Ruifeng Chong

Achieving overall water splitting using titanium dioxide-based photocatalysts of different phases

Titanium dioxide (TiO2) is regarded as the benchmark semiconductor in photocatalysis, which possesses a suitable band structure and makes the overall water splitting reaction thermodynamically possible. However, photocatalytic overall water splitting (POWS) (2H2O → 2H2 + O2) can only take place on rutile but hardly on anatase and brookite TiO2. So obtaining the POWS on TiO2-based photocatalysts has remained a long-standing challenge for over 40 years. In this work, we found that the POWS on anatase and brookite TiO2 becomes feasible under prolonged UV light irradiation. Further investigation by means of electron spin resonance spectroscopy (EPR) and transient infrared absorption–excitation energy scanning spectroscopy (TRIRA-ESS) reveals that both kinetics and thermodynamics factors contributed to unique POWS activity for different phases of TiO2. Kinetically the process of photocatalysis differs on different phases of TiO2 due to the intermediates (˙OH radical for anatase and brookite TiO2, peroxy species for rutile TiO2) that are formed. Thermodynamically there are many trapped states lying near the valence band of anatase and brookite but not for rutile TiO2, which reduce the overpotential for water oxidation. These findings develop our understanding of why some semiconductors are inactive as POWS photocatalysts despite having thermodynamically suitable band structures for the proton reduction and water oxidation reactions.

Selective photocatalytic conversion of glycerol to hydroxyacetaldehyde in aqueous solution on facet tuned TiO2-based catalysts

Glycerol is selectively converted to hydroxyacetaldehyde (HAA) and H2 in aqueous solution on TiO2-based photocatalysts. The product selectivity was verified to be strongly dependent on the facets of TiO2. Rutile with high percentage of {110} facets results in over 90% superior selectivity of HAA, while anatase with {001} or {101} facets gives only 16% and 49% selectivity for HAA, respectively.

The role of glutathione on shape control and photoelectrical property of cadmium sulfide nanorod arrays.

Well-aligned CdS nanorod arrays (CdS NRs) with ~100 nm in diameter and ~700 nm in length were fabricated on FTO (fluorine-doped tin oxide) substrate by using glutathione as capping agents. The growth of CdS NRs was studied in details by exploring the roles of each active binding group in glutathione. The thiol group in glutathione plays an important role in forming a compact CdS nanocrystal film, upon which the nanorods grow subsequently via the synergetic effect of thiol and dicarboxyl groups in glutathione. The influence of surface passivation with glutathione on the photoelectrical property of CdS NRs was also tested. The results revealed that glutathione ligands encapsulated in the surfaces of CdS NRs act as insulating barriers between CdS NRs and solution, hindering charge transport. Hybrid photovoltaic cells of FTO/CdS NRs/P3HT (poly(3-hexylthiophene))/Au were then assembled. The performance of the photovoltaic devices was increased with increasing the length of the as-prepared CdS nanorods and further enhanced to the highest efficiency of 0.373% after the thermal sulfuration treatment.

Cl- making overall water splitting possible on TiO2-based photocatalysts

Overall water splitting on a TiO2-based photocatalyst has been extensively investigated. However, in most cases, the products are not in a stoichiometric ratio, thus the reaction is not really overall water splitting. In this work, we found that in the presence of Cl−, the evolution of O2 and H2 over Pt/TiO2 can be successfully achieved, and the activity can be enhanced up to 3 times compared to having no Cl− present. Furthermore, the H2 : O2 ratio can be close to 2.0, i.e. the stoichiometric ratio of overall water splitting. It is proposed that the Cl− ion is involved with the reaction intermediate of O2 evolution from water oxidation. Our work not only reported overall water splitting on a TiO2-based photocatalyst, but also provided experimental evidence for understanding the possible reaction process and the mechanism of photocatalytic water splitting.

Transition metal (Ni, Fe, and Cu) hydroxides enhanced α-Fe2O3 photoanode-based photofuel cell

Photofuel cells have been demonstrated to be a promising strategy for generating electricity using biomass. Here, we present a photofuel cell with a visible light α-Fe2O3 based photoanode that can be directly powered by a variety of biomasses such as methanol, glycerol, glucose, cellubiose and starch. The photocurrent density and power density of the photofuel cell are significantly enhanced by loading cocatalysts (metal hydroxides, e.g. Ni(OH)2) on the α-Fe2O3 photoanode. The power density of the photofuel cell powered by glucose is enhanced over two times from 0.082 mA cm−2 for α-Fe2O3 to 0.18 mW cm−2 for Ni(OH)2/α-Fe2O3 photoanode.

Dual-functional CoAl layered double hydroxide decorated α-Fe2O3 as an efficient and stable photoanode for photoelectrochemical water oxidation in neutral electrolyte

As a promising photoanode material, hematite (α-Fe2O3) can be used for photoelectrochemical (PEC) water oxidation but there are still great challenges, such as low efficiency and poor stability in neutral pH electrolyte, before it can be used in practice. Herein, a CoAl layered double hydroxide (CoAl-LDH) has been deposited in situ onto α-Fe2O3 nanoarrays. The PEC performances of the resulting composite photoanodes (CoAl-LDH/α-Fe2O3) towards water oxidation were investigated in neutral pH electrolyte. Contrastive experiments indicated that Co2+ in CoAl-LDH provided active catalytic sites for water oxidation, while Al3+ was inactive and offered support for the layered skeleton. CoAl-LDH could have a dual role as an efficient cocatalyst to lower the onset potential and enhance the photocurrent of α-Fe2O3, and as an effective protector of α-Fe2O3 from corrosion in neutral pH electrolyte. Relative to that of bare α-Fe2O3, a ∼250 mV negatively shifted onset potential and an almost 9-fold enhancement in photocurrent density at 1.23 V vs. RHE were observed with CoAl-LDH/α-Fe2O3. Moreover, CoAl-LDH/α-Fe2O3 showed excellent stability at 1.23 V vs. RHE with a steady photocurrent of ∼2 mA cm−2 even after 2 h of irradiation, which was very close to 100% of the initial value. Electrochemical impedance spectroscopy analysis revealed that the charge transfer resistance at the interface of α-Fe2O3/electrolyte was significantly decreased by CoAl-LDH decoration. Using H2O2 as a hole scavenger, the charge injection efficiency at the interface of CoAl-LDH/α-Fe2O3 and electrolyte was recorded as 84% at 1.23 V vs. RHE, which was 4-fold higher than that at the interface of α-Fe2O3 and electrolyte. This work demonstrates that CoAl-LDH decorated semiconductors have great potential as efficient and stable photoanodes towards high PEC performance in neutral pH electrolyte.

A tetragonal tungsten bronze-type photocatalyst: Ferro-paraelectric phase transition and photocatalysis

Although ferroelectrics have potential applications in photocatalysis due to their highly efficient charge separation, their mechanism of charge separation is still unknown. A ferroelectric Sr 0.7 Ba 0.3 Nb 2 O 6 (SBN-70) semiconductor with a low ferro-paraelectric phase transition (65 ℃) was studied. The photocatalytic activity for H 2 production by ferroelectric and paraelectric SBN-70 was examined. The spontaneous polarization in the ferroelectric phase strongly affected the photocatalytic performance and parallel ferroelectric domains significantly promoted photogenerated charge separation to result in better photocatalytic H 2 production. This knowledge provides an important basis for the fabrication of ferroelectric photocatalysts with improved charge separation ability.

Conversion of Biomass Derivatives to Electricity in Photo Fuel Cells using Undoped and Tungsten-doped Bismuth Vanadate Photoanodes.

The photo fuel cell (PFC) is a promising technology for simultaneously converting solar energy and bioenergy into electricity. Here, we present a miniature air-breathing PFC that uses either BiVO4 or W-doped BiVO4 as the photoanode and a Pt/C catalyst as the air-breathing cathode. The PFC exhibited excellent performance under solar illumination and when fed with several types of biomaterial. We found the PFC performance could be significantly enhanced using W-doping into the BiVO4 photoanode. With glucose as the fuel and simulated sunlight (AM 1.5 G) as the light source, the open-circuit voltage increased from 0.74 to 0.92 V, the short-circuit current density rose from 0.46 to 1.62 mA cm(-2) , and the maximum power density was boosted from 0.05 to 0.38 mW cm(-2) , compared to a PFC using undoped BiVO4 as the anode.