Ge-doped ZnO nanorods grown on FTO for photoelectrochemical water splitting with exceptional photoconversion efficiency

In this study, a considerable effort has been devoted for the synthesis of Ge-doped ZnO nanorods on FTO as an efficient and robust photoanode material for solar water splitting. A unique, optimized, and ultra-rapid fabrication method to produce uniform nanorods (30–70 nm in diameter) has been demonstrated using radio frequency sputtering followed by electrochemical anodization. The effect of Ge doping on the conductivity, charge carrier concentration, optical, and photoelectrochemical properties of ZnO was investigated using scanning electron microscope (SEM), glancing angle X-ray diffraction (GAXRD), UV–Vis spectrometer, and Mott Schottky analysis. Glancing angle XRD confirmed the presence of wurtzite structure with a preferable orientation around (101) plane, which is of particular interest for many applications. As evidenced by the photoelectrochemical and transient photocurrent measurements, the fabricated Ge-doped ZnO nanorods exhibited enhanced photocurrent (12 mA/cm²) with an exceptional open circuit voltage of ?1.07 V_SCE (?0.416 V_RHE) under AM1.5G illumination, compared to the undoped ZnO based-photoanodes. Moreover, the Ge-doped ZnO nanorods showed unprecedented photoconversion efficiency of 3.6% under AM1.5G illumination. Therefore, the fabricated Ge-doped ZnO nanorods could be a promising conductive photoanode for water splitting.     

Surface treated TiO2 nanorod arrays for the improvement of water splitting

Water splitting is widely employed for the hydrogen production for its abundant sources of water and sunlight. The TiO₂ nanostructures are the most promising materials because of their properties of the non-toxicity and relatively low cost. Surface treatments with TiCl₄ solution and titanium butoxide solution are applied on the TiO₂ nanorod arrays respectively. On the surface of the TiO₂ nanorods, TiO₂ nanoparticles are prepared through hydrolysis of TiCl₄ and homogeneous phase of TiO₂ synthesized with assist of second hydrothermal synthesis in titanium butoxide, resulting in the increase of the surface area of the TiO₂. Comparing with that of the original TiO₂ nanorod arrays, the incident photon-to-electron conversion efficiency (IPCE) of the TiO₂–TiCl₄ and TiO₂–H₂O samples is greatly enhanced by 25% and 250% in the ultraviolet region, respectively. The obviously enhanced activity is due to the larger surface structure after treatments, which could contribute to the improved performance in the water splitting. These surface treatments provide an efficient way to regulate the properties of the TiO₂ nanorod arrays for their extensive applications in the solar device for the hydrogen production.     

Template synthesis of porous hierarchical Cu2ZnSnS4 nanostructures for photoelectrochemical water splitting

Environmentally friendly and low-cost Cu₂ZnSnS₄ (CZTS) is a promising light absorber for photoelectrochemical (PEC) hydrogen production from water splitting due to the earth-abundant elements, high absorption coefficient, and narrow bandgap. Herein, the hierarchical CZTS film with porous nanostructures was successfully synthesized by a template method. The hierarchical CZTS film was composed of flower-like particles, which were assembled with thin CZTS nanosheets. Macropores were generated owing to the aggregation of flower-like spheres, and mesopores were formed from the stacking of CZTS nanosheets. Compared to the dense CZTS film, the porous hierarchical CZTS film showed a much higher PEC property for water splitting. The improved performance could be attributed to three merits of the porous hierarchical morphology: enhanced light absorption, improved charge separation and transfer, and enlarged electrochemically active surface area. This study provides a useful idea to design efficient semiconductor photoelectrodes for water splitting with delicately controlled morphology.     

Zn-doped hematite modified by graphene-like WS2: A p-type semiconductor hybrid photocathode for water splitting to produce hydrogen

A p-type Zn-doped hematite (α-Fe₂O₃(Zn)) in spindle-shape with an acceptor density of ca. 4.21 × 10¹⁸ cm^?3 were synthesized by a facile hydrothermal method. After α-Fe₂O₃(Zn) was modified with graphene-like WS₂ (α-Fe₂O₃(Zn)/WS₂), the photoelectrochemical performances of the composite can be further enhanced. A photocell composed of the p-type α-Fe₂O₃(Zn)/WS₂ nanocomposite as photocathode and n-type α-Fe₂O₃ as photoanode was assembled to estimate the photocatalytic activity of α-Fe₂O₃(Zn)/WS₂. The amount of the hydrogen and oxygen produced from this tandem cell with the optimal electrodes under 2 h simulated solar light irradiation is 12.5 μmol and 4.3 μmol, respectively.     

Photoresponse and stability improvement of ZnO nanorod array thin film as a single layer of photoelectrode for photoelectrochemical water splitting

ZnO nanorod array thin film with Al-doping and hydrogen treatment was developed as a photoelectrode combining the functions of transparent conducting oxide thin film and photoactive 1-dimensional nanostructured semiconductor into a single layer for photoelectrochemical water splitting. It was demonstrated that hydrogen treatment and Al-doping enhanced the dark currents, photocurrents, and hydrogen generation efficiencies largely and the enhancement by hydrogen treatment was more significant. The maximum photoinduced hydrogen generation efficiency was about 0.020%. Furthermore, hydrogen treatment also improved the photosensitivity and the stability under illumination significantly. The minimum decay time constant and rise time constant were 1.71 and 1.22 s, respectively. And after current-voltage scanning upon illumination for 50 cycles, the 1-dimensional morphology still remained unchanged but those without Al-doping and/or hydrogen treatment were altered seriously. The good photoresponse and stability made the Al-doped ZnO nanorod array thin film with hydrogen treatment have wide applications in the photoelectrochemical field.     

Coaxial Ni3S2@CoMoS4/NiFeOOH nanorods for energy-saving water splitting and urea electrolysis

High efficiency and energy-saving electrochemical hydrogen production has always been a research challenge, mainly due to the limitation of the sluggish kinetics of anodic reactions. Excellent performance depends largely on the clever design of nano-architectures and smart hybridization of active components. It is also very important to establish the relationship between structure and performance of materials. Form perspectives of chemical composition and nanostructure, we developed a novel heterostructure of Ni₃S₂@CoMoS₄/NiFeOOH coaxial nanorods on NF scaffold, in which the two-dimensional CoMoS₄ nanoplates and ultrathin NiFeOOH nanosheets vertically coil around the Ni₃S₂ nanorods by hydrothermal reaction combined with electrodepostion process. Such hierarchical nanorods can provide the heterointerface with highly open surface, ensuring the maximization of synergistic interaction. This heterostructure results in prominent bifunctional activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline water electrolysis systems, and HER coupled with urea oxidation reaction in urea electrolysis devices. It exhibits very low cell voltages for alkaline water splitting (1.732 V) and urea electrolysis (1.660 V) to afford a current density of 100 mA cm^?2 in the two-electrode system as well as excellent long-term stability. Our work provides a new construction for combining various active materials to competitive bifunctional electrocatalysts applied in energy-relative electrochemical devices.     

RF sputter deposition of the high-quality intrinsic and n-type ZnO window layers for Cu(In,Ga)Se2-based solar cell applications

Transparent ZnO films were prepared by rf magnetron sputtering, and their electrical, optical, and structural properties were investigated under various sputtering conditions. Aluminum-doped n-type(n-ZnO) and undoped intrinsic-ZnO (i-ZnO) layers were deposited on a glass substrate by incorporating different targets in the same reaction chamber. The n-ZnO films were strongly affected by argon ambient pressure and substrate temperature, and films deposited at 2 mTorr and 100°C showed superior properties in resistivity, transmission, and figure of merit (FOM). The sheet resistance of ZnO film was less dependent on film thickness when the substrate was heated during deposition. These positive effects of elevated substrate temperature are presumably attributed to the rearrangement of the sputtered atoms by the heat energy. Also, the films are electrically uniform through the 5 cm×5 cm substrate. The maximum deviation in sheet resistance is less than 10%. All of the films showed strong (0 0 2) diffraction peak near 2θ =34°. The undoped i-ZnO films deposited in the mixture of argon and oxygen gases showed high transmission properties in the visible range, independent of the Ar/O₂ ratio, while resistivity rose with increased oxygen partial pressure. The Cu(In,Ga)Se₂ solar cells, incorporating bi-layer ZnO films (n-ZnO/i-ZnO) as window layer, were finally fabricated. The fabricated solar cells showed 14.48% solar efficiency under AM 1.5 conditions (100 mW/cm²).     

Recent progress in the development of visible light-driven powdered photocatalysts for water splitting

Many photocatalyst materials for water splitting have been developed, particularly since the second half of the 1990s. Highly efficient water splitting on tantalate photocatalysts under UV irradiation has been achieved. Moreover, band engineering by doping, valence band formation, and synthesis of solid solutions, has led to the development of a large number of visible light-driven photocatalysts for _H2${\mathrm{H}}_2$ or _O2${\mathrm{O}}_2$ evolution from aqueous solutions containing electron donors or acceptors. Z-scheme photocatalyst systems for water splitting to _H2${\mathrm{H}}_2$ and _O2${\mathrm{O}}_2$ under visible light irradiation have been developed. This progress in the development of visible light-driven photocatalysts for water splitting is reviewed.     

Thermodynamic efficiency analysis of zinc oxide based solar driven thermochemical H2O splitting cycle: Effect of partial pressure of O2, thermal reduction and H2O splitting temperatures

In this paper, the thermodynamic efficiency analysis of ZnO-based solar-driven thermochemical H₂O splitting cycle is performed and compared with the SnO₂-based H₂O splitting cycle. The HSC Chemistry 7.1 software is used for this analysis and effects of thermal reduction ($T_H$) and water splitting temperature ($T_L$) on various thermodynamic parameters associated with the ZnO-based H₂O splitting cycle are explored. The thermodynamic equilibrium compositions allied with the ZnO reduction and re-oxidation of Zn via H₂O splitting reaction are identified by varying the $T_H$, $T_L$, and $P_{O_2}$ in the inert gas. The efficiency analysis indicates that the highest cycle and solar-to-fuel energy conversion efficiency equal to 41.1 and 49.5% can be achieved at $T_H$ = 1340 K and $T_L$ = 650 K. Both efficiencies can be increased further by more than 10% via employing heat recuperation (50%). Based on the cycle and solar-to-fuel energy conversion efficiency values, the ZnO-based H₂O splitting cycle seems to be more attractive than SnO₂-based H₂O splitting cycle.     

Alternate synthetic strategy for the preparation of CdS nanoparticles and its exploitation for water splitting

Cadmium sulphide nanoparticles (6–12 nm) are prepared by a precipitation process using different zeolite matrices as templates. The nanoparticles were characterized by UV-Vis, XRD, SEM, TEM and sorptometric techniques. XRD study shows the presence of hexagonal and cubic phases for the nanoparticles whereas in case of the bulk samples only the hexagonal phase is observed. These nanomaterials have been used as catalysts for the photocatalytic decomposition of water. The nanoparticles show a higher hydrogen evolution rate compared to the bulk samples which correlates well with the particle size and surface area. Noble metal (Pt, Pd, Rh, Ru)-loaded samples were subsequently prepared and tested for hydrogen evolution reaction. The presence of Pt metal is found to enhance the hydrogen production rate whereas the hydrogen production rate is retarded in the presence of Ru metal. This has been explained on the basis of metal hydrogen bond, redox potential and work function of the noble metal.     

Photocatalytic effect of ZnO on carbon gasification with CO2 for high temperature solar thermochemistry

A photocatalytic effect of ZnO on carbon gasification with CO₂ was studied using a concentrated Xe beam to enhance the gasification rate in solar/chemical energy conversion process. The sample, activated carbon impregnated with ZnO (5 wt%), was heated at 873 K by a Xe beam irradiation with UV (<400 nm). The gasification rate at 873 K increased 2 folds in comparison with the Xe irradiation without UV, but, the difference of the rate of CO evolution decreased with the increasing temperature from 873 to 1073 K. The carbothermal reduction of ZnO (ZnO+C→Zn+CO) proceeded at above 950 K, which was demonstrated by XRD analysis and thermodynamic calculation. These results indicate that the photocatalytic effect of ZnO with the UV irradiation enhance the gasification rate of carbon at low temperature (873 K).     

Simulation and optimization of CdS-n/Cu2ZnSnS4 structure for solar cell applications

In this work, the performance of solar cell based on CdS-n/Cu₂ZnSnS₄-p hetero-junction is numerically simulated. The aim of the study is to investigate the influence of thickness, defects density and bandgap energy of absorber layer CZTS and the thickness of the buffer layer CdS of the solar cell on electrical parameters J_sc, V_oc, FF and efficiency η of the cell. The results of our simulation allowed us to optimize the parameters above mentioned in order to get the best efficiency at the optimal band gap which corresponds to the maximum of the solar spectrum with optimal values of the electrical performances of the cell. This results lead to develop CZTS solar cells with high efficiency and low cost and give a help full indication for fabrication process.     

Tuning the catalytic property of TiO2 nanotube arrays for water splitting

Doping is an important approach to modulate the catalytic properties, for example, the change of the overpotential, of TiO₂ semiconductor for water splitting. In this study, by systematically investigating the thermodynamic properties of one-electron water splitting, we found that the required overpotentials can be divided into two groups: one is a high overpotential (∼1.0 V) on pristine TNTAs, Pt/TNTAs and N-TNTAs; and the other is a low overpotential (∼0.6 V) on F-TNTAs, Pt/N-TNTAs and Pt/F-TNTAs. And two kinds of linear relations between the binding energies of O, HO and HOO intermediates are further identified, which are unambiguously ascribed to the bonding characteristics between the reaction intermediates and the two types of TNTAs with the high and low overpotential, respectively. Therefore, the current work will make a step towards understanding the mechanism of water splitting on various doped TNTAs and designing the superior TiO₂-based photoanode materials with lower overpotentials.     

SiO2 loading combined with high temperature calcination of kesterite Cu2ZnSnS4 nanocrystals towards enhanced photocatalytic H2 evolution

In this paper, highly efficient Cu₂ZnSnS₄ (CZTS) towards photocatalytic H₂ evolution was successfully fabricated by SiO₂ loading and high temperature calcination. X-ray diffraction (XRD) and Raman shifts revealed CZTS was pure phase with kesterite structure. Transmission electron microscope (TEM) and high resolution TEM showed CZTS could maintain nanoscale size and kesterite structure after being calcined. The SiO₂ loading was found to be an efficient approach to avoid the aggregation of CZTS nanocrystals during high temperature calcination. As the elevated calcination temperature, the crystallinity of CZTS increased without observable size change, which suggested the no change on the specific surface area. All CZTS/SiO₂ exhibited superior photocatalytic H₂ evolution. Especially, the photocatalytic H₂ evolution rates for per unit mass CZTS was significantly enhanced, which was mainly attributed to the good dispersion, the improved crystallinity and the less loss of specific surface area during high temperature calcination.     

Self-organized TiO2 nanotube arrays with uniform platinum nanoparticles for highly efficient water splitting

Highly efficient water splitting electrode based on uniform platinum (Pt) nanoparticles on self-organized TiO₂ nanotube arrays (TNTAs) was prepared by a combination of multi-step electrochemical anodization with facile photoreduction process. Uniform platinum (Pt) nanoparticles with an average diameter of 8 nm are distributed homogeneously on nanoporous top layer and underneath TiO₂ nanotube wall. In comparison to pristine TNTAs, Pt@TNTAs show substantially enhanced photocurrent density and the incident photon-to-current conversion efficiency (IPCE) in the entire wavelength window. The maximum photocurrent density and IPCE from the optimized Pt@TNTAs photoelectrode (Pt, ～1.57 wt%) were about 24.2 mA cm⁻² and 87.9% at 350 nm, which is much higher than that of the pure nanotubes sample (16.3 mA cm⁻² and 67.3%). The resultant Pt@TNTAs architecture exhibited significantly enhanced photoelectrochemical activities for solar water splitting with hydrogen evolution rate up to 495 μmol h⁻¹ cm⁻² in 2 M Na₂CO₃ + 0.5 M ethylene glycol under the optimal external bias of −0.3 V_SCE.     

Optimization of surface charge transfer processes on rutile TiO2 nanorods photoanodes for water splitting

Electrochemical impedance spectroscopy (EIS) has been performed to investigate the photocatalytic properties of titanium dioxide nanorods in a photoelectrochemical water splitting system. A two-channel transmission line model has been proposed to interpret the frequency response of the main charge transfer processes that occur at nanorod/electrolyte and platinum/electrolyte interfaces. EIS was then employed to determine that the dramatic effect of the annealing treatment on the photocurrent density had its origin on a poor charge transfer at the titania/electrolyte interface. X-ray photoelectron spectroscopy and thermogravimetry measurements have been used to prove the relevance of the presence of chlorine coming from the synthesis process of TiO₂ nanorods.     

Photocatalytic hydrogen evolution of nanoporous CoFe2O4 and NiFe2O4 for water splitting

The spinels CoFe₂O₄ and NiFe₂O₄ of nanoporous photocatalysts were prepared by dealloying and calcination. The photocatalytic performance for the hydrogen generation rate via water splitting was measured. The results revealed that CoFe₂O₄ exhibits a sheet-like nanoporous structure and that abundant mesopores are distributed in the nanosheets. NiFe₂O₄ shows a typical pore-ligament structure. The measurements show that hydrogen generation is exhibited by both oxides because the bandgap of CoFe₂O₄ and NiFe₂O₄ is higher than the water oxidization potential. The hydrogen generation rate is approximately 0.088 mmol h^?1g^?1 for CoFe₂O₄ and 0.026 mmol h^?1g^?1 for NiFe₂O₄ when the TEOA (10 vol%) sacrificial agent is adopted. This performance is significantly higher than that of methanol as the scavenger because TEOA increases the pH value of the solution, changes the negative shift in the conduction band energy level and improves the electron transport efficiency. The higher performance of CoFe₂O₄ is attributed to its larger specific surface area, ample unimpeded penetration diffusion paths and higher electron transfer rate.     

Influence of gas ambient on the synthesis of co-doped ZnO:(Al,N) films for photoelectrochemical water splitting

Al and N co-doped ZnO thin films, ZnO:(Al,N), are synthesized by radio-frequency magnetron sputtering in mixed Ar and N₂ and mixed O₂ and N₂ gas ambient at 100 °C. The ZnO:(Al,N) films deposited in mixed Ar and N₂ gas ambient did not incorporate N, whereas ZnO:(Al,N) films grown in mixed O₂ and N₂ gas ambient showed enhanced N incorporation and crystallinity as compared to ZnO:N thin films grown in the same gas ambient. As a result, ZnO:(Al,N) films grown in mixed O₂ and N₂ gas ambient showed higher photocurrents than the ZnO:(Al,N) thin films deposited in mixed Ar and N₂ gas ambient. Our results indicate that the gas ambient plays an important role in N incorporation and crystallinity control in Al and N co-doped ZnO thin films.     

H2 production by thermochemical water splitting with reticulated porous structures of ceria-based mixed oxide materials

In this work, we present for the first time the preparation and evaluation of Ceria-based mixed oxides reticulated porous ceramic (RPC) structures for H₂ production by thermochemical water splitting. After appropriate screening of the powder materials, ceria-based materials modified with Co, Mn and Zr were discarded due to their low cyclability and/or hydrogen productivity, derived from segregation of active phases or sintering during the thermal reduction and reoxidation. Sponge replica method has been optimized to allow obtaining a Ce_0.9Fe_0.1O_y RPC sponge structure with an outstanding hydrogen production of 15 STPcm³/g_material·cycle at a maximum temperature of 1300 °C. This better performance, comparing to the powder, can be attributed to the open macroporosity of the reticulated porous structure which enhances both heat and mass transfer. The H₂ production is maintained along several consecutive cycles without loss of activity, reinforcing the favorable prospects for large-scale hydrogen production.     

Integrating Co3O4 nanoparticles with MnO2 nanosheets as bifunctional electrocatalysts for water splitting

Developing high-efficiency and low-cost electrocatalyst is significant for the application of water splitting technology. Herein, Co₃O₄ nanoparticles and MnO₂ nanosheets are separately synthesized and subsequently assembled into a unique 0/2-dimensional heterostructure via van der Waals interactions. The consequent composites expose abundant accessible active sites and expedite the reaction kinetics, which can be testified by the superiorities in Tafel slope, exchange current density and double-layer capacitance, only requiring overpotentials of 355 and 129 mV for oxygen and hydrogen evolution reactions in 1.0 M KOH at 10 mA cm^?2, respectively. Moreover, a cell voltage of 1.660 V can drive the electrolyzer at 10 mA cm^?2. Benefitted from robust integration, the original aggregation and restacking of individual materials have been overcome, thereby leading to superior elelctrocatalysis durability. This facile and universal strategy may inspire the researchers on the design and construction of advanced functional composites.