Scholarly article on topic 'Targeting low-cost type-II heterostructures: Synthesis, structure and photoreactivity'

Targeting low-cost type-II heterostructures: Synthesis, structure and photoreactivity Academic research paper on "Nano-technology"

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{"SnO2 nanosphere" / "PbS nanocube" / "Rietveld refinement" / Heterostructure / Photochemistry}

Abstract of research paper on Nano-technology, author of scientific article — Arik Kar, Sumanta Sain, David Rossouw, Benjamin R. Knappett, Swapan Kumar Pradhan, et al.

Abstract One design strategy by which to iterate the photocatalytic efficiency of semiconducting nanomaterials for harvesting solar energy involves the synthesis of type-II heterostructured materials. In this article, a straightforward, facile and environmentally benign route to heterostructures in which SnO2 nanospheres are capped by PbS nanocubes is reported. It offers to address current limitations to photocatalytic efficiency brought about by electron-hole recombination and narrow photoresponsive ranges in many existing systems. PbS nanocubes are grown in the presence of preformed SnO2 nanospheres by functionalizing the surface of the latter using cetyltrimethylammonium bromide (CTAB). Heterostructure formation is confirmed by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area analysis, and transmission electron microscopy (TEM) analysis. Rietveld refinement has been exploited to simultaneously elucidate the atomic and microstructures of these materials, allowing the quantitative determination of particle structure and stability. The combination of narrow band-gap semiconductor (PbS) and wide band-gap semiconductor (SnO2) endows the heterostructured nanomaterial with potential as a photocatalyst and, in the degradation of Rhodamine B (RhB) dye under solar simulation, it showed superior photocatalytic activity to that of its separate SnO2 and PbS components. A strong type-II interaction is demonstrated by the heterostructure and a charge separation mechanism has been utilized to clarify this behaviour.

Academic research paper on topic "Targeting low-cost type-II heterostructures: Synthesis, structure and photoreactivity"

Accepted Manuscript

Targeting low-cost type-II heterostructures: Synthesis, structure and photoreactivity

Arik Kar, Sumanta Sain, David Rossouw, Benjamin R. Knappett, Swapan Kumar Pradhan, Gianluigi A. Botton, Andrew E.H. Wheatley

PII: S0925-8388(16)34085-3

DOI: 10.1016/j.jallcom.2016.12.167

Reference: JALCOM 40089

To appear in: Journal of Alloys and Compounds

Received Date: 10 June 2016 Revised Date: 2 December 2016 Accepted Date: 14 December 2016

Please cite this article as: A. Kar, S. Sain, D. Rossouw, B.R. Knappett, S.K. Pradhan, G.A. Botton, A.E.H. Wheatley, Targeting low-cost type-II heterostructures: Synthesis, structure and photoreactivity, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2016.12.167.

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Targeting low-cost type-II heterostructures: synthesis, structure and photoreactivity

Arik Kar a'*, Sumanta Sain b, David Rossouw c, Benjamin R. Knappett a, Swapan Kumar Pradhan b, Gianluigi A. Botton c, Andrew E. H. Wheatley a'*

a Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW,

b Materials Science Division, Department of Physics, The University of Burdwan, Golapbag,

Burdwan, West Bengal-713 104, India. c Department of Materials Science and Engineering, Brockhouse Institute for Materials Research and Canadian Centre for Electron Microscopy, McMaster University, Hamilton, Ontario,

L8S 4L8, Canada.

*Author to whom correspondence should be addressed; electronic mail: ak899@cam.ac.uk, Tel.: 01223 336335

ABSTRACT

One design strategy by which to iterate the photocatalytic efficiency of semiconducting nanomaterials for harvesting solar energy involves the synthesis of type-II heterostructured materials. In this article, a straightforward, facile and environmentally benign route to heterostructures in which SnO2 nanospheres are capped by PbS nanocubes is reported. It offers to address current limitations to photocatalytic efficiency brought about by electron-hole recombination and narrow photoresponsive ranges in many existing systems. PbS nanocubes are grown in the presence of preformed SnO2 nanospheres by functionalizing the surface of the latter using cetyltrimethylammonium bromide (CTAB). Heterostructure formation is confirmed by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area analysis, and transmission electron microscopy (TEM) analysis. Rietveld refinement has been exploited to simultaneously elucidate the atomic and microstructures of these materials, allowing the quantitative determination of particle structure and stability. The combination of narrow band-gap semiconductor (PbS) and wide band-gap semiconductor (SnO2) endows the heterostructured nanomaterial with potential as a photocatalyst and, in the degradation of Rhodamine B (RhB) dye under solar simulation, it showed superior photocatalytic activity to that of its separate SnO2 and PbS components. A strong type-II interaction is demonstrated by the heterostructure and a charge separation mechanism has been utilized to clarify this behaviour.

Keywords: SnO2 nanosphere; PbS nanocube; Rietveld refinement; heterostructure; photochemistry

1. Introduction

The use of more than 8000 types of dye in a range of industries is long established and applications continue to develop [1]. For example, dyes have lately achieved prominence as metal-free catalysts for transformations achieved in continuous microflow [2]. However, in spite of their immense importance, dyes suffer from significant toxicological and environmental drawbacks [1,3]. One response has been the direction of efforts towards their chemical passivation, with photocatalysis gaining traction as a 'green' technique with enormous potential for achieving this [4]. Semiconducting metal oxides can degrade a variety of dyes under UV-Vis light irradiation and to this end many types of such oxide have been developed [5]. Nevertheless, enhancing the photocatalytic effectiveness of such systems remains a considerable challenge on account of poor quantum yields caused by the fast recombination of photogenerated electrons and holes [6].

Recent work has shown that the heterostructuring of semiconductors represents one route by which to improve their photochemical properties. This has led to new materials based on oxideoxide [7,8,9,10,11,12] and oxide-sulfide [13,14,15] combinations. However, while such recently reported systems have offered improved photocatalytic capabilities they have also exhibited significant limitations. These have included (i) the agglomeration of core nanoparticles, (ii) preferential formation of separate sets of individually monometallic particles, (iii) incomplete coverage of the core by the second semiconductor, (iv) inappropriate charge separation, and/or (v) still only moderate photoresponsive ranges. One promising avenue by which to potentially overcome these problems involves the fabrication of SnO2-based core@shell heterostructures. These promise to use a commercially appealing oxides (core) in conjunction with a second semiconductor (shell) to raise photocatalytic efficiency by increasing charge separation and extending the photoresponsive range of the catalyst into the visible region of the electromagnetic spectrum. However, a survey of the limited literature [15,16] in the field shows that the goal of fabricating compelling SnO2-based core@shell heterostructures remains elusive. For example, in terms of oxide-sulfide combinations, SnO2-based heterostructures containing PbS are, to the best of our knowledge, unreported. We have therefore sought to develop a SnO2@PbS system. The desirability of such a combination is based upon there being a large band gap difference (approximately 3.2 eV) between the two materials. This offers increased charge separation and promises enhanced electrical, optical, electrochemical, photocatalytic and gas sensing properties.

It also promises a type-II system that exhibits charge separation that is improved relative to that seen in either component individually. This would dervive from the heterojunction between SnO2 and PbS hampering the recombination of photoinduced charge carriers [17].

This article examines the formation of a type-II system in which preformed SnO2 supports PbS. A simple solution approach is used. It involves only relatively inexpensive substrates [18] under facile conditions to make SnO2 nanospheres (NSs) which are then coated with PbS nanocubes (NCs; Figure 1). The structure, morphology and electronic properties of the heterostructured product are comprehensively elucidated by X-ray diffraction (XRD)/Rietveld refinement, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area analysis, and transmission electron microscopy (TEM) analysis. Rietveld refinement has been employed to probe lattice strain, lattice parameters and relative phase abundances. Detailed atomic structure modeling has also been undertaken for each phase. The photocalalytic activity of an as-synthesized heterostructured material is estimated in term of the degradation of Rhodamine B dye. The effects of crystallinity and specific surface area on the photocatalytic activity of the new heterostructured material are discussed. An existing mechanistic proposal [19] is able to explain the photocatalytic behavior observed.

«Figure 1 near here>>

2. Experimental

2.1. Materials and preparation

General synthetic and analytical details

Chemicals were obtained from Sigma-Aldrich (analytical grade reagents, HPLC grade solvents) and used without further purification. Aqueous solutions used HPLC water (Millipore). PbS NCs were prepared using a literature method [19].

SnO2 nanosphere synthesis SnCl45H2O (0.351 g, 1.0 mmol) was dissolved in water (25 ml) and a solution of cetyltrimethylammonium bromide (CTAB, 0.365 g, 1.0 mmol) in water (5 ml) was added. The mixture was stirred for 1 hr to give a colourless solution. NaOH (0.8 g, 20.0 mmol) was dissolved in water (10 ml) and 10 ml of EtOH was added. This solution was added dropwise to the stirred SnCl45H2O solution to give a white precipitate, which dissolved once a pH of 11-12

was reached. The solution was heated in a microwave reactor (CEM, Discover, operating at 300 W) at 110°C for 5 min. to give a white colloidal suspension. Centrifugation and washing with water (2x25 ml) and absolute EtOH (3x25 ml) before drying at 60 °C in air gave SnO2 NSs.

SnO2 nanosphere/PbS nanocube heterostructure synthesis A soft chemical process [20] was used in which SnO2 NSs (0.015 g, 0.1 mmol) [21] were dispersed in water (25 ml) by ultrasonication. CTAB (0.036 g, 0.1 mmol) was added with stirring. The resulting dispersion was slowly heated to 50 °C, kept at this temperature for 2 hrs. It was then left to cool to room temperature, during which time flocculation occurred. A 5 ml aqueous solution of Na2S 9H2O (0.024 g, 0.1 mmol) was added dropwise and the resulting mixture was stirred for 30 min. Finally, a 5 ml aqueous solution of Pb(NO3)2 (0.033 g, 0.1 mmol) was added dropwise and the mixture stirred for 2 hrs. at room temperature. The product was obtained by centrifugation. Washing with water (2x25 ml) and absolute EtOH (3x25 ml) before vacuum drying for 6 hrs. gave a grey powder. See Figure 1.

2.2. Characterization

X-ray Diffraction (XRD)

XRD profiles were recorded using Ni-filtered CuKa radiation from a highly stabilized and automated PAN-analytical X-ray generator operated at 40kV and 40mA. The X-ray generator was coupled with a PW3071/60 bracket goniometer for sample mounting. Step-scan data (step size 0.02° 29, counting time 2 s/step) were recorded for 20°-75° 29.

Rietveld analysis

Rietveld refinement [22] utilized MAUD [23] to refine (micro)structural parameters using an analytical function, the intensity of which was directly related to the structure of each phase [24]. XRD peaks were quite broad and not perfectly Gaussian, leading to the use of pseudo Voigt (pV) as a profile function [25,26] that accounted for broadening due both to particle size and strain. Shape parameters, e.g. full-width-at-half-maximum values (FWHM) and the Gaussianity for true line broadening profile functions, were generated by refining crystallite size and microstrain values. The background was simulated using a four-degree polynomial. This procedure has previously been shown to be appropriate to systems of the type analyzed here [27,28].

Transmission Electron Microscopy (TEM)

A Technai 20 TEM was used to screen nanoparticle size and morphology. Data were analyzed using Digital Micrograph 3.6.5. Both high-angle annular dark field (HAADF) imaging and energy-dispersive X-ray (EDX) techniques were utilized to characterize individual heterostructures. High-resolution (HR) electron microscopy was performed using a Titan 80-300 TEM. Sample preparation was by droplet coating of ethanolic suspensions on carbon-coated Cu grids (Agar Scientific, 300 mesh).

Raman spectroscopy

Data were collected at room temperature on a Thermo Scientific DXR Raman microscope using a helium-neon laser with an excitation wavelength of 532 nm (laser power 10 mW).

X-ray photoelectron spectroscopy (XPS) XPS measurements were obtained using a Thermo Scientific K-Alpha system. Binding energy was calibrated internally with respect to the C1s line.

UV-Vis spectroscopy

UV-Visible absorption spectra were measured in ethanolic solution on a UV-vis spectrophotometer (Perkin Elmer LAMBDA 265).

Brunauer-Emmett-Teller (BET) surface area analysis BET surface areas were analyzed by nitrogen adsorption in a Tristar 3000 analyzer. Samples were degassed at 77 K before measurements were commenced. Surface areas were determined by a multipoint BET method using the adsorption data in the P/P0 range 0.0-1.0 (where P and P0 are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption).

Photocatalytic measurements

The room temperature degradation of aqueous Rhodamine B (RhB) under simulated solar irradiation was studied without using sacrificial reagents. Typically, 5.0 mg of catalyst was added to 25 ml of a 1.0 x 10 5 M aqueous solution of RhB (pH 7), and the mixture stirred in the dark for 30 min. to allow dye adsorption to the catalyst surface [29]. A 3.0 ml aliquot of the mixture was centrifuged and the absorption of the dye therein determined to obtain dye concentration before photocatalysis (C0). The remaining solution was irradiated with a 300 W xenon lamp (Solar Simulator model LSO306, AM 1.5 G filter, 1 sun illumination at 100 mW cm ). Degradation of the RhB absorption maximum at 555 nm was monitored using a UV-vis spectrophotometer (Perkin Elmer LAMBDA 265) to obtain the concentration (C) of dye as a function of time in subsequent aliquots. Use of an ice bath prevented evaporation during

catalysis and avoided degradation due to heating. Degradation and apparent quantum yield (*pX) were calculated according to:

where d[x]/dt is the rate of change of the concentration of RhB and d[hv]inc/dt is the total optical power impinging on the sample [30].

Hydroxyl radicals (OH*) produced during photocatalysis were estimated by fluorescence spectroscopy using terephthalic acid (TA) as a probe molecule. Catalyst (5.0 mg) was dispersed in an aqueous solution of 30 ml of TA (5 x 10-4 M) and NaOH (2 x 10-3 M). The resulting suspension was exposed to simulated solar radiation and at regular intervals 3.0 ml of the suspension was collected and centrifuged. The maximum fluorescence emission intensity of the supernatant was measured with an excitation wavelength of 315 nm. This method relies on the fluorescence at 425 nm of 2-hydroxyterephthalic acid (TAOH) and was undertaken using an Edinburgh Instruments FLS980 photoluminescence spectrometer.

3. Results and discussion

XRD phase identification

Observed and Rietveld simulated XRD patterns for SnO2 NSs are shown in Figure 2a. Peaks are unambiguously identified and indexed using reflections from tetragonal SnO2 (ICSD 154960, space group: P42/mnm, a = 4.7331A, c = 3.1815A). Signal breadth implied very small particle sizes and significant lattice strain. Rietveld refinement yielded mean particle sizes (MPSs) for individual nanocrystallites by considering the line broadening of reflections across the entire angular range studied. Results suggested a mean particle size of 2.19 nm and a r.m.s lattice strain of 1.2 x 10-3 (Table 1). Analysis of oxygen atom occupancy in the SnO2 lattice revealed oxygen deficiencies in the unit cell consistent with the recently recorded phenomenon of SnO2 nanocrystals agglomerating to form spheres [28].

Figure 2b shows the observed and calculated XRD patterns for cubic PbS (ICSD 62190, space group Fm-3m, a = 5.9143A). Broad signals again suggest particle size in the nanometer regime, though the peak broadening is now non-uniform and the FWHM values differ with (hkl)

Degradation (%) = (1 - C/C0) x 100

(px = ±(d[x]/ dt)/d[hv] inc/dt

values. This points to the presence of anisotropy in the distribution of crystallite size and r.m.s strain. Analysis suggests that the crystallite size along the <111> direction is the smallest (~4.92 nm) and that the value of the corresponding r.m.s lattice strain is 1.39 x 10-2, whereas particle size is largest value along <200> direction with a r.m.s lattice strain value of 1.36 x 10- . The r.m.s lattice strain in the cubic PbS lattice in all crystallographic directions is greater than that seen above for SnO2 [19].

Figure 2c shows the observed and simulated XRD patterns for heterostructured SnO2/PbS (see below for microscopic corroboration of this morphology). Data comprise reflections attributable to both tetragonal SnO2 and cubic PbS phases with the (110), (211), (112) and (301) peaks of tetragonal SnO2 overlapping with those of the (111), (311), (222) and (400) planes of cubic PbS. Analysis of such severely overlapping reflections necessitates use of the Rietveld whole profile fitting method [22]. Using this, the volume percentages of tetragonal SnO2 phase and cubic PbS phase were found to be 61.42 and 38.58 %, respectively. Calculated values of the lattice parameters of the core in heterostructured SnO2/PbS are a = 4.8937 A and c =

3.0993 A.

These values show an increase and a decrease, respectively, in relation to the corresponding values for bulk SnO2 (ICSD 154960). These data gave unit cell volumes for SnO2 of 75.343 A3 and 74.223 A3

in the SnO2 NSs and heterostructured SnO2/PbS, respectively. Both size of the primary SnO2 nanocrystallites and r.m.s strain in the heterostructured material were calculated to be higher than those for pure SnO2 NSs (Tables 1 and 2), and this is attributed to coverage of the core by PbS in the heterostructure [31]. In our earlier report of a homogeneous rather than a heterogeneous system [19], the most intense (200) reflection of both PbS and of nanocomposite SnO2/PbS was perfectly simulated by considering the pV analytical function from the cubic PbS lattice. However, in the present case the peak shape of the (200) reflection of cubic PbS in heterostructured SnO2/PbS differs from that of the pure PbS phase in Figure 2b. Deconvolution (Figure 3) suggests that this has its origins in the presence in heterostructured SnO2/PbS of two PbS polymorphs with marginally different interplanar distances and lattice constants (a = 5.4621 A and 5.9894 A in PbS(1) and PbS(2), respectively, such that apbS(2)>apbS>apbS(1)). Similarly, whereas the (200) reflection is the most intense reflection in cubic PbS, its relative intensity is smaller in the heterostructured material. This is attributable to overlap of the (110) reflection of tetragonal SnO2 with the (111) reflection of cubic PbS in heterostructured SnO2/PbS. The ratio of volume fractions of PbS(1) and PbS(2) is found to be 3:4. Like cubic PbS in Figure 2b, both

PbS(1) and PbS(2) phases exhibit anisotropy in particle size and r.m.s lattice strain. The values of particle size and r.m.s lattice strain are again found to be maximal along the <111> direction for both PbS(1) and PbS(2) phases (Table 2).

<<Figures 2 and 3 and Tables 1 and 2 near here>>

To understand the changes in the atomic arrangement in the crystal lattice of nanospherical SnO2 and cubic PbS in both pristine form and in heterostuctured SnO2/PbS, the atomic modeling of tetragonal SnO2 and cubic PbS phases was carried out. Analysis used refined structural parameters obtained from the Rietveld analysis.

Atomic modeling

To understand changes in the atomic arrangement in the crystal lattices of both pristine SnO2 NSs and PbS NCs and in heterostuctured SnO2/PbS, the atomic modeling of tetragonal SnO2 and cubic PbS phases was undertaken using refined structural parameters obtained from the Rietveld analysis. The crystal structure of a SnO2 NS is shown in Figure 4a. The refined fractional coordinates of the atomic sites of O atoms are (0.306, 0.305, 0), with the Sn atoms occupying voids at the centres of oxygen octahedra. The tetragonal unit cell of nanospherical SnO2 is shown in Figure 4b while Figure 4c shows the atomic arrangement of Sn and O atoms in more detail; Sn-O = 2.05 A, Sn- ■ ■ Sn = 3.18 A, O-Sn-O (in the equatorial plane) = 78.3° and 101.7°. These last data indicate that the octahedra formed are irregular. Similar data for the SnO2 octahedra in heterostructured SnO2/PbS (Figure 4d) demonstrate equatorial Sn-O bond lengths that remain unchanged with respect to those in the uncoated SnO2 NSs. In contrast, the axial Sn-O bonds increase from 2.05 A to 2.12 A along the a-axis. Lastly, the O-Sn-O bond angles in the equatorial plane have also changed on account of formation of the heterostructure.

The cubic PbS unit cell is shown in Figure 5a. The atomic coordinates of Pb and S atoms in the PbS6 octahedra are (0.5, 0.5, 0.5) and (0, 0, 0), respectively. These octahedra in the cube-shaped PbS nanostructure reveal Pb-S = 2.96 A (Figure 5b). Moving to SnO2/PbS, the Pb-S bonds in PbS(1) decrease to 2.73 A, resulting in a decreased a. The reverse phenomenon is observed in PbS(2), with the Pb-S bonds expanding equally to 2.99 A to increase a.

<<Figures 4 and 5 near here>>

TEM analysis

HRTEM images of selected elongated PbS NCs are shown in Figure 6. The inter-planar spacing of the resolved lattice fringes in the representative NCs shown was measured to be 3.3±0.2 A. This spacing is within experimental error of that expected for the {111} crystallographic planes of PbS (3.41 A). Assuming the long axes of the NCs lie in the imaging plane, the particles are imaged close to their <110> zone axes and the growth of both NCs has occurred in the <001> direction, this assignment is in excellent agreement with the XRD analysis reported above. Figure 7a-d shows representative TEM images of SnO2 NSs (a/b) and SnO2/PbS heterostructures (c/d). Figure 7a confirms the formation of a narrow SnO2 NS size distribution with a mean sphere diameter of approximately 180 nm (Figure 7b). Figure 7c corroborates the deposition of PbS NCs on the surface of the preformed SnO2 NSs to yield heterostructures typically 250 nm in diameter (Figure 7d). While Figure 7c/d indicate near complete uptake of PbS by the SnO2 NSs, a low level of remnant isolated PbS NCs can be seen. Interestingly, a survey of particle sizes reveals a bimodal distribution. The isolated PbS NCs in Fig. 7c reveal a monocrystalline habit and a MPS of 24.0±2.5 nm (Figure 7e), which corresponds with the MPS previously documented in the Sn-free synthesis of pure PbS nanocubes using Na2S9H2O and Pb(NO3)2 [19]. In contrast, PbS NCs that are attached to SnO2 NSs appear significantly larger, demonstrating a MPS which, at 39.5±2.0 nm (Figure 7f), reinforces the Rietveld analysis of PbS phase(s) in heterostructured SnO2/PbS (Table 2). This TEM analysis of either type of PbS NC suggests that in this system PbS NC formation can occur independently of coexisting SnO2 NSs. The observation of larger PbS NCs coating the NSs is consistent with the limited agglomeration of nanocrystalline PbS NCs at the SnO2 NS surface, though the formation of a bimodal distribution of monocrystalline PbS NCs of which the larger show a greater tendency for nucleation or deposition on SnO2 cannot be ruled out. Additional TEM data for heterostructured SnO2/PbS is provided in ESI Figure S1.^

Figures 8a and b display HAADF-STEM images of representative heterostructure clusters in which aggregates of small NCs appear to decorate the surface of larger spherical particles to varying degrees. The so called 'Z-contrast' inherent to the HAADF-STEM imaging technique suggests that the NCs and larger spherical particles are uniform in composition. EDX analysis reinforces this view by confirming the presence of lead and sulfur in the smaller NCs (Figure 8c). Carbon, oxygen, silicon and copper X-rays are also detected but likely arise from the thin

amorphous carbon membrane, a thin oxide layer, internal fluorescence in the silicon detector and the copper grid bars of the sample grid, respectively. The EDX spectrum obtained from a large spherical particle confirms the presence of tin and oxygen. Lead and sulfur X-rays are also detected from decorating PbS nanoparticles in addition to the extraneous X-rays aforementioned. Elemental mapping data from the HAADF-STEM image of a SnO2/PbS heterostructure is provided in ESI Figure S2.^

Raman spectroscopy

Figure 9a shows the room temperature Raman spectrum of SnO2 NSs. The shoulder at 486 cm-1 corresponds to the Eg mode and is consistent with rutile bulk SnO2. Meanwhile, the 573 cm-1 absorption is a surface defect mode [32] mostly due to surface oxygen vacancies [33]. It is large compared to the Eg mode and this is consistent with the large surface area of the small primary nanocrystallites identified as comprising the sphere (see above). Figure 9b shows the Raman spectrum of heterostructured SnO2/PbS. It contains bands at 429 cm-1, 602 cm-1 and 967 cm-1. These are attributable to 2LO, 3LO and the laser-induced degradation of PbS (see ref. 19 for data on PbS). Notably, the 573 cm-1 SnO2 surface defect mode is no longer present, consistent with modification of the SnO2 NS surface by PbS.

XPS analysis

Figure 10a shows peaks at 485.7 and 494.1 eV corresponding to Sn3d5/2 and Sn3d3/2, respectively [34]. These are attributable to Sn(IV) (indexed Standard ESCA Spectra of the Elements and Line Energy Information, $Co., USA) [35] and are consistent with SnO2 [36]. The characteristic O1s peak at 529.6 eV (Figure 10b) is attributed to the lattice oxygen in SnO2. XPS data for heterostructured SnO2/PbS are shown in Figure 11. Figure 11a demonstrates the high resolution spectrum for the Sn3d region and reveals symmetric signals attributable to the Sn3d5/2 and Sn3d3/2 binding energies of Sn(IV). An O1s peak is observed at 530.0 eV (Figure 11b), while peaks at 137.6 eV and 142.4 eV are attributable to Pb4f7/2 and Pb4f5/2, respectively (Figure 11c). Lastly, S2p is characterized by peaks at 161.6 eV and 159.4 eV alongside a S(VI) peak at 167.5 eV (Figure 11d). Taken together with XRD and TEM results, XPS data point to the SnO2/PbS heterostructures being composed of Sn(IV), Pb(II), O, and S. Recent work revealed that the Pb4f7/2 and Pb4f5/2 peaks due to nanoscopic PbS showed high energy shoulders consistent with limited PbO formation [19]. The corresponding signals in heterostructured SnO2/PbS reside predominantly at the high-energy positions, with the presence of low energy shoulders now

signifying that heterostructure formation results in a greater proportion of Pb residing in close proximity to O [37]. This behaviour differs again from that seen in homogeneous SnO2/PbS nanocomposites, in which Pb-based regions were essentially amorphous and Pb signals had completely migrated to the high energy positions [19].

UV-Vis spectroscopy

Consistent with previous work [38], Figure 12a shows absorption edges at 289 and 221 nm for ethanolic SnO2 NSs. Meanwhile the spectrum of pure PbS NCs has been reported to reveal absorption edges at 362 nm and 278 nm [19]. The UV-Vis absorption spectrum of heterostructured SnO2/PbS (Figure 12b) exhibits two absorption bands; 224 nm (assigned to SnO2) [39] and 267 nm (assigned to PbS) [40].

Photocatalytic activity tests Photocatalytic activity was measured in triplicate for the reaction of each catalyst (5.0 mg) with 25 ml of 1.0 x 10 5 M aqueous RhB under simulated solar irradiation. Figure 13 shows RhB 1max = 555 nm as a function of time, with the decrease in 1max clearly greatest for the use of heterostructured photocatalyst. Figure 14a shows the decomposition rate for RhB in each test, C0 and C being the concentrations of RhB before and after irradiation, respectively. Remarkably, 81.6 (±0.8) % of the dye was degraded by the presence of SnO2/PbS heterostructures after 300 min. irradiation, whereas only 44.3 (±3.9) % and 43.1 (±4.4) % degradation was observed for the use of pure SnO2 NSs and PbS NCs, respectively. Reaction rate constants (k) were calculated assuming pseudo first-order kinetics (Eqn. 3) for a low initial pollutant concentration [41]:

ln(C0/C) = kt (3)

Figure 14b confirms first order kinetics, with k = 1.8 x 10-3 (±0.2 x 10-3), 1.4 x 10-3 (±0.1 x 10-3) and 4.6 x 10-3 (±0.2 x 10-3) min-1 for SnO2 NSs, PbS NCs and SnO2/PbS heterostructures, respectively. For the photodegradation of aqueous RhB by heterostructured SnO2/PbS, was determined to be 0.0469 (±0.0019) mol min-1 W-1.

The stability of the photocatalytic performance of the SnO2/PbS heterostructured catalyst was evaluated since this is an important factor in practical applications. Catalyst cycling experiments were undertaken with the catalyst recovered by centrifugation between tests. Figure 15a suggests that after three RhB photodegradation cycles the activity of SnO2/PbS

heterostructures had undergone some loss of activity; the efficiency of the catalyst being 81.6, 64.1, and 49.1% in the first, second, and third tests, respectively. The decrease in activity could be assigned to washout loss of some catalyst during the recovery steps. We also consider that the reduced effectiveness of the catalyst after recycling is contributed to by photobleaching of the catalyst surface [42]. The crystalline structure of the SnO2/PbS heterostructured catalyst has been investigated after photodegradation experiments. Figure 15b illustrates the XRD patterns of photocatalysts before and after three RhB decomposition cycles, with the lack of observable changes indicating that both crystalline phase and structure remain intact.

Semiconductor photocatalysts absorb sunlight when the energy of the incident photons is equal to or larger than the band gap of the semiconductor [43]. Thus, an electron is excited from the valence band (VB) of the semiconductor into its conduction band (CB) [44]. The photoexcited electron can be used to carry out electrochemical reduction reactions at the photocatalyst surface whereas the photoexcited hole can execute electrochemical oxidation of compounds with oxidation potentials more negative than that of the VB maximum [45]. To enable the photolytic creation of active hydroxyl radicals (OH*), the VB and the CB of the semiconductor photocatalyst should encapsulate both the oxidation potential of OH* and the reduction potential of superoxide radicals (O2-*) [46]. According to recent work [19], neither SnO2 nor PbS independently meet the thermodynamic requirements for OH* generation. However, heterostructured SnO2/PbS is postulated to be capable of both the separation and transportation of photoinduced charge carriers. Figure 16 illustrates how this heterostructure reduces the electron-hole recombination probability through the migration of photogenerated electrons from the CB of PbS to that of SnO2 at the same time as the photogenerated holes in the VB of SnO2 move into that of PbS. Electron acceptors such as adsorbed O2 can easily trap electrons at the SnO2 surface to produce superoxide radical anions, which undergo protonation to produce hydroperoxy radicals (HO2*). In water, these then give OH*, which is strongly oxidizing and can decompose RhB [47]. Finally, the photoinduced holes in PbS can also be trapped by OH , so acting as an additional OH* source. Overall, the mechanism by which RhB undergoes photocatalytic degradation can be summarized as:

PbS + hv ^ PbS (eCB- + hyB+)

PbS (eCB-) + SnO2 ^ SnO2 (eCB-) + PbS

SnO2 (eCB-) + O2 ^ SnO2 + O2* O2* + H2O ^ HO2* + OH-HO2* + H2O ^ OH* + H2O2 H2O2 ^ 2OH*

PbS (hvB+) + OH- ^ OH* + PbS OH* + RhB ^ CO2 + H2O

To verify that the photocatalytic degradation pathway afforded by the current heterostructured SnO2/PbS system proceeds via photoinduced OH* radical creation, a TA probe was employed whereby reaction with OH* radicals gave fluorescent TAOH [48]. Upon excitation at 315 nm, the highest intensity at 425 nm progressively increased with irradiation time (Figure 17), demonstrating OH* photogeneration.

BET surface area analysis BET surface areas and pore size distributions of the SnO2 NSs and SnO2/PbS heterostructures were determined by measuring nitrogen adsorption-desorption isotherms (Figure 18). Samples exhibited type IV behaviour and distinct H2-type hysteresis loops [49], suggesting an interconnected network of mesopores. Pore size distributions were determined by applying Barrett-Joyner-Halenda (BJH) analysis to the adsorption branch of each isotherm (Table 3). Interestingly, results showed that the SnO2/PbS heterostructures exhibit a lower surface area than either SnO2 NSs (Table 3) or PbS NCs. This points to the importance of the type-II electronic structure in defining the superior photocatalytic activity of the heterostructure.

4. Conclusions

In summary, we have developed a simple and inexpensive approach to the controlled synthesis of a heterostructured SnO2 NS/PbS NC photocatalyst. Its structure has been unambiguously demonstrated by an exhaustive combination of XRD, TEM, BET, XPS and Raman spectral studies. Detailed Rietveld analysis has established the occurrence of both cubic and tetragonal phases in the heterostructure and (micro)structural parameters have been elucidated. The

heterostructured material demonstrated superior photocatalytic activity in the degradation of RhB under simulated solar irradiation when compared to either SnO2 or PbS. In the light of BET surface area measurements, effective electron-hole separation at the interface of the two semiconductors is believed to be mainly responsible for this enhanced performance. In turn, this idea of the facile production of heterojunction-containing semiconducting photocatalysts with elevated activity suggests a pioneering approach to catalyst design and fabrication that addresses environmental concerns. That said, it is clear that the performance of heterostructured SnO2/PbS currently shows some subsidence when the catalyst is recycled. This may be attributable to incomplete catalyst recovery. The further development of heterostructured SnO2/PbS as a potential photocatalyst will therefore focus on immobilization studies. The development of novel matricies that offer new possibilities in this respect is now being initiated.

Acknowledgements

A. K. acknowledges support from the Royal Society's Newton International Fellowship scheme (NF130808). B. R. K. thanks the UK EPSRC for financial support (EP/J500380/1). Thanks go also to Drs. Tim Jones and Jill Geddes of Schlumberger Gould Research for help with the acquisition of Raman and X-ray photoelectron spectra, to Dr. Zlatko Saracevic of the Department of Chemical Engineering and Biotechnology (University of Cambridge) for help with BET surface area analysis, and to Mr. Tom Jellicoe for help with the photoluminescence measurements. Open Data for this paper are available at the University of Cambridge data repository (see http://dx.doi.org/xxxx/CAM.yyyy).

References

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Figure 1. Synthetic scheme for the preparation of SnO2/PbS heterostructures.

Sn02 III | ||

PbS I I I II III

PbS(1) II III I

PbS(2)l I I II III

I ^ I 1 I 1 I 1 I 1 T

20 30 40 50 60 70 75 Angle (29)

Figure 2. Experimental (red circles) and simulated (black lines) XRD

patterns of (a) SnO2 NSs, (b) PbS NCs and (c) SnO2/PbS heterostructures. Peaks are indexed according to blue markers (|) and PbS(1) and PbS(2) represent cubic PbS phases with different lattice parameters. Difference patterns between observed and calculated intensity (Iq-Ic) are shown in green.

4000 -3000 -

>> -i—»

c 2000 -CD

1000 -

Sn02 III I II

PbS(1) II III I

PbS(2)l I_I II I II

I-1-Ï-1-1-T-Ï-1-1-T-1-1

20 30 40 50 60 70 75 Angle (20)

Figure 3. The contribution of different phases (tetragonal SnO2 and PbS(1) and PbS(2)) to the XRD pattern of heterostructured SnO2/PbS.

(a) J J (b)

Qsn (d) Oo

Q- tO O *

3.I8A W ^2.05 A

Figure 4. Atomic arrangements in (a) the nanospherical SnO2 crystal lattice and (b) the corresponding unit cell. Distorted SnO6 octahedra in (c) SnO2 NSs and (d) SnO2/PbS heterostructures.

Figure 5. (a) Unit cell of PbS NCs. Atomic arrangement of Pb and S atoms forming PbS6 octahedra in (b) PbS NCs, and heterostructured (c) PbS(1) and (d) PbS(2).

Figure 6. HRTEM images of representative PbS NCs oriented close to their <110> zone axes. The {111} crystallographic planes are resolved in the elongated NCs imaged in both (a) and (b). The superimposed crystal model (Pb = grey, S = yellow), tilted slightly from the <110> zone axis, displays the orientation relationship between the unit cell (insert) and the image.

Particle Diameter (nm) Particle Diameter (rim)

Figure 7. Representative TEM images of SnO2 NSs (a, b) and SnO2/PbS heterostructures (c, d). Scale bars 100 nm (a, b, d) and 200 nm (c). Size distribution analysis for isolated (e) and NS-coating (f) PbS NCs seen in (c).

200 nm

Energy (keV)

Figure 8. (a,b) Representative HAADF-STEM images of SnO2/PbS heterostructure clusters. (c) EDX spectra obtained from a region enclosing (red) PbS NCs and (green) a SnO2 NS decorated by PbS NCs. Spurious copper X-rays originating from the supporting copper grid are also detected.

Raman Shift (cm-1)

Figure 9. Raman spectra of (a) SnO2 NSs and (b) SnO2/PbS heterostructures. For PbS data see ref. [19].

Figure 10. XPS data for SnO2 NSs. Binding energy spectra for (a) Sn3d; (b) O1s. For data on PbS NCs see ref. [19].

Figure 11. XPS data for SnO2/PbS heterostructures: binding energy spectra for (a) Sn3d; (b) O1s; (c) Pb4f; (d) S2p.

Wavelength (nm)

Figure 12. UV-Vis absorption spectra of (a) SnO2 NSs and (b) SnO2/PbS heterostructures in ethanol. For PbS data see ref. [19].

0.0 400

Rhodamine B + SnO2 nanospheres

A 0 min

30 min

iyOl 60 min

ffl/vl 120 min

JO 180 min

240 min

300 min

500 600

Wavelength (nm)

500 600

Wavelength (nm)

Rhodamine B + PbS nanocubes

j\ 0 min

/a\ 30 min

IA 60 min

ml 120 min

/Ml 180 min

/111 240 min

300 min

Rhodamine B + SnO2/PbS heterostructures

30 min

60 min

120 min

180 min

240 min

300 min

500 600

Wavelen gth (nm)

Figure 13. Photolytically-induced changes to the RhB absorption maximum at ca. 555 nm catalyzed by (a) SnO2 NSs, (b) PbS NCs and (c) SnO2/PbS heterostructures.

Figure 14. (a) Plot of C/C0 (%) (where Co and C are the concentrations of dye before and after irradiation, respectively) for RhB as a function of irradiation time in the presence of SnO2 NSs, PbS NCs and SnO2/PbS heterostructures. (b) Plot of ln(Co/C) as a function of

irradiation time.

Figure 15. (a) Recyclability of heterostructured SnO2/PbS and (b) XRD patterns of the catalyst before and after three cycles of RhB photodecomposition.

RhB CO2 + H2O

.OH O2 .O2-

0.41 eV

3.6 eV

OH- .OH

Figure 16. Proposed model for the photocatalytic activity of SnO2/PbS heterostructures.

1ex = 315 nm 240 min 180 min

120 min

3 , 60 min

« ' n et

400 425 450 475

Wavelength (nm)

Figure 17. Fluorescence spectral changes measured during illumination of heterostructured SnO2/PbS in a basic TA solution (excitation at 315 nm).

—■— SnO. nanospheres

SnO,-PbS heterostructures

0.0 0.2 0.4 0.6 0.8

Relative Pressure (P/Pq)

Figure 18. Nitrogen adsorption-desorption isotherms at 77 K (P and Po are the equilibrium and the saturation pressures of N2 at the temperature of adsorption) for SnO2 NSs and SnO2/PbS heterostructures.

Table 1 (Micro)structural parameters for SnO2 NSs.

Sp. Gr. Lattice Particles r.m.s. Atomic coordinates Oxygen parameters (A) size (nm) microstrain _of oxygen_occupancy

a c x y z

P42lmnm 4.8058 3.2622 2.19 1.2x10-3 0.306 0.305 0 0.996

Table 2 (Micro)structural parameters for SnO2lPbS heterostructures.

Compounds Vol. Lattice (%) parameters (Â) Particles size (nm) r.m.s. microstrain Atomic coordinates of oxygen

a c x y z

SnO2 nanosphere (P42lmnm) 61.42 4.8937 3.0993 4.95 0.0269 0.334 0.297 0

Anisotropic particle size (nm) Planar direction r.m.s. microstrain

68.22 (111) 0.0607

5.57 (200) 0.0146

PbS(1) 16.60 5.4621 67.12 Y (220) 0.0504

(Fm-3m) 55.84 (311) 0.0563

68.22 (222) 0.0607

5.57 (400) 0.0146

66.39 (331) 0.0599

56.13 (420) 0.0338

60.02 (422) 0.0578

46.16 (111) 0.1388

3.75 (200) 0.0007

18.76 (220) 0.0214

PbS(2) (Fm-3m) 41.56 (311) 0.0908

21.98 5.9894 46.16 (222) 0.1388

3.75 (400) 0.0007

33.69 (331) 0.1081

12.22 (420) 0.0170

43.10 (422) 0.1075

Table 3 Nitrogen sorption porosimetry studies" of SnO2 NSs and SnO2/PbS _heterostructures. For PbS NC data see ref. [19]._

Photocatalyst Sbet (m g- ) Pore volume (cm g- ) Mean pore size (nm)

SnO2 10.0698 0.034046 10.8651

SnO2lPbS 7.1488 0.157042 69.4260

"Surface areas determined by the BET technique, mean pore diameters by BJH theory (applied to the adsorption branch), and pore volumes by single-point analysis.

Table of Contents Entry

0-1-.---.---.-

0 100 200 300

irradiation lime (min)

The deposition of PbS nanocubes onto pre-formed, activated SnO2 nanospheres yields a heterostructured nanomaterial that incorporates a type-II junction, the presence of which is aimed at combating electron-hole recombination and narrow photoresponse range.

• We report a cost-effective route to a new class of type-II SnO2/PbS heterostructure

• Our new route to type-II heterostructures is environmentally benign

• A detailed understanding of structure, morphology and stability is developed

• Our heterostructures display excellent activity in the degradation of Rhodamine B

• Our new heterostructures suggest a simple route for improving exciton lifetime