Vyacheslav N. Kuznetsov

On the genesis of heterogeneous photocatalysis: a brief historical perspective in the period 1910 to the mid-1980s

The concept Photocatalysis and, of greater import here, Heterogeneous Photocatalysis were first introduced in the second decade (1910-1920) of the 20th century according to the CAPLUS and MEDLINE databases (SciFinder). This review reports a brief historical perspective on the origins of the two concepts, whether implied or explicitly stated, in some detail up to about the mid-1980s when heterogeneous photocatalysis witnessed the beginning of an exponential growth, with particular emphasis on the use of nanosized TiO(2) particles in powdered form as the (so-called) photocatalyst of choice in environmental applications because of its inherent properties of abundance and chemical stability in acidic and alkaline aqueous media (in the dark), in contrast to ZnO that had been the metal oxide of choice in the early days. The early workers in this area often used the term photosensitization rather than the current popular term photocatalysis, used since the early 1980s. The term Photocatalysis appeared in the literature as early as 1910 in a book by Plotnikow (Russia) and a few years later it was introduced in France by Landau. The review also reports on contributions during the early years by Terenin at the University of St. Petersburg (previously Leningrad, Soviet Union), and in the decade spanning 1975-1985 contributions by Bards group at the University of Texas at Austin (USA) as well as those of other groups. Some activities into the conversion of light energy to chemical fuels (e.g. H(2)) during the 1975-1985 decade are also considered.

On the way to the creation of next generation photoactive materials

IntroductionTransition from first- to second-generation photocatalysts has followed the notion that greater absorption of light in the visible region would yield greater spectral sensitivity and greater photoactivity. Though a promising strategy, in practice, it did not meet expectation because of various side issues, which in many cases has led to loss of photoactivity and chemical reactivity. This article examines some earlier notions that arose from applications of different metal oxides (e.g., TiO2, ZnO, MgO among others) that made these oxides good photocatalysts in many processes.DiscussionPhenomena that proved relevant in developing next generation photoactive materials are considered: the dependence of the activity of photocatalysts on the band gap energy, the spectral variations of the activity of photoactive materials, and the spectral variations of selectivity of photoactive materials. The tendency to decrease the energy of actinic photons through doping in forming second-generation photocatalysts is completely opposite the fundamental observation in first-generation photocatalysts whereby the activity increased with increasing band gap energy. Extension of spectral sensitivity of second-generation photoactive materials also caused a decrease of their photoactivity; hence, some notions are reconsidered to produce next(third) generation photoactive materials.SummaryThe article proposes the following concepts to develop next generation photocatalysts: (1) multi(two)-photon excitation of photoactive materials with lower energy photons to achieve the same excited state as with higher energy photons, (2) utilization of heterojunctions to drive electronic processes in the desired direction, and (3) selective photoexcitation of localized electronic states to gain better selectivity.

DFT model cluster studies of O2 adsorption on hydrogenated titania sub-nanoparticles

AbstractIn the present paper, we examine the general applicability of different TiO2 model clusters to study of local chemical events on TiO2 sub-nanoparticles. Our previous DFT study of TiO2 activation through H adsorption and following deactivation by O2 adsorption using small amorphous Ti8O16 cluster were complemented by examination of rutile-type and spherical Ti15O30 nanoclusters. The obtained results were thoroughly compared with experimental data and results of related computational studies using other TiO2 models including periodic structures. It turned out that all considered model TiO2 model systems provide qualitatively similar results. It was shown that atomic hydrogen is adsorbed with negligible activation energy on surface O atoms, which is accompanied by the appearance of reduced Ti3+ species and corresponding localized band gap 3d-Ti states. Oxygen molecule is adsorbed on Ti3+ sites spontaneously forming molecular O2– species by capturing an extra electron of Ti3+ ion, which results in disappearance of Ti3+ species and corresponding band gap states. Calculated g-tensor values of Ti3+ and O2– species agree well with the results of EPR studies and do not depend on the used TiO2 model cluster. Additionally, it was shown that the various cluster calculations provide results comparable with the calculations of periodic structures with respect to the modeling of chemical processes under study. As a whole, the present study approves the validity of molecular cluster approach to study of local chemical events on TiO2 sub-nanoparticles. FigureElectronic structure diagrams for small Ti8O16H and large Ti15O30H hydrogenated clusters

Second Generation Visible-Light-Active Photocatalysts: Preparation, Optical Properties, and Consequences of Dopants on the Band Gap Energy of TiO2

First generation metal-oxide photocatalysts based mostly on nominally pure, pristine titanium dioxide have been the object of great debate in the past 30 years with regard (i) to the nature of the oxidative agent (•OH radicals vs. holes h+); (ii) to the site at which the reaction takes place (surface vs. bulk solution); (iii) to whether TiO2 is indeed a photocatalyst since turnover numbers are difficult to determine owing to the nature of the particle surface; and (iv) to how the process efficiency can be ascertained, among many other issues yet to be resolved satisfactorily. One issue that has taken some time to be resolved is the notion of how we can make better use of sunlight’s visible radiation seeing that the absorption edge of TiO2 is at 387 nm (ca. 3.2 eV – the band gap energy) for the anatase polymorph. A successful strategy that is gaining some momentum is to dope this metal oxide with suitable dopants (e.g., metal ions and/or non-metals) to shift the absorption edge to longer wavelengths. Doping has been achieved using various physical and chemical strategies, which have led to materials whose absorption edges have been red-shifted to wavelengths ~550 nm (and beyond in some cases). The debate that now occupies discussions of doped-TiO2 materials regards the causes for this red shift. Several reports, based on density functional theory (DFT), have asserted that the band gap of doped-TiO2 is narrowed because of interactions between the dopant states and the O 2p states of the valence band, thereby pushing the valence band edge upward. Others have proposed isolated dopant states located within the band gap to explain the red shift of the absorption edges of doped-TiO2 systems through excitation of the electrons in these states to the conduction band of TiO2. Absorption spectra, calculated from several diffuse reflectance spectra (DRS) reported in the literature for both metal ion-doped TiO2s and systems doped with non-metals (e.g., carbon, sulfur, nitrogen, and fluorine), are remarkably similar if not identical in the visible spectral region. The broad spectral envelope observed at wavelengths greater than 400 nm can be deconvoluted into 2–3 single bands, which indicate different species give rise to these bands. This chapter is therefore concerned, albeit in a very restrictive way, with the various strategies used to dope TiO2, with their modeling by DFT methods, and finally with their optical properties with which we shall argue that the absorption edge red-shift originates from a singular source involving mostly the formation of (additional to existing) oxygen vacancies in the metal-oxide lattice (both surface and bulk) that can act as electron traps to yield F-type color centers and/or Ti3+ color centers.

In situ study of photo- and thermo-induced color centers in photochromic rutile TiO2 in the temperature range 90–720 K

This article reports an in situ UV-Vis-NIR diffuse reflectance (DR) spectroscopic and kinetic study of the photoformation and thermal annealing of light absorbing electronic point defects (color centers) in photochromic TiO2 in the temperature range 90-720 K using a simple laboratory-made cryostat-type accessory (for a Cary 5000 spectrophotometer equipped with an integrating sphere). The accessory also allowed for UV-Vis-NIR DR studies to be undertaken either in vacuum or in an oxygen atmosphere at significantly high temperatures (to 720 K) to assess dark chemical events occurring in photochromic titania with the participation of color centers. The DR spectral and kinetic measurements provided the opportunity to examine the separation of photoinduced charge carriers at traps and thermally stimulated carrier detrapping and recombination, as well as the response of color centers to oxidative/reductive treatments of photochromic TiO2. Kinetic results also demonstrate the applicability of the fabricated DR accessory as a high-temperature reaction cell in the systematic study of the principal regularities in the formation and destruction of color centers in titania at various temperatures and gaseous atmospheres.

Water Will Be the Coal of the Future—The Untamed Dream of Jules Verne for a Solar Fuel

This article evokes the futuristic visions of two giants, one a writer, Jules Verne, who foresaw water as the coal of the future, and the other a scientist, Giacomo Ciamician, who foresaw the utilization of solar energy as an energy source with which to drive photochemical and photocatalytic reactions for the betterment of mankind. Specifically, we examine briefly the early work of the 1960s and 1970s on the photosplitting of free water and water adsorbed on solid supports, based mostly on metal oxides, from which both hydrogen and oxygen evolve in the expected stoichiometric ratio of 2 to 1. The two oil crises of the 1970s (1973 and 1979) spurred the interest of researchers from various disciplines (photochemistry, photo-catalysis and photoelectrochemistry) in search of a Holy Grail photocatalyst, process, or strategy to achieve efficient water splitting so as to provide an energy source alternative to fossil fuels. Some approaches to the photosplitting of water adsorbed on solid insulators (high bandgap materials; Ebg ≥ 5 eV) and semiconductor photocatalysts (metal oxides) are described from which we deduce that metal oxides with bandgap energies around 5 eV (e.g., ZrO2) are more promising materials to achieve significant water splitting on the basis of quantum yields than narrower bandgap photocatalysts (e.g., TiO2; Ebg ≈ 3.0–3.2 eV), which tend to be relatively inactive by comparison. Although proof of concept of the photosplitting of water has been demonstrated repeatedly in the last four decades, much remains to be done to find the Holy Grail photocatalyst and/or strategy to achieve significant yields of hydrogen.

Spectra of thermoprogrammed annealing of photoinduced color centers

The kinetics of photoinduced formation and thermoprogrammed annealing of color centers in photochromic rutile ceramics has been studied in situ with the aid of a specially designed attachment for a spectrofluorimeter. Using a regime of constant heating rate, the spectra of color center annealing have been measured and the energy depths of hole traps responsible for the annealing of these centers have been determined.

Photo- and thermoinduced color centers in TiO 2 ceramics

Using a cryostat chamber developed for a spectrophotometer equipped with an integrating sphere, we have studied color centers (CCs) in photochromic titanium dioxide ceramics induced by radiation in the UV and visible spectral ranges. The channel of CC formation and annihilation during reductive-oxidation treatment is revealed, and the effect of thermoinduced growth of CC absorption has been found.

CHAPTER 9:Interplay Between Physical and Chemical Events in Photoprocesses in Heterogeneous Systems

The principal objectives of this chapter are to demonstrate those interplay phenomena that take place between physical and chemical events in heterogeneous photochemistry and photocatalysis through an examination of some relevant processes starting from the initial photoexcitation of solids to subsequent events occurring on absorption of photons by semiconductor/insulator photoactive materials, followed by competition and interconnection between physical and chemical relaxation of heterogeneous systems. Particular examples of such interplay are presented as a competition between physical and chemical decay of the active state of surface-active centers through charge recombination and chemical interaction, as an effect of catalytic and non-catalytic surface photochemical processes on the photostimulated formation of defects, and the interconnection between photocatalyst activity and selectivity. It is deduced that the creation of new generations of photoactive materials will lead to successful applications if and only if such interplay between physical and chemical processes is fully taken into account.