Iron doped nanostructured TiO2 for photoelectrochemical generation of hydrogen

international journal of hydrogen energy 33 (2008) 5363–5368

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Iron doped nanostructured TiO2 for photoelectrochemical generation of hydrogen Aadesh P. Singha, Saroj Kumaria, Rohit Shrivastavb, Sahab Dassb, Vibha R. Satsangia,* a

Department of Physics & Computer Science, Faculty of Science, Dayalbagh Educational Institute, Dayalbagh, Agra 5, India Department of Chemistry, Faculty of Science, Dayalbagh Educational Institute, Dayalbagh, Agra 5, India

b

article info

abstract

Article history:

This paper describes the photoelectrochemical studies on nanostructured iron doped tita-

Received 4 April 2008

nium dioxide (TiO2) thin films prepared by sol-gel spin coating method. Thin films were

Received in revised form

characterized by X-ray diffraction, Raman spectroscopy, spectral absorbance, atomic force

14 July 2008

microscopy and photoelectrochemical (PEC) measurements. XRD study shows that the films

Accepted 14 July 2008

were polycrystalline with the photoactive anatase phase of TiO2. Doping of Fe in TiO2 resulted

Available online 20 September 2008

in a shift of absorption edge towards the visible region of solar spectrum. The observed bandgap energy decreased from 3.3 to 2.89 eV on increasing the doping concentration upto

Keywords:

0.2 at.% Fe. 0.2 at.% Fe doped TiO2 exhibited the highest photocurrent density, w0.92 mA/cm2

Photoelectrochemical

at zero external bias. Flatband potential and donor density determined from the Mott–

Hydrogen

Schottky plots were found to vary with doping concentration from 0.54 to 0.92 V/SCE and

Iron doping

1.7  1019 to 4.3  1019 cm3, respectively.

TiO2

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Sol-gel

1.

Introduction

Titanium dioxide (TiO2) is one of the most favoured metal oxide semiconductor for its use as photoanode in photoelectrochemical (PEC) splitting of water into hydrogen and oxygen, due to its band edges matching with the redox level of the water, relatively low cost, chemical stability, and photostability [1,2]. However, the low efficiency of hydrogen production with TiO2 is mainly due to its large bandgap (w3.2 eV) lying in the UV region of solar radiation, which accounts only for 4% of the incoming solar energy [3,4], rendering the overall process impractical. Another difficulty with TiO2 is the high recombination rate of the photoexcited electron–hole pairs [5]. Thus, for efficient production of hydrogen, it is necessary to not only extend the absorbance of TiO2 into visible regions but also reduce the recombination of photo-generated electrons and holes. This paper is

an effort in this direction to improve the PEC behaviour of TiO2. Recently many studies have been carried out to change the bandgap and thereby electronic properties of TiO2 by doping it with transition metal ions such as Fe, Nb, Mn, Co, Sn, Cd and Ni [6–9]. Presence of metal ion in quantum sized TiO2 is also reported [10] to influence the photoreactivity, charge carrier recombination rates and interfacial electron transfer rates in TiO2. Among the various dopants, substitution of iron (III) ions in the titania lattice is most favoured [11] due to similar size of Fe3þ and Ti4þ ions. Many methods such as sol-gel [12], hydrothermal [13], wet impregnation [14], ion-implantation [15], and metal organic chemical vapor deposition (MOCVD) [16] have been reported for the preparation of Fe doped nanostructured TiO2 catalysts. Most of these papers deal with photocatalytic application [6,13,17–19]. In the present study

* Corresponding author: Tel.: þ91 9319104320; fax: þ91 562 2801226. E-mail address: [email protected] (V.R. Satsangi). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.07.041

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international journal of hydrogen energy 33 (2008) 5363–5368

photoelectrochemical properties of Fe3þ doped nanocrystalline TiO2 thin films with respect to photo splitting of water were undertaken systematically. Thin films of TiO2, doped with Fe3þ, were prepared by simple, economical, sol-gel spin coating technique. The effect of iron doping on PEC behaviour have been studied with respect to absorption edge, bandgap, flatband potential, charge carrier concentration and presented.

counter electrode (platinum electrode) and a reference electrode (saturated calomel electrode, PAR, Model: K0077, USA) immersed in 13 pH NaOH electrolyte. Capacitance (Csc) at semiconductor/electrolyte junction at AC signal frequency of 1 kHz was measured by LCR meter (Agilent Technology, Model: 4263 B, Singapore) at varying electrode potentials.

3.

Results and discussion Crystalline structure of TiO2 thin films

2.

Experimental

3.1.

2.1.

Synthesis of thin films

Fig. 1 shows the XRD (X-ray diffraction) pattern of undoped and 0.2 at.% Fe doped TiO2 thin films. There are weak intensity peaks corresponding to anatase phase of TiO2 for all samples of Fe3þ doped TiO2. The XRD pattern also shows that high crystalline material was obtained for 0.2 at.% Fe doped TiO2. The crystallite size was calculated from the XRD data using Scherrer’s equation for the anatase phase and presented in Table 1. It can be seen from Table 1 that the sizes of TiO2 particles are in nanosize for different concentration of dopant. The obtained patterns also have predominance of sharp, intense peak corresponding to underlayer of SnO2:In.

Titanium tetraisopropoxide (TTIP, 97% pure), ethanol, diethanolamine and iron nitrate (98þ% pure), obtained from Sigma–Aldrich, were used for preparation of thin films. Indium doped tin oxide (SnO2:In) conducting glass was used as substrate. A transparent gel solution of doped/undoped titanium dioxide was prepared by mixing of 3 ml TTIP and calculated amount of iron nitrate in 20 ml ethanol in the presence of diethanolamine. The solution was stirred for 4 h at room temperature to enhance the reaction rate between diethanolamine and TTIP and finally it gets converted into gel. This gel solution was applied on nearly two third area of the conducting glass substrate (1.0  1.0 cm2) and uniformly coated with spin coating unit. Multiple layers were achieved by successive deposition of the gel over the previously deposited layer. After each successive layer the film was allowed to dry for 10 min at 80  C. In the present study all the samples were deposited on conducting glass plate with four layers of coatings to optimize photocurrent density [20]. Final sintering was done at 500  C for 2 h. Doping concentration was varied from 0.1 to 0.5 at.%. Thickness of the film at the edge, as measured by alpha-step (Tencor Alpha step 500) profilometer for four layers of coating was w1.30 mm. A negligible effect of doping concentration on thickness was observed. To utilize TiO2 thin films as photoelectrode, in PEC cell, ohmic electrical contacts were obtained using silver paste and copper wire, from the uncoated area of the conducting glass substrate. The area of contact was later covered with non-transparent and non-conducting epoxy-resin (Hysol, Singapore).

Raman spectroscopy

Doped TiO2 films on ITO glass substrate were additionally analyzed by Raman Spectroscopy. Advantage of this technique over XRD is that Raman is sensitive to the TiO2 film but not to the ITO support, as exciting energy in the near IR region is less penetrating than X-rays. Raman spectra for undoped and 0.2 at.% Fe doped nanocrystalline titania has been shown in Fig. 2. Well-resolved TiO2 Raman peaks were observed at 398 cm1 (B1g), 515 cm1 (Eg), and 640 cm1 (Eg) in the spectra of all samples, indicating that anatase nanoparticles are the predominant species, except for 147 cm1 (Eg), which is suppressed by a much stronger TiO2 peak at 144 cm1 (B1g) line [21]. No effects of dopant and doping concentration were

a: Undoped TiO2 b: 0.2 at.% Fe doped TiO2

*

*

*

*

(200)

*

(004)

The crystalline phases of TiO2 thin films were characterized by X-ray powder diffractometer (Bruker AXS, Model: D8 ˚ ). Raman specAdvanced) using Cu Ka radiation (l ¼ 1.5418 A trum were recorded by a Horiba JY HR 800 Raman system using an Argon 488 nm laser pulse. The surface morphology of the films were examined using atomic force microscopy (Digital Nanoscope 3A). The bandgap energy was determined using absorption data of thin films recorded by UV–Visible Spectrophotometer (Shimadzu, UV-2450, Japan). The photoelectrochemical measurements were carried out using a potentiostat (PAR, Model: Versa state II, USA) and UV–Vis light source (Xenon Arc Lamp, 150 W, Oriel, 66901). The photoelectrochemical measurements involved current– voltage (I–V) characteristics of PEC cell consisting of three electrodes: the working electrode (semiconductor thin films),

(101)

Characterization Intensity (a.u.)

2.2.

3.2.

*

b a

20

25

30

35

40

45

50

55

60

2 (Degree) Fig. 1 – XRD patterns of undoped and 0.2 at.% Fe doped TiO2 thin films deposited on conducting glass plate. *The peaks corresponding to underlying SnO2:In layer on the substrate.

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Table 1 – Measured properties of Fe doped TiO2 thin films at different doping concentration Samples

Bandgap (eV)

Particle Size (nm)

Photocurrent density (mA/cm2 at 0 V/SCE)

Donor density, ND (cm3)

Flatband Potential, VFB (mV/SCE)

3.3 3.1 2.89 3.12

25 30 32 35

0.09 0.21 0.92 0.10

1.70  1019 2.11  1019 4.3  1019 1.86  1019

540 800 920 640

Undoped TiO2 0.1 at.% Fe–TiO2 0.2 at.% Fe–TiO2 0.5 at.% Fe–TiO2

observed on the position of peaks in Raman spectra i.e. anatase structure is retained after doping at different concentrations. This suggests that Fe3þ ions occupy the substitutional sites in the TiO2 lattice [22].

Optical characterization 3.4.

UV–visible spectra clearly show (Fig. 3) that the incorporation of Fe3þ ions into TiO2 matrix leads to a red shift in the optical response, as well as reduction in the bandgap energy. The onset of absorption on doping shifts towards the visible region. The largest shift occurred for 0.2 at.% Fe doped TiO2, starting from 550 nm wavelength of the solar spectrum. The bandgap energy were calculated by Spectrophotometeric absorbance data using following equation [22].
m (1) ahy ¼ A hy  Eg where a is the absorption coefficient, hy is the photon energy in eV, and Eg is the band gap energy in eV. A is a constant related to the effective mass of the electrons and holes and m being equal to 0.5 for allowed direct transition and 2 for an allowed indirect transition. A plot between (ahy)1/2 and hy (for udoped TiO2 shown in the inset of Fig. 3), resulted in a linear graph, which suggested the indirect nature of the bandgap in TiO2 samples. The bandgap energy of nanostructured undoped TiO2 thin film was calculated as 3.3 eV (Fig. 3), which is in good agreement with bandgap for bulk-phase anatase. It was observed to reduce from 3.3 to 2.89 eV with increase in doping concentration (Table 1). The red shift in absorption spectrum and reduction in bandgap are attributed to the transfer of

AFM images of TiO2 for some representative samples have been shown in Fig. 4A and B for undoped and 0.2 at.% Fe doped films. The surface of the films appears uniform and homogeneous. AFM images of TiO2 thin films exhibited granular nanostructured morphology. The primary particles to construct the thin films of TiO2 are observed to be smaller than those of the Fe doped TiO2 thin films. The estimates of the crystallite sizes are in good agreement with the sizes calculated from XRD data using Scherrer’s equation (Table 1).

3.5.

2.0

(515)

(640)

b

a

200

300

400

500

600

700

800

Raman Shift (cm-1) Fig. 2 – Raman spectra of undoped and 0.2 at.% Fe doped thin films.

3.5 3.0 2.5 2.0 1.5 1.0 0.5

1.0

3.0

3.5

4.0

4.5

5.0

Photon energy (eV)

Undoped Tio2 0.1 at% Fe doped Tio2 0.2 at% Fe doped Tio2 0.5 at% Fe doped Tio2

0.5

0.0

300 100

Undoped TiO2

4.0

1.5

Absorbance (a.u)

Intensity (a.u.)

(398)

Photoelectrochemical measurements

Samples of nanostructured TiO2, undoped and doped with iron were used as photoelectrode in PEC cell and current– voltage characteristics under darkness and illumination using a three-electrode system in a quartz cell were recorded. Calculated value of photocurrent density as function of electrode potential is presented in Fig. 5. Photocurrent density in no bias condition is summarized in Table 1. Fe doping in TiO2, was observed to improve the photoresponse (Fig. 5). 0.2 at.%

a: Undoped TiO2 b: 0.2 at.% Fe doped TiO2

(144)

Morphology

( h )1/2 (a.u)

3.3.

3d-electrons from Fe3þ to the conduction band of TiO2. XRD, Raman and UV–visible studies in combination suggest that Fe3þ ions have been incorporated into the lattice of TiO2 nanoparticles, which is inconsistent with results reported by Wang et al. [23].

350

400

450

500

550

600

Wavelength (nm) Fig. 3 – Absorption spectra of Fe doped TiO2 thin films with different doping concentrations. Inset shows the bandgap calculation curve for undoped TiO2 thin film.

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international journal of hydrogen energy 33 (2008) 5363–5368

Undoped Tio2 0.1 at% Fe doped Tio2 0.2 at% Fe doped Tio2 0.5 at% Fe doped Tio2

Photocurrent Density (mA/cm2)

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -1.0

-0.5

0.0

1.0

0.5

Applied Potential (V/SCE) Fig. 5 – Photocurrent density–voltage (V/SCE) characteristics for Fe doped TiO2 at different doping concentrations (at.%) using 150 W UV–Vis light source in 13 pH NaOH electrolyte solution.

The donor density were calculated from the slope of the plot (1/C2sc vs. electrode potential) using the relation: Slope ¼

Fe doped TiO2 exhibited the highest photocurrent density, which was w0.92 mA/cm2 at zero external bias, above and below this doping concentration photocurrent was decreased. To investigate the stability of the electrode prepared, the photoelectrochemical measurements were repeated 12 times after 8 h interval. The photocurrent density was found approximately same up to 8 cycle of operation, after which a decrease was noticed. Donor density and flatband potential of semiconductor at semiconductor/electrolyte junction have been obtained from the Mott–Schottky plot (1/C2sc vs. electrode potential) using the following equation [24–26].   1 2 KB T Vapp  VFB  ¼ (2) 2 Csc q3s 30 ND q where 30 is the permittivity of the free space and 3s is permittivity of semiconductor electrode, q is the charge on the carrier, ND is the donor concentration, T is temperature of operation, KB is Boltzmann’s constant, Csc is space charge capacitance, VFB is flatband potential at semiconductor/electrolyte junction and Vapp is externally applied electrode potential.

(3)

Fig. 6 shows Mott–Schottky plots obtained for all the samples of Fe doped TiO2 under darkness. The flatband potential was increased from 0.54 to 0.92 V/SCE on increasing doping concentration from 0 to 0.2 at.%, but further increase in doping concentration (0.5 at.%) the flatband potential decreased up to 0.64 V/SCE (Table 1). The donor density increased from 1.7  1019 to 4.3  1019 cm3 with increase in doping level from 0 to 0.2 at.% (Table 1). Efficiency of PEC system using TiO2 photoelectrode is mainly limited by its large bandgap [3,4] lying in UV region. The increase in photocurrent density in doped TiO2 may be attributed to shift of bandgap of titania towards the visible 0.12 Undoped Tio2 0.1 at% Fe doped Tio2 0.2 at% Fe doped Tio2 0.5 at% Fe doped Tio2

0.10

1/C2x1012 (cm4F-2)

Fig. 4 – AFM images for: (A) undoped TiO2 (B) 0.2 at.% Fe doped TiO2.

2 30 3s qND

0.08

0.06

0.04

0.02 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Applied Potential (V/SCE) Fig. 6 – Mott–Schottky plots for Fe doped TiO2 with different doping concentrations (at.%) at frequency 1 kHz.

international journal of hydrogen energy 33 (2008) 5363–5368

region by changing its electronic properties through the formation of a shallow trap within the titania matrix. These shallow traps created within the forbidden gap of titania due to doping act as traping center for photogenerated electron– hole pair [27], which may increase the photogenerated electron–hole charge recombination times, thereby increasing PEC response. However, decrease in photoelectrochemical response of doped TiO2 for too heavy concentration of Fe3þ, is probably because at high doping concentration the recombination of photogenerated electron–hole pairs in semiconductor become easier. Moreover, the energy levels of the Fe4þ/Fe3þ couple is just above the titania conduction band and the energy level of the Fe3þ/Fe2þ couple is just above the valence band [28]. Thus, the photoresponse of doped TiO2 is strongly dependent on the dopant concentration since the Fe3þ dopant can serve as a mediator of interfacial charge transfer and also as a recombination center. Doping with 0.2 at.% Fe may be the optimal doping concentration for Fe3þ ions in TiO2 nanoparticles for the separation of photogenerated electron–hole pairs. At this optimal concentration of dopant ion, the thickness of space charge region may be nearly equal to the light penetration depth. Increase in charge carrier density with doping may also be one of the factors (Table 1) which favour the PEC response. The more negative the flatband potential, better is the ability of semiconductor film to facilitate the charge separation at interface. It can be seen that 0.2 at.% Fe doped TiO2 exhibited the maximum flatband potential, which also supports the maximum photocurrent density exhibited by this sample.

4.

Conclusion

Fe doped TiO2 thin films prepared by sol-gel method were used as photoelectrodes in photoelectrochemical cell for solar water splitting. The incorporation of Fe3þ ions in TiO2 matrix reduced the bandgap of TiO2 from 3.3 to 2.89 eV, increased flatband potential, donor density, which in turn increased the photocurrent density upto w0.92 mA/cm2 at zero external bias. The present study reveals that the nanosize Fe doped TiO2 with optimal doping of 0.2 at.% may be a promising solar energy harvesting material for applications in photoelectrochemical splitting of water for generation of hydrogen.

Acknowledgements This research has been supported by Department of Science and Technology, Government of India, New Delhi, wide project No. SR/S2/CMP-47/2005. We are thankful to InterUniversity Accelerator Centre, New Delhi for providing the characterization facilities.

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