Ag Heterostructure

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A Facile Electrochemical Approach to Form TiO2/Ag Heterostructure Films with Enhanced Photocatalytic Activity Xiang Lv, Fengquan Gao, Yong Yang, and Tianhe Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02867 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 27, 2015

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Industrial & Engineering Chemistry Research

A Facile Electrochemical Approach to Form TiO2/Ag Heterostructure Films with Enhanced Photocatalytic Activity Xiang Lv, Fengquan Gao, Yong Yang, Tianhe Wang* Chemicobiology and Functional Materials Institute, Nanjing University of Science and Technology, Nanjing, 210094, PR. China

The corresponding author’s e-mail address: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: We report a simple but novel technique to prepare chemically durable TiO2/Ag films on FTO substrates via coating of P25 TiO2 with colloidal silica as a high temperature binder and subsequent electrochemical deposition of silver nanoparticles (AgNPs). The porous nanostructure facilitated implantation of AgNPs by electrochemical deposition. Extensive measurements on photocatalytic activities of the films prepared under varied conditions showed that TiO2/Ag enhanced MB degradation. Possible mechanisms of such enhancement were discussed in terms of delayed recombination of photo-generated electrons/holes via Schottky barriers and beneficial effects of Surface Plasmon. Experimental results of both electrochemical impedance spectra and transient photocurrent responses on the films supported the observed photocatalytic enhancement by TiO2/Ag heterostructures. The study offers an effective technique to implant uniformly dispersed AgNPs around 3 nm in size within a porous structure, and may also provide a strategy for preparations of other functional films on FTO substrates.

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1. INTRODUCTION Semiconductor nanoporous films are widely used in photoelectric devices, thin-film solar cells, light-emitting diodes and gas sensors1-6 and have attracted significant academic and industrial interests. Nano-scaled interconnected pores in porous films not only increase adsorption capacities, but also act as effective electron-transmission pathways.7 A further advantage is that functional chemicals can be conveniently immobilized into the porous films via simple techniques such as vacuum impregnation and electrical deposition. Recently, Lok-kun Tsui et al8 prepared TiO2 nanotubes/Cu2O composite films by anodization of Ti foils followed by Cu2O modification，the composite films displayed a much improved photocatalytic activity. In another study,9 platinum modified TiO2 films prepared by micro emulsion-templating also significantly improved photo-degradation of the organic dyes in the visible region, compared with the pure TiO2 films. Immobilization of nano functional materials on two-dimensional porous surfaces is more conveniently achievable than on bulk materials and has a number of advantages including minimal functional material requirement and better controllability. In particular, the utilization of the solar energy in cells and environmental remediation has triggered great attention.10-15Various techniques such as sputtering,16 pulsed laser deposition17 and chemical solution process,18 have been developed for preparation of TiO2 films. However, the photo-excited electrons and holes within the TiO2 structure recombine rapidly, restricting the efficiency of TiO2.19 Recently several efforts were attempted to decelerate

the

de-excitation

speed

of

photocatalytic

3


reactions,

including


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semiconductor composite methods20 and noble metal doping21 methods. Nano-scale noble metal deposition in the TiO2 structure could substantially reduce the electron–hole recombination speed in photocatalysis, as a result of formation of the Schottky barriers at the semiconductor/metal interfaces. Meanwhile, the surface plasmonic effect of a noble metal could also promote the photocatalysis.22-27 Electrochemical deposition of nanoparticles into the porous semiconductor film was suggested to be a low-cost and convenient approach for surface modification as long as the film can be formed on a conducting substrate.28 A functional material in form of porous film allows it to function more effectively, in particular for large-scale industrial applications. In this study, we describe a simple but novel technique to form titanium dioxide nanoparticle film on commercial FTO glass substrate using colloidal silica as an inorganic binder. The porous TiO2 film exhibited excellent adhesion and good chemical stability. In order to enhance the photocatalytic performance of the film, AgNPs were deposited into porous structure of the film by electric pulsed current deposition. Furthermore, recycling experiments were performed to test the durability and reusability of the TiO2/Ag film. 2. EXPERIMENTAL SECTION 2.1. Chemicals and materials. Sodium dodecyl sulfate (SDS, 99% AR), silver nitrate (AgNO3, 99.8% AR), methylene blue (MB, 98.5% AR), sodium nitrate (NaNO3, 99% AR) and sodium sulphate anhydrous (Na2SO4, 99% AR) were purchased from Aladdin and used as received. P25 TiO2 was from Degussa UK Ltd 4


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and colloidal silica (25 wt % SiO2 and 75 wt % water) was obtained from Zhejiang Jiahe Chemical Ltd, China. 2.2. Preparation of TiO2 film. FTO substrate in size of 15 × 20 mm was washed with cleaning detergent and subsequently rinsed with ethanol before use. 6 g P25 TiO2 and 12 g colloidal silica (25 wt % SiO2 and 75 wt % water) were mixed with 12 ml deionized water containing 0.1 wt % sodium dodecyl sulfate（SDS）. SDS acted as an effective dispersing agent. The mixture was then ball milled (QM-3SP04, Rotational frequency of 40Hz) for 2 h to obtain a uniform and well dispersed TiO2 slurry. Then the FTO substrate was spin-coated (KW-4A) with the slurry. The rotational speed of spin-coating was 3000 rev/min lasting 30 s. Immediately after spin-coating, the substrate was placed in an oven at 90oC drying for 1 h. Next the sample was sintered at 545℃ for 30 minutes in air with a ramping rate of 5 ℃/min. Thus obtained sample was translucent and with a uniform TiO2 coating layer thickness of about 3 microns (Figure 1). 2.3. Electrical deposition of Ag nanoparticles. The mixture with 10 mM AgNO3 and 100 mM NaNO3 was electrolyzed at room temperature to prepare the AgNPs. A two-electrode setup was adopted in this work. The TiO2 film coated on FTO was taken as the working electrode, and a Pt sheet as the counter electrode. A pulsed current of 10 mA•cm-2 with 0.1 s on and 0.5 s off was applied continuously for 300 pulses. Afterwards the sample was washed with deionized water and dried in vacuum for 2 h. Finally, it was heat-treated in a tube furnace at 200℃ for 0.5 h in N2 atmosphere. This sample was donated as TiO2/Ag (std) throughout this paper. In order 5



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to establish the optimum preparation conditions for TiO2/Ag films that lead to maximum photocatalytic activity, the number of pulse, current density, off-time and annealing temperature were deliberately varied and photocatalytic performances of each corresponding sample were evaluated. Respectively, the TiO2/Ag composite films were donated as TiO2/Ag(X pulses), TiO2/Ag(X mA·cm-2), TiO2/Ag(X s) and TiO2/Ag(X ℃), where X is a varied number. For example, TiO2/Ag (400 pulses) represents the sample that was electrolyzed for 400 pulses. 2.4.

Characterizations

of

TiO2/Ag films.

Morphologies and element

compositions of the films were studied by a field-emission scanning electron microscopy

(FESEM,

HITACHI

S-4800) and

an

energy-dispersive

X-ray

spectroscopy (EDS, Oxford instruments X-Max). The particle sizes of AgNPs were estimated by a transmission electron microscopy (FEI Tecnai F20) at an accelerating voltage of 100 kV. TEM samples were prepared by scratching the films off the FTO with a sharp zirconia blade, dispersed in ethanol and then placed on a Cu grid (200 mesh) allowing the ethanol to evaporate at room temperature. The crystalline structures were characterized by an X-ray diffraction (XRD Brucker D8) equipped with Cu Kα radiation source (λ=1.54056Å). Degradation efficiency of methyl blue (MB) was measured by a UV-vis spectroscopy (UV-vis-DRS Shimadzu UV 2600) at the wavelength of 664 nm (characteristic absorption for MB) at room temperature. The element characterization was carried out by an X-ray photoelectron spectroscopy (XPS, an RBD upgraded PHI-5000C ESCA system, Perkin-Elmer) with Mg Kα radiation (hν = 1253.6 eV). Specifically, the binding energy of C1s（284.6 eV）was 6


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used to calibrate the spectrum. Photo-electrochemical density and electrochemical impedance spectroscopy (EIS) were measured on an electrochemical workstation (CHI660E) with a standard three-electrode system. 0.1 M Na2SO4 solution was used as the electrolyte. The TiO2/Ag composite films prepared in this work were taken as the working electrode, a Pt sheet as the counter electrode and saturated calomel electrode (SCE) as the reference electrode. Respectively, photo-electrochemical density was recorded by switching on and off the UV light with periodic intervals of 50s. Experimental studies for electrochemical impedance spectroscopy were carried out by using a potentiostat with a sinusoidal perturbation voltage of 5 mV rms in the frequency range of 0.01 Hz to 10 MHz. The open circuit potential was -0.2256 V. 2.5. Photocatalytic activity measurement. Photocatalytic activities of various films were measured by degradating MB under a 100 W mercury UV lamp equipped with a water-cooling quartz jacket. Visible light irradiation tests were performed under a 300 W Xe lamp with a 420nm cutoff filter and a water-cooling quartz jacket. In a typical procedure, a 15×20 mm TiO2/Ag film was immersed in 20 ml of MB solution (concentration of 10mg/L) in a beaker at room temperature. Before irradiation started, the system was kept in dark for 3 h for MB to reach the adsorption–desorption equilibrium. After irradiation for a fixed period, the concentration of MB solution was measured at the characteristic absorption wavelength of 664 nm with a UV-vis diffuse reflectance spectroscopy spectrometer. 3. RESULTS AND DISCUSSION 3.1. Macrograph of the samples. Figure 1 shows the photographic images of 7



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FTO substrate, TiO2 film (on FTO) and TiO2/Ag (std) film (on FTO). Compared with the uncoated FTO shown in Figure 1a, the transparency of FTO substrate coated with TiO2 film hardly changed (Figure 2b). After electrical deposition of silver, the film changed to dark grey as a result of reduction of silver ions and formation of AgNPs in the film, as shown in Figure 1c.

Figure 1. Photographs of the (a) FTO substrate, (b) TiO2 film (on FTO) and (c) TiO2/Ag (std) film (on FTO).

3.2. Structure and morphology of TiO2/Ag nanocomposite films. The X-ray diffraction patterns of FTO substrates without and with TiO2 alone and TiO2/Ag (std) films were shown in Figure 2a. As it can be seen, although some peaks were overlapped with the FTO substrate, the peaks at 2θ = 25, 55 and 63° are clearly shown up corresponding to the primary diffractions of (1 0 1), (2 1 1) and (2 0 4) crystalline planes in the phase of anatase (JCPDS no.21-1272). This accorded well with the P25 TiO2. For the TiO2/Ag (std) film, besides the diffraction peaks of FTO and TiO2, the XRD pattern displays the diffraction peaks assigned to the (2 0 0) and (2 2 0) 8


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crystalline planes of silver (JCPDS no. 04-0783). Moreover, the small peak at 81° significantly enlarged, which corresponds to the diffraction of (2 2 2) crystalline planes of silver. These XRD results were in good agreement with the elemental analysis to be presented later, confirming successful electrical disposition of AgNPs. UV-vis spectroscopy was used to characterize the TiO2/Ag (std) film. Figure 2b shows the UV-vis spectra of FTO, TiO2 film and TiO2/Ag (std) film. FTO is often used as a transparent electrode material for its high transmittance of visible light. The increase of the absorption at short wavelengths is related to the fundamental absorption caused by band-to-band electron transition. The absorption edge of TiO2 coated FTO exhibits a significant red-shift, which is in agreement with several previous reports.29-31 For TiO2/Ag (std) film (on FTO), the absorption edge extends further to longer wavelength, approximately doubling the red-shift achieved by TiO2 alone.

This effect is primarily due to the existence of AgNPs. As it can been seen

(inset of Figure 2b), compared to the UV-visible spectra of TiO2 film and FTO, there is a continuous and diffused absorption hump from 375 nm to 500 nm for TiO2/Ag(std) film. This is owing to the contribution of AgNPs' surface plasmon,32 and for photocatalysis, this effect is beneficial. Although the observed absorption hump from 375 nm to 500 nm for TiO2/Ag(std) film is not as strong as that reported in an earlier study, 22 this is regarded as reasonable because the entire profile of the absorption from 375 nm to 500 nm may depend on the sizes of the AgNPs and their distribution.32 The effect of AgNPs' surface plasmon will be further discussed in the section of photocatalytic activities later. 9



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Figure 2. (a) XRD and (b) UV-visible spectra of FTO, TiO2 film and TiO2/Ag (std) film.

XPS is an effective technique for the detection of surface compositions and chemical states. Figure 3 is the XPS spectrum of TiO2/Ag (std) film. The wide scan spectrum (Figure 3a) indicates that the sample consisted of silver, titanium, silicon, oxygen and carbon. The weak C1s peak resulted from the adsorbed reactants and gaseous molecules in the atmosphere. The two peaks at 158 eV and 107 eV corresponding to Si2s and Si2p were attributed to the colloidal silica used as a binder in the coating. Figure 3b shows the photoelectron peak of Ag 3d. The peaks located at 368.21 eV and 374.21 eV can be indexed to Ag 3d5/2 and Ag 3d3/2 respectively.33 The 6.0 eV difference between the binding energies of these photoelectron peaks is the characteristic of metallic Ag, and this can be regarded as a further conformation of the reduction of Ag+ within the film. The Ti 2p core (Figure 3c) split into 2p3/2 (458.53 eV) and 2p1/2 (464.23 eV) peaks, in accordance with the reported values for Ti4+ in TiO2. Moreover, the peak of O1s from 525 eV to 535 eV shown in Figure 3d was asymmetric. It could be divided into two peaks at 529.85 eV and 532.82 eV, which were designated to Ti-O and Si-O respectively. 10


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Figure 3. XPS of TiO2/Ag (std) film (a) the wide scan spectrum, (b) Ag 3d peaks, (c) Ti 2p peaks and (d) O 1s peaks

The surface and cross section morphologies of the bare TiO2 film and TiO2/Ag composite films were examined by SEM. As displayed in Figure 4a, the surface of FTO substrate was covered completely with uniformly distributed titania particles with an average diameter approximately 30nm which is close to the particle size of P25 TiO2. The gaps or tunnels between particles constituted the porous structure of the film. This porous feature allowed Ag+ to be impregnated into the porous film, and subsequently reduced into AgNPs by feeding electrons to Ag+. As the FTO retained conductivity after TiO2 film attachment, electrons are conveniently fed through an electric source. The cross section image (Figure 4b) shows that film adhered to the 11



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FTO substrate well and there was no gap between the film and FTO. The thickness of the TiO2 film was about 2.7 µm, with a variation +/- 0.2µm. Figure 4c shows the morphology of TiO2/Ag (std) film that was prepared with following parameters: current density of 10 mA•cm-2, pulse on-time 0.1 s, pulse off- time 0.5 s and 300 cycles of pulse. The microstructure of TiO2/Ag (std) film showed little difference when compared with that of the bare TiO2 film (Figure 4a). This is because the AgNPs were very fine, in the range of 2-4 nm (as discussed later). The EDS analysis of TiO2/Ag (std) film (Figure 4d) further confirmed the coexistence of TiO2 and Ag within the film. According to the EDS analysis, the Ag/Ti atomic ratio was estimated to be 1/5.8 on the top surface of the film. Figure 4e shows the cross-sectional morphology of TiO2/Ag film after electrodeposition. To confirm existence of AgNPs inside the structure of the film, elements of the rectangular region were analyzed by EDS (Figure 4f). The Ag/Ti atomic ratio was 1/8.9. This suggests that the AgNPs were deposited and embedded in the TiO2 composite film. From Figure 4g–4j, it can be seen that an increase in number of pulse, an increase of current density or an increase in annealing temperature, resulted in the coarsening of the AgNPs. In these cases, the silver particles continued to precipitate and eventually grew out of the surfaces to form particles as large as 100 nm. Figures 4h and 4i indicate that the yield of the Ag dramatically increased, however the formation of large Ag particles might cause a decrease in specific surface area of the film, and this is not expected to favor the adsorption and photocatalysis. Besides, it is worth noting that the element of Si, mainly resulting from the colloidal silica (as a high temperature binder for TiO2), was 12


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in the form of amorphous silica and existed both in the surface and cross section of TiO2/Ag (std) film. When the pure colloidal silica (25 wt % SiO2 and 75 wt % water) were dried in an oven and sintered at various temperatures up to 750 ℃ for 2 hours, then tested by XRD, the amorphous structure remained. Only when the sintering temperature reached 800 ℃, crystobalite started to form. This experimental result can be taken as a confirmation that the silica within the film was indeed amorphous. It is known in technical ceramic fields that colloidal silica is a high temperature binding agent and can significantly strengthen the ceramic body by sintering at relatively low temperature.34,35 Amorphous silica should position themselves in the boundaries between TiO2 NPs. This is interesting because amorphous or fused silica is UV transparent and hence a UV wave guide, which is capable of guiding a UV light into the interior of the film and thus should have helped to enhance the photocatalysis. Although the detailed effect of this was not investigated in this work, it would be very interesting to explore this in the further work.

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Figure 4. (a and b) SEM image of TiO2 film’s surface and cross section, (c and d) SEM image and EDS of TiO2/Ag(std) film (current density: 10 mA•cm-2, with impulse 0.1 s on-time and 0.5 s off-time, the cycles of pulse was 300, heat- treatment temperature 200 ℃), (e and f) SEM image and EDS of TiO2/Ag(std) film’s cross section, (g) SEM image of TiO2/Ag film’s surface (400 pulses), (h) SEM image of TiO2/Ag film’s surface (1 s), (i) SEM image of TiO2/Ag film’s surface (20 mA·cm-2), (j) SEM image of TiO2/Ag film’s surface(300 ℃).

More detailed analysis of structure for TiO2/Ag (std) film were carried out by TEM, HRTEM and SAED. Figure 5a shows a typical TEM image of debris scratched off from the surface of the film. The TiO2 particles were about 30 nm in diameter which is in agreement with the SEM analysis. For the TiO2/Ag (std) composite film (Figure 5b), AgNPs with an average diameter of approximately 3 nm were uniformly distributed throughout the bulk of the film. The electron diffraction pattern (inset of Figure 5b) shows typical diffraction patterns of crystalline Ag and TiO2. The 4.241 nm-1 corresponds to the (1 1 1) of Ag and 2.841 nm-1 corresponds to the (1 0 1) of anatase TiO2. HRTEM image of the TiO2/Ag (std) film was shown in Figure 5c, the fringe spaces determined were 0.236 nm and 0.352 nm, corresponding to (1 1 1) lattice spacing of Ag and (1 0 1) lattice spacing of anatase TiO2, respectively. These results further indicate that AgNPs as fine as 3 nm had been electrically deposited or reduced within the structure of porous TiO2 coated on the FTO.

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Figure 5. (a) TEM image of TiO2 film, (b and c) TEM and HRTEM of TiO2/Ag (std) film

3.3. Photocatalytic activities. The photocatalytic performances of TiO2/Ag composite films prepared with varied parameters were evaluated by degradation of MB under UV irradiation, as well as under visible light irradiation. Figure 6a shows the degradation rate of MB with the bare TiO2 and TiO2/Ag composite films prepared with different number of pulse at a fixed current density, where C is the residual MB concentration after irradiation time t, and C0 was the initial concentration of MB. Before UV irradiation, the film immersed in the MB solution was kept in dark for 3 hours for it to reach the adsorption-desorption equilibrium. During this period, only slight reduction in MB concentration was observed. It is worth noticing that under 16


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dark condition, the bare TiO2 film possessed the higher adsorption efficiency than the TiO2/Ag composite films and caused a faster reduction of MB concentration. This can be interpreted from the fact that the metallic Ag crystals have a poor intrinsic adsorption towards MB compared with the porous TiO2 film. Under UV irradiation, remarkable improvement of degradation rate was observed in TiO2/Ag composite films compared with the bare TiO2 film. Under identical conditions, TiO2/Ag (std) film almost completely degraded MB after 2.5 h irradiation, while the bare TiO2 film only degraded 78% MB (Figure 6a). Apparently this improvement in photocatalysis for the composite film was mainly caused by the formation of heterostructures between Ag and TiO2, as well as by the surface plasmonic effects of AgNPs. The formation of Ag/TiO2 junctions allowed Schottky barriers at interfaces to be established.36 As illustrated in Figure 6b, the photo-excited electrons would transfer from TiO2 to Ag through the interface because the Fermi level of AgNPs was more positive and situated directly below the conduction band of the TiO2, allowing a built-in electric field to be formed in the space charge region at the interface. This would create an energy barrier to prevent the fast recombination between the photo-generated electrons and holes. As a result, the injected electrons on AgNPs were more readily trapped by ubiquitous oxygen molecules to form superoxide radical anions (•O2−), which were the key active ingredient for the oxidation of MB. At the same time, the holes on the surface of TiO2 were also reactive. They could directly oxidize MB by capturing electrons from MB and this process regenerated the titanium dioxide itself and promoted its photocatalytic activity continuously. The plasmonic 17



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effects were caused by surface plasmon polaritons which are electromagnetic waves traveling along a silver-air interface coupled to oscillate free electrons on the AgNPs.37 In this study, when AgNPs were irradiated under visible light region (especially under the characteristic wavelength), electrons of AgNPs were photoexcited resulting in a surface with high energy electrons, which was desirable for activating molecules on AgNPs for the photocatalytic reactions.38 Meanwhile, the Ag deposition parameters including the off-time, current density and annealing temperature were all investigated in relation to photocatalytic efficiency of TiO2/Ag films and the results are presented in Figure 6c - 6e. In general, photocatalytic activities strongly depend on the electron transfer efficiency. With the increase of AgNPs in the porous TiO2 structure, more metal/semiconductor junctions were created and more photo-generated electrons transferred cross the junctions. However, the superfluous AgNPs could also act as recombination sites, causing a decrease in photocatalytic activity.39 TiO2/Ag (std) film exhibited the best photocatalytic degradation ability, primarily due to the very fine AgNPs electrically deposited within the porous TiO2 structure (Figure 5b). It may also be tentatively concluded that oversized AgNPs are not favorable to the improvement of photocatalysis. Figure 6f shows that the photocatalytic activity of TiO2/Ag (std) film is significantly higher than that of the bare TiO2 film. This result was mainly due to the surface plasmonic effects of AgNPs (as mentioned in the Figure 2b earlier).

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Figure 6. Photocatalytic degradation efficiencies of MB under UV irradiation with TiO2 film and TiO2/Ag heterostructure films prepared at different conditions: (a) number of pulse; (b) proposed mechanism for the photocatalysis of TiO2/Ag heterostructure film; (c) off-time length; (d) current density and (e) annealing temperature; (f) Photocatalytic degradation efficiencies of MB with TiO2 film and TiO2/Ag heterostructure film (std) visible light illumination (300 W Xe lamp with a 420nm cutoff filter) and TiO2 film in dark

The kinetics of photocatalytic degradation can be described by pseudo-first-order 19



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kinetics represented by equation (1). The equation is well recognized for describing photocatalytic degradation process. 40 ln(C0/C) = kKt = kapp t

(1)

where C0 and C are respectively the concentrations of MB solution at time of 0 and t, and kapp is the apparent rate constant. The apparent rate constant can be deduced from the linear fitting of ln(C0/C) versus reaction time. As given in Table 1, the TiO2/Ag (std) film exhibited the best photocatalytic activity with kapp = 0.0159 min−1, which revealed the most significant synergistic effect among the variations investigated.

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Table 1 kapp of different samples

(a)

TiO2

TiO2/Ag

TiO2/Ag

TiO2/Ag

TiO2/Ag

sample

(0 pulse)

(100pulses)

(200 pulses)

(400pulses)

(300 pulses std)

kapp

0.0089

0.0102

0.0115

0.0118

0.0159

(b)

TiO2/Ag

TiO2/Ag

TiO2/Ag

TiO2/Ag

TiO2/Ag

sample

(0s)

(0.25s)

(0.75s)

(1s)

(0.5s std)

kapp

0.0087

0.0116

0.0101

0.0097

0.0159

(c)

TiO2/Ag

TiO2/Ag

TiO2/Ag

TiO2/Ag

sample

(5mA·cm-2)

(15mA·cm-2)

(20mA·cm-2)

(10mA·cm-2 std)

kapp

0.0115

0.0119

0.0064

0.0159

(d)

TiO2/Ag

TiO2/Ag

TiO2/Ag

TiO2/Ag

sample

(25℃)

(100℃)

(300℃)

(200℃ std)

kapp

0.0122

0.0129

0.0024

0.0159

( a ). Data obtained from experiments of TiO2/Ag (10mA·cm-2), TiO2/Ag(0.5 s) , TiO2/Ag(200℃) and TiO2/Ag(X pulses), where X is a varied number. ( b ). Data obtained from experiments of TiO2/Ag (10mA·cm-2), TiO2/Ag(X s) , TiO2/Ag(200℃) and TiO2/Ag( 300 pulses) , where X is a varied number . ( c ). Data obtained from experiments of TiO2/Ag (X mA·cm-2), TiO2/Ag(0.5 s) , TiO2/Ag(200℃) and TiO2/Ag( 300 pulses) , where X is a varied number. ( d ). Data obtained from experiments of TiO2/Ag (10mA·cm-2), TiO2/Ag(0.5 s) , TiO2/Ag(X ℃) and TiO2/Ag( 300 pulses) , where X is a varied number

An electrochemical impedance spectrum (EIS) is an effective technique to 21



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examine the charge separation and transport processes.41 Figure 7 shows the impedance plots of various samples without UV irradiation. A smaller radius of the arc in the EIS spectra implies a lower electron transfer resistance at the surface of photo-electrodes, as a result of more effective separation of the photogenerated electron/hole pairs and faster interfacial charge transport.42 In Figure 7, the arc radius of the EIS Nyquist plot exhibits a similar trend to that of photocatalytic activity (as displayed in Figure 6). The arc radius of the bare TiO2 sample was the largest whereas the value of the TiO2/Ag (std) was the smallest. From these results, it is concluded that the introduction of AgNPs by pulsed current deposition with nano-size distribution is beneficial for light harvest, efficient separation of the photo-generated charges and electron transport.

Figure 7. Electrochemical impedance spectra (EIS) of the samples

The synergistic effect of the AgNPs imbedded in TiO2 was further investigated by the chronoamperometry (CA) measurements with an open circuit potential (-0.2256 V) in 0.1 M Na2SO4 aqueous solution and under a periodic irradiation of UV 22


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light. The photo-generated current in a CA largely represents the degree of the charge separation during photocatalysis. Figure 8 shows the photocurrent densities of different samples under UV irradiation, and the transient photocurrent densities were recorded by repeating on/off cycles with durations of 50 s. Without UV illumination, all samples displayed low electrochemical currents. However when they were irradiated in UV, the current increased dramatically, caused by a sudden increase of photo-generated electrons. The bare TiO2 produced the lowest photo-current value while the TiO2/Ag (std) the highest, exhibiting an enhancement by a factor of 3.2. This result further supports the hypothesis of heterojunctions within the structure of the film, and it is consistent with the experimental results of photocatalytic performance.

Figure 8. Transient photocurrent responses of TiO2 film and different TiO2/Ag

film electrodes under UV and dark

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3.4. Durability of the TiO2/Ag composite film. The stability of the porous TiO2/Ag nanocomposite film is of great concern for practical application. Under identical conditions, cycled photo-degradation experiments of MB under UV were carried out for 5 times to test the durability and reusability of the TiO2/Ag (std) film. After each degradation test, the films were rinsed with deionized water and dried by a nitrogen gun before the next test. Figure 9 shows that the photocatalytic activities were not weakened after five cycles of test, suggesting that colloidal silica acting as a binder for TiO2 may have contributed towards improving the durability and reusability of the film. This also implies that the TiO2/Ag film prepared in this work may have practical applications.

Figure 9. The photo-degradation of MB in solution under UV radiation for five cycles using TiO2/Ag (std) film.

4. CONCLUSION A TiO2/Ag composite film was prepared by spin-coating of commercial P25 TiO2 powder using colloidal silica as a high temperature binder on FTO, followed by

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electrochemical deposition of AgNPs. The 3 µm thick TiO2 films exhibited the porous morphology and facilitated implantation of AgNPs via electrochemical deposition. Uniformly distributed AgNPs with sizes less than 5 nm were electrically deposited within the porous structure of TiO2 film. The heterostructures of TiO2/Ag enhanced MB degradation under UV irradiation and visible region significantly when compared with the bare TiO2 film. Possible mechanism of such enhancement was discussed in terms of Schottky barriers as well as effects of Surface Plasmon. Experimental results of both electrochemical impedance spectra and transient photocurrent responses provided further supporting evidence for the observed photocatalytic enhancement by TiO2/Ag heterostructures. Colloidal silica, as a high temperature binder, endowed robust characteristics to the TiO2/Ag films, offering efficiency, stability and reusability for photocatalytic applications. The electrochemical deposition technique was demonstrated to be a simple and low cost approach to implant functional chemical species within the porous structure. This study offers a new photo-catalyst with potential industrial applications and may also provide a strategy for preparations of other functional films. AUTHOR INFORMATION Corresponding Author *Telephone: +86 25 84315042; Fax: +86 25 84315042; E-mail: [email protected]. Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Nature Science Foundation of China (51303083) and the Scientific Research Foundation of Nanjing University of Science and Technology (AE89909) for financial support.

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For Table of Contents Only

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Ag Heterostructure

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