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Inside-Out: The Role of Buried Interfaces in Hybrid CuZnSnS–Noble Metal Photo-Catalysts 2

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Anal Kr Ganai, Pradipta Sankar Maiti, Lothar Houben, Ronen Bar-Ziv, and Maya Bar Sadan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01733 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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The Journal of Physical Chemistry

Inside-Out: The Role of Buried Interfaces in Hybrid Cu2ZnSnS4–Noble Metal Photo-Catalysts Anal Ganai a, Pradipta Sankar Maiti a, Lothar Houbenb, Ronen Bar-Ziva,c and Maya Bar Sadan a,* a

Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 84105,

Israel. b

Department of Chemical Research Support, Weizmann Institute of Science, Rehovot

76100, c

Nuclear Research Center Negev, Beer-Sheva 84190, Israel.

AUTHOR INFORMATION Corresponding Author *Maya Bar Sadan [email protected]

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ABSTRACT Metal–semiconductor hybrids are a promising architecture for functional nanostructures because they efficiently promote charge separation. The morphology of the hybrid supports two mechanisms of charge generation and transfer, namely, the excitation of electrons to the conduction band of the semiconductor or the induction of surface plasmon resonance on the metal. Here, we compared the photocatalytic activity of nanoparticles with a core–shell or a dimer morphology, using Pt, Pd, or Au as the metal and Cu2ZnSnS4 (CZTS), which comprises abundant and environmental-friendly elements, as the semiconductor. Their performance as photocatalysts was evaluated by using Methylene Blue (MB) degradation under light irradiation. We found that, although large Au cores improved the photocatalytic activity of the CZTS nanoparticles, the highest catalytic activity was that of a Pt–CZTS and Pd–CZTS dimers. Conversely, using small metal particles as cores degraded the activity of the CZTS due to the formation of an internal boundary and the occupation of potentially optically active volume. In addition, the results point out that depositing multiple metal particles is not beneficial for photocatalysis.


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1. Introduction Photoinduced charge generation and charge transfer are fundamental phenomena, which are extensively studied and commonly exploited for solar-driven applications. One specific combination of materials, which is used for photo-induced charge generation and transfer has received much attention recently, is hybrid nanostructured metal– semiconductor heterojunctions.1-3 Although each of the components of the hybrid may be excited by an appropriate radiation, the interface between them produces additional synergistic properties that allow controlling the nature of charge generation, charge separation, and even the direction of charge transfer.4 The synergistic properties of metal– semiconductor hybrids can be tuned by controlling their size and morphology. Such tuning is important for applications that rely on charge generation and charge separation to

retard

recombination,

making

the

metal–semiconductor

hybrids

appealing

photocatalysts. Upon excitation, the charge transfer mechanism in metal–semiconductor photocatalysis occurs through two major pathways (Figure 1).1-2 The first pathway is the transfer of photoinduced charges from the conduction band of the semiconductor to the Fermi level of the metal, which improves charge separation.5 Spatial separation is obtained due to the distinctive semiconductor–metal interface, where the Fermi level of the metal component is located within the band gap of the semiconductor valence and conduction band levels. The presence of the metal provides an electron sink to promote the transfer of the excited electrons from the semiconductor conduction band into the metal energy levels.6 In this case, the metal is mounted onto the semiconductor and is exposed to the solution to drive the redox chemical reactions with the substrate, while the hole remains confined to the 3 ACS Paragon Plus Environment


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semiconductor component. Previous research has shown that noble d-transition metals nanoparticles, such as Pt or Pd, usually outperform sp-metals such as Au and Ag.7-8 However, this is not a general rule, and in some specific systems, small metal particles of Au, Pd and Pt on a semiconductor support show a deviation from this tendency.9-10 The second charge separation pathway can be considered a “plasmon-induced” pathway, as it involves a coupling interaction between the surface plasmons of the metal nanoparticle and the excitons of the semiconductor, which results from the excitonic dipole moment generated due to the local electromagnetic modes of the plasmons.11-12 In this case, the metal particle may be either embedded within the semiconductor or mounted onto it. Metals such as Au and Ag are preferred over other metals due to their stronger plasmon resonances – which are conveniently located within the intense range of the solar spectrum and are thus aligned with the bandgap of the semiconductor. The plasmon-induced mechanism has not been thoroughly characterized, and further research is required to understand it in depth.13 In practice, there are cases where both pathways are plausible, when both semiconductor and the metal are excited by the emission spectra of the source.14


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Figure 1. Schematic representation of the electron transfer mechanisms in metal– semiconductor hybrid photocatalysts. In Pathway I, the photo-excited electrons are transferred from the conduction band of the semiconductor to the metal, depending on the work function of metal. In Pathway II, the coupling between the conduction band and the plasmonic metals can excite charge carriers in semiconductors through plasmon-induced resonance energy transfer (PIRET) and hot electron injection process.14 Hybrid metal–semiconductor particles have been synthesized extensively and numerous variations have been examined. The architecture of the hybrids usually comprises either a metal core with a semiconductor shell (e.g., Au@Cu2S or FePt@CdSe15-16) or a dimer, in which the semiconductor is mounted with a metal tip (e.g., Au/Pt/Pd–tipped CdSe@CdS, Au–ZnO, Au–Bi2S3, Au–Cu2ZnSnS4, or Pt–CuInS2 7-8, 17-21. The proper combination of the two components, semiconductor-metal, allows tailoring of the band alignment of the system and therefore its potential to match the reduction potential of the desired redox reaction.14 In the core–shell structure, the metal particles are not directly exposed to the solution and, therefore, the photocatalytic process is mainly driven by the plasmoninduced mechanism. Thus, in this architecture the desired redox reaction, e.g. H+/H2, occurs at the interface between the semiconductor and the solution. In the other morphology, when the metals are deposited onto the semiconductor particles, it is generally assumed that the reduction reaction occurs at the surface of the metal due to the accumulation of the negative charges and the favorable binding energies of hydrogen to the metal, i.e. lower over-potential. Although different material combinations were reported, to date, no direct comparison was performed between the different 5 ACS Paragon Plus Environment


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morphologies of metal–semiconductor junctions with plasmonic or non-plasmonic metals (under visible light range). Specifically, the role of the buried internal interface between the core and the shell was not discussed. In the current study, we prepared the two architectures of a metal–semiconductor hybrid: a metal core with a semiconductor shell, and a dimer. We chose to use Pd and Pt, which are known as efficient co-catalysts and exhibit low overpotentials but poor plasmonic resonance in the visible, and compared their photocatalytic activity (in degrading methylene blue) with that of Au, which is a good plasmonic metal but usually has a lower catalytic activity. As the semiconductor, we chose to use copper zinc tin sulfide (CZTS) – a multinary material, based on the wurtzite structure of ZnS, in which some of the Zn sites are substituted by Cu and Sn. CZTS absorbs in the spectral solar window (Eg= 1.40 eV22) and is non-toxic and of low cost, making it is an emerging material for use in various photovoltaic applications.23-26 Very recently, quaternary CZTS has been coupled with Au nanoparticles in a core–shell or in a dimer architecture,20, 27 and demonstrated potential for the photodegradation of pollutants and photocatalytic generation of hydrogen.28-29

2. Experimental Metal–CZTS core–shell and dimer nanostructures were prepared using a method based on the procedure published previously,20, 28-29 with slight modifications (see Supporting Information for detailed reaction protocols). Pre-formed Au, Pd, and Pt metal nanoparticles were used as seeds to form both the core–shell and the dimer structures. Different reducing agents were used for the Au nanoparticles synthesis in order to prepare either Au@CZTS core-shell or Au-CZTS dimers. For the synthesis of core–shell and 6 ACS Paragon Plus Environment

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dimer architectures with Pt or Pd, the non-coordinating solvent 1-octadecene (1-ODE) was used to form Pd@CZTS and Pt@CZTS core–shell particles, whereas oleylamine was used to form Pd–CZTS and Pt–CZTS dimers. For the photocatalytic dye degradation experiments, 1 mg of nanoparticles was dispersed in 10 ml of de-ionized water, containing 10 mg/L methylene blue at pH 11. The solution was stirred in a glass vial in dark for 30 mins at room temperature to achieve the adsorption-desorption equilibrium. A 300 W Xe lamp (Newport) was used to irradiate the sample (under continuous stirring in glass vial open to air). At a given time interval of irradiation, aliquots were withdrawn from the suspension, and all the catalyst was removed. The concentration of the remnant dye was spectrophotometrically monitored by optical absorption at 660 nm of UV–vis spectra, using UV-Vis spectrophotometer during the photodegradation process. The suspension was centrifuged for 2 min to remove the photocatalysts before measurement. The photocatalytic activity was studied by the time profiles of C0/C, where C is the concentration of methylene blue at the irradiation time t and C0 is the concentration just after the absorption equilibrium before irradiation, respectively. The final degradation efficiency of MB was calculated by the formula: Degradation (%) = (C0- C)/C0 × 100, where C0 represents the original concentration of MB and C represents the concertation of MB at a given time t. Blank measurements of dye solution without the catalyst, irradiated at the same conditions, were taken. The results showed a degradation of less than

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