Aqueous Synthesis and Photocatalytical Property Study of Bismuth-based Semiconductors and Composites

Author:Wu Gong Juan

Supervisor:zhao jing zhe


Degree Year:2018





Bismuth-based semiconductor materials have great potential in solving the problems of environmental pollution and energy shortage.Therefore,the synthesis and application of bismuth-based semiconductor and its composites have been a hot topic in the field of photocatalytic materials research.With the continuous development of this field,the researches on the development of new bismuth-based photocatalytic materials to improve the photocatalytic performance and to realize the simple and controllable synthesis have become the focus.In this work,bismuth-based semiconductor photocatalytic materials with different morphologies and compositions were successfully synthesized by simple liquid phase technique,including polycrystalline Bi2O3 and its composites,Bi5O7I nanowire and its assembled structure,a series of bismuth-rich Bix Oy Iz materials,Bi4O5Br2 nanosheets,Bi5O7Br crystal and its composite materials,etc.The morphology,composition,synthesis mechanism and photocatalytic degradation towards organic pollutants were systematically studied.On the basis of experimental data and results,the photocatalytic mechanisms were discussed.The main research contents are summarized as follows:(1)The round-cake-likeα-Bi2O3 photocatalytic material with nanorod as assembly units was successfully prepared in an aqueous phase with sodium tartrate as inducing agent.By changing the dosage of sodium tartrate and NaOH,the nucleation,growth and assembly processes of crystalline nanorod units were controlled,and the synthesis ofα-Bi2O3 hierarchical structures with different assembly degrees was realized.On the basis of relevant theoretical and experimental analysis,the formation mechanism of the round-cake-likeα-Bi2O3 was proposed.Furthermore,the composite structures of Bi/α-Bi2O3 and BiOCl/α-Bi2O3 were successfully synthesized by usingα-Bi2O3 as template.The photocatalytic activity of the assembled round-cake-likeα-Bi2O3 was studied.With the increase of the assembly degree ofα-Bi2O3,the basic unit size of nanorods decreased,and the catalytic degradation efficiency of colorless pollutant bisphenol A(BPA)and ciprofloxacin was improved under visible-light irradiation.The study on the mechanism of photocatalytic degradation showed that round-cake-likeα-Bi2O3 absorbed visible-light to generate electron(e–)and hole(h+)pairs,and photoinduced e–combined with O2 to generate the active speice·O2–,so as to realize the visible-light photocatalytic degradation of pollutant molecules in solution by h+and·O2–radicals.In addition,plate-likeα-Bi2O3,cubic-likeβ-Bi2O3 and cubic-likeγ-Bi2O3 crystals were prepared in low-temperature aqueous solution with other small organic molecules as auxiliary agents.(2)The Bi5O7I nanowire and its assembly structure are rapidly prepared by an aqueous method.By successively introducing KI and NaOH to the reaction system,assembled BiOI precursors were first generated,and then under the attack of OH–,hierarchical Bi5O7I assembly structure was obtained by morphology inheritance with nanowire,and its basic assembly unit is nanowire with 80120 nm diameter.If simultaneously adding NaOH and KI into the reaction system,isolated Bi5O7I nanowires could be formed.The formation mechanism of Bi5O7I nanowire assembly was revealed from the transformation process on crystalline structure and morphology.The formation of hierachical BiOI precursor and the appropriate substitution of OH–for I–facilitated the formation of spherical Bi5O7I achitectures.HRTEM results showed that the assembled and separated Bi5O7I nanowire samples had the same crystal orientation,which were preferentially grown along[010]direction.And the nanowire units in the Bi5O7I assembly structure have a superlattice structure with a periodicity of 0.84 nm.Compared with the BiOI precursor,the Bi5O7I nanowires with different assembly styles exhibited good photocatalytic activity for degradation of rhodamine B(RhB)dye and BPA under visible-light irradiation.In neutral(pH=7)photocatalytic degradation system,the degradation rates of RhB and BPA over Bi5O7I nanowire assembly reached 96%in 3 hours and 90%in 4 hours.Compared with the Bi5O7I nanowire sample,the assembled structure sample shows superior catalytic activity for degradation of organic pollutants under visible light.The degradation mechanism of Bi5O7I nanowire was revealed by the energy band structure and quencher experiments.(3)A series of bismuth-rich Bix Oy Iz(Bi4O5I2,Bi7O9I3 and Bi5O7I)catalysts were successfully synthesized by adjusting the pH value of the reaction solution system and the ratio of Bi(NO3)2 to KI was 2:1.The results show that the crystal structure of Bix Oy Iz is closely related to the acid-base environment of the reaction system.Bi4O5I2(pH=5)was formed in weakly acidic environment and Bi7O9I3 and Bi5O7I were formed in alkaline medium(pH=8 and 11),respectively.The morphologies of Bi4O5I2,Bi7O9I3 assembled structure and rod-like Bi5O7I microstructures were different,showing different crystal growth characteristics.UV-vis DRS results show that Bi4O5I2and Bi7O9I3 have excellent visible light absorption and suitable band structure compared with Bi5O7I microrod and BiOI microsphere.The low electrochemical impedance of Bi7O9I3 microflower further promotes the effective separation of electron-hole pairs during degradation.Therefore,in the series of Bix Oy Iz samples,the assembled structure of Bi7O9I3 based on nanosheets exhibited excellent visible-light photocatalytic degradation of BPA.(4)Bi4O5Br2 nanosheets were prepared by a simple aqueous precipitation method by introducing a large amount of NaBr under alkaline condition.The morphology,crystal structure,optical properties and electronic structure of the samples were discussed.During the reaction,it was found that the amount of NaBr and NaOH had a significant effect on the composition and structure of the samples.The photocatalytic activity of the prepared samples was studied by degrading BPA in visible-light.Bi4O5Br2 nanosheets exhibit better photocatalytic activity than that of BiOBr and Bi5O7Br due to their stronger visible-light absorption and low interfacial charge transport resistance.The photocatalytic degradation mechanism of Bi4O5Br2nanosheets was studied on the basis of quencher experiments and EPR results.It shows that h+and·O2–radicals are the main photoactive species in the process of visible-light catalysis.This study is expected to provide a new idea for the preparation and application of bismuth-rich bismuth bromide photocatalyst.(5)In this study,Bi5O7Br crystals can be rapidly prepared under strongly alkalic circumstances by control Bi/Br=1:1.51:0.5.The results revealed thatβ-Bi2O3 was the intermediate for the reaction.The conversion rate ofβ-Bi2O3 to Bi5O7Br can be controlled by adjusting the amount of NaOH and the reaction temperature,thus achieving the formation ofβ-Bi2O3/Bi5O7Br composites.The photocatalytic experiments were performed on rhodamine B(RhB)and bisphenol A(BPA)under visible-light irradiation.The degradation rate of RhB dye by Bi5O7Br crystal was 99%.The results showed that Bi5O7Br crystal was an indirect dye-sensitization mechanism.β-Bi2O3/Bi5O7Br composite structures were designed and synthesized by combining metastable bismuth oxide with non-stoichiometric bismuth bromide.By using BPA as model pollutant,the degradation rates of single component Bi5O7Br andβ-Bi2O3 were48%and 23%,respectively.Under parallel conditions,the degradation rate ofβ-Bi2O3/Bi5O7Br composite catalyst reached 90%.The improvement of photocatalytic performance ofβ-Bi2O3/Bi5O7Br is attributed to the combination of strong visible-light absorption,effective photo-generated charge separation and rapid charge transfer rate due to the formation of composite structures.Free radical quencher experiments confirmed that·O2–and h+were effective active speices for photocatalytic degradation of RhB and BPA.Finally,the photodegradation mechanism of RhB on Bi5O7Br crystal and BPA onβ-Bi2O3/Bi5O7Br composite structure were explained.In a word,the low temperature aqueous synthestic method used in this paper is simple,cheap and non-toxic,which is conducive to large-scale industrial production.Various bismuth-based semiconductor and composite photocatalytic materials were synthesized by the method.The structure,morphology,size and composition of the samples can be controlled by adjusting the reaction parameters.The prepared material has good superiority in the field of photocatalysis and has important application prospect.