【作者】 李明雪；

【导师】 邹志刚； 于涛； 罗文俊；

【作者基本信息】 南京大学 ， 物理学， 2013， 博士

【摘要】 开发可替代化石能源的可再生的清洁新能源是人类社会可持续发展的迫切需要。太阳能是重要的可再生清洁能源之一。利用光电化学电池进行水分解产氢和氧,直接把太阳能转化为可存储的清洁化学能——氢能,具有诱人的应用前景。开发性能优异的光电极(包括光阳极和光阴极)材料是研发高效光电化学太阳能水分解电池的关键技术之一。理想半导体光电极材料要具有合适的带隙,能够利用能量占太阳光中43%的可见光。氮化物半导体与广泛研究的氧化物半导体相比具有更窄的带隙,成为近年来重点关注的一类光电极材料。研究表明,半导体光电极表面光生载流子复合是制约其光电化学性能提高的一个重要因素,寻找简单、高效的消除光电极表面光生载流子复合中心的方法是目前光电化学太阳能水分解研究领域的热点和难点。本论文以提高光电化学太阳能水分解电池的太阳能转化效率为目标,围绕光电化学水分解电池中的光阳极展开研究。针对不同光阳极材料,提出了新颖有效的消除表面光生载流子复合中心的化学、物理方法;通过消除表面光生载流子复合中心大幅提高了 InxGa1-xN、Ta3N5光阳极的光电化学性能。在此基础上,本文提出了不同方法制备的Ta3N5薄膜中光生载流子的传输机理,为进一步提升光电化学水分解电池的太阳能转化效率提供参考依据。本文主要研究内容和结论如下:通过电化学腐蚀消除InxGa1-xN光阳极表面光生载流子复合中心,大幅提高其光电化学性能。InxGai-xN半导体是具有潜力的光阳极材料之一,其带隙在0.70～3.42 eV之间连续可调,其吸收光谱基本上能够覆盖太阳光谱中的整个可见光波段和部分红外光波段。从可见光利用角度考虑,InxGa1-xN薄膜越厚、In含量越高,其可见光利用率越高;但是在制备过程中薄膜厚度增加及In含量提高不可避免的会引起InxGai-xN薄膜表面In偏析相的产生。我们通过研究发现InxGa1-xN表面的In偏析相作为光生载流子的复合中心,造成大量载流子在薄膜表面发生复合,导致光阳极的光电化学性能很低。我们通过巧妙的电化学腐蚀方法成功消除了 InxGa1-xN表面的In偏析相,InxGa1-xN光生载流子的分离效率大大提高,光电流及量子转化效率提高了约2倍。表面钝化层剥离及Co(OH)x担载制备高效Ta3N5光阳极,其光电流为目前报道的光阳极材料光电流的最高值。与InxGa1-xN相比,Ta3N5薄膜原料价格便宜、制备简单,并且其带隙为2.1 eV,吸收光谱响应到600 nm,导价带位置合适,理论上能够在无外加偏压的条件下分解水,是更具应用潜力的光阳极材料。我们运用Ta金属片氧化氮化法制备了 Ta3N5薄膜,但由于表面载流子复合中心的存在,Ta3N5光电化学性能较低。我们设计了新颖的表面钝化层机械剥离、自动热剥离、化学腐蚀剥离三种方法,成功移除了 Ta3N5薄膜表面含有大量光生载流子复合中心的钝化层,大大减少了光生载流子在薄膜表面的复合,提高了 Ta3N5光阳极的载流子分离效率,从而大幅提高其光电流。另外,基于提高光生载流子的传输速率从而减少载流子复合的考虑,我们在Ta3N5薄膜表面担载Co(OH)x作为助催化剂,进一步降低了 Ta3N5光电流的开启电势、提高了其光电流和光稳定性。在1.23VRHE的外加偏压下,担载Co(OH)x的Ta3N5光阳极的光电流可达5.5 mA/cm2,是目前报道的光阳极材料光电流的最高值。并且担载Co(OH)x后,Ta3N5的光稳定性大幅提升。通过对不同方法制备的Ta3N5光阳极光生载流子输运特性研究,分析制约太阳能转化效率提升的因素。我们以探寻制约太阳能转化效率提高的因素为目标,以Ta3N5光阳极为研究对象,运用Ta片氧化氮化法及粉末电泳沉积法制备了Ta3N5半导体光阳极,分析了光阳极表面及体相光生载流子的输运特性,经过分析发现小颗粒紧密堆积形成的光电极薄膜有利于光生电子的传输,从而减少载流子体相复合,获得较高的太阳能转化效率。通过上述研究,本文设计了一系列巧妙的消除光阳极表面光生载流子复合中心,提高光生载流子分离效率的方法,效果显著。本论文设计、探索的消除复合中心的方法也可能有助于太阳电池、光探测器等半导体光电器件性能的提升。值得指出的是,本文制备的Co(OH)x担载的Ta3N5光阳极在1.23 VRHE的光电流为5.5mA/cm2,打破了之前报道的3.8mA/cm2的记录值,是目前报道的光阳极材料光电流的最高值。更多还原

【Abstract】 Renewable and clean new energy that will replaced the use of fossil fuels is imminently needed for the sustainable development of human society.A photoelectrochemical cell(PEC)is a potential way to convert solar energy into clean energy by water splitting.The key of an efficient PEC solar water splitting cell is developing a perfect photoelectrochemical electrode material.The most important quality of a photoelectrochemical material is a suitable band gap,which is suitable to make the best use of the visible light.Compared with oxide semiconductors,nitride semiconductors have narrower band gaps,which are beneficial to visible light utilization.Moreover,the carriers’ recombination on the surface of a photoelectrode is the limiting step for the photoelectrochemical performance.Therefore,finding a simple and efficient method to eliminate the surface recombination centers is the emphasis and difficulty in a PEC water splitting cell.In this paper,we intend to increase the solar conversion efficiency of a PEC cell.The InxGa1-xN and Ta3N5,which can use the visible light,have been prepared as the photoanode in a PEC water splitting cell.Chemical and physical methods for removal of surface recombination centers have been developed for different semiconductors.The photoelectrochemical performance of InxGa1-xN and Ta3N5 has been greatly increased after the removal of surface recombination centers.Moreover,the charge transfer mechanism of Ta3N5 films prepared by different methods has been studied to further increase the solar energy conversion efficiency.The main conclusions are as follows:The photoelectrochemical performance of InxGa1-xN has been greatly improved by electrochemical etching of surface recombination centers.InxGa1-xN semiconductors(band gaps ranging from 0.7 to 3.4 eV by varying indium composition)are promising photoelectrode materials for solar hydrogen production because their absorption can match the solar spectrum very well.However,indium segregation and phase separation happen during the growth of InxGa1-xN with higher In content or thicker film.The In-rich segregation layer on the surface of InxGa1-xN,which acts as recombination centers of photo-generated carriers,will decrease the photoelectrochemical performance.The photocurrent and IPCE of InxGa1-xN were greatly increased after the removal of indium segregation by the electrochemical etching method.Ta3N5 photoanode achieved record solar photocurrent after being modified by surface recombination center removal and Co(OH),loading.Compared with InxGa1-xN,Ta3N5 is cheaper,more simple to prepare,has a narrower band gap and a more suitable position of conduction and valence band for water splitting.Ta3N5 film has been prepared by oxidation and nitridation of Ta foil.However,the photoelectrochemical performance of Ta3N5 film is poor because there are carriers’recombination centers on surface of the films.The carrier transfer efficiency and performance has been greatly improved after the surface layer removal by mechanical exfoliation,automatic thermal exfoliation or HF etching.Moreover,the photoelectrochemical performance and photostability of Ta3N5 is further increased by Co(OH)x loading.After recombination center removal and Co(OH),loading,the solar photocurrent of Ta3N5 @ 1.23 VRHE is about 5.5 mA/cm2,which is the record photocurrent for the photoanode.The restrictive factors for the solar energy conversion efficiency have been explored by investigating the charge transport mechanism of photo-generated carriers in Ta3N5 films.The photo-generated carriers’ transport mechanism on the surface and in the bulk of the Ta3N5 films prepared by different methods has been studied.The results show that the Ta3N5 film composed of close packed small particles was beneficial to the transport of photo-generated electrons in the bulk of the film,which could decrease the photo-generated carriers’ recombination and improve the phtotoelectrochemical performance of Ta3N5.In this paper,the photoelectrochemical performance of photoanodes has highly enhanced after the removal of surface recombination centers by inspired methods.These surface recombination center removal methods may also favor improving the performances of other optoelectronic devices,such as solar cells and photodetectors.After recombination center removal and Co(OH)x loading,the solar photocurrent of Ta3N5 @ 1.23 VRHE is about 5.5 mA/cm2,which is the record photocurrent for the photoanode.更多还原