A new strategy for insulators to improve the ability of semiconductors to reduce Cr (VI)

Insulators have always been a kind of material that has not been valued in the field of catalysis. In order to expand its application in the field of photocatalytic treatment of heavy metal ions, a series of CdS/SrCO3 catalysts are prepared. XRD proves the successful synthesis of the composite catalyst, and TEM reveals the tight connection between CdS and SrCO3. Transient photocurrent and electrochemical impedance spectroscopy show that the composite catalyst has a better electron-hole pair separation effect and electron transport rate than the two single-phase materials. The catalyst has excellent photocatalytic properties and good stability. Finally, the reaction kinetics in the process of photocatalytic reduction of composite catalyst is discussed.


Introduction
The problem of environmental pollution brought about by scientific and technological progress is becoming more and more serious.Human health is under threat.Photocatalysis technology is born under this background that the problem of environmental governance is imminent.As an environmentally friendly material with sunlight as driving energy, photocatalytic materials have been a concern for conservative scientists in the 21st century.It is widely used in the treatment of antibiotics, dyes, and heavy metal ions.Semiconductor materials represented by TiO 2 , ZnO, and MoS 2 are the main components of photocatalytic materials because the internal electron-hole pairs of semiconductor materials can be excited and separated quickly under sunlight, which is beneficial to the photocatalytic redox reaction.Wu used a simple chemical precipitation method to construct a CuS/TiO 2 composite photocatalyst to solve the efficient degradation of Cr(VI) in a wide range of pH [1] .Wang prepared ZnO/Na 0.5 Bi 0.5 TiO 3 Z type heterojunction by hydrothermal method and achieved 100% MB degradation of 10 mg/L in 20 minutes [2] .The flower-shaped MnFe 2 O 4 /MoS 2 catalyst with a high specific surface area was prepared by Wei for tetracycline degradation.The catalyst has the advantages of magnetic recovery and good cycle life [3] .
Under the background of the rapid development of photocatalyst technology, people pay attention to insulator materials such as CaCO 3 and SrCO 3 .Compared with semiconductor materials, insulator materials have significant advantages such as large reserves and low prices.At present, the application of insulator materials in the field of photocatalysis is mainly in the field of organic matter degradation and gas air purification.Qi designed a one-step calcination method to prepare g-C 3 N 4 /CaCO 3 photocatalyst and CaCO 3 , increasing the efficiency of nicotine degradation of g-C 3 N 4 by 10 times

Raw materials and preparation methods
All the solvents and reagents used in this paper are experimental grade.All reagents are purchased from Aladdin, and the water used in the experiment is deionized water.Synthesis of CdS is conducted based on previous methods [6] .First, 0.005 mol CdCl 2 •2.5H 2 O and 0.01 mol NaOH were added to 50 mL deionized water respectively, and stirred at room temperature until completely dissolved.Second, it was mixed and stirred for 30 minutes, named solution A. 0.005 mol Na 2 S•9H 2 O and the appropriate amount of SrCO 3 were added to 50 mL deionized water and stirred at room temperature for 30 minutes, and the solution was named B. Finally, after stirring the A solution and B solution at room temperature for 12 hours, a large number of orange precipitates appeared in the solution.After stirring, the orange precipitate was extracted and filtered.The sample was dried in an oven at 60℃ for 12 hours.The orange CdS/SrCO 3 -X powder was obtained.The molar ratio of CdS and SrCO 3 is expressed by X.

Characterization method and performance determination
The phase structure of the catalyst was analyzed based on the D8-Advance diffractometer.The bonding state of the materials was studied by Thermo ESCALAB 250XI with Al Kα radiation XPS.Quanta 450FEG scanning electron microscope and Tecnai G20 transmission electron microscope revealed the microstructure.Photocurrent data and Electrochemical Impedance Spectra of compounds were measured by CHI-660C.The calculation of photocatalytic efficiency is based on previous studies [7] .The X-ray diffraction patterns of CdS, SrCO 3 , and CdS/SrCO 3 -X are shown in Figure 1.The diffraction peaks of CdS correspond to (111), (220), and (331) planes, and the diffraction peaks of SrCO 3 correspond to (111), (021), and (221) planes respectively.In CdS/SrCO 3 -X, the corresponding diffraction peak becomes stronger with the increase of strontium carbonate content.We found that there are diffraction peaks of CdS and SrCO 3 in CdS/SrCO 3 -X and there are no other miscellaneous peaks, which indicates that we have obtained a pure catalyst.2d) belong to Sr 2+ [8] .The double peaks of 289.4 eV and 284.6 eV in the C 1s region are generated by carbonate and carbon standards [9] .The two fitting peaks at 283.1 eV and 279.1 eV correspond to C=C and C-C [10][11]    Photocatalytic reduction of Cr(VI) was used to evaluate the photocatalytic performance of the composite catalyst.A solution with a total amount of 200 mL and a hexavalent chromium ion concentration of 2.5 mg/L was used to simulate industrial wastewater.The photoreaction test was carried out after 30 minutes of dark reaction.SrCO 3 with a wide band gap was not tested because of its poor response to visible light.Figure 5a reflects the relationship between catalytic efficiency and light duration.The catalytic efficiency of CdS is not good, which is related to its easy agglomeration and photo corrosion.In addition, three different ratios of composite photocatalysts show better photocatalytic performance than CdS, which is beneficial to show that the interaction between SrCO 3 and CdS can effectively improve photocatalytic properties.Sample CdS/SrCO 3 -5/1 is the best, and the Cr(VI) reduction rate reaches 93% after 30 minutes of photoreaction.From Figure 6b, we can calculate that the reduction rate constant of CdS/SrCO 3 -5/1 is 0.1064 min -1 , which is 4.88 times higher than that of CdS.The absorption peak intensity of Cr(VI) at 540 nm gradually attenuates with the change of light time (Figure 5c).Based on the trapping agent experiment, the active groups that play a leading role in the photoreaction stage were analyzed.IPA, EDTA-2Na and BQ captured ‧OH, h + and ‧O 2 -, respectively.It can be seen that the addition of IPA and EDTA-2Na has little effect on the photocatalytic reaction, and the addition of BQ seriously affects the efficiency of the photocatalytic reaction, which indicates that ‧O 2 -is the main active substance in the photocatalytic stage.The cycle life of the catalyst is an important factor in evaluating the activity of the catalyst.After four rounds of photocatalytic cycling experiments, CdS/SrCO 3 -5/1 still has a catalytic efficiency of 77.5%, which shows that the catalyst has excellent stability.The EIS-Nyquist diagram is a favorable way to analyze the carrier transport efficiency of photocatalysts.The arc pattern of CdS/SrCO 3 -5/1 in Figure 6b has the smallest arc radius, which means higher electron transfer efficiency [12] .Photocurrent transient response is an important method to evaluate the separation efficiency of photogenerated carriers in the catalyst, and the photocurrent density of CdS/SrCO 3 -5/1 is about twice that of CdS.The photocurrent response of SrCO 3 is almost a straight line because of its wide band gap.After a series of analyses above, we proposed a catalytic reduction method of Cr(VI) under the synergistic action of CdS and SrCO 3 (Figure 7).Under the action of chemical synthesis, a close connection is established between CdS and SrCO 3 which helps to increase the contact area between them and provides an effective path for the rapid transfer of electrons and holes.SrCO 3 has a wide band gap and fully includes the band gap of CdS [13][14] .The tight binding of SrCO 3 suppresses the recombination of electron holes produced on the surface of CdS [15][16] .Under the action of visible light, free oxygen in the liquid is oxidized to ‧O 2 -by electrons on the CdS conduction band.The source of oxygen also comes partly from the combination of water molecules with holes [17] .Cr(VI) can combine with electrons or superoxide radicals and be reduced to Cr(III) [18] .

Conclusions
In this paper, we successfully prepared a CdS/SrCO 3 composite photocatalyst by a simple chemical precipitation synthesis method.Under the transmission electron microscope, it can be seen that the disclike CdS and rod-shaped SrCO 3 combine closely, which is beneficial to the rapid electron transfer between them.The best photocatalytic efficiency of CdS/SrCO 3 -5/1 is 4.88 times higher than that of CdS, and the catalytic efficiency is as high as 93% after 25 minutes.Superoxide free radicals are the main active substances, and the catalyst still has a catalytic efficiency of 77.5% after four life cycles, which proves that the material has good stability.Finally, we discuss the catalytic effect of SrCO 3 on CdS and the photoreaction process in which Cr(VI) is reduced to Cr(III).This study provides a new idea for insulator materials in the field of heavy metal ion treatment and is expected to expand more applications of this neglected material in the field of photocatalysis.

Figure 2 .
Figure 2. (a) XPS spectra of CdS/SrCO 3 -5/1 nanocomposites, XPS spectra of the survey scan: (b) S 2p, (c) Cd 3d, (d) Sr 3d, (e) C 1s, (f) O 1s.The chemical composition and bonding valence of CdS/SrCO 3 -5/1 samples were measured by XPS. Figure 2a displays the measured spectrum of the composite catalyst, showing the presence of S, Cd, Sr, C, and O elements, without other impurity elements.The peaks at 160.9 eV and 159.8 eV in Figure 2b correspond to S 2p 3/2 and S 2p 1/2 , respectively.The Cd 3d spectrum (figure 2c) consists of two peaks at 410.3 eV and 403.5 eV belonging to Cd 3d 3/2 and Cd 3d 5/2 .The 134.7 eV and 133.0 eV peaks of Sr 3d (figure2d) belong to Sr 2+[8] .The double peaks of 289.4 eV and 284.6 eV in the C 1s region are generated by carbonate and carbon standards[9] .The two fitting peaks at 283.1 eV and 279.1 eV correspond to C=C and C-C[10][11] .The 530.1 eV fitting peak in the O1s map is attributed to O 2-.

Figure 3
Figure 3 reveals the microstructure images of CdS, SrCO 3 , and CdS/SrCO 3 -5/1.The CdS in Figure3ais an agglomerated structure composed of nanosheets about 50 nm in diameter.SrCO 3 (figure3b) has a typical crystal structure and different shapes, most of which are short rods.Figures3c-dclearly show that the short rod-shaped SrCO 3 is attached to the agglomerated CdS particles, which proves the successful preparation of the composite catalyst.

Figure 4 . 5 3. 3 Figure 5 .
Figure 4. TEM images of CdS/SrCO 3 -5/1 (a-c).Taking 5/1-CdS/SrCO 3 as the research object, the micromorphology of the sample was further analyzed.Figures 4a-b show flaky and slightly agglomerated CdS nanowires adhering closely to the rod-shaped SrCO 3 which helps to prove the strong interaction between CdS and SrCO 3 and provides an effective path for charge transfer.The lattice stripes in the interface between CdS and SrCO 3 are shown in Figure 4c.The lattice stripes with SrCO 3 spacing of 0.205 nm correspond to the (221) crystal planes, and the lattice stripes with CdS spacing of 0.335 nm belong to the (111) crystal planes.