Photoanodes For Water Splitting Research Articles

Highly efficient photoanode in visible light water splitting through development of Z-scheme structure between compositing TiO2 with GQDs and Ba doped VO2 (m) with smart selection of Ag nanoparticles sites

A highly efficient photoanode for water splitting under visible light is designed by placing Ag nanoparticles (NPs) at the interface of Ba doped VO2 (m) (BaV) and compositing hierarchical porous TiO2 NPs with a graphene quantum dots layer (TG). In fact, the electron flow direction in the photoanode is simply adjusted to develop the Z-scheme structure through solution processing assembly. A very high photocurrent of 2.15 mA cm−2 at 1.23 V vs. RHE is achieved for sequentially assembled reduced graphene oxide (RGO)/BaV/Ag/Ag@TG photoanode. This value is 30 times higher than that of pristine TiO2 photoanode (0.07 mA cm−2 at 1.23 V vs. RHE) and a 70 % increase over type-II heterojunction with RGO/Ag@TG/BaV assembly. Importantly, the stability of this Z-scheme photoanode is 10 times higher than its type-II heterojunction counterpart, making it a promising option for practical applications in water splitting under visible light.

Li sensitized CdS/TiO2 nanocomposite photoanode for solar water splitting, hydrogen generation and photoelectrochemical (PEC) performance

Li–CdS/TiO2 nanocomposite with different composition 10Li–CdS/TiO2, 20Li–CdS/TiO2, 30Li–CdS/TiO2, 40Li–CdS/TiO2, 50Li–CdS/TiO2 were coated on FTO surface by using doctor blade followed by successive ionic layer adsorption and reaction (SILAR) method. As synthesized Li–CdS/TiO2 photoanode is characterized with field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX), high resolution transmission electron microscopy (HR-TEM), diffuse reflectance spectroscopy (UV-DRS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). FE-SEM analysis suggests that, Li–CdS/TiO2 film is 10.2 μm thick, porous, spongy and uniformly covered on FTO surface. The HR-TEM images reveal that the CdS nanoparticles are in intimate contact with the TiO2 nanocrystal shells with the CdS/TiO2 interfaces and the particle size of CdS and Li2S lies in the range of 20 nm–50 nm. UV-DRS analysis shows that, as the loading of Li in CdS/TiO2 films increases, there is decrease in band gap energy. The XPS analysis supports the presence of Li+ in Li–CdS/TiO2 nanocomposite. The PL spectra suggest that, with the increase in extend of Li+ ions into Li–CdS/TiO2, intensity of green emission band increases and it is shifted to higher wavelength. This reflects the low recombination rate of photo generated charge carrier in Li–CdS/TiO2 which is responsible for higher photocatalytic H2 production. The visible light photoanodic performance of Li–CdS/TiO2 was investigated for H2 evolution using aqueous Na2SO3 and Na2S solution under natural sunlight. Among the all-synthesized nanocomposite material, the highest amount of hydrogen produced over 20Li–CdS/TiO2 was found to be 38.61 ml/g/h in natural sunlight. Photoelectrochemical (PEC) performance of Li–CdS/TiO2 was examined. A possible mechanism for photocatalytic hydrogen generation is also discussed.

Fabrication and research of bi-functional CuNi2S4 nanosheets decorated TiO2/CuNi2S4 heterojunction photoanode for photoelectrochemical water splitting

As a traditional n-type semiconductor, TiO2 has good UV absorption ability and stable physical and chemical properties. However, its wide band gap and low oxygen evolution reaction (OER) activity limit its application in the field of photoelectrochemical (PEC) water splitting. In this work, a type-II TiO2/CuNi2S4 heterojunction photoanode is successfully constructed, which expanded the light absorption range to visible and enhanced the OER activity. Firstly, TiO2 nanotubes (NTs) thin films are prepared on Ti substrates by two-step anodization, and then the bi-functional electrocatalytic material CuNi2S4 is grown on TiO2 NTs in the shape of nanosheets (NSs) in situ by solvothermal method. As a bi-functional electrocatalytic material, CuNi2S4 has good visible light absorption property as well as OER catalytic activity. Compared with TiO2, the IPCE value of TiO2/CuNi2S4 is 2.59% at 635 nm, and that of TiO2 is a mere 0.002%. The separation efficiency and injection efficiency increase from 2.49% and 31.52% to 3.61% and 87.77%, respectively. At 1.23 V vs. RHE, the maximum photocurrent density is 0.26 mA/cm2, which is 2.6 times than that of TiO2 (0.11 mA/cm2), and can be maintained at 0.25 mA/cm2 for at least 2 h under light illumination. Moreover, a hydrogen production rate of 4.21 μmol⋅cm−2⋅h−1 is achieved within 2 h. This work provides a new idea for the application of TiO2 in the field of PEC water splitting and the construction of efficient and stable photoelectronic devices.

(M,N) codoping (M = Nb or Ta) and CoO nanoparticle decoration of TiO2 nanotubes: synergistic enhancement of visible photoelectrochemical water splitting

The (M,N) codoping of TiO2 and the formation of heterojunctions by association with co-catalyst nanoparticles are two approaches that can improve the efficiency of TiO2 used as photoanodes for PEC water splitting. In this study, reactive pulsed laser deposition (PLD) of CoO nanoparticles on the surface of (M,N)-codoped TiO2 nanotubes has been used to combine these two methods. The (M,N) codoping reduced the band gap of TiO2 nanotubes and the subsequent nanodecoration significantly increased the photocurrent production under simulated sunlight and visible light conditions. The most significant improvement was obtained for (Nb,N) TiO2-NTs decorated with CoO-NPs, which showed a factor of 3 increase, and a significant contribution from the visible part of the spectrum, which accounts for 40% of the photocurrent generated in this sample. Our results suggest that this enhancement is due to a synergistic effect between the (M,N) codoping of TiO2-NTs and the presence of CoO-NPs through the formation of a high density of local triple phase interface (TPI) (n-TiO2/p-CoO/electrolyte). A modification of the band energy structure at these TPIs allows better extraction of charge carriers generated by visible light in (M,N) TiO2-NTs thanks to the presence of CoO nanoparticles.

Y2O3 electrodeposited TiO2 nanotube arrays as photoanode for enhanced photoelectrochemical water splitting

Y2O3 deposited TiO2 nanotubes (Y-TNTs) are investigated as photoanodes for efficient photoelectrochemical water splitting. Y2O3 is electrodeposited on TiO2 nanotubes for 5–15 min. The physicochemical and optical properties of Y-TNTs reveal that Y2O3 anchored on the TNTs enhances the visible light absorption. The Y-TNTs show good photocurrent response compared to pristine TNTs. Y2O3 deposition for 10 min on TNTs produces highest photocurrent of 609 μA cm−2 at 1.23 VRHE. Presence of Y2O3 increases the lifetime of the charge carriers as revealed by the Nyquist plot analysis and Mott-Schottky analysis. Y-TNTs-10 min sample shows 2.75 times higher photoconversion efficiency compared to pristine TNTs. Computational analysis shows that Y–TiO2 has negative free energy of adsorption for OER intermediates as compared to bare TiO2 which has positive values. The rare earth metal electrodeposition on TiO2 nanotubes is another approach to improve the photoelectrochemical water splitting activity without distorting the nanotube structure.

Nitrogen incorporated oxygen vacancy enriched MnCo2Ox/BiVO4 photoanodes for efficient and stable photoelectrochemical water splitting

Oxygen vacancies in oxygen evolution cocatalysts (OECs) can significantly improve the photoelectrochemical (PEC) water splitting performance of photoanodes. However, OECs with abundant oxygen vacancies have a poor stability when exposing to the highly-oxidizing photogenerated holes. Herein, we partly fill oxygen vacancies in a MnCo2Ox OEC with N atoms by a combined electrodeposition and sol-gel method, which dramatically improves both photocurrent density and stability of a BiVO4 photoanode. The optimized N filled oxygen vacancy-rich MnCo2Ox/BiVO4 photoanode (3 at.% of N) exhibits an outstanding photocurrent density of 6.5 mA·cm−2 at 1.23 VRHE under AM 1.5 G illumination (100 mW·cm−2), and an excellent stability of over 150 h. Systematic characterizations and theoretical calculations demonstrate that N atoms stabilize the defect structure and modulate the surface electron distribution, which significantly enhances the stability and further increases the photocurrent density. Meanwhile, other heteroatoms such as carbon, phosphorus, and sulfur are confirmed to have similar effects on improving PEC water splitting performance of photoanodes.

NiCo3-LDH catalysts decorated with p-n heterojunction CuCoO2/BiVO4 photoanodes for enhanced photoelectrochemical water oxidation

Due to the suitable bandgap and valence band for photoelectrochemical (PEC) water oxidation, BiVO4 has been regarded as a promising photoanode material. However, poor charge-separation and -transport behavior as well as sluggish surface oxygen evolution reaction (OER) kinetics caused the BiVO4 photoanode for water splitting to be significantly lower than its theoretical value. In this study, the synergistic effect of constructing an n-BiVO4/p-CuCoO2 heterojunction and NiCo3-LDH as an OER cocatalyst was designed to enhance the PEC performance of BiVO4 photoanode. The resultant BiVO4/CuCoO2/NiCo3-LDH demonstrates a maximum photocurrent density of 6.95 mA/cm2 at 1.8 V vs. RHE, which is approximately 24.8 times that of BiVO4 (0.28 mA/cm2) and 5.5 times that of BiVO4/CuCoO2 (1.27 mA/cm2) with good stability (70 % remained after 2 h of operation). This work broadens the construction strategy of photoanode heterojunctions to enhance PEC performance.

Modelling TiO2 photoanodes for PEC water splitting: Decoupling the influence of intrinsic material properties and film thickness

Semiconductor metal oxides are intensively studied in electrodes for photoelectrochemical (PEC) water splitting. On a series of nanoparticulate TiO2 photoanodes, we analyze specific fabrication variables by means of data fitting. First, the experimental outcome is gathered using PEC characterization techniques, mostly cyclic voltammetry and transient photocurrent measurements. Subsequently, we apply models to gain insights into the involved charge trapping and transfer phenomena. We find that capacitance coefficients and the switch-on transient kinetics depend on the TiO2 layer thickness, respectively indicating surface mechanisms and stationary regimes that are mediated by light accessibility. On the contrary, exponential factors of capacitance are independent of thickness, but reflect changes in the density of electron states with different sintering atmospheres. Also, the transfer resistance in the electrolyte side is indirectly influenced by sintering. Through meticulous quantitative analysis of trends, we stablish simple mathematical relationships that connect thickness-dependent parameters. This knowledge delves into fundamental mechanisms governing the TiO2 photoelectrode behaviour, and aims to facilitate further improvements in the efficiency of materials and electrodes for green hydrogen production.

Investigation of charge dynamics in dinuclear cobalt phthalocyanine ammonium sulfonate (PDS) modified Ti-Fe2O3 photoanodes for photoelectrochemical water oxidation

The kinetic competition between water oxidation/electron extraction processes and recombination behaviors is a key consideration in the development of efficient photoanodes for solar-driven water splitting. Investigating the photogenerated charge behaviors could guide the construction of high-efficiency photoanodes. In this study, the charge carrier kinetics involved in photoelectrochemical water oxidation of PDS/Ti-Fe2O3 were analyzed using surface photovoltage (SPV), transient photovoltage (TPV), short-pulse transient photocurrent (TPC) and photoelectrochemical impedance spectra (PEIS). The TPC results indicate the interfacial electric field introduced by the PDS loading increases the electron extraction and suppresses the bulk recombination, enhancing the spatial separation of photogenerated charges, which is consistent with the SPV and TPV results. Besides, the surface recombination of the back electron (BER) is also attenuated, which enhances the long-lived holes at the surface of PDS/Ti-Fe2O3 photoanode. Similarly, as obtained by PEIS fitting, the loading of PDS accelerates holes transfer at the photoanode/electrolyte interface, and increases the utilization of long-lived holes. In other word, the recombination behaviors of photogenerated charges are restrained both in the bulk and surface of the photoanode after the deposition of PDS, leading to enhanced PEC performance. These findings highlight the importance of understanding charge carrier dynamics in the design of high-efficient photoanodes.

A Synergic Effect of Bi-metallic Layered Hydro-Oxide Cocatalyst on 1-D TiO2 Driven Photoelectrochemical Water Splitting

ABSTRACT Photoelectrochemical (PEC) water splitting is one of the most sustainable approaches for converting solar energy into hydrogen fuel. Affordable and robust photoelectrodes are crucial for the commercialization of PEC technologies. Recently, transition metal-based co-catalysts, especially Ni- and Fe-based catalysts, have attracted much interest owing to their exceptional OER characteristics. Given this, we here proposed the decoration of a Fe-Ni-based cocatalyst on the surface of the TiO2 photoanode for PEC water splitting. The TiO2 photoanode was hydrothermally synthesized and then decorated by Fe-Ni hydroxide catalyst using photo-assisted electrodeposition. The optimized TiO2/FeNiOOH photoanode exhibited the maximum photocurrent density value of 1.36 mA cm−2, which is almost twice the value obtained for bare TiO2, at 1.23 V vs RHE under the AM 1.5 G illumination. Due to the enhanced light absorption in the UV region, the optimized photoanode exhibited remarkable IPCE and photoconversion efficiency of 87.8% and 0.93%, respectively. Furthermore, excellent faradaic efficiencies of ∼90% for H2 and ∼70% for O2 generations were obtained. Predominantly, the enhancement in the photocurrent potentials was explained in detail. Our study shows the roles and benefits of using bimetallic catalysts with TiO2 photoanodes for sustainable water-splitting applications.

Nanostructured AlFeO3 thin films as a novel photoanode for photoelectrochemical water splitting

Nanostructured AlFeO3 thin films as a novel photoanode for photoelectrochemical water splitting

Rugged Forest Morphology of Magnetoplasmonic Nanorods that Collect Maximum Light for Photoelectrochemical Water Splitting.

A feasible nanoscale framework of heterogeneous plasmonic materials and proper surface engineering can enhance photoelectrochemical (PEC) water-splitting performance owing to increased light absorbance, efficient bulk carrier transport, and interfacial charge transfer. This article introduces a new magnetoplasmonic (MagPlas) Ni-doped Au@Fex Oy nanorods (NRs) based material as a novel photoanode for PEC water-splitting. A two stage procedure produces core-shell Ni/Au@Fex Oy MagPlas NRs. The first-step is a one-pot solvothermal synthesis of Au@Fex Oy . The hollow Fex Oy nanotubes (NTs) are a hybrid of Fe2 O3 and Fe3 O4 , and the second-step is a sequential hydrothermal treatment for Ni doping. Then, a transverse magnetic field-induced assembly is adopted to decorate Ni/Au@Fex Oy on FTO glass to be an artificially roughened morphologic surface called a rugged forest, allowing more light absorption and active electrochemical sites. Then, to characterize its optical and surface properties, COMSOL Multiphysics simulations are carried out. The core-shell Ni/Au@Fex Oy MagPlas NRs increase photoanode interface charge transfer to 2.73 mAcm-2 at 1.23V RHE. This improvement is made possible by the rugged morphology of the NRs, which provide more active sites and oxygen vacancies as the hole transfer medium. The recent finding may provide light on plasmonic photocatalytic hybrids and surface morphology for effective PEC photoanodes.

WO3@Fe2O3 Core-Shell Heterojunction Photoanodes for Efficient Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) hydrogen production from water splitting is a green technology to convert solar energy into renewable hydrogen fuel. The construction of host/guest architecture in semiconductor photoanodes has been proven to be an effective strategy to improve solar-to-fuel conversion efficiency. In this study, WO3@Fe2O3 core-shell nanoarray heterojunction photoanodes are synthesized from the in-situ decomposition of WO3@Prussian blue (WO3@PB) and then used as host/guest photoanodes for photoelectrochemical water splitting, during which Fe2O3 serves as guest material to absorb visible solar light and WO3 can act as host scaffolds to collect electrons at the contact. The prepared WO3@Fe2O3 shows the enhanced photocurrent density of 1.26 mA cm−2 (under visible light) at 1.23 V. vs RHE and a superior IPEC of 24.4% at 350 nm, which is higher than that of WO3@PB and pure WO3 (0.43 mA/cm−2 and 16.3%, 0.18 mA/cm−2 and 11.5%) respectively, owing to the efficient light-harvesting from Fe2O3 and the enhanced electron-hole pairs separation from the formation of type-II heterojunctions, and the direct and ordered charge transport channels from the one-dimensional (1D) WO3 nanoarray nanostructures. Therefore, this work provides an alternative insight into the construction of sustainable and cost-effective photoanodes to enhance the efficiency of the solar-driven water splitting.

Photoelectrochemical water oxidation properties of Mo-doped CuWO4: Effect of p-type sulfide loading and annealing

Photoelectrochemical water splitting has received much attention from the perspective of the production of hydrogen as a next-generation source of energy. Herein, we focus on CuWO4 as a photoanode for water splitting because it is highly responsive to visible light. Mo doping led to longer wavelength shift of absorption edge in UV–vis absorption spectrum. Linear sweep voltammetry revealed that Mo-doped CuWO4 showed a 47-times higher photo-current density than pristine CuWO4 due to enhanced visible light absorption efficiency. The photo-current density of the Mo-doped sample also doubled when loaded with p-type MoS2.

Electronic Structure of Orthorhombic Bi4V2O11 and Role of Gap States in Photoelectrochemical Water Splitting

A group of bismuth vanadium oxides shows promise as an excellent material for photoanodes in water splitting. However, the efficiency of Bi4V2O11 in water splitting is not satisfactory compared to BiVO4. In this study, we investigated the electronic structure of orthorhombic Bi4V2O11 using density functional theory (DFT) and synchrotron soft X-ray spectroscopy techniques to clarify the causes of inefficiency. Our DFT calculations of the two-fold superlattice, as confirmed by near-edge X-ray absorption fine structure measurements, revealed its indirect-band-gap nature. An angle-resolved photoelectron spectroscopy study identified the states within the band gap, and the resonant photoemission technique and DFT simulation clarified that the origin of the gap states was anisotropic localization of electrons around V–O tetrahedra. Water absorption enhances the gap states, which dramatically impedes the kinetics of water oxidation reactions. Our results reveal the role of gap states in water oxidation reactions and offer new insights into manipulating gap states in 3d transition metal oxides.

Long-and short-range structure of SnO2 nanoparticles: Synthesis and photo(electro)catalytic activity

We report herein a detailed study on the influence of hydrothermal treatment temperature on both long- and short-range structures of SnO2 nanoparticles (NPs) applied as photocatalysts for the discoloration of organic pollutants and as photoanodes for water splitting. Synchrotron X-ray diffraction and X-ray absorption near-edge spectroscopy measurements confirmed the enhancement of the structural order of SnO2 NPs as a function of hydrothermal temperature. Fourier transform infrared spectroscopy revealed that the hydrothermal treatment increased the amount of hydroxyl groups on the SnO2 NPs surface. Regarding the photocatalytic activity, the NPs were able to promote the discoloration of different dyes that can act as potential organic pollutants. The photoelectrocatalytic performance of the samples depended on the hydrothermal treatment temperature, with the degree of crystallinity and surface hydroxyl groups playing a significant role in their performance as photoanodes. In particular, the NPs treated at a higher temperature presented a better degree of crystallinity, in addition to many hydroxyls on their surface, leading to increased mobility of the photogenerated charge carriers and improving the interaction between the molecules degraded and the material surface. The results demonstrated that the hydroxyls adsorbed on the SnO2 surface favor the formation of hydroxyl radicals, a species that indirectly participate in the photocatalytic oxidation of rhodamine B dye. The photoelectrocatalytic tests showed that the NPs treated at 200 °C increased oxygen evolution reaction performance.

Construction of CdS/Ti-Nb-O composite photoanode with favorable optical absorption and charge transfer for dramatically boosted photoelectrochemical water splitting

Rational design and construction of highly effective Ti-based photoanodes with advantageous structures are crucial to enhance photoelectrochemical (PEC) water splitting. In this work, we reported a CdS/Ti-Nb-O composite photoanode with satisfactory charge transfer and optical absorption for dramatically boosted PEC performance, which was constructed by bulk-phase Nb-doping synergistically with CdS quantum dots (QDs) sensitization of Ti-based oxide nanostructures. The composite system exhibited a remarkable photocurrent density of 16.11 mA cm-2, which was 34 times higher than pure TiO2 (0.47 mA cm-2). The electron lifetime was substantially increased and the solar-to-hydrogen (STH) conversion efficiency could be as high as 9.45%. Bulk-phase Nb-doping improved the conductivity of Ti-Nb-O, which enabled speedy and sustained charge separation in the composite-electrolyte solid-liquid interface. Meanwhile, CdS sensitization extended light absorption from 410 nm to 640 nm, thereby generating more photogenerated carriers. Simultaneously, Nb-doping negatively shifted the valence band (VB) and conduction band (CB) of Ti-Nb-O, which made the energy band more compatible with CdS. And the well-matched energy band structures of CdS and Ti-Nb-O not only improved separation-transfer properties of photogenerated electrons and holes but also increased overall optical absorption property of the composite system. This work involved dual-regulation charge separation strategy through bulk-phase doping synergetically with QDs sensitization may contribute to constructing high-efficient Ti-based photoanodes for PEC water splitting.

Synthesis and photoelectrochemical performance of Co doped SrTiO3 nanostructures photoanode

AbstractIt is pertinent to realize that scientific research indicates that the most promising method for producing H2 is photo electrochemical water splitting through a photo anode. Cobalt‐doped SrTiO3 (Co‐SrTiO3) composite nanostructures were created in this study via hydrothermal synthesis. The impact of cobalt concentration change on Co‐SrTiO3 has been identified using morphological, structural, and photo electrochemical research. Surface morphology of pure SrTiO3 nanoparticles using SEM and TEM reveals that the particles are intermittently agglomerated. The inclusion of Cobalt lowered the particle size of the nanostructures to 23 nm than pure SrTiO3 (41 nm). In addition, the peak profile has been influenced by cubic phase also identified from the x‐ray diffraction analysis. The purity and composition of the materials were revealed by XPS analysis. The Co‐SrTiO3 composite's produced the best charge transfer and recombination capabilities at 3% Co doping, according to electrochemical chemical impedance (EIS) spectroscopy. At 0.2 V applied potential, the obtained 3% Co‐doped SrTiO3 photoanode system displays a photocurrent density of around 3.45 mA/cm2. The outcomes show that a promising application for the Co‐doped SrTiO3 photoanode in photoelectrochemical water splitting.

Utilizing a Siloxane-Modified Organic Semiconductor for Photoelectrochemical Water Splitting

We explore the potential of employing diketopyrrolopyrrole (DPP) based π-conjugated OSs as a hole transport layer material in heteroatom-doped hematite (Ti–Fe2O3/Ge–Fe2O3) photoanodes for efficient photoelectrochemical water splitting. The siloxane-modified π-conjugated polymer (PSi) with a high carrier mobility and crystallinity revealed great potential to extract holes by forming a built-in potential with hematite photoanodes while showing high stability in an alkaline electrolyte for photoelectrochemical water oxidation. Because of the easy hole extraction and subsequent fast hole transport property of the PSi interlayer between NiFe(OH)x and Ge-doped porous Fe2O3(Ge-PH), NiFe(OH)x/PSi/Ge-PH showed a 1.8-fold increase in photocurrent density (4.57 mA cm–2 at 1.23 VRHE) with a cathodic shift of the onset potential (0.735 VRHE) and good stability for 65 h compared to Ge-PH. This study demonstrates the successful use of inherently unstable π-conjugated OSs as a hole extracting/transport medium in a photoanode, addressing the intrinsic recombination issues of hematite for efficient and stable water splitting.

Three-dimensional WO3 nanorod arrays on Si as photoanodes for efficient photoelectrochemical water splitting

Arrays of WO3–Si forest hierarchical structures have been successfully synthesized by growing WO3 nanorod on the surface of vertically aligned n-Si micropillar arrays using a chemical vapor deposition technique. The three-dimensional (3D) WO3–Si nanorod forest improves photoelectrochemical performances; it exhibits higher photocurrent density and a more negative onset potential of 0.5 V vs. reversible hydrogen electrode compared with the WO3 nanorod array on planar Si substrate. More importantly, this directly grown 3D hierarchical structure exhibits extraordinary stability for water oxidation with negligible losses of photoactivity for 24 h in a neutral pH solution. The enhanced photostability is attributed not only to the formation of O vacancies in WO3, which is resistive to photocorrosion but also the multiple internal reflection in 3D hierarchical structure. Our work demonstrates the importance of structural innovation in improving solar water efficiency.