【作者】 龙明策；

【导师】 蔡伟民； Kisch Horst；

【作者基本信息】 上海交通大学 ， 环境工程， 2007， 博士

【摘要】 环境污染和能源问题目前仍是困扰人类可持续发展的难题。半导体光催化技术是利用光照激发半导体产生的导带电子和价带空穴,进行氧化还原降解有机污染物或者分解水获取氢能的高级技术。由于该过程本质是将太阳能转化为化学能,所以半导体光催化技术有望成为解决环境和能源问题的一条有效途径。半导体光催化的核心是研制宽谱响应的长效高效光催化材料。光催化材料的吸收光谱与太阳光谱匹配是充分利用太阳能的前提,而高效使得在动力学上光催化材料的应用成为可能,长效是保证光催化材料的稳定性以使得该技术廉价可行。本文围绕宽谱响应的长效高效光催化材料的研制为中心,结合材料表征手段、密度泛函理论计算和光电化学研究方法,探索光催化活性的调控机制。本文研究了几种掺杂TiO₂的晶体结构、能带结构与光催化性能的关系。采用密度泛函理论的Castep代码,计算了未掺杂、氮掺杂、碘掺杂和铂掺杂的二氧化钛的晶体结构和电子能带结构,结果发现氮和碘掺杂TiO₂的带隙中新增三条能带,而铂掺杂TiO₂新增五条能带,新增的能带使其产生了可见光响应性能。价带和导带组成的分析表明,碘掺杂TiO₂的价带以O_2p为主,混合了I_5p和少量Ti_3d,导带以Ti_3d轨道为主,混合了少量I_5p和O_2p。铂掺杂TiO₂中Pt_5d轨道分裂成两部分,分别参与价带和导带的构成。铂掺杂和碘掺杂TiO₂能带位置负移使得光生空穴具有更强的氧化性。偶极矩计算显示掺杂TiO₂的TiO₆八面体的畸变程度依次为:碘掺杂TiO₂>铂掺杂TiO₂>氮掺杂TiO₂。由于偶极矩产生的内部电场有助于电子和空穴分离,因此铂掺杂和碘掺杂TiO₂具有较高的光催化活性。研究了钴氧化物负载的BiVO₄复合光催化材料的制备、结构表征及光催化性能。通过溶液沉淀法制备了大粒径的单斜BiVO₄,通过浸渍法制备了BiVO₄/CoOx复合光催化材料。采用XRD、XPS、DRS、BET、SEM、TEM、FTIR对其材料晶相、化合价态、光学性能、颗粒形貌等进行表征分析。XPS结果表明0.8wt%Co含量300℃退火的复合物中钴以Co₃O₄形式存在;DRS测定BiVO₄的带隙宽度为2.35 eV,而Co负载后光吸收范围延伸到800 nm;SEM和TEM显示20-50 nm的Co₃O₄颗粒分散在不规则的微米级的BiVO₄颗粒表面;BET测定负载前后的比表面积从0.74 m²g^-1增加到1.38 m²g^-1。苯酚降解的光催化性能测试显示300℃退火0.8wt% Co含量的BiVO₄/Co₃O₄具有最高光催化活性,3 h苯酚去除率达到96%。该催化剂具有较好的稳定性,在水溶液中的沉淀性能较好。分析了类p-n纳米异质结构复合物的三种能带位置的光生载流子的迁移规律,认为p型半导体的价带和导带位置均比n型半导体更正时不利于电子和空穴的有效分离。根据能带结构计算和绝对电负性估算了BiVO₄和Co₃O₄的价带和导带组成和位置,结合发光光谱PL结果,确定复合物高效光催化活性的本质是光生载流子的有效分离。采用SDS为模版的水热法制备了h-BiVO₄纳米片。TEM和HTEM结果显示BiVO₄是100 nm左右的沿（010）方向优先增长的纳米片。h-BiVO₄/Co₃O₄复合物光催化反应2h苯酚去除率达到99%。比较了BiVO₄与BiVO₄/Co₃O₄复合物半导体电极的光电化学性质。测定了BiVO₄和Co₃O₄的平带电位分别为:-0.3 V vs. NHE（pH=7.0）和+0.54 V vs. NHE（pH=7.0）,结果表明p型半导体Co₃O₄的价带和导带均较n型半导体BiVO₄更负。通过刮刀法制备了性能优良的BiVO₄、Co₃O₄和BiVO₄/Co₃O₄复合物压制电极。BiVO₄电极光电分解水的中间产物过氧化物可以作为复合中心降低光电效率,甚至可导致低偏压下电极从n型到p型半导体特征的转化。通过循环伏安还原中间产物可以恢复80%以上的光电活性,通过添加空穴捕集剂碘化钾或表面修饰Co₃O₄组分可以抑制中间产物的生成,提高电极稳定性。BiVO₄和BiVO₄/Co₃O₄电极光电化学性质研究表明后者具有更高的光电效率,并且其光生电子和空穴的表面复合被抑制了。发现添加还原物质如KI或甲醇不能提高复合物的光电效率,表明电子从BiVO₄导带迁移到ITO形成阳极光电流,而空穴则集中在Co₃O₄的价带进行光电氧化水被消耗。在碱性电解质NaOH中BiVO₄的光电效率较中性的Na2SO4更高,稳定性也更高。暗循环伏安表明碱性介质中的过氧化物中间物较少,有其它中间产物生成,空穴氧化水的机制发生变化。研究了钒酸铋复合物及混合物光催化材料的光催化性能。制备了Ag、Pt、Cu、Ni、Ru负载BiVO₄的系列复合光催化材料,可见光下的苯酚降解实验表明光催化活性为:BiVO₄/Co₃O₄>Ag-BiVO₄>BiVO₄/NiO_x>Pt-BiVO₄>BiVO₄/RuO₂>BiVO₄/CuO。电解质对BiVO₄/Co₃O₄光催化降解苯酚的抑制作用如下:NO₃^-<Cl^-<SO₄^2-。虽然Cl^-对苯酚去除率影响不大,但是抑制了苯环的开环。苯酚去除率随pH值增加而提高,随苯酚初始浓度增加而降低。羟基自由基捕集剂甲醇和异丙醇不影响苯酚去除,但是抑制苯环开环过程,空穴捕集剂KI促进苯酚的彻底降解,通氩气并添加电子清除剂SF₆时苯酚去除率提高。发现BiVO₄和Co₃O₄混合物具有颗粒间电子转移效应,BiVO₄和Co₃O₄的质量比为4:1时苯酚去除率最高,达41%。降解苯甲酸过程中水杨酸的生成验证光催化过程中形成了·OH自由基。研究了用于光致逻辑器件的混合n/p半导体光电极。通过刮刀法制备了BiVO₄/Co₃O₄、TiO₂/Co₃O₄、BiVO₄/CuO、TiO₂/CuO四种不同的混合n/p半导体压制电极,通过随波长变化的光电流性质研究,证实混合n/p型半导体电极实现XOR逻辑功能是一种普遍方法。确定混合n/p半导体光电极具备随波长变化的光电流方向转换特性需要满足的光电化学条件是n型半导体的导带和价带位置比p型半导体更正。通过光电化学、电学和化学上的调控,可以获得满足要求的XOR逻辑操作。更多还原

【Abstract】 Environment and energy issues are still the bottle neck for the continuable development of human being. Semiconductor photocatalysis is an advanced technology that employing the electrons on conduction band and holes on valence band in photons excitated semiconductor, pollutants can be removed via oxidization or reduction and hydrogen can be obtained via water-splitting. As its essence is to convert solar energy into chemical energy, this technology may become an available approach to settle the environment and energy issues. The focus of semiconductor photocatalysis is to develop photocatalysts with wide wavelength light response, high efficiency and stability. To make the absorption spectrum of photocatalysts match with that of the solar’s is the prerequisite for exploiting solar energy. High efficiency makes its application to be dynamically possible, and high stability ensures the technology to be cheap and feasible. In this paper, we focused on developing photocatalysts with such properties as wide wavelength light response, high efficiency and stability, and integrated the investigation techniques of material characterization, density functional theory （DFT） and photoelectrochemistry, to explore the controllable mechanism of photocatalytic activity.The relationship of the crystal structure and energy band structure of doped TiO₂ with photocatalytic performance has been investigated. Based on the DFT, the crystal structure and energy band structure of undoped, N-doped, I-doped and Pt-doped TiO₂ have been calculated by Castep code. The results showed that there are three new energy bands in the band gap of N-doped and I-doped TiO₂, and five ones in that of Pt-doped TiO₂. The new band is the origin of their visible light response. The analysis of the compositions of VB and CB showed that the dominant contribution to VB of I-doped TiO₂ is O_2p, mixed with some I_5p and Ti_3d, however the dominant contribution to CB is Ti_3d, as well as mixed with some O_2p and I_5p. The Pt_5d orbital in Pt-doped TiO₂ split into two parts which are involved in VB and CB, respectively. The band potentials of I-doped and Pt-doped TiO₂ shift downwards, which indicates that photogenerated holes have stronger oxidative power. The results of the dipole moment calculations showed the distortion of TiO₆ in doped TiO₂ is I-doped > Pt-doped> N-doped. Because the internal dipole moments promote the charge separation, Pt-doped and I-doped TiO₂ exhibited higher photocatalytic activity.The preparation, characterization and photocatalytic performance of CoOx modified BiVO₄ composite have been studied. The monoclinic BiVO₄ with micron size has been prepared by aqueous precipitation, and BiVO₄/CoOx composite has been prepared by impregnation method. The crystal, valence, morphology and optical properties have been characterized by XRD、XPS、DRS、BET、SEM、TEM、FTIR. XPS showed that Co in the composite calcined at 300 0C with 0.8wt% Cobalt content is present as Co₃O₄. DRS showed that band gap of BiVO₄ is 2.35 eV, and the optical absorption after CoOx modified extended to 800 nm. SEM and TEM showed that 20-50 nm Co₃O₄ particles dispersed on the surface of micron BiVO₄ particles. BET was improved from 0.74 m²g^-1 to 1.38 m²g^-1 after surface modification. Phenol degradation test showed the highest efficiency is observed when the sample calcined at 300 C with 0.8 wt % cobalt content. Phenol removal efficiency is 96% after 3 h irradiation. The composite photocatalyst exhibits good stability and precipitation performance in aqueous solution. The migration principle of photogenerated carriers in p-n heterojunction nanocomposite with three different kinds of energy band position has been studied. The VB and CB of p type semiconductor more anodic than those of n type one is not favorable for the separation of electrons and holes. The VB and CB of BiVO₄ and Co₃O₄ have been estimated by absolute electronegativity. Combined with the result of PL spectra, the origin of enhanced performance of the composite is the effective separation of charge carriers. Nanosheet of h-BiVO₄ have been prepared by hydrothermal method with SDS as morphology-directing template. TEM and HTEM showed that h-BiVO₄ is 100 nm nanosheet with a preferred （010） surface orientation. The phenol removal efficiency over the h-BiVO₄/Co₃O₄ composite is as high as 99% after 2 h irradiation.The photoelectrochemical properties of BiVO₄ and BiVO₄/Co₃O₄ composite electrode have been studied. The flatband potential of BiVO₄ and Co₃O₄ have been determined as -0.3 V vs. NHE（pH=7.0）and +0.54 V vs. NHE（pH=7.0）respectively. The result showed the VB and CB of p type semiconductor Co₃O₄ are more cathodic than that of n type BiVO₄. The pressed electrodes of BiVO₄、Co₃O₄ and BiVO₄/Co₃O₄ have been prepared by doctor blade method. The peroxide intermediates during water photoelectrochemical splitting on BiVO₄ electrode can serve as recombination center and decreased the efficiency, and even lead to the switch of photocurrent direction upon low bias and extending irradiation. By cyclic voltammogram reducing the peroxide species 80% photocurrent can be restored. By adding the holes scavenge KI or modified surface with Co₃O₄, the intermediates can be inhibited and the stability of the electrode can be improved. BiVO₄/Co₃O₄ electrode exhibited better photoelectrochemical performance than BiVO₄, and on the former electrode surface recombination of photogenerated electrons and holes inhibited. Adding reductive such as methanol and KI can not improve the efficiency, which indicates that electrons migrated from CB of BiVO₄ to ITO and produce enhanced anodic photocurrent, and simultaneously holes on the VB of Co₃O₄ can be consumed by oxidizing water. In alkaline electrolyte NaOH, the efficiency and stability of BiVO₄ electrode are improved. The Cyclic voltammogram after irradiation exhibited that peroxide intermediates are less in the alkaline, and other unknown intermediates produced, and the mechanism of water photooxidization by the holes is changed.The photocatalytic performance of composite and their mixtures have been studied. Ag, Pt, Cu, Ni, Ru modified BiVO₄ composites have been prepared. Their activity on phenol degradation under visible light irradiation is: BiVO₄/Co₃O₄>Ag-BiVO₄>BiVO₄/NiO_x>Pt-BiVO₄>BiVO₄/RuO₂>BiVO₄/CuO. The effect of electrolytes inhibiting the degradation of phenol over BiVO₄/Co₃O₄ is NO₃^-<Cl^-<SO₄^2-. Cl^- showed negligible effect on phenol removal, but the ring-opening process has been inhibited. Phenol removal efficiency increased with the pH value, and decreased with phenol initial concentration. The hydroxyl radical scavenges such as methanol or isopropanol did not affect the phenol removal, but inhibited the ring-opening process. Holes scavenge KI promoted the phenol photocatalytic degradation. The phenol removal can be improved by argon purging and adding with electron scavenge SF₆. The IPET （InterParticle Electron Transfer） on BiVO₄ and Co₃O₄ mixture have been found, the phenol removal efficiency is highest （as high as 41%） when the weight ratio of BiVO₄: Co₃O₄ is 4:1. The formation of salicylic acid upon irradiation of BiVO₄ and Co₃O₄ mixture in presence of benzoic acid established the presence of hydroxyl radicals.The hybrid n/p semiconductor electrode for light driven logic device has been investigated. Four kinds of hybrid n/p semiconductor pressed electrodes of BiVO₄/Co₃O₄、TiO₂/Co₃O₄、BiVO₄/CuO、TiO₂/CuO have been prepared by doctor blade method. By investigating their wavelength dependence photocurrents, it has been established that hybrid n/p semiconductor electrode used for XOR logic is a general approach. The prerequisite on photoelectrochemical properties for wavelength controlled switching of photocurrent direction is that VB and CB of n type semiconductor should be more anodic than those of p type one. Based on well-fitting photoelectrochemical, electrical and chemical properties, the satisfying switch behavior can be achieved.更多还原