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J. Phys. Chem. C , 2008, 112 (11), pp 4151–4158 ... Publication Date (Web): February 28, 2008. Copyright ... Catalysis Surveys from Asia 2008 12, 25...
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J. Phys. Chem. C 2008, 112, 4151-4158

4151

Tailoring of Phase Composition and Photoresponsive Properties of Ti-Containing Nanocomposites from Layered Precursor Xin Shu, Jing He,* and Dong Chen State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, P. R. China

Yingxia Wang State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China ReceiVed: NoVember 21, 2007; In Final Form: January 9, 2008

Titanium-containing nanocomposites have been synthesized by calcination of a Ni-Ti layered double hydroxide as precursor. The nanocomposites were characterized by powder X-ray diffraction (PXRD), high-resolution transmission electron microscopy (HRTEM), the Brunauer-Emmett-Teller (BET) method, and UV-vis diffuse reflection spectroscopy (DRS). The transformation of the metastable Ni2TiO4 spinel phase to the stable NiTiO3 ilmenite phase was observed during the thermal process. The phase transformation at a calcination temperature below and at 800 °C belongs to the intraparticle reaction, while a phase separation occurs at 900 °C. UV-vis DRS indicates that the nanocomposites are not only responsive but also activated in both the UV- and visible-light regions. Their UV-vis-induced photocatalytic activities were evaluated by the degradation of methylene blue. The results show that the Ti-containing nanocomposites exhibit good photocatalytic activity under UV-vis-light irradiation.

1. Introduction There has been much interest in the Ti-containing inorganic solids in the fields of material and catalytic science. TiO2 nanocrystals, for example, have in recent years been extensively applied in the photocatalytic oxidation of organic compounds in aqueous media.1 Titanate solids are also well-known functional inorganic materials with potential applications as ceramic capacitors (BaTiO3, for example),2 microwave dielectrics (ZnTiO3, for example),3 gas sensors (CoTiO3, for example),4 and photocatalysts (SrTiO3, for example).5 Ti-containing microporous and mesoporous materials, including Ti-β,6 TiMCM-41/Ti-MCM-48,7 Ti-HMS,8 and Ti-WMS,9 show excellent performance in selective oxidation, hydrogenation, or photocatalytic epoxidation. Recently, several researchers have reportedthesynthesisofTi-containinglayereddoublehydroxides,10-12 which make up a new class of Ti-containing inorganic materials with potential applications. Layered double hydroxides (LDHs) are a family of anionic clays whose structure is based on brucite (Mg(OH)2)-like layers in which some of the divalent cations have been replaced by trivalent ions giving positively charged sheets. This charge is balanced by intercalation of anions in the hydrated interlayer regions.13-15 In addition to divalent and trivalent cations, a wide range of cations in monovalence or higher valence such as Li+,16 Sn4+,17 Zr4+,18 Ti4+,10-12, and so forth, may also be accommodated in the octahedral sites of the layers. When the interlamellar anions are volatile species such as carbonate or nitrate, thermal decomposition of LDHs at moderate temperature easily leads to highly active mixed oxides with remarkable * Corresponding author. Tel: 8610-64425280. Fax: 8610-64425385. E-mail: [email protected].

properties, such as high surface area, good metal dispersion, and small crystallite size.19-23 Calcination of LDHs above 600 °C is, generally, known to give spinel-type oxides, AB2O4, mixed with the oxide of a divalent metal.24 The mixed metal oxides or spinels from the collapse of the layered structure of LDHs caused by calcination were previously found to preserve the particle morphology of the LDH precursors, undergoing a topotactic transformation.25-27 In an idealized topotactic transformation, the metal ions in the LDH layers undergo a diffusionless transformation to the oxide or spinel phase, offering the possibility of preparing new functional nanosized materials, in which the cations are uniformly distributed benefiting from the local cation ordering in the LDH structure because the presence of MIII-O-MIII linkages is believed to be unfavorable due to the cation avoidance rule.28 Despite the large number of studies concerning interesting multicationic oxides or spinel-type oxides from thermal decomposition of LDHs that have been reported,29-33 no report has involved the thermal derivatives of Ti-containing LDHs, to our best knowledge. Herein, we report a Ti-containing semiconductor nanocomposite from calcination of Ni-Ti LDH as precursor. The composition and structure, the texture and morphology, and especially the photoresponsive properties and photocatalytic activities of resulting nanocomposites have been investigated. 2. Experimental Section 2.1. Synthesis. The layered Ni-Ti hydroxide was synthesized following the procedure outlined earlier.12 In a typical synthetic procedure, 0.5 mL of TiCl4 solution (the solution was prepared from TiCl4 and HCl with a volume ratio of 1:1, in which TiCl4 is 0.002 mol), 0.005 mol Ni(NO3)2, and 0.1 mol urea were dissolved in 100 mL of deionized water under vigorous stirring.

10.1021/jp711091m CCC: $40.75 © 2008 American Chemical Society Published on Web 02/28/2008

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Shu et al. and a scan range between 3° and 70°. Powder X-ray diffraction data of the calcined samples were recorded in θ-2θ scans (10° < 2θ < 120°, step size 0.02°, counting time 5 s per step) on a Rigaku D/Max-2000 diffractometer using Cu KR radiation with a graphite secondary monochromator. Tube voltage and current were 40 kV and 100 mA, respectively. The PXRD data were analyzed by the Rietveld refinement method using the TOPAS program.34 Transmission electron microscopy (TEM) images were recorded on JEOL JEM-2010 high-resolution transmission electron microscopes. The accelerating voltage was 200 kV. The specific surface area determination and pore volume and size analysis were performed by BET and BJH methods using a Quantachrome Autosorb-1C-VP Analyzer. Prior to the measurements, the samples were degassed at 200 °C for 2 h. Solid-state UV-vis diffuse reflectance spectra were recorded at room temperature in air by means of a Shimadzu UV-2501PC spectrometer equipped with an integrating sphere attachment using BaSO4 as background. 2.3. Photocatalytic Reactions. The photocatalytic degradation of methylene blue on LDH-derived nanocomposites, the SSR sample, Degussa P25, and pure NiO was performed under UV- and visible-light irradiation, respectively. The UV irradiation source was a 500 W high-pressure mercury lamp (Institute of Electric Light Source, Beijing), and the visible-light irradiation source was a 125 W fluorescent mercury lamp (λ > 400 nm). The reactor was equipped with cooling water that circulates between the inner and the outer quartz tube to avoid the solution evaporation from the system. Typically, a mixture of methylene blue solution (1 × 10-5 mol/L 100 mL) and 25 mg of catalyst was vigorously stirred for 30 min to establish an adsorption/ desorption equilibrium. Then the reaction solution was stirred under light irradiation. At given time intervals, 3 mL aliquots were sampled and centrifuged to remove the particles. The filtrates were analyzed by measuring the absorption band maximum (665 nm) using a Shimadzu UV-2501PC UV-vis spectrophotometer. The blank reaction was carried out following the same procedure without adding catalyst.

Figure 1. Powder XRD pattern of the Ni-Ti LDH precursor.

The resulting suspension was stirred for 10 h at a refluxing temperature. Then the solid was separated by filter, washed with deionized water and anhydrous ethanol, and dried at 60 °C. The molar ratio of Ni/Ti assessed by chemical analysis was 2.7. Its PXRD pattern (Figure 1) indicates that the sample is a pure LDH phase. The resulting Ni-Ti LDH precursor was calcined in air at 600, 700, 800, and 900 °C, respectively, for 4 h. The heating rate was 10 °C/min. The resulting solids from calcination at 600, 700, 800, and 900 °C are denoted NT6, NT7, NT8, and NT9, respectively. A 1.0 g amount of calcined solids was subsequently treated with 100 mL of sulfuric acid solution (5 mol/L) for 24 h under moderate stirring. The final products were separated by centrifugation, washed with extensive deionized water, and dried at 60 °C. The resulting samples are denoted NT6A, NT7A, NT8A, and NT9A, respectively. The reference NiTiO3 sample was synthesized by a traditional solid-state reaction: analytically pure powders of NiO and TiO2 were mixed intimately in an equal molar ratio and heated in a muffle furnace at 1100 °C for 12 h. The resulting solid is denoted the SSR sample (the mean particle size of rough crystals is 1.5 µm with 2 m2 g-1 of specific surface area). A pure NiO sample was synthesized as follows: 0.005 mol Ni(NO3)2 and 0.1 mol urea were dissolved in 100 mL of deionized water, and the resulting mixture was stirred for 10 h at a refluxing temperature. The resulting solid was separated by filter, washed with deionized water, dried at 60 °C, and then calcined in air at 800 °C for 4 h (the mean crystallite size is 20.8 nm). 2.2. Characterization. Powder X-ray diffraction data of NiTi LDH samples were collected on a Shimadzu XRD-6000 diffractometer using a Cu KR source, with a scan step of 0.02°

3. Results and Discussion 3.1. Composition and Structure. Figure 2a shows the powder XRD patterns of the Ni-Ti LDH precursor calcined at different temperatures. The patterns of Rietveld refinement fitting to the PXRD data are shown in Figure 2b. Calcination at 600 °C completely destroys the layered structure, and the characteristic X-ray diffraction peaks of bunsenite NiO (JCPDS no. 47-1049), anatase TiO2 (JCPDS no. 21-1272), and Ni2TiO4 spinel phases (confirmed by Rietveld refinement) as well as the (104) and (110) reflection of ilmenite NiTiO3 (JCPDS no.

TABLE 1: Composition and Textural Property of Sintering Derivatives from Ni-Ti LDH as Precursor

sample

content of NiO (%)a

content of NiTO3 (%)a

content of Ni2TiO4 (%)a

content of TiO2 (%)a

NiO crystalli te size (nm)b

NiTiO3 crystalli te size (nm)b

Ni2TiO4 crystalli te size (nm)b

NT6 NT7 NT8 NT9 NT6A NT7A NT8A NT9A

41.2 38.4 24.8 25.8 20.0 15.6 18.3 21.3

6.1 8.9 59.0 69.6 11.1 47.6 66.9 74.6

35.4 44.3 16.2 4.6 17.5 26.2 14.8 4.1

16.7 8.4

7.8 11.0 21.5 30.9 5.1 8.6 15.5 25.4

9.1 15.9 28.2 36.0 8.2 15.0 28.0 35.7

5.3 7.3 15.2 17.3 4.2 5.4 13.6 16.5

51.4 10.6

TiO2 crystalli te size (nm)b 10.6 14.1 9.1 12.7

surface area (m2 g-1)

pore size (nm)

pore volume (cm3 g-1)

109 101 75 21 103 99 73 22

12.7 12.8 17.8 12.4 18.4 17.8 19.2 12.8

0.37 0.45 0.41 0.17 0.44 0.52 0.45 0.16

a The phase content was calculated from PXRD data by the Rietveld refinement method using the TOPAS program. b Mean crystallite size was determined by the Scherrer equation.

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Figure 2. (a) Powder XRD patterns of Ni-Ti LDH calcined at different temperatures. (4) TiO2 anatase; (0) Ni2TiO4 spinel; (b) NiO bunsenite; (f) NiTiO3 ilmenite. (b) Rietveld refinement fitting to powder X-ray diffraction data for sintering derivatives (experimental data: blue line; calculated: red line). The difference plot is shown below. Vertical bars at the bottom show the Bragg peak positions.

TABLE 2: Conversion and First-Order Rate Constants for the Photocatalytic Degradation of MB sample degradation percentage under UV/ % degradation percentage under visible light/% rate constant k under UV/min-1 rate constant k under visible light/ h-1

NT9A

NiO

SSR

P25

blank

97 80 0.0269 0.0632

75 40 0.0115 0.0234

76 35 0.0119 0.0178

100 27 0.1005 0.0120

33 18 0.0031 0.0075

33-0960) are observed. Increasing the calcination temperature to 700 °C results in the appearance of (111) reflection for the Ni2TiO4 phase and (102) and (113) reflections for the NiTiO3 phase. The intensity of the characteristic reflection for NiTiO3 increases. Calcination at 800 °C gives rise to the appearance of (300) and (214) reflections for NiTiO3 as well as to the disappearance of (101) for TiO2 and (331) for Ni2TiO4. Increasing the calcination temperature to 900 °C causes the disappearance of (220), (111), and (440) reflections for Ni2TiO4 and the (220) reflection for NiO. The intensity enhancement and peak sharpening of (104), (110), (113), (116), (024), and (012) reflections for the NiTiO3 phase at both 800 and 900 °C are supposed to be indicative of the increase in not only the phase content but also the crystallite size. The phase content estimated from the Rietveld refinement of PXRD data in Table 1 indicates more clearly the phase transformation during the calcination. It can be seen from Table 1 that the content of NiO decreased with increasing calcination temperature but reached a constant above 800 °C; TiO2 could not be observed any more above 700 °C, and the content of ilmenite NiTiO3 grows progressively as the calcination temperature increases, while Ni2TiO4 spinel reached a maximum at 700 °C and then a significant decrease was observed. The spinel phase was found previously stably present above 600 °C using LDH as precursor.13-15 In agreement with previous observation, this phase is also observed at a sintering temperature as low as 600 °C in this work, which

is attributed to the close structural relationship between the LDH precursor and its calcined derivatives. But distinct from what was observed previously for most LDHs, in the case of the NiTi LDH, the only phase which is stable in the whole range of sintering temperature is the ilmenite-type compound NiTiO3, and the spinel-type compound Ni2TiO4 is identified to be a metastable phase transformed to stable NiTiO3 at higher temperature. The phase transition is almost completed at 900 °C. According to the change of phase content, the main phase transformation of Ni-Ti LDH during calcination can be determined as follows:

600 to 700 °C: 2NiO + TiO2 f Ni2TiO4

(1)

700 to 800 °C: NiO + TiO2 f NiTiO3

(2)

700 to 900 °C: Ni2TiO4 f NiTiO3 + NiO

(3)

As shown above, between 600 and 700 °C (eq 1), the production of the Ni2TiO4 spinel phase dominates, in accordance with the decrease of NiO and TiO2 content. At a calcination temperature higher than 700 °C, the NiTiO3 ilmenite phase was produced through both eqs 2 and 3, consistent with the obvious decrease of the spinel phase and rapid growth of the NiTiO3 ilmenite phase. But no eq 2 took place above 800 °C because of the exhaustion of TiO2.

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Figure 3. (a) TEM image of Ni-Ti LDH calcined at 800 °C; (b) HRTEM image of Ni-Ti LDH calcined at 700 °C, showing the (111) lattice spacing of Ni2TiO4; (c) HRTEM image of Ni-Ti LDH calcined at 800 °C, showing the (104) lattice spacing of NiTiO3 and the (200) lattice spacing of NiO; (d) HRTEM image of Ni-Ti LDH calcined at 900 °C, showing the (104) lattice spacing of NiTiO3. The inset is the selected area electron diffraction (SAED) pattern for sintering derivatives.

The crystalline sizes for NiO, TiO2, Ni2TiO4, and NiTiO3, estimated from XRD reflections by the Scherrer equation, increase along with calcination temperature but are all below 40 nm, as shown in Table 1, suggesting that nanocomposite particles have been produced using Ni-Ti LDH as precursors. The presence of divalent metal oxide NiO nanoparticles is supposed to efficiently inhibit the growth of the ilmenite phase NiTiO3 and the spinel phase Ni2TiO4. It also can be observed from Table 1 that increasing the calcination temperature from 700 to 800 °C doubles the crystalline sizes of all existing phases, while changing the calcination temperature from 600 to 700 °C or from 800 to 900 °C only gives rise to an increase lower than 40% (15-40%). The calcined derivatives of Ni-Ti LDH were also investigated by transmission electron microscopy (TEM), as shown in Figure 3. High-resolution TEM (HRTEM) and selected area electron diffraction (SAED) analysis further provide structural information in more detail. As can be seen from Figure 3a, the calcined derivative of Ni-Ti LDH retains the morphology of its precursor, consisting of uniform plate-like

nanoparticles. The particle diameter is estimated to vary in the range 20-40 nm, in good agreement with the XRD results (Table 1). The lattice with a d spacing of 0.48 nm, corresponding to the (111) plane of the Ni2TiO4 spinel phase, is observed in the HRTEM image for the derivative from calcination at 700 °C, as shown in Figure 3b. The inset of Figure 3b shows the SAED pattern of the nanocrystallite for the same sample. All the diffraction rings can be indexed to NiO-Ni2TiO4NiTiO3 phases, which is consistent with the XRD results. Figure 3c shows a typical HRTEM image of the derivative from calcination at 800 °C. Two lattice images are observed with d spacings of 0.27 and 0.21 nm, corresponding to the (104) plane of ilmenite NiTiO3 and the (200) plane of bunsenite NiO, respectively. The HRTEM image of the derivative from calcination at 900 °C, shown in Figure 3d, reveals that the materials are highly crystalline, as evidenced by welldefined lattice fringes in some regions. The measured lattice spacing of the particle is 0.27 nm, which corresponds to the (104) plane of ilmenite NiTiO3. The SAED diffraction rings can be indexed to NiO-NiTiO3 phases for both deriva-

Tailoring Ti-Containing Nanocomposite Properties

Figure 4. High-resolution transmission electron micrographs (HRTEM) image of sample NT8, showing the (111) lattice spacing of NiO and the (111) lattice spacing of Ni2TiO4.

Figure 5. Powder XRD patterns of sintering derivatives after treatment with sulfuric acid solution. (4) TiO2 anatase; (0) Ni2TiO4 spinel; (b) NiO bunsenite; (f) NiTiO3 ilmenite.

J. Phys. Chem. C, Vol. 112, No. 11, 2008 4155 tives from 800 and 900 °C, all consistent with the XRD results. 3.2. Textural Features. To investigate the textural properties of nanocomposites from Ni-Ti LDH precursors, the materials were analyzed by nitrogen sorption measurements. As shown in Table 1, all the nanocomposite particles from below 800 °C show a surface area higher than 70 m2 g-1, but calcination at 900 °C caused a marked decrease in the BET specific surface area, consistent with the increase in mean crystallite size of each phase. Calcining at 900 °C also results in the decrease in total pore volume. The pore size shows a visible increase when the calcination temperature is elevated to 800 °C, which could be proposed to result from the increase in crystalline sizes from 700 to 800 °C as discussed above. But an unexpected decrease in pore size is observed at 900 °C while the crystalline size of each phase increases continuously. This might be explained as follows: the phase transformation at a calcination temperature below and at 800 °C belongs to the intraparticle reaction, while a phase separation occurs at 900 °C. The direct evidence can be detected by the HRTEM image shown in Figure 4, from which can be observed the clear crystalline interface between the (111) plane of the NiO phase and the (111) plane of the Ni2TiO4 phase in one particle. The transformation of the Ni2TiO4 spinel from bunsenite NiO follows a topotactic process of the (111) reflection of NiO to the (111) reflection of Ni2TiO4. The nanocomposites from Ni-Ti LDH precursors were further acid-treated to investigate the textural features. Figure 5 shows the powder XRD patterns, and Table 1 lists the phase composition of the resulting samples. The acid treatment caused a decrease in the intensity of (111), (200), and (220) reflections for the NiO phase and (222), (400), and (440) reflections for the Ni2TiO4 phase, as well as the crystalline sizes of NiO and Ni2TiO4 (Table 1), revealing that NiO and Ni2TiO4 were dissolved partially from the nanocomposites by acid treatment. The composition shown in Table 1 indicates that the relative content of different phases in the nanocomposite could be tailored by partial removal of some component. Interestingly, partial removal of NiO and Ni2TiO4 caused hardly any change of the BET specific surface area for all nanocomposites, but an increase in the total pore volume is observed for the samples from calcination at 600-800 °C. This change in surface area and pore volume supports the above speculation about intraparticle phase transformation and phase separation. The surface of intraparticle pores resulting from the phase dissolution compensates for the surface loss caused by particle removal,

Figure 6. UV-vis diffuse reflectance spectra of (a) nanocomposites form calcined Ni-Ti LDH and (b) after acid treatment of (a).

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Figure 7. Absorption changes of MB solution during the photodegradation process over sample NT9: (a) under UV-light irradiation, and (b) under visible-light irradiation. The inset is the concentration percent of photocatalytic MB by sample NT9.

which is absent for the sample from calcination at 900 °C because of phase separation. 3.3. Photoabsorption Properties. It is well-known that light absorption by the material and the migration of the light-induced electrons and holes are the most key factors controlling a photocatalytic reaction, which is relevant to the electronic structure characteristics of the material.35 The photoabsorption properties of the nanocomposites from calcined Ni-Ti LDH detected by UV-vis DRS are illustrated in Figure 6. As shown in Figure 6a, the nanocomposite particles are photoresponsive in the UV-light region and/or the visible-light region. In the UV-light region, the absorption spectra are similar because the absorption below 400 nm is a result of the cooperative effect of the mixed wide band gap semiconductor nanoparticles of NiO (3.6-4.0 eV),36 NiTiO3 (3.2 eV),37 Ni2TiO4 (approximately estimated to be 3.5 eV from the onset of the absorption edge around 350 nm of samples NT6 and NT7 in our case), and TiO2 (3.2 eV).1 On the basis of the composition given in Table 1, the UV-photo absorption is mainly contributed by the large amount of NiO and Ni2TiO4 nanoparticles as well as TiO2 and NiTiO3 for samples NT6 and NT7, while it is contributed by NiTiO3 and NiO nanoparticles for samples NT8 and NT9. The absorption spectra in the visible-light region are diverse depending on the sintering temperature. No absorption has been detected for samples NT6 and NT7, but an obvious change in the absorption spectrum has been observed around 450 and 510 nm for samples NT8 and NT9. It has been found in Table 1 that calcination at or above 800 °C caused a sharp increase in the content of NiTiO3 because of the transformation of the Ni2TiO4 spinel phase and the reaction between TiO2 and NiO. So it can be believed that the two absorption bands around 450 and 510 nm are contributed by the photoabsorption of NiTiO3 nanoparticles, corresponding to the transition from the impurity energy levels and additional bound exciton energy levels inside the energy gap of titanate semiconductors to the conduction band.37 Tailoring the phase composition by acid treatment causes a slight change of the absorbance in the UV region but no variation of the absorption around 450 and 510 nm is shown in Figure 6b; this is indicative of the mixed contribution for UV response and NiTiO3 is the determinant phase for the photoabsorption of the nanocomposites in both UV- and visible-light regions. 3.4. Photocatalytic Activities. The photocatalytic process is based on the generated electron/hole pairs by means of band gap radiation, which can give rise to redox reactions with species

Figure 8. Photodegradation of MB monitored as the normalized concentration change versus irradiation time using temperature series samples and reference samples: (a) under UV-light irradiation, (b) under visible-light irradiation.

adsorbed on the surface of the catalysts. Therefore, it is likely that the nanocomposite particles from calcined Ni-Ti LDH show a photocatalytic property under both UV- and visiblelight irradiation due to their UV-vis absorption properties and high surface areas. So the photodegradation of methylene blue (MB) was performed on the nanocomposites prepared in this work under UV- and visible-light irradiation. Temporal evolu-

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Figure 9. Influence of the phase content of NiTiO3 on the degradation percentage of MB and kinetic rate constant: (a) under UV-light irradiation; (b) under visible-light irradiation.

tion of the spectral changes taking place during the photodegradation of MB mediated by sample NT9 are displayed in Figure 7. Typically, MB solutions display maximal absorbance at 665 nm.38 As shown in Figure 7a, the major absorption band at 665 nm shifts considerably toward the blue region during the course of the photoassisted degradation. The similar hypsochromic shifts were also observed by Hidaka et al.39 The color of MB solutions becomes less intense (hypsochromic effect) when all or part of the auxochromic groups (methyl or methylamine) are degraded. The absorption at 292 and 245 nm decreases significantly along with irradiation time and no new bands appear, implying that full oxidative decomposition of the phenothiazine species had occurred and that other intermediates containing the phenothiazine moiety are no longer formed. Under visible-light irradiation, as shown in Figure 7b, no blueshift happened for the absorption at 665 nm, but a gradual decrease in the absorption intensity is observed, indicating that the big conjugated π-system has been destroyed. The significant temporal concentration changes of MB, illustrated in the inset of Figure 7, parts a and b, also clearly indicate the degradation of MB in the presence of sample NT9 under UV- and visiblelight irradiation. The above result indicates that the Ti-containing nanocomposites using LDH as precursors indeed exhibit good photocatalytic activity, as expected, under both UV- and visiblelight irradiation. The dependence of MB photodegradation on the nanocomposite composition under UV- and visible-light irradiation has been investigated. As shown in Figure 8a, the nanocomposites with different compositions exhibit photocatalytic activity for the degradation of MB under UV-light irradiation, which is consistent with their UV-photo absorption properties. The photocatalytic activity is gradually enhanced with increasing calcination temperature, consistent with the marked increase in the content of the NiTiO3 phase. It should be noted that sample NT6A also shows good activity because it contains 51.4% TiO2 (Table 1) which is deemed to be a good photocatalyst under UV light. The photocatalytic activities of nanocomposites under visible-light irradiation increase along with elevated calcination temperature as illustrated in Figure 8b. Especially for the samples calcined at or above 800 °C that have the higher content of the NiTiO3 phase, the photocatalytic activity exhibits a visible enhancement, consistent with the photoabsorption properties in the visible-light region as discussed in Figure 6. From the results, it is believed that the NiTiO3 phase in the nanocomposites acts as a major one of the photocatalytic active phases in the degradation of MB under both UV- and visible-light irradiation,

which can be figured out more clearly from Figure 9, illustrating the dependence of MB photodegradation on the content of NiTiO3. It can be observed that the degradation percentage of MB and kinetic rate constant k (first-order) increase obviously as the content of NiTiO3 reaches 50% or above, but approach a constant since the content of NiTiO3 reaches 70% under visible-light irradiation. Pure NiO, Degussa P25, and the SSR sample were used here as reference samples, as shown in Figure 8 and Table 2. Compared with the reference samples, the photocatalytic activity of NiTiO3 nanoparticles (sample NT9A), much higher than that of pure NiO and the SSR sample, approaches that of commercial P25 under UV-light irradiation. Under visible-light irradiation, the photocatalytic activity of NiTiO3 nanoparticles (sample NT9A) is markedly higher than that of pure NiO, SSR sample, and P25. The much better photocatalytic activity of NiTiO3 nanoparticles than the SSR sample can be attributed to its nanosized crystal and high surface area. 4. Conclusions In summary, UV- and visible-light-responsive Ti-containing nanocomposites have been synthesized by calcination of NiTi LDH as precursors. The spinel phase of Ni2TiO4 in the nanocomposites is identified to be a metastable phase and is finally transformed to the stable ilmenite phase of NiTiO3. TiO2 could not be observed any more above 700 °C because of its reaction with NiO to form NiTiO3. The investigation of textural features of the nanocomposites revealed that the phase transformation at a calcination temperature below and at 800 °C belongs to the intraparticle reaction, while a phase separation occurs at 900 °C. The photoabsorption properties of the nanocomposites can be attributed to the cooperative effect of the mixed wide band gap semiconductor nanoparticles in the UV-light region, while contributed by the photoabsorption of NiTiO3 nanoparticles in the visible-light region. The nanocomposites perform good photocatalytic activities for the degradation of MB under both UV- and visible-light irradiation. Due to the UV/visible-light-activated nanocomposites from Ni-Ti LDH precursors, we presumed that it can be used as a kind of potential photocatalyst in utilizing sunlight effectively. In addition, the stability of the crystal structure of titanate material should contribute to its application as well. Acknowledgment. The authors are grateful for the financial support from NSFC, Program for Changjiang Scholars and

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