Photocatalytic Activity Of TiO2 Nanocrystals Research Articles

Remarkable Enhancement of Photocatalytic Activity of High-Energy TiO2 Nanocrystals for NO Oxidation through Surface Defluorination.

The superior photocatalytic activity of TiO2 nanocrystals with exposed high-energy (001) facets, achieved through the use of hydrofluoric acid as a shape-directing reagent, is widely reported. However, in this study, we report for the first time the detrimental effect of surface fluorination on the photoreactivity of high-energy faceted TiO2 nanocrystals towards NO oxidation (resulting in a NO removal rate of only 5.9%). This study aims to overcome this limitation by exploring surface defluorination as an effective strategy to enhance the photocatalytic oxidation of NO on TiO2 nanocrystals enclosed with (001) facets. We found that surface defluorination, achieved through either NaOH washing (resulting in an improved NO removal rate of 23.2%) or calcination (yielding an enhanced NO removal rate of 52%), leads to a large increase in the photocatalytic oxidation of NO on TiO2 nanocrystals with enclosed (001) facets. Defluorination processes stimulate charge separation, effectively retarding recombination and significantly promoting the production of reactive oxygen species, including superoxide radicals (·O2-), singlet oxygen (1O2), and hydroxyl radicals (·OH). Both in situ diffuse reflectance infrared Fourier-transform spectroscopy and density functional theory calculations confirm the higher adsorption of NO after defluorination, thus facilitating the oxidation of NO on TiO2 nanocrystals.

Efficient Photocatalytic Activity of TiO2 Nanocrystals Modified with Organic Electron Donor and Barium Doping for Azo Group Decomposition Under UV Irradiation

Two kinds of surface-modified TiO2 photocatalysts, PEG–TiO2 nanocrystals and Ba–TiO2 nanocrystals, are prepared and characterized by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD) analysis. The modified TiO2 nanocrystals are small particles with irregular shape and size below 10 nm. Reactive Orange 72, with a –N=N– group, is selected as a model dye. The photocatalytic activities of the modified TiO2 nanocrystals for redox reactions of the azo group under UV irradiation are compared and the mechanism of the reduction of the dye photocatalyst is discussed. The results show that the photocatalysts can efficiently degrade Reactive Orange 72 (50 mg/L) within 2 min. The dye is completely broken down into small molecular compounds. The Ba–TiO2 nanocrystals have oxygen vacancies that serve as internal SEDs, and shows stronger photocatalytic reactivity than that of the unmodified TiO2 nanocrystal and PEG–TiO2. The Ba–TiO2 nanocrystals show the fastest decolorization rate and the best maintained performance for five cycles. The surface-modified TiO2 nanocrystals have potential application in the environmental, photochemical, and photovoltaics fields.

Synergy of palladium species and hydrogenation for enhanced photocatalytic activity of {001} facets dominant TiO2 nanosheets

Abstract

Effective surface disorder engineering of metal oxide nanocrystals for improved photocatalysis

Metal oxide (MO) semiconductors such as titanium dioxide (TiO2) are often used in many photo-chemical processes (e.g. waste-water remediation). However, their large band gaps limit their catalytic performances. Various techniques have been developed to improve the photocatalytic activity of TiO2 nanocrystals (NCs), e.g., via chemical doping, designing hybrid electronic structures, and creating surface disorders. Unfortunately, these techniques suffered from various disadvantages including complex/expensive operations and still poor photoactivity of modified TiO2 under near-infrared (IR) light (53% of the full solar irradiation). Here, we demonstrate a simple but universal route to introducing continous internal energy levels in MO semiconductors, including TiO2, zinc oxide (ZnO) and indium oxide (In2O3) NCs. This new technique just requires simply heating a mixture of an MO NC with potassium (K). Distingctly from conventional MO NCs, NCs prepared using the present technique were all in black color and able to adsorb nearly full solar spectrum. As-prepared black TiO2 NCs performed >300 times better than their conventional counterparts in the photocatalytic degradation of methylene blue (MB) dye under visible light irradiation. Similar levels of improvements in the photocatalytic performance were also achieved with as-prepared ZnO and In2O3 NCs. In addition, we show how NCs could be changed to black ones with tailored electronic structures. Our results suggested that the improvements in the NC’s photoactivity were mainly attributed to the surface disorder induced, oxygen (O) vacancies and metallic phases likely formed during the reaction process with K.

Synthesis of Few-Atomic-Layer BN Hollow Nanospheres and Their Applications as Nanocontainers and Catalyst Support Materials.

In this work, few-atomic-layer boron nitride (BN) hollow nanospheres were directly synthesized via a modified CVD method followed by subsequent high-temperature degassing treatment. The encapsulated impurities in the hollow nanospheres were effectively removed during the reaction process. The BN shells of most nanospheres consisted of 2-6 atomic layers. Because of the low thickness, the obtained BN hollow nanospheres presented excellent performance in many aspects. For instance, they were demonstrated as useful nanocontainers for controllable multistep release of iodine, which could diffuse and be encapsulated into the few-layer BN hollow nanospheres when heating. They were also promising support materials that could markedly increase the photocatalytic activity of TiO2 nanocrystals.

Green synthesis of anatase TiO(2) nanocrystals with diverse shapes and their exposed facets-dependent photoredox activity.

The exposed facets of a crystal are known to be one of the key factors to its physical, chemical and electronic properties. Herein, we demonstrate the role of amines on the controlled synthesis of TiO2 nanocrystals (NCs) with diverse shapes and different exposed facets. The chemical, physical and electronic properties of the as-synthesized TiO2 NCs were evaluated and their photoredox activity was tested. It was found that the intrinsic photoredox activity of TiO2 NCs can be enhanced by controlling the chemical environment of the surface, i.e.; through morphology evolution. In particular, the rod shape TiO2 NCs with ∼25% of {101} and ∼75% of {100}/{010} exposed facets show 3.7 and 3.1 times higher photocatalytic activity than that of commercial Degussa P25 TiO2 toward the degradation of methyl orange and methylene blue, respectively. The higher activity of the rod shape TiO2 NCs is ascribed to the facetsphilic nature of the photogenerated carriers within the NCs. The photocatalytic activity of TiO2 NCs are found to be in the order of {101}+{100}/{010} (nanorods) > {101}+{001}+{100}/{010} (nanocuboids and nanocapsules) > {101} (nanoellipsoids) > {001} (nanosheets) providing the direct evidence of exposed facets-depended photocatalytic activity.

Solvothermal synthesis of rice-like TiO2 nanocrystals with enhanced photocatalytic activity

Rice-like TiO2 nanocrystals with high specific surface area are synthesized by a facile and mild solvothermal route without further thermal annealing. These nanocrystals are aggregated to form a secondary mesoporous structure. The TiO2 samples possess a large surface area of 269.9m2/g, which is approximately 5 times higher than what is observed for P25. It is found that solvothermal temperature alters the crystallinity, surface area and porous structure of the sample. Photocatalytic activity of TiO2 nanocrystals for degradation of gaseous benzene is significantly higher than that of P25. The 1D structure and high porosity contribute to the superior photocatalytic activity.

Nanocrystalline TiO2-Catalyzed Photoreversible Color Switching

We report a novel photoreversible color switching system based on the photocatalytic activity of TiO2 nanocrystals and the redox-driven color switching property of methylene blue (MB). This system rapidly changes from blue to colorless under UV irradiation and recovers its original blue color under visible light irradiation. We have identified four major competing reactions that contribute to the photoreversible switching, among which two are dominant: the decoloration process is mainly driven by the reduction of MB to leuco MB by photogenerated electrons from TiO2 nanocrystals under UV irradiation, and the recoloration process operates by the TiO2-induced self-catalyzed oxidation of LMB under visible irradiation. Compared with the conventional color switching systems based on photoisomerization of chromophores, our system has not only low toxicity but also significantly improved switching rate and cycling performance. It is envisioned that this photoreversible system may promise unique opportunities for many light-driven actuating or color switching applications.

Tuning the morphology, stability and photocatalytic activity of TiO2 nanocrystal colloids by tungsten doping

The effects of tungsten doping on the morphology, stability and photocatalytic activity of TiO2 nanocrystal colloids were investigated. The nanostructure, chemical state of Ti, W, O, and the properties of tungsten doped TiO2 samples were investigated carefully by TEM, XRD, XPS, UV–vis, PL and photocatalytic degradation experiments. And the structure–activity relationship was discussed according to the analysis and measurement results. The analysis results reveal that the morphology, zeta potential and photocatalytic activity of TiO2 nanocrystals can be easily tuned by changing the tungsten doping concentration. The tungsten doped TiO2 colloid combines the characters of high dispersity and high photocatalytic activity.

Stable Au25(SR)18/TiO2 Composite Nanostructure with Enhanced Visible Light Photocatalytic Activity

We report the visible light photocatalytic properties of a composite material consisting of Au25(SR)18 nanoclusters (R:CH2CH2Ph) and TiO2 nanocrystals. The effects of Au25(SR)18 nanoclusters on the photocatalytic activity of TiO2 nanocrystals were evaluated in the reaction of photocatalytic degradation of methyl orange. The loading of Au25(SR)18 nanoclusters onto TiO2 results in strong visible light absorption by the composite and, more importantly, a 1.6 times increase in visible light photocatalytic activity. Furthermore, the Au25(SR)18/TiO2 composite nanostructure exhibits high stability in recycling tests. The Au25(SR)18 nanolusters dispersed on the TiO2 surface can act as a small-band-gap semiconductor to absorb visible light, giving rise to electron–hole separation and producing singlet oxygen (1O2). Both the generated hydroxyl radicals (HO•) and 1O2 are rationalized to be responsible for the decomposition of the dye.

Synergy of Low-Energy {101} and High-Energy {001} TiO2 Crystal Facets for Enhanced Photocatalysis

Controlled crystal growth determines the shape, size, and exposed facets of a crystal, which usually has different surface physicochemical properties. Herein we report the size and facet control synthesis of anatase TiO2 nanocrystals (NCs). The exposed facets are found to play a crucial role in the photocatalytic activity of TiO2 NCs. This is due to the known preferential flow of photogenerated carriers to the specific facets. Although, in recent years, the main focus has been on increasing the surface area of high-energy exposed facets such as {001} and {100} to improve the photocatalytic activity, here we demonstrate that the presence of both the high-energy {001} oxidative and low-energy {101} reductive facets in an optimum ratio is necessary to reduce the charge recombination and thereby enhance photocatalytic activity of TiO2 NCs.

Effect of substrate on evaluation of the photocatalytic activity of TiO2 nanocrystals with exposed {0 0 1} facets

There are increasing debates on the high photocatalytic activity of TiO2 nanocrystals with exposed high-energy facets. In this paper, the effect of substrate on evaluation of the photocatalytic activity of anatase TiO2 nanocrystals with exposed {001} facets was studied. Surface-clean anatase TiO2 nanocrystals with exposed {001} facets were prepared by a microwave-assisted hydrothermal route in a Ti(OC4H9)4-HF-H2O mixed solution (both the amount of Ti(OC4H9)4 and HF were 60mmol). It was found that the percentage of exposed {001} facets is negatively correlated with the molar ratio of water to titanium/or fluoride (x). With increasing x from 0 to 21, the percentage of exposed {001} facets steadily decreases from 81% to 36%. Anionic dye Brilliant Red X-3B (X3B) and cationic dye Methylene Blue (MB) were used as probes to evaluate the relative photocatalytic activity of the prepared TiO2 samples. TiO2 nanocrystals with exposed 40% {001} facets at x=15 shows the highest photocatalytic activity in degradation of MB. However, an overwhelmingly better performance of TiO2 sample with exposed 49% {001} facets at x=9 was found among all the photocatalysts in photocatalytic degradation of X3B, reflecting the importance of substrate on evaluation the photocatalytic activity of high-energy TiO2 nanocrystals.