Tunable Optical and Photocatalytic Performance Promoted by

Article pubs.acs.org/JPCC

Tunable Optical and Photocatalytic Performance Promoted by Nonstoichiometric Control and Site-Selective Codoping of Trivalent Ions in NaTaO3 Yiguo Su,† Liman Peng,† Jianwei Guo,† Shushu Huang,† Li Lv,‡ and Xiaojing Wang*,† †

College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, Inner Mongolia 010021, P. R. China Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, Inner Mongolia 010021, P. R. China

‡

S Supporting Information *

ABSTRACT: The present work explores a solid state route to synthesis of trivalent ions (Eu3+, La3+, etc.) doped NaTaO3 with controlled nonstoichiometric chemistry and lattice parameters with an aim to exploring electronic structure and photocatalytic performance. All samples were fully characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic absorption spectrophotometry, UV− vis diffuse reflectance spectroscopy, and photoluminescence measurement. By employing Eu3+ as a model trivalent ion doped in NaTaO3 lattice, the effects of siteselective doping and nonstoichiometric chemistry on the lattice parameters, band gap structure, photocatalytic activity toward methylene blue solution, and photocatalytic hydrogen evolution were systematically investigated. A nonstoichiometric Na/Ta molar ratio led to site-selective occupation of Eu3+ ions which was changed from sole substitution to dual substitutions. Meanwhile, the nonstoichiometric Na/Ta molar ratio and site-selective occupation of Eu3+ resulted in a monotonous lattice expansion and local symmetry distortion. Lattice variation, doping effects, and its relevant defect chemistry had a great impact on the ν3 mode vibration of the O−Ta bond, which became asymmetric and shifted toward higher wavenumbers. Moreover, contrary to theoretical predictions, Eu3+-doped NaTaO3 nanocrystals showed an abnormal narrowing of the band gap energies and weak visible light absorption with variation of Na/Ta molar ratios, which is thought to be related to doping effects, defect chemistry, and variation of lattice parameters. With well-defined lattice structure and defect centers and electronic structure via nonstoichiometric control and trivalent ions doping, the photocatalytic activity of trivalent ions-doped NaTaO3 can be well regulated and optimized.

1. INTRODUCTION Metal ions-doped semiconducting materials have garnered great interest due to their boosted optoelectronic, catalytic, luminescent, and magnetic properties originating from alteration of particle size, morphology, surface features, and selective site occupation in the crystalline structure.1−6 In this regard, research on doping of oxide semiconductors with metal ions showing controllable photocatalytic performance is emerging as a lucrative way to dissolve environmental issues and the energy crisis that humankind is facing now.7−9 A number of studies performed on simple and complex oxide semiconductors as well as their optimal photocatalytic characteristics have been reported, such as TiO2, SnO2, ZnO, SrTiO3, NaTaO3, Bi2WO6, and so on.10−16 Basically, the common characteristic of these studies is highly controversial in the cognition of the photocatalytic activity because there exists very complicated factors, such as particle size, electronic structure, dopant concentration, surface area, and localized d levels of dopants.17,18 Very often the reported studies have merely focused on a fine characterization of the chemical and physical properties of photocatalytic response but have not concerned detailed microstructural and crystallographic analyses that determine the native photocatalytic properties of © 2014 American Chemical Society

semiconductors. In order to uncover the nature of structuredependent properties, it is highly essential to realize efficient control of chemical composition, phase structure, and siteselective substitution of doped semiconductors. Tantalate and niobate perovskites of general formular ABO3 are of considerable interest as photocatalysts, ionic conductors, and luminescent host materials.19−21 Due to a large radius difference between A- and B-site ions, the dopants can be selectively located at the A or B sites, which can be used to accurately control the physicochemical properties of these oxides by selecting a suitably sized or suitably charged dopant. As a typical perovskite material, NaTaO3 exhibits close structural features to many semiconductors, such as KTaO3, GdFeO3, LaCrO3, SrCe0.9Yb0.1O3−δ, and so forth. Orthorhombic NaTaO3 shows a close-packed structure with Na+ ions within the corner connected to a TaO6 octahedral framework. Thus, the sole substitution at the Na site and/or dual substitution at Na and Ta sites can be achieved by choosing appropriate synthesis conditions.22,23 The photocatalytic Received: December 14, 2013 Revised: April 23, 2014 Published: May 12, 2014 10728

dx.doi.org/10.1021/jp412236u | J. Phys. Chem. C 2014, 118, 10728−10739

The Journal of Physical Chemistry C

Article

samples. XPS analyses were carried out on an ESCALab220iXL with a monochromatic Al Kα and charge neutralizer. An Edinburgh Instruments FLS920 spectrofluorimeter was used to determine the photoluminescence properties of the samples. A photomultiplier tube (PMT) (R955, Hamamatsu) was used to detect the signals in both UV and visible regions. The resolution of all spectra is maintained to be 0.05 nm. Lowtemperature measurement was performed on a closed cycle cryostat (DE202, Advanced Research System).6 All spectra recorded were ratio corrected to remove the wavelengthdependent intensity of the lamp. In order to get further specific electronic structural features of NaTaO3 samples, density functional theory (DFT) based calculations were carried on the CASTEP program package using ultrasolft pseudopotentials. The exchange and correlation potentials were determined using the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) and a kinetic energy cutoff of 420 eV.23 A 2 × 2 × 1 supercell of NaTaO3 was used to explore the doping effects of trivalent ions. Geometry relaxation is carried out until the residual forces were smaller than 0.01 eV Å−1, and the convergence threshold for self-consistent iteration was set at 5 × 10−7 eV.18 2.3. Photocatalytic Reactivity Test. Photodegradation of methylene blue (MB) has been extensively studied as a model reaction to evaluate the photocatalytic performance of semiconductors. Herein, the procedure of photocatalytic activity experiments was as follows: First, 25 mg of the NaTaO3 samples was dispersed in a 100 mL beaker containing 50 mL of 2 × 10−5 mol/L MB aqueous solution to form homogeneous suspensions. Then, these homogeneous suspensions were stirred in the dark for 12 h to establish the equilibrium of MB absorption/desorption on the NaTaO3 sample surfaces before illumination. All suspensions were allowed to irradiate under a 300 W mercury lamp with or without a filter (λ ≥ 420 nm). Subsequently, a certain amount of suspension was extracted from the beaker at given intervals. Extracted suspensions were centrifuged, and supernatants were dropped in a cuvette to record the UV−vis absorption spectra by a UVIKON XL/XS Spectrometer. Photocatalytic hydrogen evolution reactions were carried out in a quartz flask sealed system with a side window for irradiation. For photocatalytic H2 production, 0.15 g of NaTaO3 catalyst was dispersed in a mixture of 30 mL of ethanol and 300 mL of deionized water in a flask to form a suspension. Before irradiation, the reaction system was purged with N2 gas and evacuated several times by a mechanical pump to ensure the reactor in an inert atomosphere.16 Then the suspension was stirred and irradiated under a 300 W xenon lamp with/without a filter (λ ≥ 420 nm). The H2 evolution rate was measured on an online gas chromatograph (GC2014C, TCD, N2 as the carrier).

performance of NaTaO3 could be significantly modulated by site-selective doping of foreign ions in the lattice. Therefore, experimental identification of the lattice modification by siteselective doping and chemical composition control is advantageous for regulation of the chemical and physical properties of NaTaO3 and other perovskites. In this work, a series of trivalent ions-doped NaTaO3 with tunable Na/Ta molar ratio was synthesized via a traditional solid state method with an aim of exploring the impact of controllable nonstoichiometric chemistry and site-selective doping effects on the microstructure and optical and photocatalytic performance. By combination of XRD analyses and photoluminescence spectra using the Eu3+ ion as a structural probe, the homogeneity and site-selective substitution of trivalent ions in NaTaO 3 material were systematically investigated. This method will give a legible picture of the location of cations in the NaTaO3 lattice with variation of Na+ ions in these materials, since the local environment surrounding Eu3+ has a great impact on the photoluminescence performance of the lanthanide ion.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. All chemical reagents (analytical grade) were used without any purification. A typical synthetic procedure of Eu3+-doped NaTaO3 (initial Eu3+ concentration (x) to Ta was fixed to be 0.02) is described as follows: 1.0 mmol of TaCl5, 0.02 of mmol Eu(NO3)3, and a given amount of NaCO3 were fully mixed in an agate mortar where the initial molar ratio of Na/Ta was fixed to be 1, 1.5, 2.5, 3, 4, 4.5, and 5. Then the mixture was transferred in to a corundum crucible and calcined at 900 °C for 9 h. After cooling to room temperature, the samples were washed with hydrochloric acid and distilled H2O several times and dried at 80 °C for 3 h. For comparison, surface Eu3+-doped NaTaO3 sample was also prepared. Briefly, 1.0 mmol of TaCl5 and 3.0 mmol of NaCO3 were fully mixed in an agate mortar and then transferred into a corundum crucible and calcined at 900 °C for 9 h. After cooling to room temperature, the sample was washed with hydrochloric acid and distilled water several times and dried at 80 °C for 3 h. Then, 0.02 mmol of Eu(NO3)3 was dissolved in 20 mL of distilled H2O. The as-prepared white NaTaO3 was added into the above Eu(NO3)3 solution with vigorous stirring. The above mixture was kept at 60 °C overnight and dried at 80 °C for 3 h. Finally, the dried mixture was transferred into a corundum crucible and calcined at 300 °C for 3 h. Trivalent ions-doped NaTaO3 was prepared under similar conditions, where the initial dopant concentration of trivalent ions to Ta was fixed to be 0.02 and the initial Na/Ta molar ratio was fixed to be 2.5. For comparison, undoped NaTaO3 was also prepared with the initial Na/Ta molar ratio to be 2.5. 2.2. Sample Characterization. X-ray power diffraction (XRD) was used to characterize the phase structures and particle sizes of all as-prepared samples, which was performed on a Rigaku DMAX2500 X-ray diffractometer with a copper target. Transmission electron microscopy (TEM) was used to determine the morphology of the as-prepared samples, which was performed on a JEM-2010 apparatus with an acceleration voltage of 200 kV. The product compositions were measured by a GBC-Avanta atomic absorption spectrophotometer. A UVIKON XL/XS spectrometer was used to measure the optical diffuse reflectance spectra of the samples. A Perkin Elmer IR spectrometer was used to measure the surface structure of the

3. RESULTS AND DISCUSSION 3.1. Phase Formation and Structural Analysis. Figure 1a illustrates the XRD patterns of Eu3+-doped NaTaO3 synthesized by a simple solid state method. A preliminary study indicated that a lower Na/Ta initial molar ratio led to formation of Na2Ta4O11 structure (Figure S1, Supporting Information). When the Na/Ta initial molar ratio increased from 2.5 to 5.0, the diffraction peaks of all samples matched well with the standard data for NaTaO3. Narrowed peaks in Figure 1a gave evidence that NaTaO3 samples were fully 10729



Article

documented in many oxide systems,24−27 such as CaWO4:Zn2+, ZnO:Al3+, SnWO4:Zn2+, and TiO2:Nd3+, doping of impurity ions into a semiconducting host strongly influences the microstructure and surface features of semiconductors. Thereby, the nonstoichiometric feature of NaTaO3 would greatly influence the lattice structures and the subsequent site-selective trivalent ions doping. Using the Retica Rietveld program (Version LHPM, 2000, B. A. Hunter and C. J. Howard, Lucas Heights Research Laboratories, Menai 2234, Australia),5 the lattice volumes of Eu3+-doped NaTaO3 were determined as a function of Na/Ta molar ratio. Upon varying the Na/Ta molar ratio from 0.982 to 1.064, Eu3+-doped NaTaO3 showed a lattice expansion as indicated by the monotonous increase in lattice volume. A further increase of the Na/Ta molar ratio to 1.116 caused a slight decrease in the lattice volume. In order to clarify this observation, several factors should be considered, such as particle sizes, surface lattice relaxation, doping effects, and so forth. According to previous literature, the variation of particle sizes has a significant consequence on lattice parameters.28 Nanocrystalline semiconductors give birth to a high surface to volume ratio that leads to large amounts of surface-exposed atoms. Relaxation of the exposed atoms would generate an imbalance of the surface charges, resulting in surface defect dipoles that are responsible for lattice expansion.29 For NaTaO3 samples, the calculated particle size was ∼65 nm from the diffraction peak (020) using the Scherrer formula, indicating a much large particle size, which can also be confirmed by TEM observations. A larger particle size means less surface-exposed atoms, which often results in minor variation of lattice parameters. As for doping effects, the substitutional site of the Na+ and Eu3+ ions in the NaTaO3 host should be specified because the site-selective occupation of impurity ions into semiconductors will not only alter the lattice parameters, electronic structure, as well as defect chemistry but also result in boosting physical properties. Na+ ions tend to occupy the lattice site at a lower Na/Ta molar ratio and are highly possible to locate at the interstitial site rather than at the Ta5+ site with an increase of Na/Ta molar ratio because (1) the ionic radius for Na+ is 0.1032 nm, much larger than that of 0.064 nm for Ta5+,30 (2) Pauling’s electronegativity of Na+ is 0.956, which is much smaller than that of 1.881 for Ta5+,31 and (3) a large difference of charge state exists between Na+ and Ta5+. Meanwhile, the simulated lattice volumes of pure NaTaO3 increased after incorporation of Na+ at the interstitial site, which can confirm the above-mentioned supposition (Table S2, Supporting Information). To further specify the impact of nonstoichiometric Na/Ta on NaTaO3 lattice parameters, the Eu3+-free NaTaO3 systems were prepared under the same condition. With an increase of the initial Na/Ta molar ratio, the lattice volume of Eu3+-free NaTaO3 also gave a lattice expansion. However, a little deviation of the lattice volume occurred in comparison with that of Eu3+-doped NaTaO3 (Figure S3, Supporting Information). On the other hand, the Na content distribution in the NaTaO3 lattice also has a consequence on the variation of lattice parameters. The XPS technique was conducted by etching the Eu3+-doped NaTaO3 with a Na/Ta molar ratio of 1.041 for 40 s (Figure S4, Supporting Information). After Ar+ ion etching, a decrease of the XPS peak intensities for Eu3+-doped NaTaO3 was observed.32 Moreover, the molar ratio of Na/Ta was varied from 1.085 to 1.024 mol % after etching. However, it is still larger than the stoichiometric ratio. This observation indicated that partial Na+ ions preferred to locate at the Eu3+-doped

Figure 1. XRD patterns of Eu3+-doped NaTaO3 with different Na/Ta initial molar ratios (a). Vertical bars are the standard diffraction data of NaTaO3 (JCPSD card No. 25-0863). TEM (b) and HRTEM (c) images of Eu3+-doped NaTaO3 with a Na/Ta initial molar ratio of 2.5.

crystallized with fine nature. No impurities of other phases were observed, indicating formation of single-phase orthorhombic NaTaO3 structure. As indicated in the transmission electron microscopy (TEM) image (Figure 1b), NaTaO3 exhibited a large particle size and an aggregated feature. The highresolution TEM image verified the fine crystalline nature of NaTaO3 samples. As shown in Figure 1c, the lattice plane space of Eu3+-doped NaTaO3 was determined to be 0.394 nm, which is compatible with 0.388 nm for the (020) plane of NaTaO3. Nonstoichiometry is controlled by the variation of Eu3+ doping and Na/Ta molar ratio. It is noted that the ionic radius of Eu3+ is 0.1066 nm, being slightly larger than 0.1032 nm for Na+ and much larger than 0.064 nm for Ta5+. Thereby, it is expected that substitution of Eu3+ either at the Na+ site or at the Ta5+ site will give rise to a lattice expansion, which can be verified by calculated data (Table S2, Supporting Information). On the other hand, the variation of the Na/Ta molar ratio led to tunable characteristic of Na+ ions in NaTaO3, which also results in the variation of lattice parameters. As shown in Figure 2, the as-measured Na/Ta molar ratio increased with an increase of Na/Ta initial molar ratio, which indicated a nonstoichiometric feature of NaTaO3. The Eu3+ doping level was near identical in all samples, which is ∼1.7%. As well

Figure 2. Lattice volume, as-measured molar ratio of Na/Ta, and molar ratio of Eu/Na+Eu as a function of Na/Ta initial molar ratio for Eu3+-doped NaTaO3. 10730



Article

Eu2O3, indicating Eu3+ ions were homogeneously incorporated in NaTaO3 lattice. Nevertheless, no trace of emission peak was observed for surface-doped sample. It is well accepted that the 5 D0−7F0 transition is highly directed by local symmetry of cationic sites in the orthorhombic structure of NaTaO3 that have C1 site symmetry for Ta ions and Cs site symmetry for Na ions,35 for which the 5D0−7F0 transition of Eu3+ is allowed. Having this in mind, it is possible to specify the site-selective occupation of Eu3+ ions as a function of Na/Ta molar ratio. As shown in Figure 3, the emission in the range of 577−582 nm for Eu3+-doped NaTaO3 sample with a Na/Ta molar ratio of 0.982 showed a symmetric broad band feature, which can be reproduced by a single Gaussian line peaking at 580.2 nm. This single emission peak is thought to be related to Eu3+ ions located in the bulk site. Because Eu3+ in surface sites showed no obvious 5D0−7F0 emission band and surface doping of Eu3+ in NaTaO3 is limited due to the large particle size, it is highly necessary to shed light on whether Eu3+ ions were located at the Na+ site or Ta5+ site. Previous literature reported on La3+doped NaTaO3 nanocrystals indicated that lanthanide ions are likely to substitute at the Na+ site because of ionic radii similarity.30,36 These results suggest that Eu3+ may substitute at the Na+ site for this sample because of the difference in ionic radii as mentioned above. Moreover, a lower Na/Ta molar ratio can also promote Eu3+ to occupy at the Na+ site. Nevertheless, in view of the charge state difference of Eu3+ and Na+, the local symmetry surrounding Eu3+ could give a deviation from Cs symmetry. According to previous literature on Bi-doped NaTaO3,22 Bi dopant is likely to substitute at Na under Nadeficient conditions. Herein, the Na/Ta molar ratio is 0.982, where the amounts of Na+ and Eu3+ are a bit lower than that of Ta5+ ions, implying that substitution of Eu3+ at the Na+ site is likely favorable. Therefore, it is likely that the charge compensating pattern of Eu3+ to substitute in the Na+ site can be written via the path Eu3+ = Na+ + 2□ (where □ is a Na site vacancy). This observation is different from that of Eu3+doped LiTaO3, where Eu3+ are likely to substitute at the Ta5+ site.37 Due to similar ionic radii, the occupancy of Eu3+ at the Na+ site has little impact on lattice expansion. Nevertheless, the presence of cation vacancies will give rise to a slight lattice shrinkage. Notably, with the increase of Na/Ta molar ratio up to 1.064 in Eu3+-doped NaTaO3 samples, the 5D0−7F0 emission band became asymmetric, which can be reproduced by two Gaussian lines locating at 580.2 and 579.7 nm, respectively. This emission band at 579.7 nm can be assigned to substitution of Eu3+ at the Ta5+ site in NaTaO3 samples. Meanwhile, the integrated intensity of the band at 579.7 nm gradually increased with an increase of Na/Ta molar ratio, indicating Eu3+ ions were compelled to occupy at the Ta5+ site by excessive Na+ ions. It seems that, under Na+ excess conditions, the amounts of Na+ and Eu3+ are larger than that of Ta5+ and Eu3+ ions, suggesting that the occupancy of Eu3+ at the Ta5+ site is likely favorable. As Eu3+ occupied at the Ta5+ site, the local charge neutrality equation in the NaTaO3 lattice can be written as follows: 2Naint+ + Eu3+ = Ta5+ (where Naint+ means Na at the interstitial site). Considering the large difference of the ionic radii between Eu3+ and Ta5+ it is obvious that Eu3+ located at the Ta5+ site will result in lattice expansion. On further increase of the Na/Ta molar ratio to 1.116, a novel emission peak at 578.2 nm associated with 5D0−7F0 transition was observed. This observation indicated Eu3+ occupied at least three sites in the NaTaO3 lattice. As mentioned above, Na+ located at the interstitial site and Eu3+ occupied either at the Na+ site or at the

NaTaO3 surfaces. Moreover, the nonstoichiometric feature of NaTaO3 led to excessive Na+ ions that can result in interstitial oxygen (Oi″) in bulk and/or surface layers of NaTaO3 due to charge compensation effects. Consequently, Na+ located at interstitial/surface sites, and its relevant oxygen defects would certainly induce a lattice distortion and subsequent lattice expansion. On the other hand, the location of Na+ ions in the NaTaO3 lattice may have great consequences on the site-selective occupation of Eu3+ ions that also contribute to the lattice variation (Table S2, Supporting Information) and properties. Here, Eu3+ ions play dual roles including (1) Eu3+ ions incorporated in NaTaO3 to modulate the electronic structure and defect chemistry and (2) Eu3+ ions can serve as a probe to detect microstructural variation induced by nonstoichiometric Na/Ta. As a typical trivalent ion, Eu3+ doped in NaTaO3 host lattice at either the Na+ site or the Ta5+ site can create certain types of defective centers due to the charge difference between Eu3+ ions and Na+ and/or Ta5+ ions. It is then crucial to investigate the site occupation for Eu3+ ions in NaTaO3 and its relevance to lattice variation. The Eu3+ ion has been intensively investigated as a structural probe due to its nondegenerate 5D0 → 7F0 transition,33,34 which gives evidence of the inequivalent crystallographic sites. Using this technique, we anticipate that the local site occupation of Eu3+ ions in NaTaO3 can be resolved from the high-resolution spectra of doped Eu3+ ions. The photoluminescence emission spectra of Eu3+-doped NaTaO3 samples exhibited intense emission which is related to the typical 5D0−7F2 transition, irrespective of Eu3+ ions locating in the bulk site or on the surface site (Figure S5, Supporting Information). Figure 3 illustrates the photoluminescence emission spectra of Eu3+-doped NaTaO3 samples under 397 nm excitation at 10 K. The emission spectra of Eu2O3 and surface-doped sample (Figures S5 and S6, Supporting Information) were also given for comparison. All as-prepared samples exhibited 5D0−7F0 emission in the range from 577 to 582 nm, which is very difference from that of

Figure 3. Ten Kelvin photoluminescence emission spectra of the 5 D0−7F0 transition for Eu3+ as a function of Na/Ta initial molar ratio in Eu3+-doped NaTaO3 samples under excitation at 397 nm. 10731



Article

Ta5+ would perturb the local microstructure and lead to deviation of the site symmetry of Eu3+ surroundings. The variation of local symmetry may contribute to the new emission peak of the 5D0−7F0 transition at 578.2 nm. Associated with the nonstoichiometry, site-selective doping, and lattice variation is the modulation of optical, electronic, and photocatalytic properties. Surface chemistry and lattice perturbations in the local structure of the Eu3+-doped NaTaO3 samples were studied by Fourier transform infrared (FT-IR) spectra. Figure 4 illustrates

enhancement of the Ta−O strength and a subsequent blue shift of the ν3 vibration, whereas the atomic masses of the dopants also have consequences on the IR vibrational properties. Basically, stretching vibrations resemble the oscillations of two atoms connected by a spring, which can be treated by Hook’s law for a first approximation. The stretching vibrational frequency can be determined by the following equation: ν = 1/(2πc)(k/μ)1/2, where ν is the wavenumber, k is the force constant of the bond, and μ represents the effective mass.46 For the present Eu3+-doped NaTaO3 samples, the atomic mass for Eu3+ is larger than that for Ta. Consequently, substitution of Eu3+ at the Ta5+ site can increase the effective mass and shift the ν3 mode vibration to lower frequencies. On the other hand, interstitial Na+ ions and the relevant defects also have a great impact on the vibrational properties. Na+ located at the interstitial site in NaTaO3 lattice can modulate the local structure and defect chemistry to disrupt the local electroneutrality and electron density that will vary the electron− phonon coupling intensity, resulting in variation of the relative intensity of the ν3 mode vibration. Previous work on alkali metal ions and Er3+-codoped Y2O3 nanocrystals47 demonstrated that Li+ and Na+ ions may occupy at interstitial sites in the lattice, resulting in the distortion of local structure and subsequent blue shift of the Y−O bond vibration. Therefore, with an increase of Na/Ta molar ratio, the O−Ta−O centers, perturbed by the nearby Na+ atoms, can cause a largely shifted position of the ν3 mode vibration. Meanwhile, excessive Na+ ions occupying interstitial/surface sites may lead to formation of interstitial oxygen (Oi″) in bulk and/or surface sites of NaTaO3 due to charge compensation effects. The existence of interstitial oxygen can be verified by two new weak absorption bands at 880 and 1060 cm−1 for Eu3+-doped NaTaO3 (Na/Ta = 1.116), which is reasonable to ascribe to the O−Ta−Oi stretches, being similar to that is observed in La3+-doped PbWO4 crystals.48 Oxygen defect can weaken the Ta−O bond strength and lead to a red shift of the IR absorption peak.49 Consequently, the combination of doping effects, nonstoichiometric feature, and its relevance to defect chemistry is responsible for the variation of the ν3 mode vibration for Eu3+-doped NaTaO3 samples. 3.2. Modification of Optical Properties Via Eu3+ Doping and Nonstoichiometric Control. To specify the Eu3+ doping effects and nonstoichiometric control on band gap structure modification of NaTaO3, the UV−vis absorption performance of Eu3+-doped NaTaO3 was investigated. As illustrated in Figure 5a, all Eu3+-doped NaTaO3 samples exhibited a broad absorption band in the range of 250−320 nm, being related to the typical electronic transition from the O 2p to the Ta 5d orbital. A relatively weakened broad absorption ranging from 320 to 500 nm was also observed for all samples. Basically, the optical band gap near the band edge of a crystalline semiconductor can be described as follows

Figure 4. IR spectra of Eu3+-doped NaTaO3 samples. (Inset) Enlarged Ta−O vibration for given Na/Ta molar ratios.

the IR spectrum of Eu3+-doped NaTaO3 with a Na/Ta molar ratio of 1.041. Similar IR spectra were also observed for other samples. The broad band appearing at about 3250 cm−1 can be ascribed to the vibration of H−O of surface H2O layers.38 An intense broad band appearing in the region from 1090 to 440 cm−1 is related to the typical Ta−O vibration. Previous work39,40 on orthorhombic KNbO3 showed IR absorption peaks at 766, 623, and 538 cm−1 in the range of 450−1000 cm−1, which can be ascribed to O−Nb−O stretching vibration (ν3 mode) in the corner-shared and edge-shared NbO 6 octahedron. Due to the close structural link between NaTaO3 and KNbO3, the broad band in NaTaO3 was supposed to be originated from O−Ta−O stretching vibration (ν3 mode) TaO6 octahedron. It is noted that with increasing Na/Ta molar ratio the ν3 mode vibration of the O−Ta bond became asymmetric and shifted toward higher wavenumbers (inset of Figure 4). In order to clarify this observation, several factors should be considered. First, the lattice dimension with Eu3+ doping and variation of Na/Ta molar ratios has to be taken into consideration. XRD analysis suggested that Eu3+ doping and nonstoichiometric Na/Ta feature led to apparent lattice expansion. Basically, lattice expansion is not new and has been widely observed for many nanocrystals.41−43 Nevertheless, lattice expansion often gives rise to weakened force constants that may led to a red shift of the IR absorption band.44 This suggestion is apparently in contradicted to the blue shift of the ν3 mode for Eu3+-doped NaTaO3 samples. Therefore, it is necessary to explore some other reasons that can influence the variation of the ν3 mode. One possible reason may be the doping effects of Eu3+. On the basis of previous literature,31 the electronegativity of Eu3+ is 1.190, being much smaller than that of 1.881 for Ta5+. Consequently, the site-selective doping of Eu3+ at the Ta5+ site with an increase of Na/Ta molar ratios may result in an increase of the effective charge surrounding O2−.45 The spatial charge redistribution would lead to an

αhν = A(hν − Eg )n /2

where α, ν, Eg, A, and n are the absorption coefficient, incident light frequency, band gap, constant, and an integer, respectively.9 The value of n as determined for NaTaO3 is 1 due to a direct optical transition type for NaTaO3.50 A band gap narrowing from 4.08 to 4.01 eV was achieved with an increase of Na/Ta molar ratio as illustrated in Figure 4b. Several factors are responsible for this observation, including doping effects, structural modifications, defect chemistry, and so forth. 10732



Article

Figure 6. Band structures of pure NaTaO3 and four doping models.

smaller than the experimental data. This phenomenon has been observed in many calculated systems as a result of the wellknown intrinsic factor of DFT.52 Though DFT calculations often give data that deviate from the experimental results, it still acts as a useful tool to depict the electronic structures of semiconductors due to the fact that only relative positions of the occupied and empty obtials are considered. From Figure 7 it is seen that the minimum conduction band of NaTaO3 consisted mainly of the Ta 5d orbital and a minor hybridization of the O 2p orbital in the energy region from 1.11 to 4.58 eV. The maximum valence band consists of mainly O 2p orbital. The optical band edge is originated from the electronic transition between O 2p and Ta 5d obitals. With the model of Na+ doping at the interstitial site, there is no obvious change for the band structure compared with pure NaTaO3, expected for the highly enhanced density of states and the shifting of the conduction band and the valence band to lower energy. The band gap energy of Nai-doped NaTaO3 was estimated to be 1.59 eV, a bit larger than the data for pure NaTaO3. As documented in previous works,53 the O 2p orbital acts as the main component for the valence band of most oxide semiconductors. As a result, the O 1s chemical shift leads to the chemical shift of the O 2p obital. The decreasing O 1s binding energy gives evidence of enhanced electronic polarizability of oxygen,54 which is in accordance with our IR observations. For the model of Eu3+ substitution at the Na+ site, a new band contributed from the hybridization of Eu 5d and Eu 4f orbitals was observed, which was centered at about 0.59 eV (Figure 7). The appearance of a new electronic band can obviously lead to band gap narrowing of NaTaO3. However,

Figure 5. UV−vis reflectance spectra (a) and band gap energies (b) of Eu3+-doped NaTaO3 as a function of Na/Ta molar ratios.

As for doping effects, numerous works showed that aliovalent cationic doping can modify the local structure and moreover properties. Therefore, Understanding the nonstoichiometric control and site-selective codoping of Eu3+ on band structure modification of NaTaO3 needs detailed knowledge of band structure and partial density of states (DOS) of NaTaO3- and Eu3+-doped NaTaO3 that were calculated on the basis of DFT. On the grounds of the orthorhombic NaTaO3 unit cell, a supercell containing 80 atoms is employed for pure NaTaO3. Four simple different doping models based on this supercell are considered (Figure S7, Supporting Information): One model is obtained by substituting Na at an interstitial site (denoted as Nai). The interstitial Na located at (0.0047, 0.25, 0.0158) after optimization. One model was obtained by replacing one Na with a Eu atom (denoted as Eu@Na); the other one replacing one Ta with a Eu atom (denoted as Eu@Ta). The dual substitute model was set up by substituting Na at the interstitial site and Eu at the Ta site as denoted by Nai−Eu@Ta. The structure of all optimized systems was stable, and all doping models were energetically convergent (Figure S8, Supporting Information). The optimized lattice parameters are a = 5.8626 Å, b = 7.9890 Å, and c = 5.5313 Å for pure NaTaO3, compatible with previous results51 and a bit larger than that of experimental data (Table S2, Supporting Information). As shown in Figure 6, NaTaO3 shows a direct optical transition feature. The band gap energy for pure NaTaO3 was calculated to be 1.55 eV, which is 10733



Article

evolution of defect centers via Eu3+ doping. Aliovalent doping of Eu3+ in the Na+ lattice site of NaTaO3 would produce cation vacancies. Basically, doping with one Eu3+ could produce two Na+ vacancies. Previous work55 on AlN and AlGaN systems indicated that a deep level state was observed, attributed to cation vacancies. Khan and co-workers56 systematically investigated the structural and optical properties of ZnO thin film by Al doping. It is found that interstitial oxygen or zinc vacancy can generate defect centers in the band gap of ZnO and result in deep level emission. Therefore, it is reasonable to expect that Na+ vacancies in NaTaO3 may lead to defect centers into the band gap of NaTaO3 that are responsible for the weak absorption. On the other hand, a higher Na/Ta molar ratio leads to excessive Na+ ions and the subsequent interstitial oxygen (Oi″) either in the bulk or on the surface layers, which also give rise to a midgap band in NaTaO3. Besides weak absorption in the ultraviolet to visible region, the optical band edge from O 2p to Ta 5d transition shifted toward the lower energy side with variation of Na/Ta molar ratios, which is in contradiction to theoretical predictions. The band gap narrowing has been observed in many oxide nanocrystals, which is highly dependent on doping effects, defect chemistry, local structural modification, and so forth. For the present Eu3+-doped NaTaO3 samples, either excessive Na+ or Eu3+ can lead to rearrangement of the nearest neighbors that could cause local structural distortion, which would lead to lattice strain and subsequent variation of electronic structure. Moreover, the variation of lattice parameters also has a great impact on the modification of electronic structure and properties. Previous literature indicated that the superposition of orbital states forms the electronic bands of semiconductors. As lattice expansion occurred, the bond length got longer along with lattice atoms away from each other, which could result in the decrease of orbital overlapping and dispersion of the electron bands in k space,57 namely, narrowing of the band gap will occur with a decrease of orbital overlapping. From the correlation between the band gap energies and the lattice volume of Eu3+-doped NaTaO3 samples, a linear relationship between the band gap narrowing and the lattice volume was found (Figure S10, Supporting Information), indicating lattice expansion directly influences the changes of band gap energies of NaTaO3. As a result, it is expected that the doping effects, nonstoichiometric chemistry, and lattice expansion led to the red shift of the band gap and weak absorption in the ultraviolet to visible region in Eu3+-doped NaTaO3. 3.3. Evaluation of the Photocatalytic Activity of Eu3+Doped NaTaO3. The photocatalytic activity of Eu3+-doped NaTaO3 was first estimated by the degradation of methyl blue (MB). These homogeneous suspensions were stirred in the dark for 12 h to establish the equilibrium of MB absorption/ desorption on the NaTaO3 sample surfaces before illumination. No obvious adsorption of MB molecules was observed for all samples. MB concentration as a function of irradiation time was recorded UV−vis absorption spectra. As discussed above, Eu3+ doped in NaTaO3 can modify the lattice structure and optical properties, which led us to expect that Eu3+ should have a great impact on the photocatalytic activities of NaTaO3. A preliminary photocatalytic test on NaTaO3 samples with different Eu3+ doping concentrations indicated that the Eu3+ doping level can modulate the photocatalytic performance of NaTaO3, showing the highest photocatalytic activity with an Eu3+ concentration of ∼1.7% (Figure S11, Supporting Information). The maximal absorption at 664 nm of MB

Figure 7. Density of states (DOS) of pure NaTaO3 and four doping models.

the electronic gap between O 2p and Ta 5d states is 1.58 eV. Similar to the model of Eu3+ substitution at the Na+ site, Eu3+ substitution at the Ta5+ site also showed an intraband originating from Eu dopant. Moreover, for the model of Nai−Eu@Ta, an intraband belonging to Eu 5d and Eu 4f orbitals was also observed (Figure 7), which is similar to that of Eu@Na and Eu@Ta models. In addition, the electronic gap between O 2p and Ta 5d orbitals is about 1.60 eV. Having the theoretical results in mind, it is concluded that (1) Eu3+ substituted at either the Na+ site or the Ta5+ site can generate a midgap state that makes the energy of the valence band shift up and causes narrowing of the band gap, (2) incorporation of Na+ ions or/and Eu3+ ions into NaTaO3 matrix had little impact on the electronic transition between O 2p and Ta 5d orbitals, which was broadened in the range of 0.03−0.05 eV, and (3) incorporation of Na+ or/and Eu3+ can remarkably increase the density of states of the conduction and valence bands for NaTaO3 that is propitious to generation of photoinduced charge carrier and enhancement of photocatalytic performance. In combination with DFT and UV−vis reflectance results, it seems that the weak absorption band from 320 to 500 nm is related to the electronic transition from Eu 5d and Eu 4f orbitals. However, Eu3+-free NaTaO3 samples also exhibited similar weak absorption (Figure S9, Supporting Information). Therefore, some other reason may contribute to this absorption. On the basis of previous works, intrinsic defect centers such as vacancies and interstitial atoms are the most common defects in crystalline semiconductors. To investigate the possibility that localized defects in NaTaO3 are responsible for the band gap narrowing, we should recall nonstoichiometric chemistry and Eu3+ doping effects of NaTaO3 that play critical roles in modulating the local structure, electronic structure, as well as properties. As mentioned above, intrinsic defects could be produced via charge compensation with the variation of Na/ Ta molar ratios. A lower Na/Ta molar ratio and Eu3+ doping could result in the red shift of optical absorption due to the 10734



Article

Further comparative studies have been conducted to investigate MB concentration changes under UV−vis light irradiation time. It is seen that there existed a linear relationship between the ln(C0/C) plot and UV−vis light irradiation time. This result gives evidence of a pseudo-first-order kinetic reaction feature for MB degradation (Figure S13, Supporting Information), which can be described as follows

aqueous solution as a function of UV light irradiation time in the presence of the sample (with Na/Ta molar ratio of 0.982) is illustrated in Figure 8a. It is noted that UV light irradiation has

ln(C0/C) = kt

where C 0 is the initial MB concentration, C is MB concentration at time t, and k is the degradation rate constant. The degradation rate constants are illustrated in Figure 8d as a function of Na/Ta molar ratio. As shown in Figure 8d, the maximal degradation rate constant is 0.060 ± 0.005 min−1 for Eu3+-doped NaTaO3 with Na/Ta of 0.982 under UV light irradiation, whereas the maximal degradation rate constant is 0.0065 ± 0.0005 min−1 for Eu3+-doped NaTaO3 sample with Na/Ta of 1.041 under visible light irradiation, which is much smaller than that under UV light irradiation. Moreover, Eu3+doped NaTaO3 also exhibited photocatalytic water splitting property under UV and visible light irradiation. Although no cocatalyst was loaded, Eu3+-doped NaTaO3 showed tunable photocatalytic performance for water splitting with the variation of Na/Ta molar ratios (Figure 8e). Optimal H2 gas release rates are 70.0 μL·h−1·g−1 for Eu3+-doped NaTaO3 (Na/Ta = 1.041) under UV light irradiation and 9.1 μL·h−1·g−1 for Eu3+-doped NaTaO3 (Na/Ta = 1.064) under visible light irradiation. In principle, the photocatalytic process of semiconductors involves generation and recombination of electron−hole pairs after light irradiation. Electron−hole pairs in th valence/ conduction band could recombine in bulk or migrate to semiconductor surfaces to form active radicals by reacting with the adsorbed species. If charge separation and migration is maintained, the subsequent active radicals will induce effective degradation of organic contaminants. To shed light on nonstoichiometric control and doping effects on the photocatalytic mechanism and tunable photocatalytic performance for the present Eu3+-doped NaTaO3, the potential energy levels of the valence/conduction band should be first considered owing to the fact that the photocatalytic degradation of organic contaminants is strongly dependent on the band edge positions. The valence band potential of NaTaO3 is located at ∼3 V (vs NHE), which is more positive than those of OH•/OH− (1.9 V vs NHE) and OH•/H2O (2.73 V vs NHE),58 predicting the presence of OH• radical species in the photocatalytic process. Since the band gap energy of bulk NaTaO3 is 4.1 eV, the conduction band potential of NaTaO3 is close to −1.1 V (vs NHE), which is more negative than that of O2/O2−• (−0.33 V vs NHE).53 This result suggests that generation of active oxygen species is possible for adsorption oxygen by capturing an electron from the conduction band of NaTaO3, which led to degradation of MB decomposition. To investigate the primary photocatalytic radical species during the degradation of MB over Eu3+-doped NaTaO3, controlled experiments were conducted by adding some active species scavengers.59 Photodegradation of MB was repeated with modification by adding 1 mmol of benzoquinone (BQ) as a superoxide radical scavenger (O2−•), 10 mmol of tert-butyl alcohol (TBA) as a hydroxyl radical scavenger (OH•), 10 mmol of ammonium oxalate (AO) as a hole (h+) scavenger, or 1 mmol of AgNO3 as a electron (e−) scavenger. Under UV light irradiation, the photodegradation of MB was apparently inhibited when BQ and TBA were added, while there was no obvious change with

Figure 8. UV−vis absorption spectra of MB solution as a function of UV light irradiation time in the presence of Eu3+-doped NaTaO3 dispersion (a), MB concentration changes as a function of UV light irradiation time (b), MB concentration changes as a function of visible light irradiation time (c), degradation rate constant with the variation of Na/Ta molar ratio (d), and photocatalytic water splitting over Eu3+doped NaTaO3 in initial 3 h (e).

little impact on the self-degradation of MB in the absence of catalyst (Figure S12, Supporting Information). However, addition of Eu3+-doped NaTaO3 samples led to a continuous decrease of the characteristic absorption peaks of MB with an increase of UV light irradiation time. Moveover, MB showed photodegradation characteristic of the absence of additional peaks and hypsochromic shift under UV light irradiation. As shown in Figure 8b, with an increase of Na/Ta molar ratio, Eu3+-doped NaTaO3 exhibited decreased photocatalytic performance. As illustrated by UV−vis reflectance spectra, nonstoichiometric control and doping effects induced visible light absorption for Eu3+-doped NaTaO3 samples, which may predict visible light photocatalytic activity. Figure 8c shows MB concentration changes as a function of visible light irradiation time with addition of Eu3+-doped NaTaO3 samples. Obviously, the photocatalytic activity of Eu3+-doped NaTaO3 was greatly enhanced with an increase of the Na/Ta molar ratio to 1.041. However, further increases of the Na/Ta molar ratio could apparently reduce the photocatalytic performance of Eu3+doped NaTaO3 samples. 10735



Article

3.4. Photocatalytic Performance of Trivalent IonsDoped NaTaO3. Besides Eu3+ doping, several trivalent ions, such as La3+, Ce3+, Bi3+, and In3+, with similar ionic radii were also incorporated in the NaTaO3 lattice. Substitution of different trivalent ions can apparently have consequences on the electronic structure, which predicts efficient photocatalytic performance. Commercial Degussa P25 TiO2 and undoped NaTaO3 were used as reference for comparison. As shown in Figure 10a, under UV light irradiation, all trivalent ions-doped

addition of AO (Figure 9a). Interestingly, addition of AgNO3 can increase the photocatalytic process. This is because the

Figure 9. Effects of different scavengers on MB degradation in the presence of Eu3+-doped NaTaO3 (Na/Ta = 0.982) under UV light irradiation (a) and Eu3+-doped NaTaO3 (Na/Ta = 1.041) under visible light irradiation (b).

presence of AgNO3 enhanced the separation of electron−hole pairs and more photogenerated holes and the following active species were generated, which promoted the degradation rate.60 This gives evidence that the degradation of MB is dominated by the oxidation reaction of the generated O2−• and OH• active species taking place on photocatalyst surfaces. Under visible light irradiation, addition of BQ has little consequence on the photocatalytic process while AgNO3 can accelerate the photodegradation rate (Figure 9b). This means that photogenerated electrons are unable to react with the adsorbed O2 to produce O2−• radicals, which may be attributed to the fact that the energy potential of defective related centers is probably more positive than that of O2/O2−•. The influence of AO and TBA on photodegradation of MB is identical to that observed under UV light irradiation. Therefore, OH• acts as the main active species toward the MB degradation over Eu3+-doped NaTaO3 under visible light irradiation. Apparently, the differentiation of the photocatalytic mechanism for Eu3+doped NaTaO3 under UV and visible light irradiation leads to distinctive and tunable photocatalytic performance. Moreover, foreign impunities may also have a great influence on the charge carriers and the subsequent photocatalytic activity. Doping impurities into semiconductors exhibited complicated effects on photocatalytic activity because the impurities can serve as reactive or recombination centers.61−64 For instance, the photocatalytic activity of Zn2+-doped SnWO4 nanocrystals26 increased with the increase of dopant concentration, which however greatly decreased with further increasing Zn2+ dopant concentration. On the basis of XRD, photoluminescence, and DFT calculation results, the nonstoichiometric control and Eu3+ doping led to variation of the density of states, which may increase the mobility of photogenerated charge carriers in the valence/conduction band and subsequently enhance the photocatalytic acticity. On the other hand, the higher Na/Ta molar ratio and relevant defect centers may also act as recombination centers for electron−hole pairs, reducing the photocatalytic activity of Eu3+-doped NaTaO3. It is noted that the hydrogen evolution behavior in Eu3+-doped NaTaO3 is highly distinguished from that of photodegradation of MB due to the fact that the evolution of hydrogen is only associated with the photogenerated electrons in the conduction band. On the basis of the photocatalytic results, a lower Na/Ta molar ratio and dual substitution of Eu3+ both at the Na+ site and the Ta5+ site are beneficial for photocatalytic activities.

Figure 10. Normalized concentration of MB in aqueous trivalent ionsdoped NaTaO3 dispersion versus UV light irradiation time (a), MB concentration changes as a function of visible light irradiation time (b), and photocatalytic water splitting over trivalent ions-doped NaTaO3 (c).

NaTaO3 showed excellent photocatalytic activity that is superior to the undoped one. It is clear that La3+-doped NaTaO3 exhibited the highest photocatalytic activity, which is comparable to that in the presence of P25. P25 was almost inactive to the photodegradation of MB under visible light irradiation as shown in Figure 10b. While the trivalent-doped NaTaO3 also exhibited efficient and tunable photocatalytic activity. Figure 10c shows the photocatalytic H2 evolution spectra of trivalent-doped NaTaO3. For comparison, photocatalytic evolution of H2 for undoped NaTaO3 was also given. As illustrated Figure 10c, undoped NaTaO3 showed H2 gas release rates of 36.8 and 2.2 μL·h−1·g−1 under UV light and visible light irradiation, respectively. As for trivalent ions-doped NaTaO3, incorporation of La3+, Ce3+, and In3+ can obviously promote the H2 gas release rate, while Bi3+ doping had a negative effect on the photocatalytic activity. The optimized H2 gas release rates are 58.9 μL·h−1·g−1 for La3+-doped NaTaO3 under UV light irradiation and 20.7 μL·h−1·g−1 for La3+-doped NaTaO3 under UV light irradiation. Earlier studies on cationdoped NaTaO3 demonstrate that the photocatalytic activities of NaTaO3 are highly related to the dopant concentration, lattice distortion, electronic structure, and so forth.65−67 Kudo and coworkers systematically investigated the photocatalytic performance of La3+-doped NaTaO3 and suggested that lower La3+ dopant concentration (∼2%) exhibited the highest photo10736



Article

be well regulated and optimized. The finding in this work is important, which may offer strategies to develop novel photocatalyst.

catalytic hydrogen evolution rate. Higher La3+ dopant concentration (∼10%) led to lattice distortion and impurity phase which may act as recombination centers for electron− hole pairs, leading to a reduction of the photocatalytic activity.30 However, in Bi3+-doped NaTaO3 nanocrystals, the best photocatlytic performance was shown by the sample with a Bi3+ concentration of ∼7.5%.68 Therefore, it is expected that the difference in optimal concentration of dopant ions may result in the variation in photocatalytic performance. Moreover, it is accepted that the M−O−M angle strongly influences the photocatalytic activities of the materials. As the M−O−M angle is close to 180°, migration of electron−hole pairs will be proceeded, which will enhance the photocatalytic activity of semiconductors.69 The Ta−O−Ta angle in NaTaO3 is close to 163°, which can be altered by foreign dopant ions.70 The ionic radius of Na+ is close to La3+, Ce3+, and Bi3+ but much larger than that of In3+. On the basis of XRD and Eu3+ luminescent results, the trivalent ions doped in NaTaO3 lattice are likely to substitute at the Na+ site. Therefore, it is expected that In3+ ions may give rise to larger lattice distortion, subsequently leading to the decrease of photocatalytic activities. Moreover, as well documented in many oxide systems, the photocatalytic activity is strongly related to the dopant ions, which influences electronic structure and properties.71,72 For instance, Bi substituted at the Ta site in NaTaO3 lattice gives an intraband below the conduction band, whereas Bi substituted at the Na site induces new energy levels above the valence band.22 The appearance of new energy states in NaTaO3 will alter the optical transition behavior and influence the oxidation/ reduction capability of photogenerated electron−hole pairs and photocatalytic activity. However, substitution of La at both the Na site and the Ta site introduces no localized states in the calculated band gap.73 With all these above-mentioned reasons in mind, it is roughly suggested that the photocatalytic activities of trivalent ions-doped NaTaO3 can be well modulated, though more detailed aspects still need to be investigated.

■

ASSOCIATED CONTENT

* Supporting Information S

Figures of XRD pattern, emission spectra, supercell models, UV−vis diffuse spectrum, degradation data, band energy versus lattice volume. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-471-4344579. Fax: +86-471-4992981. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21103081, 21267041, 21367018) and the Project of Scientific and Technological Innovation Team of Inner Mongolia University (12110614).

■

REFERENCES

(1) Usai, S.; Obregón, S.; Becerro, A. I.; Colón, G. MonoclinicTetragonal Heterostructured BiVO4 by Yttrium Doping with Improved Photocatalytic Activity. J. Phys. Chem. C 2013, 117, 24479−24484. (2) Yang, C. Y.; Wang, Z.; Lin, T. Q.; Yin, H.; Lü, X. J.; Wan, D. Y.; Xu, T.; Zeng, C.; Lin, J. H.; Huang, F. Q.; et al. Core-shell Nanostructured “Black” Rutile Titania as Excellent Catalyst for Hydrogen Production Enhanced by Sulfur Doping. J. Am. Chem. Soc. 2013, 135, 17831−17838. (3) Li, Z. Q.; Zeng, Y. B.; Qian, H. S.; Long, R.; Xiong, Y. J. Facile Synthesis of GdBO3 Spindle Assemblies and Microdisks as Versatile Host Matrices for Lanthanide Doping. CrystEngComm 2012, 14, 3959. (4) Clavel, G.; Willinger, M. G.; Zitoun, D.; Pinna, N. Solvent Dependent Shape and Magnetic Properties of Doped ZnO Nanostructures. Adv. Funct. Mater. 2007, 17, 3159−3169. (5) Su, Y. G.; Li, L. P.; Li, G. S. Synthesis and Optimum Luminescence of CaWO4-Based Red Phosphors with Codoping Eu3+ and Na+. Chem. Mater. 2008, 20, 6060−6067. (6) Liao, J. S.; Zhang, S. A.; You, H. Y.; Wen, H. R.; Chen, J. L.; You, W. X. Energy Transfer and Luminescence Properties of Eu3+-Doped NaTb(WO4)2 Phosphor Prepared by a Facile Hydrothermal Method. Opt. Mater. 2011, 55, 953−957. (7) Elgh, B.; Yuan, N.; Cho, H. S.; Nilsson, E.; Terasaki, Q.; Palmqvist, A. E. C. Photochemical Kinetics of Ultrasmall Semiconductor Particles in Solution: Effect of Size on the Quantum Yield of Electron Transfer. J. Phys. Chem. C 2013, 117, 16492−16499. (8) Kumar, V.; Govind, A.; Nagarajan, R. Optical and Photocatalytic Properties of Heavily F−-Doped SnO2 Nanocrystals by a Novel Singlesource Precursor. Inorg. Chem. 2011, 50, 5637−5645. (9) Wang, D.; Tang, K. B.; Liang, Z. H.; Zheng, H. G. Synthesis, Crystal Structure, and Photocatalytic Activity of the New Three-Layer Aurivillius Phases, Bi2ASrTi2TaO12 (A = Bi, La). J. Solid State Chem. 2010, 183, 361−366. (10) Hai, Z. B.; EI Kolli, N.; Uribe, D. B.; Beaunier, P.; Jose-Yacaman, M.; Vigneron, J.; Etcheberry, A.; Sorgues, S.; Colbeau-Justin, C.; Chen, J. F.; et al. Modification of TiO2 by Bimetallic Au-Cu Nanoparticles for Wastewater Treatment. J. Mater. Chem. A 2013, 1, 10829−10835. (11) Tahir, M. N.; Natalio, F.; Cambaz, M. A.; Panthofer, M.; Branscheid, R.; Kolb, U.; Tremel, W. Controlled Synthesis of Linear and Branched Au@ZnO Hybrid Nanocrystals and Their Photocatalytic Properties. Nanoscale 2013, 5, 9944−9949.

■

CONCLUSION Trivalent ions (including Eu3+, La3+, etc.) doped NaTaO3 with controlled nonstoichiometric chemistry and lattice structures was successfully prepared via a solid state method. Eu3+ as one of the typical trivalent ions was first employed as a structural probe to testify the homogeneity and site-selective substitution of trivalent ions in NaTaO3 and the influence of site-selective trivalent ions doping and nonstoichiometric chemistry on the crystal structure, electronic structure, and photocatalytic performances. Nonstoichiometric Na/Ta molar ratio led to site-selective occupation of Eu3+ ions which were altered from sole substitution at the Na+ site to dual substitutions at both the Na+ and the Ta5+ sites. Simultaneously, the nonstoichiometric Na/Ta molar ratio and site-selective occupation of Eu3+ resulted in a monotonous lattice expansion and local symmetry distortion and high-density defect centers that had a great impact on the ν3 mode vibration of the O−Ta bond, which became asymmetric and shifted toward higher wavenumbers. Contrary to the DFT calculations, Eu3+-doped NaTaO3 nanocrystals showed a red shift of the band gap and weak visible light absorption with the variation of Na/Ta molar ratios, being related to doping effects, defect chemistry, and variation of lattice parameters. With well-defined lattice structure and defect centers and electronic structure via nonstoichiometric control and trivalent ions doping, the photocatalytic reactivity of trivalent ions-doped NaTaO3 must 10737



Article

(12) Oropeza, F. E.; Zhang, K. H. L.; Palgrave, R. G.; Regoutz, A.; Egdell, R. G. Electronic Structure of Epitaxial Sn-Doped Anatase Growth on SrTiO3 (001) by Dip Coating. J. Phys. Chem. C 2013, 117, 15221−15228. (13) Zhuang, S. D.; Xu, X. Y.; Pang, Y. R.; Hu, J. G.; Yang, C.; Tong, L.; Zhou, Y. X. PEGME-Bonded SnO2 Quantum Dots for Excellent Photocatalytic Activity. RSC Adv. 2013, 3, 20422−20428. (14) Wang, J. Q.; Su, S. Y.; Cao, M. H.; Hu, C. W. One-Pot, LowTemperature Synthesis of Self-doped NaTaO3 Nanoclusters for Visible-Light-Driven Photocatalysis. Chem. Commun. 2013, 49, 7830−7832. (15) Zhang, L. W.; Wang, Y. J.; Cheng, H. Y.; Yao, W. Q.; Zhu, Y. F. Synthesis of Porous Bi2WO6 Thin Films as Efficient Visible-LightActive Photocatalysts. Adv. Mater. 2009, 21, 1286−1290. (16) Long, J. L.; Xue, W. W.; Xie, X. Q.; Gu, Q.; Zhou, Y. G.; Chi, Y. W.; Chen, W. K.; Ding, Z. X.; Wang, X. X. Sn2+ Dopant Induced Visible-Light Activity of SnO2 nanoparticles for H2 Production. Catal. Commun. 2011, 16, 215−219. (17) Choi, W. Y.; Termin, A.; Hoffmann, M. R. The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation Between Photoreactivity and Charge Carrier Recombination Dynamics. J. Phys. Chem. 1994, 98, 13669−13679. (18) Yang, K.; Dai, Y.; Huang, B. B. First-Principles Calculations for Geometrical Structures and Electronic Properties of Si-Doped TiO2. Chem. Phys. Lett. 2008, 456, 71−75. (19) Wang, D. F.; Kako, T.; Ye, J. H. Efficient Photocatalytic Decomposition of Acetaldehyde over a Solid-Solution Perovskite (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3 under Visible-Light Irradiation. J. Am. Chem. Soc. 2008, 130, 2724−2725. (20) Mizoguchi, H.; Woodward, P. M.; Byeon, S. H.; Parise, J. B. Polymorphism in NaSbO3: Structure and Bonding in Metal Oxides. J. Am. Chem. Soc. 2004, 126, 3175−3184. (21) Shi, J. W.; Ye, J. H.; Zou, Z. G.; Li, M. T.; Guo, L. J. Hydrothermal Synthesis of Na0.5La0.5TiO3-LaCrO3 Solid-Solution Single-Crystal Nanocubes for Visible-Light-Driven Photocatalytic H2 Evolution. Chem.Eur. J. 2011, 17, 7858−7867. (22) Kanhere, P. D.; Zheng, J.; Chen, Z. Site Specific Optical and Photocatalytic Properties of Bi-Doped NaTaO3. J. Phys. Chem. C 2011, 115, 11846−11853. (23) Su, Y. G.; Wang, S. W.; Meng, Y.; Han, H.; Wang, X. J. Dual Substitutions of Single Dopant Cr3+ in Perovskite NaTaO3: Synthesis, Structure, and Photocatalytic Performance. RSC. Adv. 2012, 2, 12932− 12939. (24) Hu, W. B.; Tong, W. M.; Li, L. P.; Zheng, J.; Li, G. S. Cation Non-Stoichiometry in Multi-Component Oxide Nanoparticles by Solution Chemistry: a Case Study on CaWO4 for Tailored Structural Properties. Phys. Chem. Chem. Phys. 2011, 13, 11634−11643. (25) Chen, H. X.; Ding, J. J.; Guo, W. G.; Chen, G. X.; Ma, S. Y. Blue-Green Emission Mechanism and Spectral Shift Al-Doped ZnO Films Related to Defect Levels. RSC Adv. 2013, 3, 12327−12333. (26) Su, Y. G.; Hou, L. C.; Du, C. F.; Peng, L. M.; Guan, K.; Wang, X. J. Rapid Synthesis of Zn2+ Doped SnWO4 Nanowires with the Aim of Exploring Doping Effects on Highly Enhanced Visible Photocatalytic Activities. RSC. Adv. 2012, 2, 6266−6273. (27) Pandiyan, R.; Micheli, V.; Ristic, D.; Bartali, R.; Pepponi, G.; Barozzi, M.; Gottardi, G.; Ferrari, M.; Laidani, N. Structural and NearInfra Red Luminescence Properties of Nd-Doped TiO2 Films Deposited by RF Sputtering. J. Mater. Chem. 2012, 22, 22424−22432. (28) Hailstone, R. K.; DiFrancesco, A. G.; Leong, J. G.; Allston, T. D.; Reed, K. J. A Study of Lattice Expansion in CeO2 Nanoparticles by Transmission Electron Microscopy. J. Phys. Chem. C 2009, 113, 15155−15159. (29) Li, G. S.; Boerio-Goates, J.; Woodfield, B. F.; Li, L. P. Evidence of Linear Lattice Expansion and Covalency Enhancement in Rutile TiO2 Nanocrystals. Appl. Phys. Lett. 2004, 85, 2059−2061. (30) Kato, H.; Asakura, K.; Kudo, A. Highly Efficient Water Splitting into H2 and O2 over Lanthanum-Doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure. J. Am. Chem. Soc. 2003, 125, 3082−3089.

(31) Matar, S. F.; Gampet, G.; Subramanian, M. A. Electronic Properties of Oxides: Chemical and Theoretical Approaches. Prog. Solid State Chem. 2011, 39, 70−95. (32) Chen, Q. F.; Jiang, D.; Shi, W. M.; Yu, D.; Xu, Y. Visible-LightActivated Ce-Si Co-Doped TiO2 Photocatalyst. Appl. Surf. Sci. 2009, 255, 7918−7924. (33) Montini, T.; Speghini, A.; Rogatis, L. D.; Lorenzut, B.; Bettinelli, M.; Graziani, M.; Fornasiero, P. Identification of the Structural Phases of CexZr1‑xO2 by Eu(III) Luminescence Studies. J. Am. Chem. Soc. 2009, 131, 13155−13160. (34) Tu, D. T.; Liu, Y. S.; Zhu, H. M.; Li, R. F.; Liu, L. Q.; Chen, X. Y. Breakdown of Crystallographic Site Symmetry in LanthanideDoped NaYF4 Crystals. Angew. Chem., Int. Ed. 2012, 52, 1128−1133. (35) Teixeira, N. G.; Dias, A.; Moreira, R. L. Raman Scattering Study of the High Temperature Phase Transitions of NaTaO3. J. Eur. Ceram. Soc. 2007, 27, 3683−3686. (36) Li, X.; Zang, J. L. Hydrothermal Synthesis and Characterization of Lanthanum-Doped NaTaO3 with High Photocatalytic Activity. Catal. Commun. 2011, 12, 1380−1383. (37) Gasparotto, G.; Cebim, M. A.; Gose, M. S.; Lima, S. A. M.; Davolos, M. R.; Varela, J. A.; Paiva-Santos, C. O.; Zaghete, M. A. Correlation Between the Spectroscopic and Structural Properties with the Occupation of Eu3+ Sites in Powdered Eu3+-Doped LiTaO3 Prepared by the Pechini Method. J. Appl. Phys. 2009, 106, 063509−5. (38) Miao, Z. J.; Wu, Y. Y.; Zhang, X. R.; Liu, Z. M.; Han, B. X.; Ding, K. L.; An, G. M. Large-Scale Production of Self-Assembled SnO2 Nanospheres and Their Application in High-Performance Chemiluminescence Sensors for Hydrogen Sulfide Gas. J. Mater. Chem. 2007, 17, 1791−1796. (39) Liu, J. F.; Li, X. L.; Li, Y. D. Synthesis and Characterization of Nanocrystalline Niobates. J. Cryst. Growth 2003, 247, 419−424. (40) Shanker, V.; Samal, S. L.; Pradhan, G. K.; Narayana, C.; Ganguli, A. K. Nanocrystalline NaNbO3 and NaTaO3: Rietveld Studies, Raman Spectroscopy and Dielectric Properties. Solid State Sci. 2009, 11, 562− 569. (41) Tsunekawa, S.; Ishikawa, K.; Li, Z. Q.; Kawazoe, Y.; Kasuya, A. Origin of Anomalous Lattice Expansion in Oxide Nanoparticles. Phys. Rew. Lett. 2000, 85, 3440−3443. (42) Champion, Y.; Bernard, F.; Millot, N.; Perriat, P. Surface Adsorption Effects on the Lattice Expansion of Copper Nanocrystals. Appl. Phys. Lett. 2005, 86, 231914−3. (43) Li, X. K.; Li, Q.; Wang, L. Y. The Effects of NaNbO3 Particle Size on the Photocatalytic Activity for 2-Propanol Photodegradation. Phys. Chem. Chem. Phys. 2013, 15, 14282−14289. (44) Su, Y. G.; Li, G. S.; Xue, Y. F.; Li, L. P. Tunable Physical Properties of CaWO4 Nanocrystals via Particle Size Control. J. Phys. Chem. C 2007, 111, 6684−6689. (45) Qiu, X. Q.; Li, L. P.; Zheng, J.; Liu, J. J.; Sun, X. F.; Li, G. S. Origin of the Enhanced Photocatalytic Activities of Semiconductors: A Case Study of ZnO Doped with Mg2+. J. Phys. Chem. C 2008, 112, 12242−12248. (46) Ghosh, M.; Dilawar, N.; Bandyopadhyay, A. K.; Raychaudhuri, A. K. Phonon Dynamics of Zn(Mg,Cd)O Alloy Nanostructures and Their Phase Segregation. J. Appl. Phys. 2009, 106, 084306−5. (47) Liu, L.; Wang, Y. X.; Bai, Y. F.; Zhang, X. R.; Yang, K.; Song, Y. L. Effects of Alkali Metal Ions on Upconversion Photoluminescence Intensity of Er3+-Doped Y2O3 Nanocrystals. Appl. Phys. B: Laser Opt. 2013, 110, 111−115. (48) Li, W. F.; Feng, X. Q.; Huang, Y. L. Characteristics of the Optical Absorption Edge and Defect Structures of La3+-Doped PbWO4 Crystals. J. Phys.: Condens. Matter 2004, 16, 1325−1333. (49) Zheng, Y. H.; Zheng, L. R.; Zhang, Y. Y.; Lin, X. Y.; Zheng, Q.; Wei, K. M. Ag.ZnO Heterostructure Nanocrystals: Synthesis, Characterization, and Photocatalysis. Inorg. Chem. 2007, 46, 6980− 6986. (50) Fu, H. B.; Zhang, S. C.; Zhang, L. W.; Zhu, Y. F. Visible-LightDriven NaTaO3‑xNx Catalyst Prepared by a Hydrothermal Process. Mater. Res. Bull. 2008, 43, 864−872. 10738



Article

of Organic Contaminants in Air and Water with UV-vis Light. Ind. Eng. Chem. Res. 2012, 51, 1563−1569. (70) Kato, H.; Kudo, A. Water Splitting into H2 and O2 on Alkali Tantalate Photocatalysts ATaO3 (A = Li, Na, and K). J. Phys. Chem. B 2001, 105, 4285−4292. (71) Maeda, K.; Domen, K. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. C 2007, 111, 7851−7861. (72) Ishikawa, A.; Takata, T.; Matsumura, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Oxysulfides Ln2Ti2S2O5 as Stable Photocatalysts for Water Oxidation and Reduction under VisibleLight Irradiation. J. Phys. Chem. B 2004, 108, 2637−2642. (73) Choi, M.; Oba, F.; Tanaka, I. First-Principles Study of Native Defects and Lanthanum Impurities in NaTaO3. Phys. Rev. B 2008, 78, 014115−8.

(51) Wang, B. C.; Kanhere, P. D.; Chen, Z.; Nisar, J.; Pathak, B.; Ahuja, R. Anion-Doped NaTaO3 for Visible Light Photocatalysis. J. Phys. Chem. C 2013, 117, 22518−22524. (52) Liu, J. W.; Chen, G.; Li, Z. H.; Zhang, Z. G. Electronic Structure and Visible Light Photocatalysis Water Splitting Property of Chromium Doped SrTiO3. J. Solid State Chem. 2006, 179, 3704−3708. (53) Su, Y. G.; Zhu, B. L.; Guan, K.; Gao, S. S.; Lv, L.; Du, C. F.; Peng, L. M.; Hou, L. C.; Wang, X. J. Particle Size and Structural control of ZnWO4 Nanocrystals via Sn2+ Doping for Tunable Optical and Visible Photocatalytic Properties. J. Phys. Chem. C 2012, 116, 18508−18517. (54) Abdel-Baki, M.; EI-Diasty, F. Role of Oxygen on the Optical Properties of Borate Glass Doped with ZnO. J. Solid State Chem. 2011, 184, 2762−2769. (55) Sedhain, A.; Lin, J. Y.; Jiang, H. X. Nature of Optical Transitions Involving Cation Vacancies and Complexes in AlN and AlGaN. Appl. Phys. Lett. 2012, 100, 221107−3. (56) Rahman, M. M.; Khan, M. K. R.; Islam, M. R.; Halim, M. A.; Shahjahan, M.; Hakim, M. A.; Saha, D. K.; Khan, J. U. Effect of Al Doping on Structural, Electrical, Optical and Photoluminescence Properties of Nano-Structural ZnO Thin Films. J. Mater. Sci. Technol. 2012, 28, 329−335. (57) Ö zgür, Ü .; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Doğan, S.; Avrutin, A.; Cho, S. J.; Morkoc, H. A Comprehensive Revies of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 041301− 103. (58) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium Dioxide Photocatalysis. J. Photochem. Photobiol., C 2000, 1, 1−21. (59) Chen, L. L.; Jiang, D. L.; He, T.; Wu, Z. D.; Chen, M. In-Situ Ion Exchange Synthesis of Hierarchical AgI/BiOI Microsphere Photocatalyst with Enhanced Photocatalytic Properties. CrystEngComm 2013, 15, 7556−7563. (60) Li, W. J.; Li, D. Z.; Wang, J. X.; Shao, Y.; You, J. M.; Teng, F. Exploration of the Active Species in the Photocatalytic Degradation of Methyl Orange under UV Light Irradiation. J. Mol. Catal., A: Chem. 2013, 380, 10−17. (61) Bingham, S.; Daoud, W. A. Recent Advances in Making NanoSized TiO2 Visible-Light Active through Rare-Earth Metal Doping. J. Mater. Chem. 2011, 21, 2041−2050. (62) Colon, G.; Maicu, M.; Hidalgo, M. C.; Navio, J. A. Cu-Doped TiO2 Systems with Imporved Photocatalytic Activity. Appl. Catal., B 2006, 67, 41−51. (63) Liu, J. W.; Han, R.; Zhao, Y.; Wang, H. T.; Lu, W. J.; Yu, T. F.; Zhang, Y. X. Enhanced Photocatalytic of V-N Codoped TiO2 Derived from a Two-Step Hydrothermal Procedure for the Degradation of PCP-Na under Visible Light Irradiation. J. Phys. Chem. C 2011, 115, 4507−4515. (64) Zhang, Z. Z.; Long, J. L.; Xie, H. Q.; Zhuang, H. Q.; Zhou, Y. G.; Lin, H.; Yuan, R. S.; Dai, W. X.; Ding, Z. X.; Wang, X. X.; et al. Controlling the Synergistic Effect of Oxygen Vacancies and N Dopants to Enhance Photocatalytic Activity of N-Doped TiO 2 by H2 Reduction. Appl. Catal., A: Gen. 2012, 425−426, 117−124. (65) Wu, X.; Yin, S.; Dong, Q.; Stao, T. Preparation and Visible Light Induced Photocatalytic Activity of C-NaTaO3 and C-NaTaO3-Cl-TiO2 Composite. Phys. Chem. Chem. Phys. 2013, 15, 20633−20640. (66) Iwase, A.; Kato, H.; Kudo, A. The Effect of Alkaline Earth Metal Ion Dopants on Photocatalytic Water Splitting by NaTaO3 Powder. ChemSusChem 2009, 2, 873−877. (67) Husin, H.; Chen, H. M.; Su, W. N.; Pan, C. J.; Chuang, W. T.; Sheu, H. S.; Hwang, B. J. Green Fabrication of La-Doped NaTaO3 via H2O2 Assisted Sol-Gel Route for Photocatalytic Hydrogen Production. Appl. Catal., B 2011, 102, 343−351. (68) Kanhere, P.; Tang, Y.; Zheng, J.; Chen, Z. Synthesis, Photophysical Properties, and Photocatalytic Applications of Bi Doped NaTaO3 and Bi Doped Na2Ta2O6 Nanoparticles. J. Phys. Chem. Solids 2013, 74, 1708−1713. (69) Ullah, R.; Sun, H. Q.; Wang, S. B.; Ang, H. M.; Tadé, M. O. Wet-Chemical Synthesis of InTaO4 for Photocatalytic Decomposition 10739


Tunable Optical and Photocatalytic Performance Promoted by

Recommend Documents