TiO2 Hetero-Nanostructures for Plasmon

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Highly Ordered Periodic Au/TiO2 Hetero-Nanostructures for Plasmon-Induced Enhancement of the Activity and Stability for Ethanol Electro-oxidation Zhao Jin, Qiyu Wang, Weitao Zheng,* and Xiaoqiang Cui* Department of Materials Science, State Key Laboratory of Automotive Simulation and Control, and Key Laboratory of Automobile Materials of MOE, Jilin University, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: The catalytic electro-oxidation of ethanol is the essential technique for direct alcohol fuel cells (DAFCs) in the area of alternative energy for the ability of converting the chemical energy of alcohol into the electric energy directly. Developing highly efficient and stable electrode materials with antipoisoning ability for ethanol electro-oxidation remains a challenge. A highly ordered periodic Aunanoparticle (NP)-decorated bilayer TiO2 nanotube (BTNT) heteronanostructure was fabricated by a two-step anodic oxidation of Ti foil and the subsequent photoreduction of HAuCl4. The plasmon-induced charge separation on the heterointerface of Au/TiO2 electrode enhances the electrocatalytic activity and stability for the ethanol oxidation under visible light irradiation. The highly ordered periodic heterostructure on the electrode surface enhanced the light harvesting and led to the greater performance of ethanol electro-oxidation under irradiation compared with the ordinary Au NPs-decorated monolayer TiO2 nanotube (MTNT). This novel Au/TiO2 electrode also performed a self-cleaning property under visible light attributed to the enhanced electro-oxidation of the adsorbed intermediates. This light-driven enhancement of the electrochemical performances provides a development strategy for the design and construction of DAFCs. KEYWORDS: ethanol electro-oxidation, heterostructure, surface plasmon, charge separation, periodic structure electro-oxidation.22,24,25 Because light illumination enhances the ethanol electro-oxidation activity, it is a promising method to construct a periodic nanostructure consisting of catalysts to enhance the incident light harvesting.26 In this work, we fabricated a highly ordered periodic Au NPsdecorated bilayer TiO2 nanotube (BTNT) heteronanostructure by Ti foil anodizing and photoreduction of HAuCl4.27,28 This Au/TiO2 heterostructure exhibited excellent electrocatalytic activity for ethanol electro-oxidation under visible light for three reasons: (1) Among various noble metal NPs, Au NPs are the most promising material because of their high electrocatalytic activity toward ethanol29,30 and their excellent LSPR property in visible region.31,32 (2) BTNT grown on Ti foil can be used as working electrode directly27,33 and Au NPs synthesized by photoreduction have clean surfaces without reducing and protective agent. (3) Plasmon-induced charge separation occurs at the Au/TiO2 heterointerface under visible light illumination and enhances the electro-oxidation of ethanol.22 The mechanism schematic of ethanol electrooxidation at the Au/TiO2 heteronanostructure electrode under visible (vis) and ultraviolet (UV) light illumination is

1. INTRODUCTION Direct alcohol fuel cells (DAFCs) are attractive devices in the field of alternative energy for their ability of converting the chemical energy of alcohol into electrical energy directly.1−3 Among various alcohols used in DAFCs, ethanol is the ideal fuel because of its low toxicity, high energy density, abundant nature resources, and easy storage and transportation.4−6 However, the electro-oxidation of ethanol still remains slow and inefficient, which impedes the application of DAFCs.3,5 In recent research, noble metal nanoparticles (NPs), such as Pt,7,8 Pd,9,10 Ag,11,12 and Au,13,14 exhibited good catalytic activity for ethanol electro-oxidation and became the most common catalysts. Meanwhile, many efforts have been made to enhance the activity and stability of noble metal NPs, such as developing NPs with specific structure4,10,15 or with different contents of alloying.16−19 In addition, photoelectrocatalysis has attracted much attention because of the outstanding localized surface plasmon resonance (LSPR) properties, which enhance the catalytic activity of noble metal NPs.20 The recombination of photogenerated hot electrons and holes will reduce the effect of LSPR.21 Combining noble metal NPs with semiconductors prevents this recombination by transferring the excited hot electrons into the conduction band (CB) of the adjacent semiconductor, which is defined as plasmon-induced charge separation,22,23 and has been proved to be effectual for ethanol © XXXX American Chemical Society

Received: November 20, 2015 Accepted: February 10, 2016

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DOI: 10.1021/acsami.5b11259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

with DI water and drying in a pure nitrogen stream prior to use. The two-step anodization was carried out using a conventional twoelectrode system with Ti foil as an anode and Pt gauze as a cathode. The electrolyte was consisted of 0.3 wt % NH4F in ethylene glycol solution as well as 2 vol % water, and the reaction temperature was kept at 25 °C by a circulating water bath. The Ti foil was anodized under a constant voltage of 60 V for 1 h in the first-step anodization to form the ordinary monolayer TiO2 nanotube (MTNT). The formation of MTNT is an outcome of the competition between electric oxidation of Ti foil to TiO2 film and the electrochemical corrosion of TiO2 film with the existence of fluoride anions.40 Then, the as-prepared MTNT was ultrasonically removed in DI water, leaving a regular hexagonal shallow concave array on the Ti foil. After rinsing with DI water, the same Ti foil was anodized again under 40 V for 1 h in the second-step anodization, forming a BTNT with ordered periodic structures. During the second anodization, the concave array on Ti foil was anodized as the top layer and the remaining Ti foil was anodized as the bottom nanotube arrays with small diameter.40 The anodized samples were rinsed with ethanol, dried with pure nitrogen, and annealed in air at 480 °C for 3 h with a heating rate of 5 °C/min. 2.4. Au NP Decoration. Decoration of Au NPs on the BTNT was carried out via an in situ photocatalytic reduction method.28 The sample was soaked in 0.05 mM aqueous HAuCl4 for 2 h for the absorption of AuCl4− onto TNT surface. Then it was irradiated in situ with a 300 mW/cm2 white light for 90 min to reduce the absorbed AuCl4− into Au by the photocatalysis of TiO2. This green method provided Au NPs with the clean surface without reducing and protective agents, which enhanced the catalytic activity of Au.41 The schematic of all the fabrication process is shown in Figure 2. MTNT

shown in Figure 1a. Under visible light illumination, electrons of Au NPs are photoexcited; the electrons will be injected into

Figure 1. (a) The mechanism schematic of ethanol electro-oxidation on the Au/TiO2 heteronanostructure electrode under vis and UV light illumination. (b) The schematic of the periodic nanotructure of Au/ TiO2 electrode. This morphology increases the absorption of incident light and enhances the activity.

the CB of TiO2 and conducted away by the external circuit.34 This transfer of electrons leads to a hole-rich surface of Au NPs,35−37 which makes oxidation of the adsorbed ethanol much easier and enhances the catalytic activity.24 On the contrary, TiO2 substrate only absorbs the ultraviolet light under illumination. The electrons in the valence band (VB) of TiO2 will be photoexcited and transfer to CB, then conducted away by the external circuit.38 There is no charge separation on Au surface under ultraviolet light, so it provides no enhancement of ethanol electro-oxidation. (IV) The Au NPs decorated on BTNT formed a hexagon periodic nanostructure, as shown in Figure 1b, which helps to enhance the light harvesting26,39 compared with the MTNT. In addition, this highly ordered Au/ TiO2 heterostructure also shows a self-cleaning property under visible light illumination during long-term stability test, which is attributed to the electrocatalytic activity enhancement.

Figure 2. Schematic for the fabrication of the BTNT decorated with highly ordered Au NPs. was decorated with Au NPs under the same condition for comparison. The concentration of HAuCl4 and the irradiation time were optimized to form the evenly distributed Au NPs with an average size of 26 nm. Other sizes of Au NPs were formed using different concentration of HAuCl4 under the same irradiation condition for comparison.

2. EXPERIMENTAL SECTION 2.1. Materials. The pure Ti foil was purchased from Sigma-Aldrich (99.7%, 0.127 mm) and was cut into pieces of 0.5 × 1 cm prior to use. Ethylene glycol (EG) and chloroauric acid (HAuCl4) were from Sinopharm Chemical Reagent Co., Ltd., China. Ammonia fluoride (NH4F), potassium hydroxide (KOH), ethanol and all other chemicals were obtained from Beijing Regent Co. China. Deionized (DI) water was used throughout the experiments with a resistivity of 18.2 MΩ cm. 2.2. Instrument. Constant voltage during anodization was supplied by a SAKO DC power supply. Morphologies of the fabricated samples were characterized by a JEM-6700F (JEOL, Japan) scanning electron microscope (SEM). X-ray diffraction (XRD) patterns were performed by a D8 advanced Bragg−Brentano diffractometer (Bruker AXS, German) and the diffuse reflectance UV−vis adsorption spectra were recorded on a Cary 500 UV−vis−NIR spectrophotometer (Hitachi, Japan). A 300W Xe lamp was used as the illuminant during Au decoration and ethanol electro-oxidation. The electrochemical detection was carried out on a CHI 650D electrochemical workstation (Chenhua Co., China). 2.3. Preparation of the BTNT. The BTNT was fabricated via a two-step anodization of the pure Ti foil.40 The Ti foil was first degreased by sonication in acetone and ethanol, followed by rinsing

3. RESULTS AND DISCUSSION 3.1. Characterization. Figure 3a shows a top view SEM image of the as-prepared BTNT. A nearly hexagonal mesh periodic structure top layer of TiO2 is observed on the ordinary MTNT with an average pore diameter of 110 nm and a wall thickness of 20 nm. This periodic structure is expected to provide much more nucleation sites and work as a frame during the photocatalytic reduction of HAuCl4. Figure 3b shows the top view SEM of the Au NPs decorated BTNT. After irradiation, the in situ formed Au NPs were distributed neatly along the top layer of TiO2 framework, forming a periodic hexagon structure of Au NPs decorating onto the surface. The average size of Au NPs was 25.8 nm as shown in the size distribution of the inset of Figure 3b. XRD patterns are shown in Figure 3c. Excluded the diffraction peaks of Ti foil, the prepared BTNT could be fitted to the anatase with preferential orientation of (101). Two new diffraction peaks appeared after B

DOI: 10.1021/acsami.5b11259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

morphological change will affect the catalytic activity in electrochemical oxidation of ethanol. The concentration of HAuCl4 used for Au decorating has a decisive impact on the morphology. Figure 4c,d shows the SEM characterization of Au decorated BTNT prepared in 0.01 and 0.1 mM HAuCl4. Compared with the optimized sample prepared in 0.05 mM (Figure 3b), lower concentration of HAuCl4 provides less AuCl4− during the reaction and forms small amounts of Au NPs on BTNT with small average size of 12.7 nm. Higher concentration of HAuCl4 provides abundant AuCl4−, which is easy to aggregate during the reaction and form large size of Au NPs up to 74.5 nm. The electrochemical performances of electrodes fabricated under different conditions are discussed below. 3.2. Electrochemical Performance. The electrochemical performances without illumination were first investigated. All the cyclic voltammogram (CV) and amperometric i−t curves were carried out using a three-electrode electrochemical system with the Au/TiO2 heteronanostructure serving as working electrode, a Pt wire as the counter electrode, and a saturated Ag/AgCl as a reference electrode. Figure 5a,c shows the CVs

Figure 3. SEM characterization of BTNT (a) before and (b) after the decoration of Au NPs. (b, inset) Size distribution of Au NPs. (c) XRD and (d) diffuse reflectance UV−vis absorption spectra.

Au NPs decorated at 44.3 and 64.4°, respectively, which could be indexed to the (200) and (220) planes of Au and proved the successful decoration of Au onto BTNT. Figure 3d shows the diffuse reflectance UV−vis absorption spectra of MTNT and BTNT before and after the Au loading. The pristine MTNT and BTNT show a broad absorbance peak in visible region, which may be attributed to the impurity doping during the fabrication42 and the intrinsic defects sensitization of the nanotube.43 An absorption peak attributed to the LSPR absorption of Au NPs appears at 550−570 nm after the decoration of Au NPs for both MTNT and BTNT.31,44 The adsorption peak at 360 nm decreases after the Au decoration, indicating that the exposed TiO2 surface is reduced because of the Au NPs loading. BTNT shows better absorption than MTNT in both ultraviolet and visible region indicating that the highly ordered periodic nanostructure indeed enhances the light harvesting.40 Figure 4a,b show the SEM images of the ordinary MTNT before and after the decoration of Au NPs. Compared with the

Figure 5. Electrochemical performance of different electrodes prepared in different conditions: (a and c) CVs recorded in 0.5 M H2SO4 and (b and d) CVs recorded in 1 M KOH mixed with 1 M ethanol. Scanning rate is 50 mV/s.

recorded in 0.5 M H2SO4 used to calculate the electrochemical active surface areas (ECSAs) of different electrodes.14 The ECSA of BTNT in 1 M KOH shows a result similar to that in acid (Figure S1). Figure 5b,d shows the CVs recorded in 1 M KOH mixed with 1 M ethanol to compare the catalytic activity for ethanol electro-oxidation. Figure 5a,b shows the different electrochemical performance of pure BTNT (black), BTNT with Au NPs (red), and MTNT with Au NPs (green). Before Au NPs decorating, the CVs of pure TiO2 show no obvious peak in both panels a and b in Figure 5. After the Au NPs were decorated, apparent peak current can be observed for both BTNT and MTNT. It proves the Au NPs decorated TNT prepared by the photocatalytic reduction method can be indeed used as the ethanol electro-oxidation electrode. As shown in Figure 5a, the calculated ECSA of Au decorated MTNT and BTNT is 0.063 and 0.164 cm2, respectively. The ECSA value of BTNT is increased approximately 2.6 times than that of MTNT. The oxidation peak of Au for MTNT between 1.0 and 1.2 V is also not obvious compared with that of the BTNT. CVs of ethanol electro-oxidation in Figure 5b show the similar

Figure 4. SEM characterization of (a) MTNT, (b) MTNT with Au NPs reduced in 0.05 mM HAuCl4, (c) BTNT with Au NPs reduced in 0.01 mM HAuCl4, and (d) BTNT with Au NPs reduced in 0.1 mM HAuCl4. (b−d, insets) Size distributions of Au NPs.

BTNT, the morphology of MTNT in Figure 4a is uneven. Without the periodic top layer providing adequate nucleation sites and working as the frame, the Au NPs decorated onto the MTNT were in the small amount, disordered and clustered with an average size of 29.1 nm as shown in Figure 4b. This C

DOI: 10.1021/acsami.5b11259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Effect of illumination to the CVs measured (a, d) in the absence of and (b, e) in the presence of 1 M ethanol in 1 M KOH. (c and f) Corresponding statistic histograms of ethanol electro-oxidation peak currents. Scanning rate is 50 mV/s.

enhanced by visible light illumination.22 Figure 6c shows the statistic of peak currents with and without visible light for MTNT and BTNT, respectively. For MTNT, the histogram intuitively indicated that the net peak current has an increase of 1.29 fold from 0.24 to 0.31 mA/cm2 after visible light illumination. It was 1.82 fold from 0.61 to 1.11 mA/cm2 for BTNT. This increase of peak current is caused by the LSPR property of Au NPs, which is related to the adsorption of incident light. The net peak current of BTNT under visible light (1.11 mA/cm2) was 3.6 fold higher than that of the MTNT (0.31 mA/cm2) because of the hexagon periodic nanostructure of Au NPs enhances the visible light harvesting, as discussed above. Hence, the BTNT can excite much more electrons of Au NPs injected to the CB of TiO2 than MTNT. The Au NPs with fewer electrons and more holes on the surface oxidize ethanol much easier, leading to the greater enhancement of peak current under visible light. The effect of ultraviolet light illumination (λ = 365 ± 5 nm, 1 mW/cm2) was further investigated as a contrast shown in Figure 6d−f. In the absence of ethanol, the baselines of CVs measured in 1 M KOH are significantly increased by ultraviolet light illumination as shown in Figure 6d because of the strong absorption of TiO2 in ultraviolet region. The ultraviolet illumination effect to the current of BTNT is larger than that of MTNT, which is agreed with the absorption spectra in Figure 3d. Same phenomenon happened for the CVs measured in the presence of 1 M ethanol as shown in Figure 6e. The ultraviolet light illumination has obvious effect to the baseline currents as well. The statistics of the peak currents in Figure 6f indicates that there is almost no change of the net peak currents for both MTNT and BTNT before and after the ultraviolet illumination. This result further confirms that the ultraviolet light is absorbed by TiO2 and plenty of photoexcited electrons in CB of TiO2 were conducted away by the external circuit, which leads to the large increase in the baseline. There is no charge separation occurred on the interface of Au NPs/TiO2 under ultraviolet light illumination. Therefore, there is barely increase of the net peak current for ethanol electro-oxidation. The baseline changes of the CVs from MTNT and BTNT without Au NPs show the same trend as Figure 6a,d (Figure S2). The corresponding amperometric i−t curves are shown in

phenomenon. The measured peak current of ethanol oxidation for MTNT and BTNT is 0.22 mA/cm2 and 0.64 mA/cm2, respectively, which increased approximately 2.9 times. This result indicated that even the fabrication condition was same, BTNT with periodic hexagonal structure offers more nucleation sites for the formation of Au NPs than does MTNT. The periodic structure also works as a frame to form the highly ordered Au NPs. Therefore, the electrocatalytic activity of Au NPs decorated BTNT was almost 3 times higher than that of the ordinary Au/MTNT. Figure 5c,d shows the different electrochemical performance of Au decorated BTNT prepared in different concentration of HAuCl4. The amount and size of Au NPs will affect the catalytic activity apparently.25 For 0.01 mM HAuCl4 sample (black), there is barely ECSA in H2SO4 and very weak electrooxidation peak in ethanol because of the small amount of Au, although the smaller size of nanoparticle provides higher specific surface area. For 0.1 mM HAuCl4 sample (green), the calculated ECSA is 0.041 cm2 and the ethanol electro-oxidation peak current is 0.17 mA/cm2, both of which are a quarter of the 0.05 mM sample (red). This 0.1 mM sample contains large amount of Au NPs, but the bigger size of Au NPs (Figure 4d) makes the specific surface area even low and affects the catalytic activity for ethanol. 3.3. Effect of Illumination. Visible light illumination causes the plasmon-induced charge separation on the interface of Au/TiO2, which enhances the ethanol electro-oxidation. On the contrary, the excited electrons in CB of TiO2 generated under ultraviolet light illumination will not affect the catalytic activity of Au NPs.38 Figure 6 shows the illumination effect to the CV test on MTNT and BTNT decorated with Au NPs. The effect of visible light illumination (λ > 420 nm, 100 mW/cm2) was first investigated, as shown in Figure 6a−c. Figure 6a shows that the visible light illumination has little influence on the CVs in the absence of ethanol for both MTNT and BTNT. CVs with 1 M ethanol in 1 M KOH was shown in Figure 6b. Two typical peaks corresponding to the electro-oxidation of ethanol appear at 0.17 and 0.02 V.18,30 Under visible light illumination, the peak currents of both MTNT and BTNT are significantly increased. These significant increases in peak currents indicated that the catalytic activity of Au NPs to ethanol is indeed D

DOI: 10.1021/acsami.5b11259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. Effect of (a) visible light illumination and (b) ultraviolet light illumination to amperometric i−t curves for the ethanol electro-oxidation. The curves were recorded in 1 M KOH mixed with 1 M ethanol at 0.16 V vs Ag/AgCl.

site of catalyst continuously, leading to the decrease in the catalytic activity. Under visible light illumination, the catalytic activity of Au NPs is improved by the plasmon-induced charge separation, which also has a significant impact on the stability. The hole-rich surface of Au NPs has a strong oxidizing property for both ethanol and the intermediates. Therefore, intermediates adsorbed on Au NPs were much easier to be further oxidized under visible light illumination, leading to the release of the covered active site.24 This so-called self-cleaning property can recover the activity of Au NPs with visible light and cause the gradual increase of current. For the MTNT, after the considerable decrease in the dark, the peak current also got a momentary increase when the light source was turned on. Then, different from the BTNT, the current became basically stable instead of increasing. After 100 cycles in visible light, the current of MTNT got a little decrease less than 5%, which is much better than the decrease in the dark. This performance indicated that visible light can also affect the stability of Au/ MTNT, which is much weaker than that of BTNT. Analysis results from Figure 6 clearly revealed that the BTNT has greater enhancement of the catalytic activity under visible light than the MTNT because of the periodic hexagon structure formed by Au NPs, which means the enhancement of ethanol and intermediates oxidation caused by visible light for MTNT is limited. This limited increase of the catalytic activity is enough to prevent the active site from further coverage but not enough to oxidize the adsorbed intermediates in an effective and rapid way. Thus, the peak current of MTNT turned to be stable under visible light instead of gradually increasing. The stability tests started with visible light using fresh prepared BTNT and MTNT show that net peak currents of MTNT and BTNT are gradually decreased 13 and 6%, respectively, after the first 100 cycles (Figure S3). The stability is also obviously enhanced by visible light illumination compared with the value of ∼40−45% decrease in darkness after the first 100 cycles, as shown in Figure 8a. Figure 9 shows the effect of UV illumination on the stability for comparison. Compared with visible light, the result indicated that the effect of UV light illumination on the stability is rather small. TiO2 substrate absorbed UV light and generated hydroxyl radical (·OH) on the surface, helping to further oxidize the intermediates.46 However, as the surface of the electrode was fully covered with Au NPs, this effect is not obvious. For further investigation, 500 CV cycles of ethanol electrooxidation were carried out using the Au NPs decorated BTNT and the visible light source was turned on/off every 100 cycles. The statistic of net peak current is shown in Figure 10. The

Figure 7, which were recorded at 0.16 V with light turning on/ off every 60 s. The currents increase immediately when the light is turned on and fall back when the light is turned off for both vis and UV in Figure 7a,b. The increase of current under illumination includes the enhancement of net peak current and the baseline. The i−t curves show in Figure 7 further confirmed the illumination effect for ethanol electro-oxidation. To evaluate the effect of illumination for the working stability, we performed 200 cycles of CV for different samples in different conditions. As shown in Figure 8a, the statistic of

Figure 8. Effect of visible light illumination on stability. (a) The statistic of peak current for both BTNT and MTNT in dark and visible light and corresponding CVs of (b) BTNT and (c) MTNT. Scanning rate is 50 mV/s.

the net peak current indicated that both the MTNT and BTNT have poor stability in the dark for long time CV test as the peak current decreased obviously with the cycle number. After visible light irradiated, the stability has been improved for MTNT and BTNT. The corresponding CVs were shown in Figure 8b for BTNT and Figure 8c for MTNT. It is evident from the statistic that after 100 CV cycles in the dark, the net peak current of both BTNT and MTNT got a ∼40−45% decrease in total. Then, the visible light source was turned on. For the BTNT, the peak current got a 12% increase immediately because of the plasmonic effect and increased gradually with the cycle number goes on. The current reached a plateau after another 100 cycles, which is more than 90% of the original current, indicating that visible light has a significant effect on the stability for this Au decorated BTNT. This performance also indicates the decrease of current in the dark is reversible. Thus, the loss of catalytic activity caused by dropping off or clustering of Au NPs can be excluded. The current decrease in the dark is believed to be the adsorption of intermediates. Intermediates such as acetic acid and aldehyde produced in the electro-oxidation of ethanol30,45 are adsorbed onto the surface of Au NPs, covering the active E

DOI: 10.1021/acsami.5b11259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11259. CVs measured in 0.5 M H2SO4 and 1 M KOH, light effect to MTNT and BTNT without Au NPs, stability test started with visible light using fresh prepared electrodes, SEM characterization of BTNT and CVs measured with different potential range. (PDF)

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Figure 9. Effect of ultraviolet light illumination on the stability. (a) The statistic of peak current for both BTNT and MTNT in dark and ultraviolet light and corresponding CVs of (b) BTNT and (c) MTNT. Scanning rate is 50 mV/s.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21275064 and 51571100), the Specialized Research Fund for the Doctoral Program of Higher Education (20130061110035), and the Program for New Century Excellent Talents in University (NCET-100433).

Figure 10. Statistic of net peak current of 500 CV cycles with visible light turned on/off every 100 cycles. Scanning rate is 50 mV/s.

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REFERENCES

(1) Bianchini, C.; Shen, P. K. Palladium-Based Electrocatalysts for Alcohol Oxidation in Half Cells and in Direct Alcohol Fuel Cells. Chem. Rev. 2009, 109, 4183−4206. (2) Tiwari, J. N.; Tiwari, R. N.; Singh, G.; Kim, K. S. Recent Progress in the Development of Anode and Cathode Catalysts for Direct Methanol Fuel Cells. Nano Energy 2013, 2, 553−578. (3) Antolini, E. Catalysts for Direct Ethanol Fuel Cells. J. Power Sources 2007, 170, 1−12. (4) Xu, C.; Wang, H.; Shen, P. K.; Jiang, S. P. Highly Ordered Pd Nanowire Arrays as Effective Electrocatalysts for Ethanol Oxidation in Direct Alcohol Fuel Cells. Adv. Mater. 2007, 19, 4256−4259. (5) Kowal, A.; Li, M.; Shao, M.; Sasaki, K.; Vukmirovic, M. B.; Zhang, J.; Marinkovic, N. S.; Liu, P.; Frenkel, A. I.; Adzic, R. R. Ternary Pt/ Rh/SnO2 Electrocatalysts for Oxidizing Ethanol to CO2. Nat. Mater. 2009, 8, 325−330. (6) Lamy, C.; Lima, A.; LeRhun, V.; Delime, F.; Coutanceau, C.; Leger, J. M. Recent Advances in the Development of Direct Alcohol Fuel Cells (DAFC). J. Power Sources 2002, 105, 283−296. (7) Zhou, W. J.; Zhou, Z. H.; Song, S. Q.; Li, W. Z.; Sun, G. Q.; Tsiakaras, P.; Xin, Q. Pt Based Anode Catalysts for Direct Ethanol Fuel Cells. Appl. Catal., B 2003, 46, 273−285. (8) Xu, C.; Shen, P. k.; Liu, Y. Ethanol Electro-oxidation on Pt/C and Pd/C Catalysts Promoted with Oxide. J. Power Sources 2007, 164, 527−531. (9) Liang, Z. X.; Zhao, T. S.; Xu, J. B.; Zhu, L. D. Mechanism Study of the Ethanol Oxidation Reaction on Palladium in Alkaline Media. Electrochim. Acta 2009, 54, 2203−2208. (10) Qi, K.; Wang, Q.; Zheng, W.; Zhang, W.; Cui, X. Porous SingleCrystalline Palladium Nanoflowers with Enriched {100} Facets for Highly Enhanced Ethanol Oxidation. Nanoscale 2014, 6, 15090− 15097. (11) Fu, S.; Zhu, C.; Du, D.; Lin, Y. Facile One-Step Synthesis of Three-Dimensional Pd-Ag Bimetallic Alloy Networks and Their

peak current decreased in the dark continuously and turned to increase gradually under visible light every time, further confirming that the self-cleaning property of BTNT is repeatable during the long time test. Although the recovery of the catalytic activity cannot be 100% in each cycle, the tendency is also delightful.

4. CONCLUSION The slow and inefficient electro-oxidation of ethanol still impedes the application of DAFCs. To further enhance the catalytic activity of ethanol, we fabricated a highly ordered periodic Au/TiO2 heteronanostructure and used it for visible light induced enhancement of ethanol electro-oxidation. This novel heterostructure electrode with Au NPs periodic structure performs excellent ethanol electro-oxidation activity and distinctive self-cleaning property under visible light illumination because of the plasmon-induced charge separation and the enhanced visible light harvesting. The catalytic current of this electrode under visible light reached to 1.11 mA/cm2, which is 3.6 fold higher than that of the ordinary Au NPs-decorated MTNT. The peak current decreased in the dark was recovered to more than 90% under the visible light irradiation because of the plasmon-induced self-cleaning property. The combination of plasmonic periodic nanostructure and electro-oxidation of ethanol offers a new strategy for the design of DAFCs in the future. F

DOI: 10.1021/acsami.5b11259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.5b11259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX