无机材料学报, 2020, 35(12): 1349, 网络出版: 2021-03-10

WO3纳米花的热处理晶格调控及WO3/CdS/α-S异质结的构筑

Lattice Control of WO3 Nanoflowers by Heat Treatment and Construction of WO3/CdS/α-S Heterojuntion
作者单位

1福州大学 1. 材料科学与工程学院

2生态环境材料先进技术福建省高等学校重点实验室, 福州 350108

摘要
为研究热处理过程与异质结构筑对WO3的光电化学效应的影响机制, 采用低温溶剂热法制备纳米花状WO3, 通过热处理精确调控WO3纳米花的活性晶面、晶粒尺寸及结晶度。进一步借助循环化学浴法, 构筑WO3/CdS/α-S异质结, 并研究其光电化学性能与浓度效应。结果表明, (200)晶面是WO3纳米花的主要暴露晶面, 且比例随热处理温度升高而增大。350 ℃热处理的WO3纳米花表现出最高的光响应电流。通过构筑WO3/CdS/α-S梯形异质结, 增强材料在可见光区的吸收, 以牺牲少部分载流子的方式提高整体光生载流子的分离效率, 促进WO3的宏观光电化学效应的提升。
Abstract
In order to study the influence mechanism of heat treatment and heterostructures on the photoelectrochemical effect of WO3, monoclinic WO3 nanoflowers were synthesized by low-temperature solvothermal method. The active crystal fact, grain size and crystallinity of WO3 were controlled by heat treatment. Furthermore, WO3/CdS/α-S heterojunction was obtained by modified chemical bath deposition, and the concentration effect of its photoelectrochemical performance was studied. The results show that the (200) crystal plane with photoelectrochemical activity is the main exposed crystal plane of WO3, and the proportion of the exposed crystal plane increases with the heat treatment temperature increasing. The WO3 nanoflower treated at 350 ℃ showed the highest photoresponse current. By constructing WO3/CdS/α-S heterojunction, the material's absorption in the visible light region is enhanced, and the overall efficiency of photo-generated carrier separation is improved by sacrificing a small amount of carriers, which promotes the macroelectronic chemical effects of WO3.

利用半导体催化剂光电催化水分解析氧、制H2、还原CO2或降解污染物, 有望解决相关的能源环境问题[1,2,3,4]。光吸收、光电转换效率、光生载流子迁移和表面反应活性是影响材料光电催化性能的关键因素。三氧化钨(WO3)的带隙范围为2.5~2.7 eV, 能吸收12%的太阳光, 是一种极具潜力的半导体光电材料[5]。在WO3晶型结构中, γ-单斜晶相(γ-WO3)是室温下最稳定的相, 其WO6八面体在abc三轴方向通过共角方式连接。相邻WO6八面体在各方向上的扭曲方式和程度取决于体系温度, 并伴随着WO3的形貌发生变化, 因此, 通过热处理方式可对γ-WO3进行调控, 提高其表面暴露晶面中高活性晶面的比例, 增强γ-WO3的光电催化效应[6]。此外, 构筑表面异质结是提升WO3中载流子分离效率的有效方法。通过构筑WO3与g-C3N4异质结[7]、在WO3纳米线表面修饰In2S3包覆的Au颗粒[8]等方法, 均能促进材料内部光生载流子的有效分离, 提高异质结材料的光电转换效率。

硫化镉(CdS)材料不仅具有良好的可见光吸收性能, 而且可作为异质结组分用于增强光生载流子的分离[9]。近年来, 部分元素半导体, 如硫(S)、磷(P)等, 也被证明能够分解偶氮染料和光解水[10,11], 并且适量的α-S单质可显著提升CdS的光解水制氢效率[12]

本研究采用低温溶剂热法成功制备了γ-WO3纳米花, 通过空气热处理过程, 精确调控γ-WO3纳米花的晶格结构, 探究γ-WO3活性晶面比例、晶粒尺寸等对光电转换效应的影响机制。进一步借助改进的循环化学浴法, 构筑WO3/CdS/α-S异质结, 研究复合材料的光电化学性能及CdS/α-S的浓度效应。

1 实验方法

1.1 WO3纳米花的制备

将0.396 g WCl6加入20 mL 二甘醇中, 在70 ℃下搅拌4 h后, 将反应物在丙酮中沉淀, 经丙酮洗涤数次后, 60 ℃烘干, 制得WO3纳米花前驱体(标记为W-70)。将前驱体置于管式炉中, 热处理1 h, 获得具有不同晶格结构的WO3纳米花。根据热处理温度150、250、300、310、330、350、370、390、410、430和450 ℃, 样品标记为W-n (n=150, 250, 300, 310, 330, 350, 370, 390, 410, 430, 450)。

1.2 WO3/CdS/α-S异质结的构筑

采用改进的循环化学浴法[13], 在W-350表面修饰CdS/α-S。将30 mg W-350超声分散在2 mL去离子水中, 均匀滴在滤膜上, 抽滤除去溶剂; 加入2 mL 0.05 mol/L CdCl2水溶液浸没样品30 s后, 抽滤除去溶剂, 再加入10 mL去离子水, 洗涤并抽滤除去溶剂; 取2 mL 0.05 mol/L K2S水溶液浸没样品30 s后, 抽滤除去溶剂, 再加入10 mL去离子水, 洗涤并抽滤除去溶剂。由此, 完成一个CdS/α-S修饰循环。重复数次后, 用去离子水洗涤样品, 并70 ℃干燥。分别以W-350-10C、W-350-20C、W-350-30C和W-350-40C标记修饰循环次数为10、20、30和40次的样品。

1.3 样品表征

采用Miniflex 600 X射线衍射仪测试样品的X射线粉末衍射(XRD)图谱。采用Zeiss supra 55扫描电子显微镜(SEM)和TECNAI G2 F20透射电子显微镜(TEM)观察样品形貌, 利用能量色散X射线光谱仪(EDX)分析元素分布; 采用ThermoDXR2xi共聚焦激光拉曼光谱仪测定样品的拉曼光谱(Raman, 激发波长532 nm); 采用ESCALAB 250 X射线衍射光电子能谱仪(XPS)分析样品的表面元素组成及价态; 采用Lambda 950紫外-可见分光光度计, 测定样品的紫外-可见-近红外(UV-Vis-IR)吸收光谱。

1.4 光电化学性能测试

将10 mg样品与1 mL N,N-二甲基甲酰胺溶液(含40 μL Nafion溶液)混合后, 取200 μL均匀涂敷在氧化铟锡(ITO)导电玻璃上, 干燥后刮出0.5 cm× 0.5 cm样品区域, 在其余区域涂覆硝化纤维素。分别以该ITO玻璃、Ag/AgCl电极和铂片为工作电极、参比电极和对电极, 0.5 mol/L Na2SO4溶液为电解液, 氙灯(500 W, GLORIA-X500A)为光源, 采用电化学工作站(CHI 660D, 上海辰华)测试样品的光电流曲线(偏压1 V (vs. Ag/AgCl))和电化学阻抗谱(EIS)。光电流密度均已扣除暗电流影响, 其入射光子电流效率(IPCE)根据公式(1)计算[6]:

$IPCE=\frac{1240×I}{\lambda × J_{light}}$

其中, I是光电流密度(mA/cm2), λ是入射光波长(nm), Jlight是特定波长单色光的功率密度(mW/cm2)。

2 结果与讨论

如SEM照片(图1(a))所示, γ-WO3前驱体是由尺寸约50 nm的薄片组成的纳米花。350 ℃热处理后, 纳米花片状结构缩小, 但厚度增加, 沟壑变浅(图1(b))。不同温度热处理样品SEM表征结果见图S1。由XRD图谱(图1(c))可知, 样品均为γ-单斜晶相(JCPDS 72-0677), 无杂质峰, 但γ-WO3的(002)、(020)、(200)三个晶面衍射峰(位于2θ=23.2°、23.7°和24.2°)逐渐由重合状态转变为分离状态。图1(d~e)、图S2表1列出了Rietveld全谱拟合精修结果。随着热处理温度升高, (002)、(020)和(200)晶面衍射峰位趋向于PDF卡片中的标准值, 说明热处理消除了层间结合水和部分有机物, 提高了WO6八面体排列有序化程度和结晶度。Pearson VII峰型函数计算结果(图1(e))显示, (200)是样品的主要暴露晶面。晶粒尺寸随热处理温度升高而增大(图1(f)), 在450 ℃时晶粒尺寸变化趋势出现波动, 可能与高温相转变趋势有关。

SEM images of WO3 nanoflower precursor (a) and W-350 (b); XRD patterns of the samples heat-treated at different temperatures (c); (002), (020), and (200) crystal plane diffraction peak positions (d), diffraction peak integrated area ratios (e) and grain sizes (f) obtained by Rietveld refinement varied as functions of heat treatment temperature

图 1. WO3纳米花前驱体(a)和W-350(b)的SEM照片; 不同温度热处理样品的XRD图谱(c); Rietveld精修的(002)、(020)、(200)晶面衍射峰位(d)、衍射峰积分面积占比(e)和晶粒尺寸(f)与热处理温度的关系曲线

Fig. 1. SEM images of WO3 nanoflower precursor (a) and W-350 (b); XRD patterns of the samples heat-treated at different temperatures (c); (002), (020), and (200) crystal plane diffraction peak positions (d), diffraction peak integrated area ratios (e) and grain sizes (f) obtained by Rietveld refinement varied as functions of heat treatment temperature

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SEM images of precursor (a), W-310 (b), W-330 (c), W-370 (d), W-390 (e), W-410 (f), W-430 (g), and W-450 (h).

图 8. 前驱体(a)、W-310(b)、W-330(c)、W-370(d)、W-390(e)、W-410(f)、W-430(g)、W-450(h)的SEM照片

Fig. 8. SEM images of precursor (a), W-310 (b), W-330 (c), W-370 (d), W-390 (e), W-410 (f), W-430 (g), and W-450 (h).

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Rietveld refinement results of XRD data from W-310 (a), W-330 (b), W-350 (c), W-370 (d), W-390 (e), W-410 (f), W-430 (g), W-450 (h)

图 9. W-310 (a)、W-330 (b)、W-350 (c)、W-370 (d)、W-390 (e)、W-410 (f)、W-430 (g)、W-450 (h)的XRD数据Rietveld精修结果

Fig. 9. Rietveld refinement results of XRD data from W-310 (a), W-330 (b), W-350 (c), W-370 (d), W-390 (e), W-410 (f), W-430 (g), W-450 (h)

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表 1.

不同温度热处理样品的XRD数据Rietveld精修结果

Table 1. Rietveld refinement results of XRD data of the samples heat-treated at different temperatures

Sample(002)2θ/(°)(020)2θ/(°)(200)2θ/(°)(002)Peak area ratio/%(020)Peak area ratio/%(200)Peak area ratio/%R/%aE/%b
W-31023.30223.80523.98931.6073631.9359236.456727.967.22
W-33023.19123.74424.06328.5136231.2122540.274138.459.08
W-35023.15223.72424.11025.4827733.4155141.101718.718.87
W-37023.13623.69824.17124.1337035.2292340.637077.778.89
W-39023.12823.68524.21720.5281533.0232546.44868.748.72
W-41023.11323.64324.20331.6064129.8377338.555869.6211.08
W-43023.12623.66824.26830.2641031.8950737.840838.928.88
W-45023.06623.60424.20228.0814534.1631937.755368.849.0

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图2(a)为样品的Raman光谱图。266、704、805 cm-1处出现三个明显的峰, 分别对应γ-WO3中的δ(O-W-O)的弯曲振动模和ν(W-O-W)的伸缩振动模[14]。随着热处理温度升高, δ(O-W-O)逐渐向高波数方向移动, 说明结构扭曲使W-O键长缩短[15], WO6八面体的扭曲程度发生改变。UV-Vis-IR测试结果如图2(b)所示, 结合Kubelka-Munk方程[16], 计算样品的Eg值列于图S3。由于γ-WO3的吸收边对WO6八面体的连接方式很敏感[17], 随着热处理温度升高, WO6八面体排列对称性增强, 导带中电子离域增强, 促使导带电位降低[18,19], 带隙能逐渐从2.77 eV减小至2.65 eV。晶格畸变和氧空位缺陷, 是引起热处理过程中各WO3样品带隙变化的主要原因[20]。晶格畸变伴随着过量电子定域, 产生极化子, 引起γ-WO3在近红外波段的吸收[21,22]。热处理增强了γ-WO3对可见-近红外光的吸收, W-390在λ=500~1800 nm的光吸收最强。随着热处理温度继续升高, 样品晶格完整度增大, 极化子跃迁减少, 近红外波段光吸收减弱[23,24]。Raman(图S4)与FT-IR(图S5)光谱测试结果表明, 在热处理过程中, W-350中的碳被氧化为CO2后除去, 所以碳对样品的吸光性质和催化性能没有影响。

Raman spectra (a) and UV-Vis-IR absorption spectra (b) of the samples heat-treated at different temperatures

图 2. 不同温度热处理样品的Raman谱图(a)和UV-Vis-IR吸收光谱图(b)

Fig. 2. Raman spectra (a) and UV-Vis-IR absorption spectra (b) of the samples heat-treated at different temperatures

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Tauc plots of W-310 (a), W-330 (b), W-350 (c), W-370 (d), W-390 (e), W-410 (f), W-430 (g), W-450 (h) and the band gaps Eg obtained from the intersection of the absorption edge intercept line

图 10. W-310 (a)、W-330 (b)、W-350 (c)、W-370 (d)、W-390 (e)、W-410 (f)、W-430 (g)、W-450 (h)的Tauc曲线及根据吸收边截线交点得出的样品带隙Eg

Fig. 10. Tauc plots of W-310 (a), W-330 (b), W-350 (c), W-370 (d), W-390 (e), W-410 (f), W-430 (g), W-450 (h) and the band gaps Eg obtained from the intersection of the absorption edge intercept line

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Raman spectra of WO3 precursor and representative heat-treated samples (a) and the WO3 sample heat-treated above 350 ℃ (b)

图 11. WO3前驱体与代表性的热处理样品(a), 350 ℃以上热处理样品(b)的Raman谱图

Fig. 11. Raman spectra of WO3 precursor and representative heat-treated samples (a) and the WO3 sample heat-treated above 350 ℃ (b)

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FT-IR spectra of WO3 precursor and heat-treated samples

图 12. WO3前驱体与热处理样品的FT-IR光谱图

Fig. 12. FT-IR spectra of WO3 precursor and heat-treated samples

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样品的光电流响应曲线如图3(a)所示, 各样品无瞬态峰值, 说明表面无明显载流子积累[25,26]。样品的光电流峰值(图3(b))分析可知, W-350对外光场的响应效应最强。W-350暴露的(200)活性晶面占比最高, 且晶粒尺寸较大, 光生载流子在晶界处的复合少[27], 其光电流密度最大, 达到17.4 μA/cm2。但是随着热处理温度继续升高, 纳米花状结构坍塌, 使表面载流子与电解质的接触变弱, 导致光生载流子复合增加, 光电性能降低。IPCE测试结果也表现出类似的规律(图4(c))。在λ=300~800 nm入射光波长范围内, W-350表现出最理想的光电转换效率, 其Mott-Schottky曲线(图3(d))斜率为正, 平带电位约为0.64 V (vs. RHE, pH≈6.8)。

Photocurrent response curves (a), photocurrent response peak values (b) and IPCE plots (c) of the samples heat-treated at different temperatures; Mott-Schottky curve of W-350 (d) Colourful version is available on offical website

图 3. 不同温度热处理样品的光电流响应曲线(a)、光电流峰值(b)和IPCE谱图(c); W-350的Mott-Schottky曲线(d)

Fig. 3. Photocurrent response curves (a), photocurrent response peak values (b) and IPCE plots (c) of the samples heat-treated at different temperatures; Mott-Schottky curve of W-350 (d) Colourful version is available on offical website

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SEM image of W-350-30C (a); XRD patterns of W-350 and W-350-30C(b); TEM images of W-350-30C (c) and W-350-30C (d)

图 4. W-350-30C的SEM照片(a); W-350和W-350-30C的XRD图谱(b); W-350(c)和W-350-30C(d)的TEM照片

Fig. 4. SEM image of W-350-30C (a); XRD patterns of W-350 and W-350-30C(b); TEM images of W-350-30C (c) and W-350-30C (d)

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异质结处理后, 样品形貌基本不变(图4(a)), 但XRD图谱(图4(b))中, 在2θ=23.2°、25.9°、27.8°、31.5°、52.1°处出现新峰, 表明生成了立方晶相CdS (JCPDS 80-0019)与正交晶相α-S (JCPDS 74-1465)。TEM照片(图4(c))中, W-350结晶性良好, 晶格条纹清晰可见。W-350-30C(图4(d))表面存在结晶性较差的CdS, 并在边缘处观察到α-S (026)晶面。形成α-S是由于硫的电负性较小, 碱金属硫化物(如K2S), 易溶于水, 在水中发生水解(式(2, 3))而使溶液呈碱性[28]:

$K_2S+H_2O \rightleftharpoons KHS+KOH$ $KHS+H_2O \rightleftharpoons H_2S+KOH$

在化学浴沉积过程中, H2S发生氧化反应(式(4))生成硫单质:

$2H_2S O_2 \to 2S \downarrow +2H_2O$

综合成一个化学方程式(式(5))为:

$2K_2S+O_2+2H_2O \to 4KOH+2S \downarrow$

α-S(正交硫)是硫在常温常压下最稳定的一种晶型, 因此生成的硫为α-S。EDX结果(图S6)表明, 样品中S、Cd分布均匀, 其含量随循环次数增加而增大, W-350-30C中约含0.64wt% S和3wt% Cd。

EDX results of W-350-10C

图 13. W-350-10C(a)、W-350-20C (b)、W-350-30C (c)、W-350-40C (d)的EDX图谱

Fig. 13. EDX results of W-350-10C

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图5(a~e)为W-350-30C的XPS测试结果。W4f的XPS图谱(图5(b))可分出两组W4f5/2和W4f7/2的峰, 分别归属于W6+(35.50, 37.65 eV)和W5+(34.50, 36.66 eV), 说明样品中存在低价态钨[29]。O1s的XPS图谱(图5(c))包含晶格氧峰(530.54 eV)、水合基团氧峰(531.52 eV)和缺陷氧峰(532.72 eV)[29,30,31]。已有文献表明, 空气热处理所产生的氧空位来源于材料内部部分晶格的畸变[32,33], 且当热处理温度高于350 ℃时, 如450 ℃热处理的WO3中氧空位浓度降低[34]。当WO3中引入氧空位(尤其是表面氧空位), 在价带上方会出现缺陷能级并与价带部分重叠, 从而导致价带最大值升高, Eg变窄, 扩大光响应波长范围[34]。在图5(d)中, 位于405.40和412.15 eV的峰来自于Cd 3d5/2 和Cd 3d3/2, 说明Cd以+2价的形式存在[35,36]。在S2p XPS图谱(图5(e))中, S2p3/2 (161.65 eV)和S2p1/2(162.85 eV)的峰相距1.2 eV, 表明存在-2价的S[37,38,39], 即生成了CdS。除此之外, 163.85 eV处的峰则来源于零价态的单质S[40]。如图5(f)所示, 异质结显著增强了W-350在λ=340~600 nm的光吸收(CdS和α-S的带隙吸收), 而近红外光区吸收减弱, 这是因为S将低价态W5+氧化为W6+, 使极化子跃迁减弱。相应地, 随着表面修饰CdS/α-S的含量增大, 样品颜色由灰绿色向亮黄色转变(图5(g))。

XPS spectrum of sample W-350-30C (a); XPS high-resolution spectra of W4f (b), O1s (c), Cd3d (d) and S2p (e) for sample W- 350-30C; UV-Vis-IR absorption spectra (f) and photos (g) of γ-WO3 nanoflowers with different amounts of CdS/α-S modified on the surface Colourful version is available on offical website

图 5. W-350-30C的XPS全谱(a)、W4f (b)、O1s (c)、Cd3d(d)、S2p(e)的XPS高分辨谱; 不同CdS/α-S表面修饰浓度的γ-WO3纳米花的UV-Vis-IR吸收光谱图(f)和样品照片(g)

Fig. 5. XPS spectrum of sample W-350-30C (a); XPS high-resolution spectra of W4f (b), O1s (c), Cd3d (d) and S2p (e) for sample W- 350-30C; UV-Vis-IR absorption spectra (f) and photos (g) of γ-WO3 nanoflowers with different amounts of CdS/α-S modified on the surface Colourful version is available on offical website

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图6可知, 异质结样品的光电化学性能均优于W-350。W-350-30C的电流密度最高为62.7 μA/cm2 (图6(a))。分析样品的光电流峰值分析图(图6(b))可知, 光电流峰值随表面CdS/α-S修饰浓度增加而增大。当CdS/α-S浓度较高时, W5+被S氧化为W6+, 极化子跃迁减弱, 氧空位含量降低, 样品在可见-近红外光区的吸收减弱, 光电性能下降。但EIS测试结果(图6(d))显示, 表面CdS/α-S修饰使WO3的内部电阻增加, 样品在紫外光区的IPCE下降(图6(c))。由于W-350的平带电位为0.64 V, 则其导带电位为0.54 V, 结合W-350的Eg为2.67 eV, 可知W-350的价带电位约为3.21 V (vs. RHE, pH≈6.8)[41]

Photocurrent response curves(a), photocurrent response peak values(b), and IPCE plots(c) of γ-WO3 nanoflowers modified with different amounts of CdS/α-S on the surface; EIS plots of W-350 and W-350-30C(d) Colourful version is available on offical website

图 6. 不同CdS/α-S异质结浓度的γ-WO3纳米花的光电流响应曲线(a)、光电流峰值(b)和IPCE测试结果(c); W-350和W-350-30C的EIS曲线(d)

Fig. 6. Photocurrent response curves(a), photocurrent response peak values(b), and IPCE plots(c) of γ-WO3 nanoflowers modified with different amounts of CdS/α-S on the surface; EIS plots of W-350 and W-350-30C(d) Colourful version is available on offical website

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根据Zhang等[12]报道的S/CdS异质结材料, 结合本文异质结的晶型结构与尺寸, 推理CdS和α-S的导带分别位于-0.52和-0.17 eV, 价带分别位于1.89和2.62 eV。从动力学的角度, 采用梯形异质结理论[7], 解释异质结纳米花的光电效应机理, 如图7所示。在光照射下, WO3α-S以及CdS的电子由价带激发到导带, 在内部电场和库仑作用下, 来自WO3α-S导带的电子与来自α-S和CdS价带的空穴加速复合, 有效降低了CdS导带的电子与位于WO3价带上的空穴复合的概率。通过构筑梯形异质结, 增强γ-WO3纳米花在可见光区的吸收, 并以牺牲少部分载流子的方式提高整体光生载流子的分离效率, 促进γ-WO3宏观光电化学效应的提升。由于WO3导带电势较正(+0.54 eV), 不能将O2还原为具有高氧化活性的O2-·和HO2·, 该复合催化剂光催化降解污染物活性并不高(图S7), 其光电催化还原CO2的性能将在后续实验工作中做进一步深入研究。

Schematic diagram of charge carriers transfer in γ-WO3 nanoflowers with CdS/α-S modified on the surface

图 7. CdS/α-S表面修饰γ-WO3纳米花的光生载流子转移机理图

Fig. 7. Schematic diagram of charge carriers transfer in γ-WO3 nanoflowers with CdS/α-S modified on the surface

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UV-Vis spectra of methylene blue solution after absorbed by W-350 in the dark and UV irradiation for different time (a); Variation of methylene blue degradation rate with different time (b)

图 14. 亚甲基蓝溶液经W-350避光吸附与不同时间紫外光照射后的UV-Vis光谱图(a); 亚甲基蓝降解率随时间的变化曲线(b)

Fig. 14. UV-Vis spectra of methylene blue solution after absorbed by W-350 in the dark and UV irradiation for different time (a); Variation of methylene blue degradation rate with different time (b)

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3 结论

采用低温溶剂热法制备纳米花状γ-WO3, 并通过空气热处理过程, 精确调控γ-WO3纳米花的高活性晶面与晶粒尺寸。进一步采用改进的循环化学浴法, 成功构筑WO3/CdS/α-S异质结。结果表明, (200)晶面是热处理后γ-WO3的主要暴露晶面, 且在310~390 ℃范围内随温度升高而增大。350 ℃热处理的γ-WO3纳米花的光响应电流最高。本研究通过构筑WO3/CdS/α-S梯形异质结, 增强材料在可见光区的吸收, 并以牺牲少部分光生载流子的方式增强整体光生载流子的分离效率, 促进γ-WO3宏观光电效应的提升。

补充材料

本文相关补充材料可登陆https://doi.org/10.15541/ jim20200023查看。

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