Effect of transition metal ion substitution on structure and optical properties of ZnWO4 material

Abstract. In this presentation, we investigate the effects of transition metal (Fe, Co, Ni) substitution on structure and optical properties of ZnWO4 nanopowder prepared using the hydrothermal method. The prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering and optical absorption spectra. It is shown that the grain size and morphology of ZnWO4 nanopowder can be controlled by adjusting the substituted content in the solution. The results show that the optical property of ZnWO4 nanopowders obviously relies on their substituted contents and their size.

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JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2015-0031 Mathematical and Physical Sci., 2015, Vol. 60, No. 7, pp. 47-54 This paper is available online at EFFECT OF TRANSITION METAL ION SUBSTITUTION ON STRUCTURE AND OPTICAL PROPERTIES OF ZnWO4 MATERIAL Nguyen Van Minh1, Nguyen Manh Hung1,2 and Du Thi Xuan Thao2 1Faculty of Physics, Hanoi National University of Education 2Hanoi University of Mining and Geology Abstract. In this presentation, we investigate the effects of transition metal (Fe, Co, Ni) substitution on structure and optical properties of ZnWO4 nanopowder prepared using the hydrothermal method. The prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering and optical absorption spectra. It is shown that the grain size and morphology of ZnWO4 nanopowder can be controlled by adjusting the substituted content in the solution. The results show that the optical property of ZnWO4 nanopowders obviously relies on their substituted contents and their size. Keywords: Zinc tungstate, transition metal substitution, nanopowder, absorption, Raman spectra. 1. Introduction Zinc tungstate ZnWO4 is a kind of tungstate that has received considerable attention due to its applications as an X-ray and γ-scintillator, photoanodes and as a solid-state laser host, as well as for use in acoustic and optical fibers [1, 2]. Recently, it has been shown that ZnWO4 exhibits high photocatalytic activity in the mineralization of organic pollutants under UV irradiation [3]. Therefore, increasing the photocatalytic activity of ZnWO4 for use in practical applications would be significant and meaningful [4-6]. Many studies have looked at extending photo response and improving photoactivity by doping [7]. Doping with metal cations for photo-catalytic application has been done in an attempt to shift the threshold for photonic excitation of ZnWO4 towards the visible range. All metal dopants are conveniently substituted into the ZnWO4 lattice if their ion radii are identical or nearly identical to that of the Zn ion. In the case of transition metal ion doping into ZnWO4 crystal, some Zn in ZnWO4 will be replaced by transition metal ions if there is enough oxygen to compensate for the defects of oxygen vacancies in ZnWO4 crystal. As a result, an enhanced photocatalytic efficient of ZnWO4 can be observed. Received December 1, 2015. Accepted December 15, 2015. Contact Nguyen Van Minh, e-mail address: minhsp@gmail.com 47 Nguyen Van Minh, Nguyen Manh Hung and Du Thi Xuan Thao In this work, ZnWO4 doped with transition metal ions (Fe, Co, Ni) with different transition metal concentrations was synthesized using a hydrothermal process. Our principal aim is to understand the effects of transition metal introduction on the structure and optical properties of ZnWO4. 2. Content 2.1. Experiments Transition metal (M = Fe, Co, Ni)-doped zinc tungstate (ZnWO4) nanoparticles were prepared using a hydrothermal reaction of Zn(NO3)2.6H2O, Na2WO4.2H2O and M(NO3)2.6H2O at 180 ◦C for 6 hrs. A typical procedure for preparing these samples consist of adding 10 mL of an aqueous solution of Zn(NO3)2.6H2O (1 mmol) to 20 mL of an aqueous solution containing Na2WO4.2H2O (1mmol) andM(NO3)2.6H2O under vigorous stirring. The volume of this solution was increased to 40 mL by adding distilled water, and the pH of the solution was adjusted to 7 by a dropwise addition of a 30% aqueous ammonia solution. After that, the solution was transferred into a Teflon-lined stainless steel autoclave of 100 mL capacity. The autoclave was heated to 180 ◦C for 6 hrs. without shaking or stirring, and then allowed to slowly cool to room temperature. From this autoclave a white precipitate was collected and washed four times with distilled water, after which a solid powder was retrieved. The solid powder was heated at 80 ◦C and dried at this temperature under vacuum for 2.5 hrs. before calcination at 700 ◦C for 2 hrs. A structural characterization was performed by means of X-ray diffraction using a D5005 diffractometer with Cu Kα radiation. The FE-SEM observation were carried out using a S4800 (Hitachi) microscope. Raman measurements were taken in a back-scattered geometry using a Jobin Yvon T 64000 triple spectrometer equipped with a cryogenic charge-coupled device (CCD) array detector, using the 514.5 nm line of an Ar-ion laser. The absorption spectra were recorded by using Jasco 670 UV-vis spectrometer. 2.2. Results and discussion We investigated the diffraction patterns of the pure ZnWO4 and ZnWO4:M with different transition metal concentrations after the calcination. For clarity, we show only XRD patterns of pure ZnWO4 and ZnWO4:M nanopowders with M = 10 mol% (Figure 1). The pure zinc tungstate diffractogram is in good accordance with the standard JCPDS card (No. 73 0554, No. 89-0447) [8, 9]. The different transition metal doped ZnWO4 exhibits almost the same XRD patterns as the undoped ZnWO4. An expand view of the (011) and (110) peaks of these different materials on the right shows that the diffraction peaks shifted slightly to lower angles. This observation suggests that the transition metal ion is successfully substituting Zn2+ in the lattice and that the transition metal is homogeneously incorporated into the lattice. The minor lattice expansion resulting from the transition metal incorporation is reasonable considering the relative ionic radius of Zn2+ (0.74 A˚) and M as shown in Figure 2 and Table 1. In pure ZnWO4 crystals, zinc ions occupy an octahedral position (Z = 8) of the wolframite structure [10] and, when doped in ZnWO4, the transition metal ion should replace Zn2+ to be in Oh position [11]. 48 Effect of transition metal ion substitution on structure and optical properties of ZnWO4 material Figure 1. XRD patterns of ZnWO4 and ZnWO4:M (M = Fe, Co, Ni 10 mol%) Figure 2. a) FWHM, b) Position of the diffraction peaks corresponding to the (011), (110), (200) and (121) lattice planes Table 1. Ion radii of some elements with VI coordination in octahedron structure [11] State Ion radii (A˚) Zn2+ W6+ Fe2+ Fe3+ Fe4+ Co2+ Co3+ Co4+ Ni2+ Ni3+ Ni4+ High Spin (HS) – – 0.78 0.645 – 0.745 0.61 0.53 – 0.6 – Low Spin (LS) – – 0.61 0.55 – 0.65 0.545 – – 0.56 0.48 – 0.74 0.6 – – 0.585 – – – 0.69 – – ZnWO4 materials have a monoclinic wolframite structure with C2h point group symmetry and P2/c space group. It has two formula units per unit cell. The W-O interatomic distance is substantially smaller than that of Zn-O, therefore, to a first order approximation, the lattices can be separated into internal vibrations of the octahedra and the external vibrations in which an octahedron vibrates as a unit. Group theory analysis for wolframite-type ZnWO4 predicts 36 lattice modes, of which 18 even vibrations (8 Ag + 10 Bg) are Raman active [12]. We have recorded the Raman spectra of ZnWO4:M with various doping contents at room temperature. However, for clarity, we show only spectra of ZnWO4:Co nanopowders with various Co doping (0 - 10 mol%) in Figure 3. 49 Nguyen Van Minh, Nguyen Manh Hung and Du Thi Xuan Thao Figure 3. Raman spectra of ZnWO4 and ZnWO4:Co crystals. The inset shows a zoom on the peak at 121 cm−1 All the expected lines of 907, 787, 709, 679, 547, 408, 343, 276, 193 and 162 cm−1 are observed in the Raman spectra and shown in Figure 3. In particular, the presence of six internal vibration modes (stretching modes) of Ag and Bg (907, 787, 709, 679, 547 and 408 cm−1) should be noted as an important property of monoclinic wolframite ZnWO4; the vibration modes arise from the six internal stretching modes caused by each of the six W-O bonds in the WO6 octahedrons [13]. Note that the highest frequency line at 907 cm−1 corresponding to the Ag mode, with a linewidth of about 9 cm−1 for all samples. Figure 4. FWHM vs. Co content (2 - 10 mol %) Figure 4 shows the full width at half maximum (FWHM) of peaks at wave numbers 121, 193, 408 and 907 cm−1. The FWHM of the samples rise (meaning the peaks expand) following the increase in Co-doped concentration. It indicates that cobalt ions have replaced zinc ions in the crystals. In addition, the peak positions relating to the vibrations of Zn–O, Zn–W and Zn–O–W bondings in the Raman spectra of the HCo and HNi samples shifted to low frequencies with 50 Effect of transition metal ion substitution on structure and optical properties of ZnWO4 material increasing doped Co or Ni content, respectively. This is because of the impurity in the unit cell that given rise. To evaluate the effect of doped ions on the vibrational spectra of ZnWO4 material, the Raman spectra of the doped Fe, Co, Ni samples with a 2 % mol content are shown in Figure 5. The peak position and the FWHM changed in spectra of samples with different doped ions (see the inset of Figure 5). It was observed that there is a light shift of the peak position in the spectra of the HFe2, HCo2 and HNi2 samples compared to that of the pure ZnWO4 sample. As the doping, the full width at half maximum increases, especially in the spectra of the doped Fe and Ni samples. This peak was origined to Zn2+ ions [14]. Figure 5. Raman spectra of ZnWO4 and ZnWO4:M (2 mol %) crystals. The inset shows FWHM and zoom with peaks at 343, 408 and 907 cm−1 The shift is also explained in the framework of the size effect. When particle size decreases to the nanometer scale, the effect on the vibrational properties of these materials might occur. A volume contraction occurs within the nanoparticles due to size-induced radial pressure, which leads to increases in the force constants as a result of the decreases in the interatomic distances. In vibrational transitions, the wavenumber varies in approximate proportion to k1/2, where k is the force constant. Consequently, the Raman bands shift towards a higher wavenumber due to the increasing force constants [15]. As the analysing in the XRD pattern result, the difference in the replaced ion radii that caused the change in the peak position of XRD. This change is also systematically exhibited in the Raman spectra of those samples. It is known that the radius of Fe and Ni is smaller than that of Zn, but the radius of Co2+ and Zn2+ approximate each other. The change in the full width at half maximum of the peak in the Raman spectra of HFe2, HCo2 and HNi2 samples can be come from the Zn2+ ion and the replaced transition metal ions. Figure 6 shows the FE-SEM images of pure ZnWO4 and M-doped ZnWO4 materials prepared using the hydrothermal route at 180 ◦C from the solution of pH 7 with doping concentration of 2 mol %. FE-SEMmicrographs for pure ZnWO4 crystals show an average length of 80 - 110 nm and diameter of 20 nm. When M ions were doped with a concentration of 2 mol %, the particles became irregular and smaller with an average length of 40 - 60 nm. It is clear that the morphologies and dimensions of the samples depend on the ion M doping. 51 Nguyen Van Minh, Nguyen Manh Hung and Du Thi Xuan Thao Figure 6. FE-SEM images of ZnWO4 and ZnWO4:M (2 mol%) crystals Figure 7 shows the diffuse reflectance spectra of ZnWO4, ZnWO4:M samples with M-concentrations of 2 mol%. It was known that ZnWO4 material is an indirect-gap semiconductor [16, 17]. For a crystalline semiconductor, the optical absorption near the band edge follows the equation: ahν = A(hν - Eg)1/n, where a, ν, Eg and A are absorption coefficient, light frequency, band gap and a constant, respectively [18]. For ZnWO4, n was determined to be 2. Using this equation to calculate from one absorption band at wavelength about 350 nm, the band gap of the crystal ZnWO4 was roughly estimated to be 3.85 eV. With the ZnWO4:M samples (2 mol%), beside the band at 350 nm, there were other bands at 350 - 800 nm. These absorption bands are assigned electron transitions among energy levels of transition metal ions in an octahedral [19-21]. Compared to the band gap of pure ZnWO4, the band gap of these ZnWO4:M materials is less. Figure 7. Absorption spectra of ZnWO4 and ZnWO4:M (2 mol %) crystals In the absorption spectrum of the ZnWO4:Ni sample, using a Gauss function, the bands centered at a wavelength of 269, 453 and 513 nm have shown best fit (see inset in Figure 7). The band gap for this sample was determined to be 3.59 eV. The band gap becomes narrower in the 52 Effect of transition metal ion substitution on structure and optical properties of ZnWO4 material Ni-doped sample indicating that the Ni substitution may contribute to the absorption edge shift. This fact can be interpreted as doped nickel forming a nickel alloy in ZnWO4 crystals [22]. On the other hand, a very intense absorption band located near the absorption edge could result in the shifts, thus one can expect that the absorption from Ni3+ could be in the same region [23]. 3. Conclusion Transition metal (Fe, Co, Ni) doped ZnWO4 nano-particles were successfully synthesized at 180 ◦C using a hydrothermal route. XRD profiles present the identical monoclinic phase for all of the investigated samples. From the X-ray diffraction and Raman spectra results, it can be seen that the different ion radius and atom weights of Fe, Co and Ni transition metals have effected the lattice parameters and structural features. When there was an increase in dopant concentration, the grain size and the band gap of the material decreased. The grain size of the ZnWO4 particles was estimated, referring to the FE-SEM image, to be in the range of 80 - 110 nm. When M ions were doped, the particles became imhomologous and smaller. The average length is about 40 - 60 nm when the dopant concentration was 2 mol %. The band gap of the ZnWO4 sample was 3.85 eV. An Fe, Co or Ni substitution on ZnWO4 material leads to impurity levels and lattice defects that change the absorption edge of the material. A new absorption band related to the energy level of the dopant ions appeared. Acknowledgements. 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