Achieving Highly Efficient Photoelectrochemical Water Oxidation with a TiCl4 Treated 3D Antimony-Doped SnO2 Macropore/Branched ?-Fe2O3 Nanorod Heterojunction Photoanode.
ABSTRACT: Utilizing photoelectrochemical (PEC) cells to directly collecting solar energy into chemical fuels (e.g., H2 via water splitting) is a promising way to tackle the energy challenge. ?-Fe2O3 has emerged as a desirable photoanode material in a PEC cell due to its wide spectrum absorption range, chemical stability, and earth abundant component. However, the short excited state lifetime, poor minority charge carrier mobility, and long light penetration depth hamper its application. Recently, the elegantly designed hierarchical macroporous composite nanomaterial has emerged as a strong candidate for photoelectrical applications. Here, a novel 3D antimony-doped SnO2 (ATO) macroporous structure is demonstrated as a transparent conducting scaffold to load 1D hematite nanorod to form a composite material for efficient PEC water splitting. An enormous enhancement in PEC performance is found in the 3D electrode compared to the controlled planar one, due to the outstanding light harvesting and charge transport. A facile and simple TiCl4 treatment further introduces the Ti doping into the hematite while simultaneously forming a passivation layer to eliminate adverse reactions. The results indicate that the structural design and nanoengineering are an effective strategy to boost the PEC performance in order to bring more potential devices into practical use.
Project description:The replacement of oxide semiconducting TiO₂ nano particles with one dimensional TiO₂ nanotubes (TNTs) has been used for improving the electron transport in the dye-sensitized solar cells (DSSCs). Although use of one dimensional structure provides the enhanced photoelectrical performance, it tends to reduce the adsorption of dye on the TiO₂ surface due to decrease of surface area. To overcome this problem, we investigate the effects of TiCl₄ treatment on DSSCs which were constructed with composite films made of TiO₂ nanoparticles and TNTs. To find optimum condition of TNTs concentration in TiO₂ composites film, series of DSSCs with different TNTs concentration were made. In this optimum condition (DSSCs with 10 wt% of TNT), the effects of post treatment are compared for different TiCl₄ concentrations. The results show that the DSSCs using a TiCl₄ (90 mM) post treatment shows a maximum conversion efficiency of 7.83% due to effective electron transport and enhanced adsorption of dye on TiO₂ surface.
Project description:This study successfully manufactured a p-n heterojunction hematite (?-Fe<sub>2</sub>O<sub>3</sub>) structure with molybdenum disulfide (MoS<sub>2</sub>) to address the electron-hole transfer problems of conventional hematite to enhance photoelectrochemical (PEC) performance. The two-dimensional MoS<sub>2</sub> nanosheets were prepared through ultrasonication-assisted liquid-phase exfoliation, after which the concentration, number of layers, and thickness parameters of the MoS<sub>2</sub> nanosheets were respectively estimated by UV-vis, HRTEM and AFM analysis to be 0.37 mg/ml, 10-12 layers and around 6 nm. The effect of heterojunction ?-Fe<sub>2</sub>O<sub>3</sub>/MoS<sub>2</sub> and the role of the ultrasonication process were investigated by the optimized concentration of MoS<sub>2</sub> in the forms of bulk and nanosheet on the surface of the ?-Fe<sub>2</sub>O<sub>3</sub> electrode while measuring the PEC performance. The best photocurrent density of the ?-Fe<sub>2</sub>O<sub>3</sub>/MoS<sub>2</sub> photoanode was obtained at 1.52 and 0.86 mA.cm<sup>-2</sup> with good stability at 0.6 V vs. Ag/AgCl under 100 mW/cm<sup>2</sup> (AM 1.5) illumination from the back- and front-sides of ?-Fe<sub>2</sub>O<sub>3</sub>/MoS<sub>2</sub>; these values are 13.82 and 7.85-times higher than those of pure ?-Fe<sub>2</sub>O<sub>3</sub>, respectively. The results of electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis showed increased donor concentration (2.6-fold) and decreased flat band potential (by 20%). Moreover, the results of IPCE, ABPE, and OCP analyses also supported the enhanced PEC performance of ?-Fe<sub>2</sub>O<sub>3</sub>/MoS<sub>2</sub> through the formation of a p-n heterojunction, leading to a facile electron-hole transfer.
Project description:<h4>Objective</h4>Although photoelectrochemical (PEC) water splitting heralds the emergence of the hydrogen economy, the need for external bias and low efficiency stymies the widespread application of this technology. By coupling water splitting (in a PEC cell) to a microbial fuel cell (MFC) using <i>Escherichia coli</i> as the biocatalyst, this work aims to successfully demonstrate a sustainable hybrid PEC-MFC platform functioning solely by biocatalysis and solar energy, at zero bias. Through further chemical modification of the photo-anode (in the PEC cell) and biofilm (in the MFC), the performance of the hybrid system is expected to improve in terms of the photocurrent generated and hydrogen evolved.<h4>Methods</h4>The hybrid system constitutes the interconnected PEC cell with the MFC. Both PEC cell and MFC are typical two-chambered systems housing the anode and cathode. Au-TiO<sub>2</sub> hollow spheres and conjugated oligoelectrolytes were synthesised chemically and introduced to the PEC cell and MFC, respectively. Hydrogen evolution measurements were performed in triplicates.<h4>Results</h4>The hybrid PEC-MFC platform generated a photocurrent density of 0.35?mA/cm<sup>2</sup> (~70× enhancement) as compared with the stand-alone P25 standard PEC cell (0.005?mA/cm<sup>2</sup>) under one-sun illumination (100?mW/cm<sup>2</sup>) at zero bias (0?V vs. Pt). This increase in photocurrent density was accompanied by continuous H<sub>2</sub> production. No H<sub>2</sub> was observed in the P25 standard PEC cell whereas H<sub>2</sub> evolution rate was ~3.4??mol/h in the hybrid system. The remarkable performance is attributed to the chemical modification of <i>E. coli</i> through the incorporation of novel conjugated oligoelectrolytes in the MFC as well as the lower recombination rate and higher photoabsorption capabilities in the Au-TiO<sub>2</sub> hollow spheres electrode.<h4>Conclusions</h4>The combined strategy of photo-anode modification in PEC cells and chemically modified MFCs shows great promise for future exploitation of such synergistic effects between MFCs and semiconductor-based PEC water splitting.
Project description:Solar powered hydrogen evolution reaction (HER) is one of the key reactions in solar-to-chemical energy conversion. It is desirable to develop photocathodic materials that exhibit high activity toward photoelectrochemical (PEC) HER at more positive potentials because a higher potential means a lower overpotential for HER. In this work, the Cu<sub>2</sub>O/CuO bilayered composites were prepared by a facile method that involved an electrodeposition and a subsequent thermal oxidation. The resulting Cu<sub>2</sub>O/CuO bilayered composites exhibited a surprisingly high activity and good stability toward PEC HER, expecially at high potentials in alkaline solution. The photocurrent density for HER was 3.15?mA·cm<sup>-2</sup> at the potential of 0.40?V vs. RHE, which was one of the two highest reported at the same potential on copper-oxide-based photocathode. The high photoactivity of the bilayered composite was ascribed to the following three advantages of the Cu<sub>2</sub>O/CuO heterojunction: (1) the broadened light absorption band that made more efficient use of solar energy, (2) the large space-charge-region potential that enabled a high efficiency for electron-hole separation, and (3) the high majority carrier density that ensured a faster charge transportation rate. This work reveals the potential of the Cu<sub>2</sub>O/CuO bilayered composite as a promising photocathodic material for solar water splitting.
Project description:In this study, an anatase/rutile mixed-phase titanium dioxide (TiO<sub>2</sub>) hierarchical network deposited with Au nanoparticles (Au/TiO<sub>2</sub> ARHN) was synthesized using a facile hydrothermal method followed by a simple calcination step. Such a unique structure was designed for improving the light harvest, charge transportation/separation, and the performance of photo-electro-chemical (PEC) cells. The properties of the as-synthesized Au/TiO<sub>2</sub> ARHN in PEC cells were investigated by electrochemical measurements using a three-electrode system in a 1?M NaOH electrolyte. Remarkably, a 4.5-folds enhancement of the photocurrent for Au/TiO<sub>2</sub> ARHN was observed as compared to that for TiO<sub>2</sub> nanowire (NW), under AM1.5G solar illumination, suggesting its potential application in PEC cells. A mechanism has been proposed to explain the high photocurrent of Au/TiO<sub>2</sub> ARHN in PEC water splitting.
Project description:Addressing the intrinsic charge transport limitation of metal oxides has been of significance for pursuing viable PEC water splitting photoelectrodes. Growing a photoelectrode with conductive nanoobjects embedded in the matrix is promising for enhanced charge transport but remains a challenge technically. We herein show a strategy of embedding laser generated nanocrystals in BiVO<sub>4</sub> photoanode matrix, which achieves photocurrent densities of up to 5.15?mA?cm<sup>-2</sup> at 1.23 V<sub>RHE</sub> (from original 4.01?mA?cm<sup>-2</sup>) for a single photoanode configuration, and 6.22?mA?cm<sup>-2</sup> at 1.23?V<sub>RHE</sub> for a dual configuration. The enhanced performance by such embedding is found universal owing to the typical features of laser synthesis and processing of colloids (LSPC) for producing ligand free nanocrystals in desired solvents. This study provides an alternative to address the slow bulk charge transport that bothers most metal oxides, and thus is significant for boosting their PEC water splitting performance.
Project description:Herein, we have successfully constructed a solid-state Z-scheme photosystem with enhanced light absorption capacity by combining the optoelectrical properties of AgNPs with those of the MoS<sub>2</sub>/RGO/NiWO<sub>4</sub> (Ag-MRGON) heterostructure. The Ag-MRGON Z-scheme system demonstrates improved photo-electrochemical (PEC) water-splitting performance in terms of applied bias photon-to-current conversion efficiency (ABPE), which is 0.52%, and 17.3- and 4.3-times better than those of pristine MoS<sub>2</sub> and MoS<sub>2</sub>/NiWO<sub>4</sub> photoanodes, respectively. The application of AgNPs as an optical property enhancer and RGO as an electron mediator improved the photocurrent density of Ag-MRGON to 3.5 mA/cm<sup>2</sup> and suppressed the charge recombination to attain the photostability of ?2 h. Moreover, the photocurrent onset potential of the Ag-MRGON heterojunction (<i>i.e.,</i> 0.61 V<sub>RHE</sub>) is cathodically shifted compared to those of NiWO<sub>4</sub> (0.83 V<sub>RHE</sub>), MoS<sub>2</sub> (0.80 V<sub>RHE</sub>), and MoS<sub>2</sub>/NiWO<sub>4</sub> heterojunction (0.73 V<sub>RHE</sub>). The improved PEC water-splitting performance in terms of ABPE, photocurrent density, and onset potential is attributed to the facilitated charge transfer through the RGO mediator, the plasmonic effect of AgNPs, and the proper energy band alignments with the thermodynamic potentials of hydrogen and oxygen evolution.
Project description:Nowadays, increasing awareness of environment and fossil fuels protection stimulates intensive research on clean and renewable sources of energy. Production of hydrogen from water through solar-driven splitting reactions is one of the most promising approaches in the field of photoelectrochemistry (PEC). In this work we have fabricated well-aligned, highly-ordered, smooth-mouth TiO<sub>2</sub> nanotube arrays (TNAs) in a two-step anodization process of titanium foil, which were then used as photoelectrodes for PEC water splitting. It demonstrates for the first time correspondence between non-linear component characteristics of multiscale rough surface and crystalline structure of annealed TNAs measured at various fabrication stages and their photoelectrochemical response. The as-anodized TNAs with isotropic surface (deduced from AFM and SEM images) and largest figure of merit (according to their PEC performance) were annealed at 450?°C in air. Scale-invariant descriptors of the surface structure of the deposits involved: fractal dimension, corner frequency, roughness, size of nanostructures and their dominant habits. Moreover, X-ray diffraction data processed using the Rietveld method confirmed co-existence of various oxides, for example: TiO<sub>2</sub> in the form of anatase, TiO and Ti<sub>3</sub>O<sub>5</sub> phases in the TNAs under study pointing that previous well-established mechanisms of the TNA growth were to certain degree incomplete.
Project description:Hematite has a great potential as a photoanode for photoelectrochemical (PEC) water splitting by converting solar energy into hydrogen fuels, but the solar-to-hydrogen conversion efficiency of state-of-the-art hematite photoelectrodes are still far below the values required for practical hydrogen production. Here, we report a core-shell formation of gradient tantalum-doped hematite homojunction nanorods by combination of hydrothermal regrowth strategy and hybrid microwave annealing, which enhances the photocurrent density and reduces the turn-on voltage simultaneously. The unusual bi-functional effects originate from the passivation of the surface states and intrinsic built-in electric field by the homojunction formation. The additional driving force provided by the field can effectively suppress charge-carrier recombination both in the bulk and on the surface of hematite, especially at lower potentials. Moreover, the synthesized homojunction shows a remarkable synergy with NiFe(OH)<sub>x</sub> cocatalyst with significant additional improvements of photocurrent density and cathodic shift of turn-on voltage. The work has nicely demonstrated multiple collaborative strategies of gradient doping, homojunction formation, and cocatalyst modification, and the concept could shed light on designing and constructing the efficient nanostructures of semiconductor photoelectrodes in the field of solar energy conversion.
Project description:Hematite (?-Fe2O3) is one of the most promising candidates as a photoanode materials for solar water splitting. Owing to the difficulty in suppressing the significant charge recombination, however, the photoelectrochemical (PEC) conversion efficiency of hematite is still far below the theoretical limit. Here we report thick hematite films (?1500?nm) constructed by highly ordered and intimately attached hematite mesocrystals (MCs) for highly efficient PEC water oxidation. Due to the formation of abundant interfacial oxygen vacancies yielding a high carrier density of ?1020?cm-3 and the resulting extremely large proportion of depletion regions with short depletion widths (<10?nm) in hierarchical structures, charge separation and collection efficiencies could be markedly improved. Moreover, it was found that long-lived charges are generated via excitation by shorter wavelength light (below ?500?nm), thus enabling long-range hole transfer through the MC network to drive high efficiency of light-to-energy conversion under back illumination.