Photoanode and method of forming the same

Abstract

The present disclosure relates to a photoanode for photoelectrochemical water-splitting. The photoanode comprises a conductive substrate, an array of zirconium trisulfide (ZrS3) nanobelts deposited on at least one surface of the conductive substrate, and a layer of zirconium oxysulfide (ZrOS) deposited on the array of zirconium trisulfide (ZrS3), forming a zirconium trisulfide (ZrS3) / zirconium oxysulfide (ZrOS) nanobelts array on the conductive substrate. Various embodiments relate to a method of forming the photoanode.

Description

PHOTOANODE AND METHOD OF FORMING THE SAMECROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority to the Singapore application no. 10202302719S filed 26 September 2023, and the contents of which arc hereby incorporated by reference in their entirety for all purposes.TECHNICAL FIELD

[0002] The present disclosure relates generally to photoanodes and more particularly, to photoanodes for photoelectrochemical water splitting, and to a method of forming the photoanodes.BACKGROUND

[0003] Photoclcctrochcmical (PEC) water splitting presents a cost-effective and environmentally friendly solution for producing green hydrogen by harnessing abundant sunlight and water resources. One of the key obstacles in scaling up solar fuel production is ensuring the stability of efficient and cost-effective photoelectrodes in aqueous environments. Significant efforts have been made to develop combinational material repositories to seek the optimal band structure for photoelectrodes. However, these efforts merely reinforce the existing trade-off between performance and stability of the photoelectrodes. Achieving both high efficiency and stability in photoelectrochemical water splitting remains a challenging task.

[0004] It is therefore desirable to provide a photoelectrode, particularly a photoanode, and a method for forming the same that seeks to address at least one of the problems described hereinabove, or at least to provide an alternative.SUMMARY

[0005] In accordance with a first aspect of the present disclosure, a photoanode for photoelectrochemical water- splitting is provided. The photoanode comprises a conductive substrate; an array of zirconium trisulfidc nanobclts deposited on at least one surface ofthe conductive substrate; and a layer of zirconium oxy sulfide (ZrOS) deposited on the array of zirconium trisulfidc forming a zirconium trisulfidc (ZrSsl / zirconium oxysulfidc (ZrOS)nanobelts array on the conductive substrate.

[0006] In some embodiments, the photoanode further comprises a layer of nickel oxide hydroxide (NiOOH) deposited on the layer of zirconium oxysulfide

[0007] In some embodiments, the photoanodc further comprises a layer of iron oxide hydroxide (FeOOH) deposited on the layer of nickel oxide hydroxide (NiOOH), forming a photoanodc comprising the zirconium trisulfidc (ZrSq / zirconium oxysulfidc (ZrOS) nanobclts array withas cocatalysts, denoted as

[0008] In some embodiments, the layer of zirconium trisulfide () is an n-type layer; and the layer of zirconium oxysulfide (ZrOS) is a p-type layer, configured to form a buried n-p junction in the photoanode.

[0009] In accordance with a second aspect of the present disclosure, a method of forming a photoanode is provided. The method comprises depositing an array of zirconium trisulfide (ZrSs) nanobelts on at least one surface of a first conductive substrate; and subjecting the array of zirconium trisulfide (ZrSa) nanobelts to ozone treatment to form a layer of zirconium oxysulfide (ZrOS) on the array of zirconium trisulfide (ZrSs) nanobelts, forming a zirconium trisulfide (ZrS i.l / zirconium oxysulfide (ZrOS) nanobelts array on the first conductive substrate.

[0010] In some embodiments, the method further comprises depositing a layer of nickel oxide hydroxide (NiOOH) on the layer of zirconium oxysulfidc (ZrOS) by immersing the photoanode in an aqueous solution containing a nickel salt, to obtain a ZrSs / ZrOS / NiOOH photoanodc comprising the zirconium trisulfidc (ZrSq / zirconium oxysulfidc (ZrOS) nanobclts array with a layer of nickel oxide hydroxide (NiOOH).

[0011] In some embodiments, the method further comprises depositing a layer of nickel oxide hydroxide (NiOOH) on the layer of zirconium oxysulfidc (ZrOS) by immersing the photoanode in an aqueous solution containing a nickel salt; depositing a layer of iron oxide hydroxide (FeOOH) on the layer of nickel oxide hydroxide (NiOOH) by immersing the photoanode in an aqueous solution containing an iron salt, to obtain aphotoanode comprising the zirconium trisulfideoxysulfide (ZrOS) nanobclts array with a layer of NiOOH and a layer of FeOOH as cocatalysts.

[0012] In some embodiments, the method may further comprise transferring the zirconium trisulfide (ZrSsj / zirconium oxysulfide (ZrOS) nanobelts array to a second conductive substrate. The transfer method comprises coating a polystyrene precursor onto the zirconium trisulfideoxysulfidcnanobclts array deposited on the first conductive substrate; removing the first conductive substrate; adding a precursor solution onto the second conductive substrate to form a seed layer; affixing the polystyrene coated zirconium trisulfidcoxysulfide (nanobelts array to the seed layer on the second conductive substrate; heating the polystyrene coated zirconium trisulfideoxysulfide nanobclts array and the second conductive substrate; removing the polystyrene; andsubjecting the obtained zirconium trisulfidc (oxysulfidc (ZrOS) nanobclts array with the second conductive substrate to annealing to obtain a firmly adhered zirconium trisulfidcoxysulfidc (ZrOS) nanobclts array on the second conductive substrate.

[0013] In accordance with a third aspect of the present disclosure, a photoelectrochemical cell comprising the photoanode of the present disclosure is provided. The photoelectrochemical cell comprises a counter electrode, a photoanode and an electrolyte, wherein the photoanode comprises a conductive substrate, an array of zirconium trisulfidc) nanobclts deposited on at least one surface of the conductive substrate; and a layer of zirconium oxysulfide (ZrOS) deposited on the array of zirconium trisulfide forming a zirconium trisulfideoxysulfide (ZrOS) nanobelts array on the conductive substrate; and wherein the array of zirconium trisulfidenanobelts is an n-type layer and the layer of zirconium oxysulfide (ZrOS) is a p-type layer, configured to form a buried n-p junction in the photoanode.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.FIG. 1 is a general illustration of a photoanode comprising zirconium trisulfidenanobelts array deposited on a substrate in accordance with an embodiment of the present disclosure.FIG. 2 is a general illustration of a photoanode comprising zirconium trisulfide oxysulfidc (ZrOS) nanobclts array deposited on asubstrate in accordance with an embodiment of the present disclosure.FIG. 3 is a schematic diagram of the fabrication process and the corresponding photographs of the on a substrate.FIG. 4 is a schematic illustration of PEC water oxidation over thephotoanode (401) with buried n-p junction on a FTO glass substrate (402) in accordance with an embodiment of the present disclosure.FIG. 5 shows light harvest efficiencies (LHE) ofphotoanodc with different oxidation cycles.FIG. 6 shows SEM images of (a) ZrS’, on zirconium (Zr) foil, (b) side-view, and (c) top-view of ZrSs film coated by polystyrene, and (d) side-view of ZrSs film on the FTO substrate.FIG. 7A shows an SEM image of ZrSs nanobclts array on the FTO substrate.FIG. 7B shows XRD patterns of the free-standing ZrS < and ZrSs / ZrOS nanobelts array, and ZrSs and ZrSs / ZrOS nanobelts array on the FTO substrates.FIG. 7C shows Raman spectra of ZrSa and ZrS i / ZrOS.FIG. 8 shows the compared XRD spectra of free-standing Z1S3 NBA oxidized by ozone (O3) with 75 cycles and pure ZrOS powder. ZrOS particles were synthesized by vacuum annealing a stoichiometric mixture of Z1O2 and ZrS2 powder at 900 °C for 48 hours in a sealed silica glass tube. As shown in the XRD spectra, the synthesized white powders are confirmed as pure ZrOS in the cubic phase (ICCD PDF#04-0897). The additionally emerged peaks at 22.10and 27.10in the oxidized Z1S3 NBA can be attributed to the (110) and (111) planes of ZrOS. Besides, the (200) peak of ZrOS overlaps with the (012) peak of ZrS s.FIG. 9 shows the crystal structures of Z1S3 and ZrOS.FIG. 10 shows (a) TEM and (b) HRTEM images of a pure ZrOS particle. The TEM image displays a ZrOS particle with dimensions ranging from 400 to 600 nm. The corresponding HRTEM image reveals distinct lattice spacings measuring approximately 0.29 and 0.32 nm, which correspond to the (200) and (111) planes of cubic ZrOS, respectively. These findings provide evidence for the successful formation of the obtained powders, affirming their purity as cubic ZrOS.FIG. 11 shows (a) XRD spectra of free-standing Z1S3 nanobelts with different oxidation cycles, and (b) XRD spectra of ZrSs nanobclts with different oxidized cycles transferred onto FTO substrates.FIG. 12 shows (a) the Raman spectra of ZrS3 oxidized by ozone (O3) and pure ZrOS, and (b) the Raman spectra of ZrSs nanobelts arrays with different oxidation cycles.FIG. 13A shows an HRTEM image of a Z1S3 nanobelt.FIG. 13B shows a first TEM image of a ZrS , / ZrOS nanobelt.FIG. 13C shows a second TEM image of a Z1S3 / Z1OS nanobelt.FIG. 13D shows an HRTEM image of a ZrS VZrOS nanobclt.FIG. 13E shows a STEM (Scanning transmission electron microscopy) image of a ZrS i / ZrOS nanobclt.FIG. 13F shows the elemental mapping of oxygen of the ZrSs / ZrOS nanobelt of FIG. 13E. FIG. 13G shows the elemental mapping of sulfur of the ZrS i / Zi'OS nanobelt of FIG. 13E.FIG. 13H shows the elemental mapping of zirconium of the ZrS VZrOS nanobelt of FIG. 13E.FIG. 14A shows ultraviolet- visible absorption spectra of the? andphotoanodcs. FIG. 14B shows Tauc plots of thphotoanodes.FIG. 15 shows (a) ultraviolet-visible absorption spectrum, and (b) Tauc plot of ZrOS. Since cubic is a direct bandgap semiconductor, the bandgap ofis estimated to be 3.3 eV, respectively, based on their absorption spectra as shown in (b).FIG. 16 shows (a) ultraviolet-visible absorption spectra, and (b) Tauc plots ofphotoanode with different oxidation cycles. With the increases in the cycles of pulsed ozone-treatment, no obvious changes arc observed in the ultraviolet-visible absorption spectra, leading to the unchanged bandgaps of the ozone treated samples. This is due to the wide bandgap of ZrOS and its high transmittance to the visible light.FIG. 17A shows Mott-Schottky plots of the and photoanodes.FIG. 17B shows Mott-Schottky plot of the pure ZrOS sample. The negative slope observed in the sample confirms the p-type semiconductor characteristics of cubic ZrOS, aligning with prior research findings. In addition, the flat-band potential of the sample can be estimated by extrapolating the plots toTherefore, the flatband potential of ZrOS is estimated to be around 0.93FIG. 18 shows UPS spectra of (a) work function, and (b) valance band regions for andFIG. 19 shows schematic band structure diagram forand ZrOS.FIG. 20A shows XPS spectra of Zr 3d for the ZrS ? andphotoanodes.FIG. 20B shows XPS spectra of S 2p for the ZrS3 andphotoanodes.FIG. 20C shows O Is XPS spectra of the ZrS , NBA and theFIG. 21A shows a KPFM image of ananobelt in the dark.FIG. 21B shows a KPFM image of ananobelt under illumination.FIG. 21 C shows CPD profiles of the dark state and the light state in thenanobelt.FIG. 21D shows a KPFM image of ananobelt in the dark.FIG. 21E shows a KPFM image of ananobelt under illumination.FIG. 21F shows CPD profiles of the dark state and the light state in the Znanobelt.FIG. 22A shows an SPVS image of the ZrS? nanobclt derived from its KPFM image taken in the dark and under illumination.FIG. 22B shows an SPVS image of the nanobclt derived from its KPFM imagetaken in the dark and under illumination.FIG. 23A shows LSV profiles of the ZrSs and thephotoanodcs for sodium sulfitemeasured in 1 M KOH with and without 0.5 Munder AM 1.5 G, 100 millumination.FIG. 23B shows LSV profiles of the ZrS i and ZrSi / ZrOS photoanodes for water oxidation measured inKOH with and without 0.5 M Na2SOs under AM 1.5 G, 100 in Willumination.FIG. 23C shows LSV profiles of the Z1S3 and Z1S3 / Z1OS photoanodes with NiOOH cocatalyst for water oxidation measured in 1 M KOH.FIG. 23D shows LSV profiles of the ZrS i and ZrS ; / ZrOS photoanodes with NiOOH / FeOOH cocatalysts for water oxidation measured in 1 M KOH.FIG. 24 shows (a) photoelectrochemical Na2SO3, and (b) water oxidation of the NBA withdifferent oxidation cycles measured in 1 M KOH with and without 0.5 M under AM1.5 G, 100 mW cm'2illumination.FIG. 25A shows full XPS spectra of the ZrS3 / ZrOS / NiOOH (ZrS3 / ZrOS / Ni), ZrSs / ZrOS / FcOOH (ZrS3 / ZrOS / Fc), and ZrS3 / ZrOS / NiOOH / FcOOH (ZrSs / ZrOS / Ni / Fc) photoanodes.FIG. 25B shows FcXPS spectra of the ZrSs / ZrOS / Fc, and ZrS3 / ZrOS / Ni / Fc photoanodcs.FIG. 25C shows Ni XPS spectra of the ZrS i / ZrOS / Ni. and ZrS i / ZrOS / Ni / Fe photoanodes.FIG. 26 shows LSV profiles of the ZrSs / ZrOS photoanodes with different cocatalysts for (a) water oxidation, and (b) Na^SO;, oxidation, measured in 1 M KOH without and with 0.5 Munder AM 1.5 G, 100 mWillumination.FIG. 27 shows (a) TEM, and (b) HRTEM images of a ZrSVZrOS nanobclt with the NiOOH / FeOOH oxygen evolution cocatalysts (OECs), andSTEM and elemental mapping of a ZrS i / ZrOS nanobelt with the NiOOH / FeOOH OECs.FIG. 28 showsprofiles of the photoanodes coupled with the NiOOH / FeOOH OECs for (a) , and (b) water oxidation measured in 1 M KOH with and without 0.5 Munder AM 1.5 G, 100 mWillumination.FIG. 29A shows the Applied Bias Photon-to-current Efficiency (ABPE) conversion efficiencies for the photoanodcs with the NiOOH cocatalyst.FIG. 29B shows the ABPE conversion efficiencies for the photoanodes with the NiOOH / FeOOH OECs.FIG. 30A shows ABPEs of the ZrSs and the ZrSi / ZrOS photoanodes with the NiOOH cocatalyst.FIG. 30B shows ABPEs of the ZrSs and the ZrS . / ZrOS photoanodcs with the NiOOH / FcOOH cocatalysts.FIG. 30C shows charge separation efficiencies of the ZrSs and the ZrSVZrOS photoanodcs with the NiOOH cocatalyst.FIG. 30D shows charge separation efficiencies of the ZrSs and the ZrSs / ZrOS photoanodes with the NiOOH / FeOOH cocatalysts.FIG. 30E shows surface charge transfer efficiencies of the Z1S3 and the Z1S3 / Z1OS photoanodcs with and without the NiOOH cocatalyst.FIG. 30F shows surface charge transfer efficiencies of the ZrSs and the ZrSa / ZrOS photoanodes with and without the NiOOH / FeOOH cocatalysts.FIG. 30G shows charge carrier lifetimes of the ZrS ; and the ZrSs / ZrOS photoanodes with the NiOOH cocatalyst.FIG. 30H shows charge carrier lifetimes of the ZrSs and the ZrSVZrOS photoanodes with the NiOOH / FeOOH cocatalysts.FIG. 301 shows electron transport times of the ZrS i and the ZrSs / ZrOS photoanodcs with the NiOOH cocatalyst.FIG. 30J shows electron transport times of the ZrSs and the ZrSVZrOS photoanodcs with the NiOOH / FeOOH cocatalysts.FIG. 30K shows IPCE spectra of the ZrSs and the ZrSs / ZrOS photoanodes with the NiOOH cocatalyst at 1.23 VRHE.FIG. 30L shows IPCE spectra of the Z1S3 and the Z1S3 / Z1OS photoanodes with the NiOOH / FcOOH cocatalysts at 1.23 VRHE.FIG. 31 shows charge separation efficiencies of the photoanodes (a) without the NiOOH / FeOOH OECs, and (b) with the NiOOH / FeOOH OECs.FIG. 32 shows the surface charge catalysis efficiencies of the photoanodes with and without the NiOOH / FeOOH OECs.FIG. 33 shows typical experimental IMPS response measured in 1 M KOH at 1.0 VRHE for the ZrSs / Ni / Fe and the ZrSs / ZrOS / Ni / Fe photoanodes.FIG. 34 shows typical small-perturbation transient photocurrent decay measured in 1 M KOH at 1.0 VRHE for the ZrSs / Ni / Fc and the ZrSs / ZrOS / Ni / Fe photoanodes.FIG. 35 shows (a) the corresponding collection efficiency, and (b) electron diffusion length of the ZrSs / Ni / Fe and the ZrSs / ZrOS / Ni / Fe photoanodes versus applied potentials.FIG. 36A shows stability of the photoanode. Image (a) shows J-t plot of the ZrSs / ZrOS / NiOOH photoanode at 1.23 VRHE measured in 1 M KOH and its corresponding H2 O2 evolution. Image(b) shows long-term chronoampcromctric trace recorded at 1.23VRHE for the ZrS VNiOOH and the ZrSs / ZrOS / NiOOH photoanodes.FIG. 36B shows stability of the photoanodc. Image (a) shows long-term chronoampcromctric trace recorded at 0.8 VRHE for the ZrSs / Ni / Fe and the ZrSs / ZrOS / Ni / Fe photoanodes measured in 1 M KOH. Image (b) shows J-t plot of the ZrSs / ZrOS / Ni / Fe photoanode at 0.8 VRHF. measured in 1 M KOH and its corresponding H2 and O2 evolution. XRD patterns of the ZrSs / ZrOS / Ni / Fe photoanode are shown in (c) the (002) peak of Z1S3, and (d) the (110) peak of ZrOS before and after 650 hours of J-t measurements.FIG. 37 shows (a) ZXPS, and (b) S XPS spectra of the ZrSs / ZrOS / Ni / Fc photoanodebefore and after the 650 h PEC test.FIG. 38 shows (a) TEM image of thes photoanode after 650 hours of J-t measurements, and its corresponding (b) STEM image, and (c-g) elemental mappings, and (h) elemental intensity profiling as a function of scan distance in the STEM image.FIG. 39A shows LSV profiles of thee photoanode before and after the stability test formeasured in 1 M KOH with and without 0.5 M Na2SO3 under AM 1.5 G, 100 mW illumination.FIG. 39B shows LSV profiles of the ZrS VZrOS / Ni / Fc photoanodc before and after the stability test for water oxidation, measured in 1 M KOH with and without 0.5 Munder AM 1.5 G, 100 millumination.FIG. 40 shows SEM images of (a) the top-view, and (b) the side-view of the ZrS / ZrOS / Ni / Fe photoanode before the stability testing, and (c) the top-view, and (d) the side-view of the ZrSs / ZrOS / Ni / Fc photoanodc after the 1000 h J-t testing.FIG. 41 shows calculated band structure of (a) c-ZrOS and band structures with projection to (b) Zr, (c) S, and (d) O elements, respectively. Thicker lines represent a bigger projection value. FIG. 42 shows VBM-associated partial charge density contours of c-ZrOS.DETAILED DESCRIPTION

[0015] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0016] Features that arc described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0017] In the context of various embodiments, the articles “a”, “an” and “the” as used regarding a feature or clement include a reference to one or more of the features or elements.

[0018] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e.g. within 10% of the specified value.

[0019] As used herein, the term “and / or” includes any, and all combinations of one or more of the associated listed items.

[0020] Embodiments described in the context of one of the photoanodes are analogously valid for the other photoanodc. Similarly, embodiments described in the context of a method are analogously valid for a photoanode, and vice versa.

[0021] According to a first aspect, a photoanodc for photoclcctrochcmical water-splitting is provided. Referring to FIG. 1 and FIG. 2, the photoanode 100 comprises a conductive substrate 110, an array of zirconium trisulfidenanobelts 120 deposited onto at least one surface of the conductive substrate 110, and a layer of zirconium oxysulfide (ZrOS) 130 deposited on the anay of zirconium trisulfidenanobelts 120, forming a zirconium trisulfide oxysulfidc (ZrOS) nanobclts array 140 on the conductive substrate, denotedananobelts array (NBA).

[0022] The zirconium trisulfidc nanobclts arc elongated, ribbon-likc structures witha width of nanometers and can be several micrometers long. In various embodiments, the nanobelts within the array each has a thickness ranging from 50 to 100 nm. In some embodiments, the nanobelts within the array each has a width ranging from 100 to 1000 nm. In some embodiments, the nanobelts within the array each has a length ranging between 5 and p

[0023] In some embodiments, the layer of zirconium oxysulfide (ZrOS) 130 formed on the array of zirconium trisulfidc (ZrS3) nanobclts is a dense layer covering the entire surface of each of the nanobelts that form the layer of zirconium trisulfide The dense layer has athickness ranging from 18 to 22 nm.

[0024] In some embodiments, the photoanodc further comprises a layer of nickel oxide hydroxide (NiOOH) deposited on the layer of zirconium oxysulfide (resulting in a photoanodc, denoted as

[0025] In other embodiments, the photoanode further comprises a layer of nickel oxide hydroxide (NiOOH) deposited on the layer of zirconium oxysulfideand a layer of iron oxyhydroxide (FeOOH) deposited on the layer of nickel oxide hydroxide (NiOOH) as cocatalysts, resulting in a photoanode, denoted as Thecombination of the NiOOH andsimultaneously mitigates the recombination between the photoanode and the catalysts and facilitates the efficient surface hole transfer into an electrolyte. Such a layer improves the charge carrier dynamics of the photoanode.

[0026] In various embodiments, the array of zirconium trisulfidenanobelts forms an n-type layer, and the layer of zirconium oxysulfide (ZrOS) forms a p-type layer, configured to form an n-p buried junction in the photoanode. The n-p buried junction favors efficient charge carrier separation. Compared with conventional photoanodes, the photoanode has excellent light harvesting ability across the whole visible region to over 650 nm, and good photo-to- current conversion capability due to the buried n-p junction. Furthermore, the p-type ZrOS layer also acts as a hole collection layer for hole extraction and a protective layer for the array of zirconium trisulfidenanobelts to shield the photoanode from photocorrosion and to ensure excellent stability of the photoanode for over 700 hours, or over 1000 hours in an alkaline solution.

[0027] The conductive substrate 110 may be any suitable material with a surface that may be used as an electron collector. Examples of suitable substrate materials include, but arc not limited to, fluorine-doped tin oxide (FTO) glass, indium tin oxide (ITO), and zirconium substrate. In some embodiments, the conductive substrate is a zirconium substrate. In other embodiments, the conductive substrate is a fluorine-doped tin oxide (FTO) glass substrate. In some embodiments, the photoanode with the fluorine-doped tin oxide (FTO) glass substrate is fabricated from the photoanode with the zirconium substrate through a film transfer method.

[0028] The film transfer method and a method of forming the photoanode of the present disclosure arc provided and described hereinbelow.

[0029] According to a second aspect, a method of forming a photoanode is provided. Referring to FIG. 3, the method comprises depositing an array of zirconium trisulfidenanobelts 302 on at least one surface of a conductive substrate 304; and subjecting the array of zirconium trisulfide (ZrSs) nanobelts to ozone treatment 305 to form a layer of zirconium oxy sulfide (ZrOS) 306 on the array of zirconium trisulfidenanobelts, forming azirconium trisulfidc (ZrSo / zirconium oxysulfidc (ZrOS) nanobclts array 308, denoted asNBA on the conductive substrate.

[0030] The array of zirconium trisulfidc nanobclts can be deposited onto theconductive substrate using various suitable methods including, but not limited to, chemical vapor transport (CVT) method. In an exemplary embodiment, the array of zirconium trisulfidenanobelts is deposited via a typical chemical vapor transport (CVT) process 312. The process involves chemical vapor transport of sulfur across the conductive substrate in a vacuum for growing the nanobclts. The conductive substrate 304 and sulfur powder 310 arc first placed in a sealed tube under vacuum before the sealed tube is placed in a furnace for annealing, to form the array of zirconium trisulfide nanobelts 302 deposited on the conductivesubstrate 304.

[0031] In various embodiments, the layer of zirconium oxysulfide (is deposited on the array of zirconium trisulfidenanobelts through oxidation ofby pulsed-ozone- treatment. This treatment is performed in an atomic layer deposition (ALD) flow reactor by exposing the zirconium trisulfidcnanobclts to ozone305 at an elevated temperature for a predetermined duration, and for a predetermined number of cycles of deposition to form the layer of zirconium oxysulfidc (ZrOS). In some embodiments, the layer of zirconium trisulfide is exposed to ozone at an elevated temperature ranging from 280°C to 320°C, 290°C to 310°C or 295°C to 305°C. In some embodiments, the ALD process is cycled several times, ranging from 25 to 100 times, 25 to 50 times, 25 to 75 times, 50 to 100 times, 50 to 75 times or 75 to 100 times, depending on the thickness of the ZrOS layer to be deposited.

[0032] The pulscd-ozonc-trcatmcnt induces a thick coating of zirconium oxysulfidc (ZrOS) on the zirconium trisulfide nanobelts. The thick coating of zirconium oxysulfide (ZrOS)has a thickness ranging from 18 to 22 nm.

[0033] The pulsed-ozone-treatment facilitates stronger interaction between the oxygen ions and the layered zirconium ions, leading to the creation of a stable three-dimensional crystalline framework and enables the formation of a robust zirconium oxysulfide (ZrOS) layer. The zirconium oxysulfide (ZrOS) layer fully encapsulates the entire anay of zirconium trisulfide nanobclts. This approach yielded a p-typc ZrOS layer formed on the surface of the n- type zirconium trisulfide nanobelts, resulting in the buried n-p junction. As mentionedhereinabove, the buried n-p junction favors efficient charge carrier separation. The resulting zirconium trisulfide oxysulfide (ZrOS) nanobelts array demonstratedexcellent light harvesting ability across the entire visible region to over 650 nm, while also exhibiting good photo-to-current conversion capability due to the buried n-p junction.

[0034] The method of the present disclosure further includes depositing a catalyst or cocatalysts 315 on the layer of zirconium oxysulfide (ZrOS) to obtain a photoanode 317 comprising an array of zirconium trisulfidc nanobclts, a layer of zirconium oxy sulfide) and a layer of catalyst or cocatalysts.

[0035] In some embodiments, the method comprises depositing nickel oxide hydroxide (NiOOH) on the layer of zirconium oxysulfide (ZrOS) to obtain a photoanode comprising an anay of zirconium trisulfidenanobelts, a layer of zirconium oxy sulfide (ZrOS) and a layer of nickel oxide hydroxide (NiOOH), denoted asIn some embodiments, the layer of nickel oxide hydroxide (NiOOH) may be deposited by immersing the photoanode in an aqueous solution containing nickel salt.

[0036] In some embodiments, the method further includes depositing iron oxide hydroxide (FeOOH) on the layer of nickel oxide hydroxide (NiOOH) to obtain a photoanode comprising an array of zirconium trisulfidenanobelts, a layer of zirconium oxysulfidea layer of nickel oxide hydroxide (NiOOH) and a layer of iron oxide hydroxide (FeOOH), denoted asIn some embodiments, the iron oxide hydroxide (FeOOH) may be deposited by immersing the photoanode in an aqueous solution containing an iron salt.

[0037] In some embodiments, the zirconium trisulfideoxysulfide (ZrOS) nanobelts array 308 thus formed as a whole can be transferred from one substrate to another substrate as desired. For example, the zirconium trisulfideoxysulfide (ZrOS) nanobelts array may be transferred from a zirconium substrate to a fluorine-doped tin oxide (FTO) substrate. This step can be performed when the zirconium trisulfidcoxysulfide (ZrOS) nanobelts array tends to become easily detachable from the conductive substrate, rendering both the array and the conductive substate unsuitable for testing PEC performance.

[0038] In some embodiments, the method comprises transferring the zirconium trisulfide ( oxysulfide (ZrOS) nanobelts array from a first conductive substrate to asecond conductive substrate through a film transfer method.

[0039] Referring to FIG. 3, the film transfer method comprises coating a polystyrene precursor 314 onto the zirconium trisulfideoxysulfide (ZrOS) nanobelts array 308 deposited on the first conductive substrate 304 to obtain a polystyrene coated zirconium trisulfidem oxysulfide (ZrOS) nanobelts array 316. This is followed by removing 318 the first conductive substrate 304 from the polystyrene coated zirconium trisulfi oxysulfide (ZrOS) nanobelts array 316; adding a precursor solution320 onto a second conductive substrate 322; affixing 324 the polystyrene coated zirconium trisulfidem oxysulfide (ZrOS) nanobelts array 316 to the second conductive substrate 322; and heating 326 the entire assembly and subsequently washing 328 the assembly thoroughly to remove the polystryrene film. The obtained zirconium trisulfide (ZrSs) / zirconium oxysulfide (ZrOS) nanobelts array with the second conductive substrate is annealed 330 to obtain a firmly adhered zirconium trisulfide oxysulfidenanobelts array 308 on the second conductive substrate 322.

[0040] In some embodiments, the polystyrene precursor is coated onto the zirconium trisulfide (ZrS3) / zirconium oxysulfide (ZrOS) nanobelts array by spin coating. The spin coating process can be repeated several times to obtain a polystyrene film adhered to the zirconium trisulfideoxysulfide (ZrOS) nanobelts array deposited on the first conductive substrate. The zirconium trisulfide oxysulfide (ZrOS) nanobelts array isalmost entirely encapsulated by the polystyrene film. This comprehensive coverage ensures the integrity and structural preservation of the zirconium trisulfide (ZrS3) / zirconium oxysulfide (ZrOS) nanobclts array during subsequent transfer steps.

[0041] After the polystyrene is coated onto the zirconium trisulfideoxysulfidc (ZrOS) nanobclts array, the first conductive substrate can be removed. This can be carried out by gently detaching or peeling off the first conductive substrate from the polystyrene coated zirconium trisulfideoxysulfide (ZrOS) nanobelts array.

[0042] In some embodiments, the film transfer method employed in the present disclosure is a seed-assisted film transfer method. In this method, prior to transferring the polystyrene coated zirconium trisulfidcoxysulfidc (ZrOS) nanobclts array onto the second conductive substrate, seeds of the second conductive substrate are first deposited onto the second conductive substrate to form a seed layer. This is performed by dropping the second conductive substrate precursor solution onto the second conductive substrate before affixing the polystyrene coated zirconium trisulfide (oxysulfide (ZrOS) nanobelts array onto the second conductive substrate. This technique facilitates the secure transfer of the zirconium trisulfide (ZrS3) / zirconium oxysulfide (ZrOS) nanobelts array onto the second conductive substrate, ensuring a firm adhesion of the zirconium trisulfidcoxysulfide (ZrOS) nanobelts array to the second conductive substrate. This results in a robust and securely attached configuration that is conducive to reliable PEC testing.

[0043] In some embodiments, the first conductive substrate and the second conductive substrate may be any suitable materials with a surface that may be used as an electron collector. Examples of suitable substrate materials include, but are not limited to, fluorine-doped tin oxide(FTO) glass, indium tin oxide (ITO), and zirconium substrate. In some embodiments, the first conductive substrate is a zirconium substrate, and the second conductive substrate is a fluorinedoped tin oxide (FTO) glass substrate, and the film transfer method involves transferring the zirconium trisulfideoxysulfide (ZrOS) nanobelts array from the zirconium substrate to the fluorine-doped tin oxide (FTO) substrate.

[0044] According to a third aspect of the present disclosure, a photoelectrochemical cell comprising the photoanode of the present disclosure is provided. The photoelectrochemical cell comprises a counter electrode; a photoanodc; and an electrolyte, wherein the photoanodc comprises a conductive substrate; an array of zirconium trisulfidenanobelts deposited on at least one surface of the conductive substrate; and a layer of zirconium oxysulfide (ZrOS) deposited on the array of zirconium trisulfideforming a zirconium trisulfide zirconium oxysulfide (ZrOS) nanobelts array on the conductive substrate, wherein, thearray of zirconium trisulfide (nanobelts is an n-type layer and the layer of zirconium oxysulfide (ZrOS) is a p-type layer, forming a buried n-p junction in the photoanode.

[0045] Any suitable electrolyte may be employed including, but not limited to, potassium hydroxide (KOH), sodium hydroxide (NaOH), phosphate buffers and sodium sulfate

[0046] Any suitable counter electrode may be employed including, but not limited to, platinum (Pt) and silver (Ag).

[0047] The conductive substrate may be any suitable materials with a surface that may be used as an electron collector. Suitable substrate materials include, but are not limited to, fluorine-doped tin oxide (FTO) glass, indium tin oxide (ITO), and zirconium substrate.

[0048] In the present disclosure, a stable and efficient photoanodc comprising zirconium trisulfide / zirconium oxysulfidenanobelts array, with a p-type ZrOS layer that fully encapsulates the ZrSi nanobelts, is obtained. The obtained zirconium trisulfide zirconium oxysulfide (ZrOS) nanobelts array demonstrated excellent light harvestingability across the entire visible region to over 650 nm, while also exhibiting good photo-to- current conversion capability due to the buried n-p junction (FIG. 4). With the incorporation of NiOOH oroxygen evolution reaction (OER) cocatalysts in the photoanode, the resulting andphotoanodcs realisedefficient photoelectrochemcial water splitting by delivering a substantial photocurrent density of about 8.97mAto 9.3 mAat 1.23 VRHE, and a Applied Bias Photon-to-currcnt Efficiency (ABPE) of about 2.93% to 3.2 %> under simulated sunlight irradiation in an alkaline solution (pH of 13.6). Furthermore, the layer of zirconium oxysulfide (ZrOS) simultaneously acts as an efficient hole extraction layer and as a protective layer that prevents the photoanodefrom photocorrosion by the high-energy holes and electrolytes. Consequently, the ZrSs / ZrOS / NiOOH and the ZrSs / ZrOS / NiOOH / FeOOH photoanodes remain stable for over 700 hours or over 1000 hours, respectively, revealing its excellent durability. In one embodiment, the photoanode yields a photocurrent density of approximately 8.97mA cm'2at 1.23 VRHE, an ABPE of approximately 2.93% under simulated sunlight, and exhibits stability for over 700 hours without obvious performance degradation. In another embodiment, the photoanode yields a photocurrent density of approximately 9.3 mA cm2at 1.23 VRHE, an ABPE of approximately 3.2% under simulated sunlight, and exhibits stability for over 1000 hours.

[0049] To facilitate a better understanding of the present disclosure, the following examples of specific embodiments are given. In no way should the following examples be read to limit or define the entire scope of the disclosure. One skilled in the art will recognize that the examples set out below are not an exhaustive list of the embodiments of this disclosure.EXAMPLESExample 1: Fabrication of a zirconium trisulfide (ZrSs) nanobelts array and a zirconium trisulfide (ZrS3) / zirconium oxysulfide (ZrOS) nanobelts array (NBA) on a substrate

[0050] Growth ofZ' rSi nanobelts array (NBA)

[0051] The ZrS ) NBA was first grown on zirconium (Zr) foil (Sigma-Aldrich, thickness 0.1 mm, 99.98% trace metals basis) by a typical chemical vapor transport process. In a typical synthesis, a piece of 2 cm by 1 cm Zr foil (99.9%; thickness: 0.25 mm) was first chemically polished in hydrofluoric acid (HF) (48-51%, Alfa Aesar) for 1 min, and washed with ethanol and water. The polished Zr foil and 60 mg sulfur powder (99.5 % purity, Alfa Aesar) were then sealed in a silica tubeunder vacuum which was subsequentlyplaced in a furnace with the end of the Zr foil positioned at the center of the furnace. After annealing at 650 °C for 4 hours, ZrSs NBA were obtained on the Zr foil.

[0052] Fabrication of zirconium trisulfide (ZrSiVzirconium oxysulfide (ZrOS) nanobells array (ZrSs / ZrOS NBA)

[0053] The synthesis of ZrSa / ZrOS NBA on Zr foil was performed in an atomic layer deposition (ALD) flow reactor (Plasma- assisted ALD system, Wuxi MNT Micro and Nanotech Co., Ltd, China) by alternatively exposing ZrS:; NBA to ozoneat 300 °C. Ozonewas produced with a Wcdcco Ozomatic Modular 4 HC Lab ozone generator from oxygen (99.9999 %) and pulsed into the reactor with a flow rate of 100 mL / min. The carrier gas utilized in the experiment was an ultrahigh purity N2 with an exceptional purity level of 99.99%,flowing at a rate of 30 mL / min. The precursor module was excluded from the experiment, keeping the precursor container at room temperature with the valve closed. After loading the NBA on Zr foil into the reaction chamber, it was heated and held at a stable temperature of 300 °C, and the timing sequence was 500, 20, 1000, and 20 seconds for the precursor module (no precursor in),purge, O exposure, and N2 purge, respectively. The ALD process wascycled 25, 50, 75, and 100 times to synthesize ZrS i coated with different thicknesses oflayers.Example 2: Transferring a zirconium trisulfide (ZrSs) nanobelts array or a zirconium trisulfide (ZrS3) / zirconium oxysulfide (ZrOS) nanobelts array (NBA) onto a fluorinedoped tin oxide (FTO) substrate

[0054] The fabrication of NBA on FTO substrate is performed through a filmtransfer method.

[0055] During the standard transfer procedure, 15 g of polystyrene (averagewabout 280,000 by GPC, Sigma- Aldrich) was dissolved in 100 mL toluene to form a polystyrene precursor. Subsequently, the polystyrene precursor was spin-coated onto the ZrSvZrOS NBA on Zr foil at 3000 rpm for 60 seconds, which was followed by baking the sample at 80 °C for 15 minutes. The identical spin-coating process was repeated three times to obtain a sample consisting of a polystyrene film adhered to the ZNBA onr foil. Thereafter, the polystyrene coated ZrSs / ZrOS NBA was gently detached from the Zr foil at the edge of the polystyrene using tweezers, yielding a polystyrene coated ZrSvZrOS NBA assembly.

[0056] To transfer the polystyrene coated ZrSs / ZrOS NBA assembly onto a FTO substrate, FTO seeds were first deposited onto the FTO substrate. Briefly, 7 g SnCLrSEbO was added to 100 ml absolute ethanol and stirred for 2 hours to yield a transparent tin (IV) chloride (SnCU) solution. 3.33 ml aqueous solution containing 148 mg ammonium fluoridewas added into the tin (IV) chloride precursor solution drop by drop to form a FTO precursorsolution. After dropping the precursor solution onto the FTO substrate, the polystyrene coated NBA assembly was affixed to the substrate. A weighty ceramic plate was placed ontop of the polystyrene film, and the entire setup was maintained at 100°C for 2 hours. Subsequently, the polystyrene film was eliminated thoroughly by rinsing with toluene multiple times. Finally, the obtained FTO substrate with the NBA were annealed at 500 °Cin an argon atmosphere for 2 hours, resulting in the production of NBA firmlyattached to the FTO substrate.

[0057] The same steps were performed for fabricating onto the FTO substrate.Example 3: Deposition of NiOOH or NiOOH / FeOOH oxygen evolution cocatalysts (OECs) on ZrS3NBA and ZrSVZrOS NBA on FTO substrates

[0058] The or were deposited on theNBA and theNBA through an immersion method.

[0059] In one example, NiOOH was deposited by directly immersing theNBA on FTO substrate into a 20 mM nickel chloride (NiCh) aqueous solution for 24 h to obtain thephotoanode. Thesample was then directly immersed into a 20 mM iron(III) chloride (FeCh) aqueous solution for 24 h to obtain the photoanode.

[0060] In another example, FeOOH was deposited by directly immersing theNBA on FTO substrate into a 20 mM iron(III) chloride( h) aqueous solution for 24 h to obtain thephotoanode. Thesample was then directly immersed into a 20 mM nickel chlorideaqueous solution for 24 h to obtain thephotoanode.

[0061] The same steps were performed for depositing NiOOH or FeOOH on NBA onFTO substrate.Example 4: Material Characterization

[0062] Various analytical techniques were utilized to investigate the microstructure and composition of the samples, including transmission electron microscopy (TEM, FEI Titan 80- 300 operated at 200 kV), scanning transmission electron microscopy (STEM: IEM-ARM200F, 200kV), UV-Vis-NIR spectrometer (Hitachi U4100), X-ray photoelectron spectroscopy (XPS, Omicron EA125 system) with Al Ka (1486.7 eV) X-ray as the excitation source, field emission scanning electron microscopy (FE-SEM, JEOL JSM6700F), and X-ray diffraction (XRD, Bruker D8 Advance).

[0063] PEC characterization

[0064] The PEC performance evaluation of the samples was conducted by an electrochemical workstation (CHI 760E) using a three-electrode system and under simulated AM 1.5G sunlight with an intensity of 100 mWThe simulated solar illumination was sourced from a 300 W Xenon lamp (Microsolar 300; Beijing Perfectlight), supplemented with an AM 1.5 G filter.

[0065] To guarantee accuracy, the light intensity directed onto the sample was calibrated to AM 1 5G one sun (100 mW cm-2) by adjusting the distance between the sample and the lightsource. This adjustment was made using a NIM-calibrated standard Si solar cell (100 mW cm" 2) for precise measurement. As for the experimental setup, the samples on FTO substrates were directly employed as the working electrode. A Pt wire served as the counter electrode, and an Ag / AgCl (saturated with potassium chloride (KC1)) electrode acted as the reference electrode. The illumination was applied from the sample side (front-side illumination).

[0066] To record the PEC performance, an H-type PEC cell was used, with the generated O2 and H2 collected separately from the anode and cathode chambers. In this configuration, both the anode and cathode electrolytes were 1 M KOH solution. The quantities of evolved H2 and O2 were measured periodically using a gas chromatograph (Model 3420A, Beifen-Ruili Co. Ltd., China) equipped with a thermal conductivity detector and a 5 A molecular sieve column. During the long-term stability testing, a custom-built cooling water recirculation system was utilized to regulate the temperature of the PEC system at 20 °C. The cooling water recirculation system was realized by using a large glass petri dish and Re-circulating Chillers (Advanced Thermal Solutions Inc. : ATS-Chill2300V). To ensure the stability of the light source, the light intensity was calibrated both during each refresh of the electrolyte and upon completion of the long-term stability test. Furthermore, Mott-Schottky plots were derived from impedance-potential tests conducted in the dark at a frequency of 1 kHz. The Zahner Zennium C-IMPS system was employed to capture data for intensity-modulated photocurrent / photovoltage spectroscopy (IMPS / IMVS). The incident photon-to-current conversion efficiency (IPCE) measurements were also conducted using the C-IMPS system, featuring a mountable and portable tunable light source (TLS03 / UV: 290 nm - 1020 nm). This light source comprises dynamic LEDs, a monochromator, and a feedback control mechanism, enabling accurate identification of light wavelength and intensity on samples during the measurements without requiring any warm-up time.

[0067] Computational details

[0068] Density functional theory (DFT) calculations about geometric relaxation and band structures were conducted by the Vienna Ab-initio Simulation Package (VASP) based on the PBEsol method. The cutoff energy was set to 612 eV in relax calculation and was set to 500 n band structure calculation. In structural relaxation, the atomic force convergence criterion

[0069] Theory of PEC photoelectrodes

[0070] The photocurrent for water oxidatio is given the following equation (1):where is the photocurrent density at 100% internal quantum efficiency, which can be determined by integrating the product of the incident spectral irradiance of AM 1.5 and the light-harvesting efficiency (LHE) at each wavelength over the range of photons with energyEg. The LHE can be calculated from the absorption spectra by using the following equation (2):

[0071] Due to that, the oxidation ofis very fast, theis nearly equal to 100%, the photocurrent for sulfite oxidation obtained was used to calculate Hence, theequation (1) can be transformed as following equation (3):Therefore, the rjtrans can be calculated by dividingby and the can be obtainedby dividin

[0072] The ABPE was calculated from thcurves obtained from assuming 100% Faradaic efficiency using the following equation, where J is the photocurrent density, isthe applied potential and is the incident illumination power density (AM 1.5G, 100 mW (equation 4).

[0073] FIG. 5 shows light harvest efficiencies (LHE) of ZrSs photoanode with different oxidation cycles. Following Equations (1-3) outlined above, the can be obtained bydividing Notably, Jabs represents the photocurrent density at 100% internalquantum efficiency. It is determined by integrating the product of the incident spectral irradiance of AM 1.5 and the light harvesting efficiency (LHE) at each wavelength over the range of photons with energy > Eg. Based on the LHE of the photoanodes (FIG. 5), thecan be calculated as 13.71, 13.53, 13.69, 13.51, and 13.80 mA cm-2, for the photoanodes of pristine ZrSs and ZrSs with 25, 50, 75, and 100 cycles oxidation, respectively. Therefore, the forall photoanodes can be obtained.

[0074] KPFM measurement

[0075] KPFM measurements were conducted using a Bruker Dimension Edge scanning probe microscope within an enclosure provided by the manufacturer, maintaining the ambient pressure, temperature, and humidity conditions. Electrical measurements were carried out via a Pt / Ir-coated probe (Bruker SCM-PIT-V2) with a resonance frequency of 75 kHz and a springconstant of approximately 3.0 N / m. The CPD distribution over the scan area of the samples under study was determined by applying an AC voltage combined with a DC voltage to the AFM tip. FTO substrate was employed to ensure proper electrical conductivity between the tip and the sample. CPD distribution maps were collected at a similar pixel density of approximately 256x256 pixels per image, with a scan rate of 0.6 pm / s. Photon-assisted KPFM measurements were conducted using a monochromatic light source from an LED light source. The excitation wavelength of the samples is 365 nm, and the light intensity on the sample surface is approximatelyExample 5: Characterization of nanobelts array

[0076] In the present disclosure, the ZrSs NBA was initially synthesized on Zr foil by chemical-vapor-transport (CVT) of sulfur across the Zr foil in a vacuum. The ZrSs / ZrOS NBA on the foil was further fabricated through the oxidation of ZrS.s NBA using a pulsed-ozone- treatment with 75 cycles. However, as those nanobelts obtained on the Zr foil tended to be easily detached from the substrates, making them unsuitable for the test of PEC performance, a seed-assisted film transfer method was implemented to transfer the ZrSa / ZrOS NBA from the Zr foil substrate to FTO substrate. This technique ensures the secure transfer of nanobelts onto FTO substrates, resulting in a robust and firmly attached configuration that supports reliable PEC testing.

[0077] Scanning electron microscopy (SEM) was initially utilized to examine and analyze the morphology and structure of the samples. The obtained film on the Zr foil was formed into nanobelts array (NBA), which the nanobelts were vertically grown on the foil with a thickness of about 20 pm ((a) of FIG. 6). SEM images (b) and (c) of FIG. 6 demonstrate the aftermath of the spin-coating step, wherein the nanobelts are almost entirely, if not completely encapsulated by the polystyrene layer. This comprehensive coverage is useful for the subsequent transfer steps to ensure the integrity and structural preservation of the nanobelts.

[0078] The SEM results further validate the successful transfer of the film onto the FTO substrate. As depicted in FIG. 7A and (d) of FIG. 6, the FTO substrate are featured by firmly- attached nanobelts with a vertical and uniform distribution. The nanobelts possess a thickness ranging from 50 to 100 nm, a width ranging from 100 to 1000 nm, and a length varying between 5 and 20 pm. The X-ray diffraction (XRD) pattern confirms the formation of nanobelts as ZrSs in the monoclinic phase (ICCD PDF no. 30-1498). After the pulscd-ozonc-trcatmcnt, two distinct peaks at 22.1° and 27.1° appear, which are assigned to the (110) and (111) planes of cubic ZrOS (ICCD PDF#04-0897) (FIG. 7B and FIG. 8). The XRD pattern of free-standingZrSs / ZrOS film, with clearly observable peaks of ZrOS at 22.1° and 27.1°, was also obtained to rule out any effects of the FTO substrate.

[0079] The generation of ZrOS through this pulsed oxidation method is likely due to the diversity of sulfur atoms within the crystal structure of ZrS i. FIG. 9 shows crystal structures of ZrS's and ZrOS. As shown in (a), two distinct forms of sulfur ions exist in the crystal structure, which are the sulfide (S2-) ions and the disulfide ions are unstable whensubjected to vacuum heating. Pulsed oxidation of Z1S3 was realized within an atomic layer deposition (ALD) flow reactor, where the ZrSs nanobclts were heated in a vacuum or under a low-pressure environment( ). Under such conditions, ozone tends to selectivelytarget and oxidize the unstableions, leading to the desulfurization of and itssubsequent repacement by oxygen ions. This process facilitates stronger interaction between the oxygen ions and the layered Zr ions, leading to the creation of a stable three-dimensional crystalline framework, as shown in (b) of FIG. 9. Consequently, this low-pressure oxidization technique enables the formation of a robust ZrOS layer.

[0080] For comparison, ZrOS particles were synthesized through a solid-phase reaction of ZrOz powder and ZrS2 powder. The transmission electron microscopy (TEM) results (FIG. 10) and the XRD results (FIG. 11) confirm that the obtained white powder is formed as pure ZrOS in the cubic phase (ICCD PDF#04-0897). Samples of ZrS3 / ZrOS-25c, ZrS3 / ZrOS-50c, ZrS s / ZrOS. and ZrSs / ZrOS- 100c were also prepared by setting the pulsed-ozone-treatment to cycle for 25, 50, 75, and 100 times, respectively, to investigate the oxidation process. As shown in (a) of FIG. 11, the peaks at 22.1° and 27.1° become gradually stronger over the ozone treatment cycles, indicating the gradual thickening of the ZrOS layer. The film transfer process did not induce any phase changes in the nanobelts as evidenced by their identical XRD peaks when they were free-standing and on FTO substrates (FIG. 7B and (b) of FIG. 11).

[0081] FIG. 7C shows the observation of four distinct Raman modes of ZrSs nanobelts, with characteristic peaks at approximately 147, 274, 315, and 524 These peaks areattributed to different vibrations instructure: I:(rigid chain vibration), 11:(internal out-of-plane vibrations), III: (internal out-of-plane vibrations), and IV:(S-S diatomic motion). After the ozone treatment, new peaks emerge, which arc assigned to the formed ZrOS layer by a further comparison with the Raman spectrum of the pure ZrOS ((a) of FIG. 12). The Raman results also reveal that the thickness of the ZrOS layer increases gradually with an increase in the number of cycles of pulsed-ozone-treatment ((b) of FIG. 12).

[0082] TEM was utilized to further analyze the morphology and composition of individual ZrSs and ZrSs / ZrOS NBA. As shown in FIG. 13A, an individual ZrSi nanobelt is confirmed asa single crystal with lattice spacings of 0.36 and 0.51 nm, corresponding to the d- spacings of monoclinicin the (010) and (100) planes, respectively. Based on these results, the orientation of nanobclt in the axial direction is verified to be aligned with the (010)direction. This reveals that the orientation of thelayer is aligned with the axial direction of the nanobelt, and therefore favors in-plane charge carrier transport. After the ozone treatment, the nanobelt was covered by an approximately 20 nm thick and highly poly crystalline layer, as confirmed by the TEM image as shown in FIG. 13B. High-resolution TEM (HR-TEM) images in FIG. 13C and FIG. 13D confirm that the polycry stallinc layer formed is cubic ZrOS, consistent with the XRD results. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was employed to directly observe the coverage of ZrOS on the As shown in FIG. 13E to FIG. 13H, the HAADF-STEM image and corresponding energy dispersive spectroscopy (EDS) mapping ofNBA demonstrate that thelayer was well coated on the ZrSs nanobelts, which is crucial to ensuring the stability of theNBA photoanode.Example 6: Characterization of charge carrier behavior of ZrOS layer

[0083] According to the diffuse reflectance ultraviolet-visible (UV-vis) spectra (FIG. 14A), the NBA absorb light with wavelengths up to about 650 nm, and the bandgap is determined to be about 1.99 eV from the Tauc plot (FIG. 14B). After the ozone treatment, no obvious changes are observed in the absorption edge and bandgap of theAdditionally, the pureshows a light absorption up to about 400 nm ((a) of FIG. 15) with a bandgap of 3.3 eV ((b) of FIG. 15). The light absorption properties of theNBA remain comparable to those of the . This similarity is attributed to the wide bandgapof ZrOS and its high transparency to visible light. The same behaviors are also observed in otherNBA with different numbers of ozone treatment cycles (FIG. 16), further evidencing that the ZrOS layer formed has little effect on the light absorption properties of inner .

[0084] As shown in FIG. 17A, a positive slope is observed in the Mott-Schottky plot of NBA, indicating the n-type behavior ofIn contrast, theNBA exhibits a distinctly different Mott-Schottky plot, which has a negative plot in the potential range of 0.3- 0.5 VRHE. Such a coexistence of positive and negative Mott-Schottky slopes indicates the presence of an n-p junction in thewhich is a signal of the p-typc behavior of ZrOS To further confirm that, the Mott-Schottky test was conducted on pure ZrOS powders as shown in FIG. 17B. A negative slope is observed in the Mott-Schottky plot of ZrOS,suggesting a p-typc behavior of ZrOS. In addition, the flat-band potentials ofandcan be estimated by extrapolating the plots toand the flat-band potentials are determined to be around -0.1 and 0.93respectively.

[0085] Ultraviolet photoelectron spectroscopy (UPS) characterizations were further employed to determine the energy level positions ofand ZrOS. As shown in (a) of FIG. 18, the work functions ofand ZrOS are measured to be about 4.31 and 5.16 eV, corresponding to the Fermi levelsof -0.13 and 0.72respectively. The valence band spectra indicate that the valence band locates 1.80 and 0.66 eV below the EF for and ZrOS ,respectively ((b) of FIG. 18). Combined with the bandgaps determined by the UV-vis spectra for (1.99 eV) and ZrOS (3.3 eV), respectively, the information of valence band spectra further confirms the n-type behavior ofand p-type behavior of Additionally, thevalence band maximumsand ZrOS are determined to be 1.67 and 1.38 while their conduction band minimums (are determined to be -0.32 and -1.92respectively. As a result, their energy band structure alignment can be confirmed as shown in FIG. 19. This means that a typical n-p heterojunction can be formed in thefavoring efficient charge carrier separation.

[0086] X-ray photoclcctron spectroscopy (XPS) analysis was utilized to characterize the effects of ozone treatment on the chemical states. After the ozone treatment, both theXPS spectra of theNBA show a 0.5 eV shift to the lower binding energy compared to those of the ZrSs NBA (FIG. 20A and FIG. 20B). Such shifts due to the downshifts of the Fermi level are commonly observed in 2D TMDs with hole doping. These XPS results arc also strong evidence of the p-typc behavior of ZrOS. After the ozone treatment, thpeaks of theNBA are almost eliminated, while thepeaks remain nearly unchanged compared to those of theNBA. Besides, a strong lattice oxygen signal is observed from theNBA (FIG. 20C). These results indicate that the pulsed-ozone- treatment can induce a thick coating of ZrOS on the NBA.

[0087] The photogenerated charge distribution of the nanobelts was visualized by kelvin probe force microscopy (KPFM) with and without light irradiation. FIG. 21A and FIG. 21B show the KPFM images of a nano belt in the dark and under illumination, respectively.The surface photovoltage spectroscopy (SPVS) image of thenanobelt was obtained by subtracting its dark KPFM image from its illuminated KPFM image (FIG. 22A). Additionally, the KPFM images in the dark, under illumination, and the SPVS image of the nanobelt are depicted in FIG. 21D, FIG. 21E, and FIG. 22B, respectively. Thenanobelt displays a lower contact potential difference (CPD) in the dark (FIG. 21D) than theNB (FIG. 21A), implying a lower Fermi level of ZrOS than that of < Underillumination, both the ZrSs nanobelt and the nanobelt show enhanced CPDs,meaning that the photogcncratcd holes stay on their surfaces. However, the illuminated nanobelt (FIG. 21E) shows a significantly higher CPD (about 150 mV) than theilluminated ZrSs nanobelt (FIG. 21B), indicating the enhanced hole extraction capability induced by the ZrOS layer. As shown in FIG. 22B, thenanobelt shows a surface photovoltage in the range of 300 to 400much larger than the Z1S3 nanobelt (< 80 mV). These SPVS results reveal that an obvious built-in electric field exists in the NBAdue to the enhanced charge carrier separation. This significantly enhanced charge carrier separation with the holes extracted to the surface and favors the development of effective photoanode for PEC water splitting.Example 7: Photoelectrochemical (PEC) performance

[0088] The PEC capabilities of the NBA were assessed using linear sweep voltammetry (LSV) experiments in 1 M KOH with and without 0.5 Munder simulated sunlight, where theA on FTO substrates were used as the photoanodes.

[0089] The electrolyte with sodium sulphite (NazSO;) was first used to optimize the PEC performance of the oxidized NBA with different ozone treatment cycles. As shown in (a) of FIG. 24, the NBA with 75 ozone treatment cycles (the NBA) show the best PECperformance, and all the oxidized NBA exhibit enhanced PEC performance compared to the ZrSi NBA, due to the n-p heterojunction formed. The photocurrent densities of the ZrSs and Z1S3 / Z1OS photoanodes for sulfiteand water oxidationare presented in FIG. 23A and FIG. 23B. After the ozone treatment, the ZrSs / ZrOS photoanodc displays a low applied potential to the saturated photocunent for sulfite oxidation (~0.8and aof 9.5± 0.2 mA cm2at 1.23 VRHE, while the ZrS ; photoanodc exhibits much lower / sulfite (3.2 ± 0.1 mA cm-2at 1.23 VRHE) and fails to reach its saturated photocurrent over the whole applied potential window. This implies that the induced ZrOS layer can promote the charge carrier dynamics during the PEC process through the buried n-p junction. However, when the and Zi'S / ZrOS photoanodes were used for PEC water oxidation, significant reductions in / 1120 are observed compared to theirA similar phenomenon is also observed in those photoanodes with different ozone treatment cycles ((b) of FIG. 24), implying the insufficient surface catalytic capabilities of the ZrOS layer for water oxidation.

[0090] Table 1 shows the average photocurrent densities at 0.8 and 1.23 VRHE obtained from J-V measurements for sodium sulphiteand water oxidation for the Z1S3 NBA withdifferent oxidation cycles measured in 1 M KOH with and without 0.5 M NazSO3under AM 1.5 G, 100 mWillumination. For each system, four measurements using five different samples were used for the statistical analysis.Table 1

[0091] The PEC NazSCh oxidation was first measured to optimize the oxidation process. As shown in (a) of FIG. 24, prior to reaching 75 cycles, as the number of oxidation cycles escalated, the photocurrent exhibited an upward trend, while subsequent to the 75-cycle mark, there was a noticeable decline in current density. Nevertheless, all the samples consistently displayed substantially reduced photocurrent levels for water oxidation, as indicated in (b) of FIG. 24. This suggests that the ZrS.s and ZrOS layers possess inadequate surface catalytic activity for facilitating water oxidation and additional OECs are needed to achieve efficient PEC water splitting.

[0092] Oxygen evolution co-catalysts (OECs) arc further needed to couple with the photoanodes to leverage their outstanding photoelectric response characteristics, achieving efficient PEC water splitting. Thus, thin layer of FeOOH or NiOOH, which is employed as OECs, was deposited on theorphotoanode via an immersion method. The successful deposition of FeOOH or NiOOH is indicated by the Fe 2p and Ni 2p XPS spectra in FIG. 25A-C.

[0093] As shown in FIG. 26, the photoanodes of ZrSVZrOS with NiOOH (ZrSs / ZrOS / Ni), with FeOOH (ZrS3 / ZrOS / Fc), with FcOOH / NiOOH (ZrS3 / ZrOS / Fc / Ni), and with NiOOH / FeOOH (ZrS3 / ZrOS / Ni / Fe) were fabricated for water oxidation and sulfite oxidation. The results reveal that neither ZrS3 / ZrOS / Ni nor ZrS3 / ZrOS / Fe fully exploits all surfacereaching holes in the OER, instead, they experienced some loss due to the recombination at the ZrS3 / ZrOS / OEC junction. ZrS3 / ZrOS / Fe suffers from severe recombination at the interface between ZrOS and FeOOH, and the bare NiOOH is not highly active for OER. Among them, the ZrS3 / ZrOS / Ni / Fe photoanode shows the best PEC performance for water oxidation (FIG. 26). This is attributed to the dual NiOOH / FeOOH OECs, which can simultaneously mitigate the recombination between the photoanode and the catalyst and facilitate the efficient surfacehole transfer into the electrolyte. The TEM-EDS analysis was further conducted on thephotoanode to examine the deposition of

[0094] Table 2 show the average photocurrent densities at 0.8 and 1.23 VRHE obtained from J-V measurements for water andoxidation for the, and photoanodes measured in 1 M KOH without and with0.5 M under AM 1.5 G, 100 mW cm-2illumination. For each system four measurements using four different samples were used for the statistical analysis.Table 2

[0095] The Jmos from the resultingandphotoanodes are higher than the bar photoanode, but still lower than the from thephotoanode ((a) of FIG. 26). This indicates that neithernorc fully utilizes all surface-reaching holes in the OER, but instead experiences some loss due to recombination at thejunction. In comparison, the photoanodeshows comparable to thephotoanode. It indicates that the deposition ofNiOOH did not induce any charge carrier recombination at the interfaces between the nanobelts and the OECs, meaning the recombination mainly occurred at the NiOOH / electrolyte interface ((b) of FIG. 26). While thephotoanode shows much lowersuggesting severe recombination at theinterface. This comparison reveals that the formed interface states between theZrOS and FcOOH can serve as recombination centers, and the NiOOH is not highly active for OER. Therefore, thewas expected to effectively mitigate recombination between the photoanode and the catalyst, while also facilitating efficient surface hole transfer. As a result, the resulting ZrS / ZrOS / Ni / l e photoanode shows the best PEC performance for water oxidation ((a) of FIG. 26). For example, the ZrSs / ZrOS / Ni / Fe photoanode also shows an Esat of approximately 0.8 VRUE for water oxidation and a high JH20 of 9.29 mA cm2at 1.23 VRHE, higher than that of the ZrSa / ZrOS / Ni photoanode (8.97 mA cm-2at 1.23 VRHE). However, with the deposition of FeOOH / NiOOH OEC, the ZrSi / ZrOS / Fe / Ni shows much lower JH2OS ((a) of FIG. 26), due to the severe recombination at the ZrOS / FeOOH interface.

[0096] In (a) and (b) of FIG. 27, the formation of a uniform layer that covers the ZrS3 / ZrOS nanobelt is observed. Follow-up analyses utilizing STEM-EDS have verified a homogeneous distribution of Ni and Fc elements throughout the nanobclt ((c-h) of FIG. 27). This confirms the uniform deposition of the NiOOH / FeOOH OECs with a thickness of about 10 nm.

[0097] With the assistance of the NiOOH and NiOOH / FeOOH OECs, both the ZrS i / N'iOOl l and ZrS s / ZrOS / NiOOI I photoanodes (FIG. 23C), and ZrSs / Ni / Fc and ZrS3 / ZrOS / Ni / Fe photoanodes (FIG. 23D) show comparable Jmo to their Jsuifite yielded from the ZrS , and ZrSVZrOS photoanodcs. For example, the ZrSVZrOS / NiOOH and ZrSVZrOS / Ni / Fe photoanodes also shows anof about 0.8for water oxidation and a JH20 of 8.97 mA and 9.3 ± 0.2 m at 1.23 VRHE, respectively, and the ZrSs / NiOOHand ZrSs / Ni / Fe photoanodes exhibit a of 3.03 mand 3.1 ± 0.2 m at 1.23VRHE, respectively. Besides, the ZrS3 / ZrOS-25c / NiOOH, ZrS3 / ZrOS-50c / NiOOH, ZrS3 / ZrOS- 25c / Ni / Fe, ZrS3 / ZrOS-50c / Ni / Fe, ZrS3 / ZrGS-100c / NiOGH and ZrS3 / ZrGS-100c / Ni / Fe photoanodes also show comparableand 7 , further implying the good contactbetween NiOOH and NBA, and between NiOOH / FeOOH and NBA, and the superior oxidation capacity of NiOOH and NiOOH / FeOOH.

[0098] Tabic 3 shows the average photocurrent densities at 0.8 and 1.23 VRHE obtained from J-V measurements for water andoxidation for photoanodes coupled with the NiOOH / FeOOH OECs measured in 1 M KOH without and with 0.5 M under AM1.5 G, 100 mllumination. For each system four measurements using five different samples were used for the statistical analysis.Table 3

[0099] Moreover, the applied bias photon -to-current efficiency (ABPE) can be calculated from the J-V curve for water oxidation. As shown in FIG. 29A and FIG. 30A, and FIG. 29B and FIG. 30B, the highest ABPE of ZrSs / ZrOS / NiOOH is 2.95% at a low bias of 0.81 VRHE anis about 3.2 % at a low bias of 0.82 which ranks among the topcompared with the recently reported state-of-the-art photoanodcs. However, the ZrSs / NiOOH and the photoanodes only yield the highest ABPE of about 0.6 % at about 0.87VRHE. The similar light absorption properties of the NBA and thNBA imply that the great variations of their PEC performances are primarily attributed to their differences in the charge carrier dynamics. The light-harvesting efficiencies (LHEs) of the photoanodcs were obtained from their UV-vis spectra to investigate the differences in their charge carrier dynamics (FIG. 5). As a result, the charge carrier efficiencies of the photoanodes can be derived from the comparison of theirand LHEs.

[0100] As shown in FIG. 30C and FIG. 30D, theandphotoanodc yields a charge separation efficiencyof about 70.19% and 70.3% at 1.23 respectively, which is much higher than theNiOOH photoanode (22.6%) and the ZrSs / Ni / Fe photoanode (23.2 %), and all the oxidized photoanodes with and without OECs show enhanced pSep than the pristine samples (FIG. 31). This demonstrates that the ZrOS layer plays a pivotal role in charge carrier separation, primarily due to the formation of a n-p heterojunction with the inner ZrS s. After loading the NiOOH or NiOOH / FeOOH OECs, both the ZrSa / ZrOS / NiOOH and ZrS3 / NiOOH photoanodes, and the ZrS3 / ZrOS / Ni / Fe and ZrSs / Ni / Fc photoanodcs exhibit similar surface charge transfer efficiencieshat arc much higher than those yielded from the photoanodes without OECs (FIG. 30E and FIG. 30F). Besides, all the photoanodes with also show greatly enhanced r]irail, with similarbehaviors (FIG. 32), indicating the necessity of the OECs for the photoanodes. Furthermore, intensity-modulated photocurrent spectroscopy (IMPS) and small perturbation transient photocurrent measurements (FIG. 33 and FIG. 34) were employed to investigate the time constants of the photoanodes.

[0101] FIG. 33 shows typical experimental IMPS response measured in 1 M KOH at 1.0 VRHE for the ZrSVNi / Fe and ZrS i / ZrOS / Ni / Fc photoanodes. IMPS, a convenient technique for assessing charge transfer and recombination rate constants, as well as the mean electron transport time of an illuminated photoelectrode, was employed in this example. In the IMPSmeasurement, all samples were exposed to a narrow-band blue LED light (448 nm, 100 mW As shown in FIG. 33, the IMPS reveals a characteristic semicircle in the fourth quadrant,which signifies the charge carrier transport processes. The value of for an illuminatedphotoclcctrodc can be derived from the frequency corresponding to the peak of this semicircle in the fourth quadrant, denoted as Consequently, for an illuminated photoelectrode canbe approximated using the optoelectrical admittance plot:

[0102] FIG. 34 shows typical small -perturbation transient photocurrent decay measured in 1 M KOH at 1.0 VRHE for the ZrSVNi / Fc and ZrSs / ZrOS / Ni / Fe photoanodes. Smallperturbation transient photocurrent decay measurements were conducted to investigate the timeconstants of the photoanodcs. In this procedure, a background light source was generated from simulated sunlight with an intensity of 100 mW cm2. and a narrow-band blue LED light (448 nm, 100 mW cm'2) was used as the perturbation source, controlled by the Zahncr Zcnnium C- IMPS system.

[0103] The photocurrent dynamics of the PEC cell, operated within a three-electrode system, were also monitored using the Zahner Zennium C-1MPS system while maintaining a potential difference in the range of 0.8 to 1.4 VRHE (FIG. 34). Parameters for the measurements included a 0.5 ms sampling time, a 200 ns response time to abrupt load changes, and a 10 ms recording time. The intensity of the perturbation light source was adjusted to a sufficiently low level to ensure that the photocurrent decay kinetics exhibited a mono-exponential behavior.

[0104] In the context of such mono-exponential current decay, it is important to note that during the charge collection process, charges are concurrently undergoing recombination within the cell. Consequently, the decay rate constant for the current signalis a composite of the decay rate constant for charge transport out of the cell( ) and the rate constant for recombination within the cell, expressed as Thus, basedon the previously determinedthe electron lifetimecan be calculated.

[0105] The ZrSs / ZrOS / NiOOH and ZrSVZrOS / Ni / Fc photoanodcs show a slightly higher electron transport timethan the ZrS3 / NiOOH and photoanodes, probably dueto the increased transport path (FIG. 30G and FIG. 30H). However, a significantly enhanced charge carrier lifetime is observed in the ZrS / ZrOS / Ni()()H and the ZrS / ZrOS / Ni / Fephotoanodes (FIG. 301 and FIG. 30J), which mainly accounts for its greatly improved charge carrier dynamics. The electron collection efficiency and( | ) diffusion lengthcan be further obtained based on the equations o (FIG. 35).The is thus determined to be around 10 pm for the ZrSs photoanode and around 11 pm for thephotoanode in the potential range of 0.8- 1.4 ((b) of FIG. 35), which issmaller than the nanobelt thickness. For the ZrSs / ZrOS / NiOOH photoanode,is increased to around 18 pm and for the ZrSa / ZrOS / Ni / Fe photoanode,is increased to around 20 pm, due to the significantly improved electron conductivity ((b) of FIG. 35). The longerof theand the photoanodcs achieve an efficienaround 70%and 71 %, respectively in the potential range of 0.8 -1.4while the smallerof the ZrSs / NiOOH and the ZrSs / Ni / Fc photoanodcs show less than 30 %. The incident photon-to-current efficiency (IPCE) spectra (FIG. 30K and FIG. 30L) show that the ZrSVZrOS / NiOOH and the ZrSs / ZrOS / Ni / Fe photoanodes have IPCE values around 75% and 78 %, respectively from the UV region to the visible region, and demonstrate good activityeven with the excitation extended to the near-infrared region of around 680 nm. This photoresponse beyond the bandgap is probably attributed to the presence of additional energy levels within the bandgap, resulting from the inherent defects commonly observed in many semiconductor materials. The process involves the light absorption by localized defect states, which can occur even at wavelengths that do not match the semiconductor's bandgap. Once absorbed, the photon energy can excite electrons from the defect states into the conduction band or from the valence band to the defect states, thus contributing to the generation of photocurrent. These results reveal that the ZrSs / ZrOS NBA coupled with NiOOH or NiOOH / FeOOH OECs are promising photoanodes for boosting PEC water splitting.

[0106] Finally, the durability of the ZrSs / NiOOH and ZrS VZrOS / NiOOH photoanodes and th photoanodes against PEC water oxidation was assessedthrough the analysis of J-t plots at 0.8 VRHE ((b) of FIG. 36A and (a) of FIG. 36B, respectively). An evident decrease in the photocurrents of the ZrS s / NiOOl I and th photoanodeswas observed during a 75-hour test, indicating the inherent instability of for wateroxidation. In contrast, theand th photoanodcs showgood stability over hundreds of hours.

[0107] To validate the occurrence of the oxygen evolution reaction (OER) at the photoanode and the photoanode, along with that of thehydrogen evolution reaction (HER) at the counter electrode, gas chromatography was employed to detect the evolution of H2 andgases. The ratio of evolved is closelymaintained at 2: 1 during the 24-hour J-t test at 1.23 VRHE for ZrS i / ZrOS / NiOOH and the 12- hour J-t test at 0.8 VRHE for as illustrated in (a) of FIG. 36A and (b) of FIG.36B, respectively. The faradaic efficiencies for the HER approach 100% for both photoanodes, indicating that the JH20 is substantially attributed to the HER and the OER. This suggests that the Zphotoanode and thephotoanode can sustain efficient PEC water splitting. In detail, throughout the initial 450-h measurement, there is hardly any observable photocurrent attenuation in the and thephotoanodes. A gradual decrease in photocurrents became noticeable after the initial 450 to 500 h J-t test duration and the photoanodcs display an approximately 6.7% to 7% loss in J1120 after the duration was prolonged to 650 to 700 h. The photoanodes exhibit identical XPS spectra (FIG. 37) and XRD patterns ((c) and (d) of FIG. 36B) before and after the 650 to 700 h J-t measurement, providing clear evidence of the inherent excellent stability ofNBA.

[0108] TEM results of thephotoanodc after 650 hours of J-t measurement reveal that thelayer experiences degradation, leading to a reduced thickness (around 5 nm) on the surface (FIG. 38). A further STEM-EDS profile analysisreveals a pronounced detection ofand S elements from the surface to the bulk, as shown in (h) of FIG. 38. Based on the aforementioned TEM EDS elemental analysis, it is inferred that the OEC experienced a leakage into the electrolyte, whereas thestructure maintained its stability throughout the extended J-t testing period. After reloading theon the photoanodc tested, the photocurrent density can be restored to its initial values, enabling the reloadedphotoanode to sustain stable operation for an extended period of up to 1000 hours. This suggests that the decrease in the photocurrent indeed arises from the instability of the, rather than that of theNBA. The LS V experiments were further conducted on the Z photoanode before and afterthe stability test. As shown in FIG. 39A and FIG. 39B, there is a noticeable decrease in J1120 after the J-t test of 650 and 100 hours. In contrast, the / sulfite remains unchanged before and after the stability tests, further evidencing the stability of theBesides, after 100 hours oftesting, the morphological integrity of the photoanode is preserved with no significant changes detected (FIG. 40). Importantly, the robust connection between the nanobelts array and the FTO substrate remains intact, reaffirming the structural and functional stability of the photoanode. As a result, ZrSs / ZrOS NBA are positioned among the topperforming photoanodes developed during recent state-of-the-art studies.

[0109] The photocorrsion of sulfide semiconductors stems from the instability of sulfur ions and sluggish extraction of charges, which causes an accumulation of high-energy holes within their surface. These accumulated holes coupled with the water further trigger self-oxidation and S leaching of the sulfide semiconductors. However, oxysulfide semiconductors have exhibited stable properties throughout the photooxidation process, due to the stabilization of S2-ions through the hybridization of theandorbitals. Therefore, the electronic band structure of theS was further calculated to ascertain the contribution of elements to the band structure (FIG. 41). The conduction band ofpredominantly originates from the Zr orbitals, whereas the valence band is chiefly derived from the andorbitals.Detailed analysis of partial charge density contours further unveils that the valence band maximum (VBM) results from the hybridization between theAs a result, ZrOS exhibits outstanding stability, even with the considerable hole accumulation at the VBM. Additionally, the impressive surface hole transfer efficiency of the photoanode (exceeding 95%) attenuates the hole accumulation, further solidifying its stable performance.Consequently, these results showcase a high efficiency photoanodc capable of sustained operation over extended periods of PEC water splitting.

[0110] Results and Discussion

[0111] Conventional photoanodes prioritize either achieving high photoelectric conversion efficiency or ensuring long-term stability. Very few manage to strike a balance between the two. Remarkably, the photoanode of the present disclosure excels in both high efficiency photoelectric conversion and long-term stability. The present disclosure provides a stable and efficient photoanodc based onwhich possesses a coaxial nanobclt structure through a pulsed-ozone-treatment ofnanobelts array. This approach yielded a p-type ZrOS layer coated on the inner . resulting in a buried n-p junction. As a result, thephotoanode demonstrated significantly improved charge carrier dynamics during the PEC process. With the incorporation of theor OER cocatalysts,the resultingand photoanodes realizedefficient PEC water splitting by delivering a substantial photocurrent density of 8.97mA cm'2to 9.3 mA at 1.23E and a ABPE of about 2.93% to to 3.2 % under simulated sunlight. Importantly, theused can simultaneously act as an efficient hole extraction layer and as a protective layer that prevents the photoanodcs from photocorrosion by the high-energy holes and electrolytes. The photoanode and the Zphotoanode provide efficient light harvesting across the entire visible region to over 650 nm, excellent photon-to-current efficiency, and are stable over 700 hours or over 1000 hours, respectivey. Additionally, the photoanodes enable reactions under mild and environmental- friendly conitions.

[0112] Although embodiments of the invention have been shown and described, the invention is not limited to the described embodiments. Instead, it would be appreciated by those skilled in the art that various modifications and variations can be made to the embodiments of the invention without departing from the scope of the invention, the scoop of which is set forth in the following claims.

Claims

Claims1. A photoanodc for photoclcctrochcmical water-splitting, the photoanodc comprising: a conductive substrate; an array of zirconium trisulfidenanobelts deposited on at least one surface of the conductive substrate; and a layer of zirconium oxysulfidedeposited on the array of zirconium trisulfide (ZrSs), forming a zirconium bisulfide( j / zirconium oxysulfidcnanobclts array on the conductive substrate.

2. The photoanode of claim 1 , further comprising: a layer of nickel oxide hydroxide (NiOOH) deposited on the layer of zirconium oxysulfide (ZrOS).

3. The photoanodc of claim 2, further comprising: a layer of iron oxide hydroxide (FeOOH) deposited on the layer of nickel oxide hydroxide (NiOOH), forming a photoanodc comprising the zirconium trisulfidc / zirconiumoxysulfide (ZrOS) nanobelts array with NiOOH / FeOOH as cocatalysts, denoted as4. The photoanode of any one of claims 1 to 3, wherein the array of zirconium trisulfidenanobclts is an n-typc layer; and the layer of zirconium oxysulfidc (ZrOS) is a p-typc layer, configured to form a buried n-p junction in the photoanode.

5. The photoanode of any one of claims 1 to 3, wherein the layer of zirconium oxysulfide (ZrOS) is a dense layer covering the entire surface of each of the zirconium trisulfidenanobelts in the array of zirconium trisulfide nanobelts.

6. The photoanodc of claim 5, wherein the layer of zirconium oxysulfidc (ZrOS) has a thickness ranging from 18 to 22 nanometers.

7. The photoanode of claim 1 , wherein the nanobelts within the array of zirconium trisulfide each has a thickness ranging from 50 to 100 nm.

8. The photoanodc of claim 1 or 7, wherein the nanobclts within the array of zirconium trisulfide each has a width ranging from 100 to 1000 nm.

9. The photoanode of claims 1 or 8, wherein the nanobelts within the array of zirconium trisulfide each has a length ranging between 5 and 20 pm.

10. The photoanode of any one of claims 1 to 3, wherein the photoanode is stable for over 700 or over 1000 hours in an alkaline solution.

11. The photoanode of claim 1, wherein the conductive substrate is made of material selected from fluorine-doped tin oxide or zirconium.

12. A method of forming a photoanode comprising: depositing an array of zirconium trisulfide (Z1S3) nanobelts on at least one surface of a first conductive substrate; and subjecting the array of zirconium trisulfide to ozone treatment to form a layer ofzirconium oxysulfidc (ZrOS) on the array of zirconium bisulfide (ZrSs) nanobclts, forming a zirconium trisulfide oxysulfide (ZrOS) nanobelts array on the first conductivesubstrate.

13. The method of claim 12, further comprising: depositing a layer of nickel oxide hydroxideon the layer of zirconium oxy sulfideby immersing the photoanode in an aqueous solution containing a nickel salt, to obtain a ZrSVZrOS / NiOOH photoanode comprising the zirconium trisulfideoxysulfidenanobelts array with a layer of nickel oxide hydroxide (NiOOH).

14. The method of claim 12, further comprising: depositing a layer of nickel oxide hydroxideon the layer of zirconium oxy sulfide (ZrOS) by immersing the photoanode in an aqueous solution containing a nickel salt; depositing a layer of iron oxide hydroxideon the layer of nickel oxide hydroxide (NiOOH) by immersing the photoanode in an aqueous solution containing an iron salt, to obtainphotoanode comprising the zirconium trisulfideoxysulfidc (ZrOS) nanobclts array with a layer of NiOOH and a layer of FeOOH as cocatalysts.

15. The method of claim 12, wherein the ozone treatment comprises exposing the layer of zirconium trisulfide (ZrSs) to ozone at an elevated temperature for a predetermined duration and for a predetermined number of cycles of deposition to form the layer of zirconium oxysulfide (ZrOS).

16. The method of claim 12, wherein the layer of zirconium oxysulfidis a dense layer covering the entire surface of each of the zirconium trisulfide ) nanobelts in the arrayof zirconium trisulfidenanobelts.

17. The method of claim 16, wherein the layer of zirconium oxysulfide (ZrOS) has a thickness ranging from 18 to 22 nm.

18. The method of any one of claims 12 to 16, wherein the layer of zirconium trisulfideis an n-typc layer; and the layer of zirconium oxysulfidc S) is a p-typc layer, forming aburied n-p junction in the photoanode.

19. The method of claim 12, wherein the nanobelts within the array of zirconium bisulfide each has a thickness ranging from 50 to 100 nm, a width ranging from 100 to 1000 nm, and a length ranging between 5 and 20 pm.

20. The method of claim 12, further comprising: transferring the zirconium trisulfideoxysulfidenanobelts array to a second conductive substrate, the method of transfer comprising: coating a polystyrene precursor onto the zirconium trisulfideoxysulfidenanobelts anay deposited on the first conductive substrate; removing the first conductive substrate; adding a precursor solution onto the second conductive substrate to form a seed layer; affixing the polystyrene coated zirconium trisulfide ( oxysulfide(ZrOS) nanobelts array to the seed layer on the second conductive substrate;heating the polystyrene coated zirconium trisulfidcoxysulfidc) nanobelts array and the second conductive substrate; removing the polystyrene; and subjecting the obtained zirconium trisulfidezirconium oxysulfide (ZrOS) nanobelts array with the second conductive substrate to annealing to obtain a firmly adhered zirconium trisulfideoxysulfide (ZrOS) nanobelts array on the second conductive substrate.

21. A photoelectrochemical cell comprising: a counter electrode; a photoanode; and an electrolyte, wherein the photoanode comprising: a conductive substrate; an array of zirconium trisulfidenanobelts deposited on at least one surface of the conductive substrate; and a layer of zirconium oxysulfidedeposited on the array of zirconium trisulfidc forming a zirconium trisulfidc zirconium oxysulfidc nanobcltsarray on the conductive substrate, wherein, the array of zirconium trisulfide (ZrSs) nanobelts is an n-type layer and the layer of zirconium oxysulfideis a p-type layer, forming a buried n-p junction in the photoanode.

22. The photoelectrochemical cell of claim 21, further comprising: a layer of nickel oxide hydroxide (NiOOH) deposited on the layer of zirconium oxysulfide (ZrOS).

23. The photoelectrochemical cell of claim 22, further comprising: a layer of iron oxide hydroxidedeposited on the layer of nickel oxide hydroxide (NiOOH), forming a photoanodc comprising the zirconium trisulfidcoxysulfide (ZrOS) nanobelts array withas cocatalysts, denoted as

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