Jenny Schneider,*,† Masaya Matsuoka,‡ Masato Takeuchi,‡ Jinlong Zhang, Yu Horiuchi,‡ Masakazu Anpo,‡ and Detlef W. Bahnemann*,† †Institut fur Technische Chemie, Leibniz Universitaẗ Hannover, Callinstrasse 3, D-30167 Hannover, Germany ‡Faculty of Engineering, Osaka Prefecture University, 1 Gakuen-cho, Sakai Osaka 599-8531, Japan Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China

Understanding TiO2 photocatalysis: mechanisms and materials.

Photoinduced reactivity of titanium dioxide

The oxidation power of the TiO2-xNx powders with low nitrogen concentrations (<0.02) was evaluated by the decomposition of gaseous 2-propanol (IPA) under the same absorbed photon number, 1.4 × 1014 quanta·cm-2·s-1, of visible (Vis) or ultraviolet (UV) light. Regardless of the x value, the quantum yield values from irradiating with Vis light was lower than with UV light, which suggests that the isolated narrow band formed above the valence band is responsible for the Vis light response in the present nitrogen doped TiO2. In addition, increasing the nitrogen concentration when irradiating with UV light lowered the quantum yields, indicating that the doping sites could also serve as recombination sites.

Nitrogen-Concentration Dependence on Photocatalytic Activity of TiO2-xNx Powders

Titanium dioxide (TiO2), as an important semiconductor metal oxide, has been widely investigated in the field of photocatalysis. The properties of TiO2, including its light absorption, charge transport and surface adsorption, are closely related to its defect disorder, which in turn plays a significant role in the photocatalytic performance of TiO2. Among all the defects identified in TiO2, oxygen vacancy is one of the most important and is supposed to be the prevalent defect in many metal oxides, which has been widely investigated both by theoretical calculations and experimental characterizations. Here, we give a short review on the existing strategies for the synthesis of defective TiO2 with oxygen vacancies, and the defect related properties of TiO2 including structural, electronic, optical, dissociative adsorption and reductive properties, which are intimately related to the photocatalytic performance of TiO2. In particular, photocatalytic applications with regard to defective TiO2 are outlined. In addition, we offer some perspectives on the challenge and new direction for future research in this field. We hope that this tutorial minireview would provide some useful contribution to the future design and fabrication of defective semiconductor-based nanomaterials for diverse photocatalytic applications.

Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications

The power conversion efficiency of hybrid solid-state solar cells has more than doubled from 7 to 15% over the past year. This is largely as a result of the incorporation of organometallic trihalide perovskite absorbers into these devices. But, as promising as this development is, long-term operational stability is just as important as initial conversion efficiency when it comes to the development of practical solid-state solar cells. Here we identify a critical instability in mesoporous TiO₂-sensitized solar cells arising from light-induced desorption of surface-adsorbed oxygen. We show that this instability does not arise in mesoporous TiO₂-free mesosuperstructured solar cells. Moreover, our TiO₂-free cells deliver stable photocurrent for over 1,000 h continuous exposure and operation under full spectrum simulated sunlight.

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Overcoming ultraviolet light instability of sensitized TiO 2 with meso-superstructured organometal tri-halide perovskite solar cells

The photocatalytic activity for NO removal under an oxidative atmosphere has been studied over commercial TiO 2 and plasma-treated TiO 2 powders. By the plasma treatment, the photocatalytic activity for NO removal appeared in the visible light region up to 600 nm without a decrease in the ultraviolet light activity. It was found that the NO was removed as nitrate (NO 3 − ) by photocatalytic oxidation over the TiO 2 powders, where NO 3 − was accumulated. No difference in the crystal structure, the crystallinity, and the specific surface area was observed between the raw TiO 2 and the plasma-treated TiO 2 photocatalysts. In electron spin resonance (ESR) measurements, a sharp signal at g =2.004, which was identified as the electrons trapped on oxygen vacancies, was detected only for plasma-treated TiO 2 under visible light irradiation. The saturated intensity of the ESR signal at g =2.004 was proportional to the removal percentage of nitrogen oxides, suggesting that the number of trapped electrons determined the activity for the photocatalytic oxidation of NO to NO 3 − . The appearance of the visible light activity in the plasma-treated TiO 2 photocatalyst was ascribed to the newly formed oxygen vacancy state between the valence and the conduction bands in the TiO 2 band structure.

Role of oxygen vacancy in the plasma-treated TiO2 photocatalyst with visible light activity for NO removal

In order to realize a visible (vis)-light-active TiO2 photocatalyst, reduction treatment using low-temperature hydrogen plasma with elevated temperatures was applied on anatase TiO2 powder of ST-01 without causing a phase transition to rutile. According to plasma-heat treatment, oxygen was evacuated from ST-01 particles and the colored ST-01, ivory to beige, was obtained. XPS results showed non-stoichiometric, expressed as TiO2−x
} and x was increased with the increase of treating time. Plasma-heat treated ST-01 showed new ESR signal at g = 2.003 assigned to electrons trapped at the oxygen-defect site. This signal was strengthened when vis-light illumination was applied. Results of vis-light activity tests, evaluated by photocatalytic oxidation of benzoic acid in liquid phase and 2-propanol in gas phase using vis-light illumination (>406 nm), showed excellent activity while low ST-01 showed almost no activity on both tests.

Preparation of a visible-light-active TiO2 photocatalyst by RF plasma treatment

The reduction of titanium dioxide (TiO2) by the hydrogen plasma treatment creates a new absorption band in the visible light region and is expected to create photocatalytic activity in this region Anatase TiO2 powder, which has photocatalytic activity in the ultraviolet (UV) region, was treated using microwave and radio-frequency (RF) plasmas The TiO2 was reduced thus having an oxygen deficiency The RF-plasma-treated TiO2 absorbed visible light between 400 and 600 nm and showed a high activity in this region, as measured by the oxidative removal of nitric oxide from the gas phase The plasma-treated TiO2, which can be easily produced on a large scale, is expected to have a higher efficiency in utilizing solar energy than the raw material

Preparation of Visible-Light-Responsive Titanium Oxide Photocatalysts by Plasma Treatment

Reduction of CO2 with H2O was carried out by microwave plasma for the formation of methanol. The results of steam chromatography and mass spectrometry showed that the plasma products contained methanol. The methanol formation was also found in H2O-plasma-cleaning process, in which materials which had been deposited in the reaction between CO2 and H2O were removed. The most adequate plasma energy density for the formation of methanol was found to be 0.26 GJ kg−1 of W/FM. The methanol yield at the system pressure of 400 Pa was 3.5 times higher than at 240 Pa for both the CO2–H2O synthetic process and the H2O-cleaning process.

Formation of Methanol by Microwave-Plasma Reduction of CO2 with H2O

A material responsive to visible light which comprises anatase-form titanium oxide and which, when examined by ESR spectroscopy in a vacuum at 77K with irradiation with light having a wavelength of 420 nm or longer, gives a main signal having a g value of 2.004 to 2.007 and two subsignals respectively having g values of 1.985 to 1.986 and 2.024, the three signals being slightly observed or substantially unobserved in the dark in a vacuum at 77K; a process for producing the material which comprises heating amorphous or incompletely crystalline titanium oxide and/or titanium hydroxide (starting titanium compound) in the presence of ammonia or a derivative thereof and terminating the heating at the time when the absorption of light having a wavelength of 450 nm by the material being yielded is greater than that by the starting titanium compound; and a light-responsive material which responds even to visible light and a process for producing the material.

Tatsuhiko Ihara

Papers

Role of oxygen vacancy in the plasma-treated TiO2 photocatalyst with visible light activity for NO removal

Preparation of a visible-light-active TiO2 photocatalyst by RF plasma treatment

Preparation of Visible-Light-Responsive Titanium Oxide Photocatalysts by Plasma Treatment

Formation of Methanol by Microwave-Plasma Reduction of CO2 with H2O

Material responsive to visible light and process for producing the same