Thermal formation of silicon-doped TiO2 thin films with enhanced visible light photoelectrochemical response

Electrochemistry Communications 16 (2012) 26–29

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Thermal formation of silicon-doped TiO2 thin films with enhanced visible light photoelectrochemical response Mingxuan Sun a, Xiaoyan Zhang a, Jing Li a, Xiaoli Cui a,⁎, Dalin Sun a, Yuehe Lin b a b

Department of Materials Science, Fudan University, Shanghai, 200433, China Pacific Northwest National Laboratory, Richland, Washington, 99352, USA

a r t i c l e

i n f o

Article history: Received 30 November 2011 Received in revised form 19 December 2011 Accepted 19 December 2011 Available online 24 December 2011 Keywords: Titania Si-doped Thermal treatment Visible light response Photoelectrochemical

a b s t r a c t Silicon-doped TiO2 thin films were fabricated by annealing titanium metal sheet embedded in SiO2 powders and characterized by X-ray photoemission spectroscopy and photoelectrochemical measurements. The results showed that the content of silicon in the doped TiO2 thin films was proportional to the annealing time and temperature. Enhanced visible light response, more negative flat band potential and higher carrier density were demonstrated by the electrochemical measurement. The technique proposed in this paper can be also applicable to fabricate other doped TiO2 thin films based on the corresponding oxide bath. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Titanium dioxide (TiO2) has been widely studied in recent years [1–4], but its full potential application was hampered by activation only under ultraviolet (UV) light irradiation. Doping TiO2 with metal and non-metal ions has been demonstrated to be an effective route to extend the absorption of TiO2 to visible light region. Significant studies incline to dope TiO2 with nonmetals elements, such as silicon [5–9], nitrogen [10–13], carbon [14–17], sulfur [18,19], boron [20,21] and so on [22]. Among the nonmetal dopants, Si is one of the appropriate extensions of visible light photocatalytic activity of TiO2 films. Su et al. [5] successfully prepared Si-doped TiO2 nanofilm using chemical vapor deposition. Enhanced photocurrent density and higher photocatalytic property was demonstrated. Yang et al. [8] studied the electronic properties of substitutional Si-doped TiO2 and indicated that Si-doped TiO2 had a band gap narrowed about 0.25 eV than that of TiO2. Bao et al. [9] prepared Si-doped mesoporous TiO2 continuous fibers by a sol– gel method combined with centrifugal spinning. They found that the presence of suitable amount of silicon into TiO2 could improve the surface texture and enhance the thermal stability and crystal stability. Though various methods have been also developed for the preparation of Si-doped TiO2 [6,7], some of these procedures involve expensive equipment or multiple steps.

⁎ Corresponding author. Tel./fax: + 86 21 65642397. E-mail address: [email protected] (X. Cui). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.12.015

In the present work, we proposed a novel and simple method based on one-step “oxide bath” for the preparation of Si-doped TiO2 thin films by thermal treatment of titanium metal plates embedded in silicon dioxide powders. Silicon can diffuse and dope into the TiO2 matrix during the formation of TiO2 itself. Enhanced photocurrent density of resulted thin film was demonstrated by photoelectrochemical measurements. 2. Experimental section Titanium sheets (0.1 mm thickness, 99.6%), nitric acid, hydrofluoric acid (HF, 40%), silicon dioxide (analytical reagent) were used as received. Deionized water was used to prepare all the solution. The titanium sheets were mechanically polished to mirror and then chemically etched in an HF (40%)-HNO3 (65%)-H2O (1:4:5 in volume) solution for 20s. The cleaned Ti sheets were partly embedded in SiO2 powders in porcelain crucible. The thermal treatment was carried out in muffle furnace at atmospheric pressure in air by heating the samples for various times (1, 2, 3, 6 and 15 h) at 400, 450, 500, 550 °C, respectively. A RBD upgraded PHI-5000 C ESCA system (Perkin Elmer) with Al/Mg–K radiation was used to measure X-ray photoelectron spectroscopy (XPS) at pass energy of 93.9 eV, the binding energies were calibrated based on the containment carbon (C1s 284.6 eV). The photocurrent measurement was performed in a three-electrode cell on a CHI 660 electrochemical workstation (Shanghai Chenhua Instruments, China). The working electrode was illuminated with a 500 W Xenon lamp (CHF-XM35, Trusttech Co., Ltd. Beijing), from which infrared wavelengths were removed by a quartz water filter,

M. Sun et al. / Electrochemistry Communications 16 (2012) 26–29

and the UV–vis irradiance was measured to be 140 mW·cm− 2. The light was further passed through an optical filter, which cut off wavelength below 420 nm. The visible light intensity was 90 mW·cm− 2. The electrochemical impedance spectra (EIS) and the flat band potential measurement were carried out with EG&G (EG&G5210) Potentionstat/Galvanostat Model 273A. The impedance measurement was recorded over a frequency range from 100 kHz to 100mHz. The flat band potential was measured by the Mott–Schottky analysis method at a scanning rate 50 mV.s − 1 with the potential range from −0.8 to 1.5 V at 100 Hz. 3. Results and discussion XRD patterns were recorded for the resulted samples (not shown). Most of the diffraction peaks came from the Ti substrate. It was difficult to obtain complete diffraction peak of TiO2 for the samples through thermal treatment of titanium plates [5,23]. Ti peaks with obviously much stronger intensity also implied that the thermal resulted TiO2 film is too thin. The presence of TiO2 was confirmed by XPS measurements. Fig. 1 illustrated the XPS spectra of Si-doped TiO2 samples, which exhibited the presence of C, O, Si and Ti elements. The peak of C1s corresponds to C\H and C\C bond of hydrocarbons, which caused by the containment carbon. The peak for Si 2p around 101 eV is very tiny for all samples which may be caused by the little Si content (Fig. 1A). The content of silicon can be detected to be 0.8at.%, 0.9at.%, 1.4at.% and 1.7at.% for the samples obtained at 400, 450, 550 °C for 2 h and at 450 °C for 15 h, respectively.

A

intensity(a.u.)

O 1s Ti 2p

d c b a

C 1s Si 2p

600

500

400

300

200

100

EB/eV

B

intensity(a.u.)

Si 2p

c b a d 108

105

102

99

EB/eV Fig. 1. XPS spectra of survey spectrum (A), Si 2p (B) for Si-doped TiO2 thin films; The samples obtained at 400(a), 450(b), 550 °C(c) for 2 h and at 450 °C (d) for 15 h, respectively.

27

Obviously, the content of silicon in the Si-doped TiO2 samples is proportional to the thermal treatment time and temperature. To further investigate the states of the doped Si elements, high resolution of Si 2p XPS spectra of the samples were recorded and are shown in Fig. 1B. The binding energy of Si 2p for Si-doped TiO2 was 102.6 eV, 0.8 eV smaller than that of pristine SiO2, which was reported to be 103.4 eV [24]. Since the electronegativity of Si 4 + (1.80) is higher than that of Ti 4 + (1.54), the lower binding energy of Si 2p was due to the formation of the Si\O\Ti bonds which causes a less positive charge on Si atoms, as compared to that of pristine SiO2. This can be an indication of strong interaction of silicon atoms with TiO2 lattice. In addition, the ionic radium of Si 4 + (0.040 nm) is smaller than that of Ti 4 + (0.061 nm), which allows Si 4 + to move into TiO2 matrix easily. Therefore, Si 4 + can be introduced to the matrix of TiO2 and form Si\O\Ti bonds [25] in the resulted samples. The transient currents were measured for samples prepared at different temperatures. As shown in Fig. 2A, the photocurrent density of doping samples increased from 2.53 to 24.9 μA ·cm − 2 as the annealing temperature was raised from 400 to 450 °C and then it decreased from12.6 to10.1 μA·cm − 2 by changing temperature from 500 to 550 °C. The highest photocurrent density was 24.9 μA·cm − 2 for the sample prepared by thermal treatment at 450 °C for 6 h. The influence of thermal time to the photocurrent response was also investigated. Fig. 2 C presented the photocurrent response of the samples obtained at 450 °C with different thermal time. The photocurrent is 19.0 μA ·cm − 2 for the sample annealed for 1 h. Enhanced photocurrent was observed with the increasing of thermal time and the highest photocurrent value was 31.8 μA·cm− 2 when the annealing time is 2 h. Further increasing the thermal time to 3 h, the photocurrent decreased to 27.1 μA·cm− 2.This may be related to the increasing of the thickness of thin films which was proportional to the thermal treatment time and temperature. Photogenerated electrons conductivity became worse due to formation of a thicker and more aged oxide film on the electrode as the time or temperature increased [26,27]. Fig. 2B and 2D showed the comparison of photocurrent of Si-doped TiO2 and pristine TiO2 thin film electrodes. It can be seen that the photocurrent of Si-doped TiO2 electrode was larger than that of TiO2 thin film electrodes. It demonstrated that the introducing of Si into the TiO2 thin films could increase the photoresponse. Fig. 3 illustrated the photoelectrochemical behavior under visible light illumination. The photocurrent densities were 0.114 and 0.106 μA·cm − 2 for the doped samples prepared at 450 and 550 °C, whereas the values were 0.0819 and 0.0624 μA·cm − 2 for the undoped ones, respectively. Enhanced photoelectrochemical response of the Si-doped electrodes can be observed under visible light illumination. Si dopant may be more appropriate for the extension of visible light photoelectrochemical activity of TiO2. Electrochemical impedance spectroscopy (EIS) provides a powerful method for the study of charge transfer and recombined processes at the semiconductor/electrolyte interfaces [28–31]. Typical EIS Nyquist and Bode plots of the TiO2 electrodes are shown in Fig. 4A and 4B. The value of Rct (the electron-transfer resistance) was very large as implied by the large arc (an approximate line). It can be found that Si-doping promoted a decrease of the charge-transfer resistance, as implied by the smaller size of the arc diameter in Nyquist plot compared to TiO2 at the same condition. With the value of Rct decreasing, the charges transfer across the interface between the semiconductor and solution becomes easy, thus the value of photocurrent can be improved. This is consistent with the photocurrent results (Figs. 2B, 2D and 3). The corresponding Bode plot provides an obvious indication for decrease of impedance. As shown in Fig. 4B, the value of \Z\ is smaller for Si-doped TiO2 compared to pristine TiO2. The introduction of Si provides a more effective separation of electron–hole pair and/or a faster interfacial charge transfer to the electron donor/acceptor [32,33]. Fig. 4C presented the Mott–Schottky plots of pristine TiO2 and Si-doped TiO2 films under dark conditions. The value of flat band

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M. Sun et al. / Electrochemistry Communications 16 (2012) 26–29

A

C

70

60

50

Photocurrent/(µA/cm2)

Photocurrent/(µA/cm2)

60

40 30

b

20

c d a

10 0 -10 0

on

off

on

off

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100

150

200

40

b c d e a

20

0

on

off

on

off

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100

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-20

250

0

Time/s

B

D

42

TiO2 24

TiO2

35

Si-doped TiO2

Photocurrent/(µA/cm2)

Photocurrent/(µA/cm2)

250

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8

Si-doped TiO2 28 21 14 7 0

0 400

450

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550

2

4

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Fig. 2. (A) Transient photocurrent curves of Si-doped TiO2 thin film electrodes prepared after annealed for 6 h at 400(a), 450(b), 500 (c) and 550 °C (d), respectively.(C) Transient photocurrent curves of Si-doped TiO2 thin film electrodes prepared at 450 °C annealed for 1 (a), 2(b), 3(c), 6(d), 15 h (e), respectively.(B), (D) The comparison of photocurrent of Si-doped TiO2 and TiO2 thin film electrodes prepared at same condition.

Photocurrent/(µA/cm2)

A

potential (Vfb) is approximately equal to the intersection of the tangent of Mott–Schottky curves and the potential axis. The Vfb of Si-doped TiO2 (− 0.64 V) was more negative than that of TiO2 (−0.58 V),which means smaller barrier to charge transfer after Si-doping. Charge carrier densities (Nd) were calculated according to the following equation.

0.20 0.15

a 0.10

b

1 ¼ C 2SC

0.05

0

on

off

50

100

on 150

off 200

250

Time/s

Photocurrent/(µA/cm2)

2 εε0 qNd

  KT E−Efb − q

0.00 -0.05

B



0.20 0.15

a

0.10

4. Conclusions

b

0.05 0.00

on

-0.05 0

50

off 100

on 150

off 200

Where E is the applied potential (V), ε is the dielectric constant, ε0 the permittivity of free space (F·cm − 1), q is the electronic charge (C), Efb is the flat band potential (V), Csc is the capacitance of the space charge layer. The Si-TiO2 sample showed a charge carrier density of 1.3 × 10 19 cm − 3, which is larger than the values obtained for TiO2 sample (6.2 × 10 18 cm − 3). These values were calculated assuming a relative permittivity of TiO2 ε = 80 according to the literature [34]. The increase in charge carrier densities was caused by Si-doping into TiO2 lattice.

250

Time/s Fig. 3. Transient photocurrent of Si-doped TiO2 (a) and TiO2 thin film electrodes (b) under visible illumination. The samples prepared at 450 °C (A) and 550 °C (B) for 6 h.

In conclusion, we have first reported that “solid-state oxide bath” method can be directly used to prepare the Si-doped TiO2 thin film from Ti sheets and SiO2 powders. Enhanced response in the visible light region, more negative flat band potential and higher carrier density were demonstrated for the doped samples compared to the undoped ones. XPS results showed that the content of silicon in the Si-doped TiO2 films was proportional to the thermal treatment time and temperature. Such a novel and simple fabrication technology demonstrated in this work is also feasible to the preparation of

M. Sun et al. / Electrochemistry Communications 16 (2012) 26–29

A

other doped TiO2 based on the corresponding oxides. This work may open a new avenue to prepare doped TiO2 materials for efficient utilization of solar energy toward practical application.

1500

.cm2)

1200

-Z"/(

29

Acknowledgments

a

b

This work was supported by the National Basic Research Program of China (Nos. 2011CB933300, 2012CB934300) and the Shanghai Science and Technology Commission (No. 1052nm01800). Y. Lin acknowledges the financial support by a LDRD program at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for US-DOE under Contract DE-AC05-76RL01830. We also appreciate the referee's very valuable comments, which have greatly improved the quality of the manuscript.

900

600

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References 0

0

300

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900

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Z'/( .cm2)

B 6

5

log( Z )

a 4

b 3

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log(f/Hz)

C

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10-11C-2/(cm4.F-2)

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-0.5

0.0

0.5

1.0

1.5

Potential(V vs SCE) Fig. 4. Nyquist (A), Bode (B) and Mott–Schottky (C) plots of TiO2 (a) and Si-doped TiO2 (b) thin film electrodes in 0.5 M Na2SO4 under dark condition. The impedance was measured at 0.0 V vs Ag/AgCl and Mott–Schottky plots were measured at 100 Hz.

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