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Synthesis, surface structure and optical properties of double perovskite Sr2NiMoO6 nanoparticles
Accepted Manuscript Title: Synthesis, surface structure and optical properties of double perovskite Sr2 NiMoO6 nanoparticles Author: Lei Xu Yingpeng Wan Hongde Xie Yanlin Huang Li Yang Lin Qin Hyo Jin Seo PII: DOI: Reference:
S0169-4332(16)31553-7 http://dx.doi.org/doi:10.1016/j.apsusc.2016.07.117 APSUSC 33685
To appear in:
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Received date: Revised date: Accepted date:
15-6-2016 14-7-2016 14-7-2016
Please cite this article as: Lei Xu, Yingpeng Wan, Hongde Xie, Yanlin Huang, Li Yang, Lin Qin, Hyo Jin Seo, Synthesis, surface structure and optical properties of double perovskite Sr2NiMoO6 nanoparticles, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.07.117 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis, surface structure and optical properties of double perovskite Sr2NiMoO6 nanoparticles Lei Xu,a Yingpeng Wan,a Hongde Xie,a* Yanlin Huang,a Li Yang,a Lin Qin,b Hyo Jin Seo b*
a
State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of
Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China b
Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical
Engineering, Pukyong National University, Busan 608-737, Republic of Korea
*
Corresponding authors: [email protected] (Hongde Xie), [email protected]
(Hyo Jin Seo), Tel.: +82-51-629 5568; fax: +82-51-6295549.
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Graphical abstract
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Highlights Double perovskite Sr2NiMoO6 nanoparticles were prepared via sol-gel route. The nanoparticles have efficient optical absorption in visible light. The band structure and energy positions were determined. The perovskite has efficient photocatalytic on RhB photodegradation. Multivalent Mo and Ni-ions on the surfaces were investigated.
Abstract Double perovskite Sr2NiMoO6 nanoparticles were synthesized via the chemical sol-gel route. The phase formation was investigated through X-ray polycrystalline diffraction (XRD) and Rietveld refinements. The perovskite crystallized in worm-like nano-grains with the diameter of 20 to 50 nm. The optical properties were measured by the optical absorption spectra. The nanoparticles present an indirect allowed transition with a narrow band gap of 2.1 eV. Sr2NiMoO6 nanoparticles have obvious photocatalytic ability on the degradation of Rhodamine B (RhB) solutions under the irradiation of visible light. The transport behaviors of the excitons were investigated from the photoluminescence spectra and the corresponding decay lifetimes. Sr2NiMoO6 nanoparticles present several advantages for photocatalysis such as the appropriate band energy positions, the quenched luminescence, and the coexistence of multivalent ions in the lattices.
Keywords:
Inorganic compounds; Double perovskite; Photocatalysis; Electronic band
structure; Optical properties; Solar materials.
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1. Introduction Perovskite is one of the most important materials with intensively reported in optical, electrical, magnetic and biological field [1-8]. Especially synthesis, optical properties, photoactivity and applications of perovskite materials have been widely reported [9,10]. Among kinds of photocatalysts perovskites have shown very efficient photocatalytic performances under visible-light irradiation because of the unique structures and electronic properties. This crystal structure can provide a good framework in which to tune the band gap values to enable visible-light absorption and band edge potentials to suit the needs of specific photocatalytic reactions [10] . Moreover, the lattice distortion in perovskite materials can strongly influence the separation of photo-created charges. Some perovskite materials have been well-known photocatalysts such as titanate perovskites [11], tantalate perovskites [12], vanadium- and niobium-based perovskites [13], and ferrite perovskites [14] etc have shown visible-light activity. For example, double-perovskite compounds with chemical formula of A2B′B″O6−δ are derived from the simple perovskite ABO3 through arranging two different cations B′ and B″ on the B-site. A stands for alkaline earth metal ions (Ba, Ca, Sr), and B′, B″ are transition metal ions B′= Co, Fe, Ni, Cr, etc, and B″= W, Mo, Sb, etc [1]. In an ideal structure form, the framework of double-perovskite is constructed by corner-shared B′O6 and B″O6 octahedra in the lattices [2]. Double-perovskites are well-known functional materials because of the important physical properties such as thermal, electrical, optical and magnetic. Among them, double-perovskites with B=Mo have been paid great attentions, especially many reports have extensively reported the applications as electrode materials in solid oxide fuel cells [8]. For example, double perovskite Sr2NiMoO6 is one of the attractive candidates for anode materials in SOFCs [15]. In the past years, Sr2NiMoO6 has been received many attentions such as the preparation and structural refinement [16], electronic structure [17], electrical conductivity [18], defect formations [2,19], etc. In this work, the photocatalytic ability of Sr2NiMoO6 was tested because of several motivations. At first, this double perovskite has efficient optical centers MoO6 in the lattices, which are expected to induce absorption in the visible region [17]. Molybdate oxides usually show efficient abilities on degradation of organic dyes or depleting H2O. For example, scheelite-type PbMoO4 can split H2O to H2 and O2 under the excitation of UV light [20]. Bi2MoO6 is the well-known photocatalyst with the O2 evolution property by visible light irradiation [21]. Secondly, Ni2+-ions doped in a photocatalyst lattice form the local energy levels, which could enhance the charge-transfer transition. And the band energy can be greatly reduced by Ni-3d levels to permit the photocatalytic activity under the visible-light [22,23]. It has been reported that Ni substitution in Sr2Mg1–xNixMoO6−δ induces the donor levels, which move the Fermi energy to the nearer valence band. This effect results in the decrease of the band gap width [23]. The enhancements of photocatalytic abilities have been reported by doping Ni ions in the photocatalysts such as SrTiO3 [24], M3V2O8 (M = Mg, Ni, Zn) [25], Ni2+-doped TiO2 [26], NixCd1−xS [27], Ni-doped InVO4 [28], Li2Ni2(MoO4)3 [29], and K2Ni(VO3)4 [30], et al. Thirdly, the coexistence of Mo6+ with Ni2+ ions in B sites in perovskites provides the off-center distortions and polarizations of NiO6 centers, which benefits for the charge separations and enhancing the photocatalysis under visible light irradiation. The off-centering can be stabilized by 4
lowering the energy of covalent bond formations; consequently the charge-transfer from O-2p to Ni-3d can easily happen [31]. Herein, Sr2NiMoO6 was developed by the sol-gel reaction. The crystal phase and the XRD structural refinement have been conducted. The surface properties were checked by SEM, TEM, BET and XPS measurements. The luminescence spectra and decay lifetime were obtained. The photo-degradation on the degradation of RhB dyes was confirmed. The photo-active properties were discussed on the structure.
2. Experimental Sr2NiMoO6 nanoparticles were synthesized via the chemical sol-gel method. The raw reactants of Sr(NO3)2, Mg(NO3)2·6H2O, Ni(NO3)3·6H2O, and (NH4)6Mo7O24·4H2O were firstly dissolved in the water. Some citric acid monohydrate as the chelating agent was added in the solutions. Then, to adjust the viscoelasticity the PVA solutions were dropwise added in the mixtures. The solutions could become viscous after constant stirring in a water bath of 90 °C. The sticky solutions could be slowly coated on the glass substrates to get the precursor thin films, which contain Ni2+, Mo6+, and Sr2+ ions together with the organic polymer compounds. Finally, the dried thin films were peeled off and moved to a furnace for heat treatment at 900°C. in this way, the organic components were decomposed, while, Ni2+, Mo6+, and Sr2+ ions in stoichiometric ration formed Sr2NiMoO6 nanoparticles. The XRD measurements were finished on the Rigaku D/Max-2000 XRD diffractometer (40 kV, 35 mA) equipped by a Cu Kα target (λ=1.5405 Å). The SEM, EDS and TEM measurements were applied to investigate the surface morphologies. The N2-desorption-adsorption isotherms were measured at liquid nitrogen temperature by instrument of Nova 2000e. Brunauer-Emmett-Teller (BET) was used in the specific area measurements. The luminescence spectra and the decay curve of the as-prepared samples were measured to evaluate the dynamic properties of the excitons. The excitation was third harmonic (355 nm) of a pulsed Nd:YAG laser. The luminescence was dispersed by the 75 cm monochromator (Acton Research Corp. Pro-750) and multiplied by the PMT (Hamamatsu R928). The data was displayed and recorded with the LeCloy 9301 digital storage oscilloscope. The photocatalysis measurements were performed under irradiated by visible light from the 500 W-Xe UV-lamp. A 420 nm filter was used to get the visible wavelength for the irradiation. The air was pumped into the reactor in a speed of 500 mL min−1. In a typical photocatalysis experiment, the weight of Sr2NiMoO6 nanoparticles is about 0.05 g, while the corresponding RhB solution (10 mgL−1) was 300 mL. It is important that the initial test solution was stirred in a dark place for 1 h to reach desorption/adsorption equilibrium. In a time interval, 5 mL RhB solution was extracted and was taken for the UV-visible optical absorption. The effects were determined by comparing the absorption intensities for each measurement at different time.
3. Results and Discussions 3.1 Phase formation The phase formation, crystal structure and experimental unit cell parameters of the Sr2NiMoO6 nanoparticles were confirmed by XRD measurement. Fig. 1 is XRD pattern indexed on tetragonal Sr2NiMoO6 with double perovskite structure. No impurity phases such as SrMoO4 and NiO were 5
detected in the XRD pattern. The Rietveld refinement of the XRD pattern obtained by GSAS software is shown in Fig. 1, and the obtained structure data and the atomic positions are given in Table 1 and 2, respectively. Fig. 2 shows the structure sketch of Sr2NiMoO6 along [001] modeled by the refined atomic coordinate’s in Table 2. It has a tetragonal lattice with the space group of I4/m, which is in agreement with previous structural studies. It is a characteristic double perovskite with the NiO6 and MoO6 octahedra ordered in the lattices. The two kinds of octahedra are corner-shared forming the structure in Sr2NiMoO6. Sr atoms are located betwen the MoO6 and NiO6.
3.2 The surface characteristics The typical surface morphologies of Sr2NiMoO6 nanoparticles were tested by SEM measurements as displayed in Fig. 3 (a, b, c). On the view from the SEM with low-amplification, The sample shows aggregations. The particles seem to connect with each other with aggregations. It appears a sponge-like profile. The worm-like nano-grains could be observed at high amplification as shown in Fig. 3 (c). The diameters of the particles are 20 nm to 50 nm. Fig. 3 (d) shows the EDS results on the surfaces of Sr2NiMoO6 nanoparticles. The O, Mo, Ni and Sr elements were detected in Sr2NiMoO6 surfaces. And the result shows that Ni and Mo have a molar ratio of around 1.0, which in in agreement with the theoretical result in Sr2NiMoO6. The surface microstructure of Sr2NiMoO6 nanoparticles was confirmed by the TEM measurements in Fig. 4 (a, b, c). The HRTEM images in Fig. 4 (d) confirms the single-crystalline property of Sr2NiMoO6 nanoparticles. The interplanar distance presented on the lattice fringe is about 0.1966 nm, which is corresponding to the (220) preferable growth plane. The selected area electron diffraction (SAED) has a clear pattern (Fig. 4 d), which demonstrates that Sr2NiMoO6 is single crystalline and belongs to tetragonal phase. In addition, N2-adsorption–desorption isotherm of Sr2NiMoO6 nanoparticles was measured (Fig. 5). The specific surface area of Sr2NiMoO6 nanoparticles was obtained to be 31.7 m2/g by the Brunauer–Emmett–Teller (BET) results. The distribution of the pore size was around 7 nm.
3.3 Optical absorption properties and band gap structure The optical absorption spectrum in UV-Vis region of Sr2NiMoO6 nanoparticles is displayed in Fig. 6 (a). It presents two broad bands in the wavelength from 200 to 800 nm. The absorption presents a cut-off edge at about 585 nm obtained from its intercept on x-axis. The point of intersect on x-axis (lg) is the rough energy value for band-gap energy (Eg), which was calculated by the equation of Eg=1240/lg. The band energy of Sr2NiMoO6 nanoparticles is 2.1 eV in this estimation. The red color of the sample in Fig. 6 (a) is induced by the rich optical centers. To further understand the nature of the band-gap transition in this double perovskite, it was discussed in the formula h (h Eg )k . Here ν denotes frequency of incident light, α is optical absorbance, h is Planck constant. k is also a constant with four possible values of 1/2, 2, 3/2 or 3, which decides the transition nature. It could be direct allowed, indirect allowed, direct forbidden and indirect forbidden types when k is 1/2, 2, 3/2 or 3, respectively. The band energy (Eg) of Sr2NiMoO6 nanoparticles can be well taken at k=2, indicating the indirect allowed transitions. The linear portion of 2.1 eV can be obtained in Fig. 6 (b) by extrapolating the curve to the intercept of the x-axis. The narrow permits Sr2NiMoO6 to use the visible light in the
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photocatalysis. This shows the advantage than the well-known photocatalysts such as TiO2 (Eg=3.2 eV) [9,10], ZnO (Eg = 3.37 eV) [32] , which only use the UV light because of the wide band gap. This value is also narrower than that of the well-known molybdate photocatalyst Bi2MoO6 (2.63 eV) [33] and Lu2MoO6 (2.79-2.81 eV) [34]. Usually the CT optical absorption in alkali earth metal molybadate double perovskites is in the region of 350-450 nm. For example, the optical absorption cut-off edge of Sr2CaMoO6 was reported to be at 450 nm. This shows the contribution of Ni to the band gap of electronic components [35]. It is well known that Ni2+ has a 3d8 electronic configurations, which usually presents the fully occupied split Ni 3d-t2g orbitals and the partially occupied split Ni 3d-eg. Ni 3d-eg orbitals can split into the sub-bands because of the distortion in NiO6 octahedral [22]. It has been confirmed that the band structure of oxide semiconductors containing transition metal ions is defined as follows: the valence band is the O 2p from MoO6 and CB is the d levels in transition metal octahedra. For examples, the donor local energy level can be created by Ni2+ ions in Sr2Mg1–xNixMoO6−δ to make the Fermi level move closer to the valence band [23]. The absorption band at 530-800 nm (2.34 – 1.55 eV) is created by the local energy level from Ni2+ ions [8]. Fig. 7 shows the energy diagram of the band structure in Sr2NiMoO6. It can be suggested that the VB in Sr2NiMoO6 is dominated from the Ni-3d and O-2p electronic components, while CB is made up by the Mo-5d and Ni-3d states. Similar absorption profiles have been reported in photocatalysts such as Ni3V2O8 [25] and CsLaSrNb2NiO9 [22], Ca2NiWO6 [36]. As displayed in Fig. 7, the valence band (VB) of Sr2NiMoO6 nanoparticles is suggested from the partly mixed O-2p and Ni-3d components, while the Mo-5d and Ni-3d form the conduction band (CB). The positions for VB and CB were theoretically calculated by the following equations:
EVB X E e 0.5Eg
(1)
ECB X E e 0.5Eg
(2)
here Eg is band energy, Ee is free electrons energy on hydrogen scale (∼4.5 eV vs SHE), X is absolute electronegativity (geometric mean of absolute electronegativity of constituent atoms). The VB top and CB bottom of Sr2NiMoO6 nanoparticles band gap are determined to be 1.66 and -0.5 eV vs SHE, respectively, as shown in Fig. 7. It can be suggested that Sr2NiMoO6 nanoparticles could split H2O into H2 under visible light irradiation because the CB position is more negative than H2 reduction energy; while O2 evolution is also possible due to the fact that the VB is more positive than O2 oxidation energy. However, due to the limitation of the experimental condition, we cannot complete the H2 and O2 evolution. Alternatively, the photodegradation for organic dye solutions were finished as the introduction below. To evaluate the nature of the light induced exciton in Sr2NiMoO6 nanoparticles, the luminescence was test. However, at room temperature, no luminescence spectra could be detected excited by UV or near UV light. It could be suggested that a weak luminescence in a semiconductor indicates that the recombination possibility of the excitons could be much low [37]. This is beneficial for the photo-activity in a degradation reaction. Usually the weak emission could have several reasons, for example, the emission could be quenched due to the non-irradiation to defects. Or if the efficient photocatalysis could results in the luminescence quenching because of the weak recombination between the electrons and holes (exciton). When the
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luminescence measurement of Sr2NiMoO6 was performed at a temperature lower than 100 K, the weak emission signals were detected. The typical luminescence spectrum for Sr2NiMoO6 nanoparticles is shown in Fig. 8 (a), which presents a broad band (580 to 950 nm) peaked at 750 nm. The full-width at half-maximum (FWHM) is about 110 nm. The luminescence decay shown in Fig. 8 (a) has been investigated for the further understanding of the optical activities. The decay shows a non-exponential decay behavior, which could be due to the non-radiative process, for example the excitation energy could be transferred to some defects. The decay can be fitted with at least two exponential functions. The average decay time τavg were evaluated using the equation.
avg
0 0
tI (t )dt (3)
I (t )dt
where I(t) represents the intensity at time t. The emission shows the average lifetime of 11.2 μs. This lifetime is much longer than the reported value in the molybdate double perovskites such as Sr2CaWxMo1−xO6 (3.48, 4.16, 5.24, 6.93, and 254.9 ns for x=0, 0.2, 0.4, 0.6 and 1, respectively) [35]. This could be suggested the Ni ions in Sr2NiMoO6 lattices play an important role in delaying the lifetime of excitons. Certainly the long lifetime in Sr2NiMoO6 nanoparticles supplies more opportunities for the separation between the holes and the electrons. This is a great benefit for the good photocatalysis ability of Sr2NiMoO6 nanoparticles.
3.4 Photocatalytic abilities and stability The photocatalytic abilities of Sr2NiMoO6 nanoparticles were confirmed by photodegradation on RhB solutions. The excitation is the visible-light with wavelength about 420 nm. Fig. 9 (a) shows the changes of absorption spectra of the RhB after the photocatalysis by Sr2NiMoO6 nanoparticles. The absorption has a lower and lower intensity with the increase of irradiation, while the spectra keep the same profile. The photocatalytic effects of the RhB-solutions by Sr2NiMoO6 photocatalyst are displayed in Fig. 9 (b). The photodegradation rate by Sr2NiMoO6 nanoparticles is about 90 % under visible light irradiation for two hours. This indicates the sample is active to the photocatalytic response under visible light. The absorption shown in Fig. 6 (a), clearly shows that the sample has also some absorption in the red to near IR region. The photodegradation on RhB solutions by Sr2NiMoO6 was also tested by using a 580 nm filter. The results indicate that the light irradiation with the wavelength <600 nm cannot initiates the photocatalysis. This could be due to the weak optical absorption intensity or the unfavorable created charges for the photocatalytic reactions. In order to evaluate the repetitive stability of Sr2NiMoO6 nanoparticles for photodegradation, three experiments were finished as shown in Fig. 9 (c). The results confirm that the photodegradation of Sr2NiMoO6 nanoparticles is stable. The crystal phase of Sr2NiMoO6 nanoparticles after the photocatalysis is also detected in Fig. 9 (d), which keeps the same profile with the as-prepared samples. This indicates the good optical stability of the photocatalyst.
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4 Discussions The redox happened on the surfaces of the particles in the process of photocatalysis. The chemical properties on the surfaces such as the element valences have obvious effects on the electrochemical effects. In this experiment, the valent states of the elements were detected by XPS measurements. Fig. 10 is the XPS spectra of Sr2NiMoO6 nanoparticles presenting the O 1s, Mo 3d, Ni 2p, and Sr 3d signals in the sample. The XPS spectrum for Sr-3d has an asymmetrical profile in Fig. 10 (a), which could be decomposed in two separated peaks related to 3d5/2 (136 eV) and 3d3/2 (138.5 eV). The result indicates the presence of Sr2+. The XPS spectrum for Ni-2p in Fig. 10 (b) shows two peaks at 856.2 eV and 862.6 eV. The latter signal is assigned to a typical satellite peak, which usually presents in oxides containing Ni because of the multiple splits for the energy levels. Another broad and asymmetry peak was decomposed in two sub-peaks. This indicates that there are two species of Ni2+ and Ni3+. Ni3+ ions have minor component judged by the integrated areas of the Ni3+ and Ni2+ peaks. Usually the mixed valence states in Ni-containing oxides can act as the advanced electrocatalysts [38]. The XPS spectrum for Mo-3d of Sr2NiMoO6is presented in Fig. 10 (c), which is divided into two sub-peaks related to 3d3/2 and 3d5/2 with binding energies at 236 and 231 eV, respectively. The asymmetrical spectrum was fit into two pairs of peaks assigned to Mo5+ and Mo6+ states [39]. The coexistence of Mo5+/Mo6+ is very commonly observed in molybdate perovskites [18]. This has been confirmed to be one of the reasons for the enhancement of the electronic conductivity in double perovskite Sr2Ni1+xMo1-xO6 [18]. The XPS information of O (Fig. 10 d) ions in the lattices was measured. The XPS curves show an asymmetric profile, which is deconvoluted into at least two Gaussian sub-peaks. The dominated signal is the oxygen components in the lattices. While the sub-peak (at 535.6 eV) could be from the adsorbed water, the adsorbed oxygen, and some oxygen defects on the surfaces. For example, the oxygen defects could be the oxygen vacancies (VO). VO has been confirmed to be inevitable in molybdate double perovskites [18]. While Dorai et al [2] reported that oxygen vacancies are generated in Sr2MgMoO6−δ because of the reduction of Mo6+ to Mo5+. Similar results have been reported in the well-known photocatalyst BiVO4. Rossell et al [40] reported that the VO vacancies are easily created due to vanadium reduction from +5 to +4. It is possible that the oxygen vacancies could reach to 15% due to the charge neutrality demands near the surface. The VO can support the photocatalytic reactions with the adsorption of O2− and OH− active species on the surface. It is noted that there are multivalent for the Ni and Mo ions in the lattices. This is in agreement with the structural requirements in the lattices because of the mixture of Ni and Mo inos. Actually, the investigation of valence states of B-ions in A2B′B″O6 double-perovskites has been one of the hot topics. The band-structure calculation in Sr2FeMoO6 has confirmed that there is a strong mixture of d electron from pentavalent Mo and minority spin t2g band of trivalent Fe ions [41]. So there are mixed-valence states of II/III (Fe) and V/VI (Mo). This situation has been fully confirmed by the experimental results such as Mössbauer [42] and nuclear magnetic resonance (NMR) technique [43]. The photocatalytic activity of Sr2NiMoO6 nanoparticles can be elucidated from the multi-valences of Ni2+/Ni3+ and Mo6+/Mo5+. And the multivalent ions can help the improvement of the 9
photodegradation. This suggestion could be explained in Fig. 11. Ni3+ ions act as a donor and Mo5+ ions act as an acceptor altering the recombination of the photo-created hole-electron: Mo6 eCB Mo5electron trap
(3)
Ni 2 hVB Ni 3 hole trap
(4)
It could be suggested that Ni2+ ions and electrons could generate O2− and hydroperoxide radicals. Meanwhile, a Mo6+ ions and a hole could react with hydroxyl groups (or H2O) to create OH decomposing organic dye:
Ni 2 O2 Ni 3 O2
(5)
CCB O2 O2
(6)
Mo6 OH Mo5 OH
(7)
hVB OH OH
(8)
The suggested radical specie ˙OH was further verified according to the reports by Rawal et al [44]. The20 mg Sr2NiMoO6 nanoparticles were mixed in the 60 mL solution containing 0.01 M NaOH and 3 mM 1,4-terephthalic acid (TA). This is based on the fact that the ˙OH radical reacts with TA in basic solution and creates 2-hydroxy terephthalic acid (TAOH), which could give the luminescence band with the maximum at about 426 nm [45,46]. Fig. 10 is the emission spectra of the solution before and after photo-degeneration. The TA solution does not show the detectable signal for the pure TA solution (Fig. 10 a). It shows the characteristic emission after the photo-degeneration by Sr2NiMoO6 nanoparticles visible light irradiation (Fig. 10 b, c). And the emission signal was enhanced with the increase of the irradiation time. The test confirms that the major active species in the process of photocatlysis could be the hydroxyl radicals.
5. Conclusions In summary, the Sr2NiMoO6 nanoparticles were prepared via the chemical sol-gel route. The nanoparticles develop in worm-like nano-grains with diameters of 20 nm to 50 nm and the specific surface area value of 31.7 m2/g. Sr2NiMoO6 nanoparticles have the indirect allowed characteristic by the band energy 2.1 eV. The VB top and CB bottom of Sr2NiMoO6 nanoparticles were decided to be at 1.66 and -0.5 eV vs SHE, respectively. The luminescence for Sr2NiMoO6 only can be observed at a temperature below 100 K with a long decay time of 11.2 μs. As expected, Sr2NiMoO6 has a more efficient photocatalytic activity on RhB photodegradation. The desired photocatalysis was discussed on its specialties such as appropriate band energy positions, quenched luminescence, long-lived exciton, and the coexistence of multivalent Mo and Ni-ions on the surfaces of the nanoparticles.
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Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013RA1A2009154) and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China.
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[43] C. Kapusta, P.C. Riedi, D. Zajac, M. Sikora, J.M. De Teresa, L. Morellon, M.R. Ibarra, NMR study of double perovskite Sr2FeMoO6, J. Magn. Magn. Mater. 242 (2002) 701-703. [44] Sher Bahadur Rawal, Sandipan Bera, Daeki Lee, Du-Jeon Jang and Wan In Lee, Design of visible-light photocatalysts by coupling of narrow bandgap semiconductors and TiO 2: effect of their relative energy band positions on the photocatalytic efficiency. Catal. Sci. Technol., 3 (2013) 1822-1830. [45] Sher Bahadur Rawal, Sandipan Bera, Daeki Lee, Du-Jeon Jang and Wan In Lee, Design of visible-light photocatalysts by coupling of narrow bandgap semiconductors and TiO 2: effect of their relative energy band positions on the photocatalytic efficiency. Catal. Sci. Technol., 3 (2013) 1822-1830. [46] G. Liu, C. Sun, L. Cheng, Y. Jin, H. Lu, L. Wang, S. C. Smith, G. Q. Lu and H. M. Cheng, Efficient Promotion of Anatase TiO2 Photocatalysis via Bifunctional Surface-Terminating Ti−O−B−N Structures, J. Phys. Chem. C 113 (2009) 12317-12324
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Figure captions Fig. 1 Rietveld refinement for the calculated and the experimental XRD patterns of Sr2NiMoO6 nanoparticles. Fig. 2 the schematic view of the double-perovskite-type Sr2NiMoO6 structure along c-direction. Fig. 3 the SEM images in different amplified scales (a, b, c) and the EDS spectrum (d) of Sr2NiMoO6 nanoparticles Fig. 4 TEM photos (a, b, c), HRTEM images (d), and inset in (d) is the SAED pattern of Sr2NiMoO6 nanoparticles. Fig. 5 N2-adsorption–desorption isotherms of Sr2NiMoO6 nanoparticles. Inset is distribution curve for the pore-size. Fig. 6 (a): UV–Vis absorption spectra of Sr2NiMoO6 nanoparticles; inset is nature color of the nanoparticle; (b): the estimation for the band gap energy. Fig. 7 the suggested energy schematic diagram of the band structure of Sr2NiMoO6 nanoparticles Fig. 8 the emission spectrum (a) and decay curve in Sr2NiMoO6 nanoparticles under pulsed 355 nm YAG:Nd laser at 100 K. Fig. 9 UV-vis absorption (a), photo-degradation effects (b), three repetitive photo-degradation experiments (c) for the photocatalysis of Sr2NiMoO6 nanoparticles, and XRD profiles of the as-prepared sample after the photocatalysis (d). Fig. 10 XPS spectra for elements of Sr-3d (a), Ni-2p (b), Mo-3d (c) and O 1s (d) measured in Sr2NiMoO6 nanoparticles. Fig. 11 The suggested structure for multi-valent Mo and Ni-ions in Sr2NiMoO6 nanoparticles Fig. 12 The spectra measured for the pure TA solution (a) and TA solutions with Sr2NiMoO6 nanoparticles under visible-light irradiation for 40 min (b) and 120 min (c).
14
Fig. 1 Rietveld refinement for the calculated and the experimental XRD patterns of Sr2NiMoO6.
Fig. 2 the schematic view of the double-perovskite-type Sr2NiMoO6 structure along c-direction.
15
Fig. 3 SEM images in different amplified scales (a, b, c) and the EDS spectrum (d) of Sr2NiMoO6 nanoparticles
16
Fig. 4 TEM photos (a, b, c), HRTEM images (d), and inset in (d) is the SAED pattern of Sr2NiMoO6 nanoparticles.
20 15
2
160
Ds/Dd/m /g/nm
Volume adsorbed ml/g
200
120
10
80
5 0 0
10
20
30
40
Pore diameter/nm 40 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0) Fig. 5 N2-adsorption–desorption isotherms of Sr2NiMoO6 nanoparticles. Inset is distribution curve for the pore-size.
17
Optical absorption (a. u.)
1.0
(a)
0.8
0.6
0.4
1.55eV 0.2
2.1eV
0.0 200
300
400
500
600
700
800
Wavelength (nm)
(h)1/2 (eV)1/2
(b)
2.1 eV 2
3
4
5
6
h (eV) Fig. 6 (a): UV–Vis absorption spectra of Sr2NiMoO6 nanoparticles; inset is nature color of the nanoparticle; (b): the estimation for the band gap energy.
18
Fig. 7 the suggested energy schematic diagram of the band structure of Sr2NiMoO6 nanoparticles
19
(a)
Intensity (a. u.)
0.00012
ex=355 nm 0.00010
0.00008
0.00006
0.00004
500
600
700
800
900
1000
Wavelength (nm)
ex=355 nm
(b)
Ln[Intensity (a. u.)]
em=750 nm =11.2s
0
40
80
120
Time (s) Fig. 8 Emission spectrum (a) and decay curve in Sr2NiMoO6 nanoparticles under pulsed 355 nm YAG:Nd laser at 100 K.
20
Fig. 9 UV-vis absorption (a), photo-degradation effects (b), three repetitive photo-degradation experiments (c) for the photocatalysis of Sr2NiMoO6 nanoparticles, and XRD profiles of the as-prepared sample after the photocatalysis (d).
21
Fig. 10 XPS spectra for elements of Sr-3d (a), Ni-2p (b), Mo-3d (c) and O 1s (d) measured in Sr2NiMoO6 nanoparticles.
22
Fig. 11 The suggested structure for multi-valent Mo and Ni-ions in Sr2NiMoO6 nanoparticles
3.0 2.8
(c)
2.6
Intensity (a. u.)
2.4 2.2 2.0 1.8
(b)
1.6 1.4 1.2 1.0 0.8
(a)
0.6 0.4 0.2 200
300
400
500
600
700
Wavelength (nm) Fig. 12 The spectra measured for the pure TA solution (a) and TA solutions with Sr2NiMoO6 nanoparticles under visible-light irradiation for 40 min (b) and 120 min (c).
23
Table 1 the refined crystallographic data of Sr2NiMoO6 nanoparticles formula
Sr2NiMoO6
radiation
Cu Ka
2θ range(degree)
10-120o
symmetry
tetragonal
space group#
I4/m (87)
a/Å
5.5335(3)
b/Å
5.5335(3)
c/Å
7.8763(5)
α/°
90
β/°
90
γ/°
90
Z
2
Rp
0.1046
Rwp
0.1102
X2
20.97
V
241.16(4)Å3
24
Table 2 Refined atomic coordinate parameters data of Sr2NiMoO6 nanoparticles at room temperature. Atom
Wyck.
Site
x/a
y/b
z/c
U [Å2]
Sr1 Ni1 Mo1 O1 O2
4d 2b 2a 8h 4e
-4.. 4/m. 4/m. m.. 4..
0 0 0 0.27030 0
1/2 0 0 0.21580 0
1/4 1/2 0 0 0.24170
0.0537(2) -0.0179(2) 0.1101(4) 0.0003 0.0003
25
S0169-4332(16)31553-7 http://dx.doi.org/doi:10.1016/j.apsusc.2016.07.117 APSUSC 33685
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-6-2016 14-7-2016 14-7-2016
Please cite this article as: Lei Xu, Yingpeng Wan, Hongde Xie, Yanlin Huang, Li Yang, Lin Qin, Hyo Jin Seo, Synthesis, surface structure and optical properties of double perovskite Sr2NiMoO6 nanoparticles, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.07.117 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis, surface structure and optical properties of double perovskite Sr2NiMoO6 nanoparticles Lei Xu,a Yingpeng Wan,a Hongde Xie,a* Yanlin Huang,a Li Yang,a Lin Qin,b Hyo Jin Seo b*
a
State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of
Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China b
Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical
Engineering, Pukyong National University, Busan 608-737, Republic of Korea
*
Corresponding authors: [email protected] (Hongde Xie), [email protected]
(Hyo Jin Seo), Tel.: +82-51-629 5568; fax: +82-51-6295549.
1
Graphical abstract
2
Highlights Double perovskite Sr2NiMoO6 nanoparticles were prepared via sol-gel route. The nanoparticles have efficient optical absorption in visible light. The band structure and energy positions were determined. The perovskite has efficient photocatalytic on RhB photodegradation. Multivalent Mo and Ni-ions on the surfaces were investigated.
Abstract Double perovskite Sr2NiMoO6 nanoparticles were synthesized via the chemical sol-gel route. The phase formation was investigated through X-ray polycrystalline diffraction (XRD) and Rietveld refinements. The perovskite crystallized in worm-like nano-grains with the diameter of 20 to 50 nm. The optical properties were measured by the optical absorption spectra. The nanoparticles present an indirect allowed transition with a narrow band gap of 2.1 eV. Sr2NiMoO6 nanoparticles have obvious photocatalytic ability on the degradation of Rhodamine B (RhB) solutions under the irradiation of visible light. The transport behaviors of the excitons were investigated from the photoluminescence spectra and the corresponding decay lifetimes. Sr2NiMoO6 nanoparticles present several advantages for photocatalysis such as the appropriate band energy positions, the quenched luminescence, and the coexistence of multivalent ions in the lattices.
Keywords:
Inorganic compounds; Double perovskite; Photocatalysis; Electronic band
structure; Optical properties; Solar materials.
3
1. Introduction Perovskite is one of the most important materials with intensively reported in optical, electrical, magnetic and biological field [1-8]. Especially synthesis, optical properties, photoactivity and applications of perovskite materials have been widely reported [9,10]. Among kinds of photocatalysts perovskites have shown very efficient photocatalytic performances under visible-light irradiation because of the unique structures and electronic properties. This crystal structure can provide a good framework in which to tune the band gap values to enable visible-light absorption and band edge potentials to suit the needs of specific photocatalytic reactions [10] . Moreover, the lattice distortion in perovskite materials can strongly influence the separation of photo-created charges. Some perovskite materials have been well-known photocatalysts such as titanate perovskites [11], tantalate perovskites [12], vanadium- and niobium-based perovskites [13], and ferrite perovskites [14] etc have shown visible-light activity. For example, double-perovskite compounds with chemical formula of A2B′B″O6−δ are derived from the simple perovskite ABO3 through arranging two different cations B′ and B″ on the B-site. A stands for alkaline earth metal ions (Ba, Ca, Sr), and B′, B″ are transition metal ions B′= Co, Fe, Ni, Cr, etc, and B″= W, Mo, Sb, etc [1]. In an ideal structure form, the framework of double-perovskite is constructed by corner-shared B′O6 and B″O6 octahedra in the lattices [2]. Double-perovskites are well-known functional materials because of the important physical properties such as thermal, electrical, optical and magnetic. Among them, double-perovskites with B=Mo have been paid great attentions, especially many reports have extensively reported the applications as electrode materials in solid oxide fuel cells [8]. For example, double perovskite Sr2NiMoO6 is one of the attractive candidates for anode materials in SOFCs [15]. In the past years, Sr2NiMoO6 has been received many attentions such as the preparation and structural refinement [16], electronic structure [17], electrical conductivity [18], defect formations [2,19], etc. In this work, the photocatalytic ability of Sr2NiMoO6 was tested because of several motivations. At first, this double perovskite has efficient optical centers MoO6 in the lattices, which are expected to induce absorption in the visible region [17]. Molybdate oxides usually show efficient abilities on degradation of organic dyes or depleting H2O. For example, scheelite-type PbMoO4 can split H2O to H2 and O2 under the excitation of UV light [20]. Bi2MoO6 is the well-known photocatalyst with the O2 evolution property by visible light irradiation [21]. Secondly, Ni2+-ions doped in a photocatalyst lattice form the local energy levels, which could enhance the charge-transfer transition. And the band energy can be greatly reduced by Ni-3d levels to permit the photocatalytic activity under the visible-light [22,23]. It has been reported that Ni substitution in Sr2Mg1–xNixMoO6−δ induces the donor levels, which move the Fermi energy to the nearer valence band. This effect results in the decrease of the band gap width [23]. The enhancements of photocatalytic abilities have been reported by doping Ni ions in the photocatalysts such as SrTiO3 [24], M3V2O8 (M = Mg, Ni, Zn) [25], Ni2+-doped TiO2 [26], NixCd1−xS [27], Ni-doped InVO4 [28], Li2Ni2(MoO4)3 [29], and K2Ni(VO3)4 [30], et al. Thirdly, the coexistence of Mo6+ with Ni2+ ions in B sites in perovskites provides the off-center distortions and polarizations of NiO6 centers, which benefits for the charge separations and enhancing the photocatalysis under visible light irradiation. The off-centering can be stabilized by 4
lowering the energy of covalent bond formations; consequently the charge-transfer from O-2p to Ni-3d can easily happen [31]. Herein, Sr2NiMoO6 was developed by the sol-gel reaction. The crystal phase and the XRD structural refinement have been conducted. The surface properties were checked by SEM, TEM, BET and XPS measurements. The luminescence spectra and decay lifetime were obtained. The photo-degradation on the degradation of RhB dyes was confirmed. The photo-active properties were discussed on the structure.
2. Experimental Sr2NiMoO6 nanoparticles were synthesized via the chemical sol-gel method. The raw reactants of Sr(NO3)2, Mg(NO3)2·6H2O, Ni(NO3)3·6H2O, and (NH4)6Mo7O24·4H2O were firstly dissolved in the water. Some citric acid monohydrate as the chelating agent was added in the solutions. Then, to adjust the viscoelasticity the PVA solutions were dropwise added in the mixtures. The solutions could become viscous after constant stirring in a water bath of 90 °C. The sticky solutions could be slowly coated on the glass substrates to get the precursor thin films, which contain Ni2+, Mo6+, and Sr2+ ions together with the organic polymer compounds. Finally, the dried thin films were peeled off and moved to a furnace for heat treatment at 900°C. in this way, the organic components were decomposed, while, Ni2+, Mo6+, and Sr2+ ions in stoichiometric ration formed Sr2NiMoO6 nanoparticles. The XRD measurements were finished on the Rigaku D/Max-2000 XRD diffractometer (40 kV, 35 mA) equipped by a Cu Kα target (λ=1.5405 Å). The SEM, EDS and TEM measurements were applied to investigate the surface morphologies. The N2-desorption-adsorption isotherms were measured at liquid nitrogen temperature by instrument of Nova 2000e. Brunauer-Emmett-Teller (BET) was used in the specific area measurements. The luminescence spectra and the decay curve of the as-prepared samples were measured to evaluate the dynamic properties of the excitons. The excitation was third harmonic (355 nm) of a pulsed Nd:YAG laser. The luminescence was dispersed by the 75 cm monochromator (Acton Research Corp. Pro-750) and multiplied by the PMT (Hamamatsu R928). The data was displayed and recorded with the LeCloy 9301 digital storage oscilloscope. The photocatalysis measurements were performed under irradiated by visible light from the 500 W-Xe UV-lamp. A 420 nm filter was used to get the visible wavelength for the irradiation. The air was pumped into the reactor in a speed of 500 mL min−1. In a typical photocatalysis experiment, the weight of Sr2NiMoO6 nanoparticles is about 0.05 g, while the corresponding RhB solution (10 mgL−1) was 300 mL. It is important that the initial test solution was stirred in a dark place for 1 h to reach desorption/adsorption equilibrium. In a time interval, 5 mL RhB solution was extracted and was taken for the UV-visible optical absorption. The effects were determined by comparing the absorption intensities for each measurement at different time.
3. Results and Discussions 3.1 Phase formation The phase formation, crystal structure and experimental unit cell parameters of the Sr2NiMoO6 nanoparticles were confirmed by XRD measurement. Fig. 1 is XRD pattern indexed on tetragonal Sr2NiMoO6 with double perovskite structure. No impurity phases such as SrMoO4 and NiO were 5
detected in the XRD pattern. The Rietveld refinement of the XRD pattern obtained by GSAS software is shown in Fig. 1, and the obtained structure data and the atomic positions are given in Table 1 and 2, respectively. Fig. 2 shows the structure sketch of Sr2NiMoO6 along [001] modeled by the refined atomic coordinate’s in Table 2. It has a tetragonal lattice with the space group of I4/m, which is in agreement with previous structural studies. It is a characteristic double perovskite with the NiO6 and MoO6 octahedra ordered in the lattices. The two kinds of octahedra are corner-shared forming the structure in Sr2NiMoO6. Sr atoms are located betwen the MoO6 and NiO6.
3.2 The surface characteristics The typical surface morphologies of Sr2NiMoO6 nanoparticles were tested by SEM measurements as displayed in Fig. 3 (a, b, c). On the view from the SEM with low-amplification, The sample shows aggregations. The particles seem to connect with each other with aggregations. It appears a sponge-like profile. The worm-like nano-grains could be observed at high amplification as shown in Fig. 3 (c). The diameters of the particles are 20 nm to 50 nm. Fig. 3 (d) shows the EDS results on the surfaces of Sr2NiMoO6 nanoparticles. The O, Mo, Ni and Sr elements were detected in Sr2NiMoO6 surfaces. And the result shows that Ni and Mo have a molar ratio of around 1.0, which in in agreement with the theoretical result in Sr2NiMoO6. The surface microstructure of Sr2NiMoO6 nanoparticles was confirmed by the TEM measurements in Fig. 4 (a, b, c). The HRTEM images in Fig. 4 (d) confirms the single-crystalline property of Sr2NiMoO6 nanoparticles. The interplanar distance presented on the lattice fringe is about 0.1966 nm, which is corresponding to the (220) preferable growth plane. The selected area electron diffraction (SAED) has a clear pattern (Fig. 4 d), which demonstrates that Sr2NiMoO6 is single crystalline and belongs to tetragonal phase. In addition, N2-adsorption–desorption isotherm of Sr2NiMoO6 nanoparticles was measured (Fig. 5). The specific surface area of Sr2NiMoO6 nanoparticles was obtained to be 31.7 m2/g by the Brunauer–Emmett–Teller (BET) results. The distribution of the pore size was around 7 nm.
3.3 Optical absorption properties and band gap structure The optical absorption spectrum in UV-Vis region of Sr2NiMoO6 nanoparticles is displayed in Fig. 6 (a). It presents two broad bands in the wavelength from 200 to 800 nm. The absorption presents a cut-off edge at about 585 nm obtained from its intercept on x-axis. The point of intersect on x-axis (lg) is the rough energy value for band-gap energy (Eg), which was calculated by the equation of Eg=1240/lg. The band energy of Sr2NiMoO6 nanoparticles is 2.1 eV in this estimation. The red color of the sample in Fig. 6 (a) is induced by the rich optical centers. To further understand the nature of the band-gap transition in this double perovskite, it was discussed in the formula h (h Eg )k . Here ν denotes frequency of incident light, α is optical absorbance, h is Planck constant. k is also a constant with four possible values of 1/2, 2, 3/2 or 3, which decides the transition nature. It could be direct allowed, indirect allowed, direct forbidden and indirect forbidden types when k is 1/2, 2, 3/2 or 3, respectively. The band energy (Eg) of Sr2NiMoO6 nanoparticles can be well taken at k=2, indicating the indirect allowed transitions. The linear portion of 2.1 eV can be obtained in Fig. 6 (b) by extrapolating the curve to the intercept of the x-axis. The narrow permits Sr2NiMoO6 to use the visible light in the
6
photocatalysis. This shows the advantage than the well-known photocatalysts such as TiO2 (Eg=3.2 eV) [9,10], ZnO (Eg = 3.37 eV) [32] , which only use the UV light because of the wide band gap. This value is also narrower than that of the well-known molybdate photocatalyst Bi2MoO6 (2.63 eV) [33] and Lu2MoO6 (2.79-2.81 eV) [34]. Usually the CT optical absorption in alkali earth metal molybadate double perovskites is in the region of 350-450 nm. For example, the optical absorption cut-off edge of Sr2CaMoO6 was reported to be at 450 nm. This shows the contribution of Ni to the band gap of electronic components [35]. It is well known that Ni2+ has a 3d8 electronic configurations, which usually presents the fully occupied split Ni 3d-t2g orbitals and the partially occupied split Ni 3d-eg. Ni 3d-eg orbitals can split into the sub-bands because of the distortion in NiO6 octahedral [22]. It has been confirmed that the band structure of oxide semiconductors containing transition metal ions is defined as follows: the valence band is the O 2p from MoO6 and CB is the d levels in transition metal octahedra. For examples, the donor local energy level can be created by Ni2+ ions in Sr2Mg1–xNixMoO6−δ to make the Fermi level move closer to the valence band [23]. The absorption band at 530-800 nm (2.34 – 1.55 eV) is created by the local energy level from Ni2+ ions [8]. Fig. 7 shows the energy diagram of the band structure in Sr2NiMoO6. It can be suggested that the VB in Sr2NiMoO6 is dominated from the Ni-3d and O-2p electronic components, while CB is made up by the Mo-5d and Ni-3d states. Similar absorption profiles have been reported in photocatalysts such as Ni3V2O8 [25] and CsLaSrNb2NiO9 [22], Ca2NiWO6 [36]. As displayed in Fig. 7, the valence band (VB) of Sr2NiMoO6 nanoparticles is suggested from the partly mixed O-2p and Ni-3d components, while the Mo-5d and Ni-3d form the conduction band (CB). The positions for VB and CB were theoretically calculated by the following equations:
EVB X E e 0.5Eg
(1)
ECB X E e 0.5Eg
(2)
here Eg is band energy, Ee is free electrons energy on hydrogen scale (∼4.5 eV vs SHE), X is absolute electronegativity (geometric mean of absolute electronegativity of constituent atoms). The VB top and CB bottom of Sr2NiMoO6 nanoparticles band gap are determined to be 1.66 and -0.5 eV vs SHE, respectively, as shown in Fig. 7. It can be suggested that Sr2NiMoO6 nanoparticles could split H2O into H2 under visible light irradiation because the CB position is more negative than H2 reduction energy; while O2 evolution is also possible due to the fact that the VB is more positive than O2 oxidation energy. However, due to the limitation of the experimental condition, we cannot complete the H2 and O2 evolution. Alternatively, the photodegradation for organic dye solutions were finished as the introduction below. To evaluate the nature of the light induced exciton in Sr2NiMoO6 nanoparticles, the luminescence was test. However, at room temperature, no luminescence spectra could be detected excited by UV or near UV light. It could be suggested that a weak luminescence in a semiconductor indicates that the recombination possibility of the excitons could be much low [37]. This is beneficial for the photo-activity in a degradation reaction. Usually the weak emission could have several reasons, for example, the emission could be quenched due to the non-irradiation to defects. Or if the efficient photocatalysis could results in the luminescence quenching because of the weak recombination between the electrons and holes (exciton). When the
7
luminescence measurement of Sr2NiMoO6 was performed at a temperature lower than 100 K, the weak emission signals were detected. The typical luminescence spectrum for Sr2NiMoO6 nanoparticles is shown in Fig. 8 (a), which presents a broad band (580 to 950 nm) peaked at 750 nm. The full-width at half-maximum (FWHM) is about 110 nm. The luminescence decay shown in Fig. 8 (a) has been investigated for the further understanding of the optical activities. The decay shows a non-exponential decay behavior, which could be due to the non-radiative process, for example the excitation energy could be transferred to some defects. The decay can be fitted with at least two exponential functions. The average decay time τavg were evaluated using the equation.
avg
0 0
tI (t )dt (3)
I (t )dt
where I(t) represents the intensity at time t. The emission shows the average lifetime of 11.2 μs. This lifetime is much longer than the reported value in the molybdate double perovskites such as Sr2CaWxMo1−xO6 (3.48, 4.16, 5.24, 6.93, and 254.9 ns for x=0, 0.2, 0.4, 0.6 and 1, respectively) [35]. This could be suggested the Ni ions in Sr2NiMoO6 lattices play an important role in delaying the lifetime of excitons. Certainly the long lifetime in Sr2NiMoO6 nanoparticles supplies more opportunities for the separation between the holes and the electrons. This is a great benefit for the good photocatalysis ability of Sr2NiMoO6 nanoparticles.
3.4 Photocatalytic abilities and stability The photocatalytic abilities of Sr2NiMoO6 nanoparticles were confirmed by photodegradation on RhB solutions. The excitation is the visible-light with wavelength about 420 nm. Fig. 9 (a) shows the changes of absorption spectra of the RhB after the photocatalysis by Sr2NiMoO6 nanoparticles. The absorption has a lower and lower intensity with the increase of irradiation, while the spectra keep the same profile. The photocatalytic effects of the RhB-solutions by Sr2NiMoO6 photocatalyst are displayed in Fig. 9 (b). The photodegradation rate by Sr2NiMoO6 nanoparticles is about 90 % under visible light irradiation for two hours. This indicates the sample is active to the photocatalytic response under visible light. The absorption shown in Fig. 6 (a), clearly shows that the sample has also some absorption in the red to near IR region. The photodegradation on RhB solutions by Sr2NiMoO6 was also tested by using a 580 nm filter. The results indicate that the light irradiation with the wavelength <600 nm cannot initiates the photocatalysis. This could be due to the weak optical absorption intensity or the unfavorable created charges for the photocatalytic reactions. In order to evaluate the repetitive stability of Sr2NiMoO6 nanoparticles for photodegradation, three experiments were finished as shown in Fig. 9 (c). The results confirm that the photodegradation of Sr2NiMoO6 nanoparticles is stable. The crystal phase of Sr2NiMoO6 nanoparticles after the photocatalysis is also detected in Fig. 9 (d), which keeps the same profile with the as-prepared samples. This indicates the good optical stability of the photocatalyst.
8
4 Discussions The redox happened on the surfaces of the particles in the process of photocatalysis. The chemical properties on the surfaces such as the element valences have obvious effects on the electrochemical effects. In this experiment, the valent states of the elements were detected by XPS measurements. Fig. 10 is the XPS spectra of Sr2NiMoO6 nanoparticles presenting the O 1s, Mo 3d, Ni 2p, and Sr 3d signals in the sample. The XPS spectrum for Sr-3d has an asymmetrical profile in Fig. 10 (a), which could be decomposed in two separated peaks related to 3d5/2 (136 eV) and 3d3/2 (138.5 eV). The result indicates the presence of Sr2+. The XPS spectrum for Ni-2p in Fig. 10 (b) shows two peaks at 856.2 eV and 862.6 eV. The latter signal is assigned to a typical satellite peak, which usually presents in oxides containing Ni because of the multiple splits for the energy levels. Another broad and asymmetry peak was decomposed in two sub-peaks. This indicates that there are two species of Ni2+ and Ni3+. Ni3+ ions have minor component judged by the integrated areas of the Ni3+ and Ni2+ peaks. Usually the mixed valence states in Ni-containing oxides can act as the advanced electrocatalysts [38]. The XPS spectrum for Mo-3d of Sr2NiMoO6is presented in Fig. 10 (c), which is divided into two sub-peaks related to 3d3/2 and 3d5/2 with binding energies at 236 and 231 eV, respectively. The asymmetrical spectrum was fit into two pairs of peaks assigned to Mo5+ and Mo6+ states [39]. The coexistence of Mo5+/Mo6+ is very commonly observed in molybdate perovskites [18]. This has been confirmed to be one of the reasons for the enhancement of the electronic conductivity in double perovskite Sr2Ni1+xMo1-xO6 [18]. The XPS information of O (Fig. 10 d) ions in the lattices was measured. The XPS curves show an asymmetric profile, which is deconvoluted into at least two Gaussian sub-peaks. The dominated signal is the oxygen components in the lattices. While the sub-peak (at 535.6 eV) could be from the adsorbed water, the adsorbed oxygen, and some oxygen defects on the surfaces. For example, the oxygen defects could be the oxygen vacancies (VO). VO has been confirmed to be inevitable in molybdate double perovskites [18]. While Dorai et al [2] reported that oxygen vacancies are generated in Sr2MgMoO6−δ because of the reduction of Mo6+ to Mo5+. Similar results have been reported in the well-known photocatalyst BiVO4. Rossell et al [40] reported that the VO vacancies are easily created due to vanadium reduction from +5 to +4. It is possible that the oxygen vacancies could reach to 15% due to the charge neutrality demands near the surface. The VO can support the photocatalytic reactions with the adsorption of O2− and OH− active species on the surface. It is noted that there are multivalent for the Ni and Mo ions in the lattices. This is in agreement with the structural requirements in the lattices because of the mixture of Ni and Mo inos. Actually, the investigation of valence states of B-ions in A2B′B″O6 double-perovskites has been one of the hot topics. The band-structure calculation in Sr2FeMoO6 has confirmed that there is a strong mixture of d electron from pentavalent Mo and minority spin t2g band of trivalent Fe ions [41]. So there are mixed-valence states of II/III (Fe) and V/VI (Mo). This situation has been fully confirmed by the experimental results such as Mössbauer [42] and nuclear magnetic resonance (NMR) technique [43]. The photocatalytic activity of Sr2NiMoO6 nanoparticles can be elucidated from the multi-valences of Ni2+/Ni3+ and Mo6+/Mo5+. And the multivalent ions can help the improvement of the 9
photodegradation. This suggestion could be explained in Fig. 11. Ni3+ ions act as a donor and Mo5+ ions act as an acceptor altering the recombination of the photo-created hole-electron: Mo6 eCB Mo5electron trap
(3)
Ni 2 hVB Ni 3 hole trap
(4)
It could be suggested that Ni2+ ions and electrons could generate O2− and hydroperoxide radicals. Meanwhile, a Mo6+ ions and a hole could react with hydroxyl groups (or H2O) to create OH decomposing organic dye:
Ni 2 O2 Ni 3 O2
(5)
CCB O2 O2
(6)
Mo6 OH Mo5 OH
(7)
hVB OH OH
(8)
The suggested radical specie ˙OH was further verified according to the reports by Rawal et al [44]. The20 mg Sr2NiMoO6 nanoparticles were mixed in the 60 mL solution containing 0.01 M NaOH and 3 mM 1,4-terephthalic acid (TA). This is based on the fact that the ˙OH radical reacts with TA in basic solution and creates 2-hydroxy terephthalic acid (TAOH), which could give the luminescence band with the maximum at about 426 nm [45,46]. Fig. 10 is the emission spectra of the solution before and after photo-degeneration. The TA solution does not show the detectable signal for the pure TA solution (Fig. 10 a). It shows the characteristic emission after the photo-degeneration by Sr2NiMoO6 nanoparticles visible light irradiation (Fig. 10 b, c). And the emission signal was enhanced with the increase of the irradiation time. The test confirms that the major active species in the process of photocatlysis could be the hydroxyl radicals.
5. Conclusions In summary, the Sr2NiMoO6 nanoparticles were prepared via the chemical sol-gel route. The nanoparticles develop in worm-like nano-grains with diameters of 20 nm to 50 nm and the specific surface area value of 31.7 m2/g. Sr2NiMoO6 nanoparticles have the indirect allowed characteristic by the band energy 2.1 eV. The VB top and CB bottom of Sr2NiMoO6 nanoparticles were decided to be at 1.66 and -0.5 eV vs SHE, respectively. The luminescence for Sr2NiMoO6 only can be observed at a temperature below 100 K with a long decay time of 11.2 μs. As expected, Sr2NiMoO6 has a more efficient photocatalytic activity on RhB photodegradation. The desired photocatalysis was discussed on its specialties such as appropriate band energy positions, quenched luminescence, long-lived exciton, and the coexistence of multivalent Mo and Ni-ions on the surfaces of the nanoparticles.
10
Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013RA1A2009154) and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China.
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Figure captions Fig. 1 Rietveld refinement for the calculated and the experimental XRD patterns of Sr2NiMoO6 nanoparticles. Fig. 2 the schematic view of the double-perovskite-type Sr2NiMoO6 structure along c-direction. Fig. 3 the SEM images in different amplified scales (a, b, c) and the EDS spectrum (d) of Sr2NiMoO6 nanoparticles Fig. 4 TEM photos (a, b, c), HRTEM images (d), and inset in (d) is the SAED pattern of Sr2NiMoO6 nanoparticles. Fig. 5 N2-adsorption–desorption isotherms of Sr2NiMoO6 nanoparticles. Inset is distribution curve for the pore-size. Fig. 6 (a): UV–Vis absorption spectra of Sr2NiMoO6 nanoparticles; inset is nature color of the nanoparticle; (b): the estimation for the band gap energy. Fig. 7 the suggested energy schematic diagram of the band structure of Sr2NiMoO6 nanoparticles Fig. 8 the emission spectrum (a) and decay curve in Sr2NiMoO6 nanoparticles under pulsed 355 nm YAG:Nd laser at 100 K. Fig. 9 UV-vis absorption (a), photo-degradation effects (b), three repetitive photo-degradation experiments (c) for the photocatalysis of Sr2NiMoO6 nanoparticles, and XRD profiles of the as-prepared sample after the photocatalysis (d). Fig. 10 XPS spectra for elements of Sr-3d (a), Ni-2p (b), Mo-3d (c) and O 1s (d) measured in Sr2NiMoO6 nanoparticles. Fig. 11 The suggested structure for multi-valent Mo and Ni-ions in Sr2NiMoO6 nanoparticles Fig. 12 The spectra measured for the pure TA solution (a) and TA solutions with Sr2NiMoO6 nanoparticles under visible-light irradiation for 40 min (b) and 120 min (c).
14
Fig. 1 Rietveld refinement for the calculated and the experimental XRD patterns of Sr2NiMoO6.
Fig. 2 the schematic view of the double-perovskite-type Sr2NiMoO6 structure along c-direction.
15
Fig. 3 SEM images in different amplified scales (a, b, c) and the EDS spectrum (d) of Sr2NiMoO6 nanoparticles
16
Fig. 4 TEM photos (a, b, c), HRTEM images (d), and inset in (d) is the SAED pattern of Sr2NiMoO6 nanoparticles.
20 15
2
160
Ds/Dd/m /g/nm
Volume adsorbed ml/g
200
120
10
80
5 0 0
10
20
30
40
Pore diameter/nm 40 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0) Fig. 5 N2-adsorption–desorption isotherms of Sr2NiMoO6 nanoparticles. Inset is distribution curve for the pore-size.
17
Optical absorption (a. u.)
1.0
(a)
0.8
0.6
0.4
1.55eV 0.2
2.1eV
0.0 200
300
400
500
600
700
800
Wavelength (nm)
(h)1/2 (eV)1/2
(b)
2.1 eV 2
3
4
5
6
h (eV) Fig. 6 (a): UV–Vis absorption spectra of Sr2NiMoO6 nanoparticles; inset is nature color of the nanoparticle; (b): the estimation for the band gap energy.
18
Fig. 7 the suggested energy schematic diagram of the band structure of Sr2NiMoO6 nanoparticles
19
(a)
Intensity (a. u.)
0.00012
ex=355 nm 0.00010
0.00008
0.00006
0.00004
500
600
700
800
900
1000
Wavelength (nm)
ex=355 nm
(b)
Ln[Intensity (a. u.)]
em=750 nm =11.2s
0
40
80
120
Time (s) Fig. 8 Emission spectrum (a) and decay curve in Sr2NiMoO6 nanoparticles under pulsed 355 nm YAG:Nd laser at 100 K.
20
Fig. 9 UV-vis absorption (a), photo-degradation effects (b), three repetitive photo-degradation experiments (c) for the photocatalysis of Sr2NiMoO6 nanoparticles, and XRD profiles of the as-prepared sample after the photocatalysis (d).
21
Fig. 10 XPS spectra for elements of Sr-3d (a), Ni-2p (b), Mo-3d (c) and O 1s (d) measured in Sr2NiMoO6 nanoparticles.
22
Fig. 11 The suggested structure for multi-valent Mo and Ni-ions in Sr2NiMoO6 nanoparticles
3.0 2.8
(c)
2.6
Intensity (a. u.)
2.4 2.2 2.0 1.8
(b)
1.6 1.4 1.2 1.0 0.8
(a)
0.6 0.4 0.2 200
300
400
500
600
700
Wavelength (nm) Fig. 12 The spectra measured for the pure TA solution (a) and TA solutions with Sr2NiMoO6 nanoparticles under visible-light irradiation for 40 min (b) and 120 min (c).
23
Table 1 the refined crystallographic data of Sr2NiMoO6 nanoparticles formula
Sr2NiMoO6
radiation
Cu Ka
2θ range(degree)
10-120o
symmetry
tetragonal
space group#
I4/m (87)
a/Å
5.5335(3)
b/Å
5.5335(3)
c/Å
7.8763(5)
α/°
90
β/°
90
γ/°
90
Z
2
Rp
0.1046
Rwp
0.1102
X2
20.97
V
241.16(4)Å3
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Table 2 Refined atomic coordinate parameters data of Sr2NiMoO6 nanoparticles at room temperature. Atom
Wyck.
Site
x/a
y/b
z/c
U [Å2]
Sr1 Ni1 Mo1 O1 O2
4d 2b 2a 8h 4e
-4.. 4/m. 4/m. m.. 4..
0 0 0 0.27030 0
1/2 0 0 0.21580 0
1/4 1/2 0 0 0.24170
0.0537(2) -0.0179(2) 0.1101(4) 0.0003 0.0003
25