Iron doped nanostructured TiO2 for photoelectrochemical generation of hydrogen

This paper describes the photoelectrochemical studies on nanostructured iron doped titanium dioxide (TiO₂) thin films prepared by sol-gel spin coating method. Thin films were characterized by X-ray diffraction, Raman spectroscopy, spectral absorbance, atomic force microscopy and photoelectrochemical (PEC) measurements. XRD study shows that the films were polycrystalline with the photoactive anatase phase of TiO₂. Doping of Fe in TiO₂ resulted in a shift of absorption edge towards the visible region of solar spectrum. The observed bandgap energy decreased from 3.3 to 2.89 eV on increasing the doping concentration upto 0.2 at.% Fe. 0.2 at.% Fe doped TiO₂ exhibited the highest photocurrent density, ∼0.92 mA/cm² at zero external bias. Flatband potential and donor density determined from the Mott–Schottky plots were found to vary with doping concentration from −0.54 to −0.92 V/SCE and 1.7 × 10¹⁹ to 4.3 × 10¹⁹ cm⁻³, respectively.     

Photoelectrochemical properties of AgInS2 thin films prepared using electrodeposition

Ternary silver-indium-sulfide samples were deposited on fluorine-doped tin oxide (FTO) coated glass substrates using a one-step electrodeposition method. A new procedure for the deposition of AgInS₂ samples is reported. The effect of the [Ag]/[In] molar ratio in solution bath on the structural, morphological, and photoelectrochemical properties of samples was examined. X-ray diffraction patterns of samples show that the films are the AgInS₂ phase. The thickness, direct band gap, and indirect band gap of the films were in the ranges 209-1021 nm, 1.82-1.85 eV, and 1.44-1.51 eV, respectively. The carrier densities and flat-band potentials of films obtained from Mott-Schottky and open-circuit potential measurements were in the ranges of 4.2×10¹⁹-9.5×10¹⁹ cm⁻³ and −0.736 to −0.946 V vs. the normal hydrogen electrode (NHE), respectively. It was found that the samples with molar ratio [Ag]/[In]=0.8 in solution bath had a maximum photocurrent density of 9.28 mA/cm² with an applied bias of +1.0 V vs. an Ag/AgCl electrode in contact with electrolyte containing 0.25 M K₂SO₃ and 0.35 M Na₂S. The results show that high-quality AgInS₂ films can be deposited on FTO-coated glass substrates for photoelectrochemical (PEC) applications.     

Physical and photoelectrochemical studies for hydrogen photo-evolution over the spinel ZnCr2O4

The present work deals with the photoelectrochemical hydrogen production over the spinel ZnCr₂O₄. The photoactivity is dependent on the synthesis conditions and the oxide has been prepared by nitrate way in order to produce homogeneous powder with large active surface. The transport properties indicate p-type conductivity with activation energy of 0.21 eV. A corrosion potential of 0.404 V_SCE and an exchange current density of 50 μA cm⁻² have been determined from the semi logarithm plot. The photocurrent onset potential, assimilated to the flat band potential, was found to be −0.39 V_SCE. ZnCr₂O₄/S₂O₃²⁻ is a self driven system where absorption of light promotes electrons into the conduction band with a potential (−1 V) sufficient to reduce water into hydrogen. The activity shows a tendency toward saturation whose deceleration is the result of the competitive reductions of end products namely S₂O₆²⁻ and S₂O₄²⁻ with water. A comparative study with CuCr₂O₄ is reported.     

Photoelectrochemical performances of AgInS2 film electrodes fabricated using the sulfurization of Ag-In metal precursors

Ternary silver-indium-sulfide samples were deposited on various substrates using the sulfurization of Ag-In metal precursors. A new procedure for the deposition of AgInS₂ samples is reported. The effect of the [Ag]/[In] molar ratio in metal precursors on the structural, morphological, and photoelectrochemical properties of the samples was examined. X-ray diffraction patterns of samples show that the films are in the polycrystalline AgInS₂ phase. The thickness and direct band gap of the films were in the ranges of 1.1-1.2 μm and 1.92-1.94 eV, respectively. The conduction type of all samples was n-type. The carrier concentration, mobility, and resistivity of samples were in the ranges of 1.5×10¹³-7.0×10¹³ cm⁻³, 2.6-14.8 cm²V⁻¹s⁻¹, and 2.6×10⁴-3.5×10⁴ Ωcm, respectively. It was found that the samples with an [Ag]/[In] molar ratio of 0.89 in Ag-In metal precursors had a maximum photo-enhancement current density of 2.43 mAcm⁻² at an applied bias of +0.5 V vs. an Ag/AgCl electrode in contact with electrolyte containing 0.5 M K₂SO₄. The results show that high-quality AgInS₂ films can be obtained using the sulfurization of Ag-In metal precursors for photoelectrochemical (PEC) applications.     

Visible light induced hydrogen evolution on new hetero-system ZnFe2O4/SrTiO3

The physical properties and photoelectrochemical characterization of the spinel ZnFe₂O₄, elaborated by chemical route, have been investigated for the hydrogen production under visible light. The forbidden band is found to be 1.92 eV and the transition is indirectly allowed. The electrical conduction occurs by small polaron hopping with activation energy of 0.20 eV. p-type conductivity is evidenced from positive thermopower and cathodic photocurrent. The flat band potential (0.18 V_SCE) determined from the capacitance measurements is suitably positioned with respect to H₂O/H₂ level (−0.85 V_SCE). Hence, ZnFe₂O₄ is found to be an efficient photocatalyst for hydrogen generation under visible light. The photoactivity increases significantly when the spinel is combined with a wide band gap semiconductor. The best performance with a hydrogen rate evolution of 9.2 cm³ h⁻¹ (mg catalyst)⁻¹ occurs over the new hetero-system ZnFe₂O₄/SrTiO₃ in Na₂S₂O₃ (0.025 M) solution.     

Highly enhanced p-type electrical conduction in wide band gap Cu1+xAl1−xS2 polycrystals

As potential critical p-type transparency electrode materials applied in solar cells, a series of the Cu- and Zn-doped CuAlS₂ samples with band gaps over 3 eV have been prepared, and their optical and electrical properties have been thoroughly investigated. Conductivities as high as 250 S cm⁻¹ are achieved at a Cu doping level of 8 mol%, which are among the highest values known for p-type transparent materials and sufficient for collecting holes in solar cells. A high mobility of 21.2 cm² V⁻¹ s⁻¹ is also reached at the same doping level. The origin of conductivity enhancement by Cu doping and the structure-optoelectrical property relationship has been elucidated.     

Photoelectrochemical study of ZnIn2Se4 electrodes fabricated using selenization of RF magnetron sputtered Zn–In metal precursors

Polycrystalline ZnIn₂Se₄ samples are grown on glass substrates and fluorine-doped tin oxide coated glass substrates using the selenization of radio-frequency magnetron sputtered Zn–In metal alloys. The effect of the [Zn]/[Zn + In] molar ratio in the metal alloys on the physical and photoelectrochemical properties of the samples is investigated. X-ray diffraction patterns of samples reveal that the samples are polycrystalline tetragonal ZnIn₂Se₄. The thicknesses and direct band gaps of the samples are in the ranges of 1.15–1.44 μm and 1.68–1.81 eV, as obtained from surface profile measurements and transmittance/reflectance spectra, respectively. The flat-band potentials of the samples in 0.6 M K₂SO₃ electrolyte are in the range of −0.41 to −0.95 V vs. an Ag/AgCl reference electrode. The highest photoelectrochemical response of samples was 1.84 mA/cm² at an external potential of +1.0 V vs. an Ag/AgCl electrode in 0.6 M K₂SO₃ solution under illumination from a 300 W Xe lamp system with the light intensity set at 100 mW/cm².     

Influence of chelating agents on the growth and photoelectrochemical responses of chemical bath-synthesized AgIn5S8 film electrodes

An aqueous method for the deposition of silver-indium-sulfide ternary semiconductor film electrodes is presented. Various deposition parameters, such as reaction temperature and molar ratios of different chelating agents, were changed in order to grow uniform and adherent films on indium-tin-oxide glass substrates. With a reaction temperature higher than 65 °C, a film composed of AgIn₅S₈ was grown on the substrates in our experimental conditions. The direct and indirect energy band gaps of samples prepared in this study varied from 1.70 to 1.97 eV and 1.61 to 1.72 eV, respectively. The maximum photocurrent density of samples in this study reached 3.0 mA/cm² at an external potential of +1.0 V vs. a Pt electrode under illumination using a 300 W Xe lamp system with the light intensity set at 100 mW/cm².     

Visible-light-driven ZnIn2S4/CdIn2S4 composite photocatalyst with enhanced performance for photocatalytic H2 evolution

ZnIn₂S₄/CdIn₂S₄ composite photocatalysts (x = 0–1) were successfully synthesized via a hydrothermal route. Compositions of ZnIn₂S₄/CdIn₂S₄ composite photocatalysts were optimized according to the photocatalytic H₂ evolution rate. XRD patterns indicate the as-prepared samples are mixtures of hexagonal and cubic structures. FESEM and TEM images show that the as-prepared samples are composed of flower-like microspheres with wide distribution of diameter. There is obviously distinguishing distribution of Zn, Cd elements among the composite architectures. UV–vis absorption spectra of different compositions exhibit that absorption edges of ZnIn₂S₄/CdIn₂S₄ composites slightly move towards longer wavelengths with the increment of CdIn₂S₄ component. A typical time course of photocatalytic H₂ evolution from an aqueous Na₂SO₃ and Na₂S solution over unloaded and PdS-loaded ZnIn₂S₄/CdIn₂S₄ composite photocatalyst is carried out. The initial activity for H₂ evolution over 0.75 wt% PdS-loaded sample is up to 780 μmol h⁻¹. And the activity of unloaded sample also reaches 490 μmol h⁻¹ with consistent stability.     

Correlation between electrical properties and microstructure of nanocrystallized V2O5-P2O5 glasses

It was shown that by thermal nanocrystallization of a 90V₂O₅·10P₂O₅ glass one can obtain a novel nanomaterial exhibiting enhanced electronic conductivity. Using a combination of methods: DTA, SEM, XRD and impedance spectroscopy (IS), it was possible to find correlation between microstructure and electrical properties of the obtained material and to optimize conditions of its synthesis. The room temperature electronic conductivity of the nanocrystallized samples is σ₂₅ = 2 × 10⁻³ S cm⁻¹ and is by a factor of 25 higher than the conductivity of the as-received glass. The nanocrystallized material is thermally stable up to ca 400 °C, which is about 150 °C above the glass transition temperature of the original glass. Maximum electronic conductivity of the thermally treated samples reaches 2 × 10⁻¹ S cm⁻¹ at ca 400 °C. The activation energy for these samples (0.28 eV) are substantially lower than that found for the starting glass (0.34 eV). The experimental results were discussed in terms of a model proposed in this paper and based on a “core-shell” concept. The results obtained here can be important for the progress in the search of novel nanocrystalline cathode materials for applications in Li-ion batteries.     

TiO2 nanoparticles modified with ZnIn2S4 nanosheets and Co-Pi groups: Type II heterojunction and cocatalysts coexisted photoanode for efficient photoelectrochemical water splitting

The optimization of photoelectrode is the key issue for the efficient photoelectrochemical water splitting process. In this work, the TiO₂ photoanode is synthesized and modified with ZnIn₂S₄ nanosheets and Co-Pi cocatalyst (TiO₂/ZnIn₂S₄/Co-Pi) for a favorable photoelectrochemical performance. The synthesis and modification process of the TiO₂ photoanode are optimized. The physical and chemical characterizations indicate that the TiO₂ has a nano-cauliflower-like structure and rutile crystal form modified with a network hexagonal ZnIn₂S₄ nanosheets and amorphous Co-Pi groups. After optimization of the hydrothermal and annealing process, the optimized TiO₂ photoanode manifests a photocurrent density of 1.82 mA cm^?2, 1.73-fold of the pristine TiO₂ photoanode (1.05 mA cm^?2). With the surficial ZnIn₂S₄ and Co-Pi modification, the photocurrent density of the TiO₂/ZnIn₂S₄/Co-Pi photoanode is raised to 5.05 mA cm^?2, 5.32-fold of the optimized TiO₂ photoanode (1.82 mA cm^?2). The applied bias photon-to-current efficiency, the charge separation and injection efficiencies of the TiO₂/ZnIn₂S₄/Co-Pi photoanodes are 8.79, 3.40, and 1.64-folds of the optimized TiO₂ photoanode. Combined the Tauc plot, valence band XPS spectra, EIS and Mott-Schottky analysis, the PEC water splitting mechanism could be that: (i) the type II heterojunction formed by the TiO₂ and ZnIn₂S₄ semiconductors improves the charge separation/injection efficiencies; (ii) the Co-Pi groups facilitate the oxygen evolution kinetics; (iii) the Co-Pi groups and 2D ZnIn₂S₄ nanosheets synergistically enhance the charge separation efficiency. This investigation could offer a prospect of practical implementation for photoelectrochemical water splitting.     

Cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal synthesis of ZnIn2S4 as an efficient visible-light-driven photocatalyst for hydrogen production

A series of ZnIn₂S₄ photocatalysts was synthesized via a cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal method. These ZnIn₂S₄ products were characterized by X-ray diffraction (XRD), UV–visible absorption spectra (UV–vis) and scanning electron microscopy (FESEM). The effects of hydrothermal time and CTAB on the crystal structures, morphologies and optical properties of ZnIn₂S₄ products were discussed in detail. The photocatalytic activities of the as-prepared samples were evaluated by photocatalytic hydrogen production from water under visible-light irradiation. It was found that the photocatalytic activities of these ZnIn₂S₄ products decreased with the hydrothermal time prolonging while increased with the amount of CTAB increasing. The highest quantum yield at 420 nm of ZnIn₂S₄ photocatalyst, which was prepared through the CTAB (9.6 mmol)-assisted hydrothermal procedure for 1 h, was determined to be 18.4%. The optimum amount of Pt loaded for the ZnIn₂S₄ photocatalyst was about 1.0 wt%, under the present photocatalytic system.     

High-rate performance of all-solid-state lithium secondary batteries using Li4Ti5O12 electrode

The all-solid-state Li–In/Li₄Ti₅O₁₂ cell using the 80Li₂S·20P₂S₅ (mol%) solid electrolyte was assembled to investigate rate performances. It was difficult to obtain the stable performance at the charge current density of 3.8 mA cm⁻² in the all-solid-state cell. In order to improve the rate performance, the pulverized Li₄Ti₅O₁₂ particles were applied to the all-solid-state cell, which retained the reversible capacity of about 90 mAh g⁻¹ at 3.8 mA cm⁻². The 70Li₂S·27P₂S₅·3P₂O₅ glass–ceramic, which exhibits the higher lithium ion conductivity than the 80Li₂S·20P₂S₅ solid electrolyte, was also used. The Li–In/70Li₂S·27P₂S₅·3P₂O₅ glass–ceramic/pulverized Li₄Ti₅O₁₂ cell was charged at a current density higher than 3.8 mA cm⁻² and showed the reversible capacity of about 30 mAh g⁻¹ even at 10 mA cm⁻² at room temperature.     

Microwave-assisted hydrothermal synthesis of transition-metal doped ZnIn2S4 and its photocatalytic activity for hydrogen evolution under visible light

Aiming at the enhancement of photocatalytic activity for hydrogen evolution over ZnIn₂S₄, different transition metals (Cr, Mn, Fe, Co) are doped into the lattices of ZnIn₂S₄ to narrow the band gap. The doped ZnIn₂S₄ is characterized by XRD, Raman, UV-vis spectra, photoluminescence spectra, SEM and XPS techniques. The photocatalytic evaluation shows that Mn-doped ZnIn₂S₄ performs photocatalytic activity 20% higher than undoped ZnIn₂S₄, while Cr-, Fe-, and Co-doped ZnIn₂S₄ perform poorer activities in an order of Cr > Fe > Co. Based on the combined characterization results, the band structures of doped ZnIn₂S₄ are schematically depicted, which illustrates the different effects of transition-metal doping on the photocatalytic activity for hydrogen evolution. For Mn-doped ZnIn₂S₄, the enhancement of photocatalytic activity could be due to narrowed band gap induced by Mn doping. However, for Cr-, Fe-, and Co-doped ZnIn₂S₄, the suppressed photocatalytic activities should be attributed to the dopant-related impurity energy levels localizing the charge carriers or acting as non-radiative recombination centers for photoexcited electrons and holes. Hence, this study indicates that it is of great importance to make the in-depth investigation on the effects of band structures on the photocatalytic activity, especially for the doped semiconducting photocatalysts.     

Physical properties and photoresponse of Cu-Ag-In-S semiconductor electrodes created using chemical bath deposition

Cu-Ag-In-S solid solution semiconductor films were grown on indium-tin oxide-coated glass substrates using chemical bath deposition. New procedures for the growth of Cu-Ag-In-S semiconductor films are presented. The optical, electrical, and photoelectrochemical performances of the ternary CuInS₂, AgIn₅S₈, and their solid solutions are investigated. The X-ray diffraction patterns of samples reveal a change in the crystal phase of the samples from the tetragonal CuInS₂ to the cubic AgIn₅S₈ phase with an increase in the [Ag/(Cu+Ag)] molar ratio in precursor solutions. The thicknesses and band gaps of the samples, determined from the surface profile measurements and transmittance spectra, are in the ranges of 841-2107 nm and 1.42-1.75 eV, respectively. The highest photoresponse was observed in the sample with [Ag/(Ag+Cu)]=0.4 (sample (d)) under illumination with a white light intensity of 100 mW/cm². The results show that Cu-Ag-In-S film electrodes have potential in solar to hydrogen applications.     

Electrodeposited zirconium-doped α-Fe2O3 thin film for photoelectrochemical water splitting

Nanostructured hematite thin films were doped with zirconium successfully using electrodeposition method for their implementation as photoanode in photoelectrochemical (PEC) cell for hydrogen generation. XRD, Raman, XPS, SEM and UV-visible spectroscopy techniques were used to characterize the thin films. Highest photocurrent density of 2.1 mA/cm² at 0.6 V/SCE was observed for 2.0 at.% Zr⁴⁺ doped α-Fe₂O₃ sample with solar to hydrogen conversion efficiency of 1.43%. Flatband potential (−0.74 V/SCE) and donor density (2.6 × 10²¹ cm⁻³) were found to be maximum for the same sample. These results suggest substantial potential of hematite thin films with controlled doping of zirconium in PEC water splitting applications.     

Formation of the high lithium ion conducting phase from mechanically milled amorphous Li2S-P2S5 system

The fast ionic conducting structure similar to thio-Lithium Super Ionic Conductor (LISICON) phase is synthesized in the Li₂S-P₂S₅ system. The Li₂S-P₂S₅ glass-ceramics with the composition of xLi₂S·(100−x)P₂S₅ (75 ≤ x ≤ 80) are prepared by the heat-treatment of mechanically milled amorphous sulfide powders. In the binary Li₂S-P₂S₅ system, 78.3Li₂S·21.7P₂S₅ glass ceramic prepared by mechanical milling and subsequent heat-treatment at 260 °C for 3 h shows the highest conductivity of 6.3 × 10⁻⁴ S cm⁻¹ at room temperature and the lowest activation energy for conduction of 30.5 kJ mol⁻¹. The enhancement of conductivity with increasing x up to 78.3 is probably caused by the introduction of interstitial lithium ions at the Li sites which affects the Li ion distribution. The prepared electrolyte exhibits the lithium ion transport number of almost unity and voltage stability of 5 V vs. Li at room temperature.     

Photoaction, temperature and O2 depletion effects in fullerene photoelectrochemical solar cells

It is demonstrated that higher temperature and C₆₀ oxygen depletion increase the photocurrent of fullerene photoelectrochemical solar cells (PEC). Fullerene/iodide electrolyte PEC consisting of intrinsic single crystal C₆₀ in either aqueous 3 M KI, 0.01 M I₂, or 0.1 M tetrabutyl ammonium iodide, 0.3 M LiClO₄ in acetonitrile solution, drive regenerative photoinduced iodide oxidation. The photocurrent is increased by an order of magnitude (to 6.4 μA/cm² under 100 mW/cm² illumination) by an increase of the aqueous cell temperature from 24°C to 82°C. A similar order of magnitude increase in photocurrent is accomplished by O₂ depletion pretreatment (24 h at 400°C in Ar) of the C₆₀ to improve conductivity. However, this latter treatment also irreversibly increases the cell dark current. The spectral action of single crystal C₆₀ is also probed, through the generated photoelectrochemical current in iodide, ferricyanide and sulfuric electrolytes as a function of wavelength. Band edges are observed at 720 nm (1.7 eV) and 560 nm (2.2 eV), and a substantial peak photocurrent response occurs at 395 nm (3.1 eV) and decreases at shorter wavelengths.     

Preparation of Li2S-GeSe2-P2S5 electrolytes by a single step ball milling for all-solid-state lithium secondary batteries

Glass-ceramic and glass Li₂S-GeSe₂-P₂S₅ electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li_4−xGe_1−xP_xS_2(1+x)Se_2(1−x) with/without heat treatment (HT) from x = 0.55 to x = 1.00 were systematically investigated. Structural analysis by X-ray diffraction (XRD) showed gradual increase of the lattice constant followed by significant phase change with increasing GeSe₂. HT also affected the crystallinity. Incorporation of GeSe₂ in Li₂S-P₂S₅ kept high conductivity with a maximum value of 1.4 × 10⁻³ S cm⁻¹ at room temperature for x = 0.95 in Li_4−xGe_1−xP_xS_2(1+x)Se_2(1−x) without HT. All-solid-state LiCoO₂/Li cells using Li₂S-GeSe₂-P₂S₅ as solid-state electrolytes (SE) were tested by constant-current constant-voltage (CCCV) charge-discharge cycling at a current density of 50 μA cm⁻² between 2.5 and 4.3 V (vs. Li/Li⁺). In spite of the extremely high conductivity of the SE, LiCoO₂/Li cells showed a large irreversible reaction especially during the first charging cycle. LiCoO₂ with SEs heat-treated at elevated temperature exhibited a capacity over 100 mAh g⁻¹ at the second cycle and consistently improved cycle retention, which is believed to be due to the better interfacial stability.     

Characterization of new heterosystem CuFeO2/SnO2 application to visible-light induced hydrogen evolution

Photo-assisted H₂ evolution has been realized over the new heterosystem CuFeO₂/SnO₂ without any noble metal and was studied in connection with some physical parameters. The delafossite CuFeO₂ has been prepared by thermal decomposition from various salts. The polarity of generated voltage is positive indicating that the materials exhibit p-type conductivity whereas the electroneutrality is achieved by oxygen insertion. The plot of the logarithm (conductivity) vs. T⁻¹ gives average activation energy of 0.12 eV. CuFeO₂ is a narrow band gap semiconductor with an optical gap of 1.32 eV. The oxide was characterized photoelectrochemically; its conduction band (−1.09 V_RHE) is located below that of SnO₂ (−0.86 V_RHE) at pH ∼13.5 itself more negative than the H₂O/H₂ level leading to a thermodynamically favorable H₂ evolution under visible irradiation. The sensitizer CuFeO₂, working as an electron pump, is stable towards photocorrosion by hole consumption reactions involving the reducing agents X²⁻ (=S₂O₃²⁻ and SO₃²⁻). The photoactivity was dependent on the precursor and the best performance (0.026 ml h⁻¹ mg⁻¹) was obtained in S₂O₃²⁻ (pH ∼13.5) over CuFeO₂ synthesized from nitrate with a mass ratio (CuFeO₂/SnO₂) equal to unity. A quantum yield of 0.5% was obtained under polychromatic light. H₂ liberation occurs concomitantly with the oxidation of S₂O₃²⁻ to dithionate and sulfate. The tendency towards saturation, in a closed system, is mainly ascribed to the competitive reduction of the end product S₂O₆²⁻.