Alexei V. Emeline

Glossary of terms used in photocatalysis and radiation catalysis (IUPAC Recommendations 2011)

This glossary of terms covers phenomena considered under the very wide terms photocatalysis and radiation catalysis. A clear distinction is made between phenomena related to either photochemistry and photocatalysis or radiation chemistry and radiation catalysis. The term “radiation” is used here as embracing electromagnetic radiation of all wavelengths, but in general excluding fast-moving particles. Consistent definitions are given of terms in the areas mentioned above, as well as definitions of the most important parameters used for the quantitative description of the phenomena. Terms related to the up-scaling of photocatalytic processes for industrial applications have been included. This Glossary should be used together with the Glossary of terms used in photochemistry, 3rd edition, IUPAC Recommendations 2006: (doi:10.1351/pac200779030293) as well as with the IUPAC Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”, 2006– doi:10.1351/goldbook) because many terms used in photocatalysis are defined in these documents.

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.

Fibrous TiO2–SiO2 nanocomposite photocatalyst

The electrospinning method is employed to prepare a fibrous TiO2-SiO2 (Ti : Si = 1 : 2) nanocomposite photocatalyst, in which Degussa P25 T i O2 nanoparticles are embedded inthe body of SiO2 fibers and which shows good photocatalytic activity due to its 3-D open structure, as evidenced by photocatalytic reduction of silver ions and decomposition of acetaldehyde.

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.

Activity and selectivity of photocatalysts in photodegradation of phenols

Photodegradation of phenol and 4-chlorophenol over six different TiO(2) samples was tested in order to establish whether an interconnection between the activity and selectivity of photocatalysts exists. The obtained experimental data were analyzed using correlation analysis. Some correlations between the activity in phenol(s) photodegradation and selectivity toward formation of primary intermediate products were established. The type of correlations depends on the type of studied photoreactions. The discussion of the observed correlations between the activity and selectivity of photocatalysts is given in terms of the difference of surface concentrations of electrons and holes and corresponding surface active sites which might be dependent on the types of dominating surface faces. On the basis of the obtained results of correlation analysis it was assumed that a higher activity of photocatalysts could be achieved provided that both reduction and oxidation reaction pathways occur with equally high efficiency.

In situ picosecond transient diffuse reflectance spectroscopy of opaque TiO2 systems under microwave irradiation and influence of oxygen vacancies on the UV-driven/microwave-assisted TiO2 photocatalysis

We report a picosecond transient diffuse reflectance study of commercially available pristine Ishihara ST01 titania, which upon treatment with hydrogen gas yields an oxygen-vacancy rich VO-ST01 system. For comparison, a nitrogen-doped N-ST01 sample was also prepared using urea as the nitrogen donor. These were characterized by XRD and by diffuse reflectance spectroscopy. Transient decay kinetics at 550 nm for all three samples were determined in situ using a 150 ps Nd-YAG pulsed laser system (10 Hz) and a Xe flash lamp (2 μs pulses) probe while samples were being microwave-irradiated (2.45 GHz). The transient(s) absorbing at the probe wavelength displayed double exponential decay kinetics: a fast decay that occurred within ca. 5–12 ns ascribed to recombination of photogenerated shallow-trapped or free conduction band electrons with valence band holes, and a slower decay that occurred from hundreds of nanoseconds to several microseconds attributable to recombination of electrons trapped in deep traps (e.g., either as Ti3+ or as F color centers) with free holes. Significant differences were observed for the wet pristine ST01/H2O and VO-ST01/H2O systems when subjected to microwave irradiation; results concurred with those from the degradation of the 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide in aqueous TiO2 dispersions at 100 °C under UV/microwave irradiation (UV/MW) and UV irradiation with conventional heating (UV/CH).

Sonochemical/hydration–dehydration synthesis of Pt–TiO2 NPs/decorated carbon nanotubes with enhanced photocatalytic hydrogen production activity

Modified Pt-TiO2 NPs/decorated carbon nanotubes were synthesized utilizing sonochemical/hydration-dehydration techniques. Pt was loaded on TiO2 by a photodeposition method keeping in mind the end goal to achieve electron-hole pair separation and promote the surface reaction. The morphological and basic properties of Pt-TiO2/fCNTs were investigated by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) and Raman spectroscopy. The selected area electron diffraction (SAED) patterns of Pt-TiO2/fCNTs were obtained utilizing TEM-based energy dispersive X-ray spectroscopy (EDXS) analysis. It was found that the TiO2 nanoparticles were uniformly distributed on the fCNTs, and the Pt particles were decorated on the surface of TiO2/fCNTs. The photocatalytic hydrogen production activity of the Pt(0.5%)-TiO2/fCNTs(0.5%) nanoparticle composites was investigated using a sacrificial agent methanol solution. Pt-loaded TiO2 demonstrated a hydrogen evolution rate around 20 times that of TiO2/fCNTs(0.5%) (fSWCNTs, fMWCNTs). When compared with platinized TiO2 in methanol, which was utilized as a control material, Pt-TiO2/fCNTs demonstrated an almost 2-fold increment in hydrogen generation.

Impact of strength and size of donors on the optoelectronic properties of D–π–A sensitizers

A series of carbazole based sensitizers with either phenyl based donors (TBC, TMC, OMC, PC, TBR, TMR, OMR and PR) or aryl amine based donors (OMNC, CNC and HNC) as well as one without a donor group (CC) have been synthesized to understand the influence of the strength of the donor moiety on the optical, electrochemical and photovoltaic properties. Two different acceptor moieties such as cyano acrylic acid and rhodanine acetic acid were introduced and evaluated. Different substituents on the phenyl group have a significant impact on the light harvesting ability of the sensitizers. Among phenyl based donors, anisole based carbazole (OMC) shows the highest short circuit current (JSC) of 4.96 mA cm−2 with overall power conversion efficiency (PCE) of 2.69%. In the case of the sensitizers with aryl amine based donors, the increasing bulkiness of the donor group lead to increasing open circuit potential. Transient photocurrent and photovoltage measurements signify the importance of a bulky donor fragment in determining the open circuit potential of the dyes. Sensitizers with hexyloxy substituted phenyl amine as the donor group shows a JSC of 6.84 mA cm−2 with PCE of 3.33%. The overall investigation provides vital information about the influence of donor groups on the optoelectronic properties of the sensitizers for its photovoltaic applications.

Light-Controlled ZrO2 Surface Hydrophilicity.

In recent years many works are aimed at finding a method of controllable switching between hydrophilicity and hydrophobicity of a surface. The hydrophilic surface state is generally determined by its energy. Change in the surface energy can be realized in several different ways. Here we report the ability to control the surface wettability of zirconium dioxide nano-coatings by changing the composition of actinic light. Such unique photoinduced hydrophilic behavior of ZrO2 surface is ascribed to the formation of different active surface states under photoexcitation in intrinsic and extrinsic ZrO2 absorption regions. The sequential effect of different actinic lights on the surface hydrophilicity of zirconia is found to be repeatable and reversibly switchable from a highly hydrophilic state to a more hydrophobic state. The observed light-controllable reversible and reproducible switching of hydrophilicity opens new possible ways for the application of ZrO2 based materials.