Materials Letters 119 (2014) 104–106
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Solvothermal synthesis of N-doped ZnO microcrystals from commercial ZnO powder with visible light-driven photocatalytic activity Changle Wu a,n, Yong Cai Zhang b, Qingli Huang a a b
Testing Center of Yangzhou University, Yangzhou 225009, China College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 October 2013 Accepted 29 December 2013 Available online 8 January 2014
N-doped ZnO microcrystals were fabricated by a one-step low temperature (140 1C) solvothermal route from commercial ZnO (A.R), HNO3, and ethanol, in which HNO3 was utilized as the nitrogen source. The structure, composition, BET specific surface area and optical properties of N-doped ZnO sample were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, wavelength dispersive X-ray fluorescence, field emission scanning electron microscopy, N2 adsorption–desorption isotherms, Raman and UV–vis diffuse reflectance spectroscopy. The photocatalytic results demonstrated that assynthesized N-doped ZnO microcrystals possessed much higher photocatalytic activity than N-doped TiO2 (which was solvothermally synthesized using P25 TiO2 and HNO3) and commercial pure ZnO in the reduction of aqueous Rhodamine 6G under visible light (λ 4420 nm) irradiation. Moreover, the photocatalytic results indicated that the as-synthesized N-doped ZnO was a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes under visible light irradiation. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Semiconductors Electron microscope Optical materials and properties
1. Introduction Owing to high photocatalytic activity, low cost and environmental friendly feature [1,2], ZnO has been widely used as a photocatalyst [3]. However, due to a wide band gap of 3.37 eV, poor photon absorption of ZnO limits its application in visible light photocatalyst [2]. In order to shift the optical absorption of ZnO into the visible region, one possible approach is to dope ZnO photocatalyst with nitrogen [4]. As reported, the solvothermal route is an effective and promising approach to synthesize ZnO or metal ions doped ZnO micro/ nanoparticles [2]. However, to the best of our knowledge, the reports on the solvothermal synthesis of N-doped ZnO microcrystals at relatively low temperature (o150 1C) are scarce up to now. Herein, we report the synthesis of N-doped ZnO microcrystals by the onestep solvothermal route at 140 1C, as well as the characterization of the resultant products by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), wavelength dispersive X-ray fluorescence (WDXRF), field emission scanning electron microscopy (FESEM), BET measurement, Raman and UV–vis diffuse reflectance spectra (DRS). Furthermore, the photocatalytic activities of pure ZnO, the
n
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[email protected] (C. Wu).
0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.111
as-synthesized N-doped P25 TiO2 and N-doped ZnO samples are also studied by degrading Rhodamine 6G (R6G) in water under the visible light (λ 4420 nm) irradiation.
2. Materials and methods All the chemical reagents used in this work, including commercial pure ZnO powder (denoted as pure ZnO), ethanol, and HNO3, were of analytical grade. Pure ZnO powder was bought from Tianjin Guangfu Fine Chemical Reagent Co., Ltd. Ethanol and HNO3 (65–68 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd. 0.1 mol of pure ZnO powder was dispersed in 80 mL of ethanol and the suspension was stirred at room temperature for 10 min. Then 2 mL HNO3 was added to the ZnO suspension and stirred magnetically at room temperature for 30 min. Subsequently, the mixture was transferred into a Teflon-lined stainless autoclave of 100 mL capacity, sealed and heated at 140 1C for 12 h. The asformed precipitates were filtrated, washed with distilled water and ethanol, and finally dried in air at 90 1C for 5 h. For comparison, N-doped P25 TiO2 was also solvothermally synthesized using the same procedures and conditions, except pure ZnO powder which was replaced by P25 TiO2.
C. Wu et al. / Materials Letters 119 (2014) 104–106
ZnO(100) ZnO(002) ZnO(101)
The obtained products were characterized by XRD (Bruker D8 ADVANCE diffractometer system), FESEM (Philips S-4800), BET surface area (Micromeritics Tristar 3000), XPS (Thermo ESCALAB 250Xi), WDXRF (Shimadzu XRF-1800), Raman spectra (Renishaw Invia), and UV–vis absorption spectra (Varian Cary 5000 spectrophotometer). The photocatalytic reactivity of pure ZnO, N-doped Degussa P25 TiO2 and the as-synthesized N-doped ZnO (0.01 g) were evaluated using 0.05 g (100 mL of 0.5 g/L) R6G as a probe molecule under the irradiation by an 1000 W Xe lamp (λ 4420 nm). The detailed photocatalytic experiments were carried out as follows: 0.01 g of the samples were dispersed in 100 mL of 0.5 g/L R6G solution in a 200 mL beaker. Prior to illumination, the suspensions were magnetically stirred in dark for 2 h to ensure the
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Fig. 1. XRD patterns of (a) pure ZnO and (b) the as-synthesized N-doped ZnO.
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establishment of absorption equilibrium of R6G on the sample surfaces. Subsequently, the suspension was irradiated under a 1000 W Xe lamp (equipped with a filter of λ 4420 nm), which was positioned about 10 cm away from the breaker. UV–vis adsorption spectra (Shimazu, UV2101) were recorded at different time intervals to monitor the process.
3. Results and discussion Fig. 1(a) and (b) illustrates the XRD patterns of (a) pure ZnO and (b) the as-synthesized N-doped ZnO, respectively. Both products have a wurtzite structure and their XRD peaks were in good agreement with the Powder Diffraction Standards data (JCPDS card no. 076-0704) for ZnO. No peaks corresponding to other N-containing or Zncontaining phases were detected in its XRD patterns, which might imply that nitrogen had entered into the lattices of ZnO [4]. The BET measurement revealed that pure ZnO and the as-synthesized N-doped ZnO had surface areas of 45 m2/g, and 39 m2/g, respectively. The XPS and Raman results of the as-synthesized N-doped ZnO microcrystals are demonstrated in Supporting information. The XPS and Raman results of the as-synthesized N-doped ZnO also indicated that the product only contained the elements of Zn, O and N and nitrogen was successfully incorporated into the crystal lattice of ZnO, which is also consistent with the XRD results. The morphology of pure ZnO and N-doped ZnO is characterized by FESEM and the results are displayed in Fig. 2(a1), (a2), (b1) and (b2). From Fig. 2, it can be seen that most of the pure ZnO sample displayed irregular rod-like or cubic-like particles and most of the as-synthesized N-doped ZnO exhibited irregular sphere-like or shutter-like particles with particle size 5–20 μm. It should be noted that the WDXRF result of the as-synthesized N-doped ZnO sample indicated that the product only contained the elements of Zn, O and N, without any other impurities.
Fig. 2. FESEM images of (a) pure ZnO and (b) the as-synthesized N-doped ZnO.
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C. Wu et al. / Materials Letters 119 (2014) 104–106
1.8 a. pure ZnO b. N-doped ZnO
1.6 1.4 Absorbance (a.u)
1.2 1.0 0.8 0.6 395 nm
0.4 0.2 390 nm
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-0.2 200 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) Fig. 3. UV–vis diffuse reflectance spectra of (a) pure ZnO and (b) the as-synthesized N-doped ZnO.
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nitrogen doping of ZnO. More detailed explanation of the absorption properties of the as-synthesized N-doped ZnO microcrystals are demonstrated in Supporting information. The photocatalytic activities of the as-prepared pure and N-doped ZnO microcrystals are shown in Fig. 4. C0 and C in Fig. 4 are the initial concentration after the equilibrium adsorption and the reaction concentration of R6G, respectively. As seen in Fig. 4, R6G aqueous solution can be obviously decolorized by the N-doped ZnO photocatalyst under visible light irradiation. By contrast, when N-doped P25 TiO2 or pure ZnO substitutes for N-doped ZnO as the photocatalyst, the decolorizing of R6G takes place at a much slower rate under the same conditions. For example, the decolorization ratio of R6G is nearly 81.6% over 0.01 g of N-doped ZnO microcrystals, which is higher than only 5.4% over 0.01 g of pure ZnO, when irradiated by the visible light for 80 min. It is worth noting that the decolorization ratio of R6G is 70% over 0.01 g of this N-doped ZnO photocatalyst under visible light irradiation only for 60 min. The superior photocatalytic performance of N-doped ZnO photocatalyst can be explained by its enhanced visible light absorption ability. For a better understanding of the photocatalytic activities of pure ZnO and the as-synthesized N-doped ZnO, the kinetic analysis of R6G degradation is also discussed in terms of the Langmuir Hinshel wood model, which is shown in Supporting information. As seen in Supporting information, the photocatalytic activity of the as-synthesized N-doped ZnO is higher than N-doped P25 TiO2 or pure ZnO for the photodegradation of R6G under the same conditions. For example, the kapp of N-doped ZnO microcrystal is 0.0176, which is nearly 8.6 times as that of N-doped P25.
4. Conclusions
0.3 0.2 N-doped ZnO
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Irradiation time (min) Fig. 4. Photodegradation of R6G using the (a) pure ZnO, (b) N-doped P25 TiO2, and (c) the as-synthesized N-doped ZnO.
From Fig. 3, the band edge absorption at the wavelength of about 390 nm can be found in the diffuse reflectance spectra of both pure ZnO (390 nm, band gap¼ 3.17 eV) and N-doped ZnO (395 nm, band gap ¼3.13 eV). Comparing with pure ZnO, a slight red shift of the band edge absorption ( 5 nm) and an additional broad tail from approximately 400 nm to 750 nm appeared in the spectra of the N-doped ZnO. In other words, for N-doped ZnO, besides a band edge absorption at around 395 nm, it also displays a distinct tailing absorption covering the whole visible region, which indicated that the as-synthesized N-doped ZnO had optical capability nearly in the whole range of visible light spectrum. In Fig. 2, it can be seen that the N-doped ZnO crystals are much larger than the pure ones. Moreover, the XPS and Raman results of N-doped ZnO confirmed that nitrogen was successfully incorporated into the crystal lattice of ZnO. Thus, it can be deduced that the red shift of the absorption edge ( 5 nm) and the additional broad tail band stretching into the visible range for the assynthesized N-doped ZnO crystals compared to pure ZnO could be related to two phenomena – the increase in the crystal size and
N-doped ZnO microcrystals were successfully obtained by the one-step low temperature solvothermal process (o 150 1C), and verified by XRD, XPS, WDXRF, FESEM, BET, Raman, and DRS measurement for the first time. The proposed method was simple, mild and cost effective, which may be suitable for industrial production of N-doped ZnO microcrystals. UV–vis diffuse reflectance spectra indicate that the as-synthesized N-doped ZnO had optical capability nearly in the whole range of visible light spectrum. The photocatalytic results show that doping of nitrogen into ZnO can greatly enhance the photocatalytic efficiency of ZnO under visible light irradiation.
Appendix. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.12.111. References [1] Guo MY, Fung MK, Fang F, Chen XY, Ng AMC, Djurišić AB, et al. ZnO and TiO2 1D nanostructures for photocatalytic applications. J Alloys Compd 2011;509:1328–32. [2] Wu C, Shen L, Zhang YC, Huang Q. Solvothermal synthesis of Cr-doped ZnO nanowires with visible light-driven photocatalytic activity. Mater Lett 2011;65: 1794–6. [3] Asl SK, Sadrnezhaad SK, Kianpour M. The seeding effect on the microstructure and photocatalytic properties of ZnO nano powders. Mater Lett 2010;64:1935–8. [4] Ashwini PB, Shivaram DS, Rupali PW, Latesh KN, Bharat BK. An eco-friendly, highly stable and efficient nanostructured p-type N-doped ZnO photocatalyst for environmentally benign solar hydrogen production. Green Chem 2012;14:2790–8.
