Stability and Performance of Sulfide-, Nitride-, and Phosphide-Based Electrodes for Photocatalytic Solar Water Splitting

Emergence of Z-Scheme Photocatalysis for Total Water Splitting: An Improvised Route to High Efficiency

Photocatalytic water splitting to spontaneously produce H2 and O2 is a long-standing goal in solar energy conversion, presenting a significant challenge without using sacrificial electron donors or external biases. Inspired by natural photosynthesis, the design of artificial Z-scheme photocatalytic systems is at the forefront of this field. These systems achieve higher redox potential by separating photogenerated electrons and holes through a fast interlayer recombination process between valence and conduction band edges. Z-scheme photocatalysis involves using two different semiconductors with distinct bandgap energies. Here, we explore potential systems based on two-dimensional (2D) heterostructures composed of carbon, nitrogen, or similar main group elements. The advantages and disadvantages of these systems are discussed, with a focus on enhancing their efficiency through strategic design. Special emphasis is placed on the dynamics of excited charge carrier transfer and recombination processes, which are crucial for developing efficient photocatalytic systems for overall water splitting.

Stability and degradation of (oxy)nitride photocatalysts for solar water splitting

Advancing towards alternative technologies for the sustainable production of hydrogen is a necessity for the successful integration of this potentially green fuel in the future. Photocatalytic and photoelectrochemical water splitting are promising concepts in this context. Over the past decades, researchers have successfully explored several materials classes, such as oxides, nitrides, and oxynitrides, in their quest for suitable photocatalysts with a focus on reaching higher efficiencies. However, to pave the way towards practicability, understanding degradation processes and reaching stability is essential, a domain where research has been scarcer. This perspective aims at providing an overview on recent progress concerning stability and degradation with a focus on (oxy)nitride photocatalysts and at providing insights into the opportunities and challenges coming along with the investigation of degradation processes and the attempts to improve the stability of photocatalysts.

Enhancement of Photocatalytic and Photoelectrochemical Performance of ZnO by Mg Doping: Experimental and Density Functional Theory Insights

Doped ZnO nanostructures have shown great potential for solar energy applications. Considering the compatible ionic radius, Mg atoms can be doped into ZnO at different concentrations. The current work reports a combined experimental and density functional theory study on the influence of the Mg dopant concentration on ZnO performance simultaneously for photocatalytic dye removal and photoelectrochemical water splitting. Among all the samples, Mg(3)-ZnO (3 at. % Mg) exhibits superior sunlight-driven photocatalytic performance. The optimal Mg-ZnO shows an 8-fold increase in the photocatalytic activity compared to the pristine ZnO. Likewise, the most active photocatalyst shows high photoelectrochemical performance with a photocurrent response of 1.54 mA at the lowest onset potential, 11 times higher than the pristine ZnO. Tuning of the Mg content results in the generation of extra charge carriers and a reduced recombination rate, which are the crucial factors responsible for enhanced photocatalytic and photoelectrochemical performance.

Z-Scheme Modulated Charge Transfer on InVO4 @ZnIn2 S4 for Durable Overall Water Splitting

The charge transfer within heterojunction is crucial for the efficiency and stability of photocatalyst for overall water splitting (OWS). Herein, InVO₄ nanosheets have been employed as a support for the lateral epitaxial growth of ZnIn₂ S₄ nanosheets to produce hierarchical InVO₄ @ZnIn₂ S₄ (InVZ) heterojunctions. The distinct branching heterostructure facilitates active site exposure and mass transfer, further boosting the participation of ZnIn₂ S₄ and InVO₄ for proton reduction and water oxidation, respectively. The unique Z-scheme modulated charge transfer, visualized by simulation and in situ analysis, has been proved to promote the spatial separation of photoexcited charges and strengthen the anti-photocorrosion capability of InVZ. The optimized InVZ heterojunction presents improved OWS (153.3 µmol h^-1  g^-1 for H₂ and 76.9 µmol h^-1  g^-1 for O₂ ) and competitive H₂ production (21090 µmol h^-1  g^-1 ). Even after 20 times (100 h) of cycle experiment, it still holds more than 88% OWS activity and a complete structure.

Oxynitrides enabled photoelectrochemical water splitting with over 3,000 hrs stable operation in practical two-electrode configuration

Solar photoelectrochemical reactions have been considered one of the most promising paths for sustainable energy production. To date, however, there has been no demonstration of semiconductor photoelectrodes with long-term stable operation in a two-electrode configuration, which is required for any practical application. Herein, we demonstrate the stable operation of a photocathode comprising Si and GaN, the two most produced semiconductors in the world, for 3,000 hrs without any performance degradation in two-electrode configurations. Measurements in both three- and two-electrode configurations suggest that surfaces of the GaN nanowires on Si photocathode transform in situ into Ga-O-N that drastically enhances hydrogen evolution and remains stable for 3,000 hrs. First principles calculations further revealed that the in-situ Ga-O-N species exhibit atomic-scale surface metallization. This study overcomes the conventional dilemma between efficiency and stability imposed by extrinsic cocatalysts, offering a path for practical application of photoelectrochemical devices and systems for clean energy.

Photocatalytic Water Splitting: How Far Away Are We from Being Able to Industrially Produce Solar Hydrogen?

Solar water splitting (SWS) has been researched for about five decades, but despite successes there has not been a big breakthrough advancement. While the three fundamental steps, light absorption, charge carrier separation and diffusion, and charge utilization at redox sites are given a great deal of attention either separately or simultaneously, practical considerations that can help to increase efficiency are rarely discussed or put into practice. Nevertheless, it is possible to increase the generation of solar hydrogen by making a few little but important adjustments. In this review, we talk about various methods for photocatalytic water splitting that have been documented in the literature and importance of the thin film approach to move closer to the large-scale photocatalytic hydrogen production. For instance, when comparing the film form of the identical catalyst to the particulate form, it was found that the solar hydrogen production increased by up to two orders of magnitude. The major topic of this review with thin-film forms is, discussion on several methods of increased hydrogen generation under direct solar and one-sun circumstances. The advantages and disadvantages of thin film and particle technologies are extensively discussed. In the current assessment, potential approaches and scalable success factors are also covered. As demonstrated by a film-based approach, the local charge utilization at a zero applied potential is an appealing characteristic for SWS. Furthermore, we compare the PEC-WS and SWS for solar hydrogen generation and discuss how far we are from producing solar hydrogen on an industrial scale. We believe that the currently employed variety of attempts may be condensed to fewer strategies such as film-based evaluation, which will create a path to address the SWS issue and achieve sustainable solar hydrogen generation.

Highly Efficient Recovery of H2 from Industrial Waste by Sunlight-Driven Photoelectrocatalysis over a ZnS/Bi2S3/ZnO Photoelectrode

Hydrogen (H₂) fuel production from hazardous contaminants is not only of economic importance but also of significance for the environment and health. Hydrogen production is exemplified in this work by using bismuth sulfide (Bi₂S₃) sandwiched in between zinc sulfide (ZnS) and zinc oxide (ZnO) as dual-heterojunction photoelectrode to photoelectrochemically extract H₂ from sulfide- and sulfite-containing wastewater, which is emitted in enormous quantities from the petrochemical industries. The H₂ evolution rate over the ZnS/Bi₂S₃/ZnO photoelectrode under solar illumination amounts to 112.8 μmol cm^-2 h^-1, of which the photocurrent density in the meantime reaches 10.7 mA cm^-2, by far exceeding those reported for additional Bi₂S₃-based counterparts in the literature. Such superior performance is ascribed on one hand to the broadband sunlight-harvesting ability of Bi₂S₃ that gives rise to respectable photoexcited electron-hole pairs. These photogenerated charge carriers are subsequently rectified by the built-in electric field at the ZnS/Bi₂S₃ and Bi₂S₃/ZnO heterojunctions to flow in the opposite directions to well circumvent the recombination losses and, most importantly, in turn contribute substantially to the H₂ evolution reaction.

Photoreforming of Organic Waste into Hydrogen Using a Thermally Radiative CdOx/CdS/SiC Photocatalyst

To achieve superior efficiency for photocatalytic reactions, it is necessary to utilize visible light, which accounts for most of the solar energy. Herein, by applying a photocatalytic reaction, we aimed to develop a method for generating hydrogen by reforming organic waste, which is discharged as part of domestic, agricultural, forestry, and industrial practice. In the prepared CdS/SiC composite photocatalyst, etching of the oxide film of SiC and oxidation of the atomic-level surface of CdS proceeded in an alkaline reaction solution to form a CdO_x/CdS/SiC composite. This composite is stable under light irradiation in a high-temperature alkaline reaction solution and can steadily promote hydrogen production. CdO_x/CdS/SiC exhibits absorption in the entire ultraviolet and visible light region. In particular, the visible light region on the long-wavelength side, which is derived from the crystal defect of SiC, was used for heat radiation, and it was effective in increasing the temperature of the reaction solution. The high-temperature alkaline reaction solution promoted the hydrolysis of organic wastes with high molecular weight. Elution of small organic molecules by this process facilitated the progress of photocatalytic reactions and improved the rate of hydrogen production. Furthermore, in the absorption region derived from the interband transition below 580 nm, electron transfer between SiC and CdS suppressed recombination and improved the photocatalytic activity. Particularly, we achieved a high quantum yield of almost 20% in the ultraviolet region of 380 nm, where electron transfer from SiC was remarkable. Even in the visible light region, 2.0% was achieved at 420 nm, indicating an activity superior to that of conventional photoreforming systems. Using the developed photocatalytic system, we succeeded in producing hydrogen by photoreforming organic waste, such as cellulosic biomass, animal biomass, and plastic, under sunlight. Therefore, it is possible to solve waste disposal, environmental, and energy problems using sustainable photocatalytic processes.

Environmental occurrence, toxicity concerns, and remediation of recalcitrant nitroaromatic compounds

Nitroaromatic compounds (NACs) are considered important groups of chemicals mainly produced by human and industrial activities. The large-scale application of these xenobiotics creates contamination of the water and soil environment. Despite applicability, NACs have been caused severe hazardous side effects in animals and human systems like different cancers, anemia, skin irritation, liver damage and mutagenic effects. The effective remediation of the NACs from the environment is a significant concern. Researchers have implemented physicochemical and biological methods for the remediation of NACs from the environment. Most of the applied methods are based on adsorption and degradation approaches. Among these methods, degradation is considered a versatile method for the subsequent removal of NACs due to its exceptional properties like simplicity, easy operation, cost-effectiveness, and availability. Most importantly, the degradation process does not generate hazardous side products and wastes compared to other methods. Hence, the importance of NACs, their remediation, and supreme attributes of the degradation method have encouraged us to review the recent progress and development for the removal of these perilous materials using degradation as a versatile method. Therefore, in this review, (i) NACs, physicochemical properties, and their hazardous side effects on humans and animals are discussed; (ii) Physicochemical methods, microbial, anaerobic bioremediation, mycoremediation, and aerobic degradation approaches for the degradation of NACs were thoroughly vetted; (iii) The possible mechanisms for degradation of NACs were investigated and discussed. (iv) The applied kinetic models for evaluation of the rate of degradation were also assessed and discussed. Finally, (vi) current challenges and future prospects of proposed methods for degradation and removal of NACs were also directed.

Robust Protection of III-V Nanowires in Water Splitting by a Thin Compact TiO2 Layer

Narrow-band-gap III-V semiconductor nanowires (NWs) with a suitable band structure and strong light-trapping ability are ideal for high-efficiency low-cost solar water-splitting systems. However, due to their nanoscale dimension, they suffer more severe corrosion by the electrolyte solution than the thin-film counterparts. Thus, short-term durability is the major obstacle for using these NWs for practical water-splitting applications. Here, we demonstrated for the first time that a thin layer (∼7 nm thick) of compact TiO₂ deposited by atomic layer deposition can provide robust protection to III-V NWs. The protected GaAs NWs maintain 91.4% of its photoluminescence intensity after 14 months of storage in ambient atmosphere, which suggests the TiO₂ layer is pinhole-free. Working as a photocathode for water splitting, they exhibited a 45% larger photocurrent density compared with unprotected counterparts and a high Faraday efficiency of 91% and can also maintain a record-long highly stable performance among narrow-band-gap III-V NW photoelectrodes; after 67 h photoelectrochemical stability test reaction in a strong acid electrolyte solution (pH = 1), they show no apparent indication of corrosion, which is in stark contrast to the unprotected NWs that fully failed after 35 h. These findings provide an effective way to enhance both stability and performance of III-V NW-based photoelectrodes, which are highly important for practical applications in solar-energy-based water-splitting systems.

Efficient hydrogen production at a rationally designed MoSe2@Co3O4 p-n heterojunction

During the past several years, transition metal compounds have shown high activity in the field of photocatalysis. Therefore, the MoSe₂@Co₃O₄ with excellent photocatalytic properties through simple hydrothermal and physical mixing methods was prepared. This composite material was composed of n-type semiconductor MoSe₂ and p-type semiconductor Co₃O₄. After optimizing the loading of Co₃O₄, the optimal hydrogen production can reached 7029.2 μmol g^-1h^-1, which was 2.34 times that of single MoSe₂. In addition, some characterization methods were used to explore the hydrogen production performance of the composite catalyst under EY sensitized conditions. Among them, the UV-vis diffuse reflectance spectra suggests that MoSe₂@Co₃O₄ exhibits stronger visible light absorption performance than the single material. Fluorescence performance and photoelectrochemical characterization experiments further prove that, the special structure formed by MoSe₂ and Co₃O₄ and the existence of p-n heterojunction effectively accelerate the separation and transfer of carriers meanwhile inhibit the recombination probability of electron-hole pairs. Combined with other characterizations such as XRD, XPS, SEM and BET, the possible hydrogen production mechanism was proposed.

A rationally designed two-dimensional MoSe2/Ti2CO2 heterojunction for photocatalytic overall water splitting: simultaneously suppressing electron-hole recombination and photocorrosion

Electron–hole recombination and photocorrosion are two challenges that seriously limit the application of two-dimensional (2D) transition metal dichalcogenides (TMDs) for photocatalytic water splitting. In this work, we propose a 2D van der Waals MoSe₂/Ti₂CO₂ heterojunction that features promising resistance to both electron–hole recombination and photocorrosion existing in TMDs. By means of first-principles calculations, the MoSe₂/Ti₂CO₂ heterojunction is demonstrated to be a direct Z-scheme photocatalyst for overall water splitting with MoSe₂ and Ti₂CO₂ serving as photocatalysts for hydrogen and oxygen evolution reactions, respectively, which is beneficial to electron–hole separation. The ultrafast migration of photo-generated holes from MoSe₂ to Ti₂CO₂ as well as the anti-photocorrosion ability of Ti₂CO₂ are responsible for photocatalytic stability. This heterojunction is experimentally reachable and exhibits a high solar-to-hydrogen efficiency of 12%. The strategy proposed here paves the way for developing 2D photocatalysts for water splitting with high performance and stability in experiments.

The two challenges of electron–hole recombination and photocorrosion for two-dimensional transition metal dichalcogenides in the application of photocatalytic water splitting are simultaneously suppressed by rational design of heterojunctions.

Exploring edge functionalised blue phosphorene nanoribbons as novel photocatalysts for water splitting

1D phosphorene nanoribbon edges activating water molecules under sunlight.

Unsymmetrical dirhodium single molecule photocatalysts for H2 production with low energy light

New unsymmetrical dirhodium complexes for photocatalytic H₂ production with red light.

GaN nanowires grown by halide chemical vapour deposition as photoanodes for photo-electrochemical water oxidation reactions

Manifold morphologies of GaN nanowires (NWs) were fabricated using halide chemical vapour deposition (HCVD) on an n-Si (111) substrate and demonstrated to be a promising photoelectrode for photo-electrochemical (PEC) water splitting applications. We report a substantial enhancement in the photocurrent for vertically-grown GaN NWs on a buffer layer as compared to other counterparts such as GaN whiskers, tapered nanostructures and thin films. GaN NWs grown on Si have advantages due to the absorption of photons in a wide spectral range from ultraviolet to infrared and thus are directly involved in PEC reactions. A GaN NW photoanode was demonstrated with a saturation photocurrent density of 0.55 mA cm-2 under 1 sun of illumination, which is much greater than its counterparts. The role of the buffer layer and the carrier density on the PEC performance of vertically-grown GaN NW photoanodes is further elucidated. Photo-electrochemical impedance spectroscopy and Mott-Schottky characterizations were employed to further explain the PEC performance of GaN NW embedded photoanodes. Here, photoanodes based on diverse GaN nanostructures were examined for a better PEC evaluation in order to support the conclusion. The results may pave the way for the fabrication of efficient photoelectrodes and GaN as a protective layer against corrosion for improved photo-stability in an NaOH electrolyte for enhancing the efficiency of water splitting.

Enhanced Photocatalytic Degradation of the Imidazolinone Herbicide Imazapyr upon UV/Vis Irradiation in the Presence of CaxMnOy-TiO2 Hetero-Nanostructures: Degradation Pathways and Reaction Intermediates

The determination of reaction pathways and identification of products of pollutants degradation is central to photocatalytic environmental remediation. This work focuses on the photocatalytic degradation of the herbicide Imazapyr (2-(4-methyl-5-oxo-4-propan-2-yl-1H-imidazol-2-yl) pyridine-3-carboxylic acid) under UV-Vis and visible-only irradiation of aqueous suspensions of Ca_xMnO_y-TiO₂, and on the identification of the corresponding degradation pathways and reaction intermediates. Ca_xMnO_y-TiO₂ was formed by mixing Ca_xMnO_y and TiO₂ by mechanical grinding followed by annealing at 500 °C. A complete structural characterization of Ca_xMnO_y-TiO₂ was carried out. The photocatalytic activity of the hetero-nanostructures was determined using phenol and Imazapyr herbicide as model pollutants in a stirred tank reactor under UV-Vis and visible-only irradiation. Using equivalent loadings, Ca_xMnO_y-TiO₂ showed a higher rate (10.6 μM·h⁻¹) as compared to unmodified TiO₂ (7.4 μM·h⁻¹) for Imazapyr degradation under UV-Vis irradiation. The mineralization rate was 4.07 µM·h⁻¹ for Ca_xMnO_y-TiO₂ and 1.21 μM·h⁻¹ for TiO₂. In the Ca_xMnO_y-TiO₂ system, the concentration of intermediate products reached a maximum at 180 min of irradiation that then decreased to a half in 120 min. For unmodified TiO₂, the intermediates continuously increased with irradiation time with no decrease observed in their concentration. The enhanced efficiency of the Ca_xMnO_y-TiO₂ for the complete degradation of the Imazapyr and intermediates is attributed to an increased adsorption of polar species on the surface of Ca_xMnO_y. Based on LC-MS, photocatalytic degradation pathways for Imazapyr under UV-Vis irradiation have been proposed. Some photocatalytic degradation was obtained under visible-only irradiation for Ca_xMnO_y-TiO₂. Hydroxyl radicals were found to be main reactive oxygen species responsible for the photocatalytic degradation through radical scavenger investigations.

Intrinsic defect engineered Janus MoSSe sheet as a promising photocatalyst for water splitting

The Janus MoSSe sheet has aroused significant attention due to its band edge position and intrinsic dipole moment, making it a strong candidate for water splitting photocatalysis. However, weak water adsorption seriously prevents its further application. Here, first-principles calculations are used to explore the effect of intrinsic defects on water adsorption and conversion at the Janus MoSSe sheet. First-principles calculation results clearly show that intrinsic defects (Svac, Moanti, and Moint) can effectively alter the interaction between water and the MoSSe sheet. Except for Svac defects, the adsorption energy of water at Moanti or Moint defects can be significantly increased by -1.0 to -1.5 eV with respect to the weak water adsorption on a pristine MoSSe sheet of about -0.24 eV. More importantly, the energy barrier for water conversion can be dramatically lowered by 48% to 0.7 eV at Moanti or Moint defects, together with a more stable final state. Such significant enhancement of the adsorption energy is attributed to the red shift of water energy levels, resulting from the strong interaction between O2p orbitals and Mo3d orbitals. It is shown that the intrinsic defects have the potential to change the photocatalytic reactivity of the surface, and thus this may serve as an important way to design photocatalysts for water splitting.

Enhanced Photogenerated Electron Transfer in a Semiartificial Photosynthesis System Based on Highly Dispersed Titanium Oxide Nanoparticles

In this study, a hybrid semiartificial photosynthesis system based on chloroplast (CLP) and titanium oxide nanoparticles (TiO₂ NPs) was constructed. 2,6-Dichlorophenolindophenol (DCPIP) reduction by TiO₂/CLP complex and methylene blue (MB) reduction by TiO₂ were used to determine enhanced photogenerated electron transfer in this hybrid system. The DCPIP reduction by the TiO₂/CLP complex showed the same trend as MB reduction by TiO₂ as a function of concentration of TiO₂ NPs, indicating interception of photogenerated electrons in TiO₂ by CLP that leads to enhanced photosynthesis efficiency. Decreased photoluminescence intensity and shortened excited-state lifetime of the TiO₂/CLP complex compared to that of pure TiO₂ also support electron transfer from TiO₂ to CLP. Longer visible light absorption wavelength and increasing valence band edges reveal the narrower band gap of TiO₂/CLP, which finally results in the enhanced electron transfer from TiO₂ to CLP. Higher ferricyanide reduction and enhanced ATP formation with the TiO₂/CLP complex demonstrate the accelerated electron-transfer rate of the electron-transfer chain. This study reveals the mechanism of how TiO₂ interacts with CLP to enhance the photosynthesis via constructing a semiartificial photosynthesis system.

Synthetic Strategies for Trapping the Elusive trans-Dirhodium(II,II) Formamidinate Isomer: Effects of Cis versus Trans Geometry on the Photophysical Properties

The cis- and trans-dirhodium(II,II) complexes cis-[Rh₂(μ-DTolF)₂(μ-np)(MeCN)₄][BF₄]₂ (1; DTolF = N,N'-di-p-tolylformamidinate and np = 1,8-naphthyridine), cis- and trans-[Rh₂(μ-DTolF)₂(μ-qxnp)(MeCN)₃][BF₄]₂ [2 and 3, respectively, where qxnp = 2-(1,8-naphthyridin-2-yl)quinoxaline], and trans-[Rh₂(μ-DTolF)₂(μ-qxnp)₂][BF₄]₂ (4) were synthesized and characterized. A new synthetic methodology was developed that consists of the sequential addition of π-accepting axially blocking ligands to favor formation of the first example of a bis-substituted formamidinate-bearing trans product. Isolation of the intermediates 2 and 3 provides insight into the mechanistic requirements for obtaining 4 and the cis analogue, cis-[Rh₂(μ-DTolF)₂(μ-qxnp)₂][BF₄]₂ (5). Density functional theory calculations provide support for the synthetic mechanism and proposed intermediates. The metal/ligand-to-ligand charge-transfer (ML-LCT) absorption maximum of the trans complex 4 at 832 nm is red-shifted by 1173 cm^-1 and exhibits shorter lifetimes of the ¹ML-LCT and ³ML-LCT excited states, 3 ps and 0.40 ns, respectively, compared to those of the cis analogue 5. The shorter excited-state lifetimes of 4 are attributed to the longer Rh-Rh bond of 2.4942(8) Å relative to that in 5, 2.4498(2) Å. A longer metal-metal bond reflects a decreased overlap of the Rh atoms, which leads to more accessible metal-centered excited states for radiationless deactivation. The ³ML-LCT excited states of 4 and 5 undergo reversible bimolecular charge transfer with the electron donor p-phenylenediamine when irradiated with low-energy light. These results indicate that trans isomers are a source of unexplored tunability for potential p-type semiconductor applications and, given their distinct geometric arrangement, constitute useful building blocks for supramolecular architectures with potentially interesting photophysical properties.

Intrinsically strained noble metal-free oxynitrides for solar photoreduction of CO2

Metal oxynitrides show promising activity for photocatalytic solar water splitting and CO₂ reduction under solar irradiance. Precise control of cation ratios in oxynitrides is an inevitable challenge that needs to be overcome for achieving effective band gap tuning. Here we report the density functional theory-based calculations for the intricate structure-function relationships of Zn-Ga based oxynitrides and correlate the results with the experimental parameters. Crucial material property descriptors such as elemental composition, intrinsic lattice strain, and vacancy defects were exploited during the synthesis to achieve stable oxynitride photocatalysts that demonstrated CO₂ conversion to CO under simulated solar light, without any noble metal impregnation. The highest CO production rate surpassed that of TiO₂ under the same conditions. This work inspires future research on oxynitride materials with tailored optical properties and sustainable photocatalytic activity which enables their large scale applications.

Monitoring Transport Behavior of Charge Carriers in a Single CdS@CuS Nanowire via In Situ Single-Particle Photoluminescence Spectroscopy

Examination of the spectral and kinetic characteristics of charge carrier recombination on nanostructured semiconductors by photoluminescence (PL) plays a significant role in understanding the photocatalytic process. Here, with an in situ single-particle PL technique, we studied the transport behavior of charge carriers in individual one-dimensional (1D) core-shell structures of CdS@CuS nanowires. Through the PL intensity changes in the single-particle PL spectroscopy, effective interfacial electron transport along the interface of CdS and CuS was observed, which contributes to the significant improvement (i.e., 13.5-fold increase) of photocatalytic H₂ production compared to that for pure CdS nanowires. The present study provides visual experimental evidence for understanding restraining of charge carrier recombination in the semiconductor.

On the Theoretical and Experimental Control of Defect Chemistry and Electrical and Photoelectrochemical Properties of Hematite Nanostructures

Hematite (α-Fe₂O₃) is regarded as one of the most promising cost-effective and stable anode materials in photoelectrochemical applications, and its performance, like other transition-metal oxides, depends strongly on its electrical and defect properties. In this work, the electrical and thermomechanical properties of undoped and Sn-doped α-Fe₂O₃ nanoscale powders were characterized in situ at controlled temperatures ( T = 250 to 400 °C) and atmospheres ( pO₂ = 10^-4 to 1 atm O₂) to investigate their transport and defect properties. Frequency-dependent complex impedance spectra show that interfacial resistance between particles is negligible in comparison with particle resistance. Detailed defect models predicting the dependence of electron, hole, and iron and oxygen vacancy concentrations on temperature and oxygen partial pressures for undoped and doped α-Fe₂O₃ were derived. Using these defect equilibria models, the operative defect regimes were established, and the bandgap energy of undoped α-Fe₂O₃ and oxidation enthalpy of Sn-doped α-Fe₂O₃ were obtained from the analysis of the temperature and pO₂ dependence of the electrical conductivity. On the basis of these results, we are able to explain the surprisingly weak impact of donor doping on the electrical conductivity of α-Fe₂O₃. Furthermore, experimental means based on the results of this study are given for successfully tuning hematite to enhance its photocatalytic activity for the water oxidation reaction.

Photoelectrocatalytic reduction of nitrobenzene on Bi-doped CuGaS2 films

Nitrobenzene, a toxic nitroaromatic, a feedstock compound to the production of many commercially relevant chemicals were photoelectrocatalytically reduced into aniline on a photoelectrode comprised by a bismuth-doped CuGaS₂ nanocrystallyne thin films on molybdenum. The activity of the photoelectrodes were compared to the reaction performed on undoped-CuGaS₂ films, and they were carried out under illumination with an applied bias potential at 0.9 V. Aniline was highly selectively obtained with 83% of conversion for reaction times of 100 min when using Bi-doped CuGaS₂, representing higher conversion of nitrobenzene and yield to aniline than the undoped photoelectrode. The catalytic performance of the doped films remained stable for a set of 5 consecutive experiments. These results indicate Bi-doped CuGaS₂ as a promising material to be applied in the photoelectrocatalytic reduction of nitrobenzene into aniline through the direct pathway mechanism, using solar light illumination.

Interfacial coupling induced direct Z-scheme water splitting in metal-free photocatalyst: C3N/g-C3N4 heterojunctions

Mimicking the natural photosynthesis in green plants, artificial Z-scheme photocatalysis enables more efficient utilization of solar energy for photocatalytic water splitting. Most currently designed g-C₃N₄-based Z-scheme heterojunctions are usually based on metal-containing semiconductor photocatalysts, thus exploiting metal-free photocatalysts for Z-scheme water splitting is of huge interest. Herein, we propose two metal-free C₃N/g-C₃N₄ heterojunctions with the C₃N monolayer covering g-C₃N₄ sheet (monolayer or bilayer) and systematically explore their electronic structures, charge distributions and photocatalytic properties by performing extensive hybrid density functional calculations. We clearly reveal that the relative strong built-in electric fields around their respective interface regions, caused by the charge transfer from C₃N monolayer to g-C₃N₄ monolayer or bilayer, result in the bands bending, renders the transfer of photogenerated carriers in these two heterojunctions following the Z-scheme instead of the type-II pathway. Moreover, the photogenerated electrons and holes in these two C₃N/g-C₃N₄ heterojunctions can not only be efficiently separated, but also have strong redox abilities for water oxidation and reduction. Compared with the isolated g-C₃N₄ sheets, the light absorption in visible to near-infrared region are significantly enhanced in these proposed heterojunctions. These theoretical findings suggest that these proposed metal-free C₃N/g-C₃N₄ heterojunctions are promising direct Z-scheme photocatalysts for solar water splitting.

Nanocrystalline Boron-Doped Diamond as a Corrosion-Resistant Anode for Water Oxidation via Si Photoelectrodes

Due to its high sensitivity to corrosion, the use of Si in direct photoelectrochemical (PEC) water-splitting systems that convert solar energy into chemical fuels has been greatly limited. Therefore, the development of low-cost materials resistant to corrosion under oxidizing conditions is an important goal toward a suitable protection of otherwise unstable semiconductors used in PEC cells. Here, we report on the development of a protective coating based on thin and electrically conductive nanocrystalline boron-doped diamond (BDD) layers. We found that  BDD layers protect the underlying Si photoelectrodes over a wide pH range (1-14) in aqueous electrolyte solutions. A BDD layer maintains an efficient charge carrier transfer from the underlying silicon to the electrolyte solution. Si|BDD photoelectrodes show no sign of performance degradation after a continuous PEC treatment in neutral, acidic, and basic electrolytes. The deposition of a cobalt phosphate (CoPi) oxygen evolution catalyst onto the BDD layer significantly reduces the overpotential for water oxidation, demonstrating the ability of  BDD layers to substitute the transparent conductive oxide coatings, such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO), frequently used as protective layers in Si photoelectrodes.

Flexible InGaN nanowire membranes for enhanced solar water splitting

III-Nitride nanowires (NWs) have recently emerged as potential photoelectrodes for efficient solar hydrogen generation. While InGaN NWs epitaxy over silicon is required for high crystalline quality and economic production, it leads to the formation of the notorious silicon nitride insulating interface as well as low electrical conductivity which both impede excess charge carrier dynamics and overall device performance. We tackle this issue by developing, for the first time, a substrate-free InGaN NWs membrane photoanodes, through liftoff and transfer techniques, where excess charge carriers are efficiently extracted from the InGaN NWs through a proper ohmic contact formed with a high electrical conductivity metal stack membrane. As a result, compared to conventional InGaN NWs on silicon, the fabricated free-standing flexible membranes showed a 10-fold increase in the generated photocurrent as well as a 0.8 V cathodic shift in the onset potential. Through electrochemical impedance spectroscopy, accompanied with TEM-based analysis, we further demonstrated the detailed enhancement within excess charge carrier dynamics of the photoanode membranes. This novel configuration in photoelectrodes demonstrates a novel pathway for enhancing the performance of III-nitrides photoelectrodes to accelerate their commercialization for solar water splitting.

On the origin of the photocurrent of electrochemically passivated p-InP(100) photoelectrodes

The evaluation of faradaic efficiencies is essential to unambiguously determine the origin of photocurrents as showcased for electrochemically conditioned p-InP.

Controlled design of PtPd nanodendrite ornamented niobium oxynitride nanosheets for solar-driven water splitting

A facile road-map is developed for one-pot synthesis of PtPd nanodendrite ornamented niobium oxynitride nanosheets for efficient solar-driven water splitting.