Photochemical Hydrogen Generation Using Nitrogen-Doped TiO2–Pd

Article pubs.acs.org/JPCC

Photochemical Hydrogen Generation Using Nitrogen-Doped TiO2− Pd Nanoparticles: Facile Synthesis and Effect of Ti3+ Incorporation Farheen. N. Sayed,† O. D. Jayakumar,*,† R. Sasikala,† R. M. Kadam,‡ Shyamala. R. Bharadwaj,† Lorenz Kienle,§ Ulrich Schürmann,§ Sören Kaps,∥ Rainer Adelung,∥ J. P. Mittal,⊥ and A. K. Tyagi*,† †

Chemistry Division, ‡Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai, INDIA-400085 Synthesis and Real Structure, ∥Functional Nanomaterials, Technical Faculty of the CAU Kiel, Kaiserstrasse 2, 24143 Kiel, Germany ⊥ Bhabha Atomic Research Centre, Mumbai, INDIA-400085 §

ABSTRACT: Crystalline nanoparticles of anatase phase nitrogen-doped TiO2 (N−TiO2), boron and nitrogen codoped TiO2 (B−N−TiO2), and undoped TiO2 (TiO2) were synthesized by a facile xerogel method followed by reduction in a glycol medium to forcibly create Ti3+ ions (self-doping) in them (R−N−TiO2, R−B−N−TiO2, and R−TiO2, where R stands for reduced) and studied their photocatalytic activity for hydrogen generation from water. A clear red shift of the absorption edge and stronger absorption in the visible region was observed for both reduced and unreduced N−TiO2 and B−N−TiO2 compared to the other samples. The presence of bonded nitrogen in the N-doped samples was evidenced from their N1s X-ray photoelectron spectra. Photocatalytic activity for hydrogen generation using sunlight-type radiation showed enhanced activity for the reduced and doped samples compared to pristine TiO2. The enhanced photocatalytic activity of the R−N−TiO2 for hydrogen generation is attributed to the enhanced light absorption resulting from the narrowing of the band gap caused by the contribution of anion levels near the conduction band and 2p levels of N near the valence band of TiO2. A significant enhancement of photocatalytic activity was observed when Pd metal was present as a cocatalyst due to the efficient separation of photogenerated charge carriers in these nanoparticles.

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INTRODUCTION Photocatalytic property of titanium dioxide has been studied extensively in recent years due to its potential application in environmental cleaning, hydrogen production from water, and its good stability and catalytic activity. However, the wide band gap of TiO2 (anatase of 3.2 eV, rutile of 3.0 eV) limits the absorption wavelength to less than 387 nm, which is only 3− 5% of the sunlight energy and holds back the practical applications. To use solar radiation effectively and for making the photocatalytic process more efficient, the band gap of TiO2 has to be reduced to ≤3 eV using suitable dopants. Considerable efforts have been invested to broaden the photoresponse of TiO2 to the visible-light region using cationic1−3 or anionic4−6 dopants. TiO2 doped with cations such as Ce, V, Cu, Sn, Nd, Fe, Cr, or Co shows a red shift of the absorption band compared to pure TiO2 and enhances the visible light absorption.7−10 However, the disadvantage of using cationic dopant is that the doped cations can trap electrons and decrease the photocatalytic efficiency. Among anionic dopants such as nitrogen (N), sulfur (S), chlorine (Cl), and bromine (Br), N has received great attention and several methods have been reported for its incorporation in the TiO2 matrix. Many kinds of N sources can be used for the preparation of N−TiO2, which include organic nitrogen sources such as urea and triethylamine11,12 and inorganic nitrogen sources such as ammonia, ammonium chloride, and ammonium nitrate.13,14 Further, self-doping by Ti3+ in TiO2 has been reported to © 2012 American Chemical Society

enhance the visible light absorption and thus improving the photocatalytic activity. This enhanced activity arises from the band gap narrowing, which arises due to the presence of oxygen vacancies.15 In the present study, we have prepared TiO2, N−TiO2, and B−N−TiO2 by a novel synthesis route and studied its photocatalytic activity for hydrogen generation from water. Nanoparticles of their corresponding reduced samples (R− TiO2, R−N−TiO2, and R−B−N−TiO2 (containing Ti3+) were also synthesized by a forced solvothermal chemical reduction method using diethylene glycol (DEG) as a reducing agent. N/ B was doped or codoped into TiO2 by a simple facile chemical route using ethanolamine/boric acid as the N and B sources, respectively. The novelties of this method are: 1) for incorporation of N and B we have used a modified xerogel type of reaction using ethanolamine as the nitrogen source and boric acid as the B source, 2) self-doped/reduced (containing Ti3+) TiO2, N−TiO2, and B−N−TiO2 were prepared by a solution assisted forced reduction using DEG as the reducing agent, and 3) the impregnation of these photocatalysts with cocatalysts Pd and NiO has also been done by forced solvothermal reduction of the salts of Pd and Ni using DEG. It is worth mentioning here that N−TiO2 after forced reduction Received: March 29, 2012 Revised: May 16, 2012 Published: June 4, 2012 12462

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UV−vis spectra of all samples were recorded using a Jasco (model V-670) spectrophotometer equipped with an integrating sphere accessory. Barium sulfate was used as reference for the reflectance spectra. Photocatalytic activity was studied in a tubular quartz reactor of length 13 cm and diameter 2 cm closed with a Teflon stopcock. It was also provided with a side tube closed with silicone rubber septum through which gas mixture could be removed for analysis. 50 mg of sample was kept in contact with 20 cm3 water and 5 cm3 methanol for conducting the photocatalysis experiment. The reactor was flushed with argon gas before irradiation. Samples were irradiated by keeping it vertically in a circular chamber containing eight ordinary day-light fluorescent lamps (Wipro, 36 W each) fixed vertically on the walls. Spectrum of the lamp consisted of fluorescent emission predominantly in the visible region along with a UV contribution of ∼3%. Power of the solar type radiation (fluorescent lamp) used in the experiment is 10.2 mW/cm2. After every 1 h, the gas mixture in the reactor was analyzed using a gas chromatograph (Chromatography and Instruments Company, GC 2011) equipped with a molecular sieve 5A column and a thermal conductivity detector. The intensity of the light source was measured using a calibrated precision Gentec power meter (model: SOLO 2 (R2))

(R−N−TiO2) showed more enhanced activity than R−TiO2 or R−B−N−TiO2 samples. The effect of impregnation of Pd/NiO as cocatalyst on R−N−TiO2 has been attempted to further improve its photocatalytic hydrogen generation from the water−methanol mixture.

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EXPERIMENTAL SECTION Synthesis. In a typical synthesis of pristine TiO2 sample, a mixture of 15 mL of titanium(IV) iso-propoxide and 50 mL of absolute ethanol was stirred continuously for 30 min until a homogeneous clear solution was obtained. The clear solution thus obtained was heated (∼70 °C) with continuous stirring and subsequently evaporated until dryness. The dried product was further heated at 500 °C for 2 h in a tubular furnace followed by washing with deionized water before being dried. This heated, washed, and dried powder was once again subjected to heating in a furnace at 500 °C for 2 h before being used. For in situ forced reduction of the as prepared TiO2 sample, 50 mg of TiO2 powder was refluxed in 10 mL of DEG for 2 h at 220 °C. The product was then separated by centrifugation and washed several times using acetone and ethanol followed by drying and then heated at 400 °C for 2 h. For the synthesis of B−N−TiO 2 sample, 9 mL of ethanolamine and 0.255 g of boric acid were dissolved in 50 mL of absolute ethanol respectively. The solutions were stirred for 30 min to get homogeneous phase followed by the addition of titanium(IV) iso-propoxide. A similar procedure has been followed for the synthesis of N−TiO2 using ethanolamine. To see the effect of impregnation of cocatalysts on the photocatalytic properties of these samples, in situ forced reduction of these samples in the presence of appropriate amounts of Pd/Ni salts was done. For this, as prepared samples were refluxed in DEG in the presence of Pd/Ni salts (palladium(II) chloride and nickel(II) acetate (2% by weight) at 220 °C for 2 h followed by centrifugation, washing, and heating of samples at 400 °C for 1 h to get the desired product. Characterization. Powder X-ray diffraction (XRD) patterns of these samples were recorded at a scan speed of 1°/ minute using a Philips PW1820 X-ray diffractometer coupled with a PW 1729 generator, which was operated at 30 kV and 20 mA. A graphite crystal monochromator was used for generating monochromatic CuKα radiation. Silicon was used as an external standard for correction due to instrumental line broadening. Total surface area of all samples was measured by BET technique using nitrogen gas adsorption. XPS measurements were done using monochromatic AlKα X-ray of the Quantera SXMULVAC-PHI. The C 1s peak set at 284.6 eV was used for charge referencing. The relative oxidation states of the Ti in these compositions were further confirmed by electron paramagnetic resonance (EPR) measurements using a Bruker ESP 300 spectrometer operated at X band frequency (9.60 GHz). Frequency modulation at 100 kHz was used for recording the EPR spectra. DPPH was used for calibration of g values. Approximately 50 mg of sample was placed in quartz tube and spectra were recorded under identical spectrometer settings (RG = 4 × 104, MA = 2 G, 6.3 mW). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) studies along with the scanning transmission electron microscopy (STEM) EDX elemental mapping were carried out on Tecnai F30 STwin (300 kV, field emission gun (FEG) cathode, spherical aberration coefficient Cs = 1.2 mm) for microstructural, compositional, and morphological studies. DR-

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RESULTS AND DISCUSSION Figure 1 depicts the X-ray diffraction patterns of pristine and doped TiO2 samples before (part a of Figure 1) and after reduction (part b of Figure 1). The peaks of XRD patterns of all these samples can be indexed as anatase phase (JCPDC No. 78−2846) without any impurities even after doping and codoping. The reflections in the XRD patterns are broadened due to smaller crystallite size, which was estimated using

Figure 1. (A) XRD pattern of (a) TiO2, (b) TiO2−N, (c) TiO2−N, B samples before reduction. (B) XRD pattern of (a) TiO2, (b) TiO2−N, (c) TiO2−N, B after reduction. 12463

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Figure 2. (a) Bright field image, (b) HRTEM micrograph, (c) SAED pattern, and (d) scanning transmission electron microscopy (STEM) and EDX elemental maps of pristine TiO2.

Figure 3. (a) Bright field image, (b) HRTEM micrograph, and (c) SAED pattern recorded on sample N−TiO2.

anatase-type reported in the literature.16 The TEM (part a of Figure 3) and HRTEM (part b of Figure 3) of R−N−TiO2 also show highly crystalline particles with size ∼10 nm. The anatase structure of N−TiO2 is further confirmed from the SAED pattern (part c of Figure 3). Diffused reflectance spectra (Figure 4) were recorded on reduced samples along with pristine TiO2 (unreduced and undoped) to observe the change in the band gap due to doping/codoping and forced reduction. It was found that the band gap of R−TiO2 sample is 3.08 eV (less than the band gap 3.2 eV of pristine TiO2), which on N doping (R−N−TiO2) shifts to still lower value (2.9 eV) indicating the incorporation of N in TiO2. The shift of the band edge toward higher wavelengths (Figure 4) on N doping indicate the decrease of band gap of N-doped samples and it is attributed to the increases in the width of the valence band on mixing of the N 2p and O 2p states. The sample R−B−N−TiO2 also shows a

Scherrer’s equation. The strongest reflection (101) was taken as the representative peak to calculate the average crystallite size (D) of the samples which was found to be ∼10 nm for all the samples. It is worth mentioning here that the XRD patterns of 1% Pd or Ni impregnated sample also showed impurity free TiO2 phase. The BET surface areas obtained for all samples were almost identical (∼70 m2 g−1). TEM image of pristine TiO2 (part a of Figure 2) shows the agglomeration of the particles with size ∼10 nm, which is in agreement with the X-ray diffraction result. HRTEM (part b of Figure 2) and indexed SAED pattern (part c of Figure 2) of pristine TiO2 confirms that the samples are highly crystalline in nature with anatase structure. The SAED studies along with the scanning transmission electron microscopy (STEM) and EDX elemental mapping of an agglomeration of nanoparticles (part d of Figure 2) confirm that the TiO2 nanoparticles contain no impurities. The SAED patterns correspond to the metrics of the 12464

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that of Ti−N appearing at ∼397.5 eV. The peak at 399 eV could be attributed to the nitrogen, which has replaced the oxygen in the crystal lattice of TiO2. This is also further supported by the results of XPS spectra of these samples for the Ti2p region. Figure 6 shows the Ti2p3/2 and Ti2p1/2 XPS

Figure 4. Diffused reflectance spectra (in absorbance mode) of reduced samples of TiO2, N−TiO2, and B−N−TiO2 along with pristine TiO2.

red shift of its absorption edge, which is comparable to that of R−N−TiO2. It can also be seen from this figure that considerable absorption of visible light below ∼600 nm is seen for all the reduced samples. This is due to the large number of defects, which are present in these samples.17 Figure 5 shows the N 1s X-ray photoelectron spectra (XPS) of N−TiO2, B−N−TiO2, and their reduced samples (R−N−

Figure 6. Ti2p3/2 and Ti2p1/2 XPS spectra of TiO2, reduced TiO2, reduced N−TiO2, and reduced B−N−TiO2.

spectra of TiO2, R−TiO2, R−N−TiO2, and R−B−N−TiO2, respectively. It may be noted that the peaks at 459.4 and 465.2 eV correspond to the binding energies of Ti2p3/2 and Ti2p1/2 of pristine TiO2,24 which on reduction shifts to 457.6 and 463.2 eV, respectively. This value corresponds to the binding energy of Ti3+.25 The binding energies of Ti2p3/2 and Ti2p1/2 after nitrogen doping has been reported to decrease to 458.5 and 464.3 eV as compared with pure TiO2.18−22 The appearance of binding energies of Ti XPS spectra of R−N−TiO2 and R−B−N−TiO2 at 457.3 and 463.0 eV may be a combined effect of N replacing oxygen and Ti4+ reducing to Ti3+. The different electronic interactions of Ti with N anions cause partial electron transfer from the N to Ti. At the same time, there is an increase of the electron density on Ti due to the lower electronegativity of nitrogen compared with oxygen. This further testifies that nitrogen is incorporated into the lattice and substitutes oxygen.26 Another peak centered at higher binding energy (401 eV) in XPS spectra for the N1s region is controversial. It is hard to identify the origin of this peak (401 eV) from the N1s XPS spectra alone. However, many studies27−30 have concluded that the peak at 401 eV in N1s is due to the oxidized nitrogen of TiO−N, which gets chemically adsorbed on the surface of the catalyst. Thus there are two states for N in the R−N−TiO2. One is doped N in the lattice of TiO2 (N1s, 399 eV) and another deals with chemically adsorbed N on the surface of the catalyst (N1s, 401 eV). The two states of N-doped TiO2 are all thought to affect the photocatalytic activity of N−TiO2 under visible light.31 The EPR spectra were recorded to further confirm the presence of Ti3+ in these samples. The EPR spectra of R−TiO2, R−N−TiO2, and R−B−N−TiO2 samples were recorded at both room temperature and liquid nitrogen temperature. It may be noted that the EPR is insensitive to Ti4+, and hence no EPR signal is observed in unreduced samples. Figure 7 shows the representative EPR spectra (both experimental and simulated) of R−N−TiO2 samples, which showed maximum hydrogen generation among all samples. The EPR spectra of R−N−TiO2

Figure 5. N1s XPS spectra of reduced and pristine samples of N− TiO2 and B−N−TiO2.

TiO2 and R−B−N−TiO2). There is a broad peak from 397 to 403 eV. After fitting of the curve, two peaks are obtained at 399 and 401 eV (insert of Figure 5), which are in the range of 396− 403 eV and considered to be the typical of N-doped TiO2 (N− TiO2) by several researchers.18−23 A detailed qualitative analysis of the XPS spectra indicates that the atomic concentration of N in all N incorporated samples is about 1.5%. Generally, the peak in the range of 396−397 eV can be attributed to Ti−N bonds.18 However, like many other recent published results on N− TiO2,18−20,22 there is no peak at 396 eV in N1s region of Figure 5. Instead of that, the peak at 399 eV can be observed, which is attributed to anionic nitrogen in O−Ti−N linkages by many researchers.18,19 The electronegativity of N, doped into TiO2 lattice, is lower than oxygen leading to the reduction of electron density on the nitrogen. Therefore, the peak at 399 eV due to anionic nitrogen in O−Ti−N linkages is at higher position than 12465

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photocatalytic results reported by Grabowska et al. for B-doped TiO2.34 All of these samples (TiO2, N−TiO2, and B−N−TiO2) showed an improvement of H2 generation after forced reduction, with R−N−TiO2 showing the highest among them. Part b of Figure 8 shows the amount of H2 produced when R−TiO2 and R−N−TiO2 were used as photocatalysts. The amount of hydrogen generated for R−TiO2, and R−N− TiO2 impregnated with Pd, are shown in part c of Figure 8. It can be seen that the H2 generation increases when the cocatalyst Pd is present along with R−TiO2. When Pd was used as cocatalyst in R−N−TiO2, a significant increase of hydrogen yield was observed. However, when nickel oxide (NiO) is impregnated on R−TiO2 and R−N−TiO2 in place of Pd, the amount of H2 produced was less compared to that of Pd−R− N−TiO2 (part d of Figure 8). It is worth mentioning that, among all doped samples after forced reduction, R−N−TiO2 showed increased activity for H2 production and it improved further in the presence of Pd and nickel oxide cocatalysts. No nitrogen evolution was observed when both reduced and unreduced N−TiO2 and Pd−N−TiO2 were used as catalysts for photocatalytic hydrogen generation under the present experimental conditions. This observation suggests that both N−TiO2 and R−N−TiO2 synthesized by the present method are stable under irradiation conditions. The enhanced activity of the R−N−TiO2 sample can be attributed to the increased optical absorption of the sample, which is clearly seen from its optical absorption spectra (Figure 4). The presence of Ti3+ in TiO2 produces anion vacancies in the lattice. These anion vacancies introduce additional levels below the conduction band and, with the increase in concentration of these vacancies, these levels overlap with the conduction band.35 This results in a decrease of the band gap of TiO2. There are different views regarding the role of N in decreasing the band gap of TiO2. One group has proposed that N doping of TiO2 results in the mixing of N 2p and O 2p orbitals, which increases the width of the valence band thereby decreasing the bandgap.36 Another view is that N doping results in the formation of an isolated band within the bandgap, which can result in the absorption of visible light.37 In the present case, the shift of the band edge toward higher wavelengths and the absence of more than one band edge (Figure 4) suggest that the mixing of the N 2p and O 2p states have occurred resulting in the decrease of the bandgap. A schematic illustration of the modification of the conduction and valence bands as a result of Ti3+ and N doping is shown in Figure 9.

Figure 7. EPR spectra (experimental and simulated) of reduced N− TiO2 sample.

displays a weak signal having axial symmetry with g⊥ = 1.9990 and g∥ = 1.9590. The g values for both these signals were found to be almost independent of the nature of sample treatment and temperature. The observed g values are characteristic of a paramagnetic Ti3+ center in a distorted octahedron oxygen ligand field and also agreed well with those reported in literature32 for Ti3+ centers in both anatase and rutile phases. (g⊥ = 1.990, g∥ = 1.957, and g⊥ = 1.975, and g∥ = 1.940 for Ti3+ centers in anatase and rutile phase respectively). Photocatalytic activity of all the samples was tested under sunlight-type radiation using water−methanol mixture (4:1 by volume), where methanol is a well-known sacrificial electron donor.33 Part a of Figure 8 shows the hydrogen produced as a

Figure 8. Photocatalytic activity of different samples plotted as a function of irradiation time.

function of time for pristine and doped TiO2 samples before forced reduction. A notable increase in hydrogen generation was observed for N−TiO2 compared to undoped TiO2. However, the amount of hydrogen generated by B−N−TiO2 sample was slightly lower than that of N−TiO2. The decreased photocatalytic activity may be due to the marginal difference in the optical absorption property of B−N−TiO2 compared to N−TiO2 (Figure 4). This observation is in conformity with the

Figure 9. Schematic presentation of the modification of the conduction band (C. B.) and valence band (V. B.) of TiO2 due to N codoping followed by reduction. 12466

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The enhanced photocatalytic activity of TiO2 in the presence of Pd and nickel oxide (NiO) cocatalyst is due to the interfacial transfer of electrons from TiO2 to the cocatalyst when TiO2 is excited.38,39 This process increases the lifetime of the charge carriers and results in the improved availability of electrons resulting in enhanced photocatalytic activity. Present study indicates that Pd is a better cocatalyst than NiO for the photocatlytic hydrogen generation from water.

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CONCLUSIONS N-doped TiO2, B- and N-doped TiO2, and pristine TiO2 were synthesized by a facile xerogel method followed by a forced reduction in a glycol medium to create additional anion vacancies. Doping reduced TiO2 (R−TiO2) sample with N (R−N−TiO2) and B and N (R−B−N−TiO2) respectively has resulted in the narrowing of the band gap of TiO2. The reason for the reduced band gap in the R−N−TiO2 is due to the contribution of additional levels by Ti3+ near to the conduction band and N to the valence band. As a result, the R−N−TiO2 sample exhibits enhanced absorption of light and improved photocatalytic activity. Doping with both B and N followed by reduction of TiO2 (R−B−N−TiO2) also brings about a decrease in the band gap energy and enhanced photocatalytic activity compared to undoped TiO2 but lower than R−N− TiO2. A significant improvement of the photocatalytic activity of these samples in the presence of Pd cocatalyst is observed due to the efficient separation of photogenerated charges carriers.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +91-22-25590289. Fax: +91-22-25505151. E-mail: [email protected] (O.D.J.). Tel: +91-22-25595330. Fax: +91-22-25505151. E-mail: [email protected] (A.K.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. V.S.K. Chakravadhanula for TEM measurements and helpful discussions. REFERENCES

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