BiOBr/ZnO heterojunction type composite photocatalyst and preparation method thereof
Technical Field
The invention belongs to the field of photocatalysts, and particularly relates to a BiOBr/ZnO heterojunction type composite photocatalyst and a preparation method thereof.
Background
The semiconductor photocatalytic oxidation technology has become a very active research direction in the fields of environmental pollution treatment and solar energy conversion, at present, biological, physical and chemical methods are mostly adopted for treating organic pollutants at home and abroad, wherein the biological method generally takes microorganisms as a degradation medium, and a dilution biochemical method is a biological method generally adopted at home and has the defects of high dilution multiple, unstable performance, secondary pollution and the like; the physical method is mostly a more traditional method and can only treat surface pollution; chemical methods have certain degradation, but the cost is high, the decontamination is not thorough, and the semiconductor photocatalysis can completely degrade the pollutants into H directly or indirectly2O、CO2And the like. Therefore, the photocatalytic oxidation technology has the advantages of high efficiency, energy conservation, no toxicity, thorough pollutant degradation, no secondary pollution and the likeHas the advantages of simple process and low cost.
ZnO is a semiconductor material with photocatalytic performance, has a forbidden band width of 3.2eV at room temperature, can absorb ultraviolet light with a wavelength of less than 380 nm, excites electrons in valence states, generates photo-generated electrons and holes, further generates active groups with strong oxidizing capability, and degrades and mineralizes organic pollutants. Two major drawbacks exist when ZnO is used alone as a photocatalyst in an experiment: firstly, the pure ZnO has high recombination rate of photo-generated electrons and holes, and the quantum efficiency of the photocatalytic reaction is low; and secondly, ZnO is easy to generate optical corrosion phenomenon in the photocatalysis process, so that the stability of ZnO is reduced. These two factors greatly restrict the use of ZnO as a photocatalyst.
Disclosure of Invention
The invention aims to provide a BiOBr/ZnO heterojunction type composite photocatalyst to solve the problem of low quantum efficiency of photocatalytic reaction caused by independently adopting ZnO as a photocatalyst.
The invention also aims to provide a preparation method of the BiOBr/ZnO heterojunction type composite photocatalyst.
The technical scheme of the invention is as follows: a BiOBr/ZnO heterojunction type composite photocatalyst consists of two catalysts, namely BiOBr and ZnO, wherein the content of BiOBr is 2wt%, 4wt%, 8wt%, 16wt% or 32wt%, and the balance is ZnO.
As a further improvement of the invention, the mass fraction of BiOBr is 16 wt%. When the content of the composite BiOBr is 16wt%, the photocatalytic activity of the BiOBr/ZnO is the highest, and the photocatalytic stability of the ZnO is obviously enhanced.
A preparation method of a BiOBr/ZnO heterojunction type composite photocatalyst comprises the following steps:
A. preparing zinc hydroxide: mixing a zinc acetate solution and a sodium hydroxide solution, magnetically stirring for reaction, performing suction filtration and washing, and drying in a drying oven to obtain zinc hydroxide powder;
B. preparation of BiOBr/ZnO: weighing Bi (NO) according to the proportion of BiOBr and ZnO in the claim 13)3·5H2O) and Zn (OH)2Adding Bi (NO)3)3·5H2Adding O into deionized water, stirring for 15-60 min,to this solution were added Zn (OH) in sequence2And stirring the mixture and polyethylene glycol-6000 for 15-60 min, sequentially adding KBr and deionized water, stirring for 15-60 min, transferring the solution into a reaction kettle, sealing, reacting at the temperature of 120-200 ℃ for 4-12 h, performing suction filtration on a product after reaction, sequentially filtering and washing with absolute ethyl alcohol and deionized water, and placing a precipitate into a drying box for drying to obtain the BiOBr/ZnO heterojunction type composite photocatalyst.
As a further improvement of the invention, in the step A, the reaction time is 15-60 min by magnetic stirring.
As a further improvement of the method, in the step A, the drying temperature is 60-100 ℃, and the drying time is 3-12 h.
As a further improvement of the invention, in step B, KBr is reacted with Bi (NO)3)3·5H2The mass ratio of O is 1:4, polyethylene glycol-6000 and Bi (NO)3)3·5H2The mass ratio of O is 1-3: 1.
As a further improvement of the method, in the step B, the drying temperature is 60-100 ℃, and the drying time is 3-12 h.
As a further improvement of the invention, the content of the composite BiOBr is controlled to be 16 wt%.
Compared with the prior art, the invention has the following advantages: BiOBr is an indirect semiconductor material, the forbidden bandwidth of which is 2.69eV and is not changed by the influence of synthesis conditions, the invention compounds two conduction bands and valence bands with different energy levels of BiOBr and ZnO, and after illumination excitation, electron hole pairs effectively migrate, thereby accelerating the separation of photon-generated carriers, reducing the recombination of the electron hole pairs, expanding the light absorption range and further improving the photocatalytic activity. The BiOBr/ZnO composite photocatalyst has no pollution to the environment. The preparation method is simple and low in cost.
Drawings
FIG. 1 is a graph comparing the degradation rates of a BiOBr/ZnO heterojunction type composite photocatalyst and pure ZnO rhodamine B;
FIG. 2 is a structural diagram of the dye rhodamine B;
FIG. 3 is an X-ray powder diffractometer plot of the BiOBr/ZnO heterojunction type composite photocatalyst and BiOBr, ZnO in example 4;
FIG. 4 is a scanning electron micrograph of ZnO;
FIG. 5 is a scanning electron microscope image of the BiOBr/ZnO heterojunction type composite photocatalyst in example 2;
FIG. 6 is a scanning electron microscope image of the BiOBr/ZnO heterojunction-type composite photocatalyst in example 3.
Detailed Description
The following examples further illustrate the invention but are not intended to limit the invention in any way.
The formulation method of the zinc acetate solution used in the following examples was: 27.1 g of zinc acetate was weighed and dissolved in 500ml of deionized water to prepare a zinc acetate solution. The preparation method of the sodium hydroxide solution used in the following examples was: 20g of sodium hydroxide is weighed and dissolved in 500ml of deionized water to prepare 1 mol/L sodium hydroxide solution.
Embodiment 1 discloses a BiOBr/ZnO heterojunction-type composite photocatalyst, wherein the content of BiOBr is 2wt%, and the balance is ZnO. The preparation method comprises the following steps:
A. pouring a sodium hydroxide solution into a zinc acetate solution, magnetically stirring to react for 15min, performing suction filtration, sequentially performing suction filtration and washing for 3 times by using absolute ethyl alcohol and deionized water, and drying in a drying oven at 60 ℃ for 12h to obtain zinc hydroxide powder; the reaction formula is as follows:
B. weighing 0.064gBi (NO) according to the proportion that the BiOBr content is 2wt%3)3·5H2O and 2.393gZn (OH)2Adding Bi (NO)3)3·5H2O into 35ml of deionized water, stirred for 15min, and to this solution was added a weighed amount of Zn (OH)2Adding 0.064g of polyethylene glycol-6000, stirring for 15min, adding 0.016g of KBr, adding 35ml of deionized water, stirring for 15min, transferring the solution into a reaction kettle, sealing, reacting at 120 ℃ for 12h, filtering the product after reaction, and sequentially filtering with absolute ethyl alcohol and deionized waterWashing for 3 times, and drying the precipitate in a drying oven at 100 ℃ for 3 hours to obtain light yellow powder, namely the BiOBr/ZnO heterojunction type composite photocatalyst. The reaction formula is as follows:
and (3) observing the photocatalytic activity of the prepared composite photocatalyst by taking degraded rhodamine B (RhB) as a model reaction: 0.050g of catalyst sample dispersed in 40 mL of 2X 10~5In mol/L rhodamine B water solution. Before illumination, the sample is firstly magnetically stirred for 40min under the condition of keeping out of the sun to achieve adsorption-desorption balance, and an initial sample is taken; after the illumination is started, 3 mL of reaction solution samples are taken every 10min, the catalyst is removed by using a filter membrane, the filtrate is subjected to absorbance measurement at the characteristic absorption wavelength (553 nm) of RhB by using an ultraviolet visible spectrophotometer, and the concentration of the rhodamine B can be determined by using a rhodamine standard curve. The light irradiation is carried out for 40min, and the degradation rate is 78.19 percent.
Embodiment 2 discloses a BiOBr/ZnO heterojunction-type composite photocatalyst, wherein the content of BiOBr is 4wt%, and the balance is ZnO. The preparation method comprises the following steps:
A. pouring a sodium hydroxide solution into a zinc acetate solution, magnetically stirring to react for 30min, performing suction filtration, sequentially performing suction filtration and washing for 3 times by using absolute ethyl alcohol and deionized water, and drying in a drying oven at 80 ℃ for 6h to obtain zinc hydroxide powder;
B. 0.128g of Bi (NO) was weighed in a proportion of 4wt% of BiOBr3)3·5H2O and 2.344g Zn (OH)2Adding Bi (NO)3)3·5H2O into 35ml of deionized water, stirred for 30min, and to this solution was added weighed Zn (OH)2Adding 0.2g of polyethylene glycol-6000, stirring for 30min, adding 0.032g of KBr, adding 35ml of deionized water, stirring for 30min, transferring the solution into a reaction kettle, sealing, reacting for 8h at 140 ℃, performing suction filtration on a product after the reaction, sequentially performing filter washing for 3 times by using absolute ethyl alcohol and deionized water, and placing a precipitate into a drying oven, and drying for 6h at 80 ℃ to obtain light yellow powder, namely the BiOBr/ZnO heterojunction type composite photocatalyst.
And (3) observing the photocatalytic activity of the prepared composite photocatalyst by taking degraded rhodamine B as a model reaction, wherein the evaluation conditions are the same as those of example 1, the illumination is 40min, and the degradation rate is 74.54%.
Embodiment 3 discloses a BiOBr/ZnO heterojunction-type composite photocatalyst, wherein the content of BiOBr is 8wt%, and the balance is ZnO. The preparation method comprises the following steps:
A. pouring a sodium hydroxide solution into a zinc acetate solution, magnetically stirring to react for 60min, performing suction filtration, sequentially performing suction filtration and washing for 3 times by using absolute ethyl alcohol and deionized water, and drying in a drying oven at 100 ℃ for 3h to obtain zinc hydroxide powder;
B. 0.256g of Bi (NO) was weighed in a proportion of 8wt% of BiOBr3)3·5H2O and 2.247g Zn (OH)2Adding Bi (NO)3)3·5H2O into 35ml of deionized water, stirred for 45min, and to this solution was added a weighed amount of Zn (OH)2Adding 0.385g of polyethylene glycol-6000, stirring for 45min, adding 0.064g of KBr, adding 35ml of deionized water, stirring for 45min, transferring the solution into a reaction kettle, sealing, reacting for 6h at 160 ℃, performing suction filtration on a product after the reaction, sequentially performing filter washing for 3 times by using absolute ethyl alcohol and deionized water, and placing a precipitate into a drying box, drying for 12h at 60 ℃ to obtain light yellow powder, namely the BiOBr/ZnO heterojunction type composite photocatalyst.
And (3) observing the photocatalytic activity of the prepared composite photocatalyst by taking degraded rhodamine B as a model reaction, wherein the evaluation conditions are the same as those in example 1, the illumination is 40min, and the degradation rate is 92.62%.
Embodiment 4 discloses a BiOBr/ZnO heterojunction-type composite photocatalyst, wherein the content of BiOBr is 16wt%, and the balance is ZnO. The preparation method comprises the following steps:
A. pouring a sodium hydroxide solution into a zinc acetate solution, magnetically stirring to react for 30min, performing suction filtration, sequentially performing suction filtration and washing for 3 times by using absolute ethyl alcohol and deionized water, and drying in a drying oven at 80 ℃ for 6h to obtain zinc hydroxide powder;
B. 0.512g of Bi (NO) was weighed in a proportion of 16wt% of BiOBr3)3·5H2O and 2.051g Zn (OH)2Adding Bi (NO)3)3·5H2O into 35ml of deionized water, stirred for 60min, and to this solution was added a weighed amount of Zn (OH)2Adding 1.024g of polyethylene glycol-6000, stirring for 60min, adding 0.128g of KBr, adding 35ml of deionized water, stirring for 60min, transferring the solution into a reaction kettle, sealing, reacting at 180 ℃ for 6h, performing suction filtration on the product after the reaction, sequentially performing filter washing for 3 times by using absolute ethyl alcohol and deionized water, and placing the precipitate into a drying oven to dry at 80 ℃ for 6h to obtain light yellow powder, namely the BiOBr/ZnO heterojunction type composite photocatalyst.
And (3) observing the photocatalytic activity of the prepared composite photocatalyst by taking degraded rhodamine B as a model reaction, wherein the evaluation conditions are the same as those of example 1, the illumination is 40min, and the degradation rate is 95.97%.
Embodiment 5 discloses a BiOBr/ZnO heterojunction-type composite photocatalyst, wherein the content of BiOBr is 32wt%, and the balance is ZnO. The preparation method comprises the following steps:
A. pouring a sodium hydroxide solution into a zinc acetate solution, magnetically stirring to react for 30min, performing suction filtration, sequentially performing suction filtration and washing for 3 times by using absolute ethyl alcohol and deionized water, and drying in a drying oven at 80 ℃ for 6h to obtain zinc hydroxide powder;
B. weighing 1.052g of Bi (NO) according to the proportion that the BiOBr content is 32wt%3)3·5H2O and 1.66g Zn (OH)2Adding Bi (NO)3)3·5H2O into 35ml of deionized water, stirred for 60min, and to this solution was added a weighed amount of Zn (OH)2Adding 3.156g of polyethylene glycol-6000, stirring for 30min, adding 0.258g of KBr, adding 35ml of deionized water, stirring for 60min, transferring the solution into a reaction kettle, sealing, reacting for 4h at 200 ℃, filtering and washing the product for 3 times by using absolute ethyl alcohol and deionized water in sequence, and drying the precipitate for 6h at 80 ℃ in a drying oven to obtain light yellow powder, namely the BiOBr/ZnO heterojunction type composite photocatalyst.
And (3) observing the photocatalytic activity of the prepared composite photocatalyst by taking degraded rhodamine B as a model reaction, wherein the evaluation conditions are the same as those of example 1, the illumination is 40min, and the degradation rate is 72.08%.
The comparison of the degradation rates of the BiOBr/ZnO heterojunction-type composite photocatalyst prepared in the above embodiments and pure ZnO to rhodamine B is shown in FIG. 1, and it can be seen from FIG. 1 that the photocatalytic activity of the BiOBr/ZnO heterojunction-type composite photocatalyst is obviously improved compared with that of pure ZnO, and when the content of the composite BiOBr is 16wt%, the photocatalytic activity is highest.
The structure of the dye rhodamine B is shown in figure 2.
FIG. 3 is an X-ray powder diffractometer diagram of the BiOBr/ZnO heterojunction type composite photocatalyst and pure BiOBr and pure ZnO of the invention, as can be seen from FIG. 3, 16% BiOBr/ZnO has characteristic diffraction peaks appearing on (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystal faces, the positions of the diffraction peaks are consistent with JCPDS card Nos. 36-1451, which shows that the peaks correspond to ZnO with hexagonal wurtzite structure, and all the peaks have relatively sharp peak types and relatively narrow half-peak widths, which shows that the sample has very high crystallinity under the hydrothermal treatment at 150 ℃. No characteristic diffraction peak of BiOBr was observed, mainly due to too low content of BiOBr, or to the fact that BiOBr is in a highly dispersed state, exceeding the detection limit of XRD.
Fig. 4, fig. 5 and fig. 6 are scanning electron micrographs of pure ZnO, the BiOBr/ZnO heterojunction-type composite photocatalyst prepared in example 2 and example 3 respectively, and it can be seen that SEM images of the three samples are substantially similar. The pure ZnO particles subjected to hydrothermal treatment at 140 ℃ and 160 ℃ are large and have poor dispersibility. The dispersibility of the samples of 4 percent BiOBr-ZnO and 8 percent BiOBr-ZnO subjected to hydrothermal treatment at 140 ℃ and 160 ℃ is improved, and the samples also show a certain shape, are short rod-shaped structures and have the length of about 1-2 microns.