Study On The Controlled Synthesis Of Novel BiOCl Photocatalysts And Their Enhanced Performance

Recent years, semiconductor-based photocatalytic oxidative technology hasattracted great interest for its application in degrading organic pollutants inwaters. The relationship between the excitation light energy and band gapenergy of semiconductor decides the applicability of photocatalytic oxidativetechnology. Only when the excitation light energy is equal or higher than theband gap energy of semiconductor, the electrons in valence band can be excitedand jump to conduction band, leaving photogenerated holes (h+) and electrons(e–) in valence band and conduction band, respectively. These photo-generatedcarriers possess oxidizing and reducing capacity, and can react with H2O andOH–to form hydroxyl radical (OH) and super oxygen anion free radical (O–2);they also can react with the reactants and degrade them directly. However, therecombination of photo-generated holes and electrons is inevitable during theoccurring of these processes, and this will restrain the photocatalytic activity ofphotocatalyst. Hence, how to enhance the migration rate of photo-generatedcarriers in catalyst bulk phase and surface, and inhibit the recombination ofphoto-generated carriers are the important problems to be solved. On the otherhand, the band gap energy of photocatalyst is closely bound up with itsphotocatalytic activity. The larger the band gap energy, the higher the oxidizingcapacity of holes and reducing capacity of electrons, so the semiconductor withlarger band gap energy exhibits stronger photocatalytic activity, but reduces the utilization efficiency of solar energy. Hence, intensive attempts have dedicatedto enhance the light utilization efficiency of wide band gap semiconductor.Based on the above consideration, the purposes of this dissertation are thesynthesis of new BiOCl photocatalyst through liquid phase reaction at roomtemperature, the improvements of their photocatalytic activity and lightutilization efficiency through surface regulation and structure control, and theeffect of preparation and degradation process on the enhancement ofphotocatalytic activity.The main contents of this dissertation are focused on the following severalaspects:1. Firstly, flower-like structured BiOCl photocatalyst is synthesizedthrough ethylene glycol-water liquid phase synthetic method at roomtemperature and its band gap energy is3.2eV. The as-prepared BiOCl exhibitsgood simulated sunlight photocatalytic activity, and can degrade resorcinol,methyl orange, and bisphenol A efficiently. In this aspect, the effect of BiOCldosage, the initial concentration of resorcinol, the pH value and the temperatureon the photocatalytic degradation reaction is investigated systematically. Theresults show that the optimal experiment condition for resorcinol degradation iswBiOCl=1g/L, c0=20mg/L, and pH=9. After irradiation for60min, thedegradation efficiency of resorcinol is99%; The temperature exhibits littleinfluence on the reaction. The scavenger experiment results show thatphoto-generated holes and O–2are responsible for the high photocatalyticactivity of BiOCl. Simultaneously, the BiOCl photocatalyst also can degradeRhB under visible light irradiation; the degradation efficiency of RhB is99.2%after20min light irradiation, and the COD change also attains92%. Thisvisible-light catalytic activity originates from the dangling bond existing inexposed facets.2. Secondly, in order to improve the visible light absorption property, theH2O2and KI are introduced in BiOCl preparation process. H2O2and KI can play the role of regulating band gap energy and exposed facets of BiOClphotocatalyst. The band gap energy of BiOCl decreases from3.19eV to2.5eV.The assistance of H2O2and KI can also increase the surface hydroxyl content.The as-synthesized BiOCl can degrade phenol under visible light irradiation.After300min light irradiation,35.8%of phenol is degraded, greatly higher thanP25which only3.2%of phenol is degraded with the same reaction condition.During phenol degradation process, the h+and O–2are the main oxidative activespecies. Besides, BiOCl also exhibits high photocatalytic activity for methylorange and rhodamine B degradation. The BiOCl prepared though this methodshows high chemical stability, and this is of great significance to its practicalapplication.3. I-doped BiOCl was synthesized through oxidation-reduction method.The existence of Iodine in BiOCl can regulate the facet and band gap energy ofBiOCl, change the exposed facet of BiOCl, and reduce its band gap energy from3.19eV to2.3eV. The as-prepared BiOCl photocatalyst exhibits bettervisible-light-induced catalytic activity for bisphenol-A degradation; When thedosage of BiOCl is1g L-1, the degradation efficiency of bisphenol A achieve87.1%after90min light irradiation. Besides, the as-prepared BiOCl alsoexhibits higher visible-light induced photocatalytic activity for phenol, methylorange, and rhodamine B degradation. The iodide ion can also expand theoptical absorption property of BiOCl conspicuously, increase the specificsurface area and surface hydroxy content, which can benefit the enhancement ofits photocatalytic activity.4. The BiOClxBr1-xcomposite photocatalysts are constructed throughhydrolysis method at room temperature, the structure, morphology, lightabsorption property and photocatalytic activity of as-prepared photocatalysts arerelated with its composition and the used organic solvent. The organic solventcan regulate the morphology of photocatalyst, and the as-prepared BiOClexhibits microspheric structure when glycol is used as reaction medium. Following the decrease of chlorine content, the microspheric structure continuesto decline, and the lamellar structure constantly escalates. The as-preparedcomposite photocatalysts are all lamellar structured when absolute ethyl alcoholis used as reaction medium. The composition of photocatalysts can influence thecompetition between oxidizing capacity and the light utilization; the decrease ofchlorine content will narrow the band gap of photocatalyst, therefore weaken theredox ability of photogenerated carriers. However, the decrease of band gapenergy can improve the light utilization efficiency of photocatalyst. Thecompetition between them will make the composition photocatalyst exhibitcomposition-dependent catalytic activity. Simultaneously, the BiOClxBr1-xcomposite photocatalysts also show different light response behavior, andinfluence the degradation mechanism of rhodamine B on its surface. Thedifferent light response behavior is due to the different active species generatedin catalytic system, and the spontaneity of excited electrons transferring fromdye to conduction band.5. The sensitization behavior of dyes can enhance the light energy anddissolved oxygen utilization of photocatalysts. Based on this consideration, acombined cataytic system of BiOCl-sensitizer was designed to make full use ofthe sensitization behavior and photocatalytic activity of BiOCl to enhance thedegradation efficiency of organics. Therefore, the rhodamine B (RhB) wasselected as the sensitizer. Under visible light irradiation, the double electronsources of RhB excited electron and BiOCl photogenerated electron can makefull use of dissolved oxygen, and enhance the photocatalytic activity forbisphenol A (BPA) degradation greatly. With the addition of RhB, thedegradation efficiency of BPA increases from24.7%to96.2%. At the same time,the influences of BiOCl dosage, RhB dosage, BPA initial concentration, andsolution pH value on degradation reaction were investigated. The results showthat the optimal experiment condition for BPA degradation is cBiOCl=1.5g L-1,cRhB=5mg L-1, cBPA=20mg L-1, pH=6. The scavenger experiment results exhibit that photogenerated holes and O–2are the main active species in BPAdegradation. The monochromatic light experiment results further convince theexistence of double electron source. However, the dye sensitization can notreduce the recombination of photogenerated carriers. During this process, theadsorption capacity of RhB on BiOCl surface plays the key role in theenhancement of BiOCl photocatalytic activity.6. With glycerol as the reaction medium, the BiOCl photocatalyst isprepared through hydrolysis method at room temperature, and the structure ischaracterized by X-ray diffraction, scanning electron microscope, transmissionelectron microscope, nitrogen adsorption, and UV-vis diffuser reflectance. Theresults show that the BiOCl is tetragonal flower-like structure, the specificsurface area is35.30m2g-1, and the band gap energy is3.24eV. Consideringthat the surface complex can be formed via dehydration reaction betweencatechol and BiOCl photocatalyst, and polyhydroxy organics can increasesurface hydroxyl content obviously, the photodegradation behavior of catecholon BiOCl is investigated. The catechol can react with surface-adsorbed hydroxylto form surface complex, and this surface complex can broaden the opticalabsorption of BiOCl from the ultraviolet light to the visible light. The XPSresults indicate that the surface complex is linked to BiOCl through Bi-O-Cbond. After120min visible light irradiation,86.7%of catechol is degraded, andthe reaction rate constant is9.0×10-3min-1. Based on the scavenger experimentalresults, the possible reaction mechanism is proposed. The surface complex isexcited by visible light first, and the excited electrons are directly transferredfrom surface complex to the conduction band of BiOCl; subsequently, theelectrons can react with dissolved O2to generate O–2species, which finallycause a series of oxidative degradation reactions of catechol.