Metal-free carbon dots-polydopamine heterojunction photocatalyst for hydrogen peroxide production and method for manufacturing the same

Abstract

The present invention relates to a carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide production and a method for manufacturing the same. More specifically, the carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide production is a powder particle containing carbon dots and polydopamine, wherein carbon dots are formed on the surface of polydopamine, and carbon dots and polydopamine form a second type heterojunction structure.

Description

Technology Field

[0001] The present invention relates to a carbon dot-polydopamine heterojunction metal-free photocatalyst for generating hydrogen peroxide using a heterojunction of carbon dots and polydopamine, and a method for manufacturing the same. Background Technology

[0002] Hydrogen peroxide (H2O2) is an essential, eco-friendly oxidizing agent in various industrial and environmental fields. It is used as a bleaching agent in semiconductor cleaning and the paper and pulp industries, and plays an important role in environmental purification processes such as the decomposition of organic pollutants and wastewater treatment. In addition, it is widely used as an antimicrobial and disinfectant in the medical and hygiene industries.

[0003] However, current production of hydrogen peroxide relies mainly on the anthraquinone (AQ) process, which uses organic solvents and precious metal catalysts such as palladium (Pd). This process has disadvantages such as high energy consumption, complexity, and the generation of harmful byproducts, which cause environmental problems.

[0004] To address these issues, photocatalytic-based technology that utilizes light energy to directly produce hydrogen peroxide from water and oxygen is attracting attention as an eco-friendly and sustainable alternative. However, existing inorganic photocatalysts (such as TiO2 and BiVO4) have limitations in practical application due to issues such as rapid recombination of electrons and holes, low stability, and the rapid decomposition of the generated hydrogen peroxide, even if they exhibit excellent initial activity.

[0005] As an alternative to overcome these limitations, carbon dots (CDs) have recently garnered attention as promising candidates for organic-based photocatalysts, based on their advantages such as excellent light absorption characteristics, oxygen-rich functional groups, efficient charge transport capabilities, and eco-friendliness. However, carbon dots also face limitations in that they are difficult to apply as high-efficiency photocatalysts alone due to the limited absorption of visible light and the rapid recombination of photogenerated charge carriers.

[0006] Therefore, the development of carbon dot-based photocatalyst technology capable of producing hydrogen peroxide sustainably, economically, and with high efficiency is emerging as an urgent task at present. Prior art literature

[0007] Korean Patent Publication No. 10-1858007, published May 9, 2018. Korean Patent Publication No. 10-1743945, published May 31, 2017. The problem to be solved

[0008] The present invention was developed to solve the aforementioned problems, and aims to provide a carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide production with an expanded absorption range and excellent hydrogen peroxide production efficiency, and a method for manufacturing the same.

[0009] The objectives of the present invention are not limited to those mentioned above, and other unmentioned objectives may be clearly understood from the descriptions below and may be sufficiently included in the objectives of the present invention. means of solving the problem

[0010] A carbon dot-polydopamine heterojunction metal-free photocatalyst for generating hydrogen peroxide according to one aspect of the present invention for achieving the above objective is a powder particle comprising carbon dots and polydopamine, wherein carbon dots are formed on the surface of polydopamine and carbon dots and said polydopamine form a second type heterojunction structure.

[0011] A method for manufacturing a carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide generation according to another aspect of the present invention for achieving the above objective comprises the steps of: preparing carbon dot powder; dispersing the carbon dot powder in deionized water to form a carbon dot solution; preparing a polydopamine solution; reacting the carbon dot solution with the polydopamine solution to synthesize a carbon dot-polydopamine heterojunction metal-free photocatalyst; purifying the carbon dot-polydopamine heterojunction metal-free photocatalyst through centrifugation; and washing and drying the carbon dot-polydopamine heterojunction metal-free photocatalyst. Effects of the invention

[0012] The carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide generation and the method for manufacturing the same according to the present invention can be expected to have the following effects.

[0013] First, by combining two organic materials, carbon dots and polydopamine, to form a Type II heterojunction structure, a two-electron oxygen reduction reaction (2e) is performed without a separate sacrificial agent. - Hydrogen peroxide can be selectively generated by inducing ORR.

[0014] In addition, photocatalytic performance can be improved by maximizing the separation efficiency of photo-excited electrons and holes through the optimization of surface functional group interactions and band alignment between carbon dots and polydopamine. In particular, since carbon dots act as electron-active sites and polydopamine promotes hole annihilation through catechol groups on the surface, high hydrogen peroxide production efficiency can be achieved without an external electron donor. Brief explanation of the drawing

[0015] FIG. 1 is a flowchart of a method for manufacturing a carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide generation according to an embodiment of the present invention. Figures 2a and 2b are SEM, TEM images and average particle size (diameter) analysis results according to an embodiment of the present invention. FIGS. 3a and 3b are FT-IR spectra, XPS survey spectra, and deconvolution-processed high-resolution XPS spectra according to an embodiment of the present invention. Figure 4 is a UV-Vis spectrum according to an embodiment of the present invention and a Tauc plot derived therefrom. FIGS. 5a and 5b are a photoluminescence spectrum, electrochemical impedance spectrum, Mott-Shotsky plot, and energy band structure according to an embodiment of the present invention. Figure 6 is the result of a radical capture experiment according to an embodiment of the present invention. Figure 7 is a comparative result of quantitatively analyzing the amount of hydrogen peroxide produced for 2 hours according to an embodiment of the present invention. Figure 8a shows the results of confirming the generation mechanism by comparing and analyzing the amount of hydrogen peroxide produced under light / dark conditions and oxygen / air / nitrogen atmospheres. Figure 8b shows the results of the evaluation of stability during long-term light irradiation and sustainability in a light / dark alternating repeated environment. Figures 9a and 9b show the results of the hydrogen peroxide decomposition experiment and the reusability evaluation experiment over time. Specific details for implementing the invention

[0016] The present invention relates to a carbon dot-polydopamine heterojunction metal-free photocatalyst for generating hydrogen peroxide (H2O2) using a heterojunction of carbon dots (CDs) and polydopamine (PDA), and a method for manufacturing the same.

[0018] A carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide generation and a method for manufacturing the same, according to a preferred embodiment of the present invention, will be described in detail below with reference to the drawings.

[0020] A carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide generation according to one embodiment of the present invention may be composed of powder particles containing carbon dots and polydopamine.

[0021] And polydopamine can be formed through a self-polymerization reaction from dopamine hydrochloride dissolved in an alkaline solution at a concentration of 4 to 10 mg / mL, exhibits excellent light absorption performance over a broad wavelength range from ultraviolet to visible light, and undergoes a two-electron oxygen reduction reaction (2e - It has photoactivity capable of inducing ORR.

[0022] At this time, carbon dots can be formed on the surface of polydopamine.

[0023] And carbon dots and polydopamine can form a heterojunction structure to mutually complement their respective limitations.

[0024] However, carbon dots and polydopamine can form a Type II heterojunction structure in which energy bands are staggered among heterojunction structures.

[0025] According to the Type II heterojunction structure, electrons move toward the carbon dot with the lower conduction band, and holes move toward the polydopamine with the higher valence band, thereby inducing spatial separation of charge carriers.

[0026] As a result, the recombination of electron-hole pairs generated by light irradiation is suppressed, and the reaction with oxygen molecules (O2) is promoted without loss of electrons, thereby improving the selectivity and efficiency of hydrogen peroxide production and effectively enhancing the photocatalyst-based hydrogen peroxide production performance.

[0027] In addition, the light absorption range of a single semiconductor-based photocatalyst is expanded through a type 2 heterojunction structure, and photochemical stability is also improved through the interaction between semiconductors, allowing for the continuous production of hydrogen peroxide without performance degradation even under long-term reaction conditions.

[0028] It can be confirmed that the amount of hydrogen peroxide produced is improved by more than three times compared to when single components, such as carbon dots or polydopamine, are used according to the type 2 heterojunction structure.

[0029] At this time, the carbon dot-polydopamine heterojunction metal-free photocatalyst can have a size of 254.3±63.7~494.51±123.5 nm in the form of powder particles.

[0031] A method for manufacturing a carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide generation according to another embodiment of the present invention may be configured to include, as illustrated in FIG. 1, a carbon dot powder preparation step (S110), a carbon dot solution formation step (S120), a polydopamine solution preparation step (S130), a carbon dot-polydopamine heterojunction metal-free photocatalyst synthesis step (S140), a carbon dot-polydopamine heterojunction metal-free photocatalyst purification step (S150), and a carbon dot-polydopamine heterojunction metal-free photocatalyst washing and drying step (S160).

[0033] The carbon dot powder preparation step (S110) includes the step of forming a reaction solution by dissolving citric acid and ethylenediamine in deionized water in a molar ratio of 1:1, the step of forming a carbon dot precursor solution by reacting the reaction solution under hydrothermal conditions at 180°C, the step of dialyzing the carbon dot precursor solution by placing it in deionized water, and the step of obtaining carbon dot powder by freeze-drying the dialyzed carbon dot precursor solution.

[0034] At this time, the step of dialyzing the carbon dot precursor solution in deionized water may be performed for 2 weeks.

[0036] The subsequent carbon dot solution formation step (S120) involves dispersing carbon dot powder in deionized water to form a carbon dot solution.

[0037] At this time, the carbon dot solution may have a concentration of 2 mg / mL.

[0039] The polydopamine solution preparation step (S130) includes the step of dissolving dopamine hydrochloride in a solvent mixed with deionized water and ethanol, and the step of adding an aqueous ammonia solution to the solvent in which dopamine hydrochloride is dissolved to induce self-polymerization of dopamine.

[0040] By adding 0.6 mL of aqueous ammonia solution to a solvent in which dopamine hydrochloride is dissolved, the pH of the solution can be adjusted to 11.3, thereby inducing a self-polymerization reaction of dopamine to form polydopamine, and thus a polydopamine solution can be prepared.

[0042] A carbon dot-polydopamine heterojunction metal-free photocatalyst synthesis step (S140) can be performed using the carbon dot solution and polydopamine solution prepared through the above steps.

[0043] The carbon dot-polydopamine heterojunction metal-free photocatalyst synthesis step (S140) may include the step of preparing a mixed solution by injecting a carbon dot solution into a polydopamine solution and the step of synthesizing a carbon dot-polydopamine heterojunction metal-free photocatalyst in the mixed solution by stirring the prepared mixed solution.

[0044] Here, stirring the mixed solution may involve reacting the mixed solution at room temperature for 30 hours.

[0046] The carbon dot-polydopamine heterojunction metal-free photocatalyst purification step (S150) is to purify the carbon dot-polydopamine heterojunction metal-free photocatalyst synthesized in the mixed solution through centrifugation.

[0048] The carbon dot-polydopamine heterojunction metal-free photocatalyst washing and drying step (S160) may include the step of sequentially washing the carbon dot-polydopamine heterojunction metal-free photocatalyst purified by centrifugation with deionized water and ethanol, and the step of drying the washed carbon dot-polydopamine heterojunction metal-free photocatalyst in a dryer.

[0049] The sequential washing step here is intended to effectively remove unreacted material that may remain on the carbon dot-polydopamine heterojunction metal-free photocatalyst.

[0051] <Example 1>

[0052] Preparation of carbon dot-polydopamine heterojunction metal-free photocatalysts

[0053] 2 mg / mL concentration carbon dot solution (15 mL) and polydopamine solution (100 mL deionized water + 35 mL ethanol + Dopamine hydrochloride 600 mg A carbon dot-polydopamine heterojunction metal-free photocatalyst was prepared by using 0.6 mL of an aqueous ammonia solution and undergoing synthesis, purification, washing, and drying steps.

[0055] <Example 2>

[0056] Preparation of carbon dot-polydopamine heterojunction metal-free photocatalysts

[0057] 2 mg / mL concentration carbon dot solution (15 mL) and polydopamine solution (100 mL deionized water + 35 mL ethanol + Dopamine hydrochloride 900 mg A carbon dot-polydopamine heterojunction metal-free photocatalyst was prepared by using 0.6 mL of an aqueous ammonia solution and undergoing synthesis, purification, washing, and drying steps.

[0059] <Example 3>

[0060] Preparation of carbon dot-polydopamine heterojunction metal-free photocatalysts

[0061] 2 mg / mL concentration carbon dot solution (15 mL) and polydopamine solution (100 mL deionized water + 35 mL ethanol + Dopamine hydrochloride 1200 mg A carbon dot-polydopamine heterojunction metal-free photocatalyst was prepared by using 0.6 mL of an aqueous ammonia solution and undergoing synthesis, purification, washing, and drying steps.

[0063] <Example 4>

[0064] Preparation of carbon dot-polydopamine heterojunction metal-free photocatalysts

[0065] 2 mg / mL concentration carbon dot solution (15 mL) and polydopamine solution (100 mL deionized water + 35 mL ethanol + Dopamine hydrochloride 1500 mg A carbon dot-polydopamine heterojunction metal-free photocatalyst was prepared by using 0.6 mL of an aqueous ammonia solution and undergoing synthesis, purification, washing, and drying steps.

[0067] <Comparative Example 1>

[0068] Preparation of carbon dot single-component photocatalysts

[0069] A carbon dot single-component photocatalyst was prepared through the step (S110) of preparing carbon dot powder.

[0071] <Comparative Example 2>

[0072] Preparation of polydopamine monocomponent photocatalyst

[0073] A polydopamine monocomponent photocatalyst was prepared by using a polydopamine solution (100 mL of deionized water + 40 mL of ethanol + 0.5 g of dopamine hydrochloride + 0.6 mL of aqueous ammonia solution) and undergoing purification, washing, and drying steps.

[0075] Analysis of the physicochemical properties of photocatalysts

[0076] ㆍMorphological and average particle size analysis (TEM, SEM)

[0077] The reduction in average particle size is attributed to the uniform polymerization of polydopamine, which can facilitate the uniform formation of carbon dots on the surface of polydopamine. To verify this, morphological and average particle size analyses were performed on the examples and comparative examples.

[0078] Figure 2a shows the SEM image of Comparative Example 1, the TEM images of Examples 1 to 4 and Comparative Example 2, and the results of analyzing the respective particle sizes. Figure 2b shows the results of a comparative analysis of the particle sizes of Examples 1 to 4 and Comparative Example 2, that is, the average particle size according to the polydopamine content.

[0080] Referring to Fig. 2a, it can be confirmed through SEM and TEM images of Comparative Example 1, Example 3, and Comparative Example 2 that all have spherical particles, and it can be confirmed that Comparative Example 1 and Comparative Example 2 have average diameters of 4.9±1.1 nm and 344.4±29.1 nm, respectively.

[0081] n in Fig. 2b, i.e., the number on the y-axis (4, 6, 8, 10), represents the concentration of polydopamine contained in the solution used for photocatalytic synthesis, and each represents Examples 1 to 4.

[0082] Referring to Figures 2a and 2b together, it can be seen that in Examples 1 to 4, the average particle size (diameter) tends to decrease significantly and then increase slightly as the polydopamine content increases. In particular, it can be confirmed that Example 3 has the smallest average particle size (diameter) of 254.3 ± 63.7 nm.

[0084] From the above results, it can be seen that controlling the polydopamine concentration is essential for carbon dot-polydopamine heterojunction metal-free photocatalysts, as aggregation occurs and particle size increases when the polydopamine concentration is less than 4 mg / mL, whereas when the polydopamine concentration exceeds 10 mg / mL, excessive growth of polydopamine is induced, resulting in large and non-uniform particle sizes.

[0085] ㆍChemical composition analysis (FT-IR, XPS)

[0086] Compositional analysis was performed to determine the elements and bonding structures of Examples 1 to 4 and Comparative Examples 1 to 2, and the results are shown in Figures 3a and 3b.

[0087] FIG. 3a shows the FT-IR spectra and XPS survey spectra of the examples and comparative examples, and FIG. 3b shows the high-resolution XPS spectra processed by deconvolution for the carbon (C) 1s and nitrogen (N) 1s peaks of Example 3 and Comparative Examples 1 to 2.

[0089] Referring to the FT-IR spectrum in Fig. 3a, vibrational characteristics for a single composition of carbon dots and polydopamine appear in Comparative Example 1 and Comparative Example 2, and the unique characteristics of each composition are confirmed.

[0090] Carbon dots and polydopamine are each 3200–3500 cm⁻¹ -1NH and OH stretching vibrations are exhibited in the region, which is the same in Examples 1 to 4, which are carbon dot-polydopamine composite samples.

[0091] In addition, in Examples 1 to 4, 1600 to 1700 cm -1 An absorption band of the carbonyl (C=O) group is observed in the region, and it can be confirmed that the intensity of this absorption band decreases as the polydopamine concentration increases.

[0092] The CN stretching vibration is 1500 cm -1 In the vicinity, the COC stretching vibration is 1300 cm -1 It is confirmed in the vicinity. This suggests strong intermolecular interactions between carbon dots and polydopamine, indicating that it is a result reflecting changes based on polydopamine content.

[0093] In addition, through the XPS survey spectrum shown in Fig. 3a, it can be confirmed once again that characteristic peaks corresponding to oxygen, nitrogen, and carbon originating from the components of carbon dots and polydopamine exist.

[0095] Referring to the high-resolution XPS spectra deconvolved with respect to the C 1s and N 1s peaks shown in Fig. 3b, in Comparative Example 1 and Comparative Example 2, distinct XPS peaks for carbon dots and polydopamine, respectively, appear, and the presence of characteristic peaks corresponding to oxygen, nitrogen, and carbon derived from each component can be confirmed.

[0096] In the XPS spectrum of the C 1s peak, peaks corresponding to CC, CN, C-OH, and C=O bonds are identified. The intensity of the C=O peak decreases in the order of Comparative Example 1, Example 3, and Comparative Example 2, which suggests an interaction between the carbonyl groups on the carbon dot surface and polydopamine.

[0097] At the same time, in Example 3, the C-OH peak intensity increases, indicating that catechol groups derived from polydopamine are bound to carbon dots.

[0098] In addition, changes in the intensity of the CC and CN peaks are also observed, which signifies chemical changes resulting from the introduction of polydopamine.

[0099] In the XPS spectrum of the N 1s peak, pyridinic, pyrroleic, and graphetic nitrogen species are identified, and in particular, in Example 3, a carbon dot-polydopamine composite sample, pyridinic nitrogen that is not present in the carbon dot appears, confirming the introduction of polydopamine.

[0100] On the other hand, the relative amount of graphene nitrogen decreases after the introduction of polydopamine.

[0101] These deconvolution-processed high-resolution XPS results are consistent with FT-IR analysis results and support the successful synthesis of carbon dots and polydopamine.

[0102] In addition, an increase in binding energy from Comparative Example 1 to Comparative Example 2, that is, from carbon dots to polydopamine, was observed in the C 1s and N 1s spectra. This suggests electron transfer from polydopamine to carbon dots and can support a Type 2 heterojunction structure.

[0103] ㆍAnalysis of light absorption and bandgap characteristics

[0104] The results of UV-Vis spectroscopy performed to investigate the interaction between carbon dots and polydopamine, and the Tauc plot constructed based on these results are shown together in Figure 4.

[0106] Referring to Fig. 4, UV-Vis spectroscopy was performed to investigate the interaction between carbon dots and polydopamine, and the results showed that carbon dots sp at 221 nm 2- π-π of the carbon network * Transition and n-π of the carbonyl group at 340 nm * It showed a characteristic absorption peak indicating a transition. The overall absorbance was relatively low, and no distinct characteristics were observed.

[0107] As the concentration of polydopamine increased, the characteristic absorption peak of carbon dot-polydopamine gradually weakened, and spectral characteristics associated with polydopamine became prominent.

[0108] In particular, Example 3, which was confirmed to have the smallest particle size in Figure 2, showed the lowest absorbance among the carbon dot-polydopamine composite samples.

[0109] This can be interpreted as the effect of minimizing light scattering due to the reduction in particle size, suggesting that polydopamine grew uniformly without excessive aggregation.

[0111] In addition, to evaluate the semiconductor properties of the photocatalyst, a Tauc plot was constructed from the UV-Vis spectrum, and the optical bandgap energy was estimated by extrapolating the linear region of the plot. The following Tauc equation was applied in this process.

[0112]

[0113]

[0114]

[0115] Here is Planck's constant (6.62607015×10⁻³⁴ kg·m² / s), is the speed of light (299,792,458 m / s), is the wavelength of light (nm), is the absorption coefficient, is the photon frequency, is a proportionality constant, is the band gap energy. Depending on the transition type, it is set to 2 for direct transition and 1 / 2 for indirect transition.

[0116] As a result, as shown in Fig. 4, it was estimated that the carbon dot had a band gap of 3.36 eV, the polydopamine had a band gap of 1.63 eV, and Example 3, a carbon dot-polydopamine composite sample, had a band gap of 2.08 eV.

[0117] This means that Example 3 is capable of absorbing light up to a wavelength of 596 nm, and accordingly, it is determined that the photocatalyst of the present invention can absorb light in both the ultraviolet and visible light regions.

[0118] These results suggest that a new band alignment favorable for the effective inhibition of electron-hole recombination has been achieved through electronic interactions and heterojunction formation between carbon dots and polydopamine.

[0119] Electrochemical property analysis

[0120] To analyze the electrochemical properties of the photocatalyst, photoluminescence (PL) measurements, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analysis were performed to comprehensively evaluate charge separation and transfer characteristics, conduction band position, carrier density, etc., and the results are shown in Figures 5a and 5b.

[0122] Referring to the photoluminescence spectrum shown in Fig. 5a, it can be confirmed that Comparative Example 1, a single-component carbon dot photocatalyst, alone exhibits a photoluminescence signal of up to 8.0 M or more.

[0123] This indicates rapid electron-hole recombination.

[0124] On the other hand, Comparative Example 2, a polydopamine single-component photocatalyst, can be confirmed to exhibit a photoluminescence signal of 40.0 K or lower.

[0125] Example 3, a carbon dot-polydopamine heterojunction metal-free photocatalyst, showed a significantly reduced photoluminescence signal even when compared to Comparative Example 2. This suggests efficient charge separation and transfer of the carbon dot-polydopamine heterojunction metal-free photocatalyst. When charge recombination is suppressed in this way, the availability of electrons for the photocatalytic reaction increases, which can improve the overall photocatalytic efficiency.

[0127] Electrochemical impedance spectroscopy was performed in a three-electrode cell system using Example 3, Comparative Example 1, and Comparative Example 2. A platinum wire was used as the reference electrode, Ag / AgCl as the reference electrode, and a 0.1 M sodium sulfate aqueous solution with pH 7 was used as the electrolyte, and each photocatalyst was coated onto a glassy carbon electrode (GCE) and used as the working electrode.

[0128] A catalyst ink was prepared by dispersing 20 mg of photocatalyst in a solution mixed with 0.45 mL of ethanol, 0.45 mL of isopropyl alcohol, and 0.1 mL of 5 wt% Nafion™ perfluoropolymer solution.

[0129] 5 µl of the prepared catalyst ink was dropped onto a glassy carbon electrode with a diameter of Φ 3 mm, and then the experiment was performed in an O2 atmosphere with a bias of -0.4 V (relative to Ag / AgCl) in a frequency range of 1 Hz to 100 kHz.

[0130] Referring to the electrochemical impedance spectrum shown in Fig. 5a, Comparative Example 1 and Comparative Example 2 show similar semicircle diameters on the Nyquist plot, which means that the two samples have similar charge transfer resistance.

[0131] In contrast, Example 3 exhibits a smaller semicircle diameter compared to Comparative Examples 1 and 2, indicating lower charge transfer resistance. This suggests that interfacial charge mobility is enhanced and electron transfer is more efficient due to the complexation between carbon dots and polydopamine.

[0132] It is believed that these characteristics inhibited charge recombination and, consequently, contributed to enhancing the photocatalytic activity of the carbon dot-polydopamine heterojunction metal-free photocatalyst.

[0134] Mott-Shotsky analysis was performed at 1000, 1500, and 2000 Hz based on the capacitance obtained from electrochemical impedance spectroscopy (EIS), and the flat band potential was derived through the relationship with the electrode potential.

[0135] Referring to the Mott-Schottky plot in Fig. 5b, both Comparative Example 1 and Comparative Example 2, which are single-component photocatalysts, showed a positive (+) slope, confirming that carbon dots and polydopamine have the same n-type semiconductor characteristics, and the flat band potential of each sample was measured as -0.53 V for carbon dots and -0.65 V for polydopamine (vs. RHE).

[0136] Generally, since the flat band potential in n-type semiconductors approximates the lower conduction band level, the energy band structure of carbon dots and polydopamine was constructed by combining this with the optical band gap derived from the Tauc plot in Figure 4.

[0137] According to the energy band structure of Fig. 5b, band alignment is induced through the composite formation of carbon dots and polydopamine, forming a heterojunction structure that facilitates charge separation and transfer, which is considered to act as a key factor in improving photocatalytic performance.

[0138] Analysis of Photochemical Active Species Contribution

[0139] In order to further verify the charge separation and transport characteristics based on the Type 2 heterojunction structure of the present invention, a radical capture experiment was performed in Example 3.

[0140] To identify the major active species contributing to the hydrogen peroxide photogeneration reaction, superoxide radicals (·O 2- Benzoquinone (BQ), a capture agent, and isopropyl alcohol (IPA), a hydroxyl radical (·OH) capture agent, were added, respectively, and the results are shown in Fig. 6.

[0141] Referring to Figure 6, under IPA treatment conditions, the amount of hydrogen peroxide produced remained at a level similar to that of the untreated condition, whereas under BQ treatment, the hydrogen peroxide production activity showed a significantly reduced trend.

[0142] These results imply that superoxide radicals act as the primary active species in the photocatalytic reaction of the present invention, suggesting that effective separation and transfer of charge between carbon dots and polydopamine are achieved.

[0144] Additionally, according to the energy band structure of Fig. 5b, the valence band of the carbon dot is located in a position where it can thermodynamically interact with hydroxyl radicals, but radical capture experiments confirmed that hydroxyl radicals do not play an important role in the photocatalytic reaction.

[0145] This means that holes in the valence band within the carbon dot do not drive the oxidation reaction. Instead, in the present invention, charge separation is induced by a Type II heterojunction mechanism, and holes generated by photoexcitation move from the valence band of the carbon dot to the valence band of polydopamine. This charge separation effectively suppresses electron-hole recombination, thereby contributing to the improvement of photocatalytic efficiency.

[0147] In addition, under conditions where 10% of ethanol, which acts as an electron donor in a general photocatalytic system, was added (10% EtOH), the amount of hydrogen peroxide produced actually decreased. This is interpreted as a result of ethanol failing to act as an electron donor in the present invention and inhibiting the oxygen reduction reaction (ORR) by competing with oxygen for photogenerated electrons.

[0148] The above results further support the fact that a Type 2 heterojunction mechanism, in which holes generated by photoexcitation move from the carbon dot to polydopamine to inhibit charge recombination and electrons remain within the carbon dot to produce hydrogen peroxide through a two-electron oxygen reduction reaction, is substantially operating in Example 3 of the present invention.

[0150] Performance evaluation of photocatalysts

[0151] The performance of the carbon dot-polydopamine heterojunction metal-free photocatalyst presented in this invention is as follows under simulated sunlight conditions (AM 1.5, 100 mW / cm²). 2The evaluation was conducted using a 150W xenon arc lamp under ) . The photocatalysts of the examples and comparative examples were dissolved in deionized water at a concentration of 30 mg / L to carry out the reaction, and the concentration of the generated hydrogen peroxide was quantitatively measured using a peroxide analysis kit. The purpose of this experiment was to evaluate the efficiency of the synthesized photocatalysts in generating hydrogen peroxide using sunlight. All photocatalyst experiments were repeated at least three times to ensure the accuracy and reproducibility of the results.

[0152] Evaluation of hydrogen peroxide production amount according to light irradiation time

[0153] The amount of hydrogen peroxide produced in Examples 1 to 4 and Comparative Examples 1 to 2 was quantitatively analyzed using a peroxide analysis kit, and the results are shown in Fig. 7.

[0154] Referring to Fig. 7, Example 3 exhibited the highest activity, producing 2815 μmol / g of hydrogen peroxide upon 2 hours of light irradiation. Examples 1 and 2 showed production amounts of 1407 μmol / g and 2125 μmol / g, respectively.

[0155] This value is significantly higher than that of Comparative Example 1 (carbon dots, 917 μmol / g) or Comparative Example 2 (polydopamine, 686 μmol / g), which are single-component photocatalysts, demonstrating the superior charge separation performance based on the synergistic effect between quantum photoactive organic materials and the heterojunction structure.

[0156] As described above, as the polydopamine content increased from Example 1 to Example 3, effects such as improved electron-hole separation efficiency, expanded light absorption range, and increased catalytic activity were observed; however, in the case of Example 4, which had an excessive polydopamine content, the photocatalytic activity actually decreased.

[0157] This is interpreted as excess polydopamine covering the major active sites on the carbon dot surface, hindering light absorption, and inhibiting charge transfer, thereby reducing overall catalytic efficiency.

[0158] Oxygen and light dependence analysis to elucidate the photocatalytic reaction mechanism

[0159] In addition, to elucidate the photocatalytic reaction mechanism that induces hydrogen peroxide production, a control experiment was performed on Example 3 in the presence or absence of oxygen and light, and the results are shown in Fig. 8a.

[0160] Referring to Figure 8a, the amount of hydrogen peroxide produced was minimal under conditions without light or oxygen, which means that this reaction requires both light irradiation and the presence of oxygen.

[0161] In particular, a higher production amount was confirmed under a high-purity oxygen atmosphere than under an air atmosphere, supporting the fact that the two-electron oxygen reduction reaction, in which oxygen acts as an electron acceptor in the present invention, is the main reaction pathway.

[0162] ㆍStability during prolonged light irradiation and persistence in environments with repeated light / dark alternation

[0163] In addition, to evaluate the stability of long-term catalytic performance due to the dual photoactivity and charge separation ability provided by the carbon dot-polydopamine heterojunction structure, a long-term light irradiation experiment was performed on Example 3, which was identified as having the optimal composition, and the results are shown in Fig. 8b.

[0164] Referring to Fig. 8b, a linear increase in the amount of hydrogen peroxide produced was observed even under continuous light irradiation conditions of more than 9 hours, which suggests that the activity of the catalyst is maintained stably without decreasing during the reaction.

[0166] Additionally, to evaluate the dependence on photoactivity and photostability, a light-dark cycle experiment was conducted for 21 hours in Example 3, and the results are shown together in Fig. 8b.

[0167] Referring to Fig. 8b, hydrogen peroxide was generated only under light irradiation conditions, and no generation was observed under dark conditions, maintaining consistent performance throughout the repetition cycle.

[0168] These results clearly demonstrate that the photocatalyst of the present invention is a light-induced reaction and support the fact that it maintains excellent photostability even under conditions of repeated long-term use.

[0170] Evaluation of hydrogen peroxide decomposition inhibition and photocatalyst reusability

[0171] Some oxygen reduction reaction (ORR)-based photocatalysts utilize a 4-electron reduction pathway (4e) that decomposes the generated hydrogen peroxide again. - It is known that it induces ORR.

[0172] Accordingly, in order to confirm whether Example 3 of the present invention exhibits such a hydrogen peroxide decomposition trend, a hydrogen peroxide decomposition experiment over time was performed, and the results are shown in FIG. 9a.

[0173] Referring to Fig. 9a, it was confirmed that in Example 3, the concentration of hydrogen peroxide did not significantly decrease over time, so the hydrogen peroxide decomposition reaction hardly occurred.

[0174] This means that organic-based photocatalysts, especially carbon dot-based organic photocatalysts, generally have low hydrogen peroxide decomposition activity, and that the stability of hydrogen peroxide can be effectively maintained through the complexation of carbon dots and polydopamine.

[0176] In addition, to evaluate the reusability of the photocatalyst of the present invention, a reuse experiment was repeated four times to recover Example 3 through centrifugation, and the results are shown in FIG. 9b.

[0177] Referring to Fig. 9b, the hydrogen peroxide production rate was maintained relatively stably during the reuse cycle, which indicates that the photocatalyst of the present invention has excellent reusability in which catalytic activity is maintained even after repeated use.

[0178] However, a tendency for the photocatalyst yield to decrease slightly was observed during repeated experiments, which is attributed to some losses during the experimental process (losses during recovery and handling) rather than structural deterioration of the photocatalyst itself.

[0179] This interpretation is consistent with the results of the previously conducted long-term stability evaluation experiment, which confirmed that Example 3 maintains structural and functional stability even under reaction conditions.

[0180] These results suggest that the carbon dot-polydopamine heterojunction metal-free photocatalyst of the present invention is a photocatalyst for two-electron oxygen reduction reactions with high selectivity and stability, capable of selectively producing hydrogen peroxide without undesirable side reactions, and has high practical value as a heterogeneous catalyst that can be used repeatedly.

[0182] The present invention is designed to efficiently perform selective hydrogen peroxide generation by realizing a metal-free organic photocatalyst through a heterojunction structure combining carbon dots and polydopamine.

[0183] Through band alignment analysis and radical capture experiments, it was confirmed that a Type II heterojunction was formed between the two materials. Consequently, electrons generated in the conduction band of polydopamine move to the carbon dot with a lower energy level, and holes generated in the valence band of the carbon dot move to polydopamine with a higher energy level, thereby effectively separating electrons and holes.

[0184] Such charge separation suppresses electron-hole recombination in the photoexcited state and extends the charge lifetime, ultimately significantly improving the stability and efficiency of the photocatalytic reaction. In particular, the carbon dot-polydopamine heterojunction metal-free photocatalyst proposed in this invention possesses the following major advantages.

[0185] (1) Based on the energy level difference between the conduction band and the valence band, a spontaneous charge separation pathway is formed, allowing electrons and holes to move stably within the carbon dot and polydopamine, respectively, thereby enabling efficient charge separation and extended lifespan.

[0186] (2) By utilizing both the unique light absorption properties of carbon dots and polydopamine, the range of light utilization is extended to the visible light region, thereby maximizing the overall photoreaction efficiency.

[0187] (3) As electrons and holes react within different materials, the reaction sites for the reduction and oxidation reactions are separated, thereby improving the selectivity and stability of the hydrogen peroxide generation reaction.

[0188] Charge transport pathways and physical and functional separation based on such heterojunction structures play an important role, particularly in the oxygen reduction reaction (ORR).

[0189] Accordingly, hydrogen peroxide selectivity is improved and side reactions are suppressed.

[0190] In Example 3 of the present invention, the generated electrons accumulate within the carbon dot to selectively reduce oxygen to hydrogen peroxide, and the holes within the polydopamine are consumed through functional groups such as hydroxyl groups present on the surface of the material, thereby realizing a spontaneous hole annihilation pathway that does not require an external electron donor.

[0191] As a result, side reactions such as hydrogen peroxide decomposition are suppressed, and the stability and reaction efficiency of the entire system are improved. Experimental results showed that Example 3 recorded a hydrogen peroxide production of 1408 μmol / g per hour, exhibiting significantly improved activity compared to single-component carbon dots and polydopamine.

[0192] Furthermore, structural and functional stability has been proven through reuse experiments and stability evaluations, demonstrating its potential as a high-efficiency metal-free photocatalyst with practical applicability.

[0193] The present invention presents a promising platform that overcomes the limitations of metal-based catalysts and can produce hydrogen peroxide in an environmentally friendly and sustainable manner.

[0195] The above-described embodiments are merely exemplary, and various other embodiments modified therefrom are possible for those skilled in the art.

[0196] Therefore, the true technical scope of protection of the present invention should include not only the above embodiments but also other embodiments that are variously modified according to the technical concept of the invention described in the following claims. Explanation of the symbols

[0197] S100: Method for manufacturing a carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide generation S110: Carbon dot powder preparation step S120: Carbon dot solution formation step S130: Polydopamine Solution Preparation Step S140: Carbon dot-polydopamine heterojunction metal-free photocatalyst synthesis step S150: Carbon dot-polydopamine heterojunction metal-free photocatalyst purification step S160: Carbon dot-polydopamine heterojunction metal-free photocatalyst washing and drying steps

Claims

Claim 1 The powder particles comprise carbon dots and polydopamine, wherein the polydopamine is a polymeric material produced by the polymerization reaction of dopamine, wherein the dopamine is formed from dopamine hydrochloride dissolved in an alkaline solution at a concentration of 4 to 10 mg / mL, wherein the carbon dots are formed on the surface of the polydopamine, and are bonded to the polydopamine by the interaction between the surface carbonyl groups of the carbon dots and the catechol groups of the polydopamine, wherein the carbon dots and the polydopamine form a Type II heterojunction structure in which energy bands are staggered, and the powder particles are at AM 1.5, 100 mW / cm² 2 A carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide production, which exhibits a hydrogen peroxide production amount of 1407–2815 μmol / g as a result of a 2-hour photocatalytic test performed under simulated sunlight conditions, and maintains a production activity of more than 40% relative to the initial hydrogen peroxide production amount even after repeating the hydrogen peroxide production experiment four times in which the powder particles are recovered and reused through centrifugation. Claim 2 delete Claim 3 In claim 1, the powder particles are a carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide production having a size of 254.3±63.7~494.51±123.5 nm. Claim 4 A step of preparing carbon dot powder; a step of forming a carbon dot solution by dispersing the carbon dot powder in deionized water; a step of preparing a polydopamine solution by dissolving dopamine hydrochloride at a concentration of 4 to 10 mg / mL in a solvent mixed with deionized water and ethanol, and adding an aqueous ammonia solution to induce self-polymerization of dopamine under alkaline conditions; a step of preparing a mixed solution by injecting the carbon dot solution into the polydopamine solution, and synthesizing a carbon dot-polydopamine heterojunction metal-free photocatalyst having a Type 2 heterojunction structure in which the carbon dot is bound to the surface of the polydopamine by the interaction between the surface carbonyl group of the carbon dot and the catechol group of the polydopamine within the mixed solution by stirring the prepared mixed solution; and a step of purifying the carbon dot-polydopamine heterojunction metal-free photocatalyst through centrifugation. and includes the step of sequentially washing the carbon dot-polydopamine heterojunction metal-free photocatalyst with deionized water and ethanol, and then drying, wherein the carbon dot-polydopamine heterojunction metal-free photocatalyst has an AM 1.5, 100 mW / cm² 2 A method for manufacturing a carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide production, which exhibits a hydrogen peroxide production amount of 1407–2815 μmol / g as a result of a 2-hour photocatalyst test performed under simulated sunlight conditions, and maintains a production activity of 40% or more relative to the initial hydrogen peroxide production amount even after repeating the hydrogen peroxide production experiment four times in which the carbon dot-polydopamine heterojunction metal-free photocatalyst is recovered and reused through centrifugation. Claim 5 A method for manufacturing a carbon dot-polydopamine heterojunction metal-free photocatalyst for hydrogen peroxide generation, wherein the step of preparing the carbon dot powder comprises: a step of forming a reaction solution by dissolving citric acid and ethylenediamine in deionized water in a molar ratio of 1:1; a step of forming a carbon dot precursor solution by reacting the reaction solution under hydrothermal conditions at 180°C; a step of dialyzing the carbon dot precursor solution by placing it in deionized water; and a step of obtaining carbon dot powder by freeze-drying the dialyzed carbon dot precursor solution. Claim 6 delete Claim 7 delete Claim 8 delete

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