Preparation of S-type heterojunction N-C3N4/BiOClxI1−x with internal electric field and enhanced photocatalytic properties
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摘要: 采用一步水热法使固溶体BiOClxI1−x静电自组装在N掺杂的氮化碳(N-C3N4)表面，制备了N-C3N4/BiOClxI1−x S型异质结。通过XRD、XPS、SEM、TEM、FTIR、UV-Vis等技术对样品的晶型、形貌、结构、元素组成、表面官能团、光学性质等进行了表征，并考察了N-C3N4/BiOClxI1−x光催化氧化有机污染物与还原Cr(VI)的活性。结果表明，N-C3N4/BiOClxI1−x具有强的光吸收，在N-C3N4与BiOClxI1−x界面处形成的内电场抑制了电子-空穴对的复合。在可见光照射下，20%N-BiOCl0.5I0.5呈现出优异的光催化活性，2.5 h内苯酚的降解率达到98.53%；1 h内Cr(VI)的还原率达到99.11%。20%N-BiOCl0.5I0.5在5次循环后表现出良好的稳定性。3 h内20%N-BiOCl0.5I0.5可见光降解苯酚的总有机碳(TOC)去除率为80.21%。结合捕获实验、ESR、DFT计算等表明，N-C3N4/BiOClxI1−x活性归因于S型异质结的形成、N-C3N4和BiOClxI1−x之间的内部电场及能带弯曲和库仑力的存在，加速了光生载流子的空间分离和有序电子流。
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关键词:
- 光催化 /
- N-C3N4/BiOClxI1−x /
- 静电自组装 /
- S型异质结 /
- 内电场
Abstract: N doped carbon nitride (N-C3N4)/BiOClxI1−x S-type heterojunctions were prepared by a facile one-step hydrothermal method. The crystal form, morphology, structure, elemental composition, surface functional groups and optical properties of the samples were characterized by XRD, XPS, SEM, TEM, FTIR and UV-Vis. The photocatalytic activity of N-C3N4/BiOClxI1−x oxidation of organic pollutants and reduction of Cr(VI) was investigated. The results show that N-C3N4/BiOClxI1−x sample exhibits the effective enhancement in light absorption. The charge carriers were generated by the transfer of the photoinduced electron from N-C3N4 to BiOClxI1−x across the interface under irradiation, which inhibited the recombination of electron-hole pairs. Under visible light irradiation, 20%N-BiOCl0.5I0.5 exhibited high activity, the degradation rate of phenol reached 98.53% with 2.5 h of visible light irradiation. Meanwhile, the reduction rate of Cr(VI) of 20%N-BiOCl0.5I0.5 reached to 99.11% with 1 h of visible light irradiation. 20%N-BiOCl0.5I0.5 showed good stability after five cycles. The total organic carbon (TOC) removal rate of degradation phenol by 20%N-BiOCl0.5I0.5 within 3 h was 80.21%. Combined with capture experiment, ESR and DFT calculation, the improvement activity of N-C3N4/BiOClxI1−x was attributed to the formation of S-type heterojunction, the internal electric field based on different Fermi levels between N-C3N4 and BiOClxI1−x, as well as band bending and Coulomb force, which together accelerated spatial separation of photogenerated carriers and orderly electron flow. -
图 1 (a) BiOClxI1−x的XRD图谱；(b) BiOClxI1−x在2θ=25°~37°的XRD图谱；(c) N-C3N4/BiOClxI1−x的XRD图谱；(d) 样品的FTIR图谱；(e) N-C3N4的Zeta谱图；(f) BiOCl0.5I0.5的Zeta谱图
Figure 1. (a) XRD patterns of BiOClxI1−x; (b) XRD patterns of BiOClxI1−x at 2θ=25°-37°; (c) XRD patterns of N-C3N4/BiOClxI1−x; (d) FTIR spectra of the as-prepared samples; (e) Zeta spectra of N-C3N4; (f) Zeta spectra of BiOCl0.5I0.5
图 2 BiOCl0.5I0.5和20%N-BiOCl0.5I0.5的HR-XPS图谱
Figure 2. HR-XPS spectra of BiOCl0.5I0.5 and 20%N-BiOCl0.5I0.5
图 3 BiOI (a)、BiOCl0.5I0.5 (b) 的SEM图像；(c) 20%N-BiOCl0.5I0.5局部放大图像
Figure 3. SEM images of BiOI (a), BiOCl0.5I0.5 (b); (c) Partial enlargement of 20%N-BiOCl0.5I0.5
d—Thickness of nanosheets
图 4 ((a)~(c)) 20%N-BiOCl0.5I0.5的TEM图像；((d)~(j)) 20%N-BiOCl0.5I0.5的EDS图谱、Bi、Cl、O、I、N、C图谱
Figure 4. ((a)-(c)) TEM images of 20%N-BiOCl0.5I0.5; ((d)-(j)) EDS, Bi, Cl, O, I, N and C mapping of 20%N-BiOCl0.5I0.5
图 5 (a) N-BiOClxI1−x的UV-Vis漫反射图谱；(b) BiOClxI1−x的带隙
Figure 5. (a) UV-Vis DRS diffuse reflection map of N-BiOClxI1−x; (b) Band gap of N-BiOClxI1−x
图 6 N-C3N4 (a) 和BiOCl0.5I0.5 (b) 的Mott-Schottky曲线；(c) 能带结构
Figure 6. Mott-Schottky curves N-C3N4 (a) and BiOCl0.5I0.5 (b); (c) Band structure
C−2—Interfacial capacitance
图 7 (a) 苯酚的光催化降解曲线；((b), (c))光催化还原六价铬曲线和表观速率常数；(d) 20%N-BiOCl0.5I0.5循环降解苯酚稳定性测试；(e) 20%N-BiOCl0.5I0.5在5个循环前后的XRD图谱；(f) 20%N-BiOCl0.5I0.5可见光降解苯酚的TOC去除率
Figure 7. (a) Photocatalytic degradation curve of phenol; ((b), (c)) Photocatalytic reduction curve and apparent rate constant of Cr(Ⅵ); (d) Recycling use of 20%N-BiOCl0.5I0.5 for phenol degradation; (e) XRD patterns of before and after five cycles of 20%N-BiOCl0.5I0.5 for for phenol degradation; (f) Removal rate of TOC of phenol over 20%N-BiOCl0.5I0.5
C0—Initial solution concentration; Ct—Solution concentration at time t; K—Apparent rate constant; TOC0—Total organic carbon of initial solution; TOC—Total organic carbon of solution at time t
图 8 20%N-BiOCl0.5I0.5的•O2−测试 (a)、•OH−测试 (b)、h+测试 (c)
Figure 8. ESR spectra of •O2− (a), •OH− (b), h+ (c) of 20%N-BiOCl0.5I0.5
图 9 (a) N-C3N4理论模型；(b) N-C3N4的功函数；(c) BiOCl0.5I0.5的功函数；((d)~(e)) 20%N-BiOCl0.5I0.5三维电子密度差图
Figure 9. (a) N-C3N4 theoretical model; (b) N-C3N4 of work function calculation; (c) BiOCl0.5I0.5 of work function calculation; ((d)-(e)) Three-dimensional charge density differences of 20%N-BiOCl0.5I0.5
Φ—Work function
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