Photo-induced processes in heterogeneous nanosystems. From photoexcitation to interfacial chemical transformations (original) (raw)
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Photoreactions occurring on metal-oxide surfaces are not all photocatalytic
Catalysis Today, 2007
Studies involving the TiO 2 -assisted photodegradation of organic substances report that the processes are photocatalytic in nature. Yet, no evidence exists confirming such assertions. Previously, we examined the usage of relative photonic efficiencies to systematize discrepant claims about process efficiencies. An experimental protocol is now available [N. Serpone, A. Salinaro, Pure Appl. Chem. 71 (1999) 303] to measure true F in heterogeneous media. Photoinduced reduction of O 2 and photooxidation of H 2 occurring on oxidized and reduced surfaces of ZrO 2 were recently examined [A.V. Emeline, G.N. Kuzmin, L.L. Basov, N. Serpone, J. Photochem. Photobiol. A: Chem. 174 (2005) 214] to probe the spectral variations of the photoactivity and photo-selectivity of ZrO 2 by determining F for the two redox reactions at various wavelengths of photoexcitation (200 < l < 400 nm). Irradiation of ZrO 2 in the intrinsic absorption region (l < 260 nm) led predominantly to photoreduction of O 2 , whereas photooxidation of H 2 predominated on irradiation in the extrinsic spectral region (260 < l < 360 nm). A difficult task in heterogeneous catalysis and photocatalysis is determination of the actual number of active sites, an issue that has heretofore been elusive to assess the (photo)catalytic activity of a given material in heterogeneous solid-liquid and solid-gas (photo)catalysis. A kinetic description of the three turnover quantities, viz., turnover number (TON), turnover rate (TOR) and turnover frequency (TOF) has been described [N. Serpone, A. Salinaro, A.V. Emeline, V.K. Ryabchuk, J. Photochem. Photobiol. A: Chem. 130 , concluding that turnover quantities are conceptually distinct, with TON and TOR requiring knowledge of the number of active sites on the (photo)catalyst's surface. Apparently, turnovers depend on the nature of the active state of the catalyst and how it is described. The number of surface-active sites on the ZrO 2 particle surface have been determined quantitatively ($10 16 active centers) through thermoprogrammed desorption spectroscopy, affording an estimate of TONs for the photooxidation of H 2 (TON > 14.5) and photoreduction of O 2 (TON > 6.6) on ZrO 2 and demonstrating for the first time that a photoreaction occurring on the surface of a metal oxide is truly photocatalytic [A.V. Emeline, A.V. Panasuk, N. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 ]. Photocoloration of a metal oxide such as ZrO 2 (process of photoinduced formation of Zr 3+ , F-and V-type color centers) during a surface photochemical reaction was also used to assess whether a reaction is photocatalytic. Our study on the influence of simple photoreactions involving the photoreduction of O 2 , photooxidation of H 2 , photooxidation of H 2 by adsorbed O 2 , and photoinduced transformation of NH 3 and CO 2 on the photocoloration of ZrO 2 concluded that photoreactions involving NH 3 and CO 2 are non-photocatalytic processes, in contrast to the photooxidation of H 2 which is photocatalytic [A.V. Emeline, G.V. Kataeva, A.V. Panasuk, V.K. Ryabchuk, N.V. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 ]. In this article, we describe the criteria and conditions by which a photoreaction taking place on the surface of a solid can be said to be photocatalytic by considering both a chemical approach and a physical approach. #
Titania is one of the most widely used benchmark standard photocatalysts in the field of environmental applications. However, the large band gap of titania and massive recombination of photogenerated charge carriers limit its overall photocatalytic efficiency. The former can be overcome by modifying the electronic band structure of titania including various strategies like coupling with a narrow band gap semiconductor, metal ion/nonmetal ion doping, codoping with two or more foreign ions, surface sensitization by organic dyes or metal complexes, and noble metal deposition. The latter can be corrected by changing the surface properties of titania by fluorination or sulfation or by the addition of suitable electron acceptors besides molecular oxygen in the reaction medium. This review encompasses several advancements made in these aspects, and also some of the new physical insights related to the charge transfer events like charge carrier generation, trapping, detrapping, and their transfer to surface are discussed for each strategy of the modified titania to support the conclusions derived. The synergistic effects in the mixed polymorphs of titania and also the theories proposed for their enhanced activity are reported. A recent venture on the synthesis and applications of anatase titania with a large percentage of reactive {001} facets and their band gap extension to the visible region via nonmetal ion doping which is a current hot topic is briefly outlined.
Journal of Catalysis, 2003
This review deals with the preparation of highly reactive titanium oxide photocatalysts and the clarification of the active sites as well as the detection of the reaction intermediates at the molecular level. Furthermore, we discuss the advancement of photofunctional systems and processes that can utilize visible and/or solar light. The photocatalytic reactivity of semiconducting TiO 2 powder was found to be dramatically enhanced by the loading of small amounts of Pt, which work to enhance the charge separation of the electrons and holes generated by light irradiation. Highly dispersed titanium oxide species prepared within zeolite frameworks or silica matrices showed unique photocatalytic performance much higher than that of conventional semiconducting TiO 2 photocatalysts. The potential for the effective utilization and conversion of solar energy makes research into the modification of the electronic properties of TiO 2 photocatalysts by such methods as advanced metal ion implantation to produce photocatalysts which are able to absorb and operate efficiently even under visible light irradiation one of the most important fields in photocatalysis research. This modification process can be applied not only to semiconducting TiO 2 photocatalysts but also to TiO 2 thin film photocatalysts, as well as titanium oxide photocatalysts highly dispersed within zeolite frameworks. Significantly, a new alternative method for directly preparing such visible-light-responsive TiO 2 thin film photocatalysts has been successfully developed by applying a RF magnetron sputtering deposition method.
International Journal of Photoenergy, 2008
This article briefly reviews some factors that have impacted heterogeneous photocatalysis with next generation TiO 2 photocatalysts, along with some issues of current debate in the fundamental understanding of the science that underpins the field. Preparative methods and some characteristics features of N-doped TiO 2 are presented and described briefly. At variance are experimental results and interpretations of X-ray photoelectron spectra (XPS) with regard to assignments of N 1s binding energies in N-doped TiO 2 systems. Relative to pristine nominally clean TiO 2 with absorption edges at 3.2 eV (anatase) and 3.0 eV (rutile), N-doped TiO 2 s display red-shifted absorption edges into the visible spectral region. Several workers have surmised that the (intrinsic) band gap of TiO 2 is narrowed by coupling dopant energy states with valence band (VB) states, an inference based on DFT computations. With similar DFT computations, others concluded that red-shifted absorption edges originate from the presence of localized intragap dopant states above the upper level of the VB band. Recent analyses of absorption spectral features in the visible region for a large number of doped TiO 2 specimens, however, have suggested a common origin owing to the strong similarities of the absorption features, and this regardless of the preparative methods and the nature of the dopants. The next generation of (doped) TiO 2 photocatalysts should enhance overall process photoefficiencies (in some cases), since doped TiO 2 s absorb a greater quantity of solar radiation. The fundamental science that underpins heterogeneous photocatalysis with the next generation of photocatalysts is a rich playing field ripe for further exploration.
Charge transfer in TiO2-based photocatalysis: fundamental mechanisms to material strategies
Nanoscale, 2024
Semiconductor-based photocatalysis has attracted significant interest due to its capacity to directly exploit solar energy and generate solar fuels, including water splitting, CO2 reduction, pollutant degradation, and bacterial inactivation. However, achieving the maximum efficiency in photocatalytic processes remains a challenge owing to the speedy recombination of electron–hole pairs and the limited use of light. Therefore, significant endeavours have been devoted to addressing these issues. Specifically, welldesigned heterojunction photocatalysts have been demonstrated to exhibit enhanced photocatalytic activity through the physical distancing of electron–hole pairs generated during the photocatalytic process. In this review, we provide a systematic discussion ranging from fundamental mechanisms to material strategies, focusing on TiO2-based heterojunction photocatalysts. Current efforts are focused on developing heterojunction photocatalysts based on TiO2 for a variety of photocatalytic applications, and these projects are explained and assessed. Finally, we offer a concise summary of the main insights and challenges in the utilization of TiO2-based heterojunction photocatalysts for photocatalysis. We expect that this review will serve as a valuable resource to improve the efficiency of TiO2-based heterojunctions for energy generation and environmental remediation.
Surface and Electronic Structure of Titanium Dioxide Photocatalysts
The Journal of Physical Chemistry B, 2000
TiO 2 films prepared by sol-gel route are active photocatalysts for the oxidation of organics in photoelectrochemical cells. The as-grown films for photocatalysis applications and those exposed to Ar + or H 2 + +Ar + ion bombardment are characterized by different spectroscopic methods, such as X-ray diffraction (XRD), atomic force microscopy (AFM), UV-vis transmittance, photothermal deflection spectroscopy (PDS) and X-ray photoelectron spectroscopy (XPS), as well as by conductance. This material has defects associated with oxygen vacancies produced during the sample preparation which support nondissociative adsorption of O 2 when films are exposed to air. Charge transfer from reduced Ti species to adsorbed dioxygen leads to Ti-O 2surface complexes that are partially removed by heating at 200°C, and fully removed after 30 min ion bombardment. By comparison with the relatively well-understood structural defects of bombarded TiO 2 we arise to a quite complete structural model of the as grown material which corresponds to an amorphous semiconductor possessing relative low disorder and density of states as compared with a pure amorphous material. These TiO 2 films are modeled as low size crystalline domain embedded in an amorphous matrix whose electronic structure exhibit exponential band tails and a narrow band close to the conduction band. The latter is fully or partially occupied depending on the presence of adsorbed electron scavengers such as dioxygen.
Insights into the TiO2-Based Photocatalytic Systems and Their Mechanisms
2019
Photocatalysis is a multifunctional phenomenon that can be employed for energy applications such as H2 production, CO2 reduction into fuels, and environmental applications such as pollutant degradations, antibacterial disinfection, etc. In this direction, it is not an exaggerated fact that TiO2 is blooming in the field of photocatalysis, which is largely explored for various photocatalytic applications. The deeper understanding of TiO2 photocatalysis has led to the design of new photocatalytic materials with multiple functionalities. Accordingly, this paper exclusively reviews the recent developments in the modification of TiO2 photocatalyst towards the understanding of its photocatalytic mechanisms. These modifications generally involve the physical and chemical changes in TiO2 such as anisotropic structuring and integration with other metal oxides, plasmonic materials, carbon-based materials, etc. Such modifications essentially lead to the changes in the energy structure of TiO2 t...