Room-temperature photodeposition of conformal transition metal based cocatalysts on BiVO4 for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using semiconductors offers a promising way to convert renewable solar energy to clean hydrogen fuels. However, due to the sluggish reaction kinetics of water oxidation, significant charge recombination occurred at the photoanode/electrolyte interface and cause decrease of its PEC performance. To reduce the surface recombination, we deposit different transition metal complexes on BiVO₄ nanocone arrays by a versatile light driven in-situ two electrode photodeposition approach without applied bias. Conformal cobalt phosphate “Co-Pi”, nickel borate “Ni-Bi” and manganese phosphate “Mn-Pi” complexes were deposited on BiVO₄ nanocone arrays to form core-shell structure photoanode, all of which lead to enhanced photoelectrochemical performance. The photocurrent of the Co-Pi/BiVO₄ photoanode under front-side illumination for 5 min is increased by 4 folds comparing to that of bare BiVO₄ photoanode at 0.6 V vs. RHE, reaching a hole transfer efficiency as high as 94.5% at 1.23 V vs. RHE. The proposed photodeposition strategy is simple and efficient, and can be extended to deposite cocatalyst on other semiconductors with a valence band edge located at a potential more positive than the oxidation potential of transition metal ion in the cocatalyst.

     

TiO2 ALD decorated CuO/BiVO4 p-n heterojunction for improved photoelectrochemical water splitting

Bismuth vanadate (BiVO₄) is a promising photoanode material owing to the narrow bandgap, appropriate band position, and excellent resistance against photocorrosion, however, the performance of photoelectrochemical (PEC) water splitting is largely limited by the poor carrier separation and transport ability. To address these issues, for the first time, we fabricate BiVO₄ film/CuO nanocone p-n junctions as photoanodes by combing a facile spin-coating process and water bath reaction. This structure strengthens the light harvesting and promotes the charge separation and transport ability. The surface defects states are passivated by coating conformally ultrathin TiO₂ onto CuO surface through atomic layer deposition (ALD) technique. Benefiting from the favorable morphology, energy band, and surface treatment, the BiVO₄/CuO/TiO₂ heterojunction generates an improved photocurrent that is much higher than pure BiVO₄. The detailed mechanism investigations indicate that the synergetic optimization of charge separation and injection efficiency in the bulk and surface of photoelectrodes can significantly improve the performance of PEC cells.     

Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting

We report the first demonstration of hydrogen treatment as a simple and effective strategy to fundamentally improve the performance of TiO(2) nanowires for photoelectrochemical (PEC) water splitting. Hydrogen-treated rutile TiO(2) (H:TiO(2)) nanowires were prepared by annealing the pristine TiO(2) nanowires in hydrogen atmosphere at various temperatures in a range of 200-550 °C. In comparison to pristine TiO(2) nanowires, H:TiO(2) samples show substantially enhanced photocurrent in the entire potential window. More importantly, H:TiO(2) samples have exceptionally low photocurrent saturation potentials of -0.6 V vs Ag/AgCl (0.4 V vs RHE), indicating very efficient charge separation and transportation. The optimized H:TiO(2) nanowire sample yields a photocurrent density of ～1.97 mA/cm(2) at -0.6 V vs Ag/AgCl, in 1 M NaOH solution under the illumination of simulated solar light (100 mW/cm(2) from 150 W xenon lamp coupled with an AM 1.5G filter). This photocurrent density corresponds to a solar-to-hydrogen (STH) efficiency of ～1.63%. After eliminating the discrepancy between the irradiance of the xenon lamp and solar light, by integrating the incident-photon-to-current-conversion efficiency (IPCE) spectrum of the H:TiO(2) nanowire sample with a standard AM 1.5G solar spectrum, the STH efficiency is calculated to be ～1.1%, which is the best value for a TiO(2) photoanode. IPCE analyses confirm the photocurrent enhancement is mainly due to the improved photoactivity of TiO(2) in the UV region. Hydrogen treatment increases the donor density of TiO(2) nanowires by 3 orders of magnitudes, via creating a high density of oxygen vacancies that serve as electron donors. Similar enhancements in photocurrent were also observed in anatase H:TiO(2) nanotubes. The capability of making highly photoactive H:TiO(2) nanowires and nanotubes opens up new opportunities in various areas, including PEC water splitting, dye-sensitized solar cells, and photocatalysis.     

RGO nanosheets coupled NaNbO3 nanorods based nanocomposite for enhanced photocatalytic and photoelectrochemical water splitting activity

In this present work, reduced graphene oxide (RGO) coupled with hydrothermally grown sodium niobate nanorods (NaNbO₃-NRs) have been successfully synthesized. The photocatalytic performance of RGO/NaNbO₃-NRs photocatalyst demonstrated faster photodegradation of organic methylene blue (MB) dye than bare NaNbO₃-NRs. A ∼6 fold enhancement in the photocatalytic activity of RGO/NaNbO₃-NRs nanocomposite than that of NaNbO₃-NRs has been demonstrated towards the degradation of MB dye under similar light illumination. Furthermore, the potentiality of the fabricated NaNbO₃-NRs and RGO/NaNbO₃-NRs nanocomposite photoanodes have been investigated for photoelectrochemical (PEC) water splitting. The fabricated RGO/NaNbO₃-NRs nanocomposite photoanode showed ∼4 times higher photocurrent density than the NaNbO₃-NRs photoanode. The electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) measurements demonstrated that coupling of RGO nanosheets in the RGO/NaNbO₃-NRs nanocomposite reduced the charge transfer resistance (R_ct) at the photoanode/electrolyte interface, increased the donor density (N_d), and reduced flat band potential (V_fb) of the RGO/NaNbO₃-NRs, thus significantly improving the PEC performance of the RGO/NaNbO₃-NRs nanocomposite. The enhancement in the PEC measurements of RGO/NaNbO₃-NRs nanocomposite is attributed to the extended absorption of the visible portion of the solar spectra and increased mobility of the photogenerated charge transport in the RGO nanosheets, which improve the separation efficiency and reduce the recombination process. The possible charge transfer mechanism has been proposed responsible for the enhanced photocatalytic and PEC water splitting performance.     

Controlled growth of vertically oriented hematite/Pt composite nanorod arrays: use for photoelectrochemical water splitting

Highly ordered and vertically grown Pt-doped α-Fe(2)O(3) nanorod arrays on a gold substrate were successfully prepared by the electrochemical co-deposition method using an anodized aluminum oxide template. The effect of the Pt doping in α-Fe(2)O(3) nanorod arrays on their water splitting ability was investigated for the first time. The elemental maps obtained by energy dispersive spectroscopy showed that the Pt was uniformly dispersed in the α-Fe(2)O(3) nanorod arrays. The photoelectrochemical properties of the α-Fe(2)O(3)/Pt composite nanorod arrays as a function of the Pt content were studied by measuring their photocurrent-potential behavior in 1 M NaOH electrolyte under AM 1.5 100 mW cm(-2) illumination. The Pt-doped α-Fe(2)O(3) nanorod arrays show an improvement in solar-to-hydrogen conversion efficiency (～5%) for photoelectrochemical water splitting compared to undoped samples. To the best of our knowledge, it is the highest value yet obtained from α-Fe(2)O(3).     

Pulse electrodeposition of Ag,Cu nanoparticles on TiO2 nanoring/nanotube arrays for enhanced photoelectrochemical water splitting

In this paper, plasmonic Ag and Cu nanoparticles were co-deposited on TiO₂ nanoring/nanotube arrays (TiO₂ R/T) by using two-step pulse electrodeposition method for investigating the optical and photoelectrochemical properties, in comparison to monometallic Ag, Cu decoration. By optimizing the electrodeposition cycle times and electrolyte concentration, bimetallic Ag–Cu/TiO₂ R/T-0.5 with moderate densities and sizes of Ag and Cu nanoparticles was fabricated and shows great photocatalytic potential, in which, Ag mainly facilitates the generation of hot electrons by absorbing visible light and Cu plays an important role in accelerating the separation and transportation of hot electrons. The hydrogen production rate was tested as 425 μL h^?1 cm^?2, which is about 1.34-fold enhanced H₂ production over TiO₂ R/T. Furthermore, molecular dynamics simulations were made for analyzing the interface electrostatic properties between plasmonic nanoparticles of Ag or Cu and the semiconductor TiO₂. It is calculated that bimetallic Ag–Cu/TiO₂/H₂O system has larger interfacial Helmholtz potential than monometallic Ag/TiO₂/H₂O, Cu/TiO₂/H₂O and pure TiO₂/H₂O systems, accelerating the four-electron reaction occurring at the semiconductor/electrolyte interface. This research put forward a feasible and simple pulse electrodeposition method to fabricate bimetallic photoanodes for enhanced hydrogen evolution and an important analysis method of semiconductor/ metal/electrolyte interface characteristics.     

Boosting carrier dynamics of BiVO4 photoanode via heterostructuring with ultrathin BiOI nanosheets for enhanced solar water splitting

Bismuth vanadate (BiVO4) has been one of the most promising candidates for solar water splitting while still suffers from poor bulk charge transport that limits its solar to hydrogen conversion efficiency.We demonstrate in this work an efficient strategy for boosting bulk charge transport of BiVO4 through the facile impregnation of as-prepared BiVO4 photoanode in the precursor solution of ultrathin BiOI nanosheets.Such impregnation creates increased oxygen vacancies in the bulk of BiVO4 through the reduction of V5+ to V4+ by I,which greatly improves bulk separation efficiency for BiVO4-BiOI up to 65.9％ at 1.23 VRHE from the original 51.9 ％ of pure BiVO4.Moreover,the decoration of the BiOI nanosheets on BiVO4 photoanode is also beneficial for addressing the carrier dynamics at surface due to the matched energy levels of BiOI nanosheets and BiVO4.The introduced plenty of oxygen vacancies in the bulk of BiVO4 and the built-in electric field in BiVO4-BiOI synergistically improve the photocurrent density at 1.23 VRHE up to 3.88 mA cm-2.We believe that such facile impregnation strategy will pave an alternative way to the development of highly efficient BiVO4 photoanode.     

High density Si/ZnO core/shell nanowire arrays for photoelectrochemical water splitting

Si/ZnO core/shell nanowire (NW) arrays were fabricated using atomic layer deposition of ZnO shell on n-Si NW arrays prepared by metal assisted electroless etching method. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were utilized to characterize the core/shell structures. Water splitting performance of the core/shell structures was preliminarily studied. The Si/ZnO core/shell NW arrays yielded significantly higher photocurrent density than the planar Si/ZnO structure due to their low reflectance and high surface area. The photoelectrochemical efficiency was found to be 0.035 and 0.002 % for 10 μm-long Si/ZnO NW array and planar Si/ZnO sample, respectively. These results suggested that core/shell structure is superior to planar heterojunction for PEC electrode design. We demonstrated the dependence of photocurrent density on the length of the core/shell array, and analyzed the reasons why longer NW arrays could produce higher photocurrent density. The relationship between the thickness of ZnO shell and the photoconversion efficiency of Si/ZnO NW arrays was also discussed. By applying the core/shell structure in electrode design, one may be able to improve the photoelectrochemical efficiency and photovoltaic device performance.     

Sn-doped hematite nanostructures for photoelectrochemical water splitting

We report on the synthesis and characterization of Sn-doped hematite nanowires and nanocorals as well as their implementation as photoanodes for photoelectrochemical water splitting. The hematite nanowires were prepared on a fluorine-doped tin oxide (FTO) substrate by a hydrothermal method, followed by high temperature sintering in air to incorporate Sn, diffused from the FTO substrate, as a dopant. Sn-doped hematite nanocorals were prepared by the same method, by adding tin(IV) chloride as the Sn precursor. X-ray photoelectron spectroscopy analysis confirms Sn(4+) substitution at Fe(3+) sites in hematite, and Sn-dopant levels increase with sintering temperature. Sn dopant serves as an electron donor and increases the carrier density of hematite nanostructures. The hematite nanowires sintered at 800 °C yielded a pronounced photocurrent density of 1.24 mA/cm(2) at 1.23 V vs RHE, which is the highest value observed for hematite nanowires. In comparison to nanowires, Sn-doped hematite nanocorals exhibit smaller feature sizes and increased surface areas. Significantly, they showed a remarkable photocurrent density of 1.86 mA/cm(2) at 1.23 V vs RHE, which is approximately 1.5 times higher than that of the nanowires. Ultrafast spectroscopy studies revealed that there is significant electron-hole recombination within the first few picoseconds, while Sn doping and the change of surface morphology have no major effect on the ultrafast dynamics of the charge carriers on the picosecond time scales. The enhanced photoactivity in Sn-doped hematite nanostructures should be due to the improved electrical conductivity and increased surface area.     

In situ growth of single-crystal TiO2 nanorod arrays on Ti substrate: Controllable synthesis and photoelectrochemical water splitting

Despite one-dimensional (1D) semiconductor nanostructure arrays attracting increasing attention due to their many advantages,highly ordered TiO2 nanorod arrays (TiO2 NR) are rarely grown in situ on Ti substrates.Herein,a feasible method to fabricate TiO2 NRs on Ti substrates by using a through-mask anodization process is reported.Self-ordered anodic aluminum oxide (AAO) overlaid on Ti substrate was used as a nanotemplate to induce the growth of TiO2 NRs.The NR length and diameter could be controlled by adjusting anodization parameters such as electrochemical anodization voltage,anodization time and temperature,and electrolyte composition.Furthermore,according to the proposed NR formation mechanism,the anodized Ti ions migrate and deposit in the AAO nanochannels to form Ti(OH)4 or amorphous TiO2 NRs under electric field,owing to the confinement effect of the template.Photoelectrochemical tests indicated that,after hydrogenation,the TiO2 NRs presented higher photocurrent density under simulated sunlight and visible light illuminations,suggesting their potential use in photoelectrochemical water splitting,photocatalysis,solar cells,and sensors.     

Z-scheme photocatalyst based on porphyrin derivative decorated few-layer BiVO4 nanosheets for efficient visible-light-driven overall water splitting

It is highly desirable to simulate natural photosynthesis by using sunlight to drive the overall water splitting without using external bias and sacrificial agent.Herein,few-layer monoclinic BiVO4 nanosheets(BVNS)with a thickness of(～)4.3 nm,exposed(010)facets and abundant oxygen vacancies are fabricated using graphene oxide dots as templating reagent.After decorating with asymmetric chromium porphyrin derivative bearing one benzoic acid and three phenyls as meso-position substituents(chromium-5-(4-carboxyphenyl)-10,15,20-triphenylporphrin,CrmTPP)and PtOx cocatalyst,the obtained two-dimensional(2D)hybrid nanocomposite(BVNS/CrmTPP/Pt)with an optimal component ratio delivers a robust overall water splitting performance with a relatively high apparent quantum yield(8.67%)at 400 nm monochromatic light.The ultrathin structure and widely distributed oxygen vacancies on the exposed(010)facets of BVNS not only endow strong and intimate contact with the decorated CrmTPP molecules to promote a two-step excitation Z-scheme charge transfer mechanism for preserving the high redox ability of the photogenerated charge carriers,but also alleviate their recombination,and thus causing the robust overall water splitting performance of the 2D hybrid nanocomposites.The present results provide a novel strategy to construct highly efficient artificial photosynthetic system for overall water splitting.     

Defect engineering of nanostructures: Insights into photoelectrochemical water splitting

Development of long-term and sustainable energy economy is one of the most significant technical challenges facing humanity. Photoelectrochemical (PEC) water splitting is regarded as the most attractive approach for conversion of solar energy to chemical energy, with H₂ and O₂ as the energy carriers. Defect engineering of photocatalytic materials has been proved effective in improving their performances in PEC water splitting process involving three basic steps, i.e., light absorption, charge transfer/separation, and surface catalytic reaction. In this paper, recent developments in using various techniques to introduce, characterize and regulate defects are summarized, based on which the important roles played by defects are highlighted in the development of high-performance defect engineered photoelectrodes for PEC water splitting application. Moreover, current challenges and future perspectives in the field of defect engineering of nanostructures for photoelectrodes are discussed.     

Preparation and characterization of nanostructured ZnO thin films for photoelectrochemical splitting of water

Nanostructured zinc oxide thin films (ZnO) were prepared on conducting glass support (SnO₂: F overlayer) via sol-gel starting from colloidal solution of zinc acetate 2-hydrate in ethanol and 2-methoxy ethanol. Films were obtained by spin coating at 1500 rpm under room conditions (temperature, 28–35°C) and were subsequently sintered in air at three different temperatures (400, 500 and 600°C). The evolution of oxide coatings under thermal treatment was studied by glancing incidence X-ray diffraction and scanning electron microscopy. Average particle size, resistivity and bandgap energy were also determined. Photoelectrochemical properties of thin films and their suitability for splitting of water were investigated. Study suggests that thin films of ZnO, sintered at 600°C are better for photoconversion than the films sintered at 400 or 500°C. Plausible explanations have been provided.     

Bottom-up synthesis of semiconductive carbonaceous nanosheets on hematite photoanode for photoelectrochemical water splitting

Coupling two-dimention carbon materials like graphene on photodelectrode can achieve high-efficiency photoelectrochemical cells.Bottom-up synthesis of carbon-based two-dimensional materials in green media from simple molecules is very attractive but remains a challenge.Carbohydrate is an ideal precursor for the synthesis but previous report requires pyrolysis at high temperature(>700℃).Herein,starting with glucose,we develop a low temperature(210℃)synthesis of carbonaceous nanosheets in aqueous solution of glucose.With the aid of ethylenediamine and Fe3+/Fe27Co27Ni2+ions,the nanosheets can grow on hematite nanorod array with very close contact.Importantly,a metallic region is formed at the interface due to atom distribution distortion,which can promote the charge transfer.The activity can be greatly enhanced by about 500%due to fast charge transfer.This is much better than that prepared by physically or chemically mixing graphene and hematite(<200%).The enhancement is mainly due to the deformation area between the nanosheets and the hematite.The effective hole diffusion length increases from 2 to 8 nm and lifetime of charge carrier also increases,as confirmed by ultrafast transient absorption spectra.This method provides more opportunity for simple,mild and cost-effective fabrication of carbon-based two-dimensional by bottom-up method.     

An advanced NiFe-LDH nanoclusters arrays for high-efficient full water splitting

Journal of Materials Science - A rational structural strategy to design rambutan-like NiFe-LDH nanocluster arrays electrode via a buffer-salt-assisted hydrothermal method was reported. For our...     

Direct growth and photoelectrochemical properties of tungsten oxide nanobelt arrays

A single-step route was developed for the direct growth of tungsten oxide nanobelt arrays by heating a tungsten sheet without additional catalysts or reactants. X-ray diffraction and Raman analysis indicated that the tungsten oxide nanobelts were monoclinic. The surface photovoltage signal and photocurrent density of the tungsten oxide nanobelt arrays clearly suggested a high photoconversion ability. Further investigation demonstrated that the photoelectrocatalytic activity of the nanobelt arrays was higher than that of a tungsten oxide film when using phenol as the probe molecule.     

Visible light active ZnIn2S4/g-C3N4 heterostructure nanocomposite photoelectrode for efficient photoelectrochemical water splitting activity

Hexagonal zinc indium sulfide coupled g-C₃N₄ (H-ZnIn₂S₄/g-C₃N₄) nanocomposites were synthesized using chemisorption method and its performance towards photoelectrochemical water splitting activity was studied. The H-ZnIn₂S₄/g-C₃N₄ (H-ZIS/CN) nanocomposites exhibited ∼ 1.9 times enhanced photoelectrochemical performance as compared to the H-ZnIn₂S₄. The enhancement in the PEC water splitting activity of H-ZIS/CN nanocomposite is ascribed to the formation of type-II heterojunction which resulted in improved separation of photogenerated charge carriers and faster transfer of charges at the photoelectrode/electrolyte interface. The electrochemical impedance study and Mott-Schottky supported these results. Moreover, during photoelectrochemical reactions, H-ZIS/CN nanocomposites showed tremendous stability under visible light. A potential mechanism of the enhanced photoelectrochemical activity of H-ZIS/CN nanocomposites was proposed and endorsed by the PEC results. This study demonstrates that establishing a heterostructure system by coupling a ternary chalcogenide semiconductor with a conducting polymer is an effective strategy for PEC water splitting applications.     

Solution combustion synthesis of iron tungstate nanoparticles for photoelectrochemical water splitting towards oxygen evolution

Journal of Materials Science: Materials in Electronics - Iron tungstate (FeWO4) nanoparticles were prepared by simple solution combustion technique. The preparation method discloses the first...     

Chemical vapor deposition of amorphous molybdenum sulphide on black phosphorus for photoelectrochemical water splitting

Non-precious metal electrocatalyst molybdenum sulphide(MoS) and black phosphorus(BP) are highly promising catalysts for H₂ evolution reaction(HER).However,BP is environmentally unstable and the basal planes of crystal MoS₂ are inactive toward HER.Herein,amorphous molybdenum sulphide(MoSx)directly on BP/BiVO4 film dramatically improves the performance of photoelectrochemical water splitting compared with pure BiVO4.Additionally,we demonstrate that a BP layer,inserted between the MoSx and BiVO4,can enhance the photoelectrochemical performance and improve the stability of the electrodes.Finally,MoS_x/B P/BVO electrode shows the excellent current density of 2.1 mA/cm² at the potential of 1.2 V(vs Ag/AgCl),which is twice higher than that of pure BVO electrode.Our novel nanostructure materials will lead to a new class of non-precious metal photocatalysts for hydrogen production.