| The photocatalytic technology provides a brand new and promising solution for energycrisis and environmental issues. Therefore, it attracts worldwide attention because it canabsorb solar light to split water for generating hydrogen, and to decompose the environmentalpollutants. However, the practical application of traditional photocatalysts (such as TiO2) isseverely limited due to their low utilization rate of solar light and poor quantum yield.Therefore, exploration of high efficient photocatalysts with large specific surface area andhigh quantum yield becomes one of the important aims for semiconductor photocatalytictechnology. The novel organic polymer semiconductor photocatalyst, graphite carbon nitride(g-C3N4), has drawn much attention for its unique properties such as visible responded, highchemical and thermal stability, environmental compatibility and economical capability.However, the photocatalytic activity of g-C3N4is low due to its small specific surface areaand low quantum yield. In this dissertation, we mainly focus on the improvement of thephotocatalytic activity of g-C3N4by using acid hydrothermal treatment for enlarging itssurface area, with further constructing g-C3N4-based heterojunction photocatalysts forimproving quantum yield. The details are as follows:1. Processing g-C3N4(1) The N-vacancy g-C3N4with large specific surface area was prepared by hydrochloricacid hydrothermal treatment and calcination under protection of N2. The composition,structure, morphology, specific surface area and light absorption ability of the samples werecharacterized by XRD, IR, XPS, SEM, BET, UV-Vis DRS and PL. The possible formationmechanism of N-vacancy g-C3N4was also discussed. The specific surface area of the g-C3N4increased from11.5to81.5m2/g, and many mesoporous pores were formed afterhydrothermal and thermal treatment. The N-vacancy g-C3N4towards degradation of RhBunder visible light is higher than the pristine g-C3N4. The kinetic rate constant of the treatedg-C3N4is6.3times of that of the pristine g-C3N4. The enhancement of the photocatalyticactivity of the catalysts can be ascribed to their improved specific surface area and theN-vacancy in g-C3N4. The N-vacancy in the structure of g-C3N4decreases the bandgap ofcarbon nitride, thus enhances the visible light absorption. The higher specific surface area ofthe treated g-C3N4enhances the adsorption towards RhB, and provides more active sites aswell, thus higher photocatalytic activity was achieved.(2) g-C3N4with high surface area was prepared by hydrochloric acid hydrothermaltreatment and calcination at low temperature in air. The effects of hydrothermal reaction temperature and time on preparing high surface area g-C3N4were discussed. The specificsurface area, composition, morphology and photocatalytic activity of the samples wereinvestigated. The possible formation mechanism of g-C3N4with high specific surface areawas also discussed. After treatment, the size of the g-C3N4decreased from severalmicrometers to several hundred nanometers, and the specific surface area of the g-C3N4increased from11.5to115.6m2/g. Meanwhile, the photocatalytic activity of g-C3N4wassignificantly improved after treatment towards degradation of4-nitrophenol and RhB undervisible light irradiation. The degradation rate constant of the small particle g-C3N4is5.7timesof that of bulk g-C3N4, which makes it a promising visible light photocatalyst for futureapplications for water treatment and environmental remediation.(3) The protonated g-C3N4(CNH) with high specific area was synthesized by one stepHNO3hydrothermal process. The specific surface area, composition and morphology of thesamples were studied by several methods such as BET, XRD, IR and SEM. After treatment,the specific surface area of the CNH increased from11.5to114.4m2/g. The protonatedg-C3N4(CNH) is favorable for the transmission of photogenerated carriers. The photocatalyticactivity of CNH towards decomposition of RhB and4-NP under visible light is higher thanthat of g-C3N4. Moreover, the CNH shows higher photocatalytic stability. This method can beused in the pratical production of CNH due to its easy operatetion and no impurities wereformed during the processing.2. Constructing g-C3N4-based heterojunction photocatalysts for improving the quantumyield(1) Heterostructured photocatalysts Ag3VO4/g-C3N4were prepared bydeposition-precipitation method for anchoring Ag3VO4on the surface of N-vacancy g-C3N4(g-C3N4-VN). Several techniques were used to characterize the structure, composition andmorphology of the photocatalysts. The photocatalytic activity of the samples was evaluatedby degradation of rhodamine B (RhB) in aqueous solution. The ratio of Ag3VO4andg-C3N4-VN affecting the photocatalytic activity of Ag3VO4/g-C3N4-VN was also discussed.Compared with the pure Ag3VO4and g-C3N4-VN, the heterojuncted photocatalyst,65wt%Ag3VO4/g-C3N4-VN exhibits optimal activity under visible light irradiation. Thephotodegradation rate constant of65wt%Ag3VO4/g-C3N4-VN is0.0556min-1, which is23.4,5.8and6.4times of that of pure Ag3VO4, pure g-C3N4-VN and P25, respectively. Theexcellent photocatalytic performance of the Ag3VO4/g-C3N4-VN catalysts can be ascribed tothe matched band structures of Ag3VO4and g-C3N4, which formed heterojunctedphotocatalyst. The unique heterostructured photocatalyst is favorable for retarding the recombination of photogenerated electrons and holes, thus the photocatalytic activity is high.Further experiment also reveals that the O2and h+are the major active species in thedegradation of RhB. The possible photocatalytic mechanism was also proposed.(2) The AgI/CNH heterojunction photocatalyst was prepared by deposition-precipitationmethod. The composition, morphology and specific surface area of the samples werecharacterized by several techniques like XRD, IR, SEM, TEM and BET. The effect of theloading amount of AgI was also discussed. When the loading amount of AgI increased to80%,the AgI/CNH heterojunction photocatalyst exhibits optimal photocatalytic activity in allsamples. The heterojunction enhances the separation of photogenerated electrons and holes.Meanwhile, the stability of AgI/CNH is improved because the heterojunction can depress thephotogenerated electron to react with AgI. Thus, AgI/CNH heterojunction shows highphotocatalytic performance towards the degradation of RhB and4-NP, which makes it to be apromising photocatalyst for pratical application.(3) Ag/AgCl/CNH heterojunction photocatalyst was prepared by deposition-precipitationand photoreduction method. The composition, structure, morphology and photocatalyticactivity of the samples were characterized. The Ag/AgCl particles combined with CNH noonly can improve the separation of photogenerated carriers, but also can depress the growthand agglomeration of AgCl particles. Compared with the pure AgCl, the size of AgClparticles in Ag/AgCl/CNH decreased from several microns to several hundred nanometers.The Ag/AgCl/CNH heterojunction photocatalyst shows high absorption in the visible lightregion due to the local surface plasmon resonances (LSPR) of Ag nanoparticles. Comparedwith the pure g-C3N4, CNH and Ag/AgCl, the Ag/AgCl/CNH heterojunction photocatalystshows much higher photocatalytic activity towards the photocatalytic degradation of RhB and4-NP, due to the high absorption of visible light and high quantum yield. This work providesa new way to prepare novel g-C3N4-based photocatalysts with high performance.In summary, enlarging the specific surface area and forming heterojunctions with othersemiconductors are the important methods for improving the separation rate ofphotogenerated electrons and holes, which are promising ways for improving thephototcatalytic activity of g-C3N4. |
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