随着人口膨胀，工业化进程的加快，环境污染越来越严重，环境污染防治己成为全球关注的重要课题[1]。自1972年Fujishima和Honda[2]发现TiO2电极在光照下分解水产生H2以来，光催化技术由于可以将低密度太阳能转化为高密度化学能，在紫外-可见光辐射下，矿化有机污染物、还原重金属离子、将温室气体CO2还原成可储存的CH4，发展成为解决当今社会能源和环境问题的关键技术之一。以TiO2为代表的宽带隙无机半导体材料的光催化性能被广泛研究。这类宽带隙半导体材料具有较高的光催化活性，但是由于对太阳能利用率低，限制了其在实际光催化过程中的应用[3,4]。例如，纳米TiO2的禁带宽度Eg=3.2 eV，可匹配的激发光源只能是波长<400 nm的紫外光，这部分光源只占太阳能辐射的5%，对太阳能中47%的可见光利用率很低。其它的一些窄带隙的无机半导体材料，如CdS、Fe2O3、Bi2O5、Ag2O等，虽然可利用可见光为激发光源，但是在光催化过程中存在着量子转换效率低、易发生光化学腐蚀等问题[5,6]。因此，探索开发新型的可见光催化体系，一直是光催化科学的研究热点和难点。
与氧化物和含氧酸盐比较，碳化物和氮化物由于C2p和N2p的轨道能级均高于O2p轨道能级，由C或N的2p轨道参与杂化构成的价带能级更高，在导带能级不变的前提下，带隙变窄，对可见光吸收增强。类石墨相g-C3N4是碳氮化物的典型代表[7,8]。单氰胺(H2NCN)是制备g-C3N4的前驱体[9,10]，同时也是良好的有机配体，[NCN]2-是由三原子组成的线性离子，与N
1 实验方法
1.1 实验材料
实验所用材料为NiCl2·6H2O (纯度>98%)、Bi(NO3)3·5H2O (纯度>99.0%)、氨水(质量分数25%)、单氰胺水溶液(质量分数50%)，以上试剂均为分析纯，使用前未进行处理。实验用水均为去离子水。
1.2 固相法合成Ni(HNCN)2/BiVO4
Ni(HNCN)2的制备：5 mmol的NiCl2·6H2O和0.025 mol H2NCN溶于5 mL的蒸馏水中，搅拌下向其中加入10 mL NH3·H2O，持续搅拌，慢慢形成绿色沉淀，去离子水洗涤3次，70 ℃下真空干燥12 h。
BiVO4的制备：0.008 mol Bi(NO3)3·5H2O溶解于80 mL乙二醇，0.008 mol NH4VO3溶解于80 mL热水。搅拌下，将NH4VO3溶液逐滴加入到Bi(NO3)3溶液中，混合溶液100 ℃下回流3 h，产生的黄色沉淀用水和乙醇溶液洗涤3次，70 ℃下真空干燥12 h。
Ni(HNCN)2/BiVO4复合颗粒合成：上述合成的Ni(HNCN)2和BiVO4按一定的摩尔比例加入到50 mL蒸馏水中，超声30 min，然后置于100 ℃的恒温水浴锅中将水蒸干。改变Ni(HNCN)2/BiVO4的摩尔比例，获得的复合颗粒命名为Ni(HNCN)2/BiVO4-x，x代表BiVO4相对于Ni(HNCN)2的摩尔分数，分别为0.3、1、2和3。
1.3 Ni(HNCN)2/BiVO4的结构表征
采用配备能谱(EDS)的SU8010冷场发射扫描电子显微镜(SEM)观察颗粒形貌和分析成分。应用D/max-2500PC X射线衍射仪(XRD，CuKα为线源，管电压50 kV，管电流100 mA，波长0.15406 nm)测定样品的XRD谱，扫描步长0.02°，扫描范围5°~70°。应用VERTEX 70红外光谱仪测定粒子Fourier变换红外 (FT-IR)光谱，KBr压片，扫描波数范围为4000~500 cm-1。应用Lambda 35紫外-可见分光光度计测定样品的紫外-可见(UV-Vis)漫反射光谱，波长范围200~800 nm。
样品Mott-Schottky曲线测定的实验过程为：称取1 mg光催化剂分散于0.8 mL水和0.2 mL乙醇混合溶剂，将20 μLNafion溶液加入到混合物溶液中，超声分散均匀。取80 μL上述溶液均匀涂覆在1 cm2的ITO导电玻璃上作为工作电极。应用CHI660E电化学工作站测定曲线，使用三电极体系(样品电极为工作电极，Pt丝为对电极，Ag/AgCl/KCl (3 mol/L)电极为参比电极)，电解质溶液为0.2 mol/L的Na2SO4溶液。
1.4 罗丹明B的光催化降解实验
以罗丹明B为降解对象，测定Ni(HNCN)2 /BiVO4复合颗粒的光催化性能。100 mL的1×10-5 mol/L的罗丹明B溶液置于光催化反应容器中，加入50 mg 光催化剂，超声分散均匀。光照前，悬浮液在黑暗中磁力搅拌1 h以达到表面的吸附平衡。采用PLS-SXE300/300 UV型氙灯光源(300 W)为激发光源，420 nm滤光片截去紫外光。每隔一定光照时间，取5 mL悬浮液，离心分离后，上清液用紫外-可见分光光度计测定罗丹明B的吸光度，最大吸收波长为566 nm，根据工作曲线，确定染料的浓度变化。
罗丹明B的降解率(D)依照
式中，C0和C1分别为罗丹明B初始浓度和降解后的浓度。
2 实验结果
2.1 Ni(HNCN)2/BiVO4复合颗粒的结构
单一Ni(HNCN)2、BiVO4以及Ni(HNCN)2/BiVO4复合颗粒的形貌如图1所示。由图可见，Ni(HNCN)2为类纺锤形，颗粒长度约0.8 μm，纺锤状颗粒表面较光滑(图1a)。BiVO4颗粒类似棒状，平均直径约为1 µm，平均长度约为2.5 µm，棒状颗粒表面粗糙(图1b)。Ni(HNCN)2和BiVO4超声共混后得到Ni(HNCN)2/BiVO4-0.3、Ni(HNCN)2/BiVO4-2和Ni(HNCN)2/BiVO4-3复合颗粒的形貌如图1c、e和g所示。可见，棒状BiVO4颗粒的形貌没有发生改变，纺锤形Ni(HNCN)2颗粒尺寸明显变小，说明固相超声共混过程破坏了Ni(HNCN)2颗粒的形貌。较小的Ni(HNCN)2颗粒沉积在棒状BiVO4颗粒表面，形成异质结构。复合颗粒的EDS分别如图1d、f和h所示。可以看出，EDS中元素Ni和Bi的相对含量随着2种晶体复合比例的变化而改变。Ni(HNCN)2/BiVO4-0.3中元素Ni和Bi的原子数比为1∶0.33，Ni(HNCN)2 /BiVO4-2中Ni和Bi元素的原子数比为1∶2.45，与2种晶体的投入摩尔比接近，说明2种晶体混杂较为均匀，利于形成紧密接触的异质结构。Ni(HNCN)2/BiVO4-3 (图1h)中元素Ni和Bi的原子数比为1:7.06，与前2种复合颗粒相比较，2种晶体混合的均匀程度有所下降。
图1
图1
Ni(HNCN)2、BiVO4 和Ni(HNCN)2/BiVO4复合颗粒的SEM像和EDS分析
Fig.1
SEM images (a~c, e, g) and EDS analyses (d, f, h) of Ni(HNCN)2 (a), BiVO4 (b), Ni(HNCN)2/BiVO4-0.3 (c, d), Ni(HNCN)2/BiVO4-2 (e, f), Ni(HNCN)2/BiVO4-3 (g, h) (The inset in Fig.1a is the particles observed under higher magnification)
图2a为单一组分颗粒和Ni(HNCN)2/BiVO4复合颗粒的XRD谱。对于单一Ni(HNCN)2，其衍射峰归属于单斜晶系Ni(HNCN)2 (ICSD-172908)。对于单一BiVO4，其衍射峰对应于单斜晶系BiVO4 (JCPDS No.14-0688)。相比较，Ni(HNCN)2/BiVO4复合颗粒所有XRD峰可以归属于Ni(HNCN)2和BiVO4，且没有杂峰的出现，表明复合颗粒仅由Ni(HNCN)2和BiVO4 2种晶体构成。且XRD峰型比较尖锐，说明2种晶体晶化程度较高。与Ni(HNCN)2/BiVO4-0.3相对比，Ni(HNCN)2/BiVO4-2中随着BiVO4摩尔含量增加，Ni(HNCN)2衍射峰的相对强度减弱。
图2
图2
Ni(HNCN)2、BiVO4和Ni(HNCN)2/BiVO4复合颗粒的XRD谱和FT-IR谱
Fig.2
XRD spectra (a) and Fourier transform infrared spectroscopy (FT-IR) spectra (b) of single Ni(HNCN)2, BiVO4 and Ni(HNCN)2/BiVO4 composite
为了进一步研究2种晶体界面的作用方式，测定了复合颗粒的FT-IR光谱，如图2b所示。对比复合颗粒和单一组分粒子的IR光谱，Ni(HNCN)2/BiVO4复合颗粒的IR谱吸收峰表现为Ni(HNCN)2和BiVO4的IR吸收峰的叠加，没有新的IR吸收峰出现，证明Ni(HNCN)2和BiVO4之间是依靠物理相互作用结合形成复合颗粒。其中，3303 cm-1为—NH的伸缩振动峰，2214和1164 cm-1分别为[N—C≡N]2-的不对称伸缩振动峰和对称伸缩振动峰，639和569 cm-1分别为[N—C≡N]的变形振动峰和面外弯曲振动峰，1208 cm-1是—CNH的变形振动峰[17,22]。721 cm-1处的宽峰为BiVO4的V—O的振动吸收峰[23]。对比Ni(HNCN)2/BiVO4-0.3和Ni(HNCN)2/BiVO4-3的IR光谱，发现随着BiVO4的相对摩尔比例增加，Ni(HNCN)2的特征吸收峰相对强度减弱，BiVO4的特征吸收峰相对强度增加。
Ni(HNCN)2/BiVO4复合颗粒的UV-Vis漫反射吸收光谱如图3a所示。单一Ni(HNCN)2在紫外和可见区均有吸收，表现出较强的紫外-可见响应活性，其最大吸收峰分别位于290、390和630 nm。单一BiVO4在波长λ<530 nm出现强宽吸收峰。Ni(HNCN)2/BiVO4复合颗粒表现出2种组分的复合吸收峰，与Ni(HNCN)2相比，3个最大吸收峰的位置均发生一定程度红移，表明增强的可见光响应能力。
图3
图3
Ni(HNCN)2/BiVO4复合颗粒的UV-Vis光谱和(Ahν)2随 (hν)变化曲线
Fig.3
UV-Vis diffuse reflectance spectra of Ni(HNCN)2/BiVO4 (a) and re-plotted curves of (Ahν)2vs (hν) for Ni(HNCN)2/BiVO4 composites (b) (h—Planck constant, A—absorbance, v—frequency)
对于直接跃迁型半导体，其吸收带边的吸光度满足
式中，A表示吸光度，h为Plank常数，ν为入射光子的频率，Eg为禁带宽度，B为比例系数。将
2.2 Ni(HNCN)2/BiVO4复合颗粒的光催化性能研究
以罗丹明B为光催化降解对象，氙灯为激发光源，420 nm的截止滤光片滤去紫外光，测定了Ni(HNCN)2/BiVO4复合颗粒的光催化性能，结果如图4所示。从图4可以看出，单一的Ni(HNCN)2和BiVO4以及不同比例的Ni(HNCN)2/BiVO4复合颗粒在前60 min的静态吸附过程吸附量很小，吸附率不到6%。在之后的180 min的光催化降解实验中，单一的Ni(HNCN)2、BiVO4的降解率分别为24.1%和15.0%。复合颗粒对罗丹明B的降解率分别为：24.5% (Ni(HNCN)2/BiVO4-0.3)、24.1% (Ni(HNCN)2/BiVO4-1)、31.0% (Ni(HNCN)2/BiVO4-2)和21% (Ni(HNCN)2/BiVO4-3)。Ni(HNCN)2/BiVO4-2的降解率优于单一组分的Ni(HNCN)2和BiVO4以及其它比例制备的复合颗粒，这是因为Ni(HNCN)2与BiVO4之间形成良好的异质结构，两相界面的电子和空穴得到有效分离，光催化效率提高。
图4
图4
Ni(HNCN)2/BiVO4复合颗粒对罗丹明B的降解曲线
Fig.4
Photocatalytic degradation of Rhodamine B by Ni(HNCN)2 /BiVO4 composites (C—concentration at equilibrium, C0—concentration at initial, t—degradation time)
2.3 Ni(HNCN)2/BiVO4复合颗粒的能带结构及光催化机理
为了探究复合颗粒增强的光催化活性机理，确定复合颗粒的能带结构，测定了Ni(HNCN)2和BiVO4的Mott-Schottky (MS)曲线。如图5所示，Ni(HNCN)2和BiVO4的MS曲线的斜率为正，说明Ni(HNCN)2和BiVO4均为n型半导体。相对于Ag/AgCl/KCl (3 mol/L)参比电极，Ni(HNCN)2和BiVO4的平带电位(EFB)分别为-0.76和-0.48 V。根据文献报道，n型半导体导带电位(ECB)一般比平带电位低-0.1~-0.2 V[24~26]；如果将参比电极替换为饱和氢电极，计算公式为ERHE=EAg/AgCl+0.059pH+E
图5
图5
Ni(HNCN)2和BiVO4颗粒的Mott-Schottky曲线
Fig.5
Mott-Schottky plots of Ni(HNCN)2 and BiVO4 particles (Csc—interfacial capacitance, E—electrode potential)
图6
图6
Ni(HNCN)2/BiVO4复合颗粒的能带结构示意图
Fig.6
Schematic of band structure of Ni(HNCN)2/BiVO4 composites (Eg—band gap energy)
3 结论
(1) 采用化学沉淀法制备了Ni(HNCN)2和BiVO4，并采用超声-固相共混法制备了不同比例的Ni(HNCN)2/BiVO4复合颗粒。与单一Ni(HNCN)2相比，Ni(HNCN)2/BiVO4复合颗粒的吸收峰位置发生红移，禁带宽度减小，禁带宽度变窄。通过电化学曲线以及UV-Vis吸收光谱确定了Ni(HNCN)2/BiVO4的能带结构。
(2) 以罗丹明B为降解对象，与单一BiVO4相比较，Ni(HNCN)2/BiVO4复合颗粒光催化降解效果均有所提高。其中，当Ni(HNCN)2与BiVO4摩尔比例为1∶2时，光降解效果最优。从复合颗粒能带结构可知，由于能带电势匹配，使得光生电子和空穴在两相界面有效流动，光催化效率提高。
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