Enhanced Photocatalytic Hydrogen Production from Water−Methanol

Beamline Research Division, Pohang Accelerator Laboratory, POSTECH, San 31, ... Division of Materials Science & Engineering, Hanyang University, Seoul...
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Enhanced Photocatalytic Hydrogen Production from Water-Methanol Solution by Nickel Intercalated into Titanate Nanotube Jum Suk Jang,† Sun Hee Choi,*,‡ Dong Hyun Kim,§ Ji Wook Jang,† Kyung Sub Lee,§ and Jae Sung Lee† Eco-friendly Catalysis and Energy Laboratory (NRL), Department of Chemical Engineering and School of EnVironmental Science and Engineering, Pohang UniVersity of Science and Technology (POSTECH), San 31, Hyojadong, Namgu, Pohang 790-784, Korea, Beamline Research DiVision, Pohang Accelerator Laboratory, POSTECH, San 31, Hyojadong, Namgu, Pohang 790-784, Korea, DiVision of Materials Science & Engineering, Hanyang UniVersity, Seoul 133-791, Korea ReceiVed: January 22, 2009; ReVised Manuscript ReceiVed: March 21, 2009

Nickel-intercalated titanate nanotube was hydrothermally synthesized and evaluated for photocatalytic hydrogen production from methanol-water solution under UV light irradiation. The nickel intercalated into the nanotube was present as a hydrated Ni complex of [NixII(OH)2x-1(OH2)]+ and was responsible for a dramatic enhancement of hydrogen evolution rate relative to that of titanate nanotube itself. The nickel species in the interlayer provided active sites for proton reduction and caused fast diffusion of photoelectrons generated from titanate layers toward the nickel sites, leading to a high photocatalytic activity. Upon annealing at 400 °C, the hydrated nickel complex was partly converted to NiO and the hydrogen evolution rate was reduced, indicating that the nickel hydroxide was a more efficient cocatalyst for titanate nanotube. A high and stable photocurrent generation was also observed from a film made of the nickel-intercalated titanate nanotube immersed in a NaOH solution. 1. Introduction Crystalline inorganic nanotubes with uniform diameters and nanoporous structures are considered as a substitute for mesoporous molecular sieves that usually suffer from hydroinstability. Titania nanotubes are of particular interest because their precursor TiO2 has a wide range of applications from catalysis to dye-sensitized solar cells or water purification. Since Kasuga et al.1,2 first developed the TiO2-derived nanotubes by a simple hydrothermal treatment of TiO2 powder in NaOH aqueous solution, many studies have followed to optimize the preparation conditions, such as reaction temperature,4-6 caustic concentration,1,4 the type of titanium oxide as a precursor,4,8-13 and its crystal size.14 Several different crystal structures have been proposed to describe the nanotube, that is titania-type nanotube like TiO2-anatase or titanate-type like H2Ti3O7. The nanotubes are generally depicted as the scrolling of an exfoliated titania or titanate-derived nanosheet into a hollow multiwall nanotube with a spiral cross section. Such a nanotube structure makes a high surface area, regular pore size distribution, and improved crystallinity compared to usual nanostructured TiO2 and can be effectively applied in photocatalysis.15-20,58 The multilayered nanotubes can be modified by intercalation of alkaline metal ions or transition-metal cations between the layers in the nanotubes via ion exchange.21,22 Transition metals of Fe and Ni can also be incorporated into titanate nanotubes through a hydrothermal procedure.23-25 The intercalated Fe and Ni decrease the band gap of H2Ti3O6 nanotube and, consequently, have high potential for optoelectronics and photocatalysis. On the other hand, nickel has been used as a cocatalyst * To whom correspondence should be addressed. E-mail: jlee@ postech.ac.kr. Tel: 82-562-279-2266. Fax: 82-562-279-5528. † Department of Chemical Engineering and School of Environmental Science and Engineering, POSTECH. ‡ Pohang Accelerator Laboratory, POSTECH. § Division of Materials Science & Engineering, Hanyang University.

for water splitting under UV light irradiation.26-30 The NiOloaded photocatalysts show higher activities than that of unloaded or, in some cases, noble metal-loaded photocatalysts because NiO provides reduction sites to promote hydrogen formation in a water-splitting reaction. Nevertheless, no study has been reported yet for Ni-intercalated titanate nanotube for photocatalytic hydrogen production. In this study, we synthesized titanate nanotube and Niintercalated titanate nanotube via one-step hydrothermal method and investigated the local structure of Ni-intercalated in the titanate nanotubes. The catalytic performances of the materials were measured in two systems, that is photocatalytic hydrogen production from water-methanol solution and photoelectrochemical measurement under UV irradiation. The active phase of nickel responsible for the enhanced photocatalytic activity compared to bare titanate nanotube was identified by detailed analysis of the local structure of nickel in the nanotube. 2. Experimental Procedures 2.1. Synthesis of Ni-Intercalated Titanate Nanotube. Amorphous powders containing ammonia and titanium were first prepared. Ammonium hydroxide solution with an ammonia content of 28-30% (99.99%, Aldrich) was added drop-by-drop to 20% titanium (III) chloride solution (TiCl3, Kanto) for 30 min under N2 flow in an ice bath while continuously stirring. After the suspension was stirred for 5 h to complete the reaction, the precipitates were filtered and washed several times with deionized water, and then dried at 70 °C for 4 h in a convection oven. For the preparation of bare and Ni-intercalated titanate nanotubes, 0.70 g of the dried precipitates only or those with 0.170 g of Ni(NO3)2 · 6H2O were stirred in 70 mL of 10 M NaOH aqueous solution for 1 h and were introduced into a Teflon-lined stainless steel autoclave. The autoclave was maintained at 150 °C for 72 h and allowed to cool down to

10.1021/jp900653r CCC: $40.75  2009 American Chemical Society Published on Web 04/22/2009

Nickel Intercalated into Titanate Nanotube room temperature. The resulting white precipitates were filtered, washed with deionized water, and then dried at 100 °C for 10 h. The dried products were treated at 400 °C for 1 h in an electrical furnace under ambient air. We refer hereafter to the as-prepared samples as TNT and Ni-TNT and the annealed samples as TNT400 and Ni-TNT400 for bare and Ni-intercalated titanate nanotubes, respectively. 2.2. Physico-Chemical Characterization. The crystalline phases of the products were determined by powder X-ray diffraction (XRD) on a diffractometer (Mac Science Co., M18XHF) with monochromatic Cu KR radiation at 40 kV and 200 mA. The morphologies of TNT and Ni-TNT were examined by TEM (JEOL JEM 2010F) operated at 200 kV and their optical properties were analyzed by a UV-vis diffuse reflectance spectrometer (Shimadzu, UV 2401). A differential scanning calorimeter (DSC) (Shimadzu, DTA-50) was applied to monitor changes during the thermal treatment of the dried nanotubes (TNT and Ni-TNT). The DSC curves were recorded in the temperature range of 50-800 °C at a heating rate of 10 °C/min under a 45 mL/min air flow. A Jasco Valor-III spectrometer was used to take FTIR spectra of disk-type samples prepared by mixing with KBr. To investigate the physical texture of the nanotube materials, the N2 adsorption-desorption isotherm at 77 K was taken in a constant-volume adsorption apparatus (Micrometrics ASAP 2010) at relative pressures (P/P0) ranging from 10-4 to 0.995. Before measuring the isotherm, the sample was degassed for 4 h at 120 °C under 10-4 Torr. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method31 and the pore size distribution (PSD) was calculated from nitrogen desorption data using the Barrett-Joyner-Halenda (BJH) method with the modified Kelvin equation.32 The pore volume was assessed on the basis of the adsorbed amount at a relative pressure (P/P0) of 0.99. The local structure of Ni in both Ni-TNT and Ni-TNT400 was investigated with XPS (X-ray photoelectron spectroscopy) and XAFS (X-ray absorption fine structure). Mg KR radiation (1253.6 eV) was used in the XPS measurement (VG Scientific, ESCALAB 220iXL) and the binding energy was calibrated by using C1s peak as the reference energy. XAFS measurements were conducted at 5A wiggler beamline of Pohang Accelerator Laboratory (2.5 GeV, stored current of 140-180 mA). The radiation was monochromatized using a Si(111) double-crystal monochromator and the incident beam was detuned by 40% to minimize higher-order reflections of the silicon crystals. Ni K-edge spectra were taken in transmission mode where incident and transmitted beam intensities were monitored with separate IC SPEC ionization chambers. Before the measurement of the sample, a scanning energy was calibrated with respect to 8333 eV of Ni K-edge energy. The obtained data were analyzed using the IFEFFIT suite of software programs.33,34 The detailed procedure for data analysis is described elsewhere.35-38 2.3. Photocatalytic and Photoelectrochemical Reactions. Photocatalytic hydrogen production was performed at room temperature under atmospheric pressure in a closed circulation system using an Hg-arc lamp (450 W) equipped with IR liquid filter (no cutoff filter). The rate of H2 evolution was determined for water-methanol solution (distilled water 70 mL and methanol 30 mL) containing 100 mg catalyst. Hydrogen and CO2 were the main products of the photocatalytic reaction with a trace amount of CO. The concentration of H2 was analyzed by gas chromatography equipped with a thermal conductivity detector (molecular sieve 5 Å column and Ar carrier).

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Figure 1. X-ray diffraction patterns of titanate nanotube and Niintercalated titanate nanotube.

The electrochemical cell was made of three electrodes of TNT or Ni-TNT electrode (1 × 1 cm2), Ag/AgCl, and Pt gauze as photoanode, reference electrode, and cathode, respectively. The TNT or Ni-TNT electrode was prepared by the screen printing method. The photoanode was illuminated with a Hg-arc lamp (450 W) equipped with IR liquid filter (no cutoff filter). The photocurrent versus potential (I-V) was measured in an aqueous electrolyte solution (70 mL) consisting of 0.1 M NaOH using a potentiostat/galvanostat (EG&G model 263A) under illumination condition. 3. Results and Discussion 3.1. Structure and Texture. Figure 1 shows XRD patterns of bare and Ni-intercalated titanate nanotube before and after annealing. All samples exhibited characteristic peaks around 2θ ) 10, 24, 28° that can be assigned to the diffraction pattern of Na2Ti2O5 · nH2O.39 No peaks corresponding to Ni metal, Ni(OH)2, or NiO were detected in the XRD patterns of Ni-TNT and Ni-TNT400. The (200) peak at 10° was invariant in position even after nickel was intercalated into TNT, indicating that the interlayer spacing was not expanded with Ni intercalation into the interlayer of titanate nanotube.23 The Ni intercalation in the interlayer was also confirmed by TEM images in Figure 2. Both TNT and Ni-TNT have similar morphologies of tubular shape with ca. 10 nm in diameter and several hundred nanometers in length. There is no serious aggregation of nanotubes. This morphology is typically observed for wellprepared titanate nanotubes reported in the literature.15-22 The annealing shifted the (200) peak to a high angle of 11.32° for both TNT and Ni-TNT. It may be due to the shrinkage of the interlayer spacing caused by dehydration of water during heat treatment.40,41 Therefore, the dehydration process was investigated by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). In Figure 3, the drastic weight loss from 50 to 450 °C is attributed to the dehydration of water hydrated in the interlayer and adsorbed on surface of the TNT and Ni-TNT. Zhang et al. reported that the dehydration of intralayered water occurs below 300 °C and the dehydration of interlayered OH groups is observed over 300 °C.42 About 16% weight in TNT and Ni-TNT was lost during the heat treatment up to 450 °C. During the dehydration process, however, the interlayered OH group was not completely removed because of strong wateradsorbing capability by capillarity in the nanotubes.22 Unlike TNT, Ni-TNT shows an inflection point in the weight trace

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Figure 2. TEM images of (A) TNT and (B) Ni-TNT.

Figure 3. TGA and DTA curves of (A) TNT and (B) Ni-TNT.

Figure 4. A. N2 adsorption-desorption isotherms for TNT (O,b), TNT400 (4,2), Ni-TNT (1,0), and Ni-TNT400 (0,9). Filled and empty symbols denote adsorption and desorption branchs of N2 isotherms, respectively. B. Pore size distributions calculated by the BJH method from the desorption branch of the N2 isotherm for TNT (b), TNT400 (2), Ni-TNT (1), and Ni-TNT400 (9).

around 250 °C. The nickel intercalated in the interlayer of titanate nanotube could strongly interact with water in the interlayer and, consequently, the water comes out from the interlayer in an instant at a high temperature (over 250 °C). The DTA curves make clear the nature of the dehydration. The endothermic peak around 160 °C is attributed to desorption of water physically adsorbed and hydrated in the interlayer of TNT. However, Ni-TNT shows an additional small endothermic peak around 290 °C originating from the dehydration of water more strongly captured by the nickel in the interlayer. Upon further heat treatment, a secondary small endothermic peak was observed around 600 °C, indicating that titanate nanotube was transformed to other titanate structures or titanium oxide particle.3,22,43

The physical texture of the tubular structure was characterized using the N2 adsorption/desorption isotherms in Figure 4. All samples exhibit similar isotherm patterns corresponding to type IV hysteresis associated with slit-shaped pores or the space between parallel plates.44 The hysteresis in the isotherm is probably caused by the inner pores, which would exist in the multiwalls of TNT and Ni-TNT. As shown in part B of Figure 4, both TNT and Ni-TNT show a narrow pore size distribution (PSD) of ca. 3.7 nm in average diameter. However, when they were annealed, the pore size decreased a little to ca. 3.55 nm with still a narrow size distribution. Correlating with the XRD results, the dehydration of TNT and Ni-TNT causes the shrinkage of the inner diameter and of the distance between interlayer in the nanotube. However, the absence of broad peaks

Nickel Intercalated into Titanate Nanotube

Figure 5. UV-vis diffuse reflectance spectra of titanate nanotube and Ni-intercalated titanate nanotube.

in the PSD after annealing is indicative of the stable structure of the nanotubes without a serious aggregation of each titanate nanotube. The BET surface area of samples decreased from 292 to 232 m2/g after nickel was intercalated into TNT. The annealing of TNT and Ni-TNT also decreased their BET surface area to 262 and 216 m2/g, respectively, which could be well correlated with the reduced pore diameters.45 It is worthy to note that TNT400 and Ni-TNT400 still represent high surface areas while maintaining Na2Ti2O5 · nH2O structure. All of the results discussed so far indicate that TNT and Ni-TNT possess the structure, morphology, and texture of typical titanate nanotubes, that is hollow multiwall Na2Ti2O5 · nH2O nanotubes with a spiral cross section. 3.2. Local Structure of Ni in the Nanotube. The influence of Ni intercalation on an optical absorption was examined with UV-diffuse reflectance spectra in Figure 5. Whereas pure titanate nanotube has an absorption edge at ca. 350 nm, the nickel intercalation induces a dramatic red shift into the visible range of 400-700 nm. The photoabsorption of Ni-TNT in the range is attributed to the existence of the Ni 3d bands above the O 2p bands. Xu et al. has reported that Ni 3d electron energy levels are higher than that of Ti 3d, leading to the levels emerging in the band gap region between the O 2p and Ti 3d dominant bands.24 This interesting result makes the Ni-TNT material a promising candidate for solar energy applications. In this recognition, we characterized the local structure of nickel in the interlayer of the nanotube. Figure 6 shows the XPS Ni 2p3/2 spectra of Ni-TNT before and after annealing. Both as-prepared and annealed samples have the binding energy of 855.7 eV, which is higher than that of NiO but lower than of Ni(OH)2.46-48 The electronic structure of the nickel in the nanotube was also investigated with Ni K-edge XANES analyses in Figure 7. Compared with XANES spectrum of reference Ni(OH)2, the as-prepared sample has close similarities in shape near an absorption edge and peak position at the second oscillation (∼8346 eV). When the nanotube was annealed, its spectrum became closer to that of NiO. The derivative spectra in part B of Figure 7, however, reveal that Ni-TNT and Ni-TNT400 do not coincide exactly with Ni(OH)2 and NiO respectively in spite of much similarity between them. We interpret that Ni-TNT consists dominantly of Ni(OH)2 phase with a little NiO phase. Annealing converted much of Ni(OH)2 in the as-prepared sample into the NiO phase, but a small quantity of Ni(OH)2 would be still present in the

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Figure 6. XPS core-level spectra of Ni 2p2/3 of titanate nanotube and Ni-intercalated titanate nanotube.

Figure 7. (A) Ni K-edge XANES spectra and (B) their derivative spectra of Ni-intercalated titanate nanotube.

Figure 8. FTIR spectra of TNT and Ni-TNT before and after annealing.

annealed sample. This explains why Ni-TNT and Ni-TNT400 have the Ni 2p3/2 binding energy between NiO and Ni(OH)2. FTIR spectra confirm the presence of the OH group in the annealed sample as well as in the as-prepared one. In Figure 8, both TNT and Ni-TNT have water-associated bands at 1630 and 3400 cm-1. These bands do not disappear even after annealing because water can physically adsorb on the surface or in the interlayer during the measurement. Particular attention

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Figure 9. Raman spectra of TNT and Ni-TNT samples before and after annealing.

should be paid to the band at 1375 cm-1, which is indicative of a OH group.49 The strong band in TNT becomes weaker in Ni-TNT but it would not disappear even after annealing. In Raman spectra of Figure 9, all samples show the bands at 278, 448, and 660 cm-1, where the first and the last peaks correspond to Ti-O-M vibration (M ) metal cation such as Na+) and the middle one denotes Ti-O bending for 3-fold oxygen.2,22 A distinct change was observed between Ni-TNT and Ni-TNT400 samples, that is the band at 810 cm-1 increased and the one at 905 cm-1 decreased. As the band at 905 cm-1 is associated with four-coordinated Ti-O involving nonbridging oxygen atoms coordinated by a metal ion, the Ni interaction has little influence on it but the successive annealing process with developing a large amount of NiO results in the reduced intensity for it. The increased intensity of the band at 810 cm-1 can be also explained as the presence of dominant NiO phase in Ni-TNT400. This band denotes a Ti-O stretching and bending vibration involving 2-fold oxygen. In addition to the electronic structure of nickel in the nanotube, its geometric structure was also characterized with Fourier-transform analyses of EXAFS as shown in Figure 10. Imaginary function (part B of Figure 10) as well as magnitude function (part A of Figure 10) for Ni-TNT coincides with the respective function for Ni(OH)2 in the position of peaks. In the case of annealed sample, the similar feature is connected to NiO. The only difference is that Ni-TNT400 exhibits less developed peaks in intensity above 3.2 Å compared with bulk NiO. The result presents that NiO formed in the interlayer of nanotube is small without extensive aggregation during annealing. 3.3. Photocatalytic Reactions. The photocatalytic performance of Ni-intercalated nanotube was examined by hydrogen production from an aqueous solution containing methanol as a hole scavenger under UV irradiation. In Figure 11, Ni-TNT and Ni-TNT400 samples show much higher photocatalytic activity than those of TNT and TNT400, respectively. The superior activities of Ni-intercalated titanate nanotubes are clearly due to the nickel acting as a cocatalyst. The nickel could effectively separate electron and hole pairs generated upon initial light absorption by providing reaction sites.26,27,50-53 As far as the stability is concerned, the Ni-intercalated samples also perform better. The hydrogen production over Ni-TNT increased steadily with reaction time and after 180 min elapsed, its amount was larger by about 3.5 times than that of TNT (the inset plot in Figure 10). Annealing at 400 °C reduced the rate of hydrogen production significantly for both bare and Ni-

Figure 10. Fourier-transforms of Ni K-edge EXAFS for Ni-TNT before and after annealing; (A) magnitude function, (B) imaginary function.

Figure 11. Average rate of H2 evolution over TNT and Ni-TNT before and after annealing from water-methanol mixed solution under UV light irradiation. Catalysts, 100 mg methanol and water mixed solution, 100 mL light source, Hg Arc lamp (450 W) equipped with IR liquid filter.

intercalated samples. The higher activities of as-synthesized samples may be attributed to a hydrated interlayer in the nanotube. Shimizu et al. have reported that the layered perovskite tantalates with hydrated interlayer space show a higher rate of H2 evolution than that of anhydrous perovskite tantalates.53,54 In this regard, it should be noted that NiO is the most common nickel species used as a cocatalyst for semiconductor photocatalysts in water-splitting reactions under UV light irradiation.26-30 Especially, it was found to be the best cocatalyst for perovskitetype photocatalysts. However, it was found that Ni(OH)2 was a more effective than cocatalysts. The difference in activity between NiO and Ni(OH)2 would have been much larger if a part of NiO did not change to Ni(OH)2 during photocatalytic reaction in water.55

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J. Phys. Chem. C, Vol. 113, No. 20, 2009 8995 a significant role of cocatalyst in the photocatalytic hydrogen production from methanol-water mixed solution under UV light irradiation. Thus, Ni-TNT showed a higher rate of hydrogen evolution than that of TNT. The nickel in the interlayer provides active sites for proton reduction and promotes fast diffusion of photoelectrons generated from titanate nanotube, leading to higher photocatalytic activity.

Figure 12. Schematic view for the reaction mechanism of H2 evolution over Ni-TNT in water-methanol mixed solution.

Acknowledgment. This work was supported by National Research Laboratory, General Motors R&D Center, for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Program, the Brain Korea 21 Project, National R&D Project for Nano Science and Technology. S. H. Choi acknowledges that this work supported by the Korea Research Foundation Grant is funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2007-313-D00157). References and Notes

Figure 13. Photocurrent density-potential curves of the thin film electrodes of TNT and Ni-TNT in 0.1 M NaOH under illumination. Light Source, 450W Hg Arc lamp with IR liquid filter.

In summary, the enhanced activity of Ni-TNT (higher by a factor of ca. 14 compared to TNT400) results from the hydrated Ni complex in the interlayer of nanotube. In the model of Figure 12, the Ni complex like [NixII(OH)2x-1(OH2)]+ would form in the interlayer of titanate nanotube and the photogenerated electrons in the layers can be easily migrate to the Ni site to reduce water. The [NixII(OH)2x-1(OH2)]+ species has been already mentioned in the metal(II) hydroxide-layer silicate intercalation56,57 and it explains well how the Ni(II) precursor can be substituted for Na+ in titanate nanotube without further treatment to balance a charge. The correlation between photocatalytic activity and photoelectrochemical response was studied as shown in Figure 13. The film electrode of TNT or Ni-TNT generated photocurrent in the direction of anodic potential under UV light irradiation, whereas no current was measured until 0.65 V (vs Ag/AgCl) under dark conditions. The Ni-TNT electrode generated photocurrent at 0.65 V about 1.7 times faster than the TNT electrode. The nickel placed in the interlayer plays a significant role in retarding electron-hole recombination on the channel surface of titanate nanotube. Considering that photocurrent generation is a critical initial step in a whole photocatalytic reaction, the faster photogeneration makes clear the potential use of Ni-TNT in photocatalytic hydrogen production. 1. Conclusions Titanate nanotube (TNT) and Ni-intercalated titanate nanotube (Ni-TNT) were successfully synthesized via hydrothermal method. The hydrated nickel complex of [NixII(OH)2x-1(OH2)]+ was formed in the interlayer of nanotube and the complex played

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