| In 21st, environmental pollution control and treatment is one of the important issues to the humanity. Semiconductor photocatalytic materials have been intensively investigated for their wide potential application in water and air purification and solar energy conversion since Fujishima and Honda discovered the photocatalytic splitting of water on TiO2 electrodes in 1972. Among various oxide photocatalysts, TiO2 has been proved to be the most suitable material for its strong oxidizing power, non-toxicity and long-term stability against photo and chemical corrosion and cost effectiveness. Due to these excellent properties, titania has applications in various fields such as deodorizers and antibacterial agents, as well as in products featuring anti-stain, hydrophilic, anti-fog, and harmful substance removal functions. However, the rapid recombination rate of photogenerated electron-hole pairs within TiO2 particles results in its low efficiency, thus limiting its practical appilications. Therefore, in order to reach the industrial purposes, several methods have been reported to improve the photocatalytic activity. Modification of TiO2 with other semiconductors(s) as one of the effective modification methods attracts considerable attention. In this work, carbon materials facilitate the separation of photogenerated charge carriers and thus enhance the photocatalytic activity of TiO2.Firstly, mesoporous multiwalled carbon nanotubes/titanium dioxide (CNTs/TiO2) composites with low loading amounts (0-0.5 wt%) of CNTs embedded inside mesoporous TiO2 aggregates has been prepared by a simple one-pot hydrothermal method using titanium sulfate as titanium source. The as-prepared CNTs/TiO2 samples are carefully characterized by XRD, BET, SEM, TEM, IR, Raman, XPS and UV-vis. In contrast to previous reports with high CNTs loading, our results indicate that a low CNTs loading slightly influences the textural properties and UV-light absorption of the mesoporous TiO2. The photocatalytic activity is evaluated by photocatalytic oxidation decomposition of acetone in air under UV light illumination. The results show that CNTs/TiO2 composites with appropriate loading amounts (<0.1 wt%) favor the separation of photogenerated electron-hole pairs and decrease of their recombination rate, and thus display significantly enhanced photocatalytic activity. In contrast, a high CNTs loading(>0.1 wt%) as well as a simple mechanical mixing of CNTs and TiO2 results in inferior photocatalytic performance because of inhibited interfacial electron transfer and a higher recombination rate of photogenerated electron-hole pairs. The mechanism about CNTs enhancing the photocatalytic activity of TiO2 has been discussed in detail. It is also further supported by PL analysis and transient photocurrent responses. Significantly, we point that PL analysis in terms of intensity of PL signals seems to not be a reliable way to monitor the recombination rated in the CNTs/TiO2 composites, due to the quenching effect of CNTs. Instead, the analysis of transient photocurrent responses is a better measurement.Secondly, fullerene modified TiO2 nanocomposites (denoted as C60/TiO2) with low C60 loadings (0-1.5 wt%) have been prepared by a simple hydrothermal method using tetrabutylorthotitanate (TBOT, Ti(OC4H9)4) as titanium precursor. The as-prepared C60/TiO2 nanocomposites were characterized by XRD, BET, SEM, TEM, IR, Raman, XPS and UV-vis. Our results have demonstrated that C6o molecules can be dispersed as a monolayer onto bimodal mesoporous TiO2 via covalent bonding. The photocatalytic activity is evaluated by photocatalytic oxidation decomposition of acetone in air under UV light illumination. The results show that C6o significantly promote photocatalytic activity of TiO2. In particular,0.5 wt% C60/TiO2 nanocomposites show the greatest photocatalytic activity with the rate constant k exceeding that of P25 by a factor of 3.3. Based on the results from current study, we propose that C6o molecules doped onto TiO2 acts as "an electron acceptor" responsible for the efficient separation of photo-generated charge carriers and the enhancement of photocatalytic activity. The proposed mechanism for the observed photocatalytic performance of C60/TiO2 nanocomposites is further corroborated by the experiments of hydroxyl radical and transient photocurrent response.As a conclusion, we show that CNTs and C60 modify titania (TiO2) is a effective method to improve the photocatalytic efficiency. However, the loading amount is an important factor for photocatalytic activity enhancement. Only the optimal loading can significantly promote the photocatalytic activity of TiO2. |
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