Vladimir K. Ryabchuk

TERMINOLOGY, RELATIVE PHOTONIC EFFICIENCIES AND QUANTUM YIELDS IN HETEROGENEOUS PHOTOCATALYSIS. PART II: EXPERIMENTAL DETERMINATION OF QUANTUM YIELDS†

In the preceding article [Serpone and Salinaro, Pure Appl. Chem., 71(2), 303-320 (1999)] we examined two principal features of heterogeneous photocatalysis that demanded scrutiny: (i) description of photocatalysis and (ii) description of process efficiencies. For the latter we proposed a protocol relative photonic efficiency which could subsequently be converted to quantum yields. A difficulty in expressing a quantum yield in heterogeneous photochemistry is the very nature of the system, either solid/liquid or solid/gas, which places severe restrictions on measurement of the photon flow absorbed by the light harvesting component, herein the photocatalyst TiO2, owing to non-negligible scattering by the particulates. It was imperative therefore to examine the extent of this problem. Extinction and absorption spectra of TiO2 dispersions were determined at low titania loadings by normal absorption spectroscopy and by an integrated sphere method, respectively, to assess the extent of light scattering. The method is compared to the one reported by Grela et al. [J. Phys. Chem., 100, 16940 (1996)] who used a polynomial extrapolation of the light scattered in the visible region into the UV region where TiO2 absorbs significantly. This extrapolation underestimates the scattering component present in the extinction spectra, and will no doubt affect the accuracy of the quantum yield data. Further, we report additional details in assessing limiting photonic efficiencies and quantum yields in heterogeneous photocatalysis.

Turnovers and photocatalysis: A mathematical description

Abstract In previous articles we dwelled on the usage of relative photonic efficiencies [N. Serpone, G. Suave, R. Koch, H. Tahiri, P. Pichat, P. Piccinini, E. Pelizzetti, H. Hidaka, J. Photochem. Photobiol. A: Chem. 94 (1996) 191; N. Serpone, J. Photochem. Photobiol. A: Chem. 104 (1997) 1] and quantum yields Φ [N. Serpone, R. Terziaw, D. Lawless, P. Kennepohl, G. Suave, J. Photochem. Photobiol. A: Chem. 73 (1993) 11]. Recently, we also provided an experimental protocol to measure Φ in heterogeneous media [N. Serpone, A. Salinaro, Pure Appl. Chem. 71 (1999) 303] to infer which of several photocatalyzed processes might be the more significant and efficient process. In this article we revisit photocatalysis and discuss how to describe mathematically (photo)catalytic activity and how to compare (photo)catalytic activities of various materials. Specifically, we address the usage and provide a kinetic description of the three turnover quantities: turnover number (TON), turnover rate (TOR) and turnover frequency (TOF) as they bear on the (photo)catalytic activity of a given material in heterogeneous solid/liquid or solid/gas (photo)catalysis. We argue that these turnover quantities are conceptually distinct. TON and TOR require knowledge of the number of active sites on the (photo)catalyst’s surface, contrary to the requirement to determine TOF. Most significant, these turnovers also depend on the nature of the active state of the catalyst, and hence on how the active centers are described. This goes back to the differences in the nature of photocatalysis and photoinduced catalysis.

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.

Photo-induced processes in heterogeneous nanosystems. From photoexcitation to interfacial chemical transformations

This article briefly reviews some of our recent work carried out both from an experimental point of view as well as from a theoretical perspective to gain further understanding of the events that take place in Heterogeneous Photocatalysis. Previously, the multitude of reports from our laboratory and from many others looked at the primary photocatalytic events as involving (a) absorption of light, (b) formation of the free (electrons and holes) and/or trapped charge carriers (Ti 3+ andOH radicals), and (c) reaction of pre-adsorbed acceptor or donor molecules with the relevant trapped carrier. Our recent work notes that this view is reasonable if the only purpose of photocatalysis is elimination of undesirable environmental pollutants. But when we begin to query how to render a process more efficient, we need to address the primary events following photoexcitation of the photocatalyst, which in most instances has been titanium dioxide (in the anatase form). Owing to the nature of light absorption by TiO2 we resorted to examining other metal oxides, most of which are dielectric insulators with very large bandgap energies, for example zirconia (ZrO2) and scandia (Sc2O3). These dielectrics have provided added information on the photophysical events, many of which are masked by the strong light absorption in titania. Despite some of our recent progress, much remains to be done for a fuller understanding of the events that occur at the surface, which we have often considered to be the greatest and most complex defect in metal oxide particulates.

Photophysical processes related to photoadsorption and photocatalysis on wide band gap solids: A review

During the last two decades, various pathways describing photoexcitation of small molecules’ surface reactions at the wide band gap metal oxides and halides (Eg>3 eV) have been recognized. Photogeneration of excitons and free charge carriers may occur in bands of: i) fundamental absorption; ii) extrinsic and intrinsic defect absorption, including those related to surface states; and iii) in UV-induced color centers. Considerable red shifts relative to the fundamental absorption threshold of wide band gap solids have been observed for the spectral limits of surface photoreactions induced in extrinsic absorption bands. This allows thinking about the wide band gap solids as a potential competitors for the relatively narrow band gap photocatalysts. This review discusses the concept of surface photoadsorption (photocatalytic) center while differentiating active and inactive states of the center. Electronically excited defect, surface self-trapped or bound exciton, and the surface defect with trapped photo carrier are considered as the active states of photoadsorption (photocatalytic) centers of different types. The decay pathway of active state determines the lifetime of a photocatalytic center, and in this connection the Langmuir-Hinshelwood kinetic approach is discussed.

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.

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.

Photoinduced Radical Processes on the Spinel (MgAl2O4) Surface Involving Methane, Ammonia, and Methane/Ammonia

The present study explored photoinduced radical processes caused by interaction of CH(4) and NH(3) with a photoexcited surface of a complex metal oxide: magnesium-aluminum spinel (MgAl(2)O(4); MAS). UV irradiation of MAS in vacuo yielded V-type color centers as evidenced by the 360 nm band in difference diffuse reflectance spectra. Interaction of these H-bearing molecules with photogenerated surface-active hole states (O(S)(-)•) yielded radical species which on recombination produced more complex molecules (including heteroatomic species) relative to the initial molecules. For the MAS/CH(4) system, photoinduced dissociative adsorption of CH(4) on surface-active hole centers produced •CH(3) radicals that recombined to yield CH(3)CH(3). For MAS/NH(3), a similar dissociative adsorption process led to formation of •NH(2) radicals with formation of NH(2)NH(2) as an intermediate product; continued UV irradiation ultimately yielded N(2). For the mixed MAS/CH(4)/NH(3) system, however, interaction of adsorbed NH(3) and CH(4) on the UV-activated surface of MAS yielded •NH(2) and •CH(3) radicals, respectively, which produced CH(3)-NH(2) followed by loss of the remaining hydrogens to form a surface-adsorbed cyanide, CN(S), species. Recombination of photochemically produced radicals released sufficient energy to re-excite the solid spinel, generating new surface-active sites and a flash luminescence (emission decay time at 520 nm, τ ~ 6 s for the MAS/NH(3) case) referred to as the PhICL effect.

Invalidity of Band Gap Engineering Concept for Bi3+ Heterovalent Doping in CsPbBr3 Halide Perovskite.

Heterovalent CsPbBr3 doping with Bi results in a significant red shift of the optical absorption of both single-crystal and powdered samples. The results of low-temperature (3.6 K) photoluminescence studies of perovskite single crystals indicate that the position of the excitonic luminescence peak remains unaffected by Bi doping that, in turn, infers that the band gap of Bi-doped perovskite is not changed as well. The position and state density distribution of the valence band and Fermi level of single-crystal perovskites were determined by another direct method of ultraviolet photoelectron spectroscopy. The obtained results show that Bi3+ doping causes no changes in the valence band structure but an increase in the Fermi level by 0.6 eV. The summary of the obtained results directly demonstrates that the concept of the band-gap engineering in Bi3+-doped CsPbBr3 halide perovskite is not valid.