Photocatalysis

About: Photocatalysis is a research topic. Over the lifetime, 67088 publications have been published within this topic receiving 2145233 citations. The topic is also known as: photocatalyst.

Photocatalytic Overall Water Splitting Promoted by Two Different Cocatalysts for Hydrogen and Oxygen Evolution under Visible Light

Abstract: Overall water splitting using a particulate photocatalyst and solar energy has attracted significant attention as a potential means of large-scale H2 production from renewable resources without carbon dioxide emission. 2] The reaction occurs in three steps: 1) the photocatalyst absorbs photon energy greater than the band-gap energy of the material and generates photoexcited electron–hole pairs in the bulk, 2) the photoexcited carriers separate and migrate to the surface without recombination, and 3) adsorbed species are reduced and oxidized by the photogenerated electrons and holes to produce H2 and O2, respectively. The first two steps are strongly dependent on the structural and electronic properties of the photocatalyst, while the third step is promoted by an additional catalyst (called cocatalyst). Therefore, it is important to develop a photocatalyst and a cocatalyst in harmony. Recently, our group has focused on active sites for H2 evolution on the surface of a photocatalyst, because most photocatalysts lack surface H2 evolution sites. [2b] Using a solid solution of GaN and ZnO (abbreviated GaN:ZnO hereafter) that can harvest visible photons up to ca. 500 nm, chromium-containing transition-metal oxides or noble-metal/ chromia (core/shell) nanoparticles (NPs) have been shown to function as H2 evolution cocatalysts, resulting in efficient water splitting under visible light. Meanwhile, also several sulfides were proposed as efficient catalysts for H2 evolution, and the role of H2 evolution cocatalysts has been explored by spectroscopic and electrochemical techniques. It would be natural to expect that loading both H2 and O2 evolution cocatalysts onto the same photocatalyst would improve water-splitting activity, compared to photocatalysts modified with either an H2 or O2 evolution cocatalyst. [8] It is easy to imagine how these two different cocatalysts would separately facilitate H2 and O2 evolution, thereby promoting overall water splitting in harmony. Unfortunately, no successful and reliable example of this has been reported since the initial reports on photocatalytic water splitting in the 1980s. The actual demonstration of the concept remains a major challenge. Herein, we show a proof-of-concept using GaN:ZnO loaded with Rh/Cr2O3 (core/shell) and Mn3O4 NPs as H2 and O2 evolution promoters, respectively, under irradiation with visible light (l> 420 nm). First, Mn oxide was introduced onto GaN:ZnO, prepared by our previous method, as O2 evolution cocatalyst. Some Mn oxides have been reported to act as O2 evolution promoters, and it is well known that a Mn complex is the O2 evolution center in the photosynthesis of green plants. MnO NPs with a mean size of (9.2 0.4) nm (Figure S1 in the Supporting Information) were adsorbed onto GaN:ZnO. It was revealed by UV/vis spectroscopy that the introduced MnO NPs (ca. 1.0 wt %) were almost quantitatively anchored on the GaN:ZnO surface, based on the change in the absorption band of the MnO NPs (Figure S2 in the Supporting Information). The as-prepared MnO/GaN:ZnO sample was then calcined in air at 673 K for 3 h to remove organic residues. Separate experiments with thermogravimetry, differential thermal analysis (TG-DTA), and X-ray diffraction (XRD) showed that the organic ligands stabilizing the MnO NPs were completely burned off by calcination in air at 673 K, and that calcination of dried MnO NP powder under the above conditions resulted in phase transformation of the MnO into Mn3O4 (Figure S3 in the Supporting Information). Transmission electron microscopy (TEM) observation revealed that the particle size of the Mn oxide was maintained, even after calcination (Figure S1 in the Supporting Information). Thus, GaN:ZnO particles were successfully decorated with Mn3O4 NPs which were expected to act as water oxidation cocatalysts. Because GaN:ZnO is an n-type semiconductor, it is possible to monitor the photooxidation reaction occurring on its surface using an electrochemical technique. Under [*] Dr. K. Maeda, A. Xiong, N. Sakamoto, Dr. T. Hisatomi, Prof. Dr. K. Domen Department of Chemical System Engineering The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan) Fax: (+ 81)3-5841-8838 E-mail: domen@chemsys.t.u-tokyo.ac.jp Homepage: http://www.domen.t.u-tokyo.ac.jp/

BiVO(4)/CeO(2) nanocomposites with high visible-light-induced photocatalytic activity.

Abstract: Preparation of bismuth vanadate and cerium dioxide (BiVO4/CeO2) nanocomposites as visible-light photocatalysts was successfully obtained by coupling a homogeneous precipitation method with hydrothermal techniques. The BiVO4/CeO2 nanocomposites with different mole ratios were synthesized and characterized by X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). Absorption range and band gap energy, which are responsible for the observed photocatalyst behavior, were investigated by UV–vis diffuse reflectance (UV–vis DR) spectroscopy. Photocatalytic activities of the prepared samples were examined by studying the degradation of model dyes Methylene Blue, Methyl Orange, and a mixture of Methylene Blue and Methyl Orange solutions under visible-light irradiation (>400 nm). Results clearly show that the BiVO4/CeO2 nanocomposite in a 0.6:0.4 mol ratio exhibited the highest photocatalytic activity in dye wastewater treatment.

Photodegradation of Graphene Oxide Sheets by TiO2 Nanoparticles after a Photocatalytic Reduction

Abstract: TiO2 nanoparticles were physically attached to chemically synthesized single-layer graphene oxide nanosheets deposited between Au electrodes in order to investigate the electrical, chemical, and structural properties of the TiO2/graphene oxide composition exposed to UV irradiation. X-ray photoelectron spectroscopy showed that after effective photocatalytic reduction of the graphene oxide sheets by the TiO2 nanoparticles in ethanol, the carbon content of the reduced graphene oxides gradually decreased by increasing the irradiation time, while no considerable variation was detected in the reduction level of the reduced sheets. Raman spectroscopy indicated that, at first, the photocatalytic reduction resulted in a significant increase in the graphitized sp2 structure over the disorders in the graphene oxides. After that, as the carbon content decreased by UV irradiation, further disorders appeared in the reduced graphene oxide sheets, confirming degradation of the reduced sheets after the photocatalytic redu...

Nanocomposite heterojunctions as sunlight-driven photocatalysts for hydrogen production from water splitting

Abstract: Hydrogen production via photocatalytic water splitting using sunlight has enormous potential in solving the worldwide energy and environmental crisis. The key challenge in this process is to develop efficient photocatalysts which must satisfy several criteria such as high chemical and photochemical stability, effective charge separation and strong sunlight absorption. The combination of different semiconductors to create composite materials offers a promising way to achieve efficient photocatalysts because doing so can improve the charge separation, light absorption and stability of the photocatalysts. In this review article, we summarized the most recent studies on semiconductor composites for hydrogen production under visible light irradiation. After a general introduction about the photocatalysis phenomenon, typical heterojunctions of widely studied heterogeneous semiconductors, including titanium dioxide, cadmium sulfide and graphitic carbon nitride are discussed in detail.