Ind. Eng. Chem. Res. 2010, 49, 1995–1996
1995
Comments on “Visible-Light-Induced Photocatalyst Based on Nickel Titanate Nanoparticles” Mamta Kharkwal, S. Uma, and R. Nagarajan* Materials Chemistry Group, Department of Chemistry, UniVersity of Delhi, Delhi 110 007, India Sir: An interesting work by Xin Shu et al.1 on the visiblelight-induced photodegradation of Methylene Blue (MB), based on TiO2-coupled NiTiO3 nanoparticles prepared via the hydroxide coprecipitation method, as well as via the solid-state reaction of NiO and TiO2, has shown that the degradation of MB by NiTiO3 is much lower, compared to that of TiO2-coupled NiTiO3 and P25 under visible light. Earlier, the same group2 reported that nanocomposites that contain NiTiO3 with NiO synthesized from Ni-Ti layered double hydroxides exhibited good photocatalytic activities under ultraviolet (UV) and visible-light irradiation. These studies attributed the increased activity due to the formation of NiTiO3 in the nanostructures or to the formation of TiO2-coupled NiTiO3 nanocomposites. In the present comment, the photocatalytic behavior of micrometersized phase-pure NiTiO3 obtained by the solid-state reaction was compared with the NiTiO3 coupled with TiO2 obtained by a solution method employing oxine as the chelating agent. This study has revealed that the active role played by the size of NiTiO3 particles is almost negligible toward the visible-light degradation of aqueous MB solutions and the presence of the TiO2-coupled NiTiO3 nanocomposite is not a necessary condition for the increased visible-light photocatalytic activity of NiTiO3. The average crystallite size of NiTiO3 obtained by heating the precipitated metal oxinates at 700 °C was in the range of 20-30 nm, as estimated by the Scherrer analysis of powder X-ray diffraction (PXRD) patterns. Transmission electron microscopy (TEM) images also confirmed this size estimation (Figure 1(i)). The crystallite size increased to several hundred nanometers upon further heating at 900 and 1100 °C. TEM images of the NiTiO3 obtained via the solid-state reaction between NiO and TiO2 at 1100 °C for 12 h revealed the presence * To whom correspondence should be addressed. E-mail:
[email protected].
of micrometer sized crystallites. (Inset of Figure 1(i).) The photodegradation of MB over solid-state NiTiO3, TiO2-coupled NiTiO3 synthesized from the oxine method (heated at 700 °C) and TiO2-coupled NiTiO3 from the oxine method (heated at 1100 °C) under visible-light irradiation are shown in Figure 1(ii). The description of the experimental setup used is available in ref 3. All the three preparations exhibited comparable rates of decomposition of the dye. For comparison, MB photolysis without any catalyst and adsorption experiment (performed in the absence of light) was also performed and plotted, along with the degradation results. This proves that the decolorization of the dye is actually due to the photocatalytic decomposition and not due to adsorption, because there is no decrease in dye concentration with time, in the absence of light. The extent of decomposition achieved was 65%-70% in all three cases in the same time interval, irrespective of the method of synthesis used. Thus, rutile TiO2-coupled NiTiO3 and the pure NiTiO3 prepared via the solid-state method are photocatalytically active for the decomposition of MB. These observations are contradictory to the earlier reports. Shu et al.2 compared the photocatalytic degradation behavior of NiTiO3 synthesized from a Ti-Ni layered double hydroxide precursor with other impurities such as Ni2TiO4, NiO, and TiO2 (anatase) and found that NiTiO3 that contained NiO showed maximum photocatalytic degradation of 85% MB for a duration of 24 h under visible light. Shu et al.1 had also previously reported higher photocatalytic activity for TiO2-coupled NiTiO3 samples synthesized by hydroxide precipitation identified with Ni:Ti ratios of (a) 1:1 and (b) 1:1.5. The corresponding PXRD patterns indicated the presence of ∼20% anatase TiO2, relative to NiTiO3 in the Ni/Ti ) 1:1 sample, and ∼40%-45% anatase TiO2, relative to NiTiO3 in the N/Ti ) 1:1.5 sample. The NiTiO3 obtained by the solidstate reaction, in the present study, showed degradation up to
Figure 1. (i) TEM image of TiO2-coupled NiTiO3 formed via the oxine method at 700 °C. Inset shows the TEM image of NiTiO3 prepared by the solid state method. (ii) Photocatalytic decomposition of Methylene Blue (MB) over NiTiO3 under visible-light irradiation (decrease in concentration of MB with time): MB photolysis (plot a, 9), adsorption experiment (performed in the absence of light) (plot b, [), solid-state-prepared NiTiO3 (plot c, b), 1100 °C-prepared TiO2-coupled NiTiO3 (plot d, 1), and 700 °C-prepared TiO2-coupled NiTiO3 (plot e, 2).
10.1021/ie9018879 2010 American Chemical Society Published on Web 01/26/2010
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Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
70%, which is almost the same range as that which has been reported for the samples made with a Ni/Ti ratio of 1:1.5.2 Literature Cited (1) Xin, S.; He, J.; Chen, D. Visible-Light-Induced Photocatalyst Based on Nickel Titanate Nanoparticles. Ind. Eng. Chem. Res. 2008, 47, 4750.
(2) Xin, S.; He, J.; Wang, D. C. Y. Tailoring of Phase Composition and Photo responsive Properties of Ti-Containing nanocomposites from Layered Precursor. J. Phys. Chem. C 2008, 112, 4151. (3) Singh, J.; Uma, S. Efficient Photocatalytic Degradation of Organic Compounds by Ilmenite AgSbO3 under Visible and UV Light Irradiation. J. Phys. Chem. C 2009, 113, 12483.
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