4:00pm - 4:30pmInvited
Copper-oxide-based Nanolayer Stack Photoelectrode for Electrochemical Solar Water Splitting
1School of Engineering, The University of Tokyo, Japan; 2Institute of Engineering Innovation, The University of Tokyo, Japan
Cu2O-based photocathodes used in photoelectrochemical water splitting with sun light suffer from poor stability and efficiency because of their large onset potential for the water reduction and self-reduction of Cu2O in electrolyte under illumination. We report important improvements of Cu2O-based photocathodes used in photoelectrochemical water splitting. A large shift of the onset potential toward positive values and a stable photocurrent over time are achieved by introducing a gallium oxide buffer layer between the Cu2O absorber layer and the titanium oxide protective layer. The gallium oxide buffer layer decreases the conduction band discontinuity at the Cu2O/Ga2O3 interface, thus improving the open circuit voltage of the structure. The obtained large energy gap between the p-type absorber and the n-type overlayers inhibits the interface recombination and provides a large driving force for the transport of photogenerated carriers in the photocathode, resulting in efficient water reduction on the structure surface with the assistance of platinum catalyst. Additionally, gallium oxide layer owing to its high thermal resistance, works as a suitable buffer layer to grow a protective titanium oxide layer with improved quality. The final Pt/TiO2/Ga2O3/Cu2O photoelectrode consists of a porous Cu2O conformally coated with gallium oxide (20 nm) and titanium oxide (15 nm) with Pt catalyst. The observed onset potential (1.02 V vs. RHE) shows a large enhancement over the previously reported Cu2O-based photocathodes. The photocurrent generated at low applied biases (-1.17 mA/cm2 at 0.6 V vs. RHE) demonstrates the potential of this multilayered structure for developing superior photoelectrodes. Finally, the fabricated photocathode exhibits a stable photocurrent for two hours under continuous illumination, providing a clear improvement over the reported Cu2O based multilayer structures.
4:30pm - 4:45pmOral
MoS2 Nanosheets Wrapped TiO2 Inverse Opals as Photoelectrochemical Anode
Nanyang Technological University, Singapore
Three dimensional (3D) TiO2-inverse opals (TIO) coating with few layered MoS2 nanosheets (MoS2-NSs) as an effective photoelectrochemical anode for hydrogen evolution has been successfully fabricated by a facile combination of dip-coating so-gel technique for fabrication of TIO and hydrothermal process for coating of MoS2-NSs on their surface. Through adjusting the hydrothermal reaction parameters, MoS2-NSs were uniformly grown inside the whole TIO without blocking their surface, which is critical to light transmission. Then TIO with different photonic bandgap were utilized to optimize light trapping. The as prepared hybrid photoanode shows enhanced photocurrent than that of pure TIO, which can be ascribed to the visible light absorption of MoS2-NSs and accelerated separation of photo-generated electron-hole pairs due to the formation of heterojunction. In addition, the periodical porous structure of TIO greatly increase light absorption of MoS2-NSs due to the multi-scattering and slow-photon effect. Our results show a promising way to increase light absorption of 2D materials by integrating them on 3D photonic crystals, which are expected to exhibit enhanced performance in the field of photocatalysis, water splitting and solar cell.
4:45pm - 5:00pmOral
Sulfurized TiO2 Nanorods: A High Performance Photoanode for Oxygen Evolution Reaction
Seoul National University, South Korea
Titanium dioxide has been studied for solar water splitting since the first scientific report appeared by Fujishima and Honda. However, a considerable enhancement of solar-to-hydrogen conversion efficiency was not observed due to the narrow rage light absorption and low photogenerated carrier kinetics. Herein, after a brief introduction in the drawback and advantage of TiO2 for photoelectrochemical water splitting, we present the development of facilely sulfurized TiO2 NRs (S-TiO2 NRS) photoanode with enhanced light absorption and improved photogenerated carrier kinetics, holes and electrons in the bulk film. We show the control of dopant concentration in the S-TiO2 NRs is a key to achieve high photocurrent density. It is also present detail structural and elemental analysis, and the corresponding electrochemical and solar water oxidation performance of S-TiO2 NRs. More importantly, we briefly address the interaction of sulfur with the host material, TiO2 NRs, and its influence in the PEC performance.
5:00pm - 5:15pmOral
Facile Synthesis of Metal-Free, Carbon Nanotubes based Nanocomposites for Enhanced Photoelectrochemical Water Splitting and Photocatalysis under Visible Light
Indian Institute of Technology Delhi, India
Since Fujishima and Honda reported that TiO2 can be used to decompose H2O to produce hydrogen, many scientists have paid attention to photocatalysis technology. TiO2 is a cheap and nontoxic photocatalyst, which however suffers from high recombination of charge carriers or a wide bandgap (~ 3.2 eV), therefore can be activated only under UV light, which restrict their applications. To address these issues, development of visible-light active photocatalyst has attracted wide attention due to their ability to maximum utilization of solar energy. Among the narrow band gap semiconductors, graphitic carbon nitride (g-C3N4), which has a two-dimensional (2D) nanostructure and good chemical stability, has emerged as a new class of visible-light-driven photocatalyst or heterojunction component in TiO2-based photocatalysts.
A ternary nanocomposite composed of TiO2, g-C3N4 and CNT has been synthesized by a facile hydrothermal method. The synthesized nanocomposites was comprehensively characterized by SEM, HRTEM, XRD, UV/vis DRS, Raman and FTIR spectroscopy. The ternary nanocomposite exhibited a high photocatalytic activity for the degradation of methylene blue dye under visible light. The PEC water splitting has also been studied using the photoanode of the nanocomposites. The photocurrent density of ternary nanocomposite photoanode was higher than binary nanocomposite and single component system under visible light illumination. The enhanced charge transfer within the ternary system is also confirmed by the electrochemical impedance spectroscopy. This approach offers a promising method for the use of multi-component photocatalytic systems that can enhances the utilization of visible light in photocatalytic activity and PEC water splitting performance.
5:15pm - 5:45pmInvited
Atomic Layer Deposition of 3D TiO2/MoS2 Nanostructures for Photoelectrochemical Solar Hydrogen Generation
Nanyang Technological University, Singapore
Pristine MoS2, TiO2/MoS2 nanostructures of various morphologies including nanorods, nanowires, nanospheres, have been earlier synthesized for photocatalytic, and supercapacitor applications[1-3]. Recently MoS2 decorated TiO2 nanostructures has been shown be a good material for Li-ion batteries[4,5]. However, the challenge of fabricating/coating a uniform MoS2 on TiO2 mesoporous structures still remains to be widely explored. Atomic layer deposition (ALD), due to its self-limiting surface reaction mechanism allows for accurate control of thickness on nanostructures and hence a suitable technique to fabricate such hybrid nanostructures. We report the fabrication of hybrid TiO2/ MoS2 nanostructures utilizing polystyrene opal structure as the sacrificial layer to produce TiO2 inverse opal nanostructure (TIO) followed by atomic layer deposition (ALD) of MoS2 in order to fabricate hybrid TiO2/ MoS2 nanostructures. Water splitting properties of ALD fabricated TIO2/MoS2 nanostructures were also studied.
Opal structures were fabricated using polystyrene (PS) particles of different diameters. ALD TiO2 was grown on PS opal structures using TiCl4/H2O vapor. ALD MoS2 was performed on TIO nanostructures using Mo(CO)6 and DMDS. PS opals structures showed perfect fcc arrangement. The presence of absorbing MoS2 layer on TIO nanostructures interfered with stopband reflectance properties of TIO nanostructures. Annealing MoS2 coated TIO however generated unique fiber like TiO2/MoS2 nanostructures. The crystallization of MoS2 layer revealed unique light absorbing properties in a broad visible light range (400 – 700nm) as a function of particle size used for fabricating TIO nanostructures. Water splitting properties were measured using three-electrode cell configuration in 1M Na2SO4 using platinum wire as the counter electrode and Ag/AgCl as the reference electrode.
5:45pm - 6:00pmOral
Solar Driven CO2 Reduction to Hydrocarbons
Lawrence Berkeley National Laboratory, United States
Solar to fuel conversion, if it could be performed in a sustainable manner, could provide an alternative to mankind’s currently unsustainable use of fossil fuels. Solar fuel generation by photoelectrochemical (PEC) methods is a potentially promising approach to address this fundamental and important challenge. To date, research into these approaches has been primarily focused on solar water splitting, which produces hydrogen. Thus, the conversion of CO2 to hydrocarbons that could displace currently used fossil fuels remains as an unmet challenge. Achieving a viable PEC CO2 reduction energy conversion efficiency requires minimizing potential losses in all aspects of the device including the cathode, anode, electrolyte, and membrane. Achieving selective products requires management of multi-electron transfer reactions. We have optimized each component (anode, cathode, electrolyte, cell design) of our CO2 electrolyzer cell to obtain higher selectivity and energy conversion efficiency at low overpotential. A nanostructured IrOx anode has been synthesized, which shows superior stability and high performance for OER over a wide range of pH. Use of a bimetallic CuAg nanocoral type cathode enables greater selectivity of hydrocarbons over a wide range of pH and cell voltage conditions. As a result, we are able to demonstrate an overall cell design for selective production of hydrocarbons at low overall cell potential. The importance of power matching to photovoltaic elements will be discussed. Using a number of different coupled solar cells, solar to hydrocarbon energy conversion values in range of a few percent are achieved.
 Ager et al., Energy Environ. Sci. 2015, 8 , 2811–2824.
 Rongé et al., Chem. Soc. Rev. 2014, 43, 7963–7981.