钛酸锶光催化增强研究：氢/氨热处理对钛酸锶光催化的影响

【作者】 孙涛；

【导师】 陆明；

【作者基本信息】 复旦大学 ， 光学工程， 2011， 硕士

【摘要】 自从1972年Fujishima和Honda发现Ti02电极光催化裂解水以来,TiO2、SrTiO3等氧化物半导体的光化学受到了广泛的关注和深入的研究,因为它们在光伏和光催化等方面有重要的潜在应用。许多氧化物半导体(如SrTiO3和Ti02)具有优良的光催化特性,但是由于较宽的禁带它们只对紫外光响应,而紫外光只占太阳能量中的一小部分(<10%)。通过带隙调制使它们的吸收边从紫外区移到可见区将提升氧化物半导体的性能。另一方面,在光催化过程中电子空穴对的利用效率往往很低,绝大部分光生载流子发生复合,最后以热的形式散失。阻止光生载流子复合,提高它们的利用效率,也是光催化研究的主要工作之一。本论文围绕以上两点,通过氢/氨热处理法对SrTiO3基片光催化增强进行了研究。首先,我们研究了氢热处理(氢化)对SrTiO3光催化的影响。SrTiO3在氢氮气(H25%/N2)中退火进行氢化,退火结束后快速冷却。我们通过测量加载SrTiO3催化剂的亚甲基蓝(MB)光照一定时间后吸收度的变化来表征SrTiO3的光催化活性。光催化实验显示氢退火温度超过～900℃时SrTiO3 (SrTiO3-H2)的紫外光催化和原始SrTiO34目比显著增强,最高增强达5倍。然而,SrTiO3-H2和原始SrTiO3一样都没有可见光催化。XPS测量暗示SrTiO3-H2 (>～900℃)的费米能级显著升高,对于1000℃氢化的SrTiO3,费米能级上升了超过1.6eV。费米能级的升高会使SrTiO3与溶液接触时内表面形成的能带弯曲更陡峭,我们认为这是紫外光催化增强的主要原因。我们也通过紫外可见吸收谱、电子顺磁共振(EPR)对氢化SrTiO3进行了表征。紫外可见吸收谱显示SrTiO3氢化后吸收边没有发生移动,只是在430和520nm处存在两个吸收峰,>600nm存在一个强的吸收带。EPR显示SrTiO3中含有商业SrTiO3中常见的Fe3+杂质。一般认为氧化物中Al、Fe的含量不少于10-100ppma。氢化(>-880℃)后SrTiO3中的Fe3+发生衰减,我们认为这也是自由电子增多的非直接证据,自由电子饱和了带正电的缺陷,类似于氢化饱和Si中的悬挂键。然后,我们研究了氨热处理(氨化)对SrTiO3可见光催化的影响。氨化的目的是为了在SrTiO3中在O的位置替换掺入N以在价带上方产生一个N的掺杂能级,从而可以吸收利用可见光来驱动光催化反应。氨化在氨气中退火进行,退火结束后自然冷却。实验发现退火温度和氨气流量会影响SrTiO3的掺N效果。可见光催化实验显示氨化SrTiO3 (SrTiO3-NH3)具有一定的可见光催化能力,800-900℃氨化SrTiO3的可见光催化效果较好。紫外可见吸收谱显示SrTiO3-NH3的吸收边发生了红移,XPS测量表明SrTiO3-NH3 (900℃,～30L/h)表面N/O比为0.06。SrTiO3-NH3中费米能级也发生了上移(-0.7eV),但远不如SrTiO3-H2显著。最后,我们研究了氢化和氨化在SrTiO3中的协同作用。我们发现SrTiO3氢化再氨化(SrTiO3-H2-NH3),可见光催化和SrTiO3-NH3接近。而氨化再氢化(SrTiO3-NH3-H2),可见光催化比SrTiO3-NH3显著增强。XPS测量显示SrTiO3-H2-NH3 (～0.4eV)的费米能级和SrTiO3-NH3 (～0.7eV)接近,而SrTiO3-NH3-H2的费米能级(-1.4eV)比SrTiO3-NH3显著提高,和SrTiO3-H2(-1.6eV)的接近。比较氢化对SrTiO3光催化的影响,可以得出结论SrTiO3-NH3-H2将氨化(氮掺杂)和氢化的效应协同在了一起,共同导致增强的可见光催化。更多还原

【Abstract】 Since the discovery of photocatalytic water splitting of TiO2 electrode by Fujishima and Honda, enormous efforts have been spent on the study of oxide semiconductors (i.e. SrTiO3 and TiO2) due to their important potential applications on photovoltaics and photocatalysis. Many oxide semiconductors have excellent photocatalytic properties, but they generally only response to ultraviolet (UV) light for a wide band gap, while UV light accounts for only a small portion of solar energy (<10%). The modulation of band gap to enable the absorption edge to move from the UV region to visible light one will enhance the performance of the oxide semiconductor. On the other hand, in the process of photocatalysis the efficiency of employing electron hole pairs is gernally low; most of the photo-generated carriers recombine and dissipate as heat. To prevent the recombination of photo-generated carriers is also one of the main researches on photocatalysis.This thesis focused on above two points, and the enhancement of photocatalysis was studied through the hydrogen and/or ammonia heat treatment method on the SrTiO3 substrate.First, we studied the effect of hydrogen heat treatment (hydrogenation) on the photocatalytic activity of SrTiO3.Hydrogenation of SrTiO3 was carried out by annealing in the hydrogen forming gas (H25%/N2), rapidly cooling after annealing. We evaluated the photocatalytic activity of SrTiO3 by measuring absorption of methylene blue (MB) with different catalysts loaded after illumination for a certain time. Photocatalytic experiments showed that SrTiO3 hydrogen annealed (SrTiO3-H2) under temperatures exceeding～900℃showed a significantly enhanced UV photocatalysis compared to as received SrTiO3, with a maximum increase of 5 times. However, SrTiO3-H2 had no visible light photocatalysis, like the received SrTiO3. XPS measurements suggested the Fermi level of SrTiO3-H2 (>～900℃) was significantly higher than as received one. For SrTiO3 hydrogenated at 1000℃, the Fermi level increased by more than 1.6 eV. Fermi level would increase the band bending in the internal surface of SrTiO3 when it contacts with the solution. This will contribute to enhanced UV photocatalysis because the steep band bending will help separate photo-generated carriers. The hydrogenated SrTiO3 was also characterized by UV-visible absorption spectroscopy and electron paramagnetic resonance (EPR). UV-visible absorption spectra showed the absorption edge of hydrogenated SrTiO3 did not shift compared with the received SrTiO3, though there were two absorption peaks at 430 and 520 nm and a strong absorption band at the region exceeding 600 nm. EPR showed Fe3+ impurities existed in as received SrTiO3 which is common for commercial SrTiO3. Al and Fe were considered to be present in oxides at levels typically no less than 10-100 ppma. After hydrogenation (>～880℃) the Fe3+ particles disappeared, which was a sidelight of free electron increase and could contributed to the saturation effect of free electrons, like dangling bond saturation happened in hydrogenated Si semiconductor.We then studied the effect of ammonia heat treatment (ammoniation) on visible light photocatalysis of SrTiO3. Ammoniation was aimed at nitrogen ion doping through substitution of lattice oxygen in SrTiO3 by nitrogen. Nitrogen incorporation can induce several doping levels above the valence band maximum, which enables the visible light photocatalytic reaction. Ammoniation was conducted in ammonia annealing, naturally cooling after annealing. It was found that annealing temperature and ammonia flow rate affected the effect of N incorporation in SrTiO3. Visible light photocatalytic experiments showed that ammoniated SrTiO3 (SrTiO3-NH3) had a visible light photocatalytic activity, and ammoniation at 800-900℃was most effective. UV-visible absorption spectra showed a red shift of the absorption edge in SrTiO3-NH3. XPS measurements showed that the N/O ratio in the surface of SrTiO3-NH3 (900℃,～30L/h) was 0.06. Fermi level in SrTiO3-NH3 had also undergone a shift (～0.7eV), but far from significant as SrTiO3-H2 (～1.6eV).Finally, we studied the synergistic effects of hydrogenation and ammoniation in the SrTiO3. We found that visible light photocatalysis in the SrTiO3 hydrogenated followed by ammoniation (SrTiO3-H2-NH3) was close to that of SrTiO3-NH3, while SrTiO3-NH3-H2 (ammoniated and then hydrogenation) exhibited a significantly enhanced visible light photocatalysis than SrTiO3-NH3. XPS measurements showd that Fermi level in SrTiO3-H2-NH3 (～0.4eV) was close to that of SrTiO3-NH3 (～0.7eV), while Fermi level in SrTiO3-NH3-H2 (～1.4eV) was significantly more than SrTiO3-NH3, and close to that of SrTiO3-H2 (～1.6eV). Comparison to the observation in hydrogenated SrTiO3, it can be concluded in SrTiO3-NH3-H2 the synergistic effects of ammoniation (nitrogen doping) and hydrogenation is achieved, and together cause an enhanced visible light photocatalysis.更多还原