Co-catalyst For Hydrogen Evolution Reaction Research Articles

Polyaniline encapsulated Ti-MOF/CoS for efficient photocatalytic hydrogen evolution

We report the study of conductive polyaniline (PANI) chain embedded Ti-MOF functionalized with CoS as a cocatalyst for hydrogen evolution reaction (HER) application. The post synthetically modified hybrid photocatalyst PANI/Ti-MOF/CoS greatly influences the redox and e− − h+ separation process and exhibits an impressive rate of HER (∼1322 μmol h−1g−1), suppressing the pristine Ti-MOF (∼62 μmol h−1g−1) with apparent quantum yield (AQY) of ∼3.2 and transient current response of ∼46.4 μA cm−2. In this system, Ti-MOF provides the circulation of Ti3+ and Ti4+ to the reaction of photocatalytic H2 generation, where the additional PANI and CoS amended the performance of H2 production through electron enrichment and thereby improving the stability and integrity of Ti-MOF. The Electrochemical studies demonstrated increased photocurrent by interweaving Ti-MOF crystal with PANI through cation-π interaction thereby enhancing interface connection and then promoting electron transfers. The charge dynamics revealed the initial charge transfer from photoexcited PANI to encapsulated MOF framework to boost the photocatalytic performance of the system. Further, the electron movement at the Ti-MOF/CoS interface is investigated through work function and electrochemical potential of electrons (Fermi level). DFT results demonstrate the importance of CoS in improving the photocatalytic performance of hybrid Ti-MOF catalyst, which leads to superior catalytic behaviour. These results establish that the encapsulation of catalytic active sites inside MOFs with desirable energy band gaps would be an ideal choice for the production of solar fuels.

Ag-Au Core-Shell Triangular Nanoprisms for Improving p-g-C3N4 Photocatalytic Hydrogen Production.

Ag–Au core–shell triangular nanoprisms (Ag@Au TNPs) have aroused extensive research interest in the field of hydrogen evolution reaction (HER) due to their strong plasmon effect and stability. Here, Ag@Au TNPs were fabricated by the galvanic-free replacement method. Then, we loaded them on protonated g-C3N4 nanoprisms (P–CN) by the electrostatic self-assembly method as an efficient plasmonic photocatalyst for HER. The hydrogen production rate of Ag@Au TNPs/P–CN (4.52 mmol/g/h) is 4.1 times higher than that of P–CN (1.11 mmol/g/h) under simulated sunlight irradiation, making it the most competitive material for water splitting. The formed Schottky junction helps to trap the hot electrons generated from Ag@Au TNPs, and the well-preserved tips of the Ag@Au TNPs can effectively generate an electromagnetic field to inhibit the photogenerated electron–holes pairs recombination. This study suggests that the rational design of Ag@Au TNPs by the galvanic-free replacement method is an effective co-catalyst for HER and boosting the additional combination of plasmonic metals and catalyst metals for the enhancement to HER.

Towards Highly Efficient Chalcopyrite Photocathodes for Water Splitting: The Use of Cocatalysts beyond Pt.

Solar radiation is a renewable and clean energy source used in photoelectrochemical cells (PEC) to produce hydrogen gas as a powerful alternative to carbon-based fuels. Semiconductors play a vital role in this approach, absorbing the incident solar photons and converting them into electrons and holes. The hydrogen evolution reaction (HER) occurs in the interface of the p-type semiconductor that works as a photocathode in the PEC. Cu-chalcopyrites such as Cu(In, Ga)(Se,S)2 (CIGS) and CuIn(Se,S)2 (CIS) present excellent semiconductor characteristics for this purpose, but drawbacks as charge recombination, deficient chemical stability, and slow charge transfer kinetics, demanding improvements like the use of n-type buffer layer, a protective layer, and a cocatalyst material. Concerning the last one, platinum (Pt) is the most efficient and stable material, but the high price due to its scarcity imposes the search for inexpensive and abundant alternative cocatalyst. The present Minireview highlighted the use of metal alloys, transition metal chalcogenides, and inorganic carbon-based nanostructures as efficient alternative cocatalysts for HER in PEC.

In-situ mediation of graphitic carbon film-encapsulated tungsten carbide for enhancing hydrogen evolution performance and stability

Tungsten carbide (WC) particles-embedded carbon nanomaterials with ultrathin edges and high degree of graphitization are very favorable co-catalysts for Pt in electrochemical fields. Herein, WC-embedded graphite carbon films (WC@GCFs) were successfully fabricated as co-catalysts for hydrogen evolution reaction (HER) via an in-situ carbonization technique. The mass specific activity of the WC@GCFs loading Pt NPs is 278 mA mg−1 Pt and 277 mA mg−1 Pt under the overpotential of 48 and 52 mV, which is 1.3 and 1.45 times higher than those of the commercial Pt-C (212.8 mA mg−1 Pt and 191.4 mA mg−1 Pt). The WC@GCFs-Pt displays excellent high stability for HER in acidic and alkaline media even after 5000 potential cycles. As expected, the GCFs originated from the surface of WC can effectively prevent WC particles from agglomeration, which contributes to high catalytic activity and stability of WC. The in-situ formed GCFs possesses ultrathin edges (~5 carbon layers), high degree of graphitization, and some oxygen-containing function groups. Charges are easily transferred from Pt NPs to WC via the linked-GCFs, which contributes to the high HER activity of WC@GCFs-Pt. We believe this work can provide a more economical and efficient route for large-scale synthesis of graphite carbon films coated transition metal carbide as co-catalysts in electrochemical fields.

Multifunctional Active-Center-Transferable Platinum/Lithium Cobalt Oxide Heterostructured Electrocatalysts towards Superior Water Splitting.

Designing cost-effective and efficient electrocatalysts plays a pivotal role in advancing the development of electrochemical water splitting for hydrogen generation. Herein, multifunctional active-center-transferable heterostructured electrocatalysts, platinum/lithium cobalt oxide (Pt/LiCoO2 ) composites with Pt nanoparticles (Pt NPs) anchored on LiCoO2 nanosheets, are designed towards highly efficient water splitting. In this electrocatalyst system, the active center can be alternatively switched between Pt species and LiCoO2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Specifically, Pt species are the active centers and LiCoO2 acts as the co-catalyst for HER, whereas the active center transfers to LiCoO2 and Pt turns into the co-catalyst for OER. The unique architecture of Pt/LiCoO2 heterostructure provides abundant interfaces with favorable electronic structure and coordination environment towards optimal adsorption behavior of reaction intermediates. The 30 % Pt/LiCoO2 heterostructured electrocatalyst delivers low overpotentials of 61 and 285 mV to achieve 10 mA cm-2 for HER and OER in alkaline medium, respectively.

Cobalt (II)-based open‐framework systems constructed on g-C3N4 for extraordinary enhancing photocatalytic hydrogen evolution

Developing a low-cost co-catalyst for hydrogen evolution reaction (HER) on a photocatalyst is a great challenge. An open-framework material, cobalt ethylenediammonium bis(sulfate) tetrahydrate, was initially used as an efficient co-catalyst to improve photocatalytic HER of g-C3N4. The unique open‐framework cobalt(II) sulfate (Co-S) lead to twice higher H2 evolution performance (10328.9 μmol g−1 h-1) compared to noble metal (Pt)-decorated g-C3N4 photocatalyst under simulated sunlight. The open network structure provides abundant active sites and excellent electron transport properties, properties and then promotes electron-hole separation, which play a synergistic role with g-C3N4 in the photocatalytic process. The loading of Co-S significantly changes the microenvironment of the catalyst surface of g-C3N4 and enhances its hydrophilicity, which benefits the release of the produced gas on the catalyst surface. The formation of the Co(Ⅰ) species in the open-framework material by the electron reduction of Co(Ⅱ) is proposed as a vital role in the extraordinary HER process.

Surface Modification of Rutile TiO2 with Metal Chalcogenides for Hydrogen Evolution

The hydrogen evolution reaction (HER) is a key reaction in water splitting and there is an intensive search for suitable HER catalysts that will promote this reaction and also show good stability, while not using precious metals. We have studied the surface modification of rutile TiO2 with metal chalcogenide nanoclusters for the promotion of the hydrogen evolution reaction (HER) using density functional theory corrected for on-site Coulomb interactions (DFT+U).Recently, metal chalcogenides have emerged as potential catalysts for the HER due to the favourable interaction of H at chalcogen sites, particularly those that are undercoordinated. In developing materials for HER the key descriptor has been the adsorption energy of hydrogen at the catalysts, with the target free energy being close to 0 eV. Analogous to noble metal loading, surface modification of TiO2 with nanoclusters of potential HER co-catalysts, based on earth-abundant materials, is an emerging strategy. This approach combines the desirable properties of a stable titania photocatalyst with active sites provided by the low-coordinated chalcogen ions of the supported clusters.Our models consist of M4X4 (M = Sn, Zn; X = S, Se) nanoclusters at the rutile (110) surface and we examine the Gibb’s free energy of H adsorption at the modified surface which is a useful material descriptor in assessing the performance of a HER catalyst. These calculations take into account changes in the entropy and the zero-point energies (ZPEs) of the reactants and products, which our results show are material specific. As expected, the interaction of H is more favourable at the titania surface and forms stable hydroxyls. However, after subsequent adsorption events the surface sites become saturated and H adsorption at chalcogen sites exhibit Gibb’s free energies in the active range, particularly for MS modifiers relative to MSe.

Mixed cobalt-nitrides CoxN and Ta2N bifunction-modified Ta3N5 nanosheets for enhanced photocatalytic water-splitting into hydrogen

A novel mixed cobalt nitrides CoxN (x = 1, 2, and 5.47) and Ta2N bifunction-modified Ta3N5 nanosheet photocatalyst was successfully synthesized through a high-temperature ammonolysis of Co2+/Co3+-adsorbed Ta3N5@Ta2O5 nanoparticles. Effects of Co2+ or/and Co3+ modifying on the structural, electronic, optical, and photoelectrochemical performances of Ta3N5 were investigated systematically. Co-modifying extended the absorption over the whole visible region due to the sub-band gap optical absorption from the increased heterojunction interfaces of CoxN/Ta3N5 or/and Ta2N/Ta3N5. Co2+/Co3+ dual-modifying inhibited the crystallite growth of Ta3N5. Uniformly dispersed Co elements on the Ta3N5 surface provided the more surface active sites. Co3+ played a key role in controlling the modifying amounts of both Ta2N and Co5.47N. Co2+/Co3+ dual-modifying produced a synergetic effect on enhancing the charge carrier separation, decreasing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) overpotentials of Ta3N5, and then efficiently increasing the photocatalytic H2-evolution activity. 1.5%Co2+/1.5%Co3+-TN possessed an excellent photocatalytic H2-evolution activity (75.69 μmol g−1 h−1) under visible light irradiation, 3.48 times higher than Ta3N5@Ta2O5 nanoparticles. Co5.47N and Ta2N as HER cocatalysts, and CoN and Co2N as OER cocatalysts, decreased the HER and OER overpotentials of Ta3N5, accelerating the kinetics of surface catalytic reaction. A synergetic mechanism for enhanced photocatalytic H2-evolution performance was discussed in detail. Moreover, self-oxidation of Ta3N5 can be suppressed by surface-modified cobalt nitrides and Ta2N. This work provides a promising photocatalyst and a new strategy for develop efficient and stable Ta3N5-based photocatalysts for solar energy conversion.

RETRACTED: Tin diselenide nanoflakes decorated hierarchical 1D TiO2 fiber: A robust and highly efficient co-catalyst for hydrogen evolution reaction

Abstract Transitional metal dichalcogenide two-dimensional materials have revealed many astonishing properties including as a substitution of noble metals as a co-catalyst for hydrogen evolution reaction. Tin diselenide (SnSe2) is the one who also received substantial consideration in many fields due to low cost, earth-abundant and environment-friendly. However, the great challenge to make heterojunction with other semiconductor material to improve its efficiency for photocatalytic water splitting. For this purpose, we have prepared a 1D nanofiber of TiO2 by electrospinning and produced a needle-like structure on the fiber by following the alkali hydrothermal method. The chemical vapor deposition method (CVD) was adopted to load nanoflakes of SnSe2 on the branched fiber of TiO2 and make a strong heterojunction. The composite interpreted excellent photocatalytic performance by producing hydrogen about 0.95 mmol g−1 h−1 in comparison with branched TiO2 (0.47 mmol g−1 h−1) and conventional TiO2 fiber (0.35 mmol g−1 h−1). The photoluminescence, time decay fluorescent spectra and photoelectrochemical results ratified that SnSe2 not only reduces the charge recombination by increasing the transfer of electron but also provides the active site for hydrogen production as a cocatalyst. This study presents an inexpensive and environmentally friendly photocatalyst for hydrogen production without the use of noble metals.

Tin diselinide a stable co-catalyst coupled with branched TiO2 fiber and g-C3N4 quantum dots for photocatalytic hydrogen evolution

Transitional metal dichalcogenide two-dimensional materials have revealed many astonishing properties including the substitution of noble metals as a co-catalyst for hydrogen evolution reaction. Tin diselenide (SnSe2) is the one who also received substantial consideration in many fields due to low cost, earth-abundant and environment-friendly. However, the great challenge to make heterojunction with other semiconductor material to improve its efficiency for photocatalytic water splitting. For this purpose, we have prepared a 1D nanofiber of TiO2 by electrospinning and produced a needle-like structure on the fiber by following the alkali hydrothermal method. The chemical vapor deposition method (CVD) was adopted, first to load g-C3N4 QDs then nanoflakes of SnSe2 on the branched fiber of TiO2 and make a strong heterojunction. The composite interpreted excellent photocatalytic performance by producing hydrogen about 2375 μmol. g−1.h-1 having a quantum efficiency of HER more than 16 % at 420 nm. The photoluminescence, time decay fluorescent spectra and photoelectrochemical results ratified that SnSe2 not only reduces the charge recombination by increasing the transfer of electron but also provides an active site for hydrogen production as a cocatalyst. This study presents an inexpensive and environmentally friendly cocatalyst for hydrogen production as a substitution of noble metals.

Hierarchical microsphere of MoNi porous nanosheets as electrocatalyst and cocatalyst for hydrogen evolution reaction

In industrial water electrolysis cell, the electrocatalyst has a sluggish water dissociation kinetics leading to low electrocatalytic activity for the generation of H2 from water splitting. This work reports a hierarchical microsphere constructed by MoNi porous nanosheets derived from NiMoO4 microsphere, as a rapid Tafel-step-decided electrocatalyst towards hydrogen evolution reaction (HER). Consequently, the synthesized MoNi hierarchical microsphere (MoNi HM) electrocatalyst exhibits an onset potential of as small as −7 mV vs. RHE, an operating potential of 72 mV for 10 mA cm−2, and a low Tafel slope of 36.6 mV per decade in 1.0 M KOH. Furthermore, MoNi HM as cocatalyst and CdS nanowires as photocatalyst are physically mixed (MoNi HM/CdS NWs) and presentes a champion photocatalytic performance for H2 production with a prominent H2 generation rate of as high as 151.7 μmol mg-1 h−1 at λ > 420 nm light illumination. The large contact area between MoNi nanosheets and CdS nanowires endows the fast separation of photogenerated charge carriers to significantly facilitate the photocatalytic H2 production.

One-step synthesis of nickel oxide/nickel carbide/graphene composite for efficient dye-sensitized photocatalytic H2 evolution

To resolve the energy crisis and environment pollution, it is urgent to explore highly efficient and inexpensive catalysts for hydrogen evolution reaction (HER). In this work, we fabricate nickel oxide/nickel carbide/graphene (labeled as NNG) composite via one-step thermal treatment of NiCl2/graphene oxide (GO) as the precursor at relatively low temperature. GO plays a role of the carbon source to form nickel carbide and increases the distribution of the nickel species. The formed nickel carbide acts as the active sites for hydrogen atoms adsorption (Hads). The NiO contacted with the nickel carbide promotes the Volmer reaction: H2O+e→Hads+OH-. The two different adsorption sites (nickel carbide for Hads, NiO for OH−) jointly lead to the HER synergistic effect in basic solution. The obtained composites exhibit much higher dye-sensitized photocatalytic and electrocatalytic HER activities than nickel carbide, NiO and RGO alone. The highest apparent quantum yield (AQY) of dye-sensitized hydrogen evolution for NNG12 at 420 nm reaches 39.4%. And the electrocatalytic HER overpotential is -0.28 V at the current density of 10 mA cm-2 in 1.0 mol L-1 KOH. To the best of our knowledge, it is the first time to report nickel oxide/nickel carbide as an efficient HER catalyst in alkaline media. This work paves a way for facile and green fabrication of the efficient HER cocatalysts.

Template synthesis of gold nanoparticles from hyperstar polymers and exploration of their catalytic function for hydrogen evolution reaction

In this report, we designed a water-soluble hyperstar polymer that contained hyperbranched polytriazole as the core and dangling linear poly(ethylene oxide)s (PEO) as the arms for use as template to synthesize gold nanoparticle. The hyperbranched polymer that was synthesized from the chain-growth copper-catalyzed azide-alkyne cycloaddition polymerization of a tetrafunctional AB3 monomer showed low-dispersity structure, tunable sizes, and consequently produced hyperstar polymers with high density of PEO dangling arms at periphery. Loading HAuCl4 precursor at varied molar ratios to the triazole units in the core directly changed the size of Au nanoparticles after reduction by NaBH4, exhibiting different intensities in X-ray diffraction spectroscopy. The obtained Au nanoparticles were further investigated as co-catalysts for hydrogen evolution reaction when hybridized with molybdenum disulfide (MoS2) nanosheets, showing enhanced electrocatalytic activity.

In Situ Photoreduction Synthesis of Fe(0)/Melamine Core–Shell Submicrocubes for Efficient Photocatalytic H2 Evolution

Exploring low-cost and noble-metal-free HER (hydrogen evolution reaction) cocatalysts, such as Fe, Co, or Ni for H2 evolution, has drawn great attention in heterogeneous photocatalysis. Herein, for the first time, we reported a novel, facile approach to synthesize a Fe(0)/melamine composite HER cocatalyst through in situ photoreduction of Fe(II)-melamine complex at room temperature in the presence of Eosin Y (EY) and trimethylamine (TMA). The Fe(0)/melamine composite displays core–shell submicrocubes, which shows superior photocatalytic H2 evolution performance and good stability. The melamine modified on Fe(0) surface not only keeps Fe from being oxidized, but also increases the electron transfer ability from EY to metal Fe(0) and decreases the HER overpotential, promoting efficient photocatalytic hydrogen evolution under visible light irradiation. This work provides a new strategy to obtain highly efficient stable photocatalyst via a facile in situ photoreduction approach by using low-cost earth-abundan...

1D metallic MoO2-C as co-catalyst on 2D g-C3N4 semiconductor to promote photocatlaytic hydrogen production

The surface carrier density and hydrogen activation ability are crucial in photocatalytic hydrogen evolution reaction (HER), which can be improved by introducing noble-metal co-catalyst such as Pt into the system. However, the noble-metals suffer from high cost in terms of upscaling, thereby the exploration of low-cost HER co-catalysts is highly demanded. In this work, we report that one-dimensional (1D) metallic MoO2-C nanowire clusters can serve as a co-catalyst in assisting the photocatalytic hydrogen production over 2D g-C3N4. The multiple merits of MoO2-C are demonstrated: (i) noble-metal-free; (ii) highly effective transfer ability of the photogenerated electrons; (iii) high-density adsorption and active sites for hydrogen evolution. As demonstrated in photocatalytic hydrogen production, MoO2-C/2D g-C3N4 exhibits significant enhancement in comparison with pure 2D g-C3N4 semiconductor. The optimal hydrogen evolution rate achieving 1071.0 μmol g−1 h−1, almost 157.5 times higher than that of pure 2D g-C3N4.

Enhanced Solar Fuel H2 Generation over g-C3N4 Nanosheet Photocatalysts by the Synergetic Effect of Noble Metal-Free Co2P Cocatalyst and the Environmental Phosphorylation Strategy

Highly active and durable earth-abundant cocatalysts and photocatalysts are under increasing demand in developing practical hydrogen-evolution reaction (HER) systems. Especially, exploiting noble-metal-free Pt-like HER cocatalysts still remains a significant challenge. In this study, the synergistic effect of metal–organic framework (MOF) derived earth-abundant Co2P cocatalysts and the robust bioinspired environmental phosphorylation strategy in boosting photocatalyic H2 generation over the graphitic carbon nitride (g-C3N4) nanosheets was thoroughly investigated and revealed. The maximum H2-evolution rate of the ternary g-C3N4-Co2P-K2HPO4 photocatalytic systems could reach 27.81 μmol h–1, which was approximately 556 times higher than that of pure g-C3N4 nanosheets. The loaded earth-abundant Co2P nanoparticles with a good electrical conductivity could not only improve visible-light absorption and decrease the recombination of the electron–hole pairs, but also mainly serve as an efficient cocatalyst to lowe...

Electrochemically deposited Cu2O cubic particles on boron doped diamond substrate as efficient photocathode for solar hydrogen generation

Herein, we report a novel photocathode for the water splitting reaction. The electrochemical deposition of Cu2O particles on boron doped diamond (BDD) electrodes and the subsequent decoration with NiO nanoparticles by a dip coating method to act as co-catalyst for hydrogen evolution reaction is described. The morphology analysis by scanning electron microscope (SEM) revealed that Cu2O particles are cubic and decorated sporadically with NiO nanoparticles. X-ray photoelectron spectroscopy (XPS) confirmed the electronic interaction at the interface between Cu2O and NiO through a binding energy shift of the main Cu 2p peak. The photoelectrochemical (PEC) performance of NiO-Cu2O/BDD showed a much higher current density (−0.33mA/cm2) and photoconversion efficiency (0.28%) compared to the unmodified Cu2O/BDD electrode, which are only −0.12mA/cm2 and 0.06%, respectively. The enhancement in PEC performance is attributable to the synergy of NiO as an electron conduction mediator leading to the enhanced charge separation and transfer to the reaction interface for hydrogen evolution as evidenced by electrochemical impedance spectroscopy (EIS) and charge carrier density calculation. Stability tests showed that the NiO nanoparticles loading content on Cu2O surface is a crucial parameter in this regard.

Highly efficient hydrogen evolution over Co(OH)2 nanoparticles modified g-C3N4 co-sensitized by Eosin Y and Rose Bengal under Visible Light Irradiation

In this work, non-noble metal cobalt hydroxide nanoparticles [Co(OH)2 NPs] implanted uniformly on graphitic carbon nitride (g-C3N4) as a co-catalyst for hydrogen evolution reaction (HER) by in-situ chemical deposition method were reported. After co-sensitized by Eosin Y (EY) and Rose Bengal (RB) dyes, this photocatalyst exhibited a comparatively low onset potential (0.26V) for HER and high photocatalytic HER activity under visible light irradiation. The g-C3N4 not only provided a large area and nanoporous structure for the confined growth of Co(OH)2 NPs, but also greatly facilitated EY and RB (ER) molecules assembling on its surface, which promoted the charge transfer from dye to co-catalyst. The PL spectra and lifetime test showed the strong interaction between Co(OH)2 NPs and g-C3N4 could enhance the transferr rate of the photogenerated electron to reduce the carries recombination. About 431.9μmol of hydrogen generated over ER co-sensitized Co(OH)2/C3N4 in 3h. The apparent quantum efficiencies (AQE) of 29.6 and 27.3% were achieved at 520nm and 550nm over Co(OH)2/C3N4, respectively. In addition, this co-sensitized photocatalyst showed satisfied stability for HER, no remarkable decay of activity was observed in 900min reaction. These results imply that Co(OH)2 is a stable and efficient cheaper co-catalyst for photo-HER.

Tunable Photodeposition of MoS2 onto a Composite of Reduced Graphene Oxide and CdS for Synergic Photocatalytic Hydrogen Generation

Recently, MoS2 as an excellent cocatalyst for hydrogen evolution reaction (HER) has attracted extensive attention. In this work, MoS2 was controllably loaded on the composite of reduced graphene oxide (rGO) and CdS (rGO/CdS) by a facile photoreduction method at different pHs. At low pH 7, MoS2 deposits on the surface of the CdS particles of the composite. However, at high pH 11, it loads on the exposed rGO. When MoS2 is on the rGO, the transfer of the photoexcited electron from CdS to rGO is compatible with the HER at MoS2 (synergic effect), whereas the transfer is incompatible with the HER when it is on the CdS (antisynergic effect). Moreover, the MoS2 deposited on the CdS decreases the photoabsorption and photoactivity of CdS, and the effect is avoided when MoS2 is on the rGO. The photocatalytic HER rate under the synergic condition is 4.3 times as high as that under antisynergic condition. This work would open a promising way to design and fabricate the efficient composite photocatalysts.