Controllable Fabrication And Performance Evaluation Of G-C₃N₄-based Visible Light Response Photocatalyst

Graphite like carbon nitride (g-C3N4) has attracted much attention as a new typeof metal-free semiconductor photocatalytic material with visible light response.Because of its water-insolubility, good chemical stability and non-toxicity, whichmake g-C3N4have broad application prospects in photocatalysis field. This thesis,therefore, based on the study of pure g-C3N4, in order to further enhance thephotocatalytic activity of g-C3N4material, and combined g-C3N4and other inorganicsemiconductor material by in-situ growth technology, such as TiO2, CdS, and so on,to construct organic-inorganic heterojunction, improve the quantum efficiency,accelerate the electron transfer, reduce the probability of electron-hole recombinationand enhance the photocatalytic activity.Pyrolytic synthesis of g-C3N4was conducted by heating dicyandiamide andmelamine, respectively, in this study. The obtained g-C3N4samples derived fromdifferent precursors were characterized by X-ray diffraction (XRD), Fouriertransformed infrared (FT-IR) spectra, scanning electron microscopy (SEM) andtransmission electron microscopy (TEM), Zeta potential measurement, specificsurface area test (SBET) and UV-vis diffusive reflectance spectroscopy (UV-vis DRS).The Methylene blue dye (MB) was selected as a photodegrading goal to evaluate thephotocatalytic activity of as-prepared g-C3N4. Through systematic analysis of theexperiments, we selected the best precursor and determined the optimal preparationconditions to ultimately realize controllable fabrication of g-C3N4. The optimalprocess parameters of photodegradation MB of g-C3N4was ascertained by responsesurface methodology (RSM). Then, the synergy mechanism of adsorption andphotocatalytic was analyzed. The results indicated that g-C3N4sample obtained frommelamine at500℃for2h (CN-M500) had higher adsorption capacity and betterphotocatalytic activity than others and could wholly degrade5mg/L of MB dyewithin120min. The RSM analysis suggested that the optimal process parameters were3.61g/L of photocatalyst dosage,6.89mg/L of MB concentration,9.74of pHvalue and41.2min of reaction time, and that model predictions agreed well with theexperimental results, which proved the reliability of model. It can be known fromphotocatalytic mechanism that CN-M500had stable photocatalytic performance andhigh recycling rate, and the synergy mechanism of adsorption and photocatalytic ofCN-M500was a key factor of accelerating the degradation of dyes, which wasmanaged by surface charge.CdS/g-C3N4heterojunction used of g-C3N4as substrate material wassuccessfully builded by means of in situ synthesis. The content of CdS amongcomposite photocatalysts was determined via a simple wet chemistry method.Several characterization (XRD, FT-IR, TEM, SBET, UV-vis DRS) were carried out tocomparative analysis the crystal structure of CdS/g-C3N4with different mass ratio.The optimum mass ratio of CdS and g-C3N4was obtained by the study ofphotodegradation MO dye. Through RSM analysis, the optimum conditions for thephotocatalytic degradation of MO by CdS/g-C3N4were determined. Atomicabsorption spectroscopy (AAS) was used to measure the content of photo-corrosionof CdS and CdS/g-C3N4composites and explore the reasons of g-C3N4block CdSphoto-corrosion. Furthermore, the enhanced mechanism of photocatalysis caused byCdS/g-C3N4heterojunction was deeply analyzed. The study results suggested thatCdS/g-C3N4with73.3%CdS content (CCN-73.3) exhibited the best photocatalyticactivity for the discoloration of MO, of which discoloration ratio could reach100%for5mg/L of MO with a reaction time of60min. The RSM analysis indicated thatthe optimum conditions for photodegradation MO were2.56g/L of photocatalystdosage,6.33mg/L of MB concentration,4.68of pH value and9.89min of reactiontime, and that the error of model predictions was very small, as compared toexperimental results, and can be ignored, which proved the reliability of model. Thephotocatalysis mechanism analysis of CdS/g-C3N4heterojunction demonstrate thatCCN-73.3had high photocatalytic stability, and that the formation of CdS/g-C3N4heterojunction would contribute to the transfer of electrons and holes, the inhibitionof CdS photo-corrosion and the enhancement of photocatalytic activity of compositephotacatalysts. It can be found that the reduction of CdS content in CdS/g-C3N4composites could effectively prevent CdS/g-C3N4photo-corrosion. TiO2and g-C3N4, which are driven by ultraviolet and visible light, respectively,were used to in-situ construct TiO2/g-C3N4heterojunction through a method calledin-situ growth. The content of TiO2among composite photocatalysts was determinedvia thermo-gravimetric analysis. In addition, some characterizations were carried out,included XRD、FT-IR、TEM、SBETand UV-vis DRS, whereby we performed acomparative analysis of TiO2/g-C3N4composite photocatalysts with different massratio. Moreover, the photocatalytic activities of all samples were researched byphotocatalytic degradation experiments using MB as the target pollution. The samplewith optimum mass ratio was determined and used in RSM optimization to get theoptimum parameters for photodegradation MB. Finally, photocatalytic enhancementmechanism of TiO2/g-C3N4heterojunction was investigated. The results showed thatTiO2/g-C3N4with74.8%TiO2content (TCN-74.8) displayed the best photocatalyticactivity for the discoloration of MB among all samples, which could achieve anexcellent discoloration effect of97.1%for10mg/L of MB within165min. Theresults of RSM optimization revealed that the optimum parameters forphotodegradation MB were3.82g/L of photocatalyst dosage,16.04mg/L of MBconcentration,10.84of pH value and110.77min of reaction time, and that there isonly a small error between model predictions and experimental results, which provedthe reliability and predictability of model. The photocatalysis mechanism analysis ofTiO2/g-C3N4heterojunction indicated that TCN-74.8was very stable on thephotocatalytic degradation of MB, and that TiO2/g-C3N4heterojunction wasfavorable for expanding the optical response region of TiO2, accelerating the transferof electrons and holes between TiO2and g-C3N4, inhibiting the recombination ofelectrons and holes, improving photocatalytic activity of CdS/g-C3N4compositephotacatalysts.