nanoparticles.

18. The method of claim 17, wherein the pH of the aqueous solution of hydrolyzed TiCl4 is adjusted to form (TiO2)159particles.

19. The method of any one of claims 15-18, wherein the dopant solution comprises a concentration of dopant metal atoms that when mixed with the aqueous solution of TiO2ultra-nano particles results in formation of doped TiO2ultra-nano particles comprising a single dopant metal atom.

20. The method of any of claims 15-19, wherein the dopant solution is prepared from a chloride salt, sodium salt, nitrate salt, sulfate salt, acetate salt, carbonate salt, or potassium salt comprising the dopant metal atom and / or the dopant metal atom is V, Cr, Mn, Fe, Co, Ni, Cu, or Zn.

21. A composition comprising a plurality of TiO2 ultra-nanoparticles prepared by the method according to claim 1.

22. The composition of claim 21, wherein the plurality of TiO2 ultra-nanoparticles is prepared by the method according to 2-7.

23. A composition comprising a plurality of catalysts prepared by the method according to claim 8. 37 QB\166118.01470\93195440.1Ref: 166118.01470 24. The composition of claim 23, wherein the plurality of catalysts is prepared by the method according to any one of claims 9-20.

25. The composition of claim 23, wherein greater than 20% of the TiO2 ultra-nanoparticles are decorated with the single catalytic metal atom.

26. A composition comprising a plurality of catalysts, wherein the catalyst comprises doped TiO2ultra-nanoparticles decorated with a single catalytic metal atom.

27. The composition of claim 26, wherein the single catalytic metal atom is Au, Ru, Os, Rh, Ir, Pd, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, W, or Mo.

28. The composition of claim 26, wherein greater than 20% of the TiO2 ultra-nanoparticles are decorated with the single catalytic metal atom.

29. The composition of claim 28, wherein the single catalytic metal atom is Au, Ru, Os, Rh, Ir, Pd, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, W, Mo, or Pt.

30. The composition of any one of claims 26-29, wherein the doped TiO2ultra-nanoparticles comprise a single V, Cr, Mn, Fe, Co, Ni, Cu, Zn, or Mo dopant metal atom positioned as an engineered defect within the doped TiO2ultra-nanoparticle.

31. The composition of any one of claims 26-29, wherein the doped TiO2 ultra-nanoparticles comprise a Cu, V, Ni, Zn, Ag, or Mo dopant metal atom positioned as an engineered defect within the doped TiO2ultra-nanoparticle.

32. A method for reducing CO2, the method comprising irradiating a catalyst comprising doped TiO2ultra-nanoparticles decorated with a single catalytic metal atom in the presence of CO2. 38 QB\166118.01470\93195440.1Ref: 166118.01470 33. The method of claim 32, wherein the catalyst comprising doped TiO2 ultra-nanoparticles decorated with the single catalytic metal atom is the catalyst according to any one of claims 26- 31.

34. A method for oxidizing an organic substrate, the method comprising irradiating a doped TiO2ultra-nanoparticles decorated with a single catalytic metal atom in the presence of an organic substrate.

35. The method of claim 34, wherein the TiO2ultra-nanoparticles decorated with a single catalytic metal atom are irradiated in the presence of oxygen.

36. The method of claim 34 or 35, wherein the catalyst comprising doped TiO2 ultra- nanoparticles decorated with the single catalytic metal atom is the catalyst according to any one of claims 26-31. 39 QB\166118.01470\93195440.1

Images