- Email: [email protected]
titanium composites
Available online at www.sciencedirect.com
Composites: Part A 39 (2008) 690–696 www.elsevier.com/locate/compositesa
Microstructure promoted photosensitization activity of dye-titania/titanium composites Yibing Xie a
a,*
, Limin Zhou
a,*
, Haitao Huang b, Jian Lu
a
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong b Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong
Received 26 March 2007; received in revised form 16 August 2007; accepted 20 August 2007
Abstract Well-constructed titania reactive layer/titanium metal–matrix composites were fabricated for photosensitization substrate application by a controlled electrochemical anodization process. A low-voltage anodization at 20 V produced titania nanotube array with 60–70 nm diameter and 540 nm height, while a high-voltage anodization at 180 V resulted in titania multiporous film with 170–260 nm pore size and 4 lm thickness. Two types of dye-titania/titanium composite electrodes were also prepared through a surface adsorption modification by orange G dye. The surface morphologies and interfacial electric properties of heterogeneous materials were examined by microstructure characterization and electrochemical impedance spectroscopy analysis. The significant decrease of interfacial charge-transfer resistance from 12,000 X for microporous titania to 5100 X for nanotubular titania accordingly contributed a much lower electrode impedance and higher polarization current for dye-titania/titanium electrode. Additionally, 4 and 1.5 times magnification of photocurrent density and photovoltage were achieved under a visible light irradiation for nanotubular dye-titania/titanium electrode in comparison with microporous one. A better photosensitization activity could be well obtained by tailoring microstructure from micro-composite to nano-composite. The corresponding photocurrent response was more significant than photovoltage in the photosensitivity evaluation of these composite electrodes. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Metal–matrix composites (MMCs); B. Microstructure; B. Chemical properties; E. Surface treatments
1. Introduction Nanocrystalline titanium dioxide has been recognized as one of the most important semiconductor materials for photoelectrochemical applications [1,2]. In the area of photosensitization, various organic and inorganic sensitizers have been exploited for heterogeneous modification to extend the spectral response of titania (TiO2) from UV to visible light range [3]. Actually, in addition to these sensitizers, the photosensitization activity is also very dependant on the properties of TiO2 functional layer, such as single crystal or polycrystalline structure, particulate or porous *
Corresponding authors. E-mail addresses: [email protected] (Y. Xie), mmlmzhou@inet. polyu.edu.hk (L. Zhou). 1359-835X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2007.08.020
film and even particle size [4]. The multiporous geometries are typically preferred as they provide a higher interfacial reaction area [5]. Concerning construction of traditional photoelectrochemical electrode, various TiO2 sol–gel films were immobilized on indium–tin-oxide or fluorine doped tin oxide conductive glass substrate by surface dip-coating technology to act as a working electrode [6]. However, an irregular arrangement of these TiO2 particulates usually diminished their valid interfacial areas. Furthermore, weak bonding structure between TiO2 and conductive glass probably inhibited interfacial electron-transfer, which finally affected photoelectric efficiency. So, architecture modification of composite electrodes became very reasonable by tailoring TiO2 nanocrystallites and their supports so as to promote photoelectrochemical performance. One of the effective ways was to convert the disordered oxides
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
layer into uniform nanostructures such as nanorods, nanowires or nanotubes, since these structures contributed both better electron-transfer and larger surface area than planar bulk films [7,8]. Recently, direct electrochemical anodization technique has been well developed and become one of simple, but feasible methods to synthesize differently structured oxide films based on various metal materials such as aluminum, titanium, zirconium, and so on [9,10]. The photoelectrochemical activity of these composites were also studied [11,12]. In this study, we mainly investigate photosensitization activity of dye modified titania/titanium (TiO2/Ti) composite electrodes under a weak visible light illumination. The well-constructed dye-titania composite acts as a functional reaction layer along with titanium metal as an electric conducting matrix, which would be synthesized by a controlled one-step anodizing process. Additionally, 7-hydroxy-8phenylazo-1,3-naphthalenedisulphonic disodium (orange G), as an ordinary anionic dye, was used as a photoactivating probe for photosensitization evaluation. The microstructure effect of TiO2 on the photosensitivity has been examined through a full measurement of photocurrent and photovoltage response. 2. Experimental Anodization experiments were carried out in a cylindrical polytetrafluoro ethylene (PTFE) cell equipped with Agilent N5751A DC power supply for a potentiostatic control and a two-electrode configuration, in which the pretreated titanium sheet was used as a working electrode for an anode role and a platinum sheet acting as a counter electrode. Two types of TiO2/Ti composites were fabricated from titanium precursor by different potentiostatic anodization in fluoride contained phosphoric acid electrolytes (HF (0.03 M)–(H2SO4 (1.0 M)–H2O2 (0.6 M)–H3PO4 (0.3 M) and HF (0.15 M)–H3PO4 (0.5 M)). Anodization processes involving either a high-voltage of 180 V or a low-voltage of 20 V for 40 min were applied for titanium metal oxidation. Then, a post-treatment by calcination at 450 °C for 2 h was followed to achieve a complete crystallization for the low-voltage anodized TiO2. Finally, dye adsorption modification on the surface of TiO2/Ti was achieved by ultrasonic treatment for 30 min and soaking for 24 h in 10 mM orange G of phosphate buffered saline solution (PBS, pH 6.8). Field emission scanning electron microscope (FESEM, JEOL JSM-6335F) was used to investigate surface morphology and microstructure. X-ray diffraction (XRD, Philips PW3020) were conducted to determine the crystal phase. Photosensitization experiments were carried out in a quartz reactor equipped with an electrochemical workstation (CHI 660C, CH Instruments Co., Ltd.) and a standard three-electrode configuration, in which dye-TiO2/Ti composites was used as a working electrode, a platinum sheet as a counter electrode and a standard saturated calomel Hg/Hg2Cl2 electrode (SCE) as a reference electrode. PBS with 1.0 mM methanol
691
was used as an electrolyte solution. Additionally, a visible light was generated from a cold light source equipped with a tungsten halogen lamp and UV–IR cut-off filters, whose average light intensity was only 1.93 mW cm 2 in the main emission range of 400–800 nm. This weak visible light was applied to vertically irradiate the surface of the dyeTiO2/Ti photoanode through the quartz window of the reactor. 3. Results and discussion 3.1. Morphological characterization Considering the potentiostatic anodization at 180 V, which was the highest reachable voltage in our experiment, a micron-sized multiporous TiO2 layer was formed with the entire film thickness of 4 lm. These micropores were irregularly distributed on Ti matrix and most pore sizes were determined in the range of 170–260 nm (see Fig. 1A, and B). However, in view of potentiostatic anodization at 20 V, a highly-ordered and vertically-aligned TiO2 nanotube array with unique open-mouth pores was directly formed on the surface of Ti metal–matrix. The inner diameter of individual tubule was about 60–70 nm and its height was approx 540 nm (see Fig. 1C–F). Intensive anodization voltage of 180 V, was much higher than the critical sparking voltage of approx 110–120 V for titania semiconductor. The electrochemical oxidization of titanium was actually a predominant process when compared to the dissolution of titania and thick film of titania could eventually be produced. Meanwhile, the high-voltage anodization could also cause the cycling breakdown-regeneration of titania barrier layer based on titanium, which would contribute the formation of microporous structure. Regarding the tailored anodization at a voltage of 20 V, the dynamic equilibrium process between electrochemical formation and chemical dissolution of titania in a fluorate-containing solution could be well established by controlling the reaction voltage, electrolyte composition and concentration. The preferential oxidation–dissolution reaction was mostly located at these initially formed pits on the titania surface layer, which finally led to the formation of titania nanotubes. So, intensive oxidation process of titanium at a high-voltage would produce a porous thick film in a micrometer scale, while moderate oxidation–dissolution equilibrium process at a low-voltage could form a uniform thin layer with nanotubular structure. Surface morphology could be purposely manipulated by controlling electrochemical anodization parameters. An enhanced degree of dye adsorption on this TiO2 nanotube array would be expected due to the high aspect ratio of their nano-sized independent channels. Additionally, the as-anodized TiO2 nanotubes only exhibited an amorphous phase and it could be completely transformed into anatase phase only after the post-calcination treatment at high temperature. However, high-voltage anodization above the sparking voltage of TiO2 semiconductor could lead to a direct crystallization
692
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
Fig. 1. (A) Plan-view and (B) cross-sectional view of FESEM images of high-voltage anodic TiO2/Ti composite, (C) plan-view, (D) enlarged plane-view, (E) cross-sectional view and (F) enlarged cross-sectional view of FESEM images of low-voltage anodic TiO2/Ti composite.
during the fabrication process of TiO2 layer (XRD figures not shown here). Since either TiO2 nanotube array or microporous film was directly grown on the Ti metal through an anodization process, these TiO2 layers could tightly connect with Ti matrix to form an integrative structure. Such TiO2/Ti electrodes exhibited a superior mechanical bonding strength in comparison with common TiO2 sol–gel film/conductive glass electrodes. Moreover, TiO2 nanotube array provided a much higher reactive areas than micron-sized multiporous film, which would benefit interfacial adsorption of dye molecules on the surface of TiO2/Ti electrode. 3.2. Impedance spectroscopy analysis The electrochemical impedance spectroscopy is one of the powerful approaches for investigating internal resistances attributable to charge transfer processes [13]. In this study, impedance spectrum analysis had been applied to
explore the electric conductivity of TiO2/Ti electrodes at 0 V vs. SCE over the frequency range of 0.01– 100,000 Hz. Along with a continuous increase of the working frequency, impedance values of both composite electrodes were dramatically decreased at a low frequency stage below 1 Hz and then gradually achieved to a steady value above 1000 Hz. Comparatively, the complex impedance of nanotubular TiO2/Ti electrode was completely lower than that of microporous one in the full frequency range (see Fig. 2B). In addition, an obvious difference of semicircle part corresponding to the electron-transfer limited process was observed in complex impedance diagrams, whose diameter represented the charge-transfer resistance at the composite electrode interface. The interaction resistance was 5100 X for nanotubular composite electrode and 12,000 X for microporous composite electrode (see Fig. 2B). Additionally, the impedance values had obviously increased for both dye-TiO2/Ti electrodes in comparison with that of their respective TiO2/Ti electrodes, which
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
6
5
4
a
3
b 2
1 -2
-1
0
1
2
3
4
5
Log Frequency(Hz)
B 5000 -Z''Imaginary (ohm)
4000
3000
2000
3.3. Photoelectrochemical polarization analysis
a
1000
b
0 0
2000
4000
6000
8000
10000
12000
Z'real (ohm)
C 40000 30000
Z'' imaginary (ohm)
exhibited a similar result as TiO2/Ti electrode (see Fig. 2C). Therefore, a significant decrease of complex impedance could be achieved for both TiO2/Ti and dyeTiO2/Ti electrodes by modifying their microstructure from the micropores to nanotube array. Additionally, the linear part corresponding to the mass-transfer limited process did not appear at low frequencies in Nyquist plots. It means the whole photoelectrochemical system was not a diffusion-controlled, but a charge-transfer-controlled process. The entire impedance is resulted from dominant resistances of charge flow across dye/TiO2, TiO2/electrolyte and TiO2/Ti interfaces, subordinate resistances of electrolyte solution, Ti matrix and counter electrode, and low capacitances of TiO2 accumulation layer and Helmholtz double layer [14,15]. The impedance value mostly reflects the overall efficiency of interfacial electron-transfer and shift process in a certain degree [15]. Lower impedance usually means better charge transfer and electronic conductivity for dye-TiO2/Ti nano-composite electrode. Such an ordered arrangement of nanotubular channels ultimately contributed to prompting electron shift as well as photosensitivity in the photoelectrochemical process.
b
Linear sweep voltammetry analysis was applied to examine photoelectrochemical polarization behaviors of dye-TiO2/Ti composites as shown in Fig. 3. The similar polarization current curves could be achieved without visible light illumination for both dye-TiO2/Ti composite electrodes, whose current densities could be negligible due to a weak interaction in a pure electrochemical process. However, obvious photoelectrochemical polarization currents were observed under visible light illumination, which were completely ascribed to the external potential-promoted photoelectric response. The polarization current density was correspondingly enhanced along with a continuous increase of anodic bias potential above 0 V vs. SCE since
20000 6
10000
a 0 0
10000
20000
30000
40000
50000
60000
Z'real (ohm) Fig. 2. (A) Bode plot of TiO2/Ti and complex impedance diagrams of (A) TiO2/Ti and (B) dye-TiO2/Ti composite electrodes with (a) microporous and (b) nanotubular structure at 0 V vs. SCE over a frequency of 0.01– 100,000 Hz.
was mostly ascribed to the additional interfacial resistance of the charge-transfer. Moreover, the nanotubular dyeTiO2/Ti electrode showed a lower impedance response than microporous one in the whole frequency range, which
Polarization current (μA cm-2)
Log Impedance (ohm)
A
693
a
Microporous dye-TiO2/Ti in dark
5 b c
Nanotubular dye-TiO2/Ti in dark
4 d
Nanotubular dye-TiO2/Ti with visible light
Microporous dye-TiO2/Ti with visible light
3
d
2 b
1
a 0 0.0
0.3
0.6
0.9
c 1.2
1.5
Anodic potential vs. SCE (V) Fig. 3. Photoelectrochemical polarization current curves of dye-TiO2/Ti composite electrodes.
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
3.4. Photosensitization activity Concerning dye-TiO2/Ti photosensitization, the apparent efficiency of electron-transfer mainly depends upon the surface microstructure of TiO2 photoanode when the same sensitizer is used. Both photocurrent and photovoltage responses are regarded as key parameters to characterize the overall photosensitization activity [17]. To study the photo-induced charge separation and electron shift on dyeTiO2/Ti composites, both photocurrent under short-circuit condition and photovoltage under open-circuit condition were measured in PBS electrolyte with 1.0 mM methanol under visible light pulsed-irradiation. Regarding both dye-TiO2/Ti composites, the responsive currents were only constant at about 0.12 lA cm 2 under dark condition when the electrode potential was controlled at a bias potential of 0 V vs. SCE (also equal to 0.268 V vs. standard hydrogen electrode). It means that pure electrochemical interaction was very insignificant under such a low potential condition. Actually, this dark current response was more regarded as the noise current in a process of the photosensitization evaluation. However, in the case of visible light irradiation, photocurrent density was drastically soared up to a high level of 0.45 lA cm 2 for micro-composite and 2.11 lA cm 2 for nano-composite, respectively. After removing visible light, this current immediately descended to initial value again as dark current (see Fig. 4). Higher photocurrent means more photoinduced electrons have been effectively transferred from dye-TiO2/Ti photoanode to counter electrode via external circuit. The experimental results also indicate that an obvious photocurrent response could well occur even in the case of a low bias potential of 0 V vs. SCE and a higher photocurrent response could be expected on the condition
2.5
b Photocurrent (μA cm-2)
this applied potential has already exceeded the optical flatband potential of TiO2 ( 0.18 V vs. SCE) [16]. Comparatively, an obviously higher photoelectrochemical current density was obtained for nanotubular dye-TiO2/Ti than microporous one under the same anodic potential. It means that morphologies and structures of TiO2 could influence the overall photoelectrochemical reactivity for dye-TiO2/Ti composite electrodes. Additionally, owing to the electrolyzing oxidation reaction of dye molecules, a sharp increase of polarization current was observed at an anodic potential above approx 1.2 V for TiO2/Ti nanocomposite electrode rather than micro-composite one, whose difference was ascribed to the microstructure effect. Generally, in the range of a low bias potential, the photoelectrochemical response was mostly ascribed to photocurrent rather than pure electrochemical current since the applied anodic potential was much below the critical potential of water decomposition and dye electro-oxidation due to the overpotential effect on TiO2/Ti composite electrode. Therefore, a suitable safe bias potential should be controlled far below this critical value for the sake of a credible photosensitization evaluation.
b
2.0
1.5 light-off
1.0
light-on
light-on
light-off
a
0.5
a
0.0 0
50
100
150
200
Time (s) Fig. 4. Photocurrent responses of: (a) microporous and (b) nanotubular dye-TiO2/Ti composite electrodes under pulse illumination of visible light.
of a higher anodic potential applied to suppress electron relaxation and recombination. Noticeably, owing to about 4 times in magnification of photocurrent density for two types of dye-TiO2/Ti composite electrodes, the total photosensitization efficiency had been intensively improved by means of a microstructure transformation from micropores to nanotubes. In addition, the photovoltage had also been measured under open-circuit condition to investigate photosensitization response of dye-TiO2/Ti composites (see Fig. 5). Once light source was turned on, the potential was drastically decreased till to a steady value after 200 s. It means that most excited photoelectrons had been transferred and accumulated on the conduction band of TiO2, which accordingly contributed the formation of the space charge layers along the pore channels. On the condition of lightoff, this potential gradually increased up to the initial value again. It indicates that these conduction band electrons
0.09 light-on light-on 0.06
Potential vs. SCE (V)
694
0.03
0.00
a
a
b
b
-0.03
-0.06
light-off
light-off 0
200
400
600
800
Time (s) Fig. 5. Photovoltage responses of: (a) microporous and (b) nanotubular dye-TiO2/Ti composite electrodes under pulse illumination of visible light.
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
must have suffered a relaxation decay through a recombination process with cationic radicals, which was due to a strong electric field distribution within the depletion layers. Significantly, a higher photovoltage was achieved for the nano-composite (0.134 V) rather than the mciro-composite (0.091 V), which was equal to about 1.5 times in magnification. It means that ground-state electrons of dye molecule could be more effectively photogenerated, separated and finally transferred to TiO2 conduction band on the both sides of tube walls, whose architecture was more favorable than microporous structure. The photovoltaic response was an important parameter other than the photocurrent response for the characterization of photosensitization activity. A higher photovoltage means a more feasible accumulation of photo-generated electrons on the conduction band of titania regardless of photoelectron relaxation decay from the excited state to ground-state. Accordingly, a higher photosensitization activity was achieved for the nano-composite photoanode. This result highly agreed with above photocurrent measurements. Regarding the photosensitization response, two composite photoanodes exhibited a similar varying law. However, the overall magnification times of photovoltage was much lower than that of photocurrent, which was mostly due to the higher probability of charge recombination without external applied potential on TiO2/Ti composite electrodes. It should be noted that this is only a very primary research of photosensitization activity and such a photosensitivity is expected to be further improved by optimizing dye sensitizers, TiO2 structure, redox electrolyte and photon flux. In view of photosensitization effect of dye-sensitized TiO2 photoanode, charge recombination process could predominantly occur between photo-induced electrons and dye cationic radicals although photoelectrons of dye sensitizer were easily able to generate and shift from groundstate to the excited state under visible light irradiation [18]. Only very few excited electrons could inject into TiO2 electrode from dye molecules. In an open-circuit state, the steady-state photovoltage was resulted from a dynamic equilibrium between photoelectron accumulation and charge-pair recombination. However, in a short-circuit state, conduction band of TiO2 semiconductor could also mediate the electrons transfer via Ti matrix till to a counter electrode. Meanwhile, methanol molecule acted as a scavenger of radicals to regenerate dye from cationic radicals to the ground-stated molecules. Especially, nanotube array provided a favorite route for the electron transport because these independent tubules could act as more efficient shifting channels in comparison with microporous film. So, the microstructure of TiO2 might play a more significant role on photocurrent than photovoltage response. The enhanced photosensitization activity of the nanotube TiO2/Ti composites was mostly ascribed to the more adsorption amount of dye through its higher available surface areas, which had been proved by UV–vis absorption spectrum experiment (Spectrum analysis not shown here). Additionally, externally applied anodic potential on dye-
695
TiO2/Ti electrode could positively drive those photoinduced electrons away from TiO2 conduction band to form out-circuit current and accordingly enhanced photocurrent density. Consequently, microstructure modification from microporous film to nanotubular array positively led to a significant improvement of effective separation of charge pairs and interfacial electron shift which eventually promoted the photosensitization activity of dye-TiO2/Ti composite photoanodes. Photocurrent response could act more suitably as a detecting parameter for the photosensitivity evaluation of composite electrodes. 4. Conclusions Both micro- and nano-composite electrodes of dyeTiO2/Ti had been prepared by the process of potentiostatic anodization along with dye adsorption modification. Micropore distribution structure was constructed by an intensive anodization at a high-voltage while nanotube array structure was well formed by a dynamic equilibrium process of corrosion-anodization at a low-voltage. The nano-composite electrode exhibited lower electrode impedance due to the more accessible charge-transfer interfaces and a higher polarization current due to the more effective electron transport through tubular channels. The high promotion of photocurrent and photovoltage responses proves that photosensitization activity could be substantially enhanced by tailoring morphology from microporous to nanotubular structure, which contributes a potential application of photoelectric conversion as a novel solar cell. Acknowledgement The authors are grateful to financial supports from the Hong Kong Polytechnic University (Grant No.: G-YE19, G-YE68 and BB90). References [1] Carp O, Huisman CL, Reller A. Photoinduced reactivity of titanium dioxide. Prog Solid State Chem 2004;32:33–177. [2] Gratzel M. Photoelectrochemical cells. Nature 2001;414:338–44. [3] Perera VPS, Pitigala P, Senevirathne MKI, Tennakone K. A solar cell sensitized with three different dyes. Solid Energ Mater Solid Cell 2005;85:91–8. [4] Shen Q, Arae D, Toyoda T. Photosensitization of nanostructured TiO2 with CdSe quantum dots: effects of microstructure and electron transport in TiO2 substrates. J Photochem Photobiol A: Chem 2004;164:75–80. [5] Aduda BO, Ravirajan P, Choy KL, Nelson J. Effect of morphology on electron drift mobility in porous TiO2. Int J Photoenerg 2004;6:141–7. [6] Rahman MYA, Salleh MM, Talib IA, Yahaya M. Effect of ionic conductivity of a PVC–LiClO4 based solid polymeric electrolyte on the performance of solar cells of ITO/TiO2/PVC–LiClO4 graphite. J Power Sour 2004;133:293–7. [7] Law M, Greene LE, Johnson JC, Saykally R, Yang PD. Nanowire dye-sensitized solar cells. Nat Mater 2005;4:455–9.
696
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
[8] Khan SUM, Sultana T. Photoresponse of n-TiO2 thin film and nanowire electrodes. Solid Energ Mater Solid Cell 2003;76:211–21. [9] Sul YT, Johansson CB, Jeong Y, Albrektsson T. The electrochemical oxide growth behavior on titanium in acid and alkaline electrolytes. Med Eng Phys 2001;23:329–46. [10] Choi JS, Wehrspohn RB, Lee J, Gosele U. Anodization of nanoimprinted titanium: a comparison with formation of porous alumina. Electrochim Acta 2004;49:2645–52. [11] Palombari R, Ranchella M, Rol C, Sebastiani GV. Oxidative photoelectrochemical technology with Ti/TiO2 anodes. Solid Energ Mater Solid Cell 2002;71:359–68. [12] Xie YB. Photoelectrochemical application of nanotubular titania photoanode. Electrochim Acta 2006;51:3399–406. [13] Faia PM, Furtado CS, Feffeira A. AC impedance spectroscopy: a new equivalent circuit for titania thick film humidity sensors. Sensors Actuat B: Chem 2005;107:353–9.
[14] Yuan S, Hu SS. Characterization and electrochemical studies of Nafion/nano-TiO2 film modified electrodes. Electrochim Acta 2004;49:4287–93. [15] Mantzila AG, Prodromidis MI. Development and study of anodic Ti/ TiO2 electrodes and their potential use as impedimetric immunosensors. Electrochim Acta 2006;51:3537–42. [16] Oliva FY, Avalle LB, Santos E, Camara OR. Photoelectrochemical characterization of nanocrystalline TiO2 films on titanium substrates. J Photochem Photobiol A: Chem 2002;146:175–88. [17] Song MY, Kim DK, Jo SM, Kim DY. Enhancement of the photocurrent generation in dye-sensitized solar cell based on electrospun TiO2 electrode by surface treatment. Synth Met 2005;155:635–8. [18] O’Regan BC, Durrant JR. Calculation of activation energies for transport and recombination in mesoporous TiO2/dye/electrolyte films – Taking into account surface charge shifts with temperature. J Phys Chem B 2006;110:8544–7.
Composites: Part A 39 (2008) 690–696 www.elsevier.com/locate/compositesa
Microstructure promoted photosensitization activity of dye-titania/titanium composites Yibing Xie a
a,*
, Limin Zhou
a,*
, Haitao Huang b, Jian Lu
a
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong b Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong
Received 26 March 2007; received in revised form 16 August 2007; accepted 20 August 2007
Abstract Well-constructed titania reactive layer/titanium metal–matrix composites were fabricated for photosensitization substrate application by a controlled electrochemical anodization process. A low-voltage anodization at 20 V produced titania nanotube array with 60–70 nm diameter and 540 nm height, while a high-voltage anodization at 180 V resulted in titania multiporous film with 170–260 nm pore size and 4 lm thickness. Two types of dye-titania/titanium composite electrodes were also prepared through a surface adsorption modification by orange G dye. The surface morphologies and interfacial electric properties of heterogeneous materials were examined by microstructure characterization and electrochemical impedance spectroscopy analysis. The significant decrease of interfacial charge-transfer resistance from 12,000 X for microporous titania to 5100 X for nanotubular titania accordingly contributed a much lower electrode impedance and higher polarization current for dye-titania/titanium electrode. Additionally, 4 and 1.5 times magnification of photocurrent density and photovoltage were achieved under a visible light irradiation for nanotubular dye-titania/titanium electrode in comparison with microporous one. A better photosensitization activity could be well obtained by tailoring microstructure from micro-composite to nano-composite. The corresponding photocurrent response was more significant than photovoltage in the photosensitivity evaluation of these composite electrodes. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Metal–matrix composites (MMCs); B. Microstructure; B. Chemical properties; E. Surface treatments
1. Introduction Nanocrystalline titanium dioxide has been recognized as one of the most important semiconductor materials for photoelectrochemical applications [1,2]. In the area of photosensitization, various organic and inorganic sensitizers have been exploited for heterogeneous modification to extend the spectral response of titania (TiO2) from UV to visible light range [3]. Actually, in addition to these sensitizers, the photosensitization activity is also very dependant on the properties of TiO2 functional layer, such as single crystal or polycrystalline structure, particulate or porous *
Corresponding authors. E-mail addresses: [email protected] (Y. Xie), mmlmzhou@inet. polyu.edu.hk (L. Zhou). 1359-835X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2007.08.020
film and even particle size [4]. The multiporous geometries are typically preferred as they provide a higher interfacial reaction area [5]. Concerning construction of traditional photoelectrochemical electrode, various TiO2 sol–gel films were immobilized on indium–tin-oxide or fluorine doped tin oxide conductive glass substrate by surface dip-coating technology to act as a working electrode [6]. However, an irregular arrangement of these TiO2 particulates usually diminished their valid interfacial areas. Furthermore, weak bonding structure between TiO2 and conductive glass probably inhibited interfacial electron-transfer, which finally affected photoelectric efficiency. So, architecture modification of composite electrodes became very reasonable by tailoring TiO2 nanocrystallites and their supports so as to promote photoelectrochemical performance. One of the effective ways was to convert the disordered oxides
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
layer into uniform nanostructures such as nanorods, nanowires or nanotubes, since these structures contributed both better electron-transfer and larger surface area than planar bulk films [7,8]. Recently, direct electrochemical anodization technique has been well developed and become one of simple, but feasible methods to synthesize differently structured oxide films based on various metal materials such as aluminum, titanium, zirconium, and so on [9,10]. The photoelectrochemical activity of these composites were also studied [11,12]. In this study, we mainly investigate photosensitization activity of dye modified titania/titanium (TiO2/Ti) composite electrodes under a weak visible light illumination. The well-constructed dye-titania composite acts as a functional reaction layer along with titanium metal as an electric conducting matrix, which would be synthesized by a controlled one-step anodizing process. Additionally, 7-hydroxy-8phenylazo-1,3-naphthalenedisulphonic disodium (orange G), as an ordinary anionic dye, was used as a photoactivating probe for photosensitization evaluation. The microstructure effect of TiO2 on the photosensitivity has been examined through a full measurement of photocurrent and photovoltage response. 2. Experimental Anodization experiments were carried out in a cylindrical polytetrafluoro ethylene (PTFE) cell equipped with Agilent N5751A DC power supply for a potentiostatic control and a two-electrode configuration, in which the pretreated titanium sheet was used as a working electrode for an anode role and a platinum sheet acting as a counter electrode. Two types of TiO2/Ti composites were fabricated from titanium precursor by different potentiostatic anodization in fluoride contained phosphoric acid electrolytes (HF (0.03 M)–(H2SO4 (1.0 M)–H2O2 (0.6 M)–H3PO4 (0.3 M) and HF (0.15 M)–H3PO4 (0.5 M)). Anodization processes involving either a high-voltage of 180 V or a low-voltage of 20 V for 40 min were applied for titanium metal oxidation. Then, a post-treatment by calcination at 450 °C for 2 h was followed to achieve a complete crystallization for the low-voltage anodized TiO2. Finally, dye adsorption modification on the surface of TiO2/Ti was achieved by ultrasonic treatment for 30 min and soaking for 24 h in 10 mM orange G of phosphate buffered saline solution (PBS, pH 6.8). Field emission scanning electron microscope (FESEM, JEOL JSM-6335F) was used to investigate surface morphology and microstructure. X-ray diffraction (XRD, Philips PW3020) were conducted to determine the crystal phase. Photosensitization experiments were carried out in a quartz reactor equipped with an electrochemical workstation (CHI 660C, CH Instruments Co., Ltd.) and a standard three-electrode configuration, in which dye-TiO2/Ti composites was used as a working electrode, a platinum sheet as a counter electrode and a standard saturated calomel Hg/Hg2Cl2 electrode (SCE) as a reference electrode. PBS with 1.0 mM methanol
691
was used as an electrolyte solution. Additionally, a visible light was generated from a cold light source equipped with a tungsten halogen lamp and UV–IR cut-off filters, whose average light intensity was only 1.93 mW cm 2 in the main emission range of 400–800 nm. This weak visible light was applied to vertically irradiate the surface of the dyeTiO2/Ti photoanode through the quartz window of the reactor. 3. Results and discussion 3.1. Morphological characterization Considering the potentiostatic anodization at 180 V, which was the highest reachable voltage in our experiment, a micron-sized multiporous TiO2 layer was formed with the entire film thickness of 4 lm. These micropores were irregularly distributed on Ti matrix and most pore sizes were determined in the range of 170–260 nm (see Fig. 1A, and B). However, in view of potentiostatic anodization at 20 V, a highly-ordered and vertically-aligned TiO2 nanotube array with unique open-mouth pores was directly formed on the surface of Ti metal–matrix. The inner diameter of individual tubule was about 60–70 nm and its height was approx 540 nm (see Fig. 1C–F). Intensive anodization voltage of 180 V, was much higher than the critical sparking voltage of approx 110–120 V for titania semiconductor. The electrochemical oxidization of titanium was actually a predominant process when compared to the dissolution of titania and thick film of titania could eventually be produced. Meanwhile, the high-voltage anodization could also cause the cycling breakdown-regeneration of titania barrier layer based on titanium, which would contribute the formation of microporous structure. Regarding the tailored anodization at a voltage of 20 V, the dynamic equilibrium process between electrochemical formation and chemical dissolution of titania in a fluorate-containing solution could be well established by controlling the reaction voltage, electrolyte composition and concentration. The preferential oxidation–dissolution reaction was mostly located at these initially formed pits on the titania surface layer, which finally led to the formation of titania nanotubes. So, intensive oxidation process of titanium at a high-voltage would produce a porous thick film in a micrometer scale, while moderate oxidation–dissolution equilibrium process at a low-voltage could form a uniform thin layer with nanotubular structure. Surface morphology could be purposely manipulated by controlling electrochemical anodization parameters. An enhanced degree of dye adsorption on this TiO2 nanotube array would be expected due to the high aspect ratio of their nano-sized independent channels. Additionally, the as-anodized TiO2 nanotubes only exhibited an amorphous phase and it could be completely transformed into anatase phase only after the post-calcination treatment at high temperature. However, high-voltage anodization above the sparking voltage of TiO2 semiconductor could lead to a direct crystallization
692
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
Fig. 1. (A) Plan-view and (B) cross-sectional view of FESEM images of high-voltage anodic TiO2/Ti composite, (C) plan-view, (D) enlarged plane-view, (E) cross-sectional view and (F) enlarged cross-sectional view of FESEM images of low-voltage anodic TiO2/Ti composite.
during the fabrication process of TiO2 layer (XRD figures not shown here). Since either TiO2 nanotube array or microporous film was directly grown on the Ti metal through an anodization process, these TiO2 layers could tightly connect with Ti matrix to form an integrative structure. Such TiO2/Ti electrodes exhibited a superior mechanical bonding strength in comparison with common TiO2 sol–gel film/conductive glass electrodes. Moreover, TiO2 nanotube array provided a much higher reactive areas than micron-sized multiporous film, which would benefit interfacial adsorption of dye molecules on the surface of TiO2/Ti electrode. 3.2. Impedance spectroscopy analysis The electrochemical impedance spectroscopy is one of the powerful approaches for investigating internal resistances attributable to charge transfer processes [13]. In this study, impedance spectrum analysis had been applied to
explore the electric conductivity of TiO2/Ti electrodes at 0 V vs. SCE over the frequency range of 0.01– 100,000 Hz. Along with a continuous increase of the working frequency, impedance values of both composite electrodes were dramatically decreased at a low frequency stage below 1 Hz and then gradually achieved to a steady value above 1000 Hz. Comparatively, the complex impedance of nanotubular TiO2/Ti electrode was completely lower than that of microporous one in the full frequency range (see Fig. 2B). In addition, an obvious difference of semicircle part corresponding to the electron-transfer limited process was observed in complex impedance diagrams, whose diameter represented the charge-transfer resistance at the composite electrode interface. The interaction resistance was 5100 X for nanotubular composite electrode and 12,000 X for microporous composite electrode (see Fig. 2B). Additionally, the impedance values had obviously increased for both dye-TiO2/Ti electrodes in comparison with that of their respective TiO2/Ti electrodes, which
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
6
5
4
a
3
b 2
1 -2
-1
0
1
2
3
4
5
Log Frequency(Hz)
B 5000 -Z''Imaginary (ohm)
4000
3000
2000
3.3. Photoelectrochemical polarization analysis
a
1000
b
0 0
2000
4000
6000
8000
10000
12000
Z'real (ohm)
C 40000 30000
Z'' imaginary (ohm)
exhibited a similar result as TiO2/Ti electrode (see Fig. 2C). Therefore, a significant decrease of complex impedance could be achieved for both TiO2/Ti and dyeTiO2/Ti electrodes by modifying their microstructure from the micropores to nanotube array. Additionally, the linear part corresponding to the mass-transfer limited process did not appear at low frequencies in Nyquist plots. It means the whole photoelectrochemical system was not a diffusion-controlled, but a charge-transfer-controlled process. The entire impedance is resulted from dominant resistances of charge flow across dye/TiO2, TiO2/electrolyte and TiO2/Ti interfaces, subordinate resistances of electrolyte solution, Ti matrix and counter electrode, and low capacitances of TiO2 accumulation layer and Helmholtz double layer [14,15]. The impedance value mostly reflects the overall efficiency of interfacial electron-transfer and shift process in a certain degree [15]. Lower impedance usually means better charge transfer and electronic conductivity for dye-TiO2/Ti nano-composite electrode. Such an ordered arrangement of nanotubular channels ultimately contributed to prompting electron shift as well as photosensitivity in the photoelectrochemical process.
b
Linear sweep voltammetry analysis was applied to examine photoelectrochemical polarization behaviors of dye-TiO2/Ti composites as shown in Fig. 3. The similar polarization current curves could be achieved without visible light illumination for both dye-TiO2/Ti composite electrodes, whose current densities could be negligible due to a weak interaction in a pure electrochemical process. However, obvious photoelectrochemical polarization currents were observed under visible light illumination, which were completely ascribed to the external potential-promoted photoelectric response. The polarization current density was correspondingly enhanced along with a continuous increase of anodic bias potential above 0 V vs. SCE since
20000 6
10000
a 0 0
10000
20000
30000
40000
50000
60000
Z'real (ohm) Fig. 2. (A) Bode plot of TiO2/Ti and complex impedance diagrams of (A) TiO2/Ti and (B) dye-TiO2/Ti composite electrodes with (a) microporous and (b) nanotubular structure at 0 V vs. SCE over a frequency of 0.01– 100,000 Hz.
was mostly ascribed to the additional interfacial resistance of the charge-transfer. Moreover, the nanotubular dyeTiO2/Ti electrode showed a lower impedance response than microporous one in the whole frequency range, which
Polarization current (μA cm-2)
Log Impedance (ohm)
A
693
a
Microporous dye-TiO2/Ti in dark
5 b c
Nanotubular dye-TiO2/Ti in dark
4 d
Nanotubular dye-TiO2/Ti with visible light
Microporous dye-TiO2/Ti with visible light
3
d
2 b
1
a 0 0.0
0.3
0.6
0.9
c 1.2
1.5
Anodic potential vs. SCE (V) Fig. 3. Photoelectrochemical polarization current curves of dye-TiO2/Ti composite electrodes.
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
3.4. Photosensitization activity Concerning dye-TiO2/Ti photosensitization, the apparent efficiency of electron-transfer mainly depends upon the surface microstructure of TiO2 photoanode when the same sensitizer is used. Both photocurrent and photovoltage responses are regarded as key parameters to characterize the overall photosensitization activity [17]. To study the photo-induced charge separation and electron shift on dyeTiO2/Ti composites, both photocurrent under short-circuit condition and photovoltage under open-circuit condition were measured in PBS electrolyte with 1.0 mM methanol under visible light pulsed-irradiation. Regarding both dye-TiO2/Ti composites, the responsive currents were only constant at about 0.12 lA cm 2 under dark condition when the electrode potential was controlled at a bias potential of 0 V vs. SCE (also equal to 0.268 V vs. standard hydrogen electrode). It means that pure electrochemical interaction was very insignificant under such a low potential condition. Actually, this dark current response was more regarded as the noise current in a process of the photosensitization evaluation. However, in the case of visible light irradiation, photocurrent density was drastically soared up to a high level of 0.45 lA cm 2 for micro-composite and 2.11 lA cm 2 for nano-composite, respectively. After removing visible light, this current immediately descended to initial value again as dark current (see Fig. 4). Higher photocurrent means more photoinduced electrons have been effectively transferred from dye-TiO2/Ti photoanode to counter electrode via external circuit. The experimental results also indicate that an obvious photocurrent response could well occur even in the case of a low bias potential of 0 V vs. SCE and a higher photocurrent response could be expected on the condition
2.5
b Photocurrent (μA cm-2)
this applied potential has already exceeded the optical flatband potential of TiO2 ( 0.18 V vs. SCE) [16]. Comparatively, an obviously higher photoelectrochemical current density was obtained for nanotubular dye-TiO2/Ti than microporous one under the same anodic potential. It means that morphologies and structures of TiO2 could influence the overall photoelectrochemical reactivity for dye-TiO2/Ti composite electrodes. Additionally, owing to the electrolyzing oxidation reaction of dye molecules, a sharp increase of polarization current was observed at an anodic potential above approx 1.2 V for TiO2/Ti nanocomposite electrode rather than micro-composite one, whose difference was ascribed to the microstructure effect. Generally, in the range of a low bias potential, the photoelectrochemical response was mostly ascribed to photocurrent rather than pure electrochemical current since the applied anodic potential was much below the critical potential of water decomposition and dye electro-oxidation due to the overpotential effect on TiO2/Ti composite electrode. Therefore, a suitable safe bias potential should be controlled far below this critical value for the sake of a credible photosensitization evaluation.
b
2.0
1.5 light-off
1.0
light-on
light-on
light-off
a
0.5
a
0.0 0
50
100
150
200
Time (s) Fig. 4. Photocurrent responses of: (a) microporous and (b) nanotubular dye-TiO2/Ti composite electrodes under pulse illumination of visible light.
of a higher anodic potential applied to suppress electron relaxation and recombination. Noticeably, owing to about 4 times in magnification of photocurrent density for two types of dye-TiO2/Ti composite electrodes, the total photosensitization efficiency had been intensively improved by means of a microstructure transformation from micropores to nanotubes. In addition, the photovoltage had also been measured under open-circuit condition to investigate photosensitization response of dye-TiO2/Ti composites (see Fig. 5). Once light source was turned on, the potential was drastically decreased till to a steady value after 200 s. It means that most excited photoelectrons had been transferred and accumulated on the conduction band of TiO2, which accordingly contributed the formation of the space charge layers along the pore channels. On the condition of lightoff, this potential gradually increased up to the initial value again. It indicates that these conduction band electrons
0.09 light-on light-on 0.06
Potential vs. SCE (V)
694
0.03
0.00
a
a
b
b
-0.03
-0.06
light-off
light-off 0
200
400
600
800
Time (s) Fig. 5. Photovoltage responses of: (a) microporous and (b) nanotubular dye-TiO2/Ti composite electrodes under pulse illumination of visible light.
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
must have suffered a relaxation decay through a recombination process with cationic radicals, which was due to a strong electric field distribution within the depletion layers. Significantly, a higher photovoltage was achieved for the nano-composite (0.134 V) rather than the mciro-composite (0.091 V), which was equal to about 1.5 times in magnification. It means that ground-state electrons of dye molecule could be more effectively photogenerated, separated and finally transferred to TiO2 conduction band on the both sides of tube walls, whose architecture was more favorable than microporous structure. The photovoltaic response was an important parameter other than the photocurrent response for the characterization of photosensitization activity. A higher photovoltage means a more feasible accumulation of photo-generated electrons on the conduction band of titania regardless of photoelectron relaxation decay from the excited state to ground-state. Accordingly, a higher photosensitization activity was achieved for the nano-composite photoanode. This result highly agreed with above photocurrent measurements. Regarding the photosensitization response, two composite photoanodes exhibited a similar varying law. However, the overall magnification times of photovoltage was much lower than that of photocurrent, which was mostly due to the higher probability of charge recombination without external applied potential on TiO2/Ti composite electrodes. It should be noted that this is only a very primary research of photosensitization activity and such a photosensitivity is expected to be further improved by optimizing dye sensitizers, TiO2 structure, redox electrolyte and photon flux. In view of photosensitization effect of dye-sensitized TiO2 photoanode, charge recombination process could predominantly occur between photo-induced electrons and dye cationic radicals although photoelectrons of dye sensitizer were easily able to generate and shift from groundstate to the excited state under visible light irradiation [18]. Only very few excited electrons could inject into TiO2 electrode from dye molecules. In an open-circuit state, the steady-state photovoltage was resulted from a dynamic equilibrium between photoelectron accumulation and charge-pair recombination. However, in a short-circuit state, conduction band of TiO2 semiconductor could also mediate the electrons transfer via Ti matrix till to a counter electrode. Meanwhile, methanol molecule acted as a scavenger of radicals to regenerate dye from cationic radicals to the ground-stated molecules. Especially, nanotube array provided a favorite route for the electron transport because these independent tubules could act as more efficient shifting channels in comparison with microporous film. So, the microstructure of TiO2 might play a more significant role on photocurrent than photovoltage response. The enhanced photosensitization activity of the nanotube TiO2/Ti composites was mostly ascribed to the more adsorption amount of dye through its higher available surface areas, which had been proved by UV–vis absorption spectrum experiment (Spectrum analysis not shown here). Additionally, externally applied anodic potential on dye-
695
TiO2/Ti electrode could positively drive those photoinduced electrons away from TiO2 conduction band to form out-circuit current and accordingly enhanced photocurrent density. Consequently, microstructure modification from microporous film to nanotubular array positively led to a significant improvement of effective separation of charge pairs and interfacial electron shift which eventually promoted the photosensitization activity of dye-TiO2/Ti composite photoanodes. Photocurrent response could act more suitably as a detecting parameter for the photosensitivity evaluation of composite electrodes. 4. Conclusions Both micro- and nano-composite electrodes of dyeTiO2/Ti had been prepared by the process of potentiostatic anodization along with dye adsorption modification. Micropore distribution structure was constructed by an intensive anodization at a high-voltage while nanotube array structure was well formed by a dynamic equilibrium process of corrosion-anodization at a low-voltage. The nano-composite electrode exhibited lower electrode impedance due to the more accessible charge-transfer interfaces and a higher polarization current due to the more effective electron transport through tubular channels. The high promotion of photocurrent and photovoltage responses proves that photosensitization activity could be substantially enhanced by tailoring morphology from microporous to nanotubular structure, which contributes a potential application of photoelectric conversion as a novel solar cell. Acknowledgement The authors are grateful to financial supports from the Hong Kong Polytechnic University (Grant No.: G-YE19, G-YE68 and BB90). References [1] Carp O, Huisman CL, Reller A. Photoinduced reactivity of titanium dioxide. Prog Solid State Chem 2004;32:33–177. [2] Gratzel M. Photoelectrochemical cells. Nature 2001;414:338–44. [3] Perera VPS, Pitigala P, Senevirathne MKI, Tennakone K. A solar cell sensitized with three different dyes. Solid Energ Mater Solid Cell 2005;85:91–8. [4] Shen Q, Arae D, Toyoda T. Photosensitization of nanostructured TiO2 with CdSe quantum dots: effects of microstructure and electron transport in TiO2 substrates. J Photochem Photobiol A: Chem 2004;164:75–80. [5] Aduda BO, Ravirajan P, Choy KL, Nelson J. Effect of morphology on electron drift mobility in porous TiO2. Int J Photoenerg 2004;6:141–7. [6] Rahman MYA, Salleh MM, Talib IA, Yahaya M. Effect of ionic conductivity of a PVC–LiClO4 based solid polymeric electrolyte on the performance of solar cells of ITO/TiO2/PVC–LiClO4 graphite. J Power Sour 2004;133:293–7. [7] Law M, Greene LE, Johnson JC, Saykally R, Yang PD. Nanowire dye-sensitized solar cells. Nat Mater 2005;4:455–9.
696
Y. Xie et al. / Composites: Part A 39 (2008) 690–696
[8] Khan SUM, Sultana T. Photoresponse of n-TiO2 thin film and nanowire electrodes. Solid Energ Mater Solid Cell 2003;76:211–21. [9] Sul YT, Johansson CB, Jeong Y, Albrektsson T. The electrochemical oxide growth behavior on titanium in acid and alkaline electrolytes. Med Eng Phys 2001;23:329–46. [10] Choi JS, Wehrspohn RB, Lee J, Gosele U. Anodization of nanoimprinted titanium: a comparison with formation of porous alumina. Electrochim Acta 2004;49:2645–52. [11] Palombari R, Ranchella M, Rol C, Sebastiani GV. Oxidative photoelectrochemical technology with Ti/TiO2 anodes. Solid Energ Mater Solid Cell 2002;71:359–68. [12] Xie YB. Photoelectrochemical application of nanotubular titania photoanode. Electrochim Acta 2006;51:3399–406. [13] Faia PM, Furtado CS, Feffeira A. AC impedance spectroscopy: a new equivalent circuit for titania thick film humidity sensors. Sensors Actuat B: Chem 2005;107:353–9.
[14] Yuan S, Hu SS. Characterization and electrochemical studies of Nafion/nano-TiO2 film modified electrodes. Electrochim Acta 2004;49:4287–93. [15] Mantzila AG, Prodromidis MI. Development and study of anodic Ti/ TiO2 electrodes and their potential use as impedimetric immunosensors. Electrochim Acta 2006;51:3537–42. [16] Oliva FY, Avalle LB, Santos E, Camara OR. Photoelectrochemical characterization of nanocrystalline TiO2 films on titanium substrates. J Photochem Photobiol A: Chem 2002;146:175–88. [17] Song MY, Kim DK, Jo SM, Kim DY. Enhancement of the photocurrent generation in dye-sensitized solar cell based on electrospun TiO2 electrode by surface treatment. Synth Met 2005;155:635–8. [18] O’Regan BC, Durrant JR. Calculation of activation energies for transport and recombination in mesoporous TiO2/dye/electrolyte films – Taking into account surface charge shifts with temperature. J Phys Chem B 2006;110:8544–7.