Sing Yang Chiam

Copper molybdenum sulfide: a new efficient electrocatalyst for hydrogen production from water

A new inorganic solid state electrocatalyst for the hydrogen evolution reaction (HER) is reported. Highly crystalline layered ternary sulfide copper-molybdenum-sulfide (Cu2MoS4) was prepared by a simple precipitation method from CuI and [MoS4]2− precursors. In aqueous solution and over a wide pH range (pH 0 to 7), this Cu2MoS4 showed very good catalytic activity for HER with an overvoltage requirement of only ca. 135 mV and an apparent exchange current density of 0.040 mA cm−2 (Tafel slope of ca. 95 mV per decade was found irrespective of the pH value). This Cu2MoS4 catalyst was found to be stable during electrocatalytic hydrogen generation. Therefore, it represents an attractive alternative to platinum.

A novel strategy for surface treatment on hematite photoanode for efficient water oxidation

In this paper, we report a novel strategy for surface treatment of hematite nanorods for efficient photo-driven water oxidation. This is the first report describing the growth of Sn treated hematite from α-FeOOH nanorod arrays in one step without substantially altering morphologies. With this treatment the photocurrent density increased from 1.24 for pristine hematite nanorods to 2.25 mA cm−2 at 1.23 V vs. RHE (i.e. 81% improvement). The increase in photocurrent density was also accompanied by improved incident-photon-to-current efficiencies and oxygen evolution. The photocurrent improvement is mainly attributed to a reduced electron–hole recombination at the hematite–electrolyte interface through the formation of FexSn1−xO4 layer at the hematite nanorod surface as shown by XPS, HRTEM, EDAX line scan analyses and PEC measurements.

Novel cobalt/nickel–tungsten-sulfide catalysts for electrocatalytic hydrogen generation from water

The potential of water (photo)electrolysis technology to provide hydrogen as a fuel on a large scale depends on how viable electrocatalysts for the water oxidation reaction (WOR) and the hydrogen evolution reaction (HER) are and whether they can be constructed from elements which are abundant in the Earths crust. Here we show that ternary sulfides of cobalt–tungsten and nickel–tungsten (MWSx where M is Co or Ni) are efficient and robust electrocatalysts for the HER in water over a wide pH range. These novel ternary sulfides were readily grown on a conducting electrode surface by employing a scalable electrodeposition process from aqueous solutions of [M(WS4)2]2−. In terms of HER activity, the MWSx catalysts represent attractive alternatives to platinum. Moreover, we show that the HER activity is governed by the nature of the metal M within M–S–W heterobimetallic sulfide centres, located in the WS2-like layered structure of MWSx. Our work provides structural and mechanistic keys to understand how HER activity is promoted in previously described nickel and cobalt-doped molybdenum and tungsten sulfide materials.

Improving the Efficiency of Hematite Nanorods for Photoelectrochemical Water Splitting by Doping with Manganese

Here, we report a significant improvement of the photoelectrochemical (PEC) properties of hematite (α-Fe2O3) to oxidize water by doping with manganese. Hematite nanorods were grown on a fluorine-treated tin oxide (FTO) substrate by a hydrothermal method in the presence on Mn. Systematic physical analyses were performed to investigate the presence of Mn in the samples. Fe2O3 nanorods with 5 mol % Mn treatment showed a photocurrent density of 1.6 mA cm(-2) (75% higher than that of pristine Fe2O3) at 1.23 V versus RHE and a plateau photocurrent density of 3.2 mA cm(-2) at 1.8 V versus RHE in a 1 M NaOH electrolyte solution (pH 13.6). We attribute the increase in the photocurrent density, and thus in the oxygen evolving capacity, to the increased donor density resulting from Mn doping of the Fe2O3 nanorods, as confirmed by Mott-Schottky measurement, as well as the suppression of electron-hole recombination and enhancement in hole transport, as detected by chronoamperometry measurements.

Novel assembly of an MoS2 electrocatalyst onto a silicon nanowire array electrode to construct a photocathode composed of elements abundant on the Earth for hydrogen generation

Mild-mannered catalyst: a novel procedure to load a MoS(2) co-catalyst onto the surface of silicon under mild-conditions (room temperature, atmospheric pressure, aqueous solution) by a photo-assisted electrodeposition process employing commercially available precursors is reported. The obtained Si-NW@MoS(2) photocathode showed similar catalytic activity for light-driven H(2) generation compared with a Si-NW@Pt photocathode.

Growth of Cu2O on Ga-doped ZnO and their interface energy alignment for thin film solar cells

Cu2O thin films are deposited by direct current reactive magnetron sputtering on borofloat glass and indium tin oxide (ITO) coated glass at room temperature. The effect of oxygen partial pressure on the structures and properties of Cu2O thin films are investigated. We show that oxygen partial pressure is a crucial parameter in achieving pure phases of CuO and Cu2O. Based on this finding, we fabricate heterojunctions of p-type Cu2O with n-type gallium doped ZnO (GZO) on ITO coated glass substrates by pulsed laser deposition for GZO thin films. The energy band alignment for thin films of Cu2O/GZO on ITO glass is characterized using high-resolution x-ray photoelectron spectroscopy. The energy band alignment for the Cu2O/GZO heterojunctions is determined to be type II with a valence band offset of 2.82 eV and shows negligible effects of variation with gallium doping. The higher conduction band of the Cu2O relative to that of GZO in the obtained band alignment shows that the heterojunctions are suitable for so...

Silicon decorated with amorphous cobalt molybdenum sulfide catalyst as an efficient photocathode for solar hydrogen generation.

The construction of viable photoelectrochemical (PEC) devices for solar-driven water splitting can be achieved by first identifying an efficient independent photoanode for water oxidation and a photocathode for hydrogen generation. These two photoelectrodes then must be assembled with a proton exchange membrane within a complete coupled system. Here we report the preparation of a Si/a-CoMoSx hybrid photocathode which shows impressive performance (onset potential of 0.25 V vs RHE and photocurrent jsc of 17.5 mA cm(-2) at 0 V vs RHE) in pH 4.25 phosphate solution and under simulated AM 1.5 solar illumination. This performance is among the best reported for Si photocathodes decorated with noble-metal-free catalysts. The electrode preparation is scalable because it relies on a photoassisted electrodeposition process employing an available p-type Si electrode and [Co(MoS4)2](2-) precursor. Investigation of the mechanism of the Si/a-CoMoSx electrode revealed that under conditions of H2 photogeneration this bimetallic sulfide catalyst is highly efficient in extracting electrons from illuminated Si and subsequently in reducing protons into H2. The Si/a-CoMoSx photocathode is functional over a wide range of pH values, thus making it a promising candidate for the construction of a complete solar-driven water splitting PEC device.

Photovoltaic characteristics of p-β-FeSi2(Al)/n-Si(100) heterojunction solar cells and the effects of interfacial engineering

Heterojunction solar cells with Al-alloyed polycrystalline p-type β-phase iron disilicide [p-β-FeSi2(Al)] on n-Si(100) were investigated. The p-β-FeSi2(Al) was grown by sputter deposition and rapid-thermal annealing. Photocurrent of ∼1.8 mA/cm2 and open-circuit voltage of ∼63 mV were obtained for p-β-FeSi2(Al)/n-Si(100)/Ti/Al control cells with indium-tin-oxide (ITO) top electrode. Open-circuit voltage increased considerably once thin Al layer was deposited before amorphous-FeSi2(Al) deposition. Furthermore, device performances were found to improve significantly (∼5.3 mA/cm2 and ∼450 mV) by introducing germanium-nitride electron-blocking layer between ITO and p-β-FeSi2(Al). The improvement is attributed to the formation of epitaxial Al-containing p+-Si at p-β-FeSi2(Al)/n-Si(100) interface and suppressed back-diffusion of photogenerated electrons into ITO.

Role of oxygen for highly conducting and transparent gallium-doped zinc oxide electrode deposited at room temperature

In this work, we found that a desirable amount of oxygen can reduce defect related scattering in enhancing carrier mobility for pulsed laser deposited zinc oxide. However, excessive oxygen can lead to formation of oxygen interstitials that can act as compensation or scattering centers. At higher oxygen pressures, structural changes that increase grain boundary scattering prove to be very important. We introduce a simple transparency index to quantify the transmission of the thin films for usage as electrodes in photovoltaic devices. An excellent resistivity of ∼3.9×10−4 Ω cm and an electron mobility of ∼19.2 cm2/V s with a transparency index of 0.84 (84% of total solar spectrum transmitted) were achieved at room temperature suggesting possible applications in plastic devices.

Band alignment of yttrium oxide on various relaxed and strained semiconductor substrates

In this work, we report on the band offsets of yttrium oxide (Y2O3) on various relaxed and strained semiconductor substrates, such as silicon (Si), germanium (Ge), and silicon germanium (SiGe). By using the example of Y2O3/Si, important experimental aspects in using photoemission to determine band offsets are discussed. We then discuss the various values of band offsets of Y2O3 on the different substrates that we obtained by using x-ray photoelectron spectroscopy. Finally, we show that presputtering 3 A of metallic Y [equivalent to 1 ML (monolayer) coverage] before the deposition of Y2O3 affects the band lineup by reducing the resultant valence band offsets. We explain the observed band offsets by using an interfacial layer model.