Synthesis and control strategies of nanomaterials for photoelectrochemical water splitting

Formation and Recombination Dynamics of Polarons in Goethite: A Time-Domain Ab Initio Study

The temperature and the coordination environment significantly affect polaron dynamics. Using goethite (FeOOH) as a model, our study examines polaron formation and recombination behavior under various conditions, including electron injection, photoexcitation, and heterovalent doping. Ab initio and nonadiabatic molecular dynamics (NAMD) simulations reveal that polaron formation in FeOOH is dependent on temperature via an adiabatic mechanism with higher temperatures leading to shorter formation times. Only electron polarons form in FeOOH, regardless of the formation method. NAMD simulations indicate that photoexcited electron polaron recombination is significantly faster in FeOOH than in Fe2O3. This difference arises from the distinct coordination environments, resulting in higher inelastic charge-phonon scattering and stronger nonadiabatic coupling in FeOOH. Our findings highlight the crucial roles of temperature and coordination environment in polaron dynamics, offering valuable insights for designing materials to optimize carrier dynamics.

Photoelectrochemical Water Splitting: A Visible-Light-Driven CoTiO3@g-C3N4-Based Photoanode Interface Follows the Type II Heterojunction Scheme

Harnessing solar energy can be efficiently used to generate hydrogen by photochemical water splitting, which is a sustainable and environmentally benign energy source. Here, a unique visible-light-driven CoTiO3@g-C3N4 (CTOCN)-based photoanode interface has been optimized and developed with modification to follow the type II heterojunction for the enhancement of photoelectrochemical water splitting. Initially, a graphitic carbon nitride-loaded CoTiO3 (with 10 wt % g-C3N4) composite was obtained using a one-pot solvothermal method. Accordingly, the type II heterojunction interface between g-C3N4 and CoTiO3 has been successfully created and confirmed by the acquired phase, morphological, and optical examinations. Thereby, heterostructure generations with interfacial interaction were enabled to decrease photogenerated electron-hole pair recombination, leading to enhanced charge transfer for water oxidation kinetics. The minimal charge transfer resistance and hole relaxation lifetime (p) shown in Nyquist and Bode plots have further confirmed the rapid electron transport across the electrode/electrolyte interfaces, which is attributed to an enhanced absorption of holes for the water splitting process. Additionally, UV-vis spectroscopy, Mott-Schottky analysis, and UPS studies were used to determine the band edge locations of g-C3N4 and CoTiO3. In comparison to previously developed nanohybrids and their equivalents, the CTOCN-d photoanode follows the type II charge transfer mechanism, resulting in a higher photocurrent density of 55.51 mA cm-2.

An Ultrasensitive Room-Temperature H2 Sensor Based on a TiO2 Rutile-Anatase Homojunction

Metal oxide semiconductor hetero- and homojunctions are commonly constructed to improve the performance of hydrogen sensors at room temperature. In this study, a simple two-step hydrothermal method was employed to prepare TiO2 films with homojunctions of rutile and anatase phases (denoted as TiO2-R/A). Then, the microstructure of anatase-phase TiO2 was altered by controlling the amount of hydrochloric acid to realize a more favorable porous structure for charge transport and a larger surface area for contact with H2. The sensor used a Pt interdigital electrode. At an optimal HCl dosage (25 mL), anatase-phase TiO2 uniformly covered rutile-phase TiO2 nanorods, resulting in a greater response to H2 at 2500 ppm compared with that of a rutile TiO2 nanorod sensor by a factor of 1153. The response time was 21 s, mainly because the homojunction formed by the TiO2 rutile and anatase phases increased the synergistic effect of the charge transfer and potential barrier between the two phases, resulting in the formation of more superoxide (O2-) free radicals on the surface. Furthermore, the porous structure increased the surface area for H2 adsorption. The TiO2-R/A-based sensor exhibited high selectivity, long-term stability, and a fast response. This study provides new insights into the design of commercially competitive hydrogen sensors.

Nanomaterials for photo-electrochemical water splitting: a review

Over the last few decades, the global rise in energy demand has prompted researchers to investigate the energy requirements from alternative green fuels apart from the conventional fossil fuels, due to the surge in CO2 emission levels. In this context, the global demand for hydrogen is anticipated to extend by 4-5% in the next 5 years. Different production technologies like gasification of coal, partial oxidation of hydrocarbons, and reforming of natural gas are used to obtain high yields of hydrogen. In present time, 96% of hydrogen is produced by the conventional methods, and the remaining 4% is produced by the electrolysis of water. Photo-electrochemical (PEC) water splitting is a promising and progressive solar-to-hydrogen pathway with high conversion efficiency at low operating temperatures with substrate electrodes such as fluorine-doped tin oxide (FTO), incorporated with photocatalytic nanomaterials. Several semiconducting nanomaterials such as carbon nanotubes, TiO2, ZnO, graphene, alpha-Fe2O3, WO3, metal nitrides, metal phosphides, cadmium-based quantum dots, and rods have been reported for PEC water splitting. The design of photocatalytic electrodes plays a crucial role for efficient PEC water splitting process. By modifying the composition and morphology of photocatalytic nanomaterials, the overall solar-to-hydrogen (STH) energy conversion efficiency can be improved by optimizing their opto-electronic properties. The present article highlights the recent advancements in cleaner and effective photocatalysts for producing high yields of hydrogen via PEC water splitting.

Flame doping of indium ions into TiO2 nanorod arrays for enhanced photochemical water oxidation

Indium (In) ions were diffused into a TiO2 (In-TiO2) photoelectrode via a facile and efficient flame doping method resulting in improved photo-induced carrier separation. The dopant concentration was systematically investigated, and a volcano-type relationship between the dopant concentration and photoelectrochemical (PEC) performance was observed. The optimum incident photon-to-current efficiency and photocurrent density of In-TiO2 were 38.6% and 0.70 mA cm-2 at 1.23 V, respectively, 2.1 and 11.2 times the values of pristine TiO2, respectively. In doping resulted in improved charge separation and lower surface adsorption energies for reactant molecules, as evidenced by experimental and computational methods.

The synergistic effect of surface and bulk O vacancies in a WO3 photoanode to advance carrier separation and light harvesting for photoelectrochemical water splitting

It is critical to fabricate a photoanode with the virtues of high carrier separation efficiency and light harvesting to reduce the recombination of carriers and enhance the utilization of solar energy in photoelectrochemical (PEC) water splitting. In this work, WO₃ nanoflake photoanodes with surface and bulk O vacancies (D-WO_3-x) were fabricated via a hydrothermal method and H₂WO₄ etching to reveal the respective roles and collaborative effect of O vacancies in the surface and bulk. The surface O vacancies leave abundant active sites to reduce the redox barrier. Furthermore, the bulk O vacancies act as electron trap centers for heightening carrier separation efficiency. More importantly, the surface and bulk O vacancies in D-WO_3-x reduce the band gap so that the resistance to electron jumping is reduced and light harvesting is increased. As expected, the photocurrent density of D-WO_3-x is 0.98 mA cm^-2 at 1.23 V vs. RHE, which is 5 times that of pristine WO₃. Moreover, the carrier separation efficiencies in the surface and bulk are 2.38 and 2.26 times that of WO₃. This work provides a promising method for the development of high-performance photoanodes via introducing surface and bulk O vacancies in semiconductors.

Simultaneous Modulation of Interface Reinforcement, Crystallization, Anti-Reflection, and Carrier Transport in Sb Gradient-Doped SnO2 /Sb2 S3 Heterostructure for Efficient Photoelectrochemical Cell

In this study, an effective quadruple optimization integrated synergistic strategy is designed to fabricate quality Sb gradient-doped SnO₂ /Sb₂ S₃ heterostructure for an efficient photoelectrochemical (PEC) cell. The experimental results and theoretical calculations reveal that i) optical absorption matching is realized by combining the anti-reflection of SnO₂ and high light absorption ability of Sb₂ S₃ in the visible region; ii) interface reinforcement is carried out by coordinating gradient-distributed Sb in SnO₂ with S in S-rich precursor of Sb₂ S₃ for improving the Sb₂ S₃ crystallization process and matching crystalline lattice of Sb:SnO₂ and Sb₂ S₃ ; iii) ultrahigh electron mobility is achieved by making Sb gradient-doped SnO₂ ; iv) carrier separation and transport are accelerated by constructing type-II heterojunction with appropriate energy level alignment and forming a high-speed electron transport channel. All of above-mentioned optimization effects are integrated into a synergistic strategy for constructing the Sb:SnO₂ /Sb₂ S₃ photoanode, achieving a photocurrent density of 2.30 mA cm^-2 , hydrogen generation rate of 30.03 µmol cm^-2 h^-1 , and decent working stability. Notably, this method can also be used in other large-scale fabrication processes, such as drop-casting, spray-coating, blade-coating, printing, slot-die, etc. Moreover, this universal integrated strategy paves an avenue to fabricate efficient photoelectrodes with excellent photoelectrochemical performances.

Thin films composed of Zr-doped In2O3 grains rich in fracture surfaces and cracks for photoelectrochemical water oxidation

Zr-doped In₂O₃ thin films are prepared on FTO substrates by a two-step method: firstly, Zr-doped In(OH)₃ thin films are hydrothermally deposited, and then converted to Zr-doped In₂O₃ films by heat treatment. It is found that during the phase transition from Zr-doped In(OH)₃ to Zr-doped In₂O₃, the cuboid-like crystal grains will fragment, resulting in a large number of new surfaces and cracks. Zr doping can introduce shallow impurity levels in the band gap of In₂O₃, which will enhance the absorption of incident light. The substitution of trivalent In³⁺ ions by tetravalent Zr⁴⁺ ions provides additional donors for In₂O₃, which reduces the charge transfer resistance of the photoelectrochemical water oxidation and thus improves the charge transfer kinetics. These factors synergistically improve the photoelectrochemical water oxidation performance of Zr-doped In₂O₃. For example, at a potential of 1.5 V versus reversible hydrogen electrode, the photocurrent density of the Zr-doped In₂O₃ electrode during photoelectrochemical water splitting can be as high as about 3.5 times that of the undoped In₂O₃. Furthermore, Zr doping will also cause changes in the nucleation of some In(OH)₃ grains, resulting in the formation of a small number of rod-bundle-shaped grains.

The synergistic effect of CuBi2O4 and Co-Pi: improving the PEC activity of BiVO4-based composite materials

Reducing the reaction barrier on the photoelectrode surface and increasing the water oxidation power of the sample surface are the key issues to improve photoelectrochemical water splitting performances.

Boosting photoelectrochemical activity of bismuth vanadate by implanting oxygen-vacancy-rich cobalt (oxy)hydroxide

Surface charge recombination is regarded as a detrimental factor that severely downgrades the photoelectrochemical (PEC) performance of bismuth vanadate (BiVO₄). In this work, we demonstrate defect-rich cobalt (oxy)hydroxides (Co(O)OH) as an excellent cocatalyst nanolayer sheathed on BiVO₄ to substantially improve the PEC water oxidation activity. The self-transformation of metal-organic framework produces an ultrathin Co(O)OH layer rich in oxygen vacancies, which could serve as a powerful hole extraction engine to promote the charge transfer/separation efficiency as well as an excellent oxygen evolution reaction catalyst to accelerate the surface water oxidation kinetics. As a result, the BiVO₄/Co(O)OH hybrid photoanode achieves remarkably inhibited surface charge recombination and presents a prominent photocurrent density of 4.2 mA cm^-2 at 1.23 V vs. RHE, which is around 2.6-fold higher than that of the pristine BiVO₄. Moreover, the Co(O)OH cocatalyst nanolayer significantly reduces the onset potential of BiVO₄ photoanodes by 200 mV. This work provides a versatile strategy for rationally preparing oxygen-vacancy-rich cocatalysts on various photoanodes toward high-efficient PEC water oxidation.

Modification of Ti-doped Hematite Photoanode with Quasi-molecular Cocatalyst: A Comparison of Improvement Mechanism Between Non-noble and Noble Metals

Loading of molecular catalyst on the surface of semiconductors is an attractive way to boost the water oxidation activity. As active sites, molecular water oxidation cocatalysts show increasing attraction and application possibility. In order to compare the advantages between molecular catalysts with non-noble and noble metals, the loading of the Fe(salen) and Ru(salen) as cocatalyst precursors on the surface of Ti-Fe₂ O₃ was investigated Quasi-Fe(salen) and Ru(salen) improved the photocurrent density by 1.5 and 1.7 times compared to that of the original Ti-Fe₂ O₃ photoanode, respectively. The quasi-Fe(salen) could improve the conductivity and reaction kinetics on the photoanode surface. By contrast, the notable advancements could be attributed to more reaction sites for quasi-Ru(salen) as cocatalysts. Thus, non-noble quasi-Fe(salen) is a promising cocatalyst to replace the noble metal salen, and further optimization can be expected with regard to the precise control of reaction sites.