Titanium dioxide (TiO2), as an important semiconductor metal oxide, has been widely investigated in the field of photocatalysis. The properties of TiO2, including its light absorption, charge transport and surface adsorption, are closely related to its defect disorder, which in turn plays a significant role in the photocatalytic performance of TiO2. Among all the defects identified in TiO2, oxygen vacancy is one of the most important and is supposed to be the prevalent defect in many metal oxides, which has been widely investigated both by theoretical calculations and experimental characterizations. Here, we give a short review on the existing strategies for the synthesis of defective TiO2 with oxygen vacancies, and the defect related properties of TiO2 including structural, electronic, optical, dissociative adsorption and reductive properties, which are intimately related to the photocatalytic performance of TiO2. In particular, photocatalytic applications with regard to defective TiO2 are outlined. In addition, we offer some perspectives on the challenge and new direction for future research in this field. We hope that this tutorial minireview would provide some useful contribution to the future design and fabrication of defective semiconductor-based nanomaterials for diverse photocatalytic applications.

Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications

Oxygen vacancies in crystal have important impacts on the electronic properties of ZnO. With ZnO2 as precursors, we introduce a high concentration of oxygen vacancies into ZnO successfully. The obtained ZnO exhibits a yellow color, and the absorption edge shifts to longer wavelength. Raman and XPS spectra reveal that the concentration of oxygen vacancies in the ZnO decreased when the samples are annealed at higher temperature in air. It is consistent with the theory calculation. The increasing of oxygen vacancies results in a narrowing bandgap and increases the visible light absorption of the ZnO. The narrowing bandgap can be confirmed by the enhancement of the photocurrent response when the ZnO was irradiated with visible light. The ZnO with oxygen vacancies are found to be efficient for photodecomposition of 2,4-dichlorophenol under visible light irradiation.

Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO.

ZnO, aside from TiO2, has been considered as a promising material for purification and disinfection of water and air, and remediation of hazardous waste, owing to its high activity, environment-friendly feature and lower cost. However, their poor visible light utilization greatly limited their practical applications. Herein, we demonstrate the fabrication of different aspect ratios of the ZnO nanorods with surface defects by mechanical-assisted thermal decomposition method. The experiments revealed that ZnO nanorods with higher aspect ratio and surface defects show significantly higher photocatalytic performances.

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Effect of aspect ratio and surface defects on the photocatalytic activity of ZnO nanorods

Visible light-active TiO2 (m-TiO2) nanoparticles were obtained by an electron beam treatment of commercial TiO2 (p-TiO2) nanoparticles. The m-TiO2 nanoparticles exhibited a distinct red-shift in the UV-visible absorption spectrum and a much narrower band gap (2.85 eV) due to defects as confirmed by diffuse reflectance spectroscopy (DRS), photoluminescence (PL), X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and linear scan voltammetry (LSV). The XPS revealed changes in the surface states, composition, Ti4+ to Ti3+ ratio, and oxygen deficiencies in the m-TiO2. The valence band XPS, DRS and PL results were carefully examined to understand the band gap reduction of m-TiO2. The visible light-responsive enhanced photocatalytic activity of m-TiO2 was demonstrated by degrading methylene blue and brilliant blue G. The EIS and LSV in the dark and under visible light irradiation further support the visible light-induced photocatalytic activities of the m-TiO2 due to a decrease in electron transfer resistance and an increase in photocurrent. This study confirms that m-TiO2 can be used effectively as a photocatalyst and photoelectrode material owing to its enhanced visible light-induced photocatalytic activity.

Band gap engineered TiO2 nanoparticles for visible light induced photoelectrochemical and photocatalytic studies

As an alternative to the gold standard TiO2 photocatalyst, the use of zinc oxide (ZnO) as a robust candidate for wastewater treatment is widespread due to its similarity in charge carrier dynamics upon bandgap excitation and the generation of reactive oxygen species in aqueous suspensions with TiO2. However, the large bandgap of ZnO, the massive charge carrier recombination, and the photoinduced corrosion–dissolution at extreme pH conditions, together with the formation of inert Zn(OH)2 during photocatalytic reactions act as barriers for its extensive applicability. To this end, research has been intensified to improve the performance of ZnO by tailoring its surface-bulk structure and by altering its photogenerated charge transfer pathways with an intention to inhibit the surface-bulk charge carrier recombination. For the first time, the several strategies, such as tailoring the intrinsic defects, surface modification with organic compounds, doping with foreign ions, noble metal deposition, heterostructuring with other semiconductors and modification with carbon nanostructures, which have been successfully employed to improve the photoactivity and stability of ZnO are critically reviewed. Such modifications enhance the charge separation and facilitate the generation of reactive oxygenated free radicals, and also the interaction with the pollutant molecules. The synthetic route to obtain hierarchical nanostructured morphologies and study their impact on the photocatalytic performance is explained by considering the morphological influence and the defect-rich chemistry of ZnO. Finally, the crystal facet engineering of polar and non-polar facets and their relevance in photocatalysis is outlined. It is with this intention that the present review directs the further design, tailoring and tuning of the physico-chemical and optoelectronic properties of ZnO for better applications, ranging from photocatalysis to photovoltaics.

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Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications

ZnO nanocrystals with different oxygen defects (type and concentration) are successfully prepared via the templating method in Nafion membranes. Photoluminescence and infrared characterizations reveal that the concentration of oxygen defects (oxygen vacancy and interstitial oxygen) is quite different for the samples prepared via different synthesis processes. Photocatalytic activity measurements show that the sample with a higher amount of oxygen defects exhibits excellent activity toward the degradation of rhodamine B. Compared with bulk ZnO, ZnO nanocrystals embedded in Nafion membranes have an antiphotocorrosion property. The possible photocatalytic mechanism is discussed in detail, and the oxygen defects are proposed to be the active sites of the ZnO photocatalyst.

Relationship between Oxygen Defects and the Photocatalytic Property of ZnO Nanocrystals in Nafion Membranes

Crystalline ZnO nanoparticles were synthesized via the templating method by taking advantage of the hydrophilic cavities of the Nafion ionomer membranes. The ZnO-embedded Nafion film was characterized by various methods, including optical absorption, X-ray diffraction and high-resolution transmission electron microscopy. The film was of high optical quality, and exhibited excellent photocatalytic activity toward rhodamine B degradation under UV irradiation. Unlike unprotected bulk ZnO nanocrystals, the ZnO nanocatalysts embedded in Nafion membranes were found to be highly stable against photocorrosion, with hardly any reduction in the activity even after 10 times of repeated reaction cycles. The implications of the results are discussed.

Nanocrystalline zinc oxide in perfluorinated ionomer membranes: Preparation, characterization, and photocatalytic properties

Monodisperse TiO2−xNxnanocrystals were successfully synthesized in Nafion membrane at low temperature in a convenient way. Several characterization techniques, X-ray diffraction (XRD), UV-vis absorption spectra and X-Ray Photoelectron Spectroscopy (XPS) are combined in order to determine the crystal phase, grain size, optical absorption, and state of the resultant oxynitride chemical bonding. The photocatalytic activity and stability of the homogeneously dispersed TiO2−xNxnanocrystals embedded in Nafion membrane were evaluated through a study of the decomposition of Methylene Blue under visible light irradiation. It was found that the photocatalytic activity of the TiO2−xNxnanocrystals can be enhanced efficiently by re-nitridation. Also, the form of Nafion, acidic or salt, affected photocatalytic activity.

Wei Han

Papers

Relationship between Oxygen Defects and the Photocatalytic Property of ZnO Nanocrystals in Nafion Membranes

Nanocrystalline zinc oxide in perfluorinated ionomer membranes: Preparation, characterization, and photocatalytic properties

Low-temperature synthesis of regenerable TiO2−xNxnanocrystals in Nafion membrane and the promotive effect of Nafion in photocatalysis and N-doping

β-In2S3 nanocrystals in Nafion membrane: Facile synthesis and visible photocatalytic performance