Electrochemically hydrogenated TiO2 nanotubes with improved photoelectrochemical water splitting performance

One-dimensional anodic titanium oxide (ATO) nanotube arrays hold great potential as photoanode for photoelectrochemical (PEC) water splitting. In this work, we report a facile and eco-friendly electrochemical hydrogenation method to modify the electronic and PEC properties of ATO nanotube films. The hydrogenated ATO (ATO-H) electrodes present a significantly improved photocurrent of 0.65 mA/cm² in comparison with that of pristine ATO nanotubes (0.29 mA/cm²) recorded under air mass 1.5 global illumination. The incident photon-to-current efficiency measurement suggests that the enhanced photocurrent of ATO-H nanotubes is mainly ascribed to the improved photoactivity in the UV region. We propose that the electrochemical hydrogenation induced surface oxygen vacancies contribute to the substantially enhanced electrical conductivity and photoactivity.     

Blue-light-excited NaCe(MoO4)2 microcrystals for photoelectrochemical water splitting

We report the structural, morphological, and optical characterization, and the application of sodium-cerium molybdate (NaCe(MoO₄)₂) as a promising photoelectroactive material for water splitting. Information on these several properties was obtained by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and UV–Vis–NIR diffuse reflectance. For the photoelectrochemical tests, NaCe(MoO₄)₂ microcrystals were deposited on conductive indium-tin oxide (ITO) glass substrate by drop coating, and the activity of the as-prepared photoanode toward oxygen evolution reaction was investigated in the absence and presence of blue light-emitting diode irradiation. Studies carried out by linear sweep voltammetry, chronoamperometry, and electrochemical impedance spectroscopy attested to a significant photoelectroactivity of molybdate associated with the fast electron-hole pairs generation. The steady-state photocurrent density recorded under irradiation achieved a remarkable increase, varying from 1.5 µA cm⁻² (light off) to 44.1 µA cm⁻² (light on), in addition, it presents high stability after on–off cycles, what proves the proper performance of NaCe(MoO₄)₂/ITO as photoanode for water splitting.     

MoS2 nanosheet/ZnO nanowire hybrid nanostructures for photoelectrochemical water splitting

As an efficient cocatalyst, the 2D MoS₂ is thought to be a promising substitute to noble metals in the hydrogen evolution reaction (HER). In the past few years, single and few‐layer MoS₂ nanosheets have attracted a wide range of concerns for HERs. In this paper, single crystalline ZnO nanowires and MoS₂ nanosheets are fabricated to form MoS₂ nanosheet/ZnO nanowire hybrid nanostructure to enhance the hydrogen photogeneration. The results show that the hybridization of ZnO with moderate MoS₂ loading could enhance the PEC activity by producing more electrons and holes and reducing their recombination. The synthesis of MoS₂/ZnO composites proposed an effective way to build low cost and highly active HER catalysts. Moreover, the study on MoS₂/ZnO composites sheds light to the deep insight into the mechanism of photoelectrocatalytic HERs.     

Controlled fabrication of Sn/TiO2 nanorods for photoelectrochemical water splitting

In this work, we investigate the controlled fabrication of Sn-doped TiO₂ nanorods (Sn/TiO₂ NRs) for photoelectrochemical water splitting. Sn is incorporated into the rutile TiO₂ nanorods with Sn/Ti molar ratios ranging from 0% to 3% by a simple solvothermal synthesis method. The obtained Sn/TiO₂ NRs are single crystalline with a rutile structure. The concentration of Sn in the final nanorods can be well controlled by adjusting the molar ratio of the precursors. Photoelectrochemical experiments are conducted to explore the photocatalytic activity of Sn/TiO₂ NRs with different doping levels. Under the illumination of solar simulator with the light intensity of 100 mW/cm², our measurements reveal that the photocurrent increases with increasing doping level and reaches the maximum value of 1.01 mA/cm² at −0.4 V versus Ag/AgCl, which corresponds to up to about 50% enhancement compared with the pristine TiO₂ NRs. The Mott-Schottky plots indicate that incorporation of Sn into TiO₂ nanorod can significantly increase the charge carrier density, leading to enhanced conductivity of the nanorod. Furthermore, we demonstrate that Sn/TiO₂ NRs can be a promising candidate for photoanode in photoelectrochemical water splitting because of their excellent chemical stability.     

ZnO-ZnGa2O4 core-shell nanowire array for stable photoelectrochemical water splitting

A dense array of vertically aligned ZnO-ZnGa(2)O(4) core-shell nanowires was synthesized on a large scale on an a-plane sapphire substrate by a simple two-step chemical vapor deposition method. The ZnO cores and ZnGa(2)O(4) shells of the nanowires are of single crystal quality and have aligned crystallographic orientations as evidenced from XRD and TEM analyses. Mott-Schottky analysis and voltage onset from the photocurrent-voltage curve confirm an n-type semiconductor property, a flat-band potential of -0.4 V (versus NHE) and a carrier density of 7 × 10(18) cm(-3) for the ZnO-ZnGa(2)O(4) core-shell nanowires. A stable and large photocurrent of 1.2 mA cm(-2) was obtained with the ZnO-ZnGa(2)O(4) core-shell nanowire array when used as a photoanode at an applied bias of +0.7 V (versus Ag/AgCl) under a 300 W xenon lamp illumination. Moreover, a low dark current of <10(-4) mA cm(-2) was obtained at an applied bias of +0.7 V (versus Ag/AgCl) without light illumination for the ZnO-ZnGa(2)O(4) nanowire array. These results suggest that the dense array of ZnO-ZnGa(2)O(4) core-shell nanowires provides enhanced electronic properties and stable anti-photocorrosion ability and, therefore, is promising as a photoanode in photoelectrochemical water splitting.     

In situ transformation of TiO2 hierarchical nanostructures toward efficient photoelectrochemical water splitting

TiO₂ hierarchical nanomaterials have been intensively investigated as a very promising photoanode in photoelectrochemical water splitting due to its combined effects of large surface area and fast electron transfer. However, the TiO₂ hierarchical nanostructures prepared by most prevailing synthesis methods currently are at distinct disadvantages such as complicated process, poor controllability, poor interfacial contact, excessive impurities and so on. Here we report a facile route to construct a well-defined TiO₂ hierarchical nanostructure via in-situ transformation of TiO₂ nanoparticles with highly exposed {001} facets in the tube wall of the pristine TiO₂ nanotube arrays. The resulted TiO₂ hierarchical nanostructures exhibit enhanced light harvesting, lowe recombination rate of photo-generated charger carrier. Upon these synergistic effects, the target hierarchical nanostructure shows significantly improved PEC performance and excellent photo-switching response with good reproducibility and stability. Corresponding photocurrent density and photon-to-current efficiency achieve 0.76 mA cm⁻² and 0.50%, respectively, which show an enhancement of 1.70 and 1.32 times to those of the control one. This work provides a possible route for fabrication of TiO₂ starting materials with decent efficiencies.     

TiO2 nanotubes modified with electrochemically reduced graphene oxide for photoelectrochemical water splitting

The photocatalytic water splitting into hydrogen and oxygen using solar light is a promising method to provide clean energy carriers in the future. Herein we report on an experimental investigation of TiO₂ nanotubes (NTs) modified with electrochemically reduced graphene oxide (ERGO) for photoelectrochemical water splitting. A photocurrent density of 1.44 mA cm⁻² at 1.23 V vs. RHE has been achieved for ERGO–TiO₂ NTs photoanode under standard reporting conditions, i.e., simulated AM 1.5G sunlight (intensity 100 mW cm⁻²), which is notably increased by ∼140% compared to the bare TiO₂ NTs. This efficiency is nearly ten times higher than that of the P25 nanoparticles based device. The enhanced photocurrent densities can be attributed to the reduced graphene oxide and Ti^{3 +} self-doping produced by an electrochemical reduction treatment. The ERGO modified photoanodes show excellent stability during light soaking under full sunlight.     

Surface plasmon resonance metal-coupled biomass carbon modified TiO2 nanorods for photoelectrochemical water splitting

Exploring efficient and stable photoanode materials is a necessary link to realize the practical application of solar-driven photoelectrochemical (PEC) water splitting.Hence,we prepared rutile TiO2 nanorods,with a width of 50 nm,which was growth in situ on carbon cloth (TiO2@CC) by hydrothermal reaction.And then,Ag nanoparticles (NPs) and biomass N,S-C NPs were chosen for the additional modification of the fabricated TiO2 nanorods to produce broccoli-like Ag-N,S-C/TiO2@CC nanocomposites.According to the result of ultraviolet-visible diffuse reflectance spectroscopy (UV-vis) and PEC water splitting per-formance tests,Ag-N,S-C/TiO2@CC broadens the absorption region of TiO2@CC from the ultraviolet region to the visible region.Under AM 1.5G solar light irradiation,the photocurrent density of Ag-N,S-C/TiO2@CC is 89.8 μA·cm-2,which is 11.8 times higher than TiO2@CC.Under visible light irradiation,the photocurrent density of Ag-N,S-C/TiO2@CC reaches to 12.6 μA·cm-2,which is 21.0 times higher than TiO2@CC.Moreover,Ag-N,S-C/TiO2@CC shows a photocurrent responses in full pH range.It can be found that Ag NPs and N,S-C NPs play key roles in broaden the absorption range of TiO2 nanorods to the visible light region and,promote the occurrence of PEC water oxidation reaction due to the surface plasmon resonance effect of Ag NPs and the synergistic effect of N,S-C NPs.The mechanism demonstrated that Ag-N,S-C/TiO2@CC can separate the photogenerated electron-hole pairs effectively and transfer the photogenerated electrons to the photocathode (Pt plate) in time.This research provides a new strategy for exploration surface plasma metal coupled biomass carbon materials in the field of PEC water splitting.     

Hydrothermal synthesis of Co3O4 nanosheets and its application in photoelectrochemical water splitting

In this study, Co₃O₄ nanosheets were synthesized through hydrothermal method using cobalt nitrate hexahydrate. X-ray diffraction, diffuse reflectance spectra, Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, and field emission scanning electron microscopy were applied to investigate the properties of as-synthesized samples. Ultimately, the electrochemical and photoelectrochemical properties were evaluated by Mott–Schottky analysis and measuring photoconversion efficiency of Co₃O₄ nanosheets. The results indicated that Co₃O₄ nanosheets exhibited a maximum efficiency of 0.92% for water electrolysis under simulated 1.5 global sunlight air mass, which further suggests the excellent potential of Co₃O₄ nanosheets for application in hydrogen generation through photocatalytic water splitting.     

SnO2 QDs-decorated V2O5 nanobelts for photoelectrochemical water splitting under visible light

Highly efficient and solvent-free SnO₂ quantum dots (QDs)-decorated V₂O₅ nanobelt catalysts were synthesized to control environmental pollution via photoelectrochemical water splitting. To confirm the formation of the nanostructures, several analyses were performed. The modification with SnO₂ QDs demonstrated a significant influence on the optical properties of the V₂O₅ nanobelts. The optical bandgap of the synthesized V₂O₅ nanobelt-based catalysts varied between 2.19 and 2.50 eV. The Sn⁴⁺ and V⁵⁺ chemical states of the pure SnO₂ QDs and V₂O₅ nanobelts, respectively, were determined using X-ray photoelectron spectroscopy. Electrochemical impedance spectroscopy revealed that the optimized SnO₂ QDs-decorated V₂O₅ nanobelts had the lowest charge transfer resistance along with capacitive behavior in 0.1 M NaOH electrolyte. The concentration of the SnO₂ QDs had a significant effect on the photocurrent densities of the V₂O₅ nanobelts. A maximum photocurrent density of 1.161 mAcm^?2 was obtained for the sample with 80 mg V₂O₅ nanobelts decorated with 20 mg SnO₂ QDs. This occurred because of the significant enhancement in the light absorption, improved contact at the photoelectrode-electrolyte interface, reduced ion-conduction path resistance, and lower charge transfer resistance of the synthesized photoelectrode.     

Electrodeposition of BiVO4 with needle-like flower architecture for high performance photoelectrochemical splitting of water

Photoelectrochemical (PEC) water splitting is a green and sustainable approach capable of driving mass hydrogen production in the future. To realize this vision, development of a well-performing photoelectrode is highly demanded. In this comprehensive study, electrodeposition technique was applied for fabricating BiVO₄ films by regulating the deposition time from 1 min until 9 min. Interestingly, the morphology, crystallinity, chemical structure, and optical properties of BiVO₄ films depend strongly on the deposition time. It is found that BiVO₄ layer deposited for 7 min with a cross-section thickness of around 321.1–326.5 nm showed the optimum performance, whereby the photocurrent reached up to ~0.32 mA/cm^?2 at 1.23 V vs. RHE. The deposited BiVO₄ represents tiny and long petals, similar to “needle” nanostructures, which is embedded closely into compact agglomerates. Such morphology enables the BiVO₄ films to perform efficiently as photoanode in PEC cells. Besides, high crystallinity is detected from the sharp peaks of XRD and Raman analysis, as well as good light absorption capability that are the main contributors to the enhancement of PEC performance. In addition to the facile fabrication offered by electrodeposition method, the non-toxic attributes and the impressive PEC performance of the optimum BiVO₄ layer could serve as an interesting option for other applications such as gas sensors, solar cells, degradation of pollutants and photocatalytic water splitting.     

Rational design on photoelectrodes and devices to boost photoelectrochemical performance of solar-driven water splitting: a mini review

As an eco-friendly, efficient, and low-cost technique, photoelectrochemical water splitting has attracted growing interest in the production of clean and sustainable hydrogen by the conversion of abundant solar energy. In the photoelectrochemical system, the photoelectrode plays a vital role in absorbing the energy of sunlight to trigger the water splitting process and the overall efficiency depends largely on the integration and design of photoelectrochemical devices. In recent years, the optimization of photoelectrodes and photoelectrochemical devices to achieve highly efficient hydrogen production has been extensively investigated. In this paper, a concise review of recent advances in the modification of nanostructured photoelectrodes and the design of photoelectrochemical devices is presented. Meanwhile, the general principles of structural and morphological factors in altering the photoelectrochemical performance of photoelectrodes are discussed. Furthermore, the performance indicators and first principles to describe the behaviors of charge carriers are analyzed, which will be of profound guiding significance to increasing the overall efficiency of the photoelectrochemical water splitting system. Finally, current challenges and prospects for an in-depth understanding of reaction mechanisms using advanced characterization technologies and potential strategies for developing novel photoelectrodes and advanced photoelectrochemical water splitting devices are demonstrated.     

In situ metal doping during modified anodization synthesis of Nb2O5 with enhanced photoelectrochemical water splitting

A new technique of in situ doping of alkali metal (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) in Nb₂O₅ was showcased by the modified anodization of Nb foils at high frequency, negative‐to‐positive pulsed voltage. At the optimized dopant concentration and synthesis condition, the doped‐Nb₂O₅ shows twofold enhancement in photoelectrochemical water splitting efficiencies compared with the undoped Nb₂O₅ electrode, as a result of improved charge carrier density and enhanced surface charge transfer. © 2015 American Institute of Chemical Engineers AIChE J, 62: 352–358, 2016     

Enhanced charge separation of CuS and CdS quantum-dot-cosensitized porous TiO2-based photoanodes for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using high-performance catalysts shows considerable promise in generating environment-friendly hydrogen energy. Its practical applications, however, suffer from several shortcomings, such as low photocurrent density, large onset-voltage value, and poor durability. In this study, CuS and CdS quantum-dot-cosensitized porous TiO₂-based PEC catalysts (CuS-CT) have been successfully synthesized via in situ sulfuration of CuO and CdO coexisting inside a porous TiO₂ monolith by a hydrothermal method. Compared to porous TiO₂, CuS-sensitized porous TiO₂ (CuS-TiO₂), and CdS-sensitized porous TiO₂ (CdS-TiO₂) in terms of PEC performance, the CuS-CT photoanode exhibited a significantly high anodic photocurrent for water splitting under simulated sunlight radiation. The photocurrent produced by the optimized sample of 7% CuS-5% CdS-TiO₂ (7% CuS-CT) was nearly 2.7 times higher than that of pure porous TiO₂ at 1.0 V versus a reversible hydrogen electrode (RHE). Porous TiO₂ possesses large surface areas that can drive fast electrolyte transport and afford more surface reaction active sites. On the other hand, CuS and CdS quantum dots not only broaden the visible light absorption range, but also improve photoinduced electron-hole separation efficiency. The co-sensitized multi-nanostructures photoanodes lead to a remarkable and promising application in PEC water splitting reactions.     

Ferroelectric polarization controlled surface potential and charge transport across ITO-Nd-doped BiFeO3 polycrystalline ceramic bulk for photoelectrochemical solar water splitting

The spontaneous polarization in ferroelectrics provides an effective way to engineer interfacial charge distribution at the ferroelectric heterojunctions, thereby depletion width in p-n junction and energy band structures. Here, we integrated p-type multiferroic 7% neodymium-doped bismuth ferrite Bi_0.93Nd_0.07FeO₃ (BFO7Nd) polycrystalline ceramic bulk with n-type transparent conductive indium tin oxide (ITO) to demonstrate the influence of ferroelectric polarization on surface potential and its effect on photoelectrochemical (PEC) water reduction. Kelvin probe force microscopy (KPFM) results reveal the surface potential change during ferroelectric polarization switching, suggesting that the polarization bound charges may have an effect on space charge region between ITO-BFO7Nd, thus dominating surface potential at the electrode/electrolyte interface. This implies that the interface barrier between ITO and BFO7Nd can be easily modulated by polarization bound charges. The electron lifetime of upward-polarization (P-up) ITO-BFO7Nd is estimated 29.30 ms, about 15 times longer than unpoled ITO-BFO7Nd. The photocurrent density of P-up ITO-BFO7Nd is ～1.2 × 10^?6 A/cm² (at 0 V vs RHE), which is one-order higher compared to that of the unpoled and P-down states under illumination with an intensity of 100 mWcm^?2. This work shows that ferroelectric polarization combined with photogenerated carriers is an effective approach for effective charge transfer in multiferroic ceramic to boost PEC solar water splitting.     

Facile growth of novel morphology correlated Ag/Co-doped ZnO nanowire/flake-like composites for superior photoelectrochemical water splitting activity

In this paper, novel morphology correlation between silver nanowires (AgNWs) and cobalt (Co)-doped ZnO (Co-ZnO) flake-like thin films (nanowire/flake-like) has been proposed for enhanced photoelectrochemical (PEC) water splitting activity. Here in, high-quality AgNWs/Co-ZnO heterostructures enabled superior visible light water splitting activity compared to the pure ZnO and AgNWs/ZnO. To address the strategic effect of AgNWs coupling and transition metal (Co-2?at%) doping into the ZnO host lattice, we have carried out the X-ray diffraction, field emission scanning microscopy, X-ray photoelectron spectroscopy, UV–Vis transmittance, water contact angle and PEC analyses. In this way, PEC water splitting activity was mainly examined by linear sweep voltammetry (I-V), amperometric I-t and photoconversion efficiency (η) studies. The experimental results provide clear evidence of morphology correlation between AgNWs and Co-ZnO flake-like structures for strong visible light absorption. Specifically, AgNWs/Co-ZnO composites exhibited significant enhancement in the photocurrent density (7.0?×?10^?4 A/cm²) than AgNWs/ZnO (3.2?×?10^?4 A/cm²) and pure ZnO (1.5?×?10^?6 A/cm²). As a result, detailed AgNWs/Co-ZnO geometry has great potential for photoconversion efficiency (0.73%). In a word, the merits of controllable AgNWs/Co-ZnO heterostructure are proposed to improve the visible light harvesting and charge carrier generation for energy conversion devices.     

Bilayered nano-hetero-structured n/n junction thin-film electrodes,WO3/Yb-Mo-BiVO4, for efficient photoelectrochemical water splitting

Journal of Applied Electrochemistry - Significant advancement in photoelectrochemical water splitting current is observed using uniquely evolved n/n junction bilayered nano-hetero-structured thin...     

Photophysical properties and photoelectrochemical performances of sol-gel derived copper stannate (CuSnO3) amorphous semiconductor for solar water splitting application

Copper tin oxide, CuSnO₃ (CSO), is an amorphous oxide semiconductor with a band-gap of 2.0–2.5 eV, and it is an attractive material for diverse applications such as transparent conducting oxides, transistors, and optoelectronic devices. In this study, we fabricated CSO thin films on fluorine-doped tin oxide (FTO)/glass substrates using a facile sol-gel process, and their optical properties, band structure and photoelectrochemical (PEC) properties were investigated using UV–Vis spectroscopy, photocurrent-density-potential (J-V) curves, electrochemical impedance spectroscopy, and Mott-Schottky analysis. The CSO film synthesized at 500 °C had an amorphous phase and a band gap of ~ 2.3 eV with n-type behavior, while the films synthesized at 550 °C and 600 °C had a phase mixture (SnO₂ + CuO). We identified for the first time that the CSO film could be applied to photoelectrodes for photoelectrochemical water-splitting systems. Importantly, when combining the CSO with nanostructured WO₃ film, i.e., the bilayer heterojunction of the a-CSO/WO₃ showed enhanced PEC performances (cathodic shift of onset potential, increase of photocurrent generation and O₂ evolution) compared to the pristine WO₃ film.