This PDF file contains the front matter associated with SPIE Proceedings Volume 9560 including the Title Page, Copyright information, Table of Contents, Authors, and Conference Committee listing.

Many important energy systems are based on the complexity of material architecture, chemistry and interactions among constituents within. To understand and thus ultimately control the energy applications calls for in-situ/operando characterization tools. Recently, we have developed the in-situ/operando soft X-ray spectroscopic systems for the studies of catalytic and electrochemical reactions, and reveal how to overcome the challenge that soft X-rays cannot easily peek into the high-pressure catalytic or liquid electrochemical reactions. The unique design of in-situ/operando soft X-ray spectroscopy instrumentation and fabrication principle and one example are presented.

A hybrid photocatalyst consisting of a catalytic Ru complex and polymeric carbon nitride (band gap, 2.7 eV) was capable of reducing CO₂ into HCOOH with ~80% selectivity under visible light (λ > 420 nm) in the presence of a suitable electron donor. Introduction of mesoporosity into the graphitic carbon nitride structure to increase the specific surface area was essential to enhancing the activity. However, higher surface area (in other words, lower crystallinity) that originated from excessively introduced mesopores had a negative impact on activity. Promoting electron injection from carbon nitride to the catalytic Ru unit as well as strengthening the electronic interactions between the two units improved the activity. Under the optimal condition, a turnover number (TON, with respect to the Ru complex used) greater than 1000 and an apparent quantum yield of 5.7% (at 400 nm) were obtained, which are the greatest among heterogeneous photocatalysts for visible-light CO₂ reduction ever reported.

My research group in the State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi’an Jiaotong University has been focusing on renewable energy, especially solar hydrogen, for about 20 years. In this presentation, I will present the most recent progress in our group on solar hydrogen production using light and heat. Firstly, “cheap” photoelectrochemical and photocatalytic water splitting, including both nanostructured materials and pilot-scale demonstration in our group for light-driven solar hydrogen (artificial photosynthesis) will be introduced. Then I will make a deep introduction to the achievements on the thermal-driven solar hydrogen, i.e., biomass/coal gasification in supercritical water for large-scale and low-cost hydrogen production using concentrated solar light.

X-rays from synchrotron radiation enable incisive spectroscopic techniques which speed up the discovery of new materials for photovoltaics and photoelectrochemistry. A particularly useful method is X-ray absorption spectroscopy (XAS), which probes empty electronic states. XAS is element- and bond-specific, with the additional capability of determining the bond orientation. Close feedback from density functional calculations makes it possible to discover and exploit systematic trends in the electronic properties. Case studies are presented, such as solar cells that combine an absorber with an electron donor and an acceptor in one molecular complex and nanowire arrays serving as photoanodes for water splitting. In addition to the energy levels the lifetimes of the charge carriers play an essential role in device performance. A new generation of laser-like X-ray sources will make it possible to follow the fate of excited charge carriers traveling across a molecular complex or through a device structure in real time.

Cu₂O is an environment-friendly p-type semiconductor with narrow band gap (2.0~2.2eV), which has become a popular sensitizer of TiO₂. The present work is focused on the preparation of Cu₂O/TiO₂ nanotube arrays heterostructures via electrochemical deposition. TiO₂ nanotube arrays were prepared by anodic oxidation method and calcined at 450°C, then Cu₂O were deposited on TiO₂ nanotube arrays in a three-electrode system with surfactants PVP in electrolyte at different deposition potentials (-0.2V and-0.3V) for deposition time 5min. The results show that Cu₂O nanoparticles deposit on TiO₂ nanotube successfully. The obtained Cu₂O nanoparticles were quite different in size at deposition potential -0.2V and -0.3V. The resulting Cu₂O/TiO₂ nanotube arrays have the significant photoresponse in visible light region. Under irradiation of solar simulator (AM1.5, 100mW/cm²), the photocurrent density of the Cu₂O/TiO₂ nanotube arrays when Cu₂O was deposited at a voltage of -0.3V is more than that of pure TiO₂ nanotube arrays.

With the advent of X-ray free electron lasers and table-top high-harmonic-generation X-ray sources, we can now explore changes in electronic structure on ultrafast time scales -- at or less than 1ps. Transient X-ray spectroscopy of this kind provides a direct probe of relevant electronic levels related to photoinitiated processes and associated interfacial electron transfer as the initial step in solar energy conversion. However, the interpretation of such spectra is typically fraught with difficulty, especially since we rarely have access to spectral standards for nonequilibrium states. To this end, direct first-principles simulations of X-ray absorption spectra can provide the necessary connection between measurements and reliable models of the atomic and electronic structure. We present examples of modeling excited states of materials interfaces relevant to solar harvesting and their corresponding X-ray spectra in either photoemission or absorption modalities. In this way, we can establish particular electron transfer mechanisms to reveal detailed working principles of materials systems in solar applications and provide insight for improved efficiency.

To solve the global energy and environmental issues highly efficient systems for solar energy conversion and storage are needed. One of them involves the photocatalytic conversion of solar energy into the storable fuel molecular hydrogen via the water splitting process utilizing metal-oxide semiconductors as catalysts. Since photocatalytic water splitting is still a rather poorly understood reaction, fundamental research in this field is required. Herein, the photocatalytic activity for water splitting was investigated utilizing La-doped NaTaO3 as a model photocatalyst. The activity of La-doped NaTaO3 was assessed by the determination of the overall quantum yield of molecular hydrogen and molecular oxygen evolution. In pure water La-doped NaTaO3 exhibits rather poor activity for the photocatalytic H2 evolution whereby no O2 was detected. To enhance the photocatalytic activity the surface of La-doped NaTaO3 was modified with various cocatalysts including noble metals (Pt, Au and Rh) and metal oxides (NiO, CuO, CoO, AgO and RuO2). The photocatalytic activity was evaluated in pure water, in aqueous methanol solution, and in aqueous silver nitrate solution. The results reveal that cocatalysts such as RuO2 or CuO exhibiting the highest catalytic activity for H2 evolution from pure water, possess, however, the lowest activity for O2 evolution from aqueous silver nitrate solution. La-doped NaTaO3 modified with Pt shows the highest quantum yield of 33 % with respect to the H2 evolution in the presence of methanol. To clarify the role of methanol in such a photocatalytic system, long-term investigations and isotopic studies were performed. The underlying mechanisms of methanol oxidation were elucidated.

The conductivity (i.e., n-type or p-type) of Cu₂O films is controlled by the electrodeposition potential. A slightly acidic solution (pH 4.93) containing cupric acetate and sodium dodecyl sulfate (SDS) is used. Photoelectrochemical measurements at zero bias indicate that the Cu₂O films deposited at the potentials of 0.00 V and -0.05 V generate the ntype photocurrents and the films deposited at the potentials negative than -0.10 V generate the p-type photocurrents. The X-ray diffraction (XRD) results show that the n-type films are pure Cu₂O, however, the metallic copper appear in the ptype Cu₂O films. Mott-Schottky measurements show that the donor concentrations of the n-type Cu₂O films decrease and the acceptor concentrations of the p-type Cu₂O films increase with the decrease of the deposition potential. The SDS molecules adsorbed on electrode surface and the SDS micelles block the diffusion of Cu²⁺ ions, resulting in a low diffusion rate of Cu²⁺ ions. Under this circumstance, the growth of Cu₂O films are affected significantly by the overpotential. When the potential is positive than -0.05 V, oxygen vacancies are formed in the films leading to the n-type conductivity; however, when the potential is negative than -0.10 V, the Cu²⁺ ions are reduced to Cu⁺ rapidly and part of Cu²⁺ are reduced to metallic copper, the diffused Cu²⁺ ions to supply to the growth of Cu₂O films are insufficient, hence copper vacancies are formed in the films resulting in the p-type conductivity.

Mastery over the surface of a nanocrystal enables control of its properties in molecular adsorption and activation, and enhances its usefulness for catalytic applications. On the other hand, hybrid systems based on semiconductors and noble metals may exhibit improved performance in photocatalysis such as water splitting, mainly determined by the efficiency in generating carriers. In the systems, perfect interface is certainly the key to efficient carrier separation and transport. Taken together, the surface and interface modulation holds the key to materials design for photocatalytic applications. Here, we will demonstrate several different approaches to designing nanocrystal-based systems with improved photocatalytic performance. For instance, a semiconductor-metal-graphene design has been implemented to efficiently extract photoexcited electrons through the graphene nanosheets, separating electron-hole pairs. Ultrafast spectroscopy characterizations exclusively demonstrate that the charge recombination occurring at interfacial defects can be substantially avoided, enabling superior efficiency in water splitting. It is anticipated that this series of works open a new window to rationally designing hybrid systems for photo-induced applications.