| Semiconductor photocatalysis has attracted significant attention as an inexpensive and environmental-friendly technique to chemically transform organic pollutants into non-hazardous compounds in the past several decades. Among numerous photocatalysts, nanosized TiO2is the most popular due to its high stability, low cost and safety toward both humans and the environment. In general, the performance of TiO2photocatalyst mainly depends on the separation of photogenerated electron-hole pairs to involve in redox reaction. It is widely accepted that the reduction half-reaction that the adsorbed O2captures photogenerated electrons is crucial for efficient photocatalytic processes. However, it is often neglected. Herein, the factors determining TiO2photocatalytic activity for degrading pollutants first has been investigated through discussing the effect of O2adsorption on photogenerated charge separation and activity. Then, the TiO2photocatalysts has been modified by inorganic acie to enhance their photoactivity for degrading pollutants.Firstly, nanosized TiO2with containing different contents of rutile phase were controllably synthesized by a hydrochloric acid-modified hydrothermal process and explored which main factor determines the photoactivity of the resulting rutile TiO2. The results have shown that the as-prepared rutile shows high activity for photodegradation of rhodamine B dye compared with the as-prepared anatase, even superior to the P25TiO2. It is mainly attributed to the residual chloride that could promote the dye adsorbed on the surfaces of TiO2, consequently accelerating the photosensitization oxidation reactions of the dye molecules. In the photodegradation of liquid-phase phenol and gas-phase acetaldehyde, the as-prepared rutile TiO2displays low activity compared to the anatase one, which is attributed to the photogenerated electrons weakly captured by the adsorbed oxygen, since the residual chloride could effectively capture photogenerated holes based on the atmosphere-controlled surface photovoltage spectroscopy results. This work demonstrates that the reductibility of nanosized rutile TiO2is the rate-determining factors in the photodegradation processes.Based on the above work, we have prepared chloride-residual rutile TiO2, and then modified further them with H3PO4and H3BO3molecules, respectively. It has been confirmed that the photocatalytic activity of rutile TiO2can be obviously improved after modifying proper amount of H3PO4and H3BO3due to the enhancement of its oxygen adsorption. Interestingly, it has been found that the modification with H3BO3can effectively control the degrees for capturing photogenerated charges by adsorbed oxygen donators and residual chloride receptors of the rutile TiO2. According to the quantitative evaluation by means of atmosphere-controlled surface photovoltage measurements, it is suggested that to capture electrons is dominant compared to capture holes in the photocatalytic degradation of colourless gas-phase acetaldehyde and liquid-phase phenol. It can be seen that the results have further confirmed that the reductibility of photocatalyst plays an important role in the pollutant photodegradation.Then, nanocrystalline anatase TiO2with different percentages of the exposed {001} facet have been controllably synthesized by a hydrothermal process using hydrofluoric acid as a morphology-directing agent and investigated the main influencing factor in the photodegrading pollutant. It is shown that the percentage of001-facet exposure could be successfully tuned by increasing the amount of used hydrofluoric acid, and meanwhile the fluoride could reside on the surface of the as-prepared TiO2. The TiO2with high percentage of {001} facet exhibits much high photocatalytic activity for degrading gas-phase acetaldehyde and liquid-phase phenol. By comparing F-residual001-facet-exposed TiO2with the F-free one, it is concluded that the exceptional photocatalytic activity of the as-prepared001-facet-exposed TiO2mainly depends on the residual hydrogen fluoride, as well as slightly on the high-energy001-facet exposure. On the basis of the O2-TPD tests, theoretical calculations, and O2electrochemical reduction behaviors, it is further suggested that the residual hydrogen fluoride could greatly enhance the adsorption of O2so as to promote the photogenerated electrons captured by the adsorbed O2, leading to the great increase in the charge separation and then in the photocatalytic activity.In addition, it has been found based on our above work that the residual fluoride species on the surfaces of the001-facet-exposed TiO2is not thermally stable. The disappearance of surface residual fluoride would influence the thermal stability of high-energy001facet so as to greatly weaken its reductibility and consequentially decrease its photocatalytic performance. It would influence the application in areas such as ceramics, glass and bricks. To overcome the expected issues, we have modified the synthesized001-facet exposed nanocrystalline TiO2with phosphoric acid. The result shows that the modification obviously enhances the thermal stability of001-exposed facet. More important, it is confirmed that the enhanced thermal stability greatly prolongs lifetime and promotes separation of photogenerated charge carriers of001-facet-exposed TiO2mainly by means of time-resolved surface photovoltage responses, leading to the improved photocatalytic activity for degrading liquid-phase phenol and gas-phase acetaldehyde. This is attributed to the synergetic effects of001-facet exposure, high anatase crystallinity, and strong ability to adsorb O2. This work would provide a new method for the design and synthesis of high-performance functional materials for degrading environmental pollutants, and could perfect the theoretical knowledge of photodegrading pollutants. |
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