Letter www.acsami.org
Light-Induced In Situ Transformation of Metal Clusters to Metal Nanocrystals for Photocatalysis Fang-Xing Xiao,† Zhiping Zeng,† Shao-Hui Hsu,† Sung-Fu Hung,‡ Hao Ming Chen,*,‡ and Bin Liu*,‡ †
School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore ‡ Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taiwan S Supporting Information *
ABSTRACT: In situ transformation of glutathione-capped gold (Aux) clusters to gold (Au) nanocrystals under simulated solar light irradiation was achieved and utilized as a facile synthetic approach to rationally fabricate Aux/Au/TiO2 ternary and Au/TiO2 binary heterostructures. Synergistic interaction of Aux clusters and Au nanocrystals contributes to enhanced visible-light-driven photocatalysis.
KEYWORDS: TiO2 nanotube arrays, metal nanocrystals, metal clusters, transformation, photocatalysis
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assembled onto one-dimensional (1D) TiO2 nanotube substrate with relatively complicated synthetic procedures. As Aux clusters are made of gold atoms, inspired by the intrinsic correlation and the same root (i.e., Au element) of the Aux clusters and Au NPs, it would be interesting to achieve the transformation of ultrasmall Aux clusters to Au NPs in a tunable and easily accessible fashion under ambient conditions, by which synergistic interaction of these two nanobuilding blocks could be fully harnessed. In this work, in situ transformation of ultrasmall glutathione (GSH)-capped Aux clusters uniformly assembled on onedimensional hierarchically ordered nanoporous TiO2 nanotube arrays (NP-TNTAs) to Au NPs was achieved under simulated solar light irradiation, which could be judiciously utilized as a facile synthetic route to fabricate well-defined Aux/Au/NPTNTAs ternary and Au/NP-TNTAs binary heterostructures. Moreover, synergistic interaction arising from plasmonic effect of Au NPs and photosensitization effect of Aux clusters concurrently contributes to enhanced visible-light-driven photocatalysis. It is expected that our current work could provide a new synthetic strategy to prepare metal/semiconductor nanocomposites based on intrinsic phase transformation property of metal clusters and, more significantly, further bridge the gap between metal clusters and metal nanocrystals. Scheme 1 illustrates the flowchart for in situ transformation of ultrasmall Aux clusters to metallic Au NPs under simulated
ver the past few years, TiO2-based photocatalysis has attracted enormous attention in various research fields including environmental cleanup, organic synthesis (e.g., selective oxidation or reduction), and solar energy conversion on account of versatile physical-co-chemical properties of TiO2.1−3 Nevertheless, wide bandgap (3.2 eV) of TiO2 restricts its photoresponse within the ultraviolet (UV) light region, which occupies merely 4−5% of solar spectrum and thus leads to low quantum efficiency.4−6 In this regard, it is of great importance to extend light absorption of TiO2 to visible region for substantial light harvesting. Metal nanoparticles (NPs) have been well-established as a pivotal sector of nanomaterials by virtue of their unique plasmonic effect, which induces a coherent oscillation of electrons, thereby producing a strong electromagnetic field and leading to significantly enhanced optical transition in neighboring semiconductors in conjunction with boosted quantum yield of photochemical reactions.7−9 Different from metal NPs, metal clusters are made of a few metal atoms, which are protected by stabilizing ligands and have characteristics of peculiar atom packing, quantum confinement effect, and discrete molecule-like band structure.10−14 Very recently, intrinsic correlation between bulk gold (Au) NPs and glutathione (GSH)-capped gold (Aux) clusters within a wideband gap semiconductor (e.g., TiO2) system was ascertained, i.e., Au NPs could act as electron relay mediator and plasmonic sensitizer for Aux clusters under visible light irradiation, therefore bridging the gap between these two different sized Au nanostructures.15 Noteworthily, in such a ternary system, tailor-made monodispersed Au NPs and Aux clusters as nanobuilding blocks were exogenously layer-by-layer (LbL) © XXXX American Chemical Society
Received: September 26, 2015 Accepted: December 7, 2015
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DOI: 10.1021/acsami.5b09091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
> 29 gold atoms), that is, Au(0)Au(I)@GSH, in which Au (I) component is tethered on the outer layer of clusters, thus making it as an active site for triggering in situ reduction reaction. On the other hand, it is worth pointing out that molecular structure of GSH is composed of various polar functional groups including carboxylic acid, amide, and carbonyl groups which are beneficial for spontaneous assembly of GSH-capped Aux clusters on the hydrophilic surface of TiO2 (e.g., NP-TNTAs) substrate with close interfacial proximity via pronounced electrostatic interaction or hydrogen bonding. Therefore, it is rationally speculated that electrons from bandgap photoexcitation of TiO2 and/or from highest occupied molecular orbital (HOMO) of Aux clusters under simulated solar light irradiation could be used to reduce Au (I) in Aux clusters and therefore induce transformation of Aux clusters to Au NPs, giving rise to Aux/Au/TiO2 ternary or Au/TiO2 binary heterostructure. As shown in Figure 1a, b and Figure 1g, field-emission scanning electron microscopy (FESEM) images of blank NPTNTAs exhibit a regular and uniform porous structure with a mean pore diameter of 90 nm. Cross-sectional FESEM image (Figure S2) corroborates the hierarchically ordered structure of NP-TNTAs substrate with thickness of ca. 9 μm. Figure 1c, d shows the FESEM images of pristine Aux/NP-TNTAs nanostructure before light irradiation, from which it clearly seen that the morphology of Aux/NP-TNTAs is similar to NPTNTAs and no Aux clusters could be observed on the surface. This is attributable to ultrasmall size of Aux clusters as evidenced by TEM (Figure S1) result. On the contrary, as displayed in panoramic and cross-sectional FESEM images in Figure 1e, f and Figure S3, morphology of Aux/Au/NP-TNTAs
Scheme 1. Flowchart for in Situ Transformation of Aux Clusters to Au NPs for the Fabrication of Aux/Au/NPTNTAs Ternary and Au/NP-TNTAs Binary Heterostructures
solar light irradiation for the fabrication of Aux/Au/NP-TNTAs ternary or Au/NP-TNTAs binary heterostructure. A two-step electrochemical anodization approach was first utilized to prepare hierarchically ordered NP-TNTAs scaffold consisting of porous layer on the top and vertically aligned tubular arrays in the bottom, which serves as an ideal substrate for decoration of Aux clusters. Subsequently, tailor-made negatively charged Aux clusters (1.5 nm, Figure S1) were uniformly deposited on NP-TNTAs by substantial electrostatic attractive interaction, resulting in Aux/NP-TNTAs hybrid nanocomposite. Finally, simulated solar light irradiation was used to induce the transformation of Aux clusters to Au NPs. Notably, GSHcapped Aux clusters used here are featured by a core−shell structure with thiolate ligand in the shell (1:1 ratio of Au: GSH,
Figure 1. FESEM images of (a, b) NP-TNTAs, (c, d) as-assembled Aux/NP-TNTAs, and (e, f) Aux/Au/NP-TNTAs ternary heterostructure. HRTEM images of (g) NP-TNTAs, (h) pristine Aux/NP-TNTAs, and (i) Au/NP-TNTAs. B
DOI: 10.1021/acsami.5b09091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 2. (a) UV−vis diffuse reflectance spectra of NP-TNTAs, pristine Aux/NP-TNTAs, and Aux/Au/NP-TNTAs ternary heterostructure, and (b) plots of transformed Kubelka−Munk function versus the energy of light. High-resolution XPS spectra of (c) Ti 2p and (d) Au 4f for Au/NP-TNTAs nanostructures.
to be 3.18, 2.80, and 2.2 eV, respectively. The narrowing of bandgap implies synergistic interaction of Aux clusters and in situ formed Au NPs with NP-TNTAs. Besides, high-resolution X-ray photoelectron spectroscopy (XPS) spectra of Ti 2p (Figure 2c) and Au 4f (Figure 2d) for Au/NP-TNTAs nanostructure are unambiguously assigned to Ti in the +4 oxidation state and metallic Au (0) corresponding to TiO2 and metallic Au components,16 respectively, which is in faithful agreement with FESEM, DRS, and TEM results. Additionally, XRD (Figure S8) and EDX (Figure S9) results of Au/NPTNTAs nanostructure also reveal the formation of metallic Au phase. Photocatalytic activities of different samples were explored by anaerobic reduction of aromatic nitro compounds (e.g., 4nitroaniline, 4-NA) to amino organics (e.g., 4-phenylenediamine) under the irradiation of visible light (λ > 420 nm).17,18 Control experiments without catalyst or light demonstrate negligible photoactivities, suggesting the reaction was driven by a photocatalytic process (Figure S10). As displayed in Figure 3a, Aux/Au/NP-TNTAs ternary heterostructure demonstrates the best photocatalytic performance toward reduction of 4-NA under visible light irradiation in comparison with NP-TNTAs, Aux/NP-TNTAs, and Au/NP-TNTAs binary nanostructures. Noteworthily, photoactivity of Au/NP-TNTAs is greatly enhanced as compared with that of NP-TNTAs due to the pronounced SPR effect of Au, but is substantially inferior to the photoactivity of Aux/NP-TNTAs, indicating that complete transformation of Aux clusters to Au NPs is detrimental to the photocatalytic performance, which could be attributable to the loss of contributing role of Aux clusters as photosensitizers. Therefore, our result clearly highlights the synergistic role of Aux clusters and in situ formed Au NPs arising from their respective photosensitization and SPR effects in promoting the photocatalytic reactions in ternary nanostructure. Moreover, as shown in Figure 3b, control experiments with and without the addition of K2S2O8 as scavenger for quenching photogenerated
ternary heterostructure is distinct from those of NP-TNTAs and pristine as-assembled Aux/NP-TNTAs binary heterostructure, wherein a large amount of discernible Au NPs with a mean particle size of around 13 nm are evenly distributed on the whole framework of NP-TNTAs, which is in line with color change of the different samples with and without light irradiation (Figure S4). Moreover, it is revealed that in situ transformation of Aux clusters to Au NPs relates closely to the irradiation time, as shown in Figure S5. The formation of metallic Au NPs from Aux clusters was further confirmed by TEM and HRTEM images (Figure 1h, i and Figure S7), in which lattice fringe of Au NPs in Au/NP-TNTAs (Figure 1i) was clearly observed but it was absent for Aux clusters in Aux/ NP-TNTAs (Figure 1h).6,15 This strongly indicates that Aux clusters assembled on NP-TNTAs undergo transformation from ultrasmall Aux clusters to Au NPs with light irradiation, which will be further corroborated by other characterization results as displayed below. Figure 2 exhibits the UV−vis diffuse reflectance spectra (DRS) of different samples, from which it is apparent to see that absorption of NP-TNTAs has been greatly extended to visible region with the deposition of Aux clusters (Figure 2a). Furthermore, under light illumination for 3.5 h, a substantial red-shift of band edge along with a pronounced surface plasmon resonance (SPR) peak (ca. 530 nm) were observed, thus suggesting partial transformation of Aux clusters to metallic Au NPs, resulting in the formation of intermediate Aux/Au/NP-TNTAs ternary heterostructures. Moreover, it was found that all Aux clusters could be completely transformed to Au NPs under continuous long-time light irradiation (10 h), leading to the formation of Au/NP-TNTAs nanostructure. This complete transformation could be revealed by DRS result (Figure S6), in which no band edge shift of NP-TNTAs was observed other than a pronounced SPR peak. As shown in Figure 2b, bandgap of NP-TNTAs, Aux/NP-TNTAs, and Aux/ Au/NP-TNTAs ternary nanostructure was roughly determined C
DOI: 10.1021/acsami.5b09091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
with a distinct highest occupied molecular orbital (HOMO)− lowest unoccupied molecular orbital (LUMO) gap, which makes it behave like a semiconductor with a small bandgap.19,20 Hence, it is highly probable that photogenerated electrons over Aux clusters could be captured by Au (I) component on the outer layer of Aux clusters (i.e., Au(0)Au(I)@GSH) to produce Au NPs. Second, besides the photogenerated electrons from Aux clusters, electrons from bandgap excitation of TiO2 (i.e., NP-TNTAs) under simulated solar light irradiation may also facilitate the reduction of Au (I) to Au (0) based on their intimate interfacial contact. Third, GSH ligands of Aux clusters tethered on the surface may be photocatalytically removed or decomposed by various in situ formed active species including hydroxyl radicals, superoxide radicals, and holes in situ produced in the reaction system during the photocatalytic process. Notably, although photogenerated holes were not quenched by a hole scavenger during the transformation from Aux clusters to Au nanoparticles in our current reaction system, they may directly decompose GSH ligand on account of their high oxidation capability or react with water to produce highly active hydroxyl radicals, which results in removal of GSH on the clusters surface. Meanwhile, some photogenerated electrons may also react with dissolved oxygen to produce superoxide radicals apart from directly participating in the reduction of Au (I) component on the outer layer, which further contributes to complete oxidation of GSH on the clusters surface.21−24 In this way, Aux clusters without the protection of surface linkers are prone to gradually merge together resulting in larger metallic Au NPs. With regard to photocatalytic mechanism of intermediate Aux/Au/NP-TNTAs ternary heterostructures under visible light irradiation, as shown in Figure S13, photogenerated electron−hole pairs are first produced over Aux clusters. Instantly, photogenerated holes on the HOMO of Aux clusters are rapidly quenched by hole scavenger (e.g., ammonium formate), and, simultaneously, photoexcited electrons on the LUMO of Aux clusters flow into the CB of TiO2 and react with aromatic nitro compound (e.g., 4-NA) adsorbed on the surface. Meanwhile, it is worth mentioning that SPR-induced hot electrons from Au NPs in situ transformed from Aux clusters additionally boosts the density of photogenerated electrons in the reaction system, thus giving rise to greatly enhanced photoreduction performances of ternary nanostructures in comparison with binary counterparts. Besides, synergistic interaction of Aux clusters and Au NPs, in which Au NPs play pivotal roles as electron relay mediator and plasmonic sensitizer for Aux clusters concurrently contribute to substantially improved photoactivities of Aux/Au/NP-TNTAs ternary nanocomposites.15 In summary, in situ transformation of Aux clusters to Au NPs under solar light irradiation was successfully demonstrated in Aux/NP-TNTAs binary system, which was further judiciously harnessed as a novel synthetic approach to rationally construct well-defined Aux/Au/NP-TNTAs ternary or Au/NP-TNTAs binary heterostructure. The synergistic interaction of Aux clusters and in situ formed Au NPs contribute to enhanced photocatalytic performances. It is hoped that our current work could offer a general methodology for rational design of a large variety of metal/semiconductor and metal cluster/metal/ semiconductor nanostructures for a diverse range of photocatalytic and PEC applications.
Figure 3. (a) Photocatalytic performances of blank NP-TNTAs, pristine Aux/NP-TNTAs, intermediate Aux/Au/NP-TNTAs ternary, and Au/NP-TNTAs binary nanostructures toward reduction of 4-NA under visible light irradiation (λ > 420 nm) by adding ammonium formate as scavenger for holes and N2 purge at ambient conditions. (b) Photocatalytic performances of Aux/Au/NP-TNTAs with and without the addition of K2S2O8 as scavenger for photogenerated electrons under visible light irradiation (λ > 420 nm).
electrons emphasize the pivotal role of electrons played in triggering the photocatalytic reactions. Furthermore, as shown in Figure S11, photostability of Aux/Au/NP-TNTAs ternary nanostructure under cycle experiments was probed and the result demonstrates decreased photoactivity of Aux/Au/NPTNTAs with reaction cycle, which can be ascribed primarily to the gradual intrinsic transformation of Aux clusters to Au NPs under continuous light irradiation. Photoelectrochemical (PEC) measurements were carried out to explore the reasons for the improved photocatalytic activity of Aux/Au/NP-TNTAs ternary heterostructure. Figure S12 shows the on/off transient photocurrent responses of different samples under intermittent visible light irradiation (λ > 420 nm). It is obvious to see that assembly of Aux clusters onto NPTNTAs is beneficial for significantly improving visible-lightdriven photocurrent of NP-TNTAs by virtue of the substantial photosensitization effect of Aux clusters. More importantly, if Aux clusters are partially transformed to Au NPs under simulated solar light irradiation, the Aux/Au/NP-TNTAs ternary nanostructure exhibits the optimal photocurrent, which agrees well with the photocatalytic performance trend, thereby verifying the synergistic contributing roles from both Aux clusters and Au NPs. The mechanism responsible for the transformation of Aux clusters to Au NPs under simulated solar light irradiation could be attributable to the following three reasons. First, various electron transitions from the occupied levels to the unoccupied levels in Aux clusters under light irradiation impart Aux cluster D
DOI: 10.1021/acsami.5b09091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09091. Experimental details, characterizations of Aux clusters and blank NP-TNTAs, experimental setup and photographs of different samples, FESEM images of Aux/NPTNTAs after irradiation for different time, TEM images of different samples, DRS and XRD results of Au/NPTNTAs, EDX results of Aux/NP-TNTAs before and after light irradiation, blank experiments for photocatalytic reaction, cycle experiments for photocatalytic reaction over Aux/Au/NP-TNTAs, transient photocurrent responses of different samples, schematic illustration of photocatalytic mechanism over Aux/Au/ NP-TNTAs (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Nanyang Technological University startup grant: M4080977.120, Singapore Ministry of Education Academic Research Fund (AcRF) Tier 1: M4011021.120 and Public Sector Funding from Agency for Science, Technology and Research of Singapore (A*Star): M4070232.120.
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DOI: 10.1021/acsami.5b09091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
