磷酸银光催化性能提升与增强机制的研究进展

2017-11-22 10:19李锋锋蔡永丰张明熹常石岩沈毅李志宏
化工学报 2017年11期
关键词:纳米线光生空穴

李锋锋,蔡永丰,张明熹,2,常石岩,沈毅,李志宏

(1华北理工大学材料科学与工程学院,河北省无机非金属材料重点实验室,河北省唐山市环境功能材料重点实验室,河北 唐山 063210;2中南大学轻合金研究院,湖南 长沙 410012;3天津大学材料科学与工程学院,天津 300072)

磷酸银光催化性能提升与增强机制的研究进展

李锋锋1,蔡永丰1,张明熹1,2,常石岩1,沈毅1,李志宏3

(1华北理工大学材料科学与工程学院,河北省无机非金属材料重点实验室,河北省唐山市环境功能材料重点实验室,河北 唐山 063210;2中南大学轻合金研究院,湖南 长沙 410012;3天津大学材料科学与工程学院,天津 300072)

Ag3PO4是目前光催化效率最高的可见光光催化剂之一,在降解有机污染物、分解水制氢和CO2还原等领域具有广泛的应用前景。但 Ag3PO4光催化性能距离实际应用还存在一定差距,化学性质也不稳定,因此对其性能提升受到了各国研究者的关注。围绕 Ag3PO4纳米化、形貌控制、异质结构等提升光催化性能的途径及其增强机制进行阐述,其中与Ag3PO4形成异质结构是目前提升其光催化性能的最主流的方法,Ag3PO4与金属氧化物、卤化物、硫化物、有机半导体、单质金属形成的异质结构均有效改善了其光催化性能,最后还对 Ag3PO4基光催化剂未来的发展趋势进行了展望。

磷酸银;催化;降解;环境;性能提升;增强机制

引 言

随着当今世界工业的快速发展,能源危机与环境污染问题日趋严重。2015年全球一次能源消耗总量已达131亿吨油当量,这些化石燃料的使用不仅加剧了能源危机,更对人类环境造成了严重的污染。光催化技术的问世,为解决环境污染问题和开发新能源提供了新途径。光催化剂通过吸收太阳光,不仅可以有效降解有机污染物,还可以应用于分解水制 H2和还原CO2等领域,是一种非常重要的能源材料。

光催化技术起源于 1972年,Fujishima 等[1]首先发现用TiO2可以光催化分解水制取氢气和氧气,随后众多研究者开始致力于光催化技术的研究。但是TiO2由于自身量子效率低(仅4%~5%)、太阳能的利用率较低等问题,制约了其产业化应用。研究在可见光下具有光催化活性的材料具有重要的实际意义,也是光催化走向实用化的关键所在。为了实现可见光催化,除了对TiO2改性使光响应拓展到可见光区域,WO3[2-3]、BiVO4[4-5]和 Ag3PO4[6]等具有可见光响应的新体系光催化材料也得到了广泛研究。

Ag3PO4是目前光催化性能最优良的光催化剂之一,为黄绿色晶体,空间结构为体心立方,晶格参数a=b=c=0.6013 nm,溶于酸、氰化钾溶液和氨水,微溶于水和稀乙酸。对可见光有强烈吸收,在多种有机染料降解实验中表现出极其优越的光催化活性,它的可见光催化能力要远优于WO3、BiVO4等其他可见光催化材料,成为近年来的研究热点。

但是,目前 Ag3PO4光催化剂的研究仍仅局限于实验室,特别是其光化学稳定性较差,不利于循环使用。为了进一步改善 Ag3PO4的光催化性能和稳定性,研究者从粒子纳米化、形貌控制、形成异质结构等方面展开了研究,并取得了初步的研究成果。本文对 Ag3PO4基可见光催化剂的国内外研究现状进行了综述,重点对各种提升光催化性能的途径和增强机制进行了总结、分析和讨论,并展望了今后的发展趋势和应用前景。

1 磷酸银基光催化剂的研究历程

2010年,Ye等[6]报道了一种新型的Ag3PO4光催剂,Ag3PO4禁带宽度为2.36 eV,价带电位2.43 eV,可以对太阳光中小于530 nm的紫外-可见光波段吸收。他们研究得出 Ag3PO4分解水制氢的能力分别是WO3、BiVO4的8.8倍和2.6倍,对亚甲基蓝的降解速率比所报道的BiVO4快几十倍。为了进一步解释 Ag3PO4的高氧化活性,他们还进行了密度泛函的计算并提出了光催化机理模型。

在这之后,他们围绕 Ag3PO4光催化剂又做了大量的研究工作,Hu等[7]合成了规则立方体、四面体等不同形貌的Ag3PO4,获得了比普通Ag3PO4和N掺杂TiO2等体系更高的可见光催化活性。Bi等[8]还通过在Ag纳米线上选择性生长Ag3PO4的亚微米立方体,形成具有项链构造的复合异质结构。这种复合光催化剂相比纯Ag3PO4立方体和Ag纳米线,对有机污染物的可见光降解活性更高,并将催化活性的提高归结于二者接触界面上高效的电荷分离以及Ag纳米线充当电子的快速出口。Guo等[9]通过原位沉淀法成功合成了 Ag3PO4/In(OH)3复合光催化剂,获得了比Ag3PO4和In(OH)3更高的可见光催化活性,Ag3PO4/In(OH)3的摩尔比为1:1.65时,反应速率常数最大,kapp= 1.75 min-1,并认为复合后,光催化剂表面能带结构被调控是多电子反应增强的主要原因。Guo等[10]合成了Ag3PO4/氮化Sr2Nb2O7异质结,在光降解实验中检测到,在该异质结材料的催化作用下,CO2的释放速率约为纯 Ag3PO4的40倍。

同时Ye的这项研究成果,也引发了世界各国研究者的极大关注。研究的热点主要集中在Ag3PO4光催化性能的提升和光催化机理研究等方面。Ag3PO4的发现为可见光催化领域开辟了新的途径,有望在太阳能转换和环境净化等方面获得实际应用。

2 磷酸银光催化性能的提升途径

2.1 磷酸银粒子的纳米化

采用纳米半导体粒子作为光催化剂,一方面,量子尺寸效应会使半导体能隙变宽,导带电位变得更负,而价带电位变得更正,这使得其获得了更强的氧化还原能力;另一方面,纳米粒子的比表面积远远大于常规材料,高比表面使得纳米材料具有强大的吸附污染物的能力,这对提高光催化反应速率是十分有利的,而且,粒径越小,电子与空穴复合概率越小,电荷分离效果越好,从而使催化活性得到提高。

2012年,Bi等[11]采用 Ag纳米线在室温下与H2O2、NaH2PO4水溶液反应,合成了具有二维树枝状结构的纳米Ag3PO4。并通过可见光照射下的有机污染物降解实验证实,二维树枝状结构的纳米Ag3PO4与形貌不规则的Ag3PO4纳米晶体、N掺杂TiO2等光催化剂相比,具有更高的光催化活性。随后,Vu等[12]在油胺环境下,用H3PO4沉淀AgNO3成功合成了 Ag3PO4纳米粒子,粒子的粒度范围分布在5~10 nm。Ag3PO4纳米粒子在470~475 nm范围内,具有强烈的可见光吸收,对罗丹明B的可见光催化性能优于TiO2纳米粒子。Wu等[13]通过控制沉淀反应制备了 Ag3PO4微/纳米微晶,制备的Ag3PO4纳米微晶包括3种不同的形态,分别为500 nm的菱形十二面体、100 nm和20 nm球状粒子。由于尺寸为20 nm的Ag3PO4纳米粒子具有较大的比表面积,因此在水、乙二醇(EG)和二甲基亚砜(DMSO)等溶剂中的分散性较好,且具有最佳的抗菌活性。

2.2 磷酸银粒子的形貌控制

一方面,通过粒子形貌的控制可以提升光催化剂与有机污染物的接触面积,加速降解反应的进行;另一方面,还可以使更多的高活性位Ag粒子暴露出来,提高光催化剂的反应活性。

图1 不同形貌的Ag3PO4晶体Fig.1 Ag3PO4 crystals with different morphologies

Xu等[14]采用阴离子交换法,合成了球状[图1(a)]、菱形十二面体状[图1(b)]、截断八面双锥状[图1(c)]和平行四边形棱镜状[图1(d)]等不同粒子形态的Ag3PO4晶体。通过可见光照射下的降解罗丹明 B实验研究发现,截断八面双锥状Ag3PO4的光催化活性最高。Wang等[15]采用一种简单的尿素辅助水热法合成了四足形态的 Ag3PO4粒子[图1(e)],四足形态的Ag3PO4粒子具有高度暴露的{110}面,因此在降解有机毒性化合物时,表现出了较高的光催化活性。Zheng等[16]成功地通过简单的湿化学法合成了具有四面体形态的 Ag3PO4单晶,通过理论计算证明了{111}晶面的表面能最高,带隙宽度最大,有利于光生电子和空穴的分离。Kumar等[17]采用银-氨配合物合成了棱角分明的梯形状Ag3PO4晶体[图1(f)],并证实了这种Ag3PO4晶体的吸附能力和可见光催化活性比常规 Ag3PO4高得多。

2.3 半导体异质结结构

通过将磷酸银与其他具有适当禁带宽度的半导体物质形成异质结,可以有效调控磷酸银能带结构,促进电荷分离,提高光催化剂的反应活性。异质结构是目前提升 Ag3PO4光催化性能方法中报道最多的一种技术手段。

2.3.1 金属氧化物复合 金属氧化物半导体在半导体领域占据重要的位置,多用于气敏元件、光敏元件,也已经有部分应用于光催化领域,最典型的是TiO2,TiO2(P25)目前已经实现工业化生产。TiO2的导带、价带电位分别为2.91 eV和-0.29 eV,满足异质结的形成条件,因此 TiO2/Ag3PO4异质结复合光催化也是目前的研究热点[18-22]。

Yao等[18]通过原位沉积法将 Ag3PO4纳米粒子修饰到 TiO2(P25)表面形成异质结构。这种复合光催化剂降低了Ag的负载量47%~77%(质量分数),节约了Ag3PO4的应用成本。Teng等[20]通过连续的化学浴沉积将Ag3PO4纳米粒子负载TiO2纳米管阵列(TiO2-NTs)上,随后又在紫外线照射下,将纳米 Ag3PO4中部分 Ag+还原为金属 Ag,形成了Ag/Ag3PO4/TiO2纳米管异质结构。并通过实验证实了纳米簇Ag/Ag3PO4的形成没有对TiO2纳米管阵列的有序结构造成损伤。由于 Ag/Ag3PO4/TiO2-NTs发光强度比TiO2纳米管低得多,也说明了Ag/Ag3PO4纳米颗粒沉积在TiO2纳米管表面能促进光生电子的转移,从而抑制电子和空穴的有效复合。

另外,Ag3PO4与 CeO2[23]、Bi2O3[24]、SnO2[25]、ZnO[26]、WO3[27-28]、CuO[29]、Fe2O3[30]、Co3O4[31]等半导体物质的异质结构也得到了研究。

2.3.2 卤化物半导体复合 AgX半导体(除价带较大的AgCl)的导带和价带电位比Ag3PO4更负,可以通过异质结构促进光生电子-空穴对的迁移和分离[32-36]。为了进一步提高 Ag3PO4的抗侵蚀性及光催化活性,Amornpitoksuk 等[33]采用 AgNO3与Na2HPO4+ KX(X = Cl-,Br-,I-)溶液的共沉淀法进一步合成了AgX(X = Cl,Br,I),并通过实验证明了AgCl和AgBr可以诱导Ag3PO4增加其光催化脱色效率。Katsumata等[34]通过原位离子交换法制备了不同摩尔比的 AgBr/Ag3PO4复合光催化剂,AgBr为 60%(摩尔分数)时,复合光催化剂对酸性橙的脱色表现出最高的光催化活性。

2.3.3 硫化物半导体复合 作为宽带隙半导体纳米,硫化物半导体纳米材料具有优异的光、电、磁等性质,一直受到科研工作者的广泛关注,MoS2、CdS、In2S3、WS2等与Ag3PO4复合形成光催化剂得到了较为广泛的研究[37-40]。

Zhu等[37]通过水和乙醇混合溶剂沉淀法合成了可见光驱动的Ag3PO4/ MoS2复合光催化剂,并通过实验得出,MoS2掺量为 0.648%(质量分数)时,光催化活性最高,可以在60 min内降解所有的MB,且化学稳定性优良。Yu等[39]通过在 WS2片上可控生长 Ag3PO4/WS2复合催化剂,WS2片的表面和边缘暴露的S原子,在Ag3PO4/WS2复合形成中起重要作用。当引入0.05 mol C2H3AgO2(AgAc)时,Ag3PO4/WS2显示出最高的光催化活性,WS2片还可以减少Ag离子的水溶性,提高Ag3PO4稳定性。

2.3.4 有机半导体复合 目前,由有机半导体材料与光催化剂复合提高光催化活性也已有报道,这些有机半导体主要包括石墨烯(GO)[41-43]、氮化碳(g-C3N4)[44-46]、聚丙烯腈(PAN)[47]和导电聚苯胺(PANI)[48]等。

2012年,Liu等[41]报道了GO/Ag3PO4复合光催化剂。他们采用离子交换法合成的GO/Ag3PO4复合光催化剂显示出优良的抗菌和可见光催化消毒能力。Panigrahy等[43]报道了一种GO/rGO(0.13 %,0.26 %和0.52%,质量分数)-Ag3PO4复合材料及其在可见光照射下的光降解染料罗丹明 B(RhB)和有机污染物氯酚(2-CP)的机理模型,证实了抗坏血酸和水合肼作还原剂制备出的 rGO比硼氢化钠作还原剂具有更好的光催化活性,并分析了复合材料的光催化活性的增强应归因于范德瓦耳斯力的相互作用以及在GO/rGO和Ag3PO4的界面上形成的势阱。

Kumar等[44]较早就开展了Ag3PO4/g-C3N4的复合研究。2013年,他们采用室温原位沉积的方法将Ag3PO4纳米粒子负载于g-C3N4表面,并通过TEM证实了Ag3PO4纳米粒子在g-C3N4表面的原位生长。当g-C3N4所占质量分数为25%时,Ag3PO4/g-C3N4光催化效果最佳,分别为纯 g-C3N4、Ag3PO4的 5倍和3.5倍。

另外,有机共轭半导体也是一种非常重要的有机半导体材料,通常是指具有共轭双键的高分子类半导体物质。有机共轭半导体典型载流子是π-π键中的空穴和电子。由于离域π-π键共轭结构,可以与光催化剂的能级完美配合,加之两者界面的混合效应,导致在电子转移过程中能够产生快速的光致电荷分离和较低的电荷复合,因此可以提高光催化效果。Yu等[47]通过聚丙烯腈(PAN)作为聚合物模板,采用静电纺丝技术制备了一种项链状结构的Ag3PO4/PAN的纳米纤维,这种纳米纤维对降解有机污染物具有优异的光催化性能。Liu等[48]通过化学吸附法合成了一种具有核壳结构的 Ag3PO4@聚苯胺(PANI)可见光催化剂。Ag3PO4@PANI(5 %,质量分数)对苯酚和 2,4-二氯酚降解效果最佳,分别达到100%和95.3%,分别是Ag3PO4的1.44倍和1.38倍。

2.3.5 肖特基异质结构 沉积在 Ag3PO4表面的金属纳米粒子可以作为电子受体,在金属纳米粒子/Ag3PO4界面上,产生较高的肖特基势垒,形成肖特基异质结,从而有效诱导界面电荷转移,提高电荷分离效率。

纳米Ag是目前研究者采用较多的一种电子受体[49-50]。Hu等[49]在室温下,通过在单晶银纳米线上生长Ag3PO4的方法,合成了Ag/Ag3PO4核-壳结构的同轴异质纳米线,Ag/Ag3PO4异质纳米线比纯Ag3PO4立方体颗粒、Ag纳米线和N掺杂TiO2光催化剂具有更高的可见光活性。Wu等[50]采用电化学法,在垂直排列的银纳米片上形成了Ag3PO4,合成了Ag/Ag3PO4纳米片光阳极,外层的Ag3PO4层作为光吸收材料产生电子-空穴对,而内层的银纳米片不仅起到结构框架的作用,同时也是连接 Ag3PO4和导电衬底之间的电子通道。

另外,其他纳米金属及复合纳米金属作为电子受体也得到了研究。Yan等[51]以硼氢化钠作为还原剂,采用化学沉积方法制备了M/Ag3PO4(M = Pt,Pd, Au)肖特基型异质结,并在可见光照射下(λ> 420 nm),通过对甲基橙、亚甲基蓝、罗丹明B 3种染料的降解实验,评价了M/Ag3PO4(M = Pt, Pd, Au)的光催化活性。研究发现这些贵金属纳米粒子高度分散在Ag3PO4表面后,极大增加了Ag3PO4在紫外和可见光区域的光吸收,且 M/Ag3PO4表面的光响应比纯Ag3PO4高得多,顺序为Pt > Au > Pd。

3 磷酸银基光催化剂的光催化机理

Ye等在初次报道 Ag3PO4的可见光催化性能时,就提出了 Ag3PO4的分解水制氢的光催化机理模型,如图2所示。图2说明了由于Ag/Ag3PO4的电极电势介于H+和Ag/AgNO3之间,因此Ag3PO4无法直接分解水制氢;但是当AgNO3作为牺牲剂存在时,Ag3PO4就表现出强的光氧化能力。同时,Ye等还采用CASTEP程序计算了Ag3PO4的能带和态密度,如图3所示。Ag3PO4高度分散的价带和导带有益于光激发电子-空穴的迁移,这反过来也有可能抑制电子-空穴对的复合,从而引发高氧化活性。另外,在 Ag3PO4导带底部主要由杂交的 Ag 5s5p以及少量的P 3s轨道组成,而价带的顶部是Ag 4d和O 2p杂化轨道。Ag2O是一种窄带隙半导体材料,添加P元素似乎可以调整Ag2O的能带结构和氧化还原能力,从而获得高可见光催化性能。

图2 Ag3PO4的可见光催化机理Fig.2 Schematic drawing for photocatalytic mechanism of Ag3PO4 under visible light

图3 基于密度泛函计算的Ag3PO4能带和态密度Fig.3 Energy-band and states density of Ag3PO4 calculated by density functional method

图4 Ag3PO4/TiO2纳米线阵列的光催化机理Fig.4 Schematic drawing of photocatalytic mechanism of Ag3PO4/TiO2 nanowire arrays

图5 Ag3PO4/BiVO4 和 ZnFe2O4-ZnO-Ag3PO4的光催化机理Fig.5 Schematic drawings for photocatalytic mechanism of Ag3PO4/BiVO4 and ZnFe2O4-ZnO-Ag3PO4

异质结构是目前用于提升光催化性能的最主要的一种手段。形成异质结构的半导体物质存在能带电位差,会造成光生电子或空穴的转移,从而促进电荷分离,提高光催化活性。第1种情况是在两种半导体物质间只存在1种电荷发生单向流动,如Ag3PO4/TiO2体系光催化剂的电荷转移过程大多是采用这种机制解释。Jin等[52]在研究 Ag3PO4/TiO2纳米线阵列的光催化机理时,指出由于 Ag3PO4的价带电位(2.9 eV)比TiO2(2.7 eV)更正,因此可见光激发Ag3PO4所产生的光生空穴会转移至TiO2价带,再与吸附H2O结合生成OH,光生电子则是迁移到Ag3PO4粒子表面,然后与吸附O2产生O2-。这样电荷转移过程促进了电荷分离,从而高效降解甲基橙 MO,机理如图4所示。Zhao等[53]、Yang等[54]、Xie等[55]也都采用了同样的机理模型解释了Ag3PO4/TiO2体系的异质结光催化剂的提升机理。另外一种情况,形成异质结构的两种或多种半导体的能带电位呈交错式排布,可以促进光生电子和空穴分离,从而抑制电荷复合。这是一种异质结光催化剂的主流光催化机制,大多异质结光催化剂是采用这种机制进行解释。如图5(a)所示,Qi等[56]指出 Ag3PO4/BiVO4光催化剂在可见光照射下,光生电子从Ag3PO4向BiVO4的CB转移,光生空穴由 BiVO4向 Ag3PO4的 VB 跃迁。Li等[57]研究了ZnFe2O4-ZnO-Ag3PO43种半导体形成的异质结光催化剂[图5(b)],并指出光生电子的流向。不同的异质结构光生电子和空穴的流向是不同的,这取决于形成异质结构的半导体物质的能带电位差,电子向正电势方向流动,空穴向负电势方向流动,这种电荷跃迁模式进一步促进了电荷分离,提升了光催化效果。另外,Ag3PO4/WO3[58]、Ag3PO4/ZnO[59]、Ag3PO4/ Ag2CO3[60]、Ag3PO4/Cr-SrTiO3[61]、Ag3PO4/Bi2WO6[62]、AgI/Ag3PO4[63]、Ag3PO4-Bi2MoO6[64]、SnSe2/Ag3PO4[65]等体系也采用这种电荷跃迁机制进行了解释。

图6 Ag纳米线/ Ag3PO4 项链状异质结的光诱导电荷分离及Ag纳米线和Ag3PO4立方体间的电荷分布Fig.6 Schematic illustrations for photo-induced charge separation of Ag nanowire/Ag3PO4 cube necklace-like heterostructures and charge distribution between Ag3PO4 cube and Ag nanowire

肖特基机制主要用来解释纳米金属或量子点与Ag3PO4复合光催化剂的光催化机理。2012年Bi等[8]针对Ag纳米线/ Ag3PO4项链状异质结光催化剂提出了其可能的光催化机理,Ag纳米线和Ag3PO4立方体复合后,形成肖特基势垒,费米能级趋向平衡,特别是纳米 Ag具有适当的费米能级[Ef(vs NHE)= 0.4 V],使其成为了优良的电子收集器,促进了Ag3PO4导带的光生电子向Ag纳米线的快速转移。如图6所示,光生电子从Ag纳米线上输出,空穴从 Ag3PO4立方体上输出。这种光催化机制显著加快了有机染料的光催化降解反应速率。Yan等[51]详细探讨了M/Ag3PO4(M = Pt,Pd,Au)肖特基型异质结的光催化机理。研究者首先通过电荷俘获实验,证明了空穴是M/Ag3PO4最主要的活性物种。同时由于 Ag3PO4的导带电位为 0.45 eV,比 O2-(-0.33 eV)更正,这意味着光生电子Ag3PO4的导带电子不能还原吸附O2成为O2-。另外,研究者还得出Pt、Pd、Au 3种贵金属的费米能级高于Ag3PO4,证明了Ag3PO4的导带(CB)电子会传递给贵金属粒子,再与吸附O2结合才能形成O2-。同时根据4种贵金属的功函数值的高低顺序Pt(5.65 eV)> Pd(5.12 eV)> Au(5.10 eV)> Ag(4.95 eV),得出同等条件下光催化活性的顺序为 Pt/Ag3PO4>Pd/Ag3PO4> Au/Ag3PO4> Ag/Ag3PO4。Zhang 等[66]系统研究了碳量子点(CQDs)增强 Ag3PO4、Ag/Ag3PO4的光催化活性和稳定性的机制,光催化机理如图7所示。首先不溶性CQDs层能有效减缓Ag3PO4和Ag/Ag3PO4的水溶解。其次,在光催化过程中,CQDs既可以作电子供体,也可以作电子受体,平衡了光生电子在CQDs和光催剂表面的浓度,缓解了Ag/Ag3PO4的光腐蚀。第三,由于量子点具有上转化性能,吸收可见光后发出短波长的光(300~530 nm),从而激发Ag3PO4形成电子-空穴对,以有效地利用太阳光的全谱,提高了光催化活性。

图7 CQDs增强Ag3PO4光催化机制Fig.7 Schematic drawings for enhanced photocatalytic mechanism of Ag3PO4

Z-scheme 机制也是提升光催化性能的一种重要的机制。Chen 等[67]制备了Ag3PO4/Ag/SiC异质结材料,并详细描述了其 Z-scheme 光催化机制,如图8(a)所示。在可见光照射下,Ag3PO4和SiC都受到了激发,分别在导带和价带产生光生电子和空穴。由于Ag3PO4CB电位比金属Ag的费米能级更负,因此在 Ag3PO4导带最低能级(CBM)上的电子很容易流入金属银(电子转移Ⅰ:Ag3PO4CBM → Ag)。同时,由于Ag的费米能级比SiC的价带电位更负,SiC价带的空穴也容易流入金属Ag(空穴转移Ⅱ:SiC VBM → Ag),流入金属银的空穴和电子会发生复合,因此促进了 Ag3PO4和SiC的电荷分离。另外Ag3PO4的价带空穴显示出了很强的氧化能力,SiC导带电子显示出强烈的还原能力。

图8 Ag3PO4/Ag/SiC、Ag3PO4Ag/WO3-x和Ag3PO4/Ag/g-C3N4 Z-机制光催化机理Fig. 8 Proposed mechanism for Z-scheme charge-carrier transfer process in Ag3PO4/Ag/SiC, Ag3PO4/Ag/WO3-x and Ag3PO4/Ag/g-C3N4 composite

Bu 等[68]针对 Ag3PO4/Ag/WO3-x体系解释了 Z机制对光催化降解性能的提升,机理如图8(b)所示。光激发后,所产生的WO3-x光生电子和Ag3PO4的光生空穴,在电位差的作用下,都会迅速转移到金属Ag表面。导致具有较弱还原性的光生电子和弱氧化性的空穴被湮没。因此,具有更强还原性的光生电子和更强氧化性的光生空穴,分别留在了Ag3PO4的导带和WO3-x的价带,这些光生电子和空穴将参与有机染料的氧化还原反应,这种电子转移过程促进了光催化降解能力的提升。

对于Z-scheme机制,在分解水制氢制氧领域也得了应用。Yang等[69]合成了Ag3PO4/g-C3N4复相材料,并通过生成的Ag纳米颗粒形成了Z-机制,机理模型如图8(c)所示。Ag纳米颗粒在复合材料中作为Ag3PO4和g-C3N4交联桥,Ag3PO4导带的电子和g-C3N4价带的空穴会在Ag纳米颗粒处重新复合,从而促进了电荷分离。留在 Ag3PO4价带处的空穴,可以有效地氧化水,而留在g-C3N4的导带处的电子可以减少硝酸银的用量,从而提高了该材料的分解水的能力。

另外其他研究者也通过 Ag3PO4/MoS2[37]、Ag3PO4/HAP[70]、Ag3PO4/SnSe2[71]、Ag3PO4/Ag/BiVO4[72]、Ag3PO4/ZnFe2O4[73]、Ag3PO4/Ag/ZnS[74]、Ag3PO4/Ag2MoO4[75]等体系进一步阐述了 Z-机制对光催化性能的促进作用。

4 总结与展望

综上所述,通过 Ag3PO4的纳米化产生量子效应,通过控制微观形貌增大与反应物的接触面积或者获得高活性晶面,采用多种半导体与 Ag3PO4形成异质结构减缓电子-空穴对的复合,均使得Ag3PO4光催化性能获得提升。其中与Ag3PO4形成异质结构是目前提升其光催化性能的最主流,也是最有效的一种研究方法,并针对典型的光催化增强机制进行了系统的分析讨论。

然而 Ag3PO4基光催化剂的研究仍然处于研究的初期阶段,实现其实用化,还需要从以下几个方面深入研究。

(1)化学不稳定性仍是制约 Ag3PO4基光催化剂发展的最大问题,尽管该问题通过目前的研究得到一定的缓解,还需要通过表面改性,形成异质结构等方法抑制光腐蚀作用,进一步提升其光化学稳定性,才有可能获得实际应用。

(2)由于 Ag3PO4中的银含量高,直接应用成本很高。对于该问题,主要是进一步提升 Ag3PO4基光催化剂的性能,从而减少使用量,另外通过合适的廉价半导体物质形成复合光催化剂,从而降低催化剂中的Ag3PO4含量。

(3)目前的光催化机理研究还不够成熟,大多是通过结构、性能测试,结合经验进行推测,这样建立起来的机理模型只能初步解释光催化行为。要对光催化剂的研究开发起到指导作用,还必须融合材料学、固体物理、光学、热力学、动力学等多学科的基本理论和研究方法,进行更深入的量化研究。

(4)Ag3PO4基光催化材料的研究大多以降解有机染料为主,在处理有机质、重金属以及分解水制氢等领域的应用研究并不多,因此其应用领域也有待进一步扩展。

如何处置环境污染和应对能源短缺已经成为当今社会的重点问题。Ag3PO4基可见光催化材料凭借其优良的可见光催化性能有望成为解决这些问题的新型材料,在环境治理和新能源等领域中发挥重要的作用。

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date:2017-05-04.

SHEN Yi, tsshenyi@sina.com

supported by the National Natural Science Foundation of China(51772099, 51572069).

Progress on photocatalytic performance improvement and enhancement mechanisms of silver phosphate

LI Fengfeng1, CAI Yongfeng1, ZHANG Mingxi1,2, CHANG Shiyan1, SHEN Yi1, LI Zhihong3
(1College of Material Science and Engineering,Key Laboratory of Inorganic Nonmetallic Materials Hebei Province,Key Laboratory of Environment Functional Materials of Tangshan City,North China University of Science and Technology,Tangshan063210,Hebei,China;2Light Alloy Research Institute,Central South University,Changsha410012,Hunan,China;3College of Material Science and Engineering,Tianjin University,Tianjin300072,China)

Due to its excellent visible light photocatalytic performance, Ag3PO4has wide application prospective in many fields, such as organic pollutant degradation, water decomposition and CO2reduction. However, there is still a great gap between photocatalytic performance of Ag3PO4and requirement for practical applications, besides its unstable chemical properties. A plenty of work has been contributed to performance improvement of Ag3PO4.This review was focused on photocatalytic performance improvement and enhancement mechanism of Ag3PO4by means of nanonization, morphology control, and structure heterogenization. So far, the most popular approach had been development of Ag3PO4heterostructure, which heterostructures with metal oxides, sulfides, halides, organic semiconductors, and metals effectively improved photocatalytic performance. Future development trend of Ag3PO4photocatalysts were also prospected.

silver phosphate; catalysis; degradation; environment; performance improvement; enhancement mechanism

TB 332

A

0438—1157(2017)11—4005—11

10.11949/j.issn.0438-1157.20170551

2017-05-04收到初稿,2017-08-07收到修改稿。

联系人:沈毅。

李锋锋(1981—),男,博士,讲师。

国家自然科学基金项目(51772099,51572069)。

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