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#-SnWO4 Photocatalyst with Controlled Morphological Transition of Cubes to Spikecubes Ying-Chu Chen, Liang-Ching Hsu, Yan-Gu Lin, A. Tarasov, Po-Tuan Chen, Michitoshi Hayashi, Jan Ungelenk, Yu-Kuei Hsu, and Claus Feldmann ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02444 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016
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ACS Catalysis
β-SnWO4 Photocatalyst with Controlled Morphological Transition of Cubes to Spikecubes Ying-Chu Chen1, Yan-Gu Lin2,*, Liang-Ching Hsu2, Alexander Tarasov3, Po-Tuan Chen3, Michitoshi Hayashi3, Jan Ungelenk1, Yu-Kuei Hsu4,*, Claus Feldmann1,* 1
Institut für Anorganische Chemie, Karlsruhe Institute of Technology (KIT), Engesserstraße 15, D-76131 Karlsruhe,
Germany 2
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
3
Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
4
Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien 97401, Taiwan
KEYWORDS. Tin tungstate, polyol, photocatalyst, hierarchical structure, multibranch ABSTRACT: A distinct morphology of β-SnWO4 with hierarchically multiarmed architecture and overall hexahedral symmetry – entitled as spikecube – is fabricated for the first time via a polyol-mediated synthesis. The growth of the βSnWO4 spikecubes is investigated and attributed to thermodynamic and kinetic control. In a sequential reaction, crystalline cubes of β-SnWO4 enclosed by {100} facets grow in a first Ostwald ripening-based step. A kinetically controlled growth process to spikecubes follows under formation of multiarmed spikes on the facets of the cubic seeds. Such growth process differs significantly from literature concerning highly branched crystals. The synergistic effect of morphological modification (i.e. introducing more surface reaction sites) and textural alteration (i.e. incorporation of the p-block Sn2+ into simple tungsten oxide to reframe its band structure) leads to an enhanced photocatalytic activity of the β-SnWO4 spikecubes being 150% higher in comparison to benchmark WO3 photocatalysts.
1.
INTRODUCTION
In the global energy-climate debate, renewables have been targeted for almost one-third of total electricity output in 2035.1 To this concern, solar energy has the potential to grow much more rapidly than any other renewable technology. Hitherto, the development of cost-effective methods to harvest solar energy remains as a fundamental challenge. This situation has stimulated a large number of experimental and theoretical studies aiming at materials capable of either transforming solar energy into electricity (photovoltaics) or inducing chemical reactions to transfer materials into useful fuels or to convert pollutants to environmentally friendly compounds (photocatalysis).2-12 Beyond simple binary compounds, meanwhile, numerous attempts have been made to develop multi-component chemical systems, including Perovskite-related (ABX3) or Scheelite-related (ABX4) materials.2-12 Recently, special attention has been paid to lone-pair containing compounds. Due to their unique twisted coordination geometries mediated by those “non-bonding” electron pairs, additional options afford in view of electronic and structural modifi-
cation.13,14 In particular, cations at the lower end of groups 13-16 in the periodic table, e.g., Pb(II), Sn(II), Bi(III), have triggered intense research activities.15-27 One notable case is m-BiVO4, which is hitherto widely recognized as a solarlight-driven photocatalyst and exhibits superior photoactivity toward water oxidation.19-27 The excellent photoactivity of m-BiVO4 results from a reframed band structure due to the “non-bonding” interaction between the electron pair on Bi(III) and O-2p orbitals. Such mediation is especially related to the elevated valence band (VB) and the distorted coordination due to second-order Jahn-Teller (SOJT) effects, generating a moderate-band gap of 2.4 eV (λonset ~500 nm) while preserving the oxidative activity of the photogenerated holes. An alternative material is β-SnWO4. Here, it was shown via first-principle calculations and subsequent spectroscopic analysis that its photoactivity arises from optical transition between anti-bonding Sn-5s and O-2p orbitals at the top of the valence band and empty W-5d orbitals mixed with Sn-5p orbitals at the bottom of the conduction band (CB).17,18,28-31 Apart from high-surface nanoparticles, here, a distinct, robust shape could not only influence the VB of
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β-SnWO4 but also result in an elevation of the CB. Altogether, a favorable photocatalytic activity can be anticipated for β-SnWO4. Apart from textural aspects, hierarchical architectures have emerged as an alternative concept for expanding the functional material scope via a straightforward manipulation of size and shape and is intensively exploited from both fundamental and technological points of view.32-34 Very recently, highly branched, multipod-shaped micro/nanocrystals formed from semiconductors developed as a class of materials that is of particular interest for catalysis, owing to their huge specific surface area and the high activity induced by high densities of reactive edges, corners, and stepped atoms present on their branches.35-41 Taking the above textural and morphological aspects together, a synergistic strategy of fabricating β-SnWO4 with a stereoactive lone pair into a multipod-shaped hierarchical architecture could be of very promising for promoting an enhanced photoactivity. To date, there are only few reports on the synthesis of β-SnWO4 due to the ambient metastability of this cubic phase.28,42,43 Thus, highsurface nanoparticles have been obtained by aqueous nucleation and micronsized dodecahedra were made via microemulsion techniques.28 In this study, we demonstrate a controlled polyol-mediated synthesis of β-SnWO4 microcrystals with both cubic shape and a novel class of highly branched hexahedral-multiarmed spikecubes. A spikecubic morphology is unique, in general, and can merge large surface (of the spikes) and high crystallinity (of the cubic seed). Nucleation and growth from nanoparticles via cubes to spikecubes are well controlled based on the conditions of the polyol synthesis. As a proof of the concept, the photoactivity of β-SnWO4 cubes and β-SnWO4 spikecubes was validated based on organic-dye degradation. Here, the βSnWO4 spikecubes outperform β-SnWO4 cubes with 243% and a commercial, visible-light activated WO3 photocatalyst with 150% higher activity. 2.
EXPERIMENTAL SECTION
2.1. Materials. Materials. Tin chloride (SnCl2×2H2O, 98%, SigmaAldrich), sodium tungstate (Na2WO4×2H2O, 99%, SigmaAldrich) and diethylene glycol (DEG, C4H10O3, 99%, Alfa Aesar) worked as the precursors and solvent for tin tungstate preparation. Zwitterionic rhodamine B (RhB, C28H31ClN2O3, 99%) and cationic methylene blue (MB, C16H18N3SCl, 95%) used as model dyes in photooxidative activity measurements were purchased from Acros Organics. All chemicals used in this study were of analytical grade and used as received without further purification. The dye solutions were prepared with deionized water of a resistivity not less than 18.2 MΩ cm. 2.2. Synthesis of β-SnWO4 photocatalysts. In a typical synthesis, the reaction was proceeding in a three-neck flask equipped with a reflux condenser and a magnetic stirring bar embedded in DURAN glass. A clear solution of tungstate precursor was first made in the reaction flask by dissolving 3.6 mmol Na2WO4×2H2O into 150 mL DEG at 60 °C. To this solution, 6 mL of an aqueous equimolar tin precursor solution (SnCl2×2H2O) was added and a yellow precipitate appeared immediately. The reaction mixture was subsequently subjected to thermal treatment under nitrogen atmosphere with different temperatures ranging
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from 120 °C to below 200 °C and kept under the respective temperature for 1 hour. The suspensions turned from golden orange to yellowish-white, which is indicative of the formation of β-SnWO4. Whereas β-SnWO4 with nanoparticulate shape could be obtained at the reaction temperature lower than 150 °C, β-SnWO4 cubes and spikecubes were formed at temperatures under 160 °C and 200 °C, respectively. In the last step, all the product was collected by centrifugation, washed with ethanol several times to remove diethylene glycol and excess precursors. 2.3. Preparation reparation of WO3 photocatalyst. The detailed procedure for preparing fine particulate WO3 photocatalysts can be found elsewhere,44 but is described here briefly. The WO3 photocatalysts with a particle size of ~200 nm was separated from commercial WO3 powder (99.8%, metals basis, Alfa Aesar) as follows. The WO3 powder (5 g) was suspended in demineralized water (100 mL) with ultrasonic irradiation for 30 min (SI: Figure S1a,b). Thereafter, the suspension was centrifuged at 1000 rpm for 10 min. After the removal of the precipitate containing large particles and aggregates, fine particulate WO3 was collected by centrifugation (10,000 rpm for 30 min) (SI: Figure S1c,d). The specific surface area of the obtained WO3 fine particles was 5.9 m2g-1, determined by nitrogen sorption at 77 K. Prior to separation, the surface area was 3.0 m2g-1. 2.4. Characterization of β-SnWO4 photocatalysts. The geometry and morphological evolution of the β-SnWO4 photocatalyst under different preparation conditions were examined using field-emission scanning electron microscopy (FESEM, Zeiss Supra 40 VP equipped with energy dispersive X-ray spectrometry (EDXS)). Transmission Xray microscopy (TXM) was further employed to exhibit the 3-dimensional tomographic reconstruction, in which the specimens were quickly irradiated with X-ray generated from the beamline 01B at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan with a photon energy in the range 8−11 keV. The micrographs were collected using a Princeton Instruments CCD detector. Crystal structure and phase identification were validated by X-ray powder diffraction (XRD, Stoe STADI-P diffractometer) with a Ge-monochromatized Cu-Kα1 radiation (40 kV, 40 mA). Extinction spectra of β-SnWO4 powders were measured using a Varian Cary 100 Spectrometer and further analyzed by means of Kubelka-Munk transformation. The particle size distribution of β-SnWO4 and WO3 powders was examined either via dynamic light scattering (DLS, Malvern Zetasizer Nano ZS) for submicrometer-sized particles or statistically analyzed from SEM micrographs. BET nitrogen sorption measurements (BEL BELSORP-max) were carried out at 77 K to obtain the specific surface area. X-ray absorption experiments on the W-L3-edge under total fluorescence yield (TFY) and O-K-edge under total electron yield (TEY) mode were employed to investigate the local coordination and corresponding band structure evolution. The synchrotron radiation experiments were also conducted at NSRRC but with beamlines 17C and 20A. The W-L3-edge extended X-ray absorption fine structure (EXAFS) was measured at ambient conditions but under a "normal incidence – grazing exist" geometry, in which the sample was illuminated by the synchrotron X-ray at nearly normal incident angle (φ ~ 900) and the detector was placed at a grazing exit to collect the fluorescence signal.
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The radiation from the storage ring was monochromatized by a Fixed-Exit Si(111) double-crystal monochromator and the intensity was measured by two ionization chambers filled with helium gas. In particular, the detection integral was further refined via a confined sensitive area of the detector by a deliberate slit (5~10 mm) on the filter positioned before the detector in order to circumvent the potential damping effect originated from self-absorption on the EXAFS oscillations. The acquired results were subsequently processed and analyzed using the Athena data analysis package implemented with the IFEFFIT codes. On the other hand, the O-K-edge soft X-ray absorption spectra (SXAS) were measured at room temperature in an ultrahigh vacuum (UHV) chamber, in which the β-SnWO4 powder was fixed by Scotch tape on the sample holder. In particular, the optical design of beamline 20A consists of a horizontal focusing mirror (HFM), a vertical focusing mirror (VFM), a spherical grating monochromator with 4 gratings, and a toroidal refocusing mirror to ensure a mean energy resolution of 5000. Theoretical fitting to the real part of the Fourier Transformed (FT) W L3-edge EXAFS data with k3 weighting were further performed with the aid of Artemis data analysis package to refine the relevant structural parameters involving the effective coordination number (N), interatomic distance (R) and Debye-Waller factor (σ2). Theoretical standard model is derived from the typical β-SnWO4 with rocksalt-analogous cubic structure (P213 space group) and lattice parameter a = 7.2989 Å. Theoretical scattering amplitude and phase-shift functions, employed in the EXAFS simulations, were calculated by the self-consistent real-space multiple scattering (RSMS) FEFF6 code using a complex Hedin-Lundqvist exchange-correlation potential with atomic muffin-tin radii defined following the Norman’s criterion.14,30,31,45 The amplitude reduction factor (VWX ) from multielectron effects is determined by fitting the theoretical first-coordinated shell (O shell) to the same shell in the experimental spectra and which along with the edge shift (E0) are constrained to be equal over first few shells during fitting procedure. The number of fitted parameters in fitting routine was justified statistically according to the criterion of the Nyquist theorem, X∆^∆_ YZ[\\ = , in which ∆k and ∆R is the width of the win` dow cut in k-space and R-space, respectively. The fits are performed over the k-range of 3.5–14.5 Å-1 and R-range of 1-5 Å. The values of goodness of fit (R-factor, aZbcde[ = ∑(ghih − kli)X / ∑ ghihX ) were also included in each fit. The real space full multiple scattering (FMS) simulations of O-K-edge spectra were performed for the same reference model. All muffin-tin spheres were automatically overlapped by 30% (AFLOP card) to reduce the effects of potential discrepancies at the muffin tins and in particular to analogize the peculiar Sn 5s lone pair, which characterized as spatially extended electronic orbital with the localized electron density directed toward the triangular face formed by O(1) atoms of the coordination [SnO6] octahedron. The numerical computations were performed for different cluster sizes and the convergence was reached for cluster sizes of about 8.4 Å corresponding to 145[152] atoms around the absorber O(1)[O(2)]. The numerical SXAS spectra were calculated in the presence of an appro-
priately screened core hole according to the final-state rule. The theoretical spectra were convoluted with a Lorentzian-shape function to account for the core-hole lifetime (Г = 0.16 eV) broadening. Additional broadening effect (0.3 eV) responsible for technical inaccuracies and lattice vibrations (Debye-Waller factor) of experimental aspect was further included during calculation procedure. The theoretical spectra were directly compared to the experimental results without fitting procedure. The estimation of the simulation quality was based on the reproduction of the significant spectral features including correct energy position and relative energy separation as well as the intensity ratio. In addition to the aforesaid computations, the projected density of states (pDOS) based on the ab initio FMS calculation for the photoabsorbing oxygen atom and all neighboring atoms (W and Sn) were also included. 2.5. Photoreactivity measurements. Photocatalytic decomposition of organic dyes was carried out in a DURAN glass reaction cell containing a suspension of the photocatalyst powder (0.3 g L-1, 12 mg) in an aqueous dye solution (5.44 mg L-1, 40 mL) with continuous agitation using a magnetic stirrer. In the degradation reaction, an 8 W UVlamp with monowavelength of 366 nm was employed as the light source. Before illumination, the suspensions were magnetically stirred in the dark for 1 h to ensure establishment of an adsorption/desorption equilibrium of the dyes on the photocatalyst surface. The experiments were then carried out at room temperature in air. The suspensions were sampled every 10 minutes. The powders were separated by centrifugation and the dye solutions were analyzed by UV-Vis spectrophotometry, where the concentration evolution was monitored by comparing the intensity variation of the featured absorption peak of the dyes (zwitterionic λRhB = 554 nm, cationic λMB = 554 nm). 3. RESULTS AND DISCUSSION 3.1. Synthesis of β-SnWO4 cubes ubes and spikecubes pikecubes βSnWO4 cubic particles with rocksalt-like crystal structure − where regular [WO4] tetrahedra (W–O distance of ~1.75 Å) form a face-centered cubic lattice with Sn2+ filling the octahedral sites − were prepared via a polyol-mediated synthesis (Figure 1).14,29 It is evident that the as-prepared βSnWO4 particles are enclosed by six sharp faces and a cube-shaped habitus (Figure 1a,c). Taking the rocksaltanalogous cubic crystal structure of β-SnWO4 into account, it can be concluded that the cubes are composed of the characteristic cube-type faces (Figure 1c). The particle size of the crystals falls mostly in the range of ~2-9 μm, estimated from scanning electron microscopy (SEM) overview images (Figure 1b) and TXM tomography (SI: Movie S1). The yield of the as-prepared cubic β-SnWO4 particles is around 90% and with high morphological uniformity. On X-ray diffraction (XRD) patterns (Figure 2a), all Bragg peaks match well with the β-SnWO4 cubic phase without other measurable Bragg peaks associated to impurity phases. Further analysis of the faceted samples via energy dispersive X-ray spectroscopy (EDXS) (Figure 2b)
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been triggered thermally beyond the cubic geometry (Figure 3a,b), in which the cubes with sharp faces now act as homogeneous seeds for the subsequent anisotropic growth of bundled multiarms including nanotip, nanopyramid and nanocone shapes. Particularly, each of the six bundles − mimicking a Karst-like landscape vertical to each face of the cube − is exclusively composed of several separate, parallel arms as evidenced by the TXM tomography (SI: Movie S2). In contrast, coalescence into a single arm or entangling together with dendrite-shapes – as it is typically described in the literature35-41 – is not observed here.
Figure 1. (a,b) SEM and (c) TXM micrographs of the asprepared β-SnWO4 cubes at high-magnitude (a; scale bar: 5 µm) and low-magnitude (b; scale bar: 10 µm) aspects. Inset: Indexed crystal facets of featured β-SnWO4 cubes.
confirms the stoichiometry with an elemental ratio of Sn to W approximated to unity. UV-Vis spectra (Figure 2c, inset) indicate that the yellowish β-SnWO4 has a visible-light absorption band with a featured edge at ~430 nm. A Tauc plot with direct transition features revealed a band gap of 2.91(±0.03) eV (Figure 2c), which was found to coincide with theoretical calculations.14,17 This excellent agreement confirms that the featured threshold observed in the absorption spectrum is ascribed to the intrinsic direct interband transition. In addition to facet engineering, further attempts were made to raise the reaction temperature during synthesis. As a result, a novel hierarchical architecture − designated as spikecube structure − has been observed once the reaction temperature exceeds certain critical degree (180 °C). A series of SEM and TXM images at different viewing angles is shown in Figure 3, illustrating the geometric features of typical β-SnWO4 spikecubes. Tilted-angle SEM and TXM images indicate a distinct structure evolution that has
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The geometric features of the elongated arms in the spikecube case were further evaluated via statistic analysis of hundreds of SEM (Figure 3c) and TXM (Figure 3b) images. Thus, they exhibit lengths (h) from ~0.7 to 2.0 μm and base diameters (db) of ~200 nm. In contrast to geochemical-like nanoisland formation during the thermallyinduced transition, no significant influence on symmetry, shape and size of the seeded cubes has been observed. A scheme of this novel hexahedral-multiarmed β-SnWO4 spikecubes and the representative geometric features are depicted in Figure 3d. In addition to the peculiar morphology presented in this study, the observed seeded-growth behavior is substantially different from what thus far has been recognized as the control for fabricating semiconductor or metal multipods, viz. is the coexistence of identical planar-buildingblocks within polytypic compounds or an epitaxial relationship between heterogeneous nuclei and branches.33-39 As further evidenced by the crystallographic and compositional homogeneity (according to XRD and EDXS), both cubes and spikecubes crystallize in the cubic rocksalt-like structure only (Figures 2a,4a). Moreover, the stoichiometry of Sn and W within the spikecubes remained close to unity without noticeable contributions of potential impurities, e.g. arising from the precursor (Figures 2b,4b). 3.2. Thermally hermallyly-triggered growth of β-SnWO4 cubes and spikecubes. Several phenomena thus far could be observed by comparing the shape evolution in different growth regimes (Figure 5a). Begin with the background of the polyol-mediated synthesis, where the reaction starts with the nucleation of a solid while heating sufficient aqueous precursors in a multivalent and high-boiling alcohol (e.g., diethylene glycol (DEG), b.p. 246 °C).46-48 Here, the nucleation occurred along the equilibria of coordination complexes of Sn2+ and [WO4]2- in the presence of water and DEG according to: VrXs (DEG, H2O) + tuvXw (DEG, H2O) ↔ Vrtuv
(1)
Controlling the shape starts from a competitive growth of nuclei from different solvents adhering on the β-SnWO4 nuclei. At lower temperature (160 °C) and in the presence of water, facet engineering occurred due to Ostwald ripening, where adatoms can migrate on the crystal surface to minimize the total surface energy, and accordingly a polyhedral cubic microcrystal covered by low-index facets is obtained (Figure 1). Apart from temperature-driven crystal facet engineering, we found that the injection of water as a mediator is essential. Hence, no β-SnWO4 was obtained without water injection (SI: Figure S3). Taken together, at a given water content (H2O/DEG = 0.04), the diameter of the underlying particles (dc) could be simply altered from a nanoscopic to the microscopic length scale just by raising the reaction temperature. Furthermore, facet engineering is triggered above certain temperature determining the reaction temperature as the crucial parameter for both crystal sizing and shaping (Figure 5a). A three-dimensional, hierarchical structure arises if the temperature exceeds a critical degree (180-200 °C). Hence, a nanoarchitecture mimicking a Karst landscape is formed, building up on the former cubes. In comparison to polyhedral crystals, the highly branched morphology of the spikecubes has a larger surface area, and thus, a higher surface energy. As a result, the formation of multiarm-shaped microcrystals is surely not favored in terms of thermodynamics and requires precise kinetic growth control. Obviously, the kinetically controlled growth of the spikes appeared beyond the thermodynamically controlled growth of cubes as a function of reaction temperature. We found that the consumption rate of water injected as a mediator crucially determines the degree of multiarmed spike growth. It is to be noted that the boiling point of low amounts of water in a pool of DEG is much higher than the boiling point of pure water at standard conditions (100 °C, 1 bar). Such observation obeys Raoult’s law and can be ascribed to: (1) the presence of metal salts (e.g. Sn2+) coordinating the H2O molecules, and (2) intermolecular interaction (e.g. hydrogen bridging) of H2O with DEG as the majority phase. Once the synthesis
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proceeds at temperatures higher than 100 °C, the absence of water during the first few minutes of synthesis causes a reduced migration rate of adatoms, where the growth of the nanoarms proceeds similar to the kinetic mechanisms previously reported for multipod-shaped metal nanocrystals.35 Anisotropy results exclusively from atomic addition, and the growth rate is limited by the concentration of the precursor remaining in the mother liquor, thereby leading to a lower aspect ratio between the diameter of seeds and the length of the arms (h) (Figure 5b). An additional influence owing to the absence of the aqueous mediator emerges in parallel to the anisotropic growth and shape evolution (Figure 5b, inset). Thus, SEM analysis of the spikecubes clearly reveals the difference between the nanoarms and the underneath cube crystals (Figure 3). In contrast to the known multipod-shaped nanocrystals, the presence of a microcrystal as the substrate on which the deposition of the nanoarms sets in is vital to rationalize a growth mechanism in the thin-filmdeposition mode rather than in the LaMer-Dinegar mode.49,50 Moreover, the anisotropic growth rate is, beyond the concentration of the precursor, further governed by the diffusion rates taking into account that the polyol medium is more viscous than water. Here, the Stokes-Einstein equation provides the diffusion rate: D=
^{ | }`~[
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where D is the diffusivity, kB is Boltzmann’s constant, T is the temperature, r is the radius of the particle, and η is the viscosity. Taken together, the anisotropic growth rate is further postponed by a significantly slower diffusion behavior at reduced precursor concentrations. This forces the branches to grow in a columnar mode with dozens of parallel and separate nanoarms instead of a coalesced single-arm form.51 An extraordinary feature of the peculiar β-SnWO4 spikecube morphology is that the dozens of separate nanoarms offer an enlarged surface area as evidenced by the Brunauer-Emmett-Teller (BET) nitrogen adsorption
analysis (Table 1). Thus, the specific surface area of the spikecubes reaches 6.52 m2g-1 and is significantly higher than for the β-SnWO4 cubes, suggesting a more facile molecular accessibility through the voids (ds) between the nanoarms. Table 1. Geometric features and specific surface area of the as-obtained β-SnWO4 and commercial WO3 photocatalyst. 2
Morphology
Size (µm)
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β-SnWO4 spikecubes Fine-particulate WO3 *
*
Commercial WO3 catalyst (SI: Figure S1).
3.3. Coordination and bonding in β-SnWO4. The thermally induced growth not only may influence the microstructural properties but also the interior coordination and bonding. To gain detailed insights into the thermal effects, W-L3-edge Extended X-ray Absorption Fine Structure (EXAFS) and O-K-edge Soft X-ray Absorption Spectroscopy (SXAS) were performed and allow for detailed probing of the local coordination (Figures 6,7) and the corresponding electronic states (SI: Figures S5,S6).52,53 In a typical polyolmediated synthesis of either cubic or spikecubic β-SnWO4, the surface is capped by the oxygen atoms of the DEG solvent. Such situation–i.e. partial substitution of near-surface oxygen atoms of β-SnWO4 by solvent oxygen atoms–only has a minor effect on the electronic structure in comparison to capping agents containing heteroatoms such as nitrogen or sulfur.52,53 The reduced white line in the W-L3edge spectra can be attributed to near-surface disorder owing to the steric hindrance of the ethylene groups of capping DEG molecules. In particular, a significantly decreased intensity is expected for the spikecubic shape in view of its nearly 5-fold higher specific surface area (Table 1). However, an increased intensity is observed while comparing the W-L3-edge spectra of both cubic and spike-
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cubic β-SnWO4 (SI: Figure S4a), which evidently rules out the influence of capping molecules (i.e. charge-transfer effects, change of near-surface bonding situation). Further quantification regarding the local stoichiometry of the β-SnWO4 cubes and spikecubes, particularly of the outmost spikes, was performed via TEM-EDXS elemental-mapping analysis showing a uniform distribution of both Sn and W without any significant inhomogeneity (SI: Figures S4b-e). This result in combination with the equivalent energy position of the absorption edges in the W-L3-edge spectra (SI: Figure S4a) clearly reveals that no chemical inhomogeneity of the surface spike forest or the underneath cubic seed was induced during morphological evolution from cubes to spikecubes.
coordination number (N), interatomic distance (R) and Debye-Waller factor (σ2) (Table 2, Figure 7). Fits were carried out in the R-space for the first FT maximum by varying the amplitude NVWX , the bond length distribution width (σ) and the bond length (R). The reference model (Figure 7a) used in fitting routines suggests four O atoms (N = 4) in the first shell around W forming a nearly regular [WO4] tetrahedron (Figure 7c).
Figure 6. (a) Nonphase-corrected Fourier-transformed (FT) 3 W-L3-edge EXAFS spectra χ(k)k for β-SnWO4 (solid red line: spikecubes; solid black line: cubes) as function of a particular R section of the concerned first few shells around W. (b) Differentiation at longer distances corresponding to the second coordination shell for cubic (hollow black square) and spikecubic (hollow red sphere) shape.
Subsequently, Fourier transforms (FT) of the W-L3edge EXAFS spectra χ(k)k3 (Figure 6) were made to study the local coordination around W. Here, the relative positions between the W, O and Sn atoms in specimens prepared at distinct temperatures were studied. The first FT maximum at 1.4 Å is attributed to an interaction of photoabsorbing W atoms with four coordinated oxygen atoms in the first coordination shell (i.e. W-O bonds within a [WO4] tetrahedron). This first-shell peak is symmetric and located at the same position for both cubic and spikecubic βSnWO4. This suggests equivalent regular [WO4] tetrahedra in distinct morphologies (Figure 6a). In contrast to the first-shell coordination, a significant deviation in the second coordination shell corresponding to the nearest Sn atoms around the [WO4] tetrahedra (i.e. W-Sn interatomic distance) with a radial distances between 3.0-4.2 Å was evidenced (Figure 6b). To this concern, Kuzmin and coworkers have reported that the [WO4] tetrahedra in βSnWO4 behave as rigid units whereas they loosely interact with the nearest [SnO6] octahedral units.30,31 Such weak interaction suggests that the structural variation, which depends on the synthesis conditions, predominantly affects the tilting of the [WO4] and [SnO6] building units relative to each other. Indeed, this is well in accordance with our observations. To this end, theoretical fitting of the real part of the FT W-L3-edge EXAFS data with k3 weighting was conducted to refine the relevant structural parameters involving the
Figure 7. (a) Projected stereodiagram of β-SnWO4 with typical cubic crystal structure acts as theoretical model in numerical XANES (SI: Figures S5,S6) and EXAFS calculations. Structural Information: (b) First oxygen coordination shell forming [WO4] tetrahedra. Nearest Sn in distorted [SnO6] octahedra in the second shell disposed around [WO4] tetrahedra via (c) diverse oxygen bridging (left: O(2); right: O(1)) 2 or (d) spatially localized Sn-5s lone pairs. Real part of W-L33 edge EXAFS spectra χ(k)k for β-SnWO4 (e) spikecubes and (f) cubes (solid line: theoretical fits; red/black hollow sphere/square: experimental scans). (e-f) Right panels demonstrate the isolated contributions of diverse coordination shells as a function of R.
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Table 2. Structural parameters obtained from W L3-Edge EXAFS fitting analysis for spikecubic and cubic β-SnWO4. Absorber-backscatter Absorber-backscatter 2 2 N R (Å) σ (Å ) N R (Å) pair pair Spikecubic shape Cubic shape W-O 4 1.780(1) 0.0010(2) W–O 4 1.780(12) W-Sn(1) 2.1(9) 4.050(74) 0.0067(8) W–Sn(1) 4.0(1.7) 4.080(35) W-Sn(2) 1.6(7) 4.240(66) 0.0043(59) W–Sn(2) 0.9(4) 4.250(25) W-W 0.6(3) 4.540(24) 0.0010(59) W–W 0.5(4) 4.540(31)
2
2
σ (Å ) 0.0010(2) 0.0147(40) 0.0029(20) 0.0010(59)
Notation: Effective coordination number (N), interatomic distance (R) and Debye-Waller parameters (σ2). Typical β-SnWO4 with cubic structure was used as the model for EXAFS calculations.45 Statistical error for the associated model parameters: N ± 20%; R: 1%; σ2: 20%. a Fitting range: k = 3.5 – 14.5 Å–1, R = 1.0 – 5.0 Å. Values in parentheses are uncertainties in the least significant digit estimated for the corresponding structural parameters (associated with fits). b Fit quality index Rf (R-factor = ∑(data-fit)2/∑data2) = 0.01. Values of other EXAFS model parameters not shown above were either fixed or fited a common value over all samples as follows: S02 = 0.72(0.03) (fixed amplitude reduction factor based on first-shell fitting to spikecubic β-SnWO with improved crystallinity); ΔE0 = −0.059 eV (fitted energy shift parameter).
The amplitude reduction factor (VWX ) is generally between 0.7 and 1.0 and graphically evaluated from scaling the average of several fits to scans, in which VWX = 0.72(3) is obtained and then fixed for the subsequent fits as a function of coordination shells (characterized as second and third FT features within a range of 3-5 Å in the FT W-L3edge EXAFS spectra (Figure 6b). The relevant structural parameters and the respective uncertainties are shown in Table 2. Theoretical fits and experimental scans in the real part of FT W-L3-edge EXAFS spectra are summarized in Figures 7e,f. At first sight, all these fits trace the original data in the real part of the FT spectra reasonably well − in particular in the cubic case − over the entire R range. Such successful reproduction of spectral features indicate that the structural framework of both cubic and spikecubic βSnWO4 are substantially equivalent to the standard model, which is consistent with the anticipations from the XRD results. Moreover, reasonable Debye-Waller factors and satisfactory goodness-of-fits ensure the reliability of the estimated structural parameters. Regarding the first coordination shell, the reference model implies the interatomic distances of four O atoms to fall into two groups with a bond-length difference of ~0.02 Å, forming a nearly regular [WO4] tetrahedron (Figure 7c). However, such differentiation is insufficient to be resolved by EXAFS analysis, and therefore, only the average bond length of 1.780(1) Å was derived (Table 2). On the contrary, a significant deviation in the second coordination shell of cubic and spikecubic β-SnWO4 became evident featuring as a bimodal (Figure 6b: red spheres) and one-hump with a shoulder-like distribution (Figure 6b: black squares), respectively (Table 2). Prior to interpreting the parameters acquired from these fits, one may first notice that the uncertainty of the respective structural parameters simultaneously increases in comparison to the first shell. In particular, some parameters are greater than the statistically anticipated errors. Such issues are frequently observed in EXAFS analysis at longer distances and ascribed to different factors: i) Statistical limitations of the EXAFS fitting procedure, in which inappropriate variables for background subtraction result in damped XAFS oscillations; ii) Cubic and spikecubic β-SnWO4 were both prepared at moderate thermal conditions with peculiar morphologies resulting in limited crystallinity, and therefore, limited intensity of EXAFS signals; iii) The associated defects and disorder in the materials can cause a greater
uncertainty of R and σ2. Due to the aforesaid uncertainty, it is more meaningful to address the trends of parameters between different scans rather than assessing specific values. The characteristic one-hump with a shoulder-like distribution of cubic β-SnWO4 can be further deconvoluted into a dominant part centered at 4.080(35) Å and a minor extension at 4.250(25) Å corresponding to distinct W−Sn interatomic distances (Table 2). The origin of the distinct W−Sn distance is ascribed to the stereoactive non-bonding Sn 5s electron pair, as schematically depicted in Figures 7b,d. The interatomic distance between Sn scatter and W absorber (W–Sn(1)) mediated by an oxygen bridge bonding (Figure 7d) – either an extended W−O(1)–Sn bond or a contracted W−O(2)–Sn bond – is responsible for the one-hump distribution at shorter radial distance (Figures 7e,f; right middle panels).45 Whereas the alternative contribution (W–Sn(2)) accounts for the shoulder-like distribution. In particular, the longer W–Sn(2) distance represents a distinct length between the off-centroid Sn and W atoms. Moreover, the off-centroid behavior is thoroughly triggered by the lone pair laying in-between (Figure 7b). The individual contribution is further characterized by an effective coordination number extracted by FEFF6 to 4.0(1.7) and 0.9(0.4), respectively. To our surprise, these values within error resemble in those suggested by the model, in which an approximate ratio of 6 is proposed (NW-Sn(1):NW-Sn(2) = 6:1). The slight reduction in N, in particular of the W–Sn(1) contribution, results from the relatively poor crystallinity as implied by the XRD results. Likewise, similar fitting routine is performed for spikecubic β-SnWO4 in the same shell, in which the peculiar bimodal distribution is categorized into two ingredients with one at 4.050(74) Å and another at 4.240(66) Å (Table 2). Upon first inspection, the distinct interatomic distances seem to remain invariable without deviations in distance can be meaningfully discernible. Nonetheless, the thermal-triggered modification on the local texture is otherwise evidenced by the effective coordination number. According to the structural information of the theoretical model, the origins of the contributions can be attributed to 7 Sn scatter surrounding W absorber. In contrast to the sufficient similarity in N observed in cubes, the effective coordination number depicted for spikecubic β-SnWO4 shows a relatively small value. Such observation is evident to demonstrate that a local texture modification is involved
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reactivity of photoelectrons, and a hierarchical highsurface framework provides facile accessibility to excitons. In sum, a promising photoreactivity is to be expected and next evaluated by the photooxidation of organic dyes as a conceptual study. 3.4. Photocatalytic characterization. The photocatalytic properties of β-SnWO4 were evaluated as a conceptual study via the photooxidation of organic dyes (Figure 8) and compared to a commercial WO3 photocatalyst (SI: Figure S1). 5
5
4
4
s-SnWO 4 TON (mmoldye mol-1cat.)
TOF:2.77 3
2 Light on
1
s-SnWO4 TOF:2.77
3
2 WO3
Light on
TOF:1.81
1
c-SnWO 4 TOF:1.14 0
0 -10 0 10 20 30 40 50 60 70 80 90 Irradiation time (min)
3
-10 0 10 20 30 40 50 60 70 80 90 Irradiation time (min)
14 250
s-SnWO4 TOF:2.65
10 8 6 WO3 4 2
TOF:1.69
200 R (%)
12
Turnover Freq. (mmoldye /molcat h)
in the morphologic evolution of cubic and spikecubic βSnWO4. The rearrangement can occur in a way, in which greater site imperfections implied by the enhanced surface introduce more defects and disorder, which is preferentially distributed over the space outside the [WO4] anions. Thus, a reduction in N for the outer shell is observed. However, this damping is antipodal for both W–Sn contributions. Instead, a reverse increment in N corresponding to contributions of W–Sn(2) is found. As a consequence, the balanced W–Sn contributions further result in the featured bimodal FT distributions (Figure 6b). Such situation is exciting due to the fact that the motion of the Sn atoms is indeed observed experimentally. In contrast to the aforesaid significant alteration, the contribution at much longer distance characterized as [WO4] units surrounding a certain W absorber doesn’t undergo any significant displacement. The estimated R of W–W distances in cubic and spikecubic β-SnWO4 are highly similar to the theoretical model with a damped amplitude as evidenced by N less than 1 (Table 2). The thermally initiated modification of cubic and spikecubic β-SnWO4 is manifested in the aforementioned series of systematic comparison of R and N. In particular, defects originating from the moderate thermal treatment and further caused by the surface-site imperfections in the spikecubes play an important role in the textural alteration. These defects occur preferentially in a medium shell, in which the distorted [SnO6] octahedral and concomitant lone pair reside. Moreover, they are prone to distribute over O sites owing to the higher ratio. The resulting free space allows motion of the residual elements (mainly O and Sn). Such rearrangement can be accelerated in spikecubic β-SnWO4 as it is prepared at higher temperatures. As a result, the displacement of the Sn atoms is clearly depicted by the varied ratio of N of distinct W–Sn contributions (Table 2). Nonetheless, the adaptable movement is limited by the stereoconfinement of lone pair. Altogether, this situation is characterized by the balanced N of diverse W– Sn contributions (Table 2) and leads to the observed bimodal FT distribution (Figure 6b). The distortion of the [SnO6] octahedra as well as nearby defects can further reduce the C3 site symmetry and stimulate an even more pronounced SOJT effect. This may strongly correlate with the band gap opening observed in UV-Vis spectra. Thus, O-K-edge SXAS spectra (SI: Figure S5) were performed next to exploit the impact on the corresponding unoccupied electronic states, namely the conduction band (CB). Meanwhile, numerical simulations were also conducted to understand the origin of the spectral features from an electronic perspective (SI: Figures S5,S6). Detailed characterizations and ab-initio XANES calculations were illustrated in the supplementary. The results manifest that the enhanced SOFT and covalence effects account for the observed band gap opening by the energy shift of conduction band minimum (CBM). Taken together, the polyol synthesis employed in this study turned out as advantageous in comparison to other strategies of synthesis, not only for preparing β-SnWO4 with tailored structure and shape but also in view of finetuning of the band structure. Altogether, the β-SnWO4 spikecubes possess both a satisfactory band structure, where a slightly elevated CBM even further promotes the
TON (mmoldye mol-1cat.)
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ACS Catalysis
TON(mmol dye mol-1 ) cat.
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2
150
Light on
100 1 0.0
0 -10 0 10 20 30 40 50 60 70 80 90 Irradiation time (min)
c-SnWO 4
WO 3
s-SnWO4
Figure 8. Photoactivity for degradation of organic dyes under monochromatic NUV light irradiation (366 nm): (a) βSnWO4 cubes (c-SnWO4) and spikecubes (s-SnWO4) for RhB degradation; β-SnWO4 spikecubes and commercial WO3 photocatalyst for (b) RhB and (c) MB degradation. (d) Photodegradation rate (left axis) of β-SnWO4 in the respective forms as well as commercial WO3 photocatalyst and the ratio (right axis) between them.
To isolate the photocatalytic oxidation from photolysis effects owing to the fact that organic dyes can also harvest light, the dye-degradation experiments were carefully examined under irradiation with specific wavelengths.54 From UV-Vis spectra, it is obvious that the organic dyes selected for photooxidative activity measurements only respond to a specific part of the visible light (SI: Figure S7a): zwitterionic rhodamine B (RhB) is sensitive to 450-600 nm; cationic methylene blue (MB) is sensitive to 600-700 nm. In contrast, both organic dyes do not absorb in the near ultraviolet (NUV) region (e.g., RhB shows low absorption at 350-450 nm; MB shows low absorption at 350-500 nm). Consequently, the experiments were conducted under monochromatic light centered in the NUV region at a wavelength of 366 nm, in which both the β-
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SnWO4 and the WO3 photocatalyst show significant absorption. As an additional verification, the self-photolysis of the organic dyes under 366 nm irradiation is negligible in absence of a photocatalyst (SI: Figure S7b,c). Taken together, the dye photodegradation under these experimental conditions can be reliably ascribed to the photocatalytic oxidation driven by β-SnWO4 or WO3. The results are shown in Figure 8a, first, using RhB as a model dye. The photooxidative capacity was evaluated in terms of the turnover number (TON) via normalization to the molar amount of the photocatalyst:7 TON =
\[ eZ [\bcd\ e\c\
\[ eZ bde b edecbdbd
(3)
The time elapsed curves exhibit a linear behavior in the period studied (Figure 8a), indicating that degradation rate and reactant concentration are independent from each other. Moreover, an enhanced photooxidation is revealed in the following order: β-SnWO4 spikecubes > βSnWO4 cubes. To elucidate this observation, a monomolecular surface reaction mechanism, which has been generally adopted to interpret the rate of a heterogeneous photochemical reaction, is given by:55 = ′
(4)
Herein, k’ is the photoxidation rate constant, Cs is the adsorption capacity of the photocatalyst, and θA is the surface coverage of the reactant. The photoxidation rate constant k’ can be further analyzed into:55 ′ =
^
(5)
^
Here, I is the incident light flux and can be regarded as constant throughout a series of equivalent measurements. The additional terms, i.e., Ф, kred and kr represent the photoabsorption efficiency, the interfacial reaction rate constant and the recombination rate constant of the photogenerated electron-hole pairs, respectively. These terms are solely related to physicochemical properties of the photocatalyst and thereby regarded as intrinsic factors. In contrast to k’, the term of the surface coverage θA, which is usually interpreted in accordance to the LangmuirHinshelwood kinetics, is as follows:56 =
(6)
s
Here, KA and CA represent the adsorption equilibrium constant and the concentration of the reactant remaining in the volume solution, respectively. Both factors strongly affect the experimental parameters (e.g., pH value, type and concentration of dye, etc.) and the course of the reaction. Taken together, a linear behavior can arise once the photocatalysts exhibit nearly full coverage conditions (θA ≈ 1). Then, the intrinsic photoactivity can be derived in ^ the form of . ^
The experimental conditions applied in this study feasibly point to a scenario, where β-SnWO4 with moderate adsorption capacity originates from a modest surface area. This is followed by measurements in which a reduced amount of photocatalyst was used. Consequently, a linear course was observed and the rates representing the intrin-
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sic photoactivity were calculated in terms of the turnover frequency (TOF) TOF =
| [\bcde [bde
(7)
Notably, the β-SnWO4 spikecubes exhibit a more than two times higher photoactivity than in the case of the βSnWO4 cubes (Figure 8a). This improvement presumably results from the substantial difference in adsorption capacity (Cs) due to the enhanced surface area and the more facile accessibility provided by the multibranched structure of the spikecubes. Similar measurements were performed to gain detailed insights regarding the photocatalytic properties based on the photooxidation of zwitterionic RhB and cationic MB over β-SnWO4 spikecubes in comparison to commercial WO3 as a photocatalyst (particle size: ~200 nm; specific surface area: 5.87 m2g-1) (Figure 8b,c). Remarkably, a linear behavior remains over the diverse combinations of dyes and photocatalysts. The superior photooxidative performance of the β-SnWO4 spikecubes, especially, in comparison to WO3 is evident. It is worth noting that the photoactivity of the β-SnWO4 spikecubes estimated in different photodecomposition reactions is nearly equal within few percent even if the adsorption behavior diverges significantly (e.g., only ~4% of RhB were adsorbed on β-SnWO4 powders, whereas ~41% of MB were adsorbed under equivalent condition). A similar result was also observed for the commercial WO3 particles, reinforcing the pivotal and reliable measurement and extraction of the intrinsic photocatalytic properties. Taking the almost identical specific surface area of the β-SnWO4 spikecubes and the commercial WO3 photocatalyst into account (Table 1), the different photoactivities definitely stem from other factors that are dominated by intrinsic terms shown in Eq. 5. As reported by Abe and co-workers, the photoexcited holes in WO3 are reactive toward oxidation owing to the deeply positive VB (about +3.1 V), whereas the bottle neck relates to the poor consumption of photoelectrons accumulated on the more positive CB level (about +0.5 V).44 In other words, the efficiency of the photogenerated electron-hole ^ pairs react with the surface-adsorbed substrate, i.e. , ^
plays another key-factor beyond the surface area on the photoactivity and relies significantly on the band structure of the photocatalyst. In relation to the observation from SXAS spectra, it was found that the comparatively more negative CB position enables the photoelectrons on β-SnWO4 spikecubes to be consumed via a single-electron pathway (e.g., O2 + e− = O2−aq with −0.284 V; O2 + H+ +e− = HO2 with −0.046 V).44,57 This supports an efficient consumption of the photoexcited electron-hole pairs and results in an enhancement factor R, defined as the ratio of the intrinsic photoactivity of the βSnWO4 spikecubes versus that on other photocatalysts. To this concern, the β-SnWO4 spikecubes differ from the commercial WO3 photocatalyst and the β-SnWO4 cubes by more than 150% and 243%, respectively (Figure 8d). 4.
CONCLUSIONS
In conclusion, we have developed a facile “one-pot” polyolmediated synthesis, in which some of the design criteria for photocatalysts, namely, high surface-to-volume ratio,
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open framework with facile accessibility, and optimal band structure, have been successfully addressed at the same time. In this way, β-SnWO4 spikecubes have been prepared following a sequential morphology transition starting from cubic particles with sharp faces to a peculiar shape where dozens of separate and parallel nanoarms are self-aligned onto each face of the cube seeds. Particularly, the nanospikes not only provide an enlarged surface area but also form a porous matrix, where the inter-particular voids lead to an improved accessibility and diffusion. Besides these morphological advantages, the distorted coordination mediated by the stereoactive lone pair can be adequately promoted in this way, by which the reframed band structure of the β-SnWO4 spikecubes supports the photoexcited electron-hole pairs to be readily consumed, as evidenced by EXAFS, SXAS and FMS calculations. For the first time, a thermal effect of growth, which gained only scarce attention to date, has turned out as highly relevant on textural and morphological modification. Last but not least, an outstanding photocatalytic activity of the β-SnWO4 spikecubes was observed that originates from the above synergistic effects. In view of the photooxidation of organic dyes, an enhancement of more than 150% to as high as 243% in comparison to commercial WO3 photocatalysts and β-SnWO4 cube-like particles were obtained. The work presented here not only renders βSnWO4 spikecubes as a promising candidate for photocatalysis but also provides a synthetic protocol, which can readily be extended to fabricate other photocatalysts that may benefit from straightforward growth of multiarmed shapes.
ASSOCIATED CONTENT Supporting Information. TXM movies showing the 3dimensional tomography of β-SnWO4 cubes and spikecubes, TEM-EDXS mapping addressing the stoichiometry at the superficial spike forest, SEM images and size distribution histogram concerning the fine particulate WO3 photocatalysts after separation from the commercial powder and nanoparticulate β-SnWO4 are available. In addition, the characteristic X-ray and optical absorption spectra of the photocatalysts and dyes are added. Detailed characterizations and ab-initio XANES calculation are also provided. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] *E-mail:
[email protected] *E-mail:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors gratefully thank the Deutsche Forschungsgemeinschaft (DFG) as well as the Ministry of Science and Technology (MOST) and the National Synchrotron Radiation Research Center (NSRRC), Taiwan for financial support under contracts MOST 103-2112-M-213-001-MY2 and 103-2221-E259-013. Y.C. Chen also acknowledges the support from the
Deutscher Akademischer Austauschdienst (DAAD) scholarship (No. A/13/92805).
for
ABBREVIATIONS CB: conduction band; VB: valence band; DEG: diethylene glycol; CBM: conduction band minimum; MB: methylene blue; RhB: Rhodamine B; NUV: near ultraviolet; TON: turnover number; TOF: turnover frequency.
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