Lequan Liu

Recent advances in TiO2-based photocatalysis

Semiconductor photocatalysis is a promising approach to combat both environmental pollution and the global energy shortage. Advanced TiO2-based photocatalysts with novel photoelectronic properties are benchmark materials that have been pursued for their high solar-energy conversion efficiency. In general, the photocatalytic efficiency is affected by the degree of light absorption, charge separation, and surface reactivity. Consequently, in this review we first discuss a series of interesting studies that aim to extend the light absorption of TiO2 from UV wavelengths into the visible or even the near-infrared region. We next focus on attempts to overcome the drawback that dopants usually act as charge recombination centres. We discuss the use of either selective local doping or the introduction of disorder together with doping, which aims to facilitate charge separation while preserving the visible-light response. We also show that crystal facet engineering can endow TiO2 with superior physicochemical properties, thus yielding high surface reactivity in photocatalytic reactions. Finally, we examine the recent theoretical advances of TiO2-based photocatalysis.

Reduced TiO2 nanotube arrays for photoelectrochemical water splitting

We report a facile one-step chemical method to synthesize partially reduced TiO2 nanotube arrays (NTAs). The NaBH4 treatment introduces oxygen vacancies on the surface and interior of TiO2. Oxygen vacancy extends the photocatalytic activity of TiO2 NTAs from the UV to visible light region, and enhances the electrical conductivity as well as charge transportation. Surface oxygen vacancies serve as charge carrier traps as well as adsorption sites where the charge transfer to adsorbed species inhibits the surface charge recombination, whereas bulk oxygen vacancies tend to act as charge carrier traps where e–h recombination occurs. The optimally reduced TiO2 NTAs yield a photocurrent density of 0.73 mA cm−2 at 1.23 VRHE and a highest photoconversion efficiency of 1.31% at a rather low bias of 0.40 VRHE under a standard AM 1.5G solar illumination. Not only does the incident photon to current conversion efficiency (IPCE) spectrum increase in the UV region, but photoactivity in visible light also emerged. Surface oxygen vacancies, serving as electron donors, cause a noticeable negative flatband shift and increase the donor density of TiO2 NTAs 2-fold. Electron paramagnetic resonance (EPR) spectra confirm the presence of oxygen vacancies on the surface and interior of TiO2. Benefitting from the oxygen vacancy, a narrowed band gap of 2.46 eV and suitable localized states for hydrogen production are observed.

Photocatalytic Reduction of Carbon Dioxide by Hydrous Hydrazine over Au–Cu Alloy Nanoparticles Supported on SrTiO3/TiO2 Coaxial Nanotube Arrays

Efficient photocatalytic conversion of CO2 into CO and hydrocarbons by hydrous hydrazine (N2H4⋅H2O) is achieved on SrTiO3/TiO2 coaxial nanotube arrays loaded with Au-Cu bimetallic alloy nanoparticles. The synergetic catalytic effect by the Au-Cu alloy nanoparticles and the fast electron-transfer in SrTiO3/TiO2 coaxial nanoarchitecture are the main reasons for the efficiency, while N2H4⋅H2O as the H source and electron donor provides a reducing atmosphere to protect the surface Cu atoms from oxidation, therefore maintaining the alloying effect which is the basis for the high photocatalytic activity and stability. This approach opens a feasible route to enhance the photocatalytic efficiency, which also benefits the development of photocatalysts and co-catalysts.

Photothermal Conversion of CO2into CH4with H2over Group VIII Nanocatalysts: An Alternative Approach for Solar Fuel Production

The photothermal conversion of CO2 provides a straightforward and effective method for the highly efficient production of solar fuels with high solar-light utilization efficiency. This is due to several crucial features of the Group VIII nanocatalysts, including effective energy utilization over the whole range of the solar spectrum, excellent photothermal performance, and unique activation abilities. Photothermal CO2 reaction rates (mol h(-1) g(-1)) that are several orders of magnitude larger than those obtained with photocatalytic methods (μmol h(-1) g(-1)) were thus achieved. It is proposed that the overall water-based CO2 conversion process can be achieved by combining light-driven H2 production from water and photothermal CO2 conversion with H2. More generally, this work suggests that traditional catalysts that are characterized by intense photoabsorption will find new applications in photo-induced green-chemistry processes.

Nanometals for Solar-to-Chemical Energy Conversion: From Semiconductor-Based Photocatalysis to Plasmon-Mediated Photocatalysis and Photo-Thermocatalysis.

Nanometal materials play very important roles in solar-to-chemical energy conversion due to their unique catalytic and optical characteristics. They have found wide applications from semiconductor photocatalysis to rapidly growing surface plasmon-mediated heterogeneous catalysis. The recent research achievements of nanometals are reviewed here, with regard to applications in semiconductor photocatalysis, plasmonic photocatalysis, and plasmonic photo-thermocatalysis. As the first important topic discussed here, the latest progress in the design of nanometal cocatalysts and their applications in semiconductor photocatalysis are introduced. Then, plasmonic photocatalysis and plasmonic photo-thermocatalysis are discussed. A better understanding of electron-driven and temperature-driven catalytic behaviors over plasmonic nanometals is helpful to bridge the present gap between the communities of photocatalysis and conventional catalysis controlled by temperature. The objective here is to provide instructive information on how to take the advantages of the unique functions of nanometals in different types of catalytic processes to improve the efficiency of solar-energy utilization for more practical artificial photosynthesis.

0D/2D Heterojunctions of Vanadate Quantum Dots/Graphitic Carbon Nitride Nanosheets for Enhanced Visible-Light-Driven Photocatalysis

0D/2D heterojunctions, especially quantum dots (QDs)/nanosheets (NSs) have attracted significant attention for use of photoexcited electrons/holes due to their high charge mobility. Herein, unprecedent heterojunctions of vanadate (AgVO3 , BiVO4 , InVO4 and CuV2 O6 ) QDs/graphitic carbon nitride (g-C3 N4 ) NSs exhibiting multiple unique advances beyond traditional 0D/2D composites have been developed. The photoactive contribution, up-conversion absorption, and nitrogen coordinating sites of g-C3 N4 NSs, highly dispersed vanadate nanocrystals, as well as the strong coupling and band alignment between them lead to superior visible-light-driven photoelectrochemical (PEC) and photocatalytic performance, competing with the best reported photocatalysts. This work is expected to provide a new concept to construct multifunctional 0D/2D nanocomposites for a large variety of opto-electronic applications, not limited in photocatalysis.

In situ synthesis of ordered mesoporous Co-doped TiO2 and its enhanced photocatalytic activity and selectivity for the reduction of CO2

Ordered mesoporous cobalt-doped titanium dioxide was successfully synthesized by a multicomponent self-assembly process. The doped Co species change the construction of the conduction band and valence band of TiO2, leading to visible-light absorption for TiO2. The designed cobalt-doped titanium dioxide exhibits a higher visible light activity for the reduction of CO2 among the commonly reported photocatalysts. In addition, the selectivity of the reduction products is improved by optimizing the energy-band configurations of cobalt-doped titanium dioxide through varying the molar ratio of Co/Ti. When the doping content of cobalt species increases to some extent, Co3O4/Co-doped TiO2 nanocomposites with oxygen vacancies were obtained, which markedly improve the generation rate of CH4.

Porous-structured Cu2O/TiO2 nanojunction material toward efficient CO2 photoreduction

Porous-structured Cu2O/TiO2 nanojunction material is successfully fabricated by a facile method via loading Cu2O nanoparticles on the network of a porous TiO2 substrate. The developed Cu2O/TiO2 nanojunction material has a size of several nanometers, in which the p-type Cu2O and n-type TiO2 nanoparticles are closely contacted with each other. The well designed nanojunction structure is beneficial for the charge separation in the photocatalytic reaction. Meanwhile, the porous structure of the Cu2O/TiO2 nanojunction can facilitate the CO2 adsorption and offer more reaction active sites. Most importantly, the gas-phase CO2 photoreduction tests reveal that our developed porous-structured Cu2O/TiO2 nanojunction material exhibits marked photocatalytic activity in the CH4 evolution, about 12, 9, and 7.5 times higher than the pure TiO2, Pt-TiO2, and commercial Degussa P25 TiO2 powders, respectively. The greatly enhanced activity can be attributed to the well designed nanojunction structure combined with the porous structure, which can simultaneously enhance the charge separation efficiency and facilitate the CO2 adsorption.

Band-structure-controlled BiO(ClBr)(1−x)/2Ix solid solutions for visible-light photocatalysis

A group of BiO(ClBr)(1−x)/2Ix solid solutions with a homogeneous layered tetragonal matlockite structure have been explored as novel visible-light-active photocatalysts. By manipulating the composition ratio of halogen elements (I/(Cl + Br)), the band gaps of these Bi-based solid solutions can be continuously modulated in a rather wide range of 2.88 to 1.82 eV. The density functional calculations demonstrate that this continuous band gap narrowing originates from the gradual increase of valence band maximum with increasing ratio of I/(Cl + Br). The photocatalytic evaluations showed these materials possess composition-dependent photoactivities for degrading 2-propanol (IPA) to acetone and CO2 under visible light (400 < λ < 800 nm). Particularly, the highest acetone evolution rate (215.6 μmol h−1 g−1) was achieved over BiO(ClBr)0.21I0.58, which was 16.5, 11.8 and 659.3 times that of BiO(ClBr)0.5, BiOI and commercial Bi2O3, respectively. And BiO(ClBr)0.375I0.25 exhibited the best photocatalytic performance for CO2 evolution (4.8 μmol h−1 g−1, 2.3 and 23.2 times that of BiO(ClBr)0.5 and BiOI, respectively). In addition, a composition-dependent photocatalysis mechanism is proposed in detail and it involves the indirect hole-induced ˙OH oxidation or direct hole oxidation of IPA molecules in valence bands and simultaneous electron reduction of oxygen to H2O2 in conduction bands. This work not only shows that BiO(ClBr)(1−x)/2Ix photocatalysts hold great promise for practical applications but also proves that fabricating solid solutions is an effective approach to develop highly efficient visible-light photocatalysts.

Constructing cubic–orthorhombic surface-phase junctions of NaNbO3 towards significant enhancement of CO2 photoreduction

A NaNbO3 photocatalyst with cubic–orthorhombic surface-junctions was synthesized by a polymerized-complex method. Compared with cubic and orthorhombic NaNbO3, the activity of mixed-phase NaNbO3 is enhanced by 30% and 200% in reducing CO2 into CH4, respectively. The enhancement of photoactivity over mixed-phase NaNbO3 was attributed to the cubic–orthorhombic surface-junctions which improved the charge separation.