Onset potential behavior in α-Fe2O3 photoanodes: the influence of surface and diffusion Sn doping on the surface states.

The onset potential is an important parameter that affects the water oxidation performance of photoanodes. Herein, we investigated the behavior of the photocurrent onset potential of hematite (α-Fe2O3) photoanodes by incorporating Sn(4+) cations via external (surface overlayer) or self (underlying FTO substrate) doping. The α-Fe2O3/FTO photoanodes fabricated at both low (550 °C) and high (800 °C) temperatures were chosen for surface Sn(4+) doping (0-10 mM SnCl4). At the lower temperature, Sn(4+) doping enriched the conductivity of α-Fe2O3/FTO, thereby improving the photocurrent response at higher applied potentials. In addition, the surface incorporation of Sn(4+) shifted the onset of the water oxidation reaction in the positive direction. In the case of high temperature-annealed photoanodes, Sn leaching (resulting from FTO deformation) also affected the water oxidation performance of the photoanodes. This was caused by the loss of FTO conductivity as well as by the unfavourable surface properties due to the excessive incorporation of Sn ions (SnOx) into the hematite matrix. The anodic shift of the onset potential in both cases was due to the decreased surface state capacitance, as revealed by electrochemical impedance spectroscopy (EIS). The different annealing conditions, where lattice distortion and deformation-directed Sn diffusion-doping occur, were also found to affect the surface states associated with hematite and its water oxidation onset potential. Crystallographic analyses made by synchrotron XRD further support the results obtained from the EIS study. Sn doping was found to be concurrent with the respective changes in the (104) and (110) planes of hematite, which are associated with the onset potential-driving surface states and the photocurrent-boosting electron mobility, respectively.

Degree of Cure, Microstructures, and Properties of Carbon/Epoxy Composites Processed via Frontal Polymerization.

The frontal polymerization (FP) of carbon/epoxy (C/Ep) composites is investigated, considering FP as a viable route for the additive manufacturing (AM) of thermoset composites. Neat epoxy (Ep) resin-, short carbon fiber (SCF)-, and continuous carbon fiber (CCF)-reinforced composites are considered in this study. The evolution of the exothermic reaction temperature, polymerization frontal velocity, degree of cure, microstructures, effects of fiber concentration, fracture surface, and thermal and mechanical properties are investigated. The results show that exothermic reaction temperatures range between 110 °C and 153 °C, while the initial excitation temperatures range from 150 °C to 270 °C. It is observed that a higher fiber content increases cure time and decreases average frontal velocity, particularly in low SCF concentrations. This occurs because resin content, which predominantly drives the exothermic reaction, decreases with increased fiber content. The FP velocities of neat Ep resin- and SCF-reinforced composites are seen to be 0.58 and 0.50 mm/s, respectively. The maximum FP velocity (0.64 mm/s) is observed in CCF/Ep composites. The degree of cure (αc) is observed to be in the range of 70% to 85% in FP-processed composites. Such a range of αc is significantly low in comparison to traditional composites processed through a long cure cycle. The glass transition temperature (Tg) of neat epoxy resin is seen to be approximately 154 °C, and it reduces slightly to a lower value (149 °C) for SCF-reinforced composites. The microstructures show significantly high void contents (12%) and large internal cracks. These internal cracks are initiated due to high thermal residual stress developed during curing for non-uniform temperature distribution. The tensile properties of FP-cured samples are seen to be inferior in comparison to autoclave-processed neat epoxy. This occurs mostly due to the presence of large void contents, internal cracks, and a poor degree of cure. Finally, a highly efficient and controlled FP method is desirable to achieve a defect-free microstructure with improved mechanical and thermal properties.

Colloidal Synthesis, Characterization, and Photoconductivity of Quasi-Layered CuCrS2 Nanosheets.

The current need to accelerate the adoption of photovoltaic (PV) systems has increased the need to explore new nanomaterials that can harvest and convert solar energy into electricity. Transition metal dichalcogenides (TMDCs) are good candidates because of their tunable physical and chemical properties. CuCrS2 has shown good electrical and thermoelectrical properties; however, its optical and photoconductivity properties remain unexplored. In this study, we synthesized CuCrS2 nanosheets with average dimensions of 43.6 ± 6.7 nm in length and 25.6 ± 4.1 nm in width using a heat-up synthesis approach and fabricated films by the spray-coating method to probe their photoresponse. This method yielded CuCrS2 nanosheets with an optical bandgap of ~1.21 eV. The fabricated film had an average thickness of ~570 nm, exhibiting a net current conversion efficiency of ~11.3%. These results demonstrate the potential use of CuCrS2 as an absorber layer in solar cells.

Bonding interface boosts the intrinsic activity and durability of NiSe@Fe2O3 heterogeneous electrocatalyst for water oxidation.

The intrinsic activity and durability of oxygen evolution reaction (OER) electrocatalysts are mainly dominated by the surface and interface properties of active materials. Herein, a core-shell heterogeneous structure (NF/NiSe@Fe2O3) is fabricated via two-step hydrothermal method, which exhibits a low overpotential of 220 mV (or 282 mV) at 10 mA/cm2 (or 200 mA/cm2), a small Tafel slope of 36.9 mV/dec, and long-term stability (~230 h) in 1 mol/L KOH for OER. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy reveal the (oxy)hydroxide-rich surface and strong coupling interface between NiSe and Fe2O3 via the Fe-Se bond. Density functional theory calculation suggests that the d-band center and electronic state of NiSe@Fe2O3 heterojunction are well optimized due to the formation of Fe-Se bond, which is favorable for the enhanced OER activity because of the easy adsorption of oxygen-containing intermediates and desorption of O2 in the OER process. In addition, the unique core-shell structure and robust bonding interface are responsible for the good stability for OER. This work provides fundamental insights on the bonding effect that determine the performance of OER electrocatalyst.

Investigating the Redox Properties of Two-Dimensional MoS2 Using Photoluminescence Spectroelectrochemistry and Scanning Electrochemical Cell Microscopy.

Control over photophysical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) is the key to advance their applications in next-generation optoelectronics. Although chemical doping and surface modification with plasmonic metals have been reported to tune the photophysical and catalytic properties of 2D TMDs, there have been few reports of tuning optical properties using dynamic electrochemical control of electrode potential. Herein, we report (1) the photoluminescence (PL) enhancement and red-shift in the PL spectrum of 2D MoS2, synthesized by chemical vapor deposition and subsequent transfer onto an indium tin oxide electrode, upon electrochemical anodization and (2) spatial heterogeneities in its photoelectrochemical (PEC) activities. Spectroelectrochemistry shows that positive electrochemical bias causes an initial ten-fold increase in the PL intensity followed by a quick decrease in the enhancement. The PL enhancement and spectrum red-shift are associated with the decrease in nonradiative decay rates of excitons formed upon electrochemical anodization of 2D MoS2. Additionally, scanning electrochemical cell microscopy (SECCM) study shows that the 2D MoS2 crystal is spatially sensitive to PEC oxidation at positive potentials. SECCM also shows a photocurrent increase caused by spatially heterogeneous edge-type defect sites of the crystal.

Precious metal-free solar-to-fuel generation: SSM-DSCs powering water splitting with NanoCOT and NiMoZn electrocatalysts.

A precious metal-free sequential series multijunction dye-sensitized solar cell (SSM-DSC)-powered water electrolysis system is demonstrated using NanoCOT and NiMoZn electrodes. A stable 3.9% solar-to-hydrogen (STH) efficiency is achieved using a recently reported black organic dye and graphene electrodes for DSCs.

High-Throughput Screening and Surface Interrogation Studies of Au-Modified Hematite Photoanodes by Scanning Electrochemical Microscopy for Solar Water Splitting.

Au-modified hematite photoanode was screened for photoelectrochemical (PEC) water oxidation by the scanning electrochemical microscopy (SECM) technique with a scanning probe of the optical fiber for visible light irradiation of the photoanode substrate. The Au-modified hematite exhibited an enhancement in the photocurrent up to 3% (at. %), and the performance drop was observed with 4-10% (at. %) of Au modification. Subsequently, pristine and Au-modified hematite thin-film photoanodes were fabricated by the spin-coating method to confirm the results of SECM. The PEC response confirms that 3% (at. %) of Au is the optimum concentration to provide the best enhancement of PEC water oxidation with a ∼6-fold increase compared to the pristine hematite sample. Direct Au oxidation, charge recombination, and strong light absorption by Au are responsible for the decrease in PEC performance when the Au percentage is above 3%. The pristine and Au-modified hematite materials were also characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. Au was found to exist in the form of embedded metallic nanoparticles in the modified hematite. Mott-Schottky analysis of the bulk samples confirms an improvement in charge carrier density for the Au-modified hematite. Additionally, there was little plasmonic enhancement as evidenced by UV-vis spectroscopy, with a minimal contribution toward photoactivity. Surface interrogation SECM quantitatively probed the reactive surface states (RSSs) such as OH• formed on hematite and Au-modified hematite surfaces during water oxidation. The coverage of RSSs was found to increase with the substrate potential. The interrogated charge under the dark condition for the 3% Au-modified hematite sample is higher than the pristine hematite sample because of the enhanced electronic conductivity of the hematite film.

Photoelectrochemical study of carbon-modified p-type Cu2O nanoneedles and n-type TiO2-x nanorods for Z-scheme solar water splitting in a tandem cell configuration.

Nanostructured photoelectrodes with a high surface area and tunable optical and electrical properties can potentially benefit a photoelectrochemical (PEC) water splitting system. The PEC performance of a nanostructured photoelectrode is usually quantified in a standard three-electrode configuration under potential-assisted conditions because of the additional overpotentials for the two half-reactions of water splitting. However, it is a necessity to fully recognize their potential to split water under unassisted conditions by designing a tandem cell that can provide sufficient voltage to split water. Herein, we present a tandem cell consisting of carbon-modified cuprous oxide (C/Cu2O) nanoneedles and oxygen-deficient titanium dioxide (TiO2-x ) nanorods for unassisted solar water splitting. The synthesized photoelectrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and electrochemical impedance spectroscopy (EIS) techniques. The tandem cell performance was analyzed by measuring the current-voltage responses in various photoelectrode configurations to validate the collective contributions of both photoelectrodes to unassisted solar water splitting. The PEC properties of C/Cu2O nanoneedles coupled with TiO2-x nanorods in a tandem configuration exhibited a photocurrent density of 64.7 µA cm-2 in the absence of any redox mediator and external bias. This photocurrent density can be further enhanced with an application of external bias. Moreover, the heterojunction formed by the above-mentioned nanostructured photoelectrodes in intimate contact and in the absence of water exhibited 2 µA cm-2 UV photoresponsivity at 1.5 V with promising rectifying characteristics of a diode.

Correction: Photoelectrochemical study of carbon-modified p-type Cu2O nanoneedles and n-type TiO2-x nanorods for Z-scheme solar water splitting in a tandem cell configuration.

[This corrects the article DOI: 10.1039/C8RA09403A.].

Nanocrystals of CuMSnS4 (M = In or Ga) for solar energy conversion applications.

Quaternary MIMIIIMIVXVI4 (I-III-IV-VI4) chalcogenides obtained by cross-substitution of binary and ternary compounds remain relatively unexplored. We have for the first-time synthesized wurtzite and defect chalcopyrite phases of CuMSnS4 (M = In or Ga) in the form of nanocrystals. Optical measurements show that the CuMSnS4 (M = In or Ga) nanocrystals exhibit strong visible light absorption with band gap values between 1.15 and 1.4 eV, suitable for solar energy conversion applications.

Data on the effect of improved TiO2/FTO interface and Ni(OH)2 cocatalyst on the photoelectrochemical performances and stability of CdS cased ZnIn2S4/TiO2 heterojunction.

This data article presents the experimental evidences of the effect of TiO2-fluorine doped tin oxide interface annealing and Ni(OH)2 cocatalysts on the photoelectrochemical, structural, morphological and optical properties of Ni(OH)2/CdS/ZnIn2S4/TiO2 heterojunction. The Raman spectroscopy exhibits the sharp features of the rutile phase of TiO2 and in agreement with the X-ray diffraction data. The band gap energy of the 500 °C sample was found to be 3.12 eV, further it was increased to 3.20, 3.22 eV for samples annealed at 600 and 700 °C respectively. The decrease in the band gap energy at 500 °C related to the oxygen vacancies and was analysed by photoluminescence spectroscopy analysis. The synthesis, characterization methods and other experimental details of TiO2 based heterostructure are also provided. The presence of CdS and ZnIn2S4 coating on surface of TiO2 electrodes providing a high surface area, extended visible absorption and helps to improve the change separation. This data article contains data related to the research article entitled "Highly efficient and stable 3D Ni(OH)2/CdS/ZnIn2S4/TiO2 heterojunction under solar light: Effect of an improved TiO2/FTO interface and cocatalyst" (Mahadik et al., 2017) [1].

Enhanced photoelectrochemical performance of internally porous Au-embedded α-Fe2O3 photoanodes for water oxidation.

We report internally porous Au-embedded hematite on FTO using CTAB. Incorporation of Au and CTAB synergistically improved the photocurrent of hematite by 63% at 1.23 VRHE in 1 M NaOH under standard illumination conditions. The performance enhancement is due to the increased donor density and optical properties of internally porous networks and plasmonic absorption of hematite.

A Synergistic Effect of Surfactant and ZrO2 Underlayer on Photocurrent Enhancement and Cathodic Shift of Nanoporous Fe2O3 Photoanode.

Augmenting the donor density and nanostructure engineering are the crucial points to improve solar water oxidation performance of hematite (α-Fe2O3). This work addresses the sluggish water oxidation reaction associated with hematite photoanode by tweaking its internal porosity. The porous hematite photoanodes are fabricated by a novel synthetic strategy via pulse reverse electrodeposition (PRED) method that involves incorporation of a cationic CTAB surfactant in a sulfate electrolyte and spin-coated ZrO2 underlayer (UL) on FTO. CTAB is found to be beneficial in promoting the film growth rate during PRED. Incorporation of Zr(4+) ions from ZrO2 UL and Sn(4+) ions from FTO into the Fe2O3 lattice via solid-state diffusion reaction during pertinent annihilation of surfactant molecules at 800 °C produced internally porous hematite films with improved carrier concentration. The porous hematite demonstrated a sustained photocurrent enhancement and a significant cathodic shift of 130 mV relative to the planar hematite under standard illumination conditions (AM 1.5G) in 1 M NaOH electrolyte. The absorption, electrochemical impedance spectroscopy and Mott-Schottky analyses revealed that the ZrO2 UL and CTAB not only increased the carrier density and light harvesting but also accelerated the surface oxidation reaction kinetics, synergistically boosting the performance of internally porous hematite photoanodes.

PRED treatment mediated stable and efficient water oxidation performance of the Fe2O3 nano-coral structure.

Herein, we demonstrate that an electrochemical surface treatment of Fe foil with simple pulse reverse electrodeposition (PRED) prior to thermal oxidation can substantially enhance the photoelectrochemical (PEC) stability and water splitting performance of Fe2O3/Fe photoanodes. Comprehensive structural (XRD, FESEM, and HRTEM), compositional (XPS depth profiling), and electrochemical (EIS and Mott-Schottky) analyses were performed to understand the effect of PRED treatment on the PEC performance of fabricated photoanodes. It is revealed that air-exposed Fe foil is prone to formation of a loosely bound surface oxide layer that, upon annealing at 800 °C, results in an unstable Fe2O3 nano-flake (2-3 µm long) morphology. In contrast, when such Fe foil is pre-treated with PRED to etch the loosely bound oxide layer, adherent inverse-opal-like nano-coral structures (60-100 nm thin) are formed. In addition to stability improvement, PRED-treatment also assists in exposing the photocatalytically active high index [104] facet sites of hematite. Thin hematite nano-coral structures with high index [104] facet sites significantly improved the separation of photo-generated charge carriers and oxygen evolution kinetics, resulting in performance enhancement with excellent photocurrent stability for extended duration in a 1 M NaOH solution under one sun illumination. The net photocurrent density for nano-coral morphology was 0.813 mA cm(-2) at 1.23 V vs. RHE, which is the highest reported value for pristine hematite photoanodes fabricated from Fe foil.

Photoelectrochemical, impedance and optical data for self Sn-diffusion doped Fe2O3 photoanodes fabricated at high temperature by one and two-step annealing methods.

The optical, morphological and photoelectrochemical (PEC) properties of transition metal oxide semiconductors are important to understand their influence on water oxidation performance. Herein, we provide experimental evidences for a better understanding of the factors that dictate the interactions of Sn-diffusion doping on the PEC properties of Fe2O3 photoanodes fabricated at high temperature by one- and two-step annealing methods. The synthesis, characterization methods and other experimental details are provided. Limited previous information on the PEC and electrochemical impedance spectroscopic studies has been published. This data article contains , figures and methods related to the research article by Shinde et al. (2015) [1]. Here, we provide a further set of the obtained experimental data results.