Rapid synthesis of Zn²⁺ doped SnWO₄ nanowires with the aim of exploring doping effects on highly enhanced visible photocatalytic activities

Abstract

In this work, we report on the rapid synthesis of Sn_1−xZn_xWO₄ nanocrystals with the aim of tailoring their structural, electronic, and photocatalytic properties. The samples were carefully characterized by X-ray diffraction, transmission electron microscopy, inductive coupled plasma optical emission spectroscopy, UV-vis diffuse reflectance spectroscopy, and the Barrett–Emmett–Teller technique. The effects of Zn²⁺ doping in SnWO₄ on the electronic structure and photogradation of methylene orange dye solution were investigated experimentally and theoretically. It was found that Zn²⁺ ions were homogeneously incorporated into the SnWO₄ host lattice with a solubility of x = 0.060, which led to a monotonous decrease in lattice volume. With Zn²⁺ doping, SnWO₄ nanocrystals showed a morphological alteration from irregular nanosheets to nanowires. Meanwhile, the BET surface areas were also greatly enlarged from 54 m² g⁻¹ to ∼100 m² g⁻¹. Contrary to the theoretical predictions of the quantum size effect, Zn²⁺ doped SnWO₄ nanocrystals showed an abnormal band gap narrowing, which can be well-defined as a consequence of the balance of quantum size effect, lattice contraction, electronegativity, and surface defect centers. With well-controlled morphology, surface area, and electronic structure via Zn²⁺ doping, the photocatalytic performance of Sn_1−xZn_xWO₄ nanocrystals was optimized at a Zn²⁺ doping level of x = 0.045.

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Introduction

Introducing impurity ions into a semiconducting host, in order to control its structure, particle size, morphology, surface features and chemical composition, has been one of the most important techniques since the necessity arose for tailored properties with promising applications in electronics, magnetics, light emitting devices and catalysis.¹ In this regard, semiconductors doped with metal or non-metal ions showing visible photocatalytic activity have attracted great interest, because they provide a potential solution to many environmental pollution problems that humankind is currently facing.²

To significantly improve the visible photocatalytic efficiency of oxide semiconductors (e.g. TiO₂ and NaTaO₃), one can tailor the particle sizes, morphologies, band gap energies and surface active sites by modifying the host structures with various transition metal dopants including La, Fe, Cu, and Bi.³ However, doping strategies have shown both positive and negative impacts for most of the semiconductors. This is because there exist very complicated factors, such as particle size, electronic structure, dopant concentration, surface area, d electronic configuration of transition ions, and the localized d-levels of dopants, which could contribute to the photocatalytic activity. For instance, the localized d-levels of transition metal dopants can not only decrease the photothreshold energy but also serve as recombination centers for photoinduced charge carriers.⁴ Apparently, clarifying the correlation between these factors and photoactivity is fundamentally important, which may facilitate the design and exploitation of novel photocatalysts showing efficient visible-light response.

Herein, multi-metal oxide semiconductors were taken as the model compounds based on the following considerations: (1) in comparison to traditional binary oxides (e.g. TiO₂, ZnO and SnO₂et al.), multi-metal oxide semiconductors possess higher tolerance to structural distortion due to their inherent lattice strain, which enables the incorporation of foreign ions into the host matrix;⁵ (2) the multi-metal oxides show higher chemical stability and fewer electron–hole pair recombination centers, so that charge carrier separation and migration is more effective;^4b (3) multi-metal oxides, such as SnWO₄, Sn₂Ta₂O₇, Cd₂Sb₂O_6.8etc. are excellent visible-light driven photocatalysts, which provide a convenient way to obtain doped materials with highly enhanced visible photocatalytic activities.⁶ As a multi-oxide, SnWO₄ offers a better environmentally visible-light driven alternative to the oxides containing harmful and expensive ions. Though bulk SnWO₄ exhibits superior photocatalytic activity to that of the nanosized WO₃ and TiON_x, the nano-chemistry of SnWO₄ is still limited by the difficulties associated with conventional solid-state synthesis.^6a With the aim of using visible-light to generate electron–hole pairs for promoting photocatalytic reactions, it is desirable to exploit a SnWO₄ based photocatalyst with a well-defined phase structure, particle size, and surface area, by a rapid, reproducible and simple method.

In this work, we report on the fabrication of Zn²⁺ doped α-SnWO₄ nanocrystals by a rapid and simple microwave-assisted hydrothermal approach. The doping effects were systematically studied in an attempt to investigate the possible reasons for the significantly improved photocatalytic activity toward the degradation of methylene orange.

Experimental section

Sample preparation

All chemicals were of analytical grade and were used without further purification. All samples were prepared in a microwave system (NOVA-1, PreeKem Scientific Instruments Co., Ltd.). The system was equipped with in situ magnetic stirring. The exposure time and temperature were programmed. The automatic temperature-control system allowed continuous monitoring and control of the internal temperature of the reaction system (1 °C). A typical synthetic procedure for Sn_1−xZn_xWO₄ (the initial molar ratios of Zn to Sn were 0, 0.02, 0.05, 0.07, 0.10 and 0.12) nanocrystals can be described briefly as follows: 1.0 mmol SnCl₂·2H₂O and a given amount of ZnCl₂ were dissolved in 10 mL degassed water. Na₂WO₄ aqueous solution (0.1 M, 15 mL) was added dropwise to the above solution under continuous stirring. A well controlled NaOH solution was used to adjust the pH value to 7. After treating the mixture at 180 °C for 10 min under microwave irradiation, it was rapidly cooled to room temperature by an air compressor. The obtained products were washed with distilled water several times and dried at 80 °C for 3 h.

Sample characterization

The phase purities of all samples were characterized by X-ray power diffraction (XRD) on a Rigaku DMAX2500 X-ray diffractometer using a copper target. The average grain size, D, was measured from the diffraction peak (121) using the Scherrer formula, D = 0.9 λ/(βcosθ), where λ is the X-ray wavelength employed, θ is the diffraction angle of the diffraction peak (121), and β is defined as the half width after subtracting the instrumental broadening. The particle sizes and morphologies of the samples were determined using transmission electron microscopy (TEM) on a JEM-2010 apparatus with an acceleration voltage of 200 kV. The chemical compositions of the samples were measured by the inductively coupled plasma (ICP) technique on a Perkin-Elmer Optima 3300DV spectrometer. Optical diffuse reflectance spectra of the samples were measured using a UVIKON XL/XS Spectrometer at room temperature. The specific surface areas of the samples were determined from the nitrogen absorption data at liquid nitrogen temperature by using the Barrett–Emmett–Teller (BET) technique on a Micromeritics ASAP 2000 Surface Area and Porosity Analyzer.

The plane-wave based density functional theory (DFT) calculations of the Sn_1−xZn_xWO₄ samples were performed with the CASTEP program package with the core orbitals replaced by ultrasoft pseudopotentials, the generalized gradient approximation (GGA) with the Perdew, Burke and Emzerhof (PBE) exchange correlation functional, and a kinetic energy cutoff of 450 eV. Ninety-six atom supercells were constructed in order to explore the doping effects by 2 × 2 × 2 expansion of the unit cell of α-SnWO₄. The Broyden, Fletcher, Goldfarb, and Shannon minimization was used to perform the structural optimization and the convergence. All band structures and the density of states were calculated on the corresponding optimized crystal geometries.

Photocatalytic activity test

A heteropolyaromatic dye, methylene orange dye (MO), was used as a probe molecule to evaluate the photocatalytic reactivity of the samples in response to visible light at room temperature. The experiments were carried out as follows: 50 mg of the sample was placed in 50 mL 2 × 10⁻⁵ M MO aqueous solution in a 100 mL beaker. Prior to illumination, the suspension was magnetically stirred in the dark for 12 h to ensure the establishment of an absorption–desorption equilibrium of MO on the sample surface. Subsequently, the suspension was irradiated under a 300 W Hg lamp with a filter (λ ≥ 420 nm) as a cutoff, about 10 cm away from the beaker. At given intervals, 5 mL of the suspension was extracted and subsequently centrifuged at a rate of 8000 rpm for 10 min. UV-vis absorption spectra of the supernatant were then measured with a UVIKON XL/XS Spectrometer.

Results and discussion

Fig. 1 shows X-ray diffraction patterns of the Sn_1−xZn_xWO₄ nanocrystals prepared by a microwave-assisted hydrothermal approach. It is found that, by varying the dopant concentration x ≤ 0.10, XRD data of all diffraction peaks could be accurately indexed to a pure orthorhombic phase of α-SnWO₄ (Joint Committee on Powder Diffraction Standards, JCPDS card, No. 29-1354). Upon further increasing the Zn²⁺ content up to x = 0.12, several additional sharp diffraction peaks appeared, which are assigned to an unknown phase with a large domain size (Fig. S1, ESI†), showing the same XRD pattern as that of the cubic Sn₂TiWO₇ (25-0978). The broad diffraction peaks indicate the fine crystalline nature of Sn_1−xZn_xWO₄. The peak broadening was used to calculate the crystalline size of the primary particles. Based on the half height width of the (121) peak, the particle size was calculated to be around 9.7 nm for pure SnWO₄ and 5.6–6.0 nm for Sn_1−xZn_xWO₄ (x ≤ 0.10) nanocrystals, respectively.

Fig. 1 XRD patterns of Sn_1−xZn_xWO₄ nanocrystals with initial dopant concentration. The vertical bars below the patterns represent the standard diffraction data from the JCPDS file for α-SnWO₄ (29-1354).

To prove the presence of Zn²⁺ in SnWO₄ nanocrystals, chemical analyses of the as-prepared samples were performed by the inductively coupled plasma-atomic emission spectrometry (ICP-AES) technique. As indicated in Fig. 2, the final molar ratios of Zn²⁺ content in Sn_1−xZn_xWO₄ nanocrystals were all smaller than the initial ones. Increasing the initial Zn²⁺ concentration causes the Zn²⁺ content in the final samples to increase. The maximum dopant concentration of Zn²⁺ is about 0.060. The inset of Fig. 2 illustrates the X-ray photoelectron spectroscopy (XPS) data of Zn for Sn_1−xZn_xWO₄ (x = 0.060). The XPS spectrum of Sn_1−xZn_xWO₄ in the Zn 2p region shows the binding energies of Zn 2p_3/2 and Zn 2p_1/2 at around 1021.7 and 1043.9 eV, respectively, suggesting that Zn exists in the chemical state of Zn²⁺ in Sn_1−xZn_xWO₄ nanocrystals.⁷ Moreover, the chemical states of Sn and W were also determined to be Sn²⁺ and W⁶⁺ by XPS data (Fig. S2, ESI†).⁸ To further specify the homogeneous incorporation of Zn²⁺ into the SnWO₄ host matrix, the variations of the lattice parameters for Sn_1−xZn_xWO₄ are supposed to be investigated. By a least-squares method using the Retica Rietveld program (Version LHPM, 2000, B. A. Hunter and C. J. Howard, Lucas Heights Research Laboratories, Menai 2234, Australia), the lattice volumes were determined. Fig. 3 shows the lattice parameters of Sn_1−xZn_xWO₄ nanocrystals as a function of dopant concentration of Zn²⁺. Upon varying the Zn²⁺ doping level from x = 0 to 0.060, a lattice contraction was observed, as indicated by the monotonous decrease in lattice volume. It is then highly necessary to clarify the occupation-site of the dopant Zn²⁺ in SnWO₄ nanocrystals. It is much easier to substitute Zn²⁺ into Sn²⁺ rather than W⁶⁺ sites, because (1) the Pauling electronegativity of Zn²⁺ is 1.58, close to 1.34 for Sn²⁺ but much smaller than that of 2.03 for W⁶⁺;⁹ (2) the ionic radius for Zn²⁺ is 0.74 Å in 6-fold coordination, smaller than that of 1.18 Å for Sn²⁺, while Zn²⁺ in 4-fold coordination has an ionic radius of 0.6 Å, larger than that of 0.42 Å for W⁶⁺;¹⁰ (3) there exists a huge charge difference between Zn²⁺ and W⁶⁺. If Zn²⁺ was doped at the W⁶⁺ sites in SnWO₄, then the structure of SnWO₄ may be destabilized and a lattice expansion would be expected. Accordingly, the majority, if not all, of the Zn²⁺ ions, are doped at Sn²⁺ sites to yield a lattice contraction, while the minority would be situated at surfaces or interfaces, resulting in particle size reduction and morphology alteration. Besides lattice contraction, the axis ratio of c/a slightly decreased with increasing Zn²⁺ doping level, indicating that a lattice distortion has occurred.

Fig. 2 Zn²⁺ content of Sn_1−xZn_xWO₄ nanocrystals measured by ICP-AES as a function of initial Zn²⁺ molar ratio. The inset shows the typical energy range related to Zn peaks.

Fig. 3 Lattice parameters of Sn_1−xZn_xWO₄ (x = 0 to 0.060) nanocrystals as a function of dopant content of Zn²⁺.

To characterize the structural and morphological properties, transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were performed. Fig. 4a shows a typical TEM image of pure SnWO₄ nanocrystals. It is seen that SnWO₄ nanocrystals mainly consist of nanosheets with irregular shapes. For a single nanosheet, the spacing between adjacent lattice fringes was 0.155 nm (Fig. 4b), which is very close to that of 0.156 nm for the (121) plane as calculated by XRD, but slightly larger than that of 0.154 nm for SnWO₄ (JCPDS card, No. 29-1354). However, after the incorporation of Zn²⁺ ions (x = 0.045), as shown in Fig. 4c, Sn_1−xZn_xWO₄ nanocrystals were tuned to be rod-like tiny crystals with high crystallinity, as verified by the HRTEM image (Fig. 4d). Moreover, as the Zn²⁺ dopant concentration increased to 0.060, the Sn_1−xZn_xWO₄ nanocrystals were entirely composed of nanowires with a diameter of ∼8 nm (inset of Fig. 4e) and a length of up to several hundreds of nanometers. The interplanar spacing of a single nanowire was about 0.154 nm (Fig. 4f), which is slightly smaller than that of the (121) plane for pure SnWO₄ nanocrystals. It is noteworthy that the incorporation of Zn²⁺ ions into the SnWO₄ host matrix led to dramatic changes in morphology. Knowledge of the shape evolution of the doped nanocrystals is still limited, although attempts to investigate the underlying mechanisms that control doping have been reported in the literature.¹¹ For example, Wang et al. suggest that, for lanthanide-doped NaYF₄ nanocrystals, the influence of lanthanide doping strongly depends on the size and dipole polarizability of the substitutional dopant ions.¹² It is supposed that Zn dopants may play an important role in the primary growth stage and influence the intraparticle ripening process,^11a which is critical for the generation of doped nanocrystals with different shapes.

Fig. 4 TEM and HRTEM images of pure SnWO₄ (a, b), Sn_1−xZn_xWO₄ (x = 0.045) (c, d) and Sn_1−xZn_xWO₄ (x = 0.060) (e, f) nanocrystals.

The energy level and the band gap of the oxide semiconductors play a crucial role in determining their photocatalytic activity. Fig. 5 shows optical band gap spectra of Sn_1−xZn_xWO₄ (x = 0 to 0.060) nanocrystals as a function of Zn²⁺ dopant concentration. It is seen that all Sn_1−xZn_xWO₄ nanocrystals exhibit a strong absorption band in the visible light region. According to previous literature, α-SnWO₄ is a semiconductor with an indirect band gap.^6a Thereby, the indirect band gap of Sn_1−xZn_xWO₄ nanocrystals is estimated from the graph of hν versus (αhν)^1/2 for the absorption coefficient α. The absorption coefficient α is related to the band gap E_g as

αhv = A(hv − E_g)^1/2 (1)

where hν is the incident photon energy and A is a constant. The energy band gap E_g is determined as 1.90 eV for pure SnWO₄ nanocrystals and 1.70 for Sn_1−xZn_xWO₄ (x = 0.018) nanocrystals. Apparently, Zn²⁺ doping can lead to the red shift of the band gap energy, which is similar to that observed in many doped nanocrystals.¹³ However, further increase of the Zn²⁺ dopant concentration leads to no additional band gap shift.

Fig. 5 Optical band gap of Sn_1−xZn_xWO₄ nanocrystals.

What then is the reason for Zn²⁺ doped SnWO₄ nanocrystals giving rise to the abnormal shift in the band gap energies? To answer this question, several factors should be taken into consideration, such as quantum size effect, lattice structure variations, electronic structure modifications, and surface structures. With regards to the quantum size effect, as stated above, all Sn_1−xZn_xWO₄ nanocrystals have compatible particle sizes in the range 5.6 to 9.7 nm (calculated XRD results), which are in the region (i.e. <10 nm) for obvious quantum size effects.¹⁴ Therefore, the band gap energies of Sn_1−xZn_xWO₄ nanocrystals are all larger than those of the bulk structure. However, for a particle size reduction by Zn²⁺ doping, we would expect a blue shift of the band gap energies, which contradicts the observed results. On the other hand, from the viewpoint of solid-state physics, the variation of lattice structures also has great impact on the static potential within the cells, which can have consequences for the electronic structures. As the neighboring atoms approach each other with lattice contraction of the solid, the basis functions overlap more strongly, producing increased dispersion of the electron bands in k space and consequently increased bandwidths along the energy axis.¹⁵ In other words, a blue shift of band gap energy is expected with a lattice contraction by Zn²⁺ doping.

To obtain further information on the electronic structures of the Sn_1−xZn_xWO₄ nanocrystals, the density of states (DOS) was calculated on the basis of the plane-wave density function theory program package CASTEP. Fig. 6 displays the variation in DOS with energy. For pure SnWO₄, as indicated in Fig. 6a, the top of the valence band maximum consists of O 2p, Sn 5s, and Sn 5p like orbitals due to the possibility of hybridization of Sn²⁺ with the O 2p orbitals, while the bottom of the conduction band minimum is predominantly composed of W 5d and Sn 5p states. Finally, the band gap energy of SnWO₄ was calculated to be 1.65 eV. For Zn²⁺ doped in SnWO₄ and substituted at the Sn²⁺ lattice site, the conduction band and valence band remain almost the same as those of pure SnWO₄ (Fig. 6b). It is noted that the Zn 4s and Zn 4p orbitals show contributions to the top of the valence band and the bottom of the conduction band. Zn 3d orbitals mainly contribute to the middle of valence band (Fig. S3, ESI†). The band gap energy for Sn_1−xZn_xWO₄ (x = 0.06) was calculated to be 1.63 eV, which is a bit smaller than that of pure SnWO₄. This small red shift of the band gap energy could be attributed to the difference in electronegativity of the Zn²⁺ and Sn²⁺ ions. Till now, numerous works have been concentrated on the semiempirical correlation between optical band gap values of oxides and the difference in electronegativities of the elements.¹⁶ For binary oxides, the general relationship between the optical band gap of crystalline oxides and the difference in electronegativity of their constituents was suggested according to Phillips:¹⁷

where E_i is the extra-ionic energy unit orbitally dependent on the hybridization configuration. X_M and X_O are the electronegativities of the metal and oxygen, respectively. The stoichiometric coefficient y is obtained from the oxide formula MO_y; D_M-M and D_O-O are the bond energies of diatomic molecules in the gas phase, and R represents a repulsive term. For d-metal oxides, by plotting the experimental band gap values versus the square of the difference between the electronegativities of metal cation and oxygen, the above formula can be rewritten as a function of (X_M − X_O)², which is described as follows:

E_g = 1.347 × (X_M − X_O)² − 1.488 (3)

Fig. 6 Density of states (DOS) of pure SnWO₄ and Sn_1−xZn_xWO₄ (x = 0.06). The horizontal dashed line represents the Fermi level.

As for multi-metal oxides (i.e. BaTiO₃, NaTaO₃, La₂Ti₂O₇),^16b the electronegativity (X_M) is replaced by the cationic group average electronegativity (X̄_c,a):

where X_A and X_B represent the electronegativity values of the two metallic cations in the oxides. Here a and b are the stoichiometric coefficients of the cations in multi-metal oxides A_aB_bO_o. Having these in mind, the band gap energies were estimated to be 2.68 and 2.64 eV for pure SnWO₄ and Sn_1−xZn_xWO₄ (x = 0.06), respectively. The calculated band gap energies here are larger than the theoretical values, which may be ascribed to the large electronegativity difference between the Sn²⁺ and W⁶⁺ ions. But the red shift value of the band gap energy is compatible with that of 0.02 eV for the theoretical one. However, the as-measured band gap shift is about 0.2 eV, while the electronegativity induced change is only about 0.02 eV. There must be some other reason that is responsible for the red shift of the band gap energies. It is worth pointing out that, as the particle size decreases, the surface to volume ratio increases, which results in an increase in the number of broken and dangling bonds. The surface area is greatly enhanced from 54 to 93.1–101.6 m² g⁻¹ for pure and doped SnWO₄ nanocrystals. The enlarged surface area of the Sn_1−xZn_xWO₄ nanocrystals would give rise to a highly disordered and defected surface structure, which gives sub-bands of defects in the gaps between nanocrystals.¹⁸ The near identical surface area of Sn_1−xZn_xWO₄ nanocrystals would predict defect sub-bands in the gaps between nanocrystals that result in a similar red shift in optical absorption. Consequently, it is concluded that the balance of quantum size effect, lattice contraction, electronegativity, and surface defect centers led to the abnormal band gap narrowing of Sn_1−xZn_xWO₄ nanocrystals by Zn²⁺ doping.

The impact of Zn²⁺ doping on the surface areas of SnWO₄ nanocrystals was investigated using nitrogen adsorption and desorption isotherm curves. As shown in Fig. 7, the isotherm curves of pure and doped SnWO₄ nanocrystals are characterized by a hybrid type between types I and IV in the Brunauer–Deming–Deming–Teller (BDDT) classification.^19,20 The hysteresis loop resembles type B in de Boer's classification, which matches with the adsorbents with slit-shaped pores between the parallel layers. It is worth noting that the BET surface area of SnWO₄ nanocrystals is only about 54 m² g⁻¹, which, however, increases to ∼100 m² g⁻¹ by Zn²⁺ doping (inset of Fig. 7). The alteration of surface area of the Sn_1−xZn_xWO₄ nanocrystals predicts significantly enhanced photocatalytic activities.

Fig. 7 Nitrogen adsorption–desorption isotherms of Sn_1−xZn_xWO₄ nanocrystals. The inset shows the BET surface area as a function of Zn²⁺ doping concentration.

The photocatalytic activity of Sn_1−xZn_xWO₄ nanocrystals was evaluated using methyl orange (MO) degradation as the model reaction. Temporal changes in the concentration of MO were monitored by examining the variations in maximal absorption in the UV-vis spectra at 464 nm. Fig. 8a shows the temporal evolution of the spectral changes of the MO mediated by Sn_1−xZn_xWO₄ (x = 0.045). It can be found that the concentration of MO decreases quickly under visible light irradiation, and the peak (λ = 464 nm) nearly disappears after 25 min, while a new peak at 245 nm emerges, indicating that MO was degraded but was not completely mineralized in 25 min,^2e which can also be verified by chemical oxygen demand (COD) data (Fig. S4, ESI†). Fig. 8b illustrates the photocatalytic activity for the degradation of MO under visible light irradiation over all Sn_1−xZn_xWO₄ (x = 0–0.060) nanocrystals. It is clear to see that the photocatalytic performance of Sn_1−xZn_xWO₄ nanocrystals can be flexibly tuned by Zn²⁺ doping. With the increase in Zn²⁺ doping concentration from 0 to 0.045, Sn_1−xZn_xWO₄ nanocrystals showed greatly enhanced photocatalytic activity, while further increase in Zn²⁺ content led to an obvious decrease in photocatalytic performance.

Fig. 8 UV-vis spectral changes of MO in aqueous Sn_1−xZn_xWO₄ (x = 0.045) dispersion as a function of irradiation time (a) and normalized concentration of methyl orange versus visible light irradiation time in the presence of Sn_1−xZn_xWO₄ nanocrystals at various Zn²⁺ doping concentrations (b).

Generally, when a semiconductor absorbs a photon with energy greater than or equal to the band gap energy, an electron would be promoted from the valence band to the conduction band, leaving a hole in the valence band. If this charge separation is valid, these electrons and holes could migrate to the surface of the semiconductor to react with the adsorbed O₂ and H₂O, respectively, in producing the redox sources of the O₂⁻· and OH· radical species, which lead to the efficient destruction of organic pollutants.²¹ On the basis of UV-visible and DFT results, one can see that all Sn_1−xZn_xWO₄ nanocrystals have a narrow band gap in the visible region. In order to fully understand the effect of the electronic structure characteristics on the photocatalytic activity, the band edge positions of the valence band (VB) and conduction band (CB) of the Sn_1−xZn_xWO₄ nanocrystals were determined because they are also strongly related to the process of photocatalytic oxidation of organic compounds. Although the band edge positions are not easily determined experimentally, a theoretical prediction is possible using concepts of Mulliken electronegativity. The conduction band edge energy of a semiconductor at the point of zero charge can be expressed by²²

E_CB = X − E^e − 0.5E_g (5)

where E_CB is the CB edge potential and X is the Mulliken electronegativity of the semiconductor, which is the geometric mean of the electronegativities of the constituent atoms. The Mulliken electronegativity of an atom is the arithmetic mean of the atomic electron affinity and the first ionization energy. Generally, the atomic electron affinity is one order of magnitude smaller than the first ionization energy. Thus, Mulliken electronegativity is approximately equal to half of the first ionization energy. E^e is the energy of free electrons on the hydrogen scale (≈4.5 eV), and E_g is the band gap energy of the semiconductor.²³ According to this expression, the rough CB edge potentials of pure SnWO₄ and Zn²⁺ doped samples were determined to be 1.10 and 1.20 eV with respect to the normal hydrogen electrode (NHE). On the basis of the relationship between the absolute vacuum energy and the normal electrode potential,²⁴ the apparent E_CB of pure and Zn²⁺ doped samples are −1.10 and −1.20 V (versus NHE), which are more negative than that of O₂/O₂⁻· (−0.33 V versus NHE).²⁵ Therefore, it is possible for the adsorption oxygen to capture an electron from the CB of Sn_1−xZn_xWO₄, resulting in an active oxygen species (O₂⁻·), subsequently leading to MO decomposition. On the other hand, the holes may react either with electron donors in the solution or with hydroxide ions to produce powerful oxidizing species like OH· radical species. According to the above-mentioned results, the VB edge potentials were obtained to be about 0.80 and 0.50 V (versus NHE) for pure and Zn²⁺ doped samples. Apparently, the VB edge potentials for all Sn_1−xZn_xWO₄ are more negative than those of OH/OH⁻ (1.9 V versus NHE) and OH/H₂O (2.73 V versus NHE),²⁶ indicating the absence of OH· radical species in the photocatalytic process for the degradation of MO. To further understand the nature of the primary active species involved in visible degradation of MO in an aqueous phase over Sn_1−xZn_xWO₄ nanocrystals, we have carried out control experiments by adding a scavenger for holes (h⁺) and OH· radical species. As shown in Fig. 9, with 0.1 g of ammonium oxalate (AO) as a hole-scavenger added to the solution,²⁷ the rate of degradation of MO over Sn_1−xZn_xWO₄ nanocrystals decreases remarkably. When 2 mL tert-butyl alcohol (TBA),²⁸ as a scavenger for the OH· radical species, is added, a slight decrease in degradation rate is observed, indicating the absence of the OH· radical species, which is in accordance with the theoretical prediction. This suggests that, under visible light irradiation, the holes play a dominant role towards the degradation of MO over Sn_1−xZn_xWO₄ nanocrystals.

Fig. 9 Photocatalytic degradation of MO in the presence of ammonium oxalate (AO, scavenger for holes) or tert-butyl alcohol (TBA, scavenger for OH· radical species) under visible irradiation over Sn_1−xZn_xWO₄ (x = 0.045) nanocrystals.

As for the enhancement of the photocatalytic activities of Sn_1−xZn_xWO₄ nanocrystals, several factors should be taken into consideration. (1) BET surface area may impose a great impact on the photocatalytic activity of the nanocrystals. The majority of photocatalytic reactions occur at semiconductor surfaces, and therefore the photocatalytic activities of semiconductor oxides are usually greatly improved by the increase in surface area.²⁹ For the present Sn_1−xZn_xWO₄ nanocrystals, the BET surface area is ∼54 m² g⁻¹, which, however, greatly increases to ∼100 m² g⁻¹ with Zn²⁺ doping. The increase in surface area will improve the surface active sites and photonic efficiency, which favors an increased ratio of surface charge carrier transfer rate to electron–hole recombination rate. (2) Doping effects may have a great influence on the charge carriers and the subsequent photocatalytic activity. Doping with transition metal ions including V, Fe, Co, and Cu has shown both positive and negative effects on the photocatalytic activity.^3a–c,30 For instance, Choi et al. reported that the photocatalytic activity of transition metal ion doped TiO₂ increases as the dopant concentration is increased, but that there exists a complex dependence on dopant concentration, dopant energy state in the lattice, d-electron configuration, distribution of dopants, and electron donor concentration.^4a On the basis of the XRD and DFT calculation results, Zn²⁺ ions were homogeneously incorporated into SnWO₄, leading to the hybridization of the W 5d and Zn 3d orbitals, which may increase the mobility of photogenerated charge carriers in the valence band, and subsequently enhance the photocatalytic activity. On the other hand, Zn²⁺ ions with higher doping concentrations may also serve as a recombination center for electrons and holes, thus diminishing the overall photocatalytic activity of Sn_1−xZn_xWO₄ nanocrystals. Considering the BET surface area and doping effects, Sn_1−xZn_xWO₄ nanocrystals showed an optimized photocatalytic activity at a Zn²⁺ doping level of x = 0.045.

Conclusion

Sn_1−xZn_xWO₄ nanocrystals were successfully prepared via a microwave-assisted hydrothermal approach. The homogeneous incorporation of Zn²⁺ ions into the SnWO₄ host lattice led to a monotonous lattice contraction. The solubility of Zn²⁺ in SnWO₄ was measured to be about 0.060 by chemical analyses. One notable result of this work is the demonstration that, by Zn²⁺ doping, SnWO₄ nanocrystals can be controlled to achieve morphological alteration, enlarged BET surface area, band gap narrowing and highly enhanced photocatalytic performance. With well-controlled morphology, surface area, and electronic structure via Zn²⁺ doping, the photocatalytic performance of Sn_1−xZn_xWO₄ nanocrystals was optimized at a Zn²⁺ doping level of x = 0.045. Such a finding is fundamentally important, and may help to provide hints for developing and designing new photocatalytic semiconductors.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (Grants 21103081, 51102129 and 20967004), the Program of Higher-Level Talents of Inner Mongolia University (Grant Z20090106), the Project of Scientific and Technological Innovation Team of Inner Mongolia University (12110614).

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Footnote

† Electronic Supplementary Information (ESI) available: Figures showing XPS spectra of Sn_1−xZn_xWO₄ nanocrystals and density of states (DOS) of Zn in Sn_1−xZn_xWO₄. See DOI: 10.1039/c2ra20401k/

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