Cocatalyst For Hydrogen Evolution Research Articles

Tuning Electronic and Proton Transfer Properties on Amino-Functionalized Co-Based MOF for Efficient Photocatalytic Hydrogen Evolution.

Efficient hydrogen (H2) production through photocatalytic water splitting was achieved by using an amino-functionalized azolate/cobalt-based metal-organic framework (MOF). While previous reports highlighted the amino group's role only as a substituent group for enabling light absorption of MOFs in the visible region, our present study revealed its dual role. The amino substituent not only acts as an electron donor to increase the electron availability at the active Co sites but also provides hydrogen-hopping sites within the pore channel, facilitating proton (H+) diffusion along the framework. This dual functionality significantly boosts the performance of this Co-MOF as a hydrogen evolution cocatalyst. When combined with fluorescein and triethylamine as the photosensitizer and sacrificial agent, respectively, the Co-MOF achieved a remarkable H2 production rate of 27 mmol g-1 over 4 h. Notably, this performance surpasses those of benchmark platinum (Pt) and titanium dioxide (TiO2) cocatalysts.

(Invited) Two-Dimensional Metal–Organic Framework Acts As a Hydrogen Evolution Cocatalyst for Overall Photocatalytic Water Splitting

Water-splitting semiconductor photocatalysts require hydrogen evolution reaction (HER) cocatalysts, the majority of which are robust metals and metal oxides. Metal complexes feature designability and reaction selectivity. This feature should be appreciated in the application as HER cocatalysts by suppressing unfavorable back reactions; however, low stability hampers their application. In this research, we demonstrate that a fully π-conjugated and conductive two-dimensional metal–organic framework (2D MOF), which possesses both virtues of metals/metal oxides and metal complexes, serves as an ideal HER cocatalyst candidate. NiBHT, a kind of 2D MOF, is deposited on the photocatalyst SrTiO3:Al with CoO x as an oxygen evolution cocatalyst to show long-term overall one-step water splitting with an apparent quantum efficiency (AQE) of 6.5% at 350 nm. In comparison to Pt as a representative HER cocatalyst, NiBHT turns out to be a suppressor of back reactions related to oxygen reduction reactions, which is proven both experimentally and theoretically.

NiMoP2 co-catalyst modified Cu doped ZnS for enhanced photocatalytic hydrogen evolution

The design of a low-cost, efficient and durable co-catalyst for hydrogen evolution is of great significance for improving photocatalytic activity. In this work, a NiMoP2/Cu-ZnS composite photocatalyst was prepared by in-situ growth of NiMoP2 on the surface of copper-doped zinc sulfide (Cu-ZnS). The test results showed that the hydrogen production rate of the optimized NiMoP2/Cu-ZnS under visible light irradiation was significantly improved. Among them, NiMoP2/Cu-ZnS-1 exhibited the highest photocatalytic activity (5.13 mmol·g−1·h−1), which was 2.47 times that of Cu-ZnS and maintained good stability during the reaction. The improvement of hydrogen production performance of the photocatalyst was mainly attributed to the synergistic effect of Cu-ZnS and co-catalyst NiMoP2. The ohmic contact formed between Cu-ZnS and NiMoP2 could improve the separation efficiency of photogenerated carriers and reduce the overpotential of H2 evolution. Besides, NiMoP2 was found to provide the active site for the reduction reaction, and accelerate the transfer efficiency of photogenerated electrons. This strategy opens up a new way for the construction of co-catalyst modified semiconductor materials to improve the photocatalytic hydrogen production rate.

Understanding the Activation Mechanism of RhCrOx Cocatalysts for Hydrogen Evolution with Nanoparticulate Electrodes.

Mixed oxides of Rh-Cr (RhCrOx), containing Rh3+ and Cr3+ cations, are commonly used as cocatalysts for the hydrogen evolution reaction (HER) on particulate photocatalysts. The precise physicochemical mechanisms of the HER at the catalytic sites of these oxides are not well understood. In this study, model cocatalyst electrodes, composed of nanoparticulate RhCrOx, were fabricated to investigate the physicochemical mechanisms of the HER. Electroanalytical and X-ray photoelectron spectroscopic measurements revealed that nanoparticulate RhCrOx produces reduced Rh (Rh0) species by maintaining an electrode potential more negative than 0.03 V versus the reversible hydrogen electrode (VRHE). This results in significant enhancement of the HER activity. The catalytic activity for the HER stems from the reduced Rh species, and the inclusion of Cr3+ (CrOx) aided in the electron transfer process at the solid/liquid interface, resulting in a higher current density during the HER. To achieve a solar-to-hydrogen efficiency of over 3%, the conduction band minimum of the particulate photocatalyst should be positioned more negatively than -0.10 VRHE. Moreover, the formation of electron trap states at potentials more positive than 0.03 VRHE should be avoided. This study highlights the importance of understanding the catalytic sites on metal oxide cocatalysts. Moreover, it offers a design strategy for enhancing the efficiency of photocatalytic water splitting.

Interface Regulation of Bi3TiNbO9/Rh Photocatalysts by Introducing Ultrathin Heterolayer to Enhance Overall Water Splitting

AbstractAurivillius compounds‐based photocatalysts have attracted extensive interest largely due to their ferroelectric properties and modifiable characteristics arising from the alternate stacking of structural units. However, the interfacial Schottky barrier between such semiconducting compounds as light absorbers and metallic cocatalysts for hydrogen evolution restrains the transfer of photogenerated electrons, resulting in low photocatalytic overall water splitting activity and stability. Here, an anions‐induced surface structure transformation strategy is employed to modulate the interface structure between Bi3TiNbO9 as a typical Aurivillius compound and the cocatalyst Rh by in situ growing ultrathin Bi2MoO6 heterolayer on the surface of Bi3TiNbO9 nanosheets. The introduction of Bi2MoO6 heterolayer lowers the energy barrier near the Bi3TiNbO9 surface, which can facilitate the transfer of the photogenerated electrons from bulk to surface and thus the reduction of Rh cocatalyst for highly active hydrogen production. Compared with the Bi3TiNbO9‐Rh photocatalyst, the proportion of low‐valence metallic Rh0 in Bi2MoO6 heterolayer‐modified Bi3TiNbO9‐Rh (Bi3TiNbO9‐Bi2MoO6‐Rh) is improved by 7.76%, giving rise to a photocatalytic overall water splitting activity enhancement by a factor of 4.74. This strategy emphasizes the importance of interface regulation in promoting the transfer of photogenerated charge carriers in Aurivillius‐type photocatalysts, providing an effective pathway for designing and fabricating high‐performance photocatalysts.

Defect-decorated Cu2MoS4 as a new efficient hydrogen evolution cocatalyst: Rich active sites, rapid charge transfer and optimized hydrogen adsorption

Herein, a new defect-decorated Cu2MoS4 cocatalyst was successfully synthesized using a sacrificial-template method. Detailed characterizations confirmed the presence of multivalent metal ions (Cu2+/Cu+, Mo6+/Mo4+) and abundant sulfur vacancies. To assess its cocatalytic performance, Cu2MoS4/Cd0.5Zn0.5S Schottky heterojunction was constructed and achieved a hydrogen production rate of 11.83 mmol∙g−1∙h−1, which is 6.33 and 2.03 times higher than pristine Cd0.5Zn0.5S and MoS2/Cd0.5Zn0.5S, respectively. The superior photocatalytic activity is attributed to multiple factors. Sulfur vacancies regulate the charge density of neighboring atoms and induce production of more coordination-unsaturated atoms as active sites to accelerate hydrogen generation. Meanwhile, the high conductivity of Cu2MoS4, Schottky heterojunction, and redox couples could synergistically boost charge transfer and separation efficiency. Theoretical calculations further illustrated the key role of sulfur vacancies in optimizing H2-evolution kinetics, which reduced the hydrogen adsorption free energy of in-plane S atoms. This work demonstrates that Cu2MoS4 has significant potential as an efficient cocatalyst material, enhancing the photocatalytic performance of metal sulfides.

Non-noble metal Cu modification ZnIn2S4 driving efficient production H2 under visible light

Copper species have been extensively researched as effective cocatalysts for hydrogen evolution because of their inexpensive nature when compared to Pt. In this work, we have successfully loaded Cu nanoparticles highly dispersed on the surface of ZnIn2S4 (ZIS) by an always simple low-temperature vacuum reduction method, which significantly improved the separation efficiency of photogenerated carriers. According to the transient photocurrent (TPR) and PL results, when the mass fraction of Cu is 0.6%, the maximum photocurrent density is approximately 0.55 μA/cm2, which is 2.75 times that of ZIS. And the separation efficiency of the photogenerated carriers is the fastest at this point. Simultaneously, under visible light and with a Cu loading of 0.6% in the Cu/ZnIn2S4 (Cu/ZIS) composite material, the efficiency of hydrogen production is 6.56 times greater than that of pure ZIS, ultimately achieving an optimal value of 4.66 mmol/g/h. This study provides a new method for the preparation of photocatalysts with a high efficiency of electron-hole separation.

Manipulating the Morphology and Electronic State of a Two-Dimensional Coordination Polymer as a Hydrogen Evolution Cocatalyst Enhances Photocatalytic Overall Water Splitting

Manipulating the Morphology and Electronic State of a Two-Dimensional Coordination Polymer as a Hydrogen Evolution Cocatalyst Enhances Photocatalytic Overall Water Splitting

Boosting the photocatalytic hydrogen generation: The multifunctional role of NiWO4 in NiS2/NiWO4/ZnIn2S4 hollow microsphere

As a promising metal sulfide co-catalyst, nickel disulfide (NiS2) has become extremely attractive for cost-effectiveness and efficiency during the photocatalytic activity of hydrogen evolution reaction (HER). However, the strong bonding strength of S-Hads poses a challenge in desorbing H atoms from the active S-site, hindering the photocatalytic hydrogen evolution performance of NiS2. Intrinsically, accelerating the photoproduction electron transfer and promoting the release of more hydrogen protons from active adsorption water is essential for enhancing photocatalytic hydrogen evolution. Herein, we propose an interface modulation strategy for promoting photocatalytic performance: fabricating amorphous NiWO4 nanoparticles modified on NiS2 cocatalysts to construct a NiS2/NiWO4/ZnIn2S4 hollow microsphere photocatalyst. The close contact arrangement between NiS2/NiWO4 microspheres and ZnIn2S4 nanosheets undeniably enhances charge separation of carriers. Meanwhile, the amorphous NiWO4 interlayer functions excellently in conductivity, as an “electron bridge,” for accelerating charge carrier transfer. Significantly, amorphous NiWO4 strongly interacts electronically with NiS2, reducing the local electron density of Ni and S atoms. This promotes the adsorption of water by Ni sites and optimizes of adsorption/desorption of hydrogen atoms at the adjacent S sites where S-Hads are weakened. The hierarchical ZnIn2S4/NiWO4/NiS2 photocatalyst remarkably boosts the hydrogen production rate. It was 11.919 mmol·g−1·h−1 compared to bare ZnIn2S4, and ZnIn2S4/NiS2 only showed 1.8 times increase, while ZnIn2S4/NiWO4/NiS2 increased by 12.5 times. The optimization of the active center in the study provides a beneficial strategy for designing efficient photocatalysts for water splitting with great potential for hydrogen evolution cocatalysts.

Porphyrin sensitized ZnWO4 nanosheets with CuO nanoparticles for photocatalytic hydrogen evolution

Photocatalytic hydrogen production is a promising technique for solar-to-chemical energy conversion. However, the efficiency of this method is greatly limited by the need to develop efficient photocatalysts. In this study, we rationally designed an organic–inorganic hybrid material as a photocatalyst for hydrogen production. ZnWO4 nanosheets were fabricated via a hydrothermal reaction, before decorating with CuO nanoparticles as a co-catalyst for hydrogen evolution and tetrakis(4-carboxyphenyl) porphyrin (TCPP) as a photosensitizer. The as-fabricated ZnWO4/CuO/TCPP obtained excellent photocatalytic hydrogen evolution (1.84 mmol g−1 h−1) under the optimized reaction conditions, which was about 4.9 times higher compared with that using the pristine ZnWO4. This significant enhancement was ascribed to the efficient charge separation by CuO and light absorption extension by porphyrin. Thus, we demonstrated a good example of efficient hydrogen production based on the rational design of photocatalysts.

Electronic coupling between metallic Ni and VN nanoparticles enables g-C3N4 nanosheet as an efficient photocatalyst for hydrogen evolution

Developing highly efficient hydrogen evolution cocatalysts is of great significance for semiconductor photocatalysts toward the large-scale conversion of solar light energy into clean hydrogen fuel. Herein, a novel Ni/VN hetero-nanosheets is reported for the first time, as precious metal-free co-catalyst for g-C3N4, and the electronic coupling between Ni and VN greatly promotes the photocatalytic hydrogen production activity of g-C3N4 nanosheet (CNNS) under visible-light irradiation. As expected, CNNS with 3 wt% Ni/VN (CNNS/Ni/VN) exhibits excellent photocatalytic activity with hydrogen production rates are as high as 417.1 μmol h−1g−1. In addition, CNNS/Ni/VN has excellent cyclic hydrogen production stability for 24 h. This excellent performance is attributed to the strong electron coupling between Ni and VN. Therefore, the Ni/VN cocatalysts will open a newfangled way for powerful solar-fuel production.

Accurate design of spatially separated double active site in Bi4NbO8Cl single crystal to promote Z - Scheme photocatalytic overall water splitting

The efficiency of photocatalytic overall water splitting was mainly limited by the slow reaction kinetics of water oxidation. How to design effective surface active site to overcome the slow water oxidation reaction was a major challenge. Here, we propose a strategy to accelerate surface water oxidation through the fabrication spatially separated double active sites. FeCoPi/Bi4NbO8Cl-OVs photocatalyst with spatially separated double active site was prepared by hydrogen reduction photoanode deposition method. Due to the high matching of the spatial loading positions of FeCoPi and OVs with the photogenerated charge distribution of Bi4NbO8Cl and corresponding reaction mechanisms of substrate, the FeCoPi and OVs on the (001) and (010) crystal planes of Bi4NbO8Cl photocatalyst provided surface active site for water oxidation reaction and electron shuttle reaction (Fe3+/Fe2+), respectively. Under visible light irradiation, the evolution O2 rate of FeCoPi/Bi4NbO8Cl OVs was 16.8 μmol h−1, as 32.9 times as Bi4NbO8Cl. Furthermore, a hydrogen evolution co-catalyst PtRu@Cr2O3 was prepared by sequential photodeposition method. Due to the introduction of Ru, the Schottky barrier between PbTiO3 and Pt was effectively reduced, which promoted the transfer of photogenerated electrons to PtRu@Cr2O3 thermodynamically, the evolution H2 rate on PtRu@Cr2O3/PbTiO3 increased to 664.8 times. On based of the synchronous enhancement of the water oxidation performance on FeCoPi/Bi4NbO8Cl-OVs and water reduction performance on PtRu@Cr2O3/PbTiO3, a novel Z - Scheme photocatalytic overall water splitting system (FeCoPi/Bi4NbO8Cl-OVs) mediated by Fe3+/Fe2+ had successfully constructed. Under visible light irradiation, the evolution rates of H2 and O2 were 2.5 and 1.3 μmol h−1, respectively. This work can provide some reference for the design of active site and the controllable synthesis of OVs spatial position. On the other hand, the hydrogen evolution co catalyst (PtRu@Cr2O3) and the co catalyst FeCoPi for oxygen evolution contributed to the construction of an overall water splitting system.

An epitaxial La2CuO4 thin film photocathode for water splitting under visible light

A semiconductive oxide, La2CuO4 (LCO), was investigated as a potential material to compose photocathode for sunlight-driven hydrogen evolution by splitting water. LCO, despite involving partially filled Cu 3d orbitals, behaves as a semiconductor and absorbs visible light on the bandgap formed by significant Coulomb repulsion between the electronic orbitals. An epitaxial LCO film was grown on a SrRuO3/SrTiO3 (SRO/STO) single-crystal substrate by pulsed laser deposition to obtain a photocathodic specimen for water photo-splitting. An LCO photocathode dressed with a Pt cocatalyst for hydrogen evolution (Pt/LCO/SRO/STO) exhibited a cathodic photocurrent with a density of 0.4 mA cm−2 at 0 VRHE under simulated AM1.5 G sunlight. This photocathode responded to incident light up to 800 nm, which is one of the longest wavelengths so far reported for an oxide photoelectrode. Together with a counter-electrode for oxygen evolution, the Pt/LCO/SRO/STO photocathode generated hydrogen with the expected H2 : O2 = 2 : 1 stoichiometric ratio with a Faradaic efficiency of approximately 80%.

Carbon Nitride Loaded with an Ultrafine, Monodisperse, Metallic Platinum-Cluster Cocatalyst for the Photocatalytic Hydrogen-Evolution Reaction.

For the realization of a next-generation energy society, further improvement in the activity of water-splitting photocatalysts is essential. Platinum (Pt) is predicted to be the most effective cocatalyst for hydrogen evolution from water. However, when the number of active sites is increased by decreasing the particle size, the Pt cocatalyst is easily oxidized and thereby loses its activity. In this study, a method to load ultrafine, monodisperse, metallic Pt nanoclusters (NCs) on graphitic carbon nitride is developed, which is a promising visible-light-driven photocatalyst. In this photocatalyst, a part of the surface of the Pt NCs is protected by sulfur atoms, preventing oxidation. Consequently, the hydrogen-evolution activity per loading weight of Pt cocatalyst is significantly improved, 53 times, compared with that of a Pt-cocatalyst loaded photocatalyst by the conventional method. The developed method is also effective to enhance the overall water-splitting activity of other advanced photocatalysts such as SrTiO3 and BaLa4 Ti4 O15 .

Phase-controllable NixMoyPz dual-cocatalyst regulates electron transfer for enhanced photocatalytic hydrogen evolution

The rational design and construction of low-cost nickel molybdenum phosphides (NixMoyPz) cocatalysts for hydrogen evolution have aroused wide concern in recent years. However, NixMoyPz involves so many stoichiometric compositions with various crystal phase, thus, fabricating NixMoyPz with excellent hydrogen evolution activity and revealing their intrinsic mechanism at atomic level become crucial problems. Herein, a novel phase-controllable phosphating method by reasonably introducing various amounts of PH3 deriving from red phosphorus is firstly proposed to synthesize NiMoP/ZnxCd1-xS (NMP/ZCS), NiMoP/NiMoP2/ZnxCd1-xS (NMPP2/ZCS) and NiMoP2/ZnxCd1-xS (NMP2/ZCS) photocatalysts. The experimental and theoretical results reveal that this novel NMPP2 dual-cocatalyst maximumly accelerates photoelectrons transfer into the outmost layer of NMP2 component, meanwhile, under the intense synergy among NiMoP and NiMoP2 components, NMPP2 dual-cocatalyst further promotes the dissociation of H2O and the desorption of hydrogen proton. Accordingly, the optimized NMPP2/ZCS heterojunction reveals the best photocatalytic hydrogen evolution (PHE) efficiency of 83.54 mmol h−1 g−1 without an evident decrease after 6 cycles of photocatalytic tests in the Na2S-Na2SO3 solution, which is 25.55, 1.6 and 1.24 times higher than these of Pt/ZCS, NMP/ZCS and NMP2/ZCS, respectively. This tactic hews out a new way for optimizing photocatalysis by the construction of dual-cocatalyst to enhance the activity of PHE.

Pt@Ni2P/C3N4 for charge acceleration to promote hydrogen evolution from ammonia-borane

Incorporating g-C3N4 with transition metal phosphides is emerging as a low-cost and robust co-catalyst for hydrogen evolution. The ammonia borane hydrolysis is an efficient method to release H2 at ambient conditions in the presence of a catalyst. An efficient and cheap catalyst is needed for practical application to achieve this benchmark. For this purpose, a catalyst Ni2P/C3N4 is synthesized by hydrothermal method and low-temperature phosphidation. The optimization reveals that the Ni2P/C3N4 with 6.5% Ni contents shows the best performance for H2 release. Furthermore, 2% Pt nanoparticles loading over Ni2P/C3N4 boosts the charge transfer and improves activity 5.7-fold compared to Ni2P/C3N4, and the Pt-loaded catalyst is depicted as Pt@Ni2P/C3N4. The reaction kinetics reveals that the hydrogen evolution rate accelerates by increasing the amount of Pt@Ni2P/C3N4 and AB concentration, and the loading of Pt nanoparticles loaded over Ni2P/C3N4 reduces the activation energy significantly. Moreover, the ionic interaction between Pt and Ni2P/C3N4 generates Ptᵟ+ and (Ni2P/C3N4)ᵟ− active sites which facilitates B–H cleavage and O–H bonds of ammonia borane and water, respectively. Incorporating transition metals phosphide and noble metals supported over g-C3N4 paves the pathway toward the efficient H2 evolution from ammonia borane, bringing cost-effective modifications to synthesize constructive catalysts.

Multiple roles of 2D conductive metal-organic framework enable noble metal-free photocatalytic hydrogen evolution

Developing advanced catalysts with high electron-hole separation efficiency and sufficient active centers for redox reaction is vital for photocatalytic hydrogen evolution. Herein, a synergetic system constructed by 2D conductive Ni3(HITP)2 MOF and TiO2 was elaborated to address this challenge for the first time. Photoelectrochemical experiments verify that Ni3(HITP)2 can greatly facilitate the charge separation and charge transfer efficiency of the catalytic heterostructure of Ni3(HITP)2/TiO2. Meanwhile, photocatalytic hydrogen evolution reaction, UPS tests and EPR measurement indicate that Ni3(HITP)2 can serve as co-catalyst which can provide reactive sites for hydrogen reduction. Benefiting from the multiple roles of Ni3(HITP)2, the photosensitive inorganic semiconductor and conductive MOF synergistically realize noble-metal free photocatalysis, rendering>7 times improved hydrogen evolution performance than pristine TiO2. This work gives insight into the mechanism of electroconductive MOF as cocatalyst for hydrogen evolution, which will further inspire the design of advanced electroconductive MOF applied for photocatalysis.

Water splitting with silicon p-i-n superlattices suspended in solution.

Photoelectrochemical (PEC) water splitting to produce hydrogen fuel was first reported 50 years ago1, yet artificial photosynthesis has not become a widespread technology. Although planar Si solar cells have become a ubiquitous electrical energy source economically competitive with fossil fuels, analogous PEC devices have not been realized, and standard Si p-type/n-type (p-n) junctions cannot be used for water splitting because the bandgap precludes the generation of the needed photovoltage. An alternative paradigm, the particle suspension reactor (PSR), forgoes the rigid design in favour of individual PEC particles suspended in solution, a potentially low-cost option compared with planar systems2,3. Here we report Si-based PSRs by synthesizing high-photovoltage multijunction Si nanowires (SiNWs) that are co-functionalized to catalytically split water. By encoding a p-type-intrinsic-n-type (p-i-n) superlattice within single SiNWs, tunable photovoltages exceeding 10 V were observed under 1 sun illumination. Spatioselective photoelectrodeposition of oxygen and hydrogen evolution co-catalysts enabled water splitting at infrared wavelengths up to approximately 1,050 nm, with the efficiency and spectral dependence of hydrogen generation dictated by thephotonic characteristics of the sub-wavelength-diameter SiNWs. Although initial energy conversion efficiencies are low, multijunction SiNWs bring the photonic advantages of a tunable, mesoscale geometry and the material advantages of Si-including the small bandgap and economies of scale-to the PSR design, providing a new approach for water-splitting reactors.

Ultrathin Pd metallenes as novel co-catalysts for efficient photocatalytic hydrogen production

Photocatalytic hydrogen production as a green and pollution-free technology plays an important role in alleviating the fossil fuel crisis, but remains a great challenge. Herein, an ultrathin two-dimensional (2D) palladium metallene (Pd-ene) is first developed as hydrogen evolution co-catalysts for g-C3N4 nanosheets. The Pd-ene and Pd-ene/g-C3N4 photocatalysts were fabricated by a hydrothermal method and deposition method using acetylacetonate palladium as Pd source, and the Pd-ene with a size of 100 nm uniformly distributed on the surface of g-C3N4 nanosheets forming a 2D/2D structure. The Pd-ene/g-C3N4 with 2 wt% Pd-ene showed the highest photocatalytic hydrogen production rate with 31,292 μmol/h/ g, which was>98 times that of the pure g-C3N4 and superior to the PdNC/g-C3N4 loading with Pd nanocluster. The unique 2D/2D structure of the Pd-ene/g-C3N4 catalyst, which promotes the transfer and separation of photogenerated charge carriers, is credited with the improved photocatalytic performance. In additions, the 2D structure of Pd-ene makes it expose more reaction sites for the photocatalytic reduction reaction, promoting the hydrogen production of the photocatalysts. This study demonstrates that the development of structurally matched 2D metallene co-catalysts for 2D photocatalysts is a promising strategy to obtain the photocatalysts with high hydrogen production performance.

ALD-Deposited NiO Approaches the Performance of Platinum as a Hydrogen Evolution Cocatalyst on Carbon Nitride

The photocatalytic hydrogen evolution reaction is one of the most promising approaches to utilize solar energy and produce hydrogen as an alternative energy source to traditional fossil fuels. In this work, we focused on further exploring the cocatalyst potential of the transition metal nickel and prepared nickel oxide/carbon nitride (NiO/CNx) heterojunctions through thermal polymerization as well as plasma-enhanced atomic layer deposition (PEALD). The optimal NiO loading thickness on the surface of the CNx was found to be 6.5 nm, prepared from 65 cycles of ALD. This NiO(65)/CNx had a photocatalytic hydrogen evolution rate of 3.9 μmol h–1 in a 2 mg mL–1 aqueous photocatalyst solution (195 μmol h–1 g–1), corresponding to an AQY of 3.1% and exhibiting 54% of the activity of 3 wt % Pt/CNx under 405 nm light illumination with a sacrificial reagent. A 3 wt % Ni/CNx prepared through a photodeposition process from a dispersion of CNx and simple NiCl2 had a photocatalytic hydrogen evolution rate of 2.8 μmol h–1 (140 μmol h g–1) and an AQY of 2.2% with 2 mg mL–1 photocatalyst in the reactant solution, corresponding to 39% of the activity of 3 wt % Pt/CNx. An induction period at the beginning of the hydrogen evolution process was found in all Ni-based photocatalysts, which we related to a Ni in situ photoreduction from NiII to Ni0 and the reverse reaction. In addition, the reproducibility of the induction period even in an oxygen-free environment suggests that electrons flow back from reduced Ni species to CNx under dark conditions. Furthermore, a large population of trap states was detected in the bulk CNx materials via photoinduced absorption spectroscopy, and the trapping severely hinders cocatalysts (even the benchmark Pt cocatalyst) from extracting charges from CNx for subsequent interfacial redox reactions. We conclude that not only does the surface/interface engineering of the photocatalyst/catalyst heterojunctions need to be considered but also trap states in CNx that severely limit the transfer of photogenerated charges to cocatalysts.