Remarkable variation of visible light photocatalytic activities of M/Sn0.9Sb0.1O2/TiO2 (M=Au, Ag, Pt) heterostructures depending on the loaded metals
Graphical abstract
Introduction
Photocatalytic oxidation process has been investigated as potential clean technologies for removing environmental organic pollutants utilizing sunlight or indoor light. To develop advanced photocatalytic systems highly effective under visible light, various types of semiconductor heterostructures have been designed, because of their advantages over single semiconductors with respect to visible light utilization, charge separation, production of energetic electron-hole pairs and others (Xu et al., 2019; Low et al., 2017; Meng et al., 2019; Zhang and Jaroniec, 2018; Liu et al., 2020; Tang et al., 2020).
Semiconductor heterostructures classified as Type-B heterojunctions which are operated by hole-transport mechanisms have attracted increasing attention as a new category of visible light photocatalysts (Bera et al., 2019; Rawal et al., 2013; Yang et al., 2014; Chai et al., 2009). As shown in Fig. 1a, a narrow bandgap semiconductor (NBS) is coupled to TiO2, which is known to be the most efficient photocatalyst working under UV-light, to form an NBS/TiO2 heterostructure, where the valence band (VB) of NBS is located more positive than the VB of TiO2. When such a system is irradiated with visible light, only the NBS is photoexcited and the holes produced in the VB of NBS can move to that of TiO2. Then, the holes in the TiO2 VB are able to produce highly oxidative •OH. In this photocatalytic system, however, the electrons left in the conduction band (CB) of NBS are difficult to be used for chemical reactions or to be scavenged, because the CB level of NBS is not sufficiently negative to convert oxygen (O2) to superoxide (•O2−). Presumably, these electrons are used up in producing hydrogen peroxide (H2O2) via multielectron-involved reactions, but the reaction rates will be very low (Bard et al., 1985; Irie et al., 2008; Shiraishi et al., 2003; Moon et al., 2014).
To expedite photocatalytic reactions for this catalytic system, electrons excited to the CB of NBS must be rapidly scavenged. Otherwise, the inter-semiconductor hole-transport from the VB of NBS to that of TiO2 will be impeded by the charge recombination reactions as described below. First, the photoexcited electrons in the CB of NBS recombine with the holes in its VB (recombination pathway 1, RP1), which decreases the quantity of holes in the VB of NBS and in turn decreases the hole-transport rate to the TiO2 VB (Fig. 1a). Second, inter-semiconductor electron transport can occur via a Z-scheme-like mechanism that causes electrons to migrate from the NBS CB to the TiO2 VB (recombination pathway 2, RP2). Although this charge-transport process has not been analyzed systematically to date, it is presumed to occur, because the NBS CB is located in the middle of TiO2 CB and VB (Xu et al., 2019; Low et al., 2017; Meng et al., 2019; Zhang and Jaroniec, 2018; Liu et al., 2020; Tang et al., 2020; Bard, 1979; Maeda, 2013; Zhou et al., 2014). According to this RP2 mechanism, holes in the TiO2 VB produced by hole-transport from NBS will be eliminated with a high rate, leading to a marked reduction of photocatalytic efficiency (Fig. 1a).
The deposition of noble metals selectively onto the NBS surface in NBS/TiO2 provides a simple means of effectively scavenging the electrons accumulated in the NBS CB. If electrons in the NBS CB are trapped by a noble metal, charge recombination via RP1 and RP2 mentioned above could be suppressed. It is well known that noble metals are excellent electron acceptors and that their Fermi levels are basically determined from their work functions, although the Fermi level can be changed according to the population of the accumulated electrons (Wood et al., 2001; Suvramanian et al., 2003; Subramanian et al., 2004). Also, the Fermi level of the metal needs to be more positive than the CB level of the NBS for effective electron trapping.
Various Type-B heterojunction systems like FeTiO3/TiO2 (Gao et al., 2008), WO3/TiO2 (Leghari et al., 2011; Wang et al., 2020), W18O49/TiO2 (Rawal et al., 2013), Bi2O3/TiO2 (Bian et al., 2008; Huang et al., 2017), Ag3PO4/TiO2 (Rawal et al., 2012a; Taheri et al., 2017), Fe2WO6/TiO2 (Rawal et al., 2014), ATO/TiO2 (Rawal et al., 2012b; Park et al., 2018), FeWO4/TiO2 (Bera et al., 2014), and others (Yang et al., 2014; Chai et al., 2009; Shamaila et al., 2011; Xu et al., 2011; Min et al., 2012; Che et al., 2016) have been designed thus far and show considerable visible light photocatalytic activities based on the hole-transport mechanism. Among them, ATO/TiO2 is a typical heterostructure with excellent material stability and catalytic performance (Bera et al., 2019; Rawal et al., 2012b). In this regard, we chose ATO/TiO2 heterostructure as a model photocatalytic system and selectively deposited Au, Ag, or Pt onto the ATO to form M/ATO/TiO2 (M = Au, Ag, Pt). The electron trapping from the ATO CB would be strongly dependent on the metals because Au, Ag and Pt have quite different work functions. Accordingly, we expected that charge recombination in ATO and resultant hole-transfer efficiency to the TiO2 VB would be greatly influenced by the loaded metals.
In the present work, we evaluated visible light activities of M/ATO/TiO2 depending on the noble metals deposited and investigated how electron-trapping at the ATO CB influences the charge transport behaviors and resultant photocatalytic properties. The results derived in this study will offer a new concept for the design of efficient visible light photocatalysts based on heterojunction of semiconductors.
Section snippets
Preparation of ATO/TiO2 heterostructures
10 mol % antimony-doped tin oxide (Sb0.1Sn0.9O2, ATO) nanoparticles (NPs) were prepared by a solid-state method by modifying the procedure reported previously (Rawal et al., 2012b; Jeon et al., 2005), while the detailed procedures are described in Supplementary Material.
3.67 g titanium tetraisopropoxide purchased from Aldrich Chemical Co. was added to a mixed solution consisting of 40 mL anhydrous ethanol (EtOH), 1.0 mL HNO3 (Aldrich) and 1.0 mL H2O. This precursor solution was then stabilized
Characterization of individual components and M/ATO/TiO2
The as-prepared dark blue ATO NPs were elliptical in shape of average size ∼25 nm with a moderately narrow size distribution and without heavy aggregation (Fig. 2a and b). They were then coupled with TiO2 by a sol-gel method to form ATO/TiO2. TEM images of ATO/TiO2-5/95 (ATO:TiO2 = 5:95 in wt% ratio) in Fig. 2c and d exhibit that ∼25 nm-sized ATO NPs are embedded in a TiO2 matrix with tight connections between ATO NPs and polycrystalline TiO2. The XRD patterns in Fig. S1 show the as-prepared
Conclusions
We investigated photocatalytic behaviors and charge transfer mechanisms of M/ATO/TiO2 (M = Au, Ag, or Pt) heterostructures. Our results provide fundamental insights into the photocatalytic mechanisms occurring in these heterostructures and offer a principle for designing highly efficient visible light photocatalysts. Two important findings can be summarized as follows. First, photocatalytic activity enhancement was observed when the Fermi levels of noble metals are more positive than the CB of
Credit author statement
Hye Jin Kang: Investigation, Methodology, Data curation, Dong-Il Won: Investigation, Methodology, Validation, Yeongsu Lim: Formal analysis, Jeongho Kim: Validation., Data curation, Writing, Wan In Lee: Conceptualization, Supervision, Writing
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Research Foundation (NRF) of Korea (NRF-2018R1A2B6004766).
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These authors contributed equally.