Enhanced Photocatalytic Hydrogen Production from Water-Methanol Solution by NickelIntercalated into Titanate Nanotube

Jum Suk Jang,† Sun Hee Choi,*,‡ Dong Hyun Kim,§ Ji Wook Jang,† Kyung Sub Lee,§ andJae Sung Lee†

Eco-friendly Catalysis and Energy Laboratory (NRL), Department of Chemical Engineering and School ofEnVironmental 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 hydrogenproduction from methanol-water solution under UV light irradiation. The nickel intercalated into the nanotubewas present as a hydrated Ni complex of [Nix

II(OH)2x-1(OH2)]+ and was responsible for a dramatic enhancementof hydrogen evolution rate relative to that of titanate nanotube itself. The nickel species in the interlayerprovided active sites for proton reduction and caused fast diffusion of photoelectrons generated from titanatelayers toward the nickel sites, leading to a high photocatalytic activity. Upon annealing at 400 °C, the hydratednickel complex was partly converted to NiO and the hydrogen evolution rate was reduced, indicating that thenickel hydroxide was a more efficient cocatalyst for titanate nanotube. A high and stable photocurrent generationwas 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 andnanoporous structures are considered as a substitute for meso-porous molecular sieves that usually suffer from hydro-instability. Titania nanotubes are of particular interest becausetheir precursor TiO2 has a wide range of applications fromcatalysis to dye-sensitized solar cells or water purification. SinceKasuga et al.1,2 first developed the TiO2-derived nanotubes bya simple hydrothermal treatment of TiO2 powder in NaOHaqueous solution, many studies have followed to optimize thepreparation conditions, such as reaction temperature,4-6 causticconcentration,1,4 the type of titanium oxide as a precursor,4,8-13

and its crystal size.14 Several different crystal structures havebeen proposed to describe the nanotube, that is titania-typenanotube like TiO2-anatase or titanate-type like H2Ti3O7. Thenanotubes are generally depicted as the scrolling of an exfoliatedtitania or titanate-derived nanosheet into a hollow multiwallnanotube with a spiral cross section. Such a nanotube structuremakes a high surface area, regular pore size distribution, andimproved crystallinity compared to usual nanostructured TiO2

and can be effectively applied in photocatalysis.15-20,58

The multilayered nanotubes can be modified by intercalationof alkaline metal ions or transition-metal cations between thelayers in the nanotubes via ion exchange.21,22 Transition metalsof Fe and Ni can also be incorporated into titanate nanotubesthrough a hydrothermal procedure.23-25 The intercalated Fe andNi decrease the band gap of H2Ti3O6 nanotube and, conse-quently, have high potential for optoelectronics and photoca-talysis. On the other hand, nickel has been used as a cocatalyst

for water splitting under UV light irradiation.26-30 The NiO-loaded photocatalysts show higher activities than that ofunloaded or, in some cases, noble metal-loaded photocatalystsbecause NiO provides reduction sites to promote hydrogenformation in a water-splitting reaction. Nevertheless, no studyhas been reported yet for Ni-intercalated titanate nanotube forphotocatalytic hydrogen production.

In this study, we synthesized titanate nanotube and Ni-intercalated titanate nanotube via one-step hydrothermal methodand investigated the local structure of Ni-intercalated in thetitanate nanotubes. The catalytic performances of the materialswere measured in two systems, that is photocatalytic hydrogenproduction from water-methanol solution and photoelectro-chemical measurement under UV irradiation. The active phaseof nickel responsible for the enhanced photocatalytic activitycompared to bare titanate nanotube was identified by detailedanalysis of the local structure of nickel in the nanotube.

2. Experimental Procedures

2.1. Synthesis of Ni-Intercalated Titanate Nanotube. Amor-phous powders containing ammonia and titanium were firstprepared. Ammonium hydroxide solution with an ammoniacontent of 28-30% (99.99%, Aldrich) was added drop-by-dropto 20% titanium (III) chloride solution (TiCl3, Kanto) for 30min 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 withdeionized water, and then dried at 70 °C for 4 h in a convectionoven.

For the preparation of bare and Ni-intercalated titanatenanotubes, 0.70 g of the dried precipitates only or those with0.170 g of Ni(NO3)2 ·6H2O were stirred in 70 mL of 10 MNaOH aqueous solution for 1 h and were introduced into aTeflon-lined stainless steel autoclave. The autoclave wasmaintained at 150 °C for 72 h and allowed to cool down to

* 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 EnvironmentalScience and Engineering, POSTECH.

‡ Pohang Accelerator Laboratory, POSTECH.§ Division of Materials Science & Engineering, Hanyang University.

J. Phys. Chem. C 2009, 113, 8990–89968990

10.1021/jp900653r CCC: $40.75 2009 American Chemical SocietyPublished on Web 04/22/2009

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 electricalfurnace under ambient air. We refer hereafter to the as-preparedsamples as TNT and Ni-TNT and the annealed samples asTNT400 and Ni-TNT400 for bare and Ni-intercalated titanatenanotubes, respectively.

2.2. Physico-Chemical Characterization. The crystallinephases of the products were determined by powder X-raydiffraction (XRD) on a diffractometer (Mac Science Co.,M18XHF) with monochromatic Cu KR radiation at 40 kV and200 mA. The morphologies of TNT and Ni-TNT wereexamined by TEM (JEOL JEM 2010F) operated at 200 kV andtheir optical properties were analyzed by a UV-vis diffusereflectance spectrometer (Shimadzu, UV 2401). A differentialscanning calorimeter (DSC) (Shimadzu, DTA-50) was appliedto monitor changes during the thermal treatment of the driednanotubes (TNT and Ni-TNT). The DSC curves were recordedin 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-IIIspectrometer was used to take FTIR spectra of disk-type samplesprepared by mixing with KBr.

To investigate the physical texture of the nanotube materials,the N2 adsorption-desorption isotherm at 77 K was taken in aconstant-volume adsorption apparatus (Micrometrics ASAP2010) at relative pressures (P/P0) ranging from 10-4 to 0.995.Before measuring the isotherm, the sample was degassed for4 h at 120 °C under 10-4 Torr. The specific surface area wascalculated using the Brunauer-Emmett-Teller (BET) method31

and the pore size distribution (PSD) was calculated fromnitrogen desorption data using the Barrett-Joyner-Halenda(BJH) method with the modified Kelvin equation.32 The porevolume was assessed on the basis of the adsorbed amount at arelative pressure (P/P0) of 0.99.

The local structure of Ni in both Ni-TNT and Ni-TNT400was 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 byusing C1s peak as the reference energy. XAFS measurementswere conducted at 5A wiggler beamline of Pohang AcceleratorLaboratory (2.5 GeV, stored current of 140-180 mA). Theradiation was monochromatized using a Si(111) double-crystalmonochromator and the incident beam was detuned by 40% tominimize higher-order reflections of the silicon crystals. NiK-edge spectra were taken in transmission mode where incidentand transmitted beam intensities were monitored with separateIC SPEC ionization chambers. Before the measurement of thesample, a scanning energy was calibrated with respect to 8333eV of Ni K-edge energy. The obtained data were analyzed usingthe IFEFFIT suite of software programs.33,34 The detailedprocedure for data analysis is described elsewhere.35-38

2.3. Photocatalytic and Photoelectrochemical Reactions.Photocatalytic hydrogen production was performed at roomtemperature under atmospheric pressure in a closed circulationsystem using an Hg-arc lamp (450 W) equipped with IR liquidfilter (no cutoff filter). The rate of H2 evolution was determin-ed for water-methanol solution (distilled water 70 mL andmethanol 30 mL) containing 100 mg catalyst. Hydrogen andCO2 were the main products of the photocatalytic reaction witha trace amount of CO. The concentration of H2 was analyzedby gas chromatography equipped with a thermal conductivitydetector (molecular sieve 5 Å column and Ar carrier).

The electrochemical cell was made of three electrodes of TNTor Ni-TNT electrode (1 × 1 cm2), Ag/AgCl, and Pt gauze asphotoanode, reference electrode, and cathode, respectively. TheTNT or Ni-TNT electrode was prepared by the screen printingmethod. The photoanode was illuminated with a Hg-arc lamp(450 W) equipped with IR liquid filter (no cutoff filter). Thephotocurrent versus potential (I-V) was measured in an aqueouselectrolyte solution (70 mL) consisting of 0.1 M NaOH usinga potentiostat/galvanostat (EG&G model 263A) under illumina-tion condition.

3. Results and Discussion

3.1. Structure and Texture. Figure 1 shows XRD patternsof bare and Ni-intercalated titanate nanotube before and afterannealing. All samples exhibited characteristic peaks around 2θ) 10, 24, 28° that can be assigned to the diffraction pattern ofNa2Ti2O5 · nH2O.39 No peaks corresponding to Ni metal,Ni(OH)2, or NiO were detected in the XRD patterns of Ni-TNTand Ni-TNT400. The (200) peak at 10° was invariant inposition even after nickel was intercalated into TNT, indicatingthat the interlayer spacing was not expanded with Ni intercala-tion into the interlayer of titanate nanotube.23 The Ni intercala-tion in the interlayer was also confirmed by TEM images inFigure 2. Both TNT and Ni-TNT have similar morphologiesof tubular shape with ca. 10 nm in diameter and several hundrednanometers in length. There is no serious aggregation ofnanotubes. This morphology is typically observed for well-prepared titanate nanotubes reported in the literature.15-22 Theannealing shifted the (200) peak to a high angle of 11.32° forboth TNT and Ni-TNT. It may be due to the shrinkage of theinterlayer spacing caused by dehydration of water during heattreatment.40,41 Therefore, the dehydration process was investi-gated by thermogravimetric analysis (TGA) and differentialthermal analysis (DTA).

In Figure 3, the drastic weight loss from 50 to 450 °C isattributed to the dehydration of water hydrated in the interlayerand adsorbed on surface of the TNT and Ni-TNT. Zhang etal. reported that the dehydration of intralayered water occursbelow 300 °C and the dehydration of interlayered OH groupsis observed over 300 °C.42 About 16% weight in TNT andNi-TNT was lost during the heat treatment up to 450 °C.During the dehydration process, however, the interlayered OHgroup was not completely removed because of strong water-adsorbing capability by capillarity in the nanotubes.22 UnlikeTNT, Ni-TNT shows an inflection point in the weight trace

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

Nickel Intercalated into Titanate Nanotube J. Phys. Chem. C, Vol. 113, No. 20, 2009 8991

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

The physical texture of the tubular structure was characterizedusing the N2 adsorption/desorption isotherms in Figure 4. Allsamples exhibit similar isotherm patterns corresponding to typeIV hysteresis associated with slit-shaped pores or the spacebetween parallel plates.44 The hysteresis in the isotherm isprobably caused by the inner pores, which would exist in themultiwalls of TNT and Ni-TNT. As shown in part B of Figure4, both TNT and Ni-TNT show a narrow pore size distribution(PSD) of ca. 3.7 nm in average diameter. However, when theywere annealed, the pore size decreased a little to ca. 3.55 nmwith still a narrow size distribution. Correlating with the XRDresults, the dehydration of TNT and Ni-TNT causes theshrinkage of the inner diameter and of the distance betweeninterlayer in the nanotube. However, the absence of broad peaks

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 emptysymbols denote adsorption and desorption branchs of N2 isotherms, respectively. B. Pore size distributions calculated by the BJH method from thedesorption branch of the N2 isotherm for TNT (b), TNT400 (2), Ni-TNT (1), and Ni-TNT400 (9).

8992 J. Phys. Chem. C, Vol. 113, No. 20, 2009 Jang et al.

in the PSD after annealing is indicative of the stable structureof the nanotubes without a serious aggregation of each titanatenanotube.

The BET surface area of samples decreased from 292 to 232m2/g after nickel was intercalated into TNT. The annealing ofTNT and Ni-TNT also decreased their BET surface area to262 and 216 m2/g, respectively, which could be well correlatedwith the reduced pore diameters.45 It is worthy to note thatTNT400 and Ni-TNT400 still represent high surface areaswhile maintaining Na2Ti2O5 ·nH2O structure. All of the resultsdiscussed so far indicate that TNT and Ni-TNT possess thestructure, morphology, and texture of typical titanate nanotubes,that is hollow multiwall Na2Ti2O5 ·nH2O nanotubes with a spiralcross section.

3.2. Local Structure of Ni in the Nanotube. The influenceof Ni intercalation on an optical absorption was examined withUV-diffuse reflectance spectra in Figure 5. Whereas pure titanatenanotube has an absorption edge at ca. 350 nm, the nickelintercalation induces a dramatic red shift into the visible rangeof 400-700 nm. The photoabsorption of Ni-TNT in the rangeis attributed to the existence of the Ni 3d bands above the O 2pbands. Xu et al. has reported that Ni 3d electron energy levelsare higher than that of Ti 3d, leading to the levels emerging inthe band gap region between the O 2p and Ti 3d dominantbands.24 This interesting result makes the Ni-TNT material apromising candidate for solar energy applications. In thisrecognition, we characterized the local structure of nickel inthe interlayer of the nanotube.

Figure 6 shows the XPS Ni 2p3/2 spectra of Ni-TNT beforeand after annealing. Both as-prepared and annealed samples havethe binding energy of 855.7 eV, which is higher than that ofNiO but lower than of Ni(OH)2.46-48 The electronic structureof the nickel in the nanotube was also investigated with NiK-edge XANES analyses in Figure 7. Compared with XANESspectrum of reference Ni(OH)2, the as-prepared sample has closesimilarities in shape near an absorption edge and peak positionat the second oscillation (∼8346 eV). When the nanotube wasannealed, its spectrum became closer to that of NiO. Thederivative spectra in part B of Figure 7, however, reveal thatNi-TNT and Ni-TNT400 do not coincide exactly withNi(OH)2 and NiO respectively in spite of much similaritybetween them. We interpret that Ni-TNT consists dominantlyof Ni(OH)2 phase with a little NiO phase. Annealing convertedmuch 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

annealed sample. This explains why Ni-TNT and Ni-TNT400have the Ni 2p3/2 binding energy between NiO and Ni(OH)2.

FTIR spectra confirm the presence of the OH group in theannealed sample as well as in the as-prepared one. In Figure 8,both TNT and Ni-TNT have water-associated bands at 1630and 3400 cm-1. These bands do not disappear even afterannealing because water can physically adsorb on the surfaceor in the interlayer during the measurement. Particular attention

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

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

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

Figure 8. FTIR spectra of TNT and Ni-TNT before and afterannealing.

Nickel Intercalated into Titanate Nanotube J. Phys. Chem. C, Vol. 113, No. 20, 2009 8993

should be paid to the band at 1375 cm-1, which is indicative ofa OH group.49 The strong band in TNT becomes weaker inNi-TNT but it would not disappear even after annealing. InRaman spectra of Figure 9, all samples show the bands at 278,448, and 660 cm-1, where the first and the last peaks correspondto Ti-O-M vibration (M ) metal cation such as Na+) and themiddle one denotes Ti-O bending for 3-fold oxygen.2,22 Adistinct change was observed between Ni-TNT and Ni-TNT400samples, that is the band at 810 cm-1 increased and the one at905 cm-1 decreased. As the band at 905 cm-1 is associated withfour-coordinated Ti-O involving nonbridging oxygen atomscoordinated by a metal ion, the Ni interaction has little influenceon it but the successive annealing process with developing alarge amount of NiO results in the reduced intensity for it. Theincreased intensity of the band at 810 cm-1 can be also explainedas the presence of dominant NiO phase in Ni-TNT400. Thisband denotes a Ti-O stretching and bending vibration involving2-fold oxygen.

In addition to the electronic structure of nickel in thenanotube, its geometric structure was also characterized withFourier-transform analyses of EXAFS as shown in Figure 10.Imaginary function (part B of Figure 10) as well as magnitudefunction (part A of Figure 10) for Ni-TNT coincides with therespective function for Ni(OH)2 in the position of peaks. In thecase of annealed sample, the similar feature is connected to NiO.The only difference is that Ni-TNT400 exhibits less developedpeaks in intensity above 3.2 Å compared with bulk NiO. Theresult presents that NiO formed in the interlayer of nanotube issmall without extensive aggregation during annealing.

3.3. Photocatalytic Reactions. The photocatalytic perfor-mance of Ni-intercalated nanotube was examined by hydrogenproduction from an aqueous solution containing methanol as ahole scavenger under UV irradiation. In Figure 11, Ni-TNTand Ni-TNT400 samples show much higher photocatalyticactivity than those of TNT and TNT400, respectively. Thesuperior activities of Ni-intercalated titanate nanotubes areclearly due to the nickel acting as a cocatalyst. The nickel couldeffectively separate electron and hole pairs generated upon initiallight absorption by providing reaction sites.26,27,50-53 As far asthe stability is concerned, the Ni-intercalated samples alsoperform better. The hydrogen production over Ni-TNT in-creased steadily with reaction time and after 180 min elapsed,its amount was larger by about 3.5 times than that of TNT (theinset plot in Figure 10). Annealing at 400 °C reduced the rateof hydrogen production significantly for both bare and Ni-

intercalated samples. The higher activities of as-synthesizedsamples may be attributed to a hydrated interlayer in thenanotube. Shimizu et al. have reported that the layered perovs-kite tantalates with hydrated interlayer space show a higher rateof H2 evolution than that of anhydrous perovskite tantalates.53,54

In this regard, it should be noted that NiO is the most commonnickel species used as a cocatalyst for semiconductor photo-catalysts inwater-splittingreactionsunderUVlight irradiation.26-30

Especially, it was found to be the best cocatalyst for perovskite-type photocatalysts. However, it was found that Ni(OH)2 was amore effective than cocatalysts. The difference in activitybetween NiO and Ni(OH)2 would have been much larger if apart of NiO did not change to Ni(OH)2 during photocatalyticreaction in water.55

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

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

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

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In summary, the enhanced activity of Ni-TNT (higher by afactor of ca. 14 compared to TNT400) results from the hydratedNi complex in the interlayer of nanotube. In the model of Figure12, the Ni complex like [Nix

II(OH)2x-1(OH2)]+ would form inthe interlayer of titanate nanotube and the photogeneratedelectrons in the layers can be easily migrate to the Ni site toreduce water. The [Nix

II(OH)2x-1(OH2)]+ species has beenalready mentioned in the metal(II) hydroxide-layer silicateintercalation56,57 and it explains well how the Ni(II) precursorcan be substituted for Na+ in titanate nanotube without furthertreatment to balance a charge.

The correlation between photocatalytic activity and photo-electrochemical response was studied as shown in Figure 13.The film electrode of TNT or Ni-TNT generated photocurrentin 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 generatedphotocurrent at 0.65 V about 1.7 times faster than the TNTelectrode. The nickel placed in the interlayer plays a significantrole in retarding electron-hole recombination on the channelsurface of titanate nanotube. Considering that photocurrentgeneration is a critical initial step in a whole photocatalyticreaction, the faster photogeneration makes clear the potentialuse of Ni-TNT in photocatalytic hydrogen production.

1. Conclusions

Titanate nanotube (TNT) and Ni-intercalated titanate nanotube(Ni-TNT) were successfully synthesized via hydrothermalmethod. The hydrated nickel complex of [Nix

II(OH)2x-1(OH2)]+

was formed in the interlayer of nanotube and the complex played

a significant role of cocatalyst in the photocatalytic hydrogenproduction from methanol-water mixed solution under UV lightirradiation. Thus, Ni-TNT showed a higher rate of hydrogenevolution than that of TNT. The nickel in the interlayer providesactive sites for proton reduction and promotes fast diffusion ofphotoelectrons generated from titanate nanotube, leading tohigher photocatalytic activity.

Acknowledgment. This work was supported by NationalResearch Laboratory, General Motors R&D Center, for theHydrogen Energy R&D Center, one of the 21st Century FrontierR&D Program, the Brain Korea 21 Project, National R&DProject for Nano Science and Technology. S. H. Choi acknowl-edges that this work supported by the Korea Research Founda-tion Grant is funded by the Korean Government (MOEHRD,Basic Research Promotion Fund) (KRF-2007-313-D00157).

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