Enhanced photoelectrochemical and sensing performance of novel TiO2 arrays to H2O2 detection

Sensors and Actuators B 211 (2015) 111–115

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Enhanced photoelectrochemical and sensing performance of novel TiO2 arrays to H2 O2 detection Lianqing Yu a,∗ , Yaping Zhang a,∗ , Qianqian Zhi a , Qingqing Wang a , Forrest S. Gittleson b , Jinyang Li b , André D. Taylor b a b

College of Science and Key Laboratory of New Energy Physics & Materials Science, China University of Petroleum, QingDao, China Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06511, United States

a r t i c l e

i n f o

Article history: Received 11 September 2014 Received in revised form 20 December 2014 Accepted 9 January 2015 Available online 28 January 2015 Keywords: Silver TiO2 NW arrays Photodeposition Photoelectrochemical property

a b s t r a c t Ag-decorated TiO2 nanowire arrays were facilely prepared by H2 O2 corrosion of Ti and subsequent photodeposition of Ag. Array structures and photoelectric properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis absorption spectroscopy and electrochemical methods. These results show that Ag quantum dots are evenly distributed on the surface of TiO2 nanowires. With an Ag content of 0.4 at%, the electrode’s photocurrent density and photoconversion efficiency obtained optimum values of 0.29 mA/cm2 and 8.55%, respectively. The enhanced photoelectrochemical properties with Ag modification can be attributed to the extended visible light absorption range and improved separation of photo-generated carriers. Additionally, the modified nanowire arrays exhibited much stronger photoelectrocatalytic reduction activity toward hydrogen peroxide in comparison with pure TiO2 arrays or previously reported Ag electrodes. © 2015 Elsevier B.V. All rights reserved.

1. Introduction In recent years, TiO2 material has attracted great interest for environmental applications [1,2], especially in the field of photocatalytic degradation of organic pollutants, because of its biological and chemical inertness, nontoxicity, strong anti-oxidizing power and long-term stability against light. However, commercial application of photocatalytic super fine TiO2 nanoparticles (NPs) is greatly limited due to the difficulty of recycling this material. TiO2 nanostructures on Ti metal substrates prepared by either direct oxidation [3–5] or an anodic oxidation method [6] can overcome this obstacle and demonstrate good photoelectrochemical (PEC) and photocatalytic properties. Simultaneously, in comparison with one-dimensional nanostructures of rod and tube, titania nanowire (NW) arrays have been confirmed to possess an advantage in charge separation over compact films [4], which is of great importance for photocatalysts and photoanodes. A wide band gap and high recombination rate for photogenerated electrons and holes with TiO2 are obstacles to photoelectric application. In order to improve the PEC activity of TiO2 [7,8] various modifications have been demonstrated including with pure metal [9,10] or non-metal dopants [11], or

∗ Corresponding authors: Tel.: +86 0532 86983372; fax: +86 0532 86983372. E-mail address: [email protected] (L. Yu). http://dx.doi.org/10.1016/j.snb.2015.01.060 0925-4005/© 2015 Elsevier B.V. All rights reserved.

surface modification with semiconductors [12–14]. Thanks to its high carrier transfer rate and plasmonic effect, Ag deposited on a TiO2 surface successfully acts as a transfer intermediate for photogenerated electrons and holes, promoting charge carrier separation [15,16]. With a view toward controlling Ag particle size and dispersion, methods of sol–gel [17–20] and photodeposition [21,22] can be used since photocatalytic activity closely depends on content, crystal size, and morphology [23–26]. Specifically, Ag quantum dots and/or nanoclusters can effectively promote the separation of the photo-generated electrons and holes [23]. However, there are still few efforts and little attention paid on the PEC properties and mechanisms of structures like Ag quantum dots modified TiO2 NW arrays. In the electrochemical biosensor field, amperometric detection of H2 O2 provides a signal transduction to indirectly recognize biomolecules such as glucose, cholesterol, and acetylcholine with oxidase modified electrodes [27–33]. Silver has previously been used in electrochemical detectors for H2 O2 , which could exhibit strong cytotoxicity toward a broad range of microorganisms [34] and has shown remarkably low toxicity compared to other heavy metal elements. In this work, we combined the excellent electrocatalytic properties of Ag quantum dots and the photocatalytic activities of TiO2 NW arrays. The resulting Ag/TiO2 array electrodes were characterized by size, structure, PEC activity as well as H2 O2 biosensor behaviors to demonstrate the role of both Ag quantum dots and TiO2 NW arrays.

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X-ray intensity/a.u.

2. Experimental 2.1. Preparation of TiO2 NW arrays Ti foils (2.0 cm × 1.0 cm × 0.2 mm) were first cleaned for 15 min by sonication in acetone and anhydrous ethanol, respectively. Then the dried Ti foil was put into an autoclave filled with 50 ml of H2 O2 . The autoclave was put in a preheated blast electric oven at 80 ◦ C for 6 h and then cooled to room temperature. The sample was washed with distilled water three times before annealing at 450 ◦ C for 2 h.

A−Anatase T-Titanium A 15mM

A

T

T

A

A

A

10mM 5mM 0

2.2. Preparation of Ag/TiO2 arrays Ag quantum dots modified TiO2 arrays were prepared by a photodeposition method. TiO2 NW arrays were immersed for 30 min in AgNO3 solution (5 mM, 10 mM, 15 mM, 20 mM) and marked as 5 mM Ag/TiO2 , 10 mM Ag/TiO2 , 15 mM Ag/TiO2 , and 20 mM Ag/TiO2 , respectively. Then Ag/TiO2 arrays were illuminated with a xenon lamp for 4 min to induce Ag reduction. 2.3. Characterization of samples The phase of TiO2 arrays was determined by X-ray diffraction (XRD) performed on an X-ray diffractometer (Model Dmax-2700, Dandong Fangyuan instrument Co., Ltd., Dandong, China.) with Cu K␣ radiation at 40 kV and 30 mA. The morphologies were investigated with a scanning electron microscope (FESEM, model S-4800, Hitachi Ltd., Shanghai, China). UV–vis absorption spectra were recorded using a Shimadzu UV-2450 spectrophotometer. 2.4. Measurements of PEC properties and H2 O2 sensing PEC performances and Mott–Schottky (MS) spectra of the asprepared samples were measured with a CHI760D electrochemical workstation in a quartz beaker using a three-electrode system with a platinum wire as a counter electrode and a saturated calomel electrode (SCE) as a reference electrode. 1 M of KOH was used as electrolyte. The working electrode was illuminated with a 350 W xenon lamp under a light power density of 35 mW/cm2 . The photocurrent was recorded concurrently with the light switching on and off without applied voltage. Photocurrent of the H2 O2 detection sensor was obtained by a three-electrode system in a stirred 0.1 M phosphate buffer (PBS) solution by applying a potential of 0.15 V to the sample. The concentration of H2 O2 was increased by 2.5 mM after testing every 50 s by injection. 3. Results and discussion 3.1. Characterization Fig. 1 shows the XRD patterns of Ag/TiO2 arrays. All diffraction peaks of the Ag-modified TiO2 arrays can be indexed as Ti (JCPDS 65-6231) or the anatase phase of TiO2 (JCPDS 21-1272). In addition, no characteristic peaks of Ag were detected, which was likely due to the low concentration and well dispersed nature of Ag quantum dots on the TiO2 NW arrays. Fig. 2 shows SEM images of free and Ag modified TiO2 NW arrays. The anatase TiO2 has a well-defined NW array structure, with a length of more than 500 nm and a diameter of approximately 20 nm. No obvious structural changes were observed on the morphology of Ag/TiO2 NW arrays after Ag quantum dots deposition at size of 5–10 nm (Fig. 2(b)). Energy-dispersive X-ray spectroscopy (EDS) indicated Ag content of 0.4 at% for the 10 mM Ag/TiO2 array sample.

20

30

40

50

60

70

2θ/degree Fig. 1. XRD pattern of Ag/TiO2 arrays made with different concentrations of Ag precursor. Table 1 Photoelectric performance of different Ag concentrations. AgNO3 concentration (mM)

Deposit amount (at%)

Photocurrent densities (mA/cm2 )

Photoconversion efficiency (%)

0 5 10 15 20

0 0.19 0.40 0.63 0.82

0.10 0.15 0.29 0.26 0.20

2.69 4.21 8.55 7.47 5.33

The UV–vis absorption spectra of TiO2 and Ag/TiO2 NW arrays are shown in Fig. 2(d). The absorption edge of the pure TiO2 NW arrays is observed at around 410 nm, corresponding to a bandgap energy of 3.12 eV. After the deposition of Ag, the absorption edge is shifted significantly toward the visible region to 468.2 nm, which can be attributed to the surface plasmon resonance absorption of Ag quantum dots. The adsorption wavelength of Ag/TiO2 NW was much higher than that of TiO2 (∼410 nm) or previous reported Ag/TiO2 (∼440 nm) [35]. 3.2. PEC properties under xenon light illumination Fig. 3(a) shows the transient photocurrent responses of TiO2 arrays with Ag quantum dots modification. Upon xenon light illumination, the photocurrent increases with Ag photodeposited on the TiO2 NW arrays (seen in Table 1). The highest photocurrent density was obtained at 0.29 mA/cm2 for NW arrays with 0.4 at% Ag content. We believe that the plasmon resonance effect of Ag quantum dots enhances the transfer and lifetime of photogenerated electrons and significantly improves the PEC activity of TiO2 NWs. However, over-deposition of silver will cover and shield the TiO2 surface from the light. This results in a decrease in the number of generated photoelectrons and holes; therefore the photocurrent decreases with Ag concentrations higher than 10 mM. After calculation (according to our previous report [36]) the photoconversion efficiency from light energy to chemical energy remarkably improved from 2.69% to 8.55% (Fig. 3(b)), after Ag quantum dots modification on TiO2 NW arrays. Fig. 4 represents Nyquist diagrams from electrochemical impedance spectroscopy (EIS) for different samples obtained in dark and under light illumination. It reveals the relationship between the real part (Z ) and the imaginary part (Z ) of the Ag/TiO2 NW array electrodes. The equivalent circuit for this cell system is depicted in the inset of Fig. 4. In the equivalent circuit, Rb is the bulk resistance of the electrolyte, separator, and electrode, corresponding to the resistance value of the high frequency intercept of the semicircle with the real axis. Cdl and Rct are the double

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Fig. 2. SEM images of (a) TiO2 NW arrays, cross-section view, (b) 10 mM Ag/TiO2 NW arrays, top-view, (c) EDS spectra of sample (b), and (d) UV–vis absorption spectra of (a) and (b).

Table 2 Equivalent circuit parameters of TiO2 arrays with different Ag concentrations. AgNO3 concentration (mM)

Rb ( cm2 )

Rsc ( cm2 )

Csc (F/cm2 )

Rct ( cm2 )

0-dark 0-light 10-light 20-light

40.07 30.68 4.153 3.51

5170 913.7 581.9 601.7

2.158E−5 2451 8.665E−5 894.6 1.184E−5 83.03 2.101E−5 383.5

Cdl (F/cm2 ) 5.011E−5 1.063E−4 4.086E−4 8.167E−5

layer capacitance and the charge transfer resistance; Csc and Rsc are the capacitance and the resistance of the solid-state interfacial layer which is formed at the highly charged state due to the passivation reaction between the electrolyte and the surface of the electrode. From the Nyquist plots and the resistance values (seen in Table 2), the Rb of Ag/TiO2 NW samples is smaller than that of the TiO2 NW electrode. Since the electrolyte and separator of the two cell systems are identical, the resistance of Ag/TiO2 NW electrodes must be reduced by increasing the conductivity. The Ag

additive is responsible for this increase in the electronic conductivity. In addition, these results indicate that the solid-state interfacial resistance (Rsc ) and the charge transfer resistance (Rct ) of Ag/TiO2 NW electrodes are smaller than those of the TiO2 array electrodes. Moreover, a metal–semiconductor contact may be formed due to Ag quantum dot-deposition on the TiO2 surface. Since the Fermi level of TiO2 is higher than that of Ag [16,37], photoelectron transfer is likely from the conduction band of TiO2 to Ag. Ag quantum dots may thus act as intermediates for photo-generated electron transfer, or as effective electron traps to inhibit the recombination of holes and electrons [38]. However, with increased silver content beyond the optimum (0.4 at%), impedance increases due to the covering of the TiO2 surface by excess Ag. 3.3. Detection of H2 O2 The photoelectrocatalytic behavior of 10 mM Ag/TiO2 NW arrays on Ti substrate toward the reduction of H2 O2 was tested. A

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μ

μ

114

Fig. 5. Ag/TiO2 NW arrays response to H2 O2 concentration increase from 0 to 22.5 mM (a) photocurrent, and (b) linearly fitted calibration plot. Fig. 3. Ag/TiO2 NW arrays with different concentrations of Ag for (a) transient photocurrent response, and (b) photoconversion efficiency.

Ω

than those obtained with H2 O2 biosensors based on cytochrome c/TiO2 nanoneedles [39], HRP/TiO2 nanoparticles [40] and HRP/ThTiO2 nanotubes [41], which obtained linear ranges from 0.85 to 24,000 ␮M, 7.5 to 123 ␮M, and 10 to 3000 ␮M, respectively.

3.4. Stability and selectivity of the H2 O2 sensor

Ω Fig. 4. Nyquist diagrams of EIS (−Z vs. Z ) in 1 M KOH solution for (a) TiO2 , in dark; (b) TiO2 , under light illumination; (c) 10 mM Ag/TiO2 , under light illumination; (d) 20 mM Ag/TiO2 , under light illumination. Inset includes the equivalent circuit used for fitting the experimental data.

steady-state photocurrent response was obtained and stepwise decreased with increases in H2 O2 concentration from 0 to 22.5 mM, while no response was observed for the pure TiO2 NW arrays in Fig. 5. The calibration plot of steady-state currents for Ag/TiO2 against concentration of H2 O2 is given in Fig. 5(b). The mechanism for this sensor involves the reaction of electron accepting H2 O2 molecules at the Ag quantum dot surface, leading to a photocurrent decrease. Our H2 O2 sensor demonstrates an excellent photoelectrocatalytic performance, including higher sensitivity and a lower detection limit than previously reported [39–41]. The dynamic curve reveals a sensitivity of 8.33 ␮A mM−1 cm−2 with a detection limit of 1.2 mM (signal to noise ratio of 3). The dynamic linear range for detection of H2 O2 from 0 to 22.5 mM, is also wider

After five successive tests, the relative standard deviation of the photocurrent response of Ag modified electrode was only 1.5%. The fabrication reproducibility of five sensors shows a receivable reproducibility with a relative standard deviation of 2.3% for the current determined at 12.5 mM H2 O2 . No obvious decrease in the response to H2 O2 is observed after 2 weeks of storage in PBS at 15 ◦ C. It even retains 92% of its initial current response after 60 days. This indicates that the Ag-modified NW arrays exhibit excellent reproducibility and stability for H2 O2 sensing. The sensitivity of PEC sensors is strongly dependent on the applied potential. We observe that the cathodic current of Ag/TiO2 NW H2 O2 sensors increases with a negative shift in the operating potentials. This observation suggests that the selectivity of PEC sensors is also potential dependent. Similar to the H2 O2 biosensors previously reported, our H2 O2 PEC sensor was free from anodic interference by NO2 − , NO3 − , and SO3 2− at negative potentials, but 1.84% of cathodic response currents were shown to come from 150 ␮M O2 , observed relative to 100 ␮M H2 O2 . On the other hand, at positive potentials (+0.03 V), 1.25% of anodic currents were shown to come from 50 ␮M glucose, relative to 100 ␮M H2 O2 [39]. Fortunately, at the suitable potential of 0 V, both anodic and cathodic potential interferences were negligible.

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4. Conclusion Ag-modified TiO2 NW arrays on Ti substrates were obtained through H2 O2 corrosion combined with a photodeposition method. Ag quantum dots of 5–10 nm were evenly distributed on the TiO2 NW surface without changing the crystal structure of the anatase phase. This electrode retains not only the catalytic activity of Ag quantum dots but also the intrinsic photocatalytic capacity of TiO2 . The deposited Ag effectively captures photo-generated electrons, reducing the rate of electron-hole recombination, as well as enhancing photoelectrochemical properties due to the extended absorption of visible light. Moreover, Ag/TiO2 NW arrays show superior catalytic activity and reproducibility in sensing H2 O2 . These novel PEC sensors with high selectivity and stability may be applicable in the food industry and biomedical fields for future physiological and pathological investigations. Acknowledgements The financial support for this study by National Natural Science Foundation of China (No. 21476262); the Technology Development Plan of Qingdao (No. 14-2-4-108-jch); and the National Natural Science Foundation of Shandong province (No. ZR2011EMQ001). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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Biographies Lianqing Yu obtained his Ph.D. in Materials Science from Zhejiang University (PR China) in 2007. He works as associate Professor at the China University of Petroleum and during the time he has worked as a visiting researcher at Peking University and Yale University in 2011 and 2014. His research interests include magnet materials, the chemical synthesis of functional nanoparticles and their electrochemical properties. Yaping Zhang works as associate professor at the China University of Petroleum. Qianqian Zhi is a master student major in Materials Science. Qingqing Wang is a master student major in Materials Science. Forrest S. Gittleson is a Ph.D. student major in Chemical & Environmental Engineering in Yale University. Jinyang Li is a Ph.D. student major in Chemical & Environmental Engineering in Yale University. André D. Taylor is an associate professor in Chemical & Environmental Engineering Department of Yale University and interested in design nanomaterials for energy conversion and storage.