A surface science perspective on TiO2 photocatalysis

Since the early development of this technology in the 1970s, TiO2 constitutes the archetypical photocatalyst due to its relatively high efficiency, low cost and availability. However, during the last decade a considerable number of new photocatalytic materials, either semiconductor or not, have been proposed as potential substitutes of TiO2, particularly in the case of solar applications, for which this standard photocatalyst is not very suitable because of its wide band gap. Semiconductors based on cations with d0 configuration such Ta5+ or Nb5+, as well as oxides or nitrides of d10 elements such as Bi3+, In3+ or Ga3+ are among the most successful novel photocatalysts, but non-semiconductor solids like cation-interchanged zeolites also produce interesting results. In addition, some classical semiconductors like ZnO or CdS, initially discarded as a consequence of their poor stability under irradiation, have been reconsidered as feasible photocatalysts for particular applications. This growing body of data requires new analysis of the challenges and opportunities facing photocatalysis in order to assess which of the photoactive materials are best for each particular application. In this review, we summarize, with an historical perspective, the main achievements obtained with photocatalyst alternatives to TiO2 in the three main niches for this technology: water splitting for hydrogen production, decontamination and disinfection processes, and organic synthesis.

Development of alternative photocatalysts to TiO2: Challenges and opportunities

Titanium Dioxide Photocatalysis in Atmospheric Chemistry

A two-dimensional layered polymeric photocatalyst, graphitic carbon nitride (g-C3N4), is becoming the rising star in the field of solar-to-fuel conversion. However, the performance of commonly prepared g-C3N4 is usually very weak because of the high recombination rate of photogenerated charge carriers and a small amount of surface active sites. Here we demonstrate simultaneous texture modification and surface functionalization of g-C3N4via a stepwise NH3-mediated thermal exfoliation approach. The resulting g-C3N4 photocatalyst possesses a hierarchical structure obtained by the assembly of amine-functionalized ultrathin nanosheets and thus exhibits remarkably enhanced light harvesting, a high redox ability of charge carriers, increased CO2 adsorption and a larger amount of surface active sites, as well as improved charge carrier transfer and separation. Therefore the aforementioned hierarchical g-C3N4 consisting of amine-functionalized ultra-thin nanosheets shows much better performance for photocatalytic CO2 reduction than unmodified conventional g-C3N4 photocatalysts.

Ultra-thin nanosheet assemblies of graphitic carbon nitride for enhanced photocatalytic CO2 reduction

A new efficient photocatalyst structure, a shell of anatase nanocrystals on the fibril core of a single TiO2(B) crystal, was obtained via two consecutive partial phase transition processes In the first stage of the process, titanate nanofibers reacted with dilute acid solution under moderate hydrothermal conditions, yielding the anatase nanocrystals on the fiber In the subsequent heating process, the fibril core of titanate was converted into a TiO2(B) single crystal while the anatase crystals in the shell remained unchanged The anatase nanocrystals do not attach to the TiO2(B) core randomly but coherently with a close crystallographic registry to the core to form a stable phase interface For instance, (001) planes in anatase and (100) planes of TiO2(B) join together to form a stable interface Such a unique structure has several features that enhance the photocatalytic activity of these fibers First, the differences in the band edges of the two phases promote migration of the photogenerated holes fr

An Efficient Photocatalyst Structure: TiO2(B) Nanofibers with a Shell of Anatase Nanocrystals

Thermo- and photoreactivities of methoxy (CH3O(a)) and ethoxy groups (C2H5O(a)) bonded, via the oxygen atom, to one Ti ion (monodentate adsorption form) or to two Ti ions (bidentate adsorption form) on TiO2 have been studied by Fourier transformed infrared spectroscopy. Regardless of the similar thermal stability for the two adsorption geometries of the adsorbed alkoxy groups, difference in photoreactivity is observed by monitoring the change of their integrated IR absorptions as a function of UV irradiation time. The monodentate photooxidation rate is ∼1.5 times that of bidentate for both methoxy and ethoxy groups. CH3O(a) on TiO2 is photooxidized to H2O(a) and HCOO(a) in the presence of O2. On the other hand, C2H5O(a) is photooxidized to H2O(a), HCOO(a), and CH3COO(a). A Russell-like mechanism is invoked to explain the formation of the reaction products. Possible reaction steps that control the photoreactivity of the monodentate and bidentate adsorption forms are discussed in terms of this mechanism.

Bonding Geometry and Reactivity of Methoxy and Ethoxy Groups Adsorbed on Powdered TiO2

FTIR study of adsorption and reactions of methyl formate on powdered TiO2

The photochemistry of CH3OH(a) and CH3O(a) adsorbed on TiO2 and the effect of O2 have been studied by infrared spectroscopy and mass spectrometry. In the absence of O2, CH3OH(a) desorbs molecularly, while CH3O(a) decomposes to form CH2O(g) under UV irradiation. The rate of CH3O(a) decomposition reaches maximum in the initial stage of UV irradiation and then decreases significantly. On the other hand, for the reaction in O2, the behavior of CH3O(a) depletion upon UV irradiation is very different, giving rise to adsorbed HCOO(a) and H2O(a). A radical mechanism is proposed to explain the variation of CH3O(a) depletion upon UV irradiation, the role of O2, and the formation of the reaction products.

Photochemistry of Methanol and Methoxy Groups Adsorbed on Powdered TiO2

The adsorption and photochemistry of acetonitrile and acetamide on TiO2 have been studied by Fourier-transform infrared spectroscopy (FTIR). Adsorbed CH3CN, CH3CONH2, η2(N,O)-CH3CONH are formed after CH3CN
 adsorption on TiO2. The last two species are due to surface hydroxy groups attacking the electron-deficient
 carbon in the cyano group of the adsorbed acetonitrile. UV exposure causes decomposition
 of CH3CN(a) to CH2CN(a) and decomposition of CH3CONH2(a) and η2(N,O)-CH3CONH(a)
 to CH3COO(a),
 HCOO(a), NCO(a) and CN-containing species on the surface.

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Adsorption and photochemistry of CH3CN and CH3CONH2 on powdered TiO2

Adsorption and reaction of N2H4 and NH3 on powdered TiO2 have been studied by Fourier-transformed
 infrared spectroscopy. N2H4 decomposes on the surface to form NH3 and surface hydroxy groups at temperatures
 higher than ∽150°C. Based on these findings, a N2H4
 reaction pathway on TiO2
 is established to be N2H4→2NH2→NH+NH3
 and NH+O(l)→N+OH(ads). NH3 decomposes on the surface and forms adsorbed
 azide N3(ads) species at temperatures higher than ∽350°C. Photooxidation of NH3 and ND3 is investigated
 as well and the mechanism is discussed in terms of the NH2 reaction pathway
 and recent reports for the reactions between NHx and NOx (x=1–2) in the gas phase.

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Chih-Chung Chuang

Papers

Bonding Geometry and Reactivity of Methoxy and Ethoxy Groups Adsorbed on Powdered TiO2

FTIR study of adsorption and reactions of methyl formate on powdered TiO2

Photochemistry of Methanol and Methoxy Groups Adsorbed on Powdered TiO2

Adsorption and photochemistry of CH3CN and CH3CONH2 on powdered TiO2

Interaction of hydrazine and ammonia with TiO2