Thin Film Photoanodes Research Articles

Electrochemically-synthesized tungstate nanocomposites γ-WO3/CuWO4 and γ-WO3/NiWO4 thin films with improved band gap and photoactivity for solar-driven photoelectrochemical water oxidation

The main aim of this study was to synthesize and characterise tungstate (WO3) nanocomposites with its metal-based nanostructures, such as copper (II) tungstate (CuWO4) and nickel tungsten oxide (NiWO4), as visible-light active thin film photoanodes for solar-driven photoelectrochemical (PEC) water oxidation. FE-SEM and AFM results showed that the bare as-deposited WO3 films were transformed into polycrystalline WO3 structure with highly agglomerated surfaces and roughness during the annealing-induced crystallisation process. XRD results suggested that the bare as-deposited WO3 films undergone phase transformation process from amorphous to the photoactive monoclinic-I (γ-WO3) at 550 °C. XPS results indicated the existence of WO42−, Ni2+ and Cu2+ ions at 35.58 eV, 856 eV and 932.4 eV, respectively. Through the formation of WO3 nanocomposites, the energy band gap was effectively lowered from 2.7 eV (γ-WO3) → 2.3 eV (γ-WO3/CuWO4) → 2.1 eV (γ-WO3/NiWO4) as estimated from the UV–Vis spectra. Finally, the corresponding photoactivity of WO3 nanocomposites was estimated by measuring the photocurrent density and γ-WO3/NiWO4 nanocomposite structure was found to give the highest photocurrent density of 400 μA/cm2 at 1.5 V vs Ag/AgCl (4 M KCl).

Al decorated ZnO thin-film photoanode for SPR-enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting has been considered to be a promising approach to ease the energy and environmental crisis. Herein, Al decorated ZnO thin films are successfully achieved through a facile dc magnetron-sputtering method followed with Al evaporation for further enhanced PEC performance. The Al/ZnO thin film with 60 s Al evaporating time exhibits the highest photocurrent density under AM1.5G and visible light irradiation, which are more than 5 and 3 times as the pure ZnO film, respectively. Such surface modification by Al not only enlarges the visible light absorption based on surface plasmonic resonance effect, but facilitates the charge separation and transportation at the electrode/electrolyte interface. Finally, a possible mechanism is proposed for the photocatalytic activity enhancement of Al/ZnO thin film photoanode.

Photoelectrocatalytic inactivation of Pseudomonas aeruginosa using an Ag-decorated TiO2 photoanode

Abstract Fast and total inactivation of Gram-negative Pseudomonas aeruginosa in suspensions at natural pH 5.9 has been achieved by photoelectrocatalyisis (PEC), using Ag-decorated TiO2 photoanodes onto transparent conducting indium tin oxide (ITO) under UVA irradiation. The assays were made with 100 mL of bacterial suspensions in an undivided cell equipped with a photoanode, a stainless steel cathode and Ag|AgCl (3 M KCl) as reference electrode. Total inactivation was obtained in only 5 min using coatings with 4 wt.% Ag, 25 mM Na2SO4 as the electrolyte and 1.70 V as applied bias potential. Comparative photocatalytic treatments reached total inactivation at much longer time, suggesting the crucial role of hydroxyl radicals in PEC. These oxidants, which were detected by electron spin resonance, attacked the outer cell wall very effectively, since the recombination of the electron/hole pairs photoinduced under UVA irradiation was reduced. As characterized by high-resolution transmission electron microscopy (HRTEM), the best synthesized Ag-TiO2 thin-film photoanode mainly contains anatase TiO2 nanopowder decorated with Ag nanoparticles of ca. 45 nm. Analyses by X-ray powder diffraction and UV/Vis spectroscopy were also performed. The potential use of PEC for bacterial disinfection was confirmed for the rod-shaped Gram-positive Bacillus atrophaeus, which was more slowly inactivated due to its different cell wall structure. Scanning electron micrographs of both bacteria showed that PEC induced a high roughness, cell lysis and accumulation of cellular debris.

Aqueous synthesis of Z-scheme photocatalyst powders and thin-film photoanodes from earth abundant elements

Solid-state narrow band gap semiconductor heterostructures with a Z-scheme charge-transfer mechanism are the most promising photocatalytic systems for water splitting and environmental remediation under visible light. Herein, we construct all-solid Z-scheme photocatalytic systems from earth abundant elements (Ca and Fe) using an aqueous synthesis procedure. A novel Z-scheme two-component Fe2O3/Ca2Fe2O5 heterostructure is obtained in a straightforward manner by soaking various iron-containing nanoparticles (amorphous and crystalline) with Ca(NO3)2 and performing short (20 min) thermal treatments at 820 °C. The obtained powder materials show high photocatalytic performances for methylene blue dye degradation under visible light (45 mW/cm2), exhibiting a rate constant up to 0.015 min−1. The heterostructure exhibits a five-fold higher activity compared to that of pristine hematite. The experiments show that amorphous iron-containing substrate nanoparticles trigger the Fe2O3/Ca2Fe2O5 heterostructure formation. We extended our study to produce Fe2O3/Ca2Fe2O5 nanoheterostructure photoanodes via the electrochemical deposition of amorphous iron-containing sediment were used. The visible-light (15 mW/cm2) photocurrent increases from 183 μA/cm2 to 306 μA/cm2 after coupling hematite and Ca2Fe2O5. Notably, the powders and photoanodes exhibit distinct charge-transfer mechanisms evidenced by the different stabilities of the heterostructures under different working conditions.

Different Roles of Fe1–xNixOOH Cocatalyst on Hematite (α-Fe2O3) Photoanodes with Different Dopants

Transparent Fe1-xNixOOH overlayers (~2 nm thick) were deposited photoelectrochemically on (001) oriented heteroepitaxial Sn- and Zn-doped hematite (Fe2O3) thin film photoanodes. In both cases, the water photo-oxidation performance was improved by the co-catalyst overlayers. Intensity modulated photocurrent spectroscopy (IMPS) was applied to study the changes in the hole current and recombination current induced by the overlayers. For the Sn-doped hematite photoanode, the improvement in performance after deposition of the Fe1-xNixOOH overlayer was entirely due to reduction in the recombination current, leading to a cathodic shift in the onset potential. For the Zn-doped hematite photoanode, in addition to a reduction in recombination current, an increase in the hole current to the surface was also observed after the overlayer deposition, leading to a cathodic shift in the onset potential as well as an enhancement in the plateau photocurrent. These results demonstrate that Fe1-xNixOOH co-catalysts can play different roles depending on the underlying hematite photoanode. The effect of the co-catalyst is not always limited to changes in the surface properties, but also to an increase in hole current from the bulk to the surface that indicates a possible crosslink between surface and bulk processes.

Operando Investigation of Mn3O4+δ Co-catalyst on Fe2O3 Photoanode: Manganese-Valency-Determined Enhancement at Varied Potentials

The development of efficient catalysts containing earth-abundant elements for the oxygen evolution reaction (OER) in photoelectrochemical (PEC) systems is highly desired for low-cost energy storage and conversion. In this work, mesoporous α-Fe2O3 thin film photoanodes coated with manganese oxide (Mn3O4+δ) co-catalysts are prepared by a dip-coating method. The co-catalyst coating significantly enhances PEC water oxidation performance as compared with the uncoated α-Fe2O3. To understand the origin of this enhancement, in situ X-ray absorption spectroscopy is employed to monitor the valence state of Mn in the Mn3O4+δ co-catalyst as a function of applied potential. It is found that the enhancement of the photocurrent is governed by the Mn valency, and the most prominent enhancement takes place at the valency of ∼3.4+, which is due to the optimal eg electron filling in Mn cations as the electrocatalyst for OER. Our investigation indicates that the contribution of Mn3O4+δ co-catalyst to OER kinetics is variable at different applied potentials.

Ultrathin Lutetium Oxide Film as an Epitaxial Hole‐Blocking Layer for Crystalline Bismuth Vanadate Water Splitting Photoanodes

AbstractHere a novel ultrathin lutetium oxide (Lu2O3) interlayer is integrated with crystalline bismuth vanadate (BiVO4) thin film photoanodes to facilitate carrier transport through atomic‐scale interface control. The epitaxial Lu2O3 interlayer fabricated by pulsed laser deposition features very few structural defects at the back contact of the heterojunction, and forms a unique band alignment that favors photohole blocking. An optimized interlayer thickness of 1.4 nm significantly enhances charge separation efficiency and photocurrent. Combined with photoelectrochemical characterization, solid‐state electronic, and localized conductive atomic force microscopy measurements, it is revealed that the Lu2O3 interlayer modulates the electronic conduction pathways along structural grain boundaries and determines the overall device performance. This study sheds light on the nature of interface‐engineered carrier transport for efficient photoelectrode heterostructure design.

Annealing temperature effect on structural, morphological and optical parameters of mesoporous TiO2film photoanode for dye-sensitized solar cell application

AbstractUse of Degussa P25 titanium-dioxide nanopowder in dye-sensitized solar cell (DSSC) photoanode improves efficiency of the DSSC cell. Annealing of titanium dioxide is required for fabrication of crystalline mesoporous thin film photoanode on transparent conducting glass using doctor blade method. Different annealing temperatures provide different structural, morphological, and optical properties of the photoanode, which may influence the efficiency of the cell. In this paper, energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and UV-Vis-NIR spectroscopicanalysis have been carried out to investigate annealing temperature effect on various structural parameters, mole-fraction, phase-content, and optical bandgap of the TiO2film photoanode. It was observed that depending on annealing temperature, theratio of polymorphs of Degussa P25 changed substantially. For the change in annealing temperature from 350 °C to 600 °C, variations occurred in crystallite size from 11.9 nm to 24.9 nm, strain from 0.006 to 0.014, specific surface area from 62.77 m2·g-1to 125.74 m2·g-1, morphology index from 0.49 to 0.64, dislocation density from 5 × 1013line/m2to 8 × 1015line/m2, crystallite per unit surface area from 2 × 1013m-2to 2.5 × 1014m-2, and optical bandgap from 2.4 eV to 3.1 eV.

Hematite Thin Film Photoanodes for Visible Light Water Photooxidation: Effects of Zn Doping and Hydrogen Treatment

Hematite Thin Film Photoanodes for Visible Light Water Photooxidation: Effects of Zn Doping and Hydrogen Treatment

Role of CuO in the modification of the photocatalytic water splitting behavior of TiO2 nanotube thin films

The role of CuO nanoparticles decorating TiO2 nanotubes (TNT) thin film photoanodes in the behavior of photo-electrocatalytic (PEC) cells for water splitting reaction is investigated. CuO is present mainly as small nanoparticles of few nanometer decorating the internal walls of the TiO2 nanotubes. Their presence improves i) the photocurrent behavior, ii) the H2 generation rate by water splitting in a full PEC device (without application of a bias) and iii) the solar-to-hydrogen (STH) efficiency. The increase is about 20% with respect to parent TNT photoanodes using open spectrum light from a solar simulator and about 50% increase using AM 1.5G filtered light from a solar simulator. An STH efficiency over 2% in the full PEC cell is observed in the best conditions. IPCE (incident photon to current conversion efficiency) measurements clearly evidence that the presence of CuO nanoparticles induce an enhanced IPCE in the 300–340nm region. The increase in the performances in water splitting is mainly associated to the transient generation of a p–n junction between the CuxO nanoparticles and TNT upon illumination, which enhances photocurrent density by promoting charge separation.

Molecular Engineering for Enhanced Charge Transfer in Thin-Film Photoanode.

We developed three types of dithieno[3,2-b;2',3'-d]thiophene (DTT)-based organic sensitizers for high-performance thin photoactive TiO2 films and investigated the simple but powerful molecular engineering of different types of bonding between the triarylamine electron donor and the conjugated DTT π-bridge by the introduction of single, double, and triple bonds. As a result, with only 1.3 μm transparent and 2.5-μm TiO2 scattering layers, the triple-bond sensitizer (T-DAHTDTT) shows the highest power conversion efficiency (η = 8.4%; VOC = 0.73 V, JSC = 15.4 mA·cm-2, and FF = 0.75) in an iodine electrolyte system under one solar illumination (AM 1.5, 1000 W·m-2), followed by the single-bond sensitizer (S-DAHTDTT) (η = 7.6%) and the double-bond sensitizer (D-DAHTDTT) (η = 6.4%). We suggest that the superior performance of T-DAHTDTT comes from enhanced intramolecular charge transfer (ICT) induced by the triple bond. Consequently, T-DAHTDTT exhibits the most active photoelectron injection and charge transport on a TiO2 film during operation, which leads to the highest photocurrent density among the systems studied. We analyzed these correlations mainly in terms of charge injection efficiency, level of photocharge storage, and charge-transport kinetics. This study suggests that the molecular engineering of a triple bond between the electron donor and the π-bridge of a sensitizer increases the performance of dye-sensitized solar cell (DSC) with a thin photoactive film by enhancing not only JSC through improved ICT but also VOC through the evenly distributed sensitizer surface coverage.

Novel composite ZnO/TiO2 thin film photoanodes for enhanced visible-light-driven photoelectrochemical water splitting activity

Visible-light-driven photoelectrochemical water splitting activity has been achieved on heterojunction ZnO/TiO2 thin films for the first time using facile RF magnetron sputtering technique. In this study, post-deposition annealed ZnO/TiO2 films with a series thickness of about 30nm (60, 95, and 120nm) were tested as a future photoanodes. Here, the post-deposition annealing treatment at 673K was mainly aimed to promote the close electronic interaction between energy levels of ZnO and TiO2 in the composite ZnO/TiO2. We find that annealing treatment induced a well connection between anatase TiO2 (101) and ZnO (002) facets, which is a determinant factor for the solar water splitting. Here, the PEC ability of the ZnO/TiO2 heterostructure was tested and achieved under visible light (λ=532nm). As a result, remarkable photocurrents were achieved. From linear sweep voltammagrams (I-V), there is no saturation of photocurrents at higher potentials suggests that spatial charge carrier separation. In addition, amperometric I-t curves revealed that photoanodes were relatively stable during the photo-oxidation process. As evidenced from I-V and I-t studies, the ZnO/TiO2-120nm photoanodes exhibited a quite remarkable photocurrents, which is about ten times higher than that of thinner (ZnO/TiO2-60nm) films. Comprehensively, the salient visible light response and sharp photocurrents suggested that composite ZnO/TiO2 in thin film form are productive photoanodes for efficient PEC water splitting activity and stability.

Electronic Structures and Photoanodic Properties of Ilmenite-type MTiO3 Epitaxial Films (M = Mn, Fe, Co, Ni)

We performed systematic study on electronic structures and photoelectrochemical (PEC) properties of ilmenite-type MTiO3 (M = Mn, Fe, Co, and Ni) thin-film photoanodes for water-oxidation reactions. Single-phase MTiO3 films were grown on α-Al2O3 substrates by using pulsed-laser deposition. Formation of the ilmenite-type structures and oxidation states of M2+Ti4+O3 were revealed by X-ray diffraction and X-ray absorption spectroscopy, respectively. The PEC performance in water was investigated by linear sweep voltammetry under Xe-lamp illumination and incident photon to current effciency measurements under monochromatic illumination. We found that NiTiO3 acted as the most effective photoanode among MTiO3. The mechanism of the different PEC performance was discussed from the viewpoint of the electronic structures investigated by photoemission spectroscopy and band calculation based on density functional theory. We revealed that the stronger hybridization between O 2p and M 3d states was responsible for the mo...

Planar D–D−π-A Organic Sensitizers for Thin-Film Photoanodes

The planarity of an organic sensitizer is one of the most crucial factors for determining molar absorptivity and intramolecular charge transfer (ICT). The photovoltaic performance of dye-sensitized solar cells dramatically changed depending on the planarity of the donor, although all dyes exhibited similar extinction coefficients, electrochemical characteristics, as well as the amount of loaded dye. The power conversion efficiency (PCE) of planar donor dyes was 3 times greater than that of twisted donor dyes because of more rapid ICT. In addition, RK-3 dye, characterized by additional donor groups on an indoline unit, exhibited broad light-harvesting ability with higher performance as compared to those observed for planar dyes with a single donor attached to the thin-film photoanode (1.8 μm transparent + 2.5 μm scattering film). The champion cell of RK-3 exhibited a PCE of 10.3% when the thickness of the active film increased to 3.5 μm, as well as when an antireflection layer was applied with an iodine-ba...

Synthesis of TiO2 NRs - ZnO Composite for Dye Sensitized Solar Cell Photoanodes

Composite of TiO2 NRs - ZnO were synthesized for DSSCs photoanode materials. TiO2 NRs was synthesized from TiO2 anatase by mechanochemical technique using ball milling process with agitation speed of 1000 rpm. While, the further hydrothermal refluxing process was conducted at 120°C under various concentration of NaOH in aqueous solution. The starting material of ZnO was prepared from ZnSO4.7H2O as a precursor. The hydrothermal treated TiO2 was added to the ZnO powder in a certain composition of 1:1, 1:2 and 2:1 (w/w), and the mixtures were then annealed at 400°C. The resulting material was characterized by X-ray diffraction (XRD), Surface area analyzer (SAA), Transmission electron microscopy (TEM), and Thermogravimetry/Differential thermal analysis (TG/DTA). The TiO2 revolution occurs from anatase phase into brookite phase. Rutile TiO2 phase was increasing when the NaOH was added at about 12 M. Nanograf of TEM showed the optimum condition for the formation of TiO2 NRs was obtained when 12 M NaOH was used. Structural transformation to 1D nanorods of TiO2 capable increase surface area up to 79 m2/g. TiO2 NRs–ZnO composite was prepared from TiO2 NRs and ZnO using comparation of TiO2 NRs: ZnO = 1:1, 1:2, dan 2:1. Anatase phase TiO2 as a single phase TiO2 was obtained in the TiO2–ZnO composite (1:1 w/w) upon heating the sample until 400°C. Difference TiO2 NRs-ZnO composite materials were investigated as good photovoltaic materials. Evaluation of the performance of DSSCs was conducted by I-V Keithley 2602A measurement indicate that photoanode built of TiO2 NRs - ZnO thin film has a higher solar cell efficiency than that of TiO2 thin film photoanode.

Domain-engineered BiFeO3 thin-film photoanodes for highly enhanced ferroelectric solar water splitting

In photoelectrochemical (PEC) water splitting, charge separation and collection by the electric field in the photoactive material are the most important factors for improved conversion efficiency. Hence, ferroelectric oxides, in which electrons are the majority carriers, are considered promising photoanode materials because their high built-in potential, provided by their spontaneous polarization, can significantly enhance the separation and drift of photogenerated carriers. In this regard, the PEC properties of BiFeO3 thin-film photoanodes with different crystallographic orientations and consequent ferroelectric domain structures are investigated. As the crystallographic orientation changes from (001)pc via (110)pc to (111)pc, the ferroelastic domains in epitaxial BiFeO3 thin films become mono-variant and the spontaneous polarization levels increase to 110 μC/cm2. Consequently, the photocurrent density at 0 V vs. Ag/AgCl increases approximately 5.3-fold and the onset potential decreases by 0.180 V in the downward polarization state. It is further demonstrated that ferroelectric switching in the (111)pc BiFeO3 thin-film photoanode leads to an approximate change of 8,000% in the photocurrent density and a 0.330 V shift in the onset potential. This study strongly suggests that domain-engineered ferroelectric materials can be used as effective charge separation and collection layers for efficient solar water-splitting photoanodes.

Solar‐Water‐Splitting BiVO4 Thin‐Film Photoanodes Prepared By Using a Sol–Gel Dip‐Coating Technique

AbstractA facile and low cost method to construct a bismuth vanadate thin film photoanode was implemented with the aim of integrating it in a tandem dual water splitting photoelectrochemical cell. Multilayer semi‐transparent thin films of BiVO4 were fabricated by a sol–gel process and deposited by dip‐coating onto transparent conducting oxide substrates with intermediate annealing treatment between layers and final calcination at a low temperature of 450 °C in air. The effect of the intermediate annealing temperature has a great impact on the porosity, and therefore density, of thin layers of BiVO4 when fabricated by sol–gel dip‐coating methods; thus, for optimal activity, the annealing temperature should be kept at 400 °C for thinner layers and 450 °C for thicker layers. The annealing temperature has a direct effect on the size of the crystallites which determines the microstructural density and porosity. In contrast, the final calcination temperature must be 450 °C in order to achieve good electrochemical performances. Optimized BiVO4 photoanodes exhibit a photocurrent of up to 2.1 mA cm−2 with an average Faradic efficiency of 85 % for oxygen evolution in neutral pH potassium phosphate buffer at 1.23 V vs. RHE under 350 mW cm−2 light irradiation.

An Investigation of Interfacial and Photoelectrochemical Performance of Thermally Prepared C,N‐codoped TiO2 Photoanodes for Water Splitting

AbstractIn the present study, C,N‐codoped TiO2 (C,N‐TiO2) thin film photoanodes are prepared by a thermal treatment of titanium substrates embedded in various materials such as aluminosilicates (zeolite) or alumina particles and heat treated at 5000 C for 3 h in a carbon contaminated muffle furnace. The prepared C,N‐codoped TiO2 photoanodes showed a significant water oxidation with the highest photo currents of about 100 μA cm−2 at zero bias versus (Ag/AgCl) using alumina as reactant medium under a standard 1 SUN illumination of solar simulator when compared to pure TiO2 (∼25μA cm−2) in 1 M KOH electrolyte. The C,N‐codoped TiO2 photoanode showed stable photocurrent for about 10 hours in the long term stability test. The physico‐chemical properties of the prepared C,N‐codoped TiO2 thin film sample was examined by Scanning electron microscopy, Energy dispersive X‐rays, X‐ray diffractions, X‐ray photoelectron spectroscopy, UV‐visible diffuse reflectance spectroscopy and electrochemical impedance measurements for light harvesting (photo absorption) characteristics. It is found that the C,N‐codoped TiO2 exhibited strong absorption to visible‐light region and improved electron donor density than pure TiO2 from UV‐Vis absorbance spectra and Mott‐Schottky analysis, respectively. The enhanced photoelectrochemical performance of the C,N‐codoped TiO2 could be ascribed to the strong visible light absorption through band gap reduction and suppression of electrons‐holes recombination process.

A Facile Electrochemical Reduction Method for Improving Photocatalytic Performance of α-Fe2O3 Photoanode for Solar Water Splitting.

Electrochemical reduction method is used for the first time to significantly improve the photo-electrochemical performance of α-Fe2O3 photoanode prepared on fluorine-doped tin oxide substrates by spin-coating aqueous solution of Fe(NO3)3 followed by thermal annealing in air. Photocurrent density of α-Fe2O3 thin film photoanode can be enhanced 25 times by partially reducing the oxide film to form more conductive Fe3O4 (magnetite). Fe3O4 helps facilitate efficient charge transport and collection from the top α-Fe2O3 layer upon light absorption and charge separation to yield enhanced photocurrent density. The optimal enhancement can be obtained for <50 nm films because of the short charge transport distance for the α-Fe2O3 layer. Thick α-Fe2O3 films require more charge and overpotential than thinner films to achieve limited enhancement because of the sluggish charge transport over a longer distance to oxidize water. Electrochemical reduction of α-Fe2O3 in unbuffered pH-neutral solution yields much higher but unstable photocurrent enhancement because of the increase in local pH value accompanied by proton reduction at a hematite surface.

Compound Homojunction:Heterojunction Reduces Bulk and Interface Recombination in ZnO Photoanodes for Water Splitting.

Photoelectrochemical water splitting is far more efficient thanks to the novel ZnOSe/ZnO/BZO thin-film photoanodes fabricated in this work. A novel structure is developed for simultaneously suppressing the charge recombination in the ZnO bulk and at the semiconductor-electrolyte interface. This structure achieves a five-fold enhancement in water-splitting performance, compared to that of pristine ZnO photoanodes, when illuminated using visible light.