ZnO与氧化石墨烯的制备及其复合材料光催化性能研究
摘要
氧化锌(ZnO)是一种宽带隙多功能半导体材料。由于其优良的物理及化学特性,ZnO在纳米发电机、气敏传感器、太阳能电池和光催化等领域具有广阔的应用前景,受到了人们的广泛关注。石墨烯(graphene)是单原子厚度的碳原子层,其结构与展开的碳纳米管相似。自2004年问世以来,因其独特的物理及化学性质,石墨烯在光学、电学、传感器、催化剂等领域表现出巨大的潜在应用价值。
随着社会经济的不断发展和环境问题日益突显,光催化剂作为一种降解有机污染物的高效、绿色环保催化剂,越来越受到人们的关注。目前应用最广泛的光催化材料是Ti02及其复合物,但是其成本较高且不易回收利用。相比之下,ZnO具有与Ti02相似的带隙,安全无毒且成本更低,甚至在降解某些污染物时表现出比TiO：更高的光催化效率,因此,ZnO成为光催化领域一个更好的选择。此外,由于独特的结构和性能,氧化石墨烯(GO)巨大的比表面积可吸附有机污染物,其特殊结构可调节复合材料的光吸收范围。因此,将ZnO和氧化石墨烯有机结合起来,采取较为简单的方法制备出更为高效和成本低廉的复合光催化剂,在光催化降解有机污染物领域具有重大意义。
本文的主要研究内容和取得的结果如下：
(1)采用低温水溶液法在玻璃基底上生长ZnO纳米棒(ZnO nanorods, ZnO NRs),通过XRD和SEM表征研究了实验参数对制备的ZnO纳米棒的形貌和结构的影响,并通过PL谱研究了ZnO纳米棒的缺陷状态。结果显示：四层种子层,硝酸锌和HMT浓度比为1：l时生长的ZnO纳米棒具有最好的结构和形貌；随着硝酸锌和HMT浓度比的增大,制备的ZnO纳米棒的均匀性越来越差,产生的氧空位缺陷和结构缺陷越来越多。
(2)为了便于研究ZnO的光催化性能,我们用无基底自组装法制备了ZnO纳米棒,用均匀沉淀法制备了ZnO纳米颗粒(ZnO nanoparticles, ZnO NPs)。通过XRD和SEM发现,无基底自组装法生长的ZnO纳米棒具有明显的C轴择优取向,其直径在500nm左右,长度在5μm左右,而且具有较多的结构缺陷；通过均匀沉淀法制备的ZnO纳米颗粒平均粒径约在50nm左右,近似球形。通过紫外-可见吸收光谱和光催化实验我们发现,ZnO纳米棒和纳米颗粒的吸收谱基本一致,都是具有较强的紫外吸收能力,而对可见光基本不吸收,ZnO纳米棒的光吸收能力稍强于ZnO纳米颗粒：二者在紫外光照射下降解甲基橙的能力相差不大,最大降解率都能达到94%左右。
(3)通过Hummers法制备了氧化石墨烯(GO),并通过硼氢化钠、DMAB、抗坏血酸对其进行还原得到石墨烯(rGO)。结果表明：硼氢化钠对氧化石墨烯的还原速率最快,但是还原后容易出现团聚现象,得到的rGO分散性较差,缺陷也较多；DMAB和抗坏血酸对氧化石墨烯的还原速率较慢,但是可以得到分散性良好的rGO悬浮液,缺陷也相对少一点。根据不同的要求可以选用不同的还原剂。
(4)分别制备了自组装ZnO纳米棒和ZnO纳米颗粒与GO的复合材料,并对两种复合材料进行了紫外-可见吸收光谱和光催化测试。结果表明：将ZnO与GO复合,可以明显提高ZnO对紫外光和可见光的吸收,在紫外光照射下两种ZnO/GO复合材料对甲基橙的降解率几乎均可达到100%,GO与ZnO的质量比不同时,对复合材料的光吸收和光催化能力有所影响,在我们的实验结果中,GO和ZnO纳米颗粒的质量比为1：20的复合物具有最好的光吸收能力和光催化性能。
ZnO is a kind of multifunctional semiconductor material with a broad band gap. Because of its excellent physical and chemical characteristics, ZnO has a broad applications in fields such as nanogenerators, gas sensors, solar cells and photocatalysts. Graphene is a layer of carbon atoms with the thickness of a single atom. Its structure is similar to that of carbon nanotube. Since its appearance in2004, it has shown great potential applications.
With the continuous development of economy and society and concomitant deterioration of environment, photocatalyst has attracted increasing attentions as a highly efficient and environment-friendly catalyst for degradation of organic contaminants. At present, the most widely applied photocatalyst materials are TiO2and its composites. However, TiO2is expensive and difficult to recycle. ZnO has a similar clearances with TiO2and it is safe, non-toxic and less expensive. At the same time, ZnO has a higher photocatalytic efficiency in degradation of certain contaminants. Therefor ZnO could be a better choice in photocatalysts. In addition, graphene oxide has a large specific surface area which can absorb organic contaminants and its special structure can adjust light absorption range. Therefore, the preparation of high-efficient and cheap complex photocatalyst can be realized through the combination of ZnO and graphene oxide, which is of great significance to photocatalytic degradation of organic contaminants.
The main contents and conclusions of this thesis are as follows:
(1) The ZnO nanorods with different thickness seed layers on glass substrates have been prepared with low-temperature aqueous solution method. The influence of experimental parameters on morphology and structure of ZnO nanorods has been studied through XRD and SEM. The defect state of ZnO nanorod has been characterized through photoluminescence spectra.The results showed that:with four seed layers, and zinc nitrate and HMT concentration ratio of1:1, the ZnO nanorods obtain the best structure and morphology; With increasing concentration ratio of zinc nitrate and HMT, the uniformity of the ZnO nanorods is getting worse, more and more oxygen vacancy defects and structural defects exist in the ZnO nanorods.
(2) In order to study the photocatalytic properties of ZnO, we prepared substrate free ZnO nanorods by self-supporting method. In comparison, we also prepared ZnO nanoparticles by homogeneous precipitation. The XRD and SEM results indicate that the substrate free ZnO nanorods with a highly preferential c-axis orientation, and its diameter is about500nm and5μm in length, with more structural defects. Meanwhile, the average size of ZnO nanoparticles is about50nm, the shape is approximately spherical. Through UV-visible absorption spectrum and photocatalytic experiments, we found that both absorption spectra of ZnO nanorods and nanoparticles have a strong capability of absorbing UV, but they barely absorb visible light. The ZnO nanorod is more capable of absorbing light than ZnO nanoparticles; the degrading abilities of methyl orange under UV irradiation of the two are of little difference and the maximum degrading ratio reaches94%.
(3) GO is prepared through the way of Hummers and rGO is reduced through NaBH4, DMAB and ascorbic acid respectively. The results show that NaBH4can reduce GO in the shortest time, but the obtained rGO is easily accumulated and retain low dispersivity and more defects. The DAMB and ascorbic acid reduce GO in a slower speed, but the acquired rGO suspending liquid shows higher dispersivity. Therefor different reductants could be chosen according to different requirements on reducing speed.
(4) The substrate free ZnO nanorods and ZnO nanoparticles composite repectively with GO. The ZnO/GO composites are then conducted ultraviolet-visible absorption spectrum photocatalytic test. The results indicate that the compound of ZnO and GO will improve the ability of ZnO to absorb ultraviolet light and visible light; besides, degradation ratio of ZnO/GO composites to methyl orange can reach up to almost100%when exposed to ultraviolet light. Difference of mass ratio in GO and ZnO can exert impacts on light absorption and light catalytic ability of composite materials. With respect to the present test, composite materials with mass ratio of1:20between GO and ZnO exhibit the best light absorption and light catalytic ability.
随着社会经济的不断发展和环境问题日益突显,光催化剂作为一种降解有机污染物的高效、绿色环保催化剂,越来越受到人们的关注。目前应用最广泛的光催化材料是Ti02及其复合物,但是其成本较高且不易回收利用。相比之下,ZnO具有与Ti02相似的带隙,安全无毒且成本更低,甚至在降解某些污染物时表现出比TiO：更高的光催化效率,因此,ZnO成为光催化领域一个更好的选择。此外,由于独特的结构和性能,氧化石墨烯(GO)巨大的比表面积可吸附有机污染物,其特殊结构可调节复合材料的光吸收范围。因此,将ZnO和氧化石墨烯有机结合起来,采取较为简单的方法制备出更为高效和成本低廉的复合光催化剂,在光催化降解有机污染物领域具有重大意义。
本文的主要研究内容和取得的结果如下：
(1)采用低温水溶液法在玻璃基底上生长ZnO纳米棒(ZnO nanorods, ZnO NRs),通过XRD和SEM表征研究了实验参数对制备的ZnO纳米棒的形貌和结构的影响,并通过PL谱研究了ZnO纳米棒的缺陷状态。结果显示：四层种子层,硝酸锌和HMT浓度比为1：l时生长的ZnO纳米棒具有最好的结构和形貌；随着硝酸锌和HMT浓度比的增大,制备的ZnO纳米棒的均匀性越来越差,产生的氧空位缺陷和结构缺陷越来越多。
(2)为了便于研究ZnO的光催化性能,我们用无基底自组装法制备了ZnO纳米棒,用均匀沉淀法制备了ZnO纳米颗粒(ZnO nanoparticles, ZnO NPs)。通过XRD和SEM发现,无基底自组装法生长的ZnO纳米棒具有明显的C轴择优取向,其直径在500nm左右,长度在5μm左右,而且具有较多的结构缺陷；通过均匀沉淀法制备的ZnO纳米颗粒平均粒径约在50nm左右,近似球形。通过紫外-可见吸收光谱和光催化实验我们发现,ZnO纳米棒和纳米颗粒的吸收谱基本一致,都是具有较强的紫外吸收能力,而对可见光基本不吸收,ZnO纳米棒的光吸收能力稍强于ZnO纳米颗粒：二者在紫外光照射下降解甲基橙的能力相差不大,最大降解率都能达到94%左右。
(3)通过Hummers法制备了氧化石墨烯(GO),并通过硼氢化钠、DMAB、抗坏血酸对其进行还原得到石墨烯(rGO)。结果表明：硼氢化钠对氧化石墨烯的还原速率最快,但是还原后容易出现团聚现象,得到的rGO分散性较差,缺陷也较多；DMAB和抗坏血酸对氧化石墨烯的还原速率较慢,但是可以得到分散性良好的rGO悬浮液,缺陷也相对少一点。根据不同的要求可以选用不同的还原剂。
(4)分别制备了自组装ZnO纳米棒和ZnO纳米颗粒与GO的复合材料,并对两种复合材料进行了紫外-可见吸收光谱和光催化测试。结果表明：将ZnO与GO复合,可以明显提高ZnO对紫外光和可见光的吸收,在紫外光照射下两种ZnO/GO复合材料对甲基橙的降解率几乎均可达到100%,GO与ZnO的质量比不同时,对复合材料的光吸收和光催化能力有所影响,在我们的实验结果中,GO和ZnO纳米颗粒的质量比为1：20的复合物具有最好的光吸收能力和光催化性能。
ZnO is a kind of multifunctional semiconductor material with a broad band gap. Because of its excellent physical and chemical characteristics, ZnO has a broad applications in fields such as nanogenerators, gas sensors, solar cells and photocatalysts. Graphene is a layer of carbon atoms with the thickness of a single atom. Its structure is similar to that of carbon nanotube. Since its appearance in2004, it has shown great potential applications.
With the continuous development of economy and society and concomitant deterioration of environment, photocatalyst has attracted increasing attentions as a highly efficient and environment-friendly catalyst for degradation of organic contaminants. At present, the most widely applied photocatalyst materials are TiO2and its composites. However, TiO2is expensive and difficult to recycle. ZnO has a similar clearances with TiO2and it is safe, non-toxic and less expensive. At the same time, ZnO has a higher photocatalytic efficiency in degradation of certain contaminants. Therefor ZnO could be a better choice in photocatalysts. In addition, graphene oxide has a large specific surface area which can absorb organic contaminants and its special structure can adjust light absorption range. Therefore, the preparation of high-efficient and cheap complex photocatalyst can be realized through the combination of ZnO and graphene oxide, which is of great significance to photocatalytic degradation of organic contaminants.
The main contents and conclusions of this thesis are as follows:
(1) The ZnO nanorods with different thickness seed layers on glass substrates have been prepared with low-temperature aqueous solution method. The influence of experimental parameters on morphology and structure of ZnO nanorods has been studied through XRD and SEM. The defect state of ZnO nanorod has been characterized through photoluminescence spectra.The results showed that:with four seed layers, and zinc nitrate and HMT concentration ratio of1:1, the ZnO nanorods obtain the best structure and morphology; With increasing concentration ratio of zinc nitrate and HMT, the uniformity of the ZnO nanorods is getting worse, more and more oxygen vacancy defects and structural defects exist in the ZnO nanorods.
(2) In order to study the photocatalytic properties of ZnO, we prepared substrate free ZnO nanorods by self-supporting method. In comparison, we also prepared ZnO nanoparticles by homogeneous precipitation. The XRD and SEM results indicate that the substrate free ZnO nanorods with a highly preferential c-axis orientation, and its diameter is about500nm and5μm in length, with more structural defects. Meanwhile, the average size of ZnO nanoparticles is about50nm, the shape is approximately spherical. Through UV-visible absorption spectrum and photocatalytic experiments, we found that both absorption spectra of ZnO nanorods and nanoparticles have a strong capability of absorbing UV, but they barely absorb visible light. The ZnO nanorod is more capable of absorbing light than ZnO nanoparticles; the degrading abilities of methyl orange under UV irradiation of the two are of little difference and the maximum degrading ratio reaches94%.
(3) GO is prepared through the way of Hummers and rGO is reduced through NaBH4, DMAB and ascorbic acid respectively. The results show that NaBH4can reduce GO in the shortest time, but the obtained rGO is easily accumulated and retain low dispersivity and more defects. The DAMB and ascorbic acid reduce GO in a slower speed, but the acquired rGO suspending liquid shows higher dispersivity. Therefor different reductants could be chosen according to different requirements on reducing speed.
(4) The substrate free ZnO nanorods and ZnO nanoparticles composite repectively with GO. The ZnO/GO composites are then conducted ultraviolet-visible absorption spectrum photocatalytic test. The results indicate that the compound of ZnO and GO will improve the ability of ZnO to absorb ultraviolet light and visible light; besides, degradation ratio of ZnO/GO composites to methyl orange can reach up to almost100%when exposed to ultraviolet light. Difference of mass ratio in GO and ZnO can exert impacts on light absorption and light catalytic ability of composite materials. With respect to the present test, composite materials with mass ratio of1:20between GO and ZnO exhibit the best light absorption and light catalytic ability.
引文
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