as

Xis Re

H2O2 sensorPhotoelectrochemical devices

iedogicby enotde

respectively. Moreover, ZnOSnO2 nanotubes electrode appears more outstanding H2O2 sensing capabil-

ism ofsignalito deas we

chemical and optical detection. Therefore, the development ofnon-enzymatic H2O2 PEC sensor has drawn more attention. PECreactions often happen on the electrode material surface, includingadsorptionreactiondesorption process, thus, the performance ofPEC depends on the effective surface area and surface defectconcentration. It is well known nanostructures possess largesurface volume ratio and ample surface defects. Recently various

erable researcheseparationicle modicture fabr

[8,2022]. It has been clearly demonstrated the photo-genelectrons and holes can be separated effectively throughenergy band alignment of semiconductor composites. Nevless, few researches have been devoted to investigate semiconduc-tor composites on the application of PEC biosensor. Therefore, it isnecessary and instructive to explore the potential of semiconduc-tor nano-composites for PEC biosensor such as H2O2 monitoring.

ZnO and SnO2 are important semiconductors with wide bandgaps. They exhibit good electron transport properties, excellentoptical properties, and good biocompatibility, leading to diverse

Corresponding authors. Tel.: +86 9318912616; fax: +86 9318913554.E-mail addresses: zhangzx@lzu.edu.cn (Z. Zhang), xieeq@lzu.edu.cn (E. Xie).

Journal of Alloys and Compounds 614 (2014) 373378

Contents lists availab

s a

.e lhas relatively high cost, short lifetime and the critical operatingsituation, which limits enzyme-biosensor applicability [10]. Onthe other hand, PEC detection is a promising analytical techniquewith a potentially higher sensitivity than conventional electro-

improve the charge transfer efciency. Considhave been focused on improving the charge ssemiconductor systems through metal nanopartcarbon materials composition and heterostruhttp://dx.doi.org/10.1016/j.jallcom.2014.06.1300925-8388/ 2014 Elsevier B.V. All rights reserved.in thecation,icationeratedpropererthe-Up till now, many methods, such as colorimetric, uorescence,surface plasma resonance, electrochemical and PEC detection havebeen applied in biological detection [59]. Among them, enzymebased electrochemical method has much higher sensitivity andselectivity compared to the other methods. However, this method

pairs which can improve their photoelectrical properties. However,fast recombination of photo-generated electronhole pairs willgreatly hinder the photoelectrical application of those semicon-ductors. Thus, it is very important to reduce the recombination ofphoto-generated electronhole pairs in the semiconductors to1. Introduction

H2O2 is necessary for the metaboletc. It plays an important role in cell[1,2]. Thus, it is of high importancebiomedical, environment chemistryity than pure SnO2 nanotubes electrode under UV radiation. These can be well claried by electron trans-fer mechanism in ZnOSnO2 nano-composite. Different reactions on SnO2 and ZnOSnO2 surface havebeen explained in details based on PEC performances of the two modied electrodes. This facile strategyextends the application of semiconductor composite for monitoring biomolecules with PEC method.

2014 Elsevier B.V. All rights reserved.

proteins, fats, vitamins,ng and communicationtect H2O2 in analytical,ll as other elds [3,4].

non-enzyme PEC sensors have been focused on metal, semiconduc-tor nanostructures and carbon materials [1114]. It has beenreported semiconductor nanostructures such as ZnO, TiO2, SnO2,MoS2 and WO3 exhibiting remarkable photosensitive propertyhave been frequently applied in PEC devices [1519]. Under suit-able light irradiation, semiconductors can generate electronholeKeywords:ZnOSnO2 composite

electrode to hydrogen peroxide (H2O2) randomly increases with increased H2O2 concentration. Whilethe dark current and photocurrent of ZnOSnO2 nanotubes electrode decrease exponentially and linearly,Enhanced photoelectrochemical sensor bnanotubes

Xiuyun An, Feng Teng, Peng Zhang, Changhui Zhao,School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People

a r t i c l e i n f o

Article history:Received 25 November 2013Received in revised form 18 June 2014Accepted 18 June 2014Available online 27 June 2014

a b s t r a c t

Composite nanotubes modgreat potentialities in biolnanotubes were preparedof ZnOSnO2 composite naof large amount of oxygen

Journal of Alloy

journal homepage: wwwed on ZnOSnO2 composite

aojun Pan, Zhenxing Zhang , Erqing Xie public of China

electrodes with enhanced photoelectrochemical (PEC) performance haveal systems monitoring and detection. In this paper ZnOSnO2 and SnO2lectrospinning. The results show that the visible light emission intensityubes is much higher than that of SnO2 nanotubes, indicating the existencefects in ZnOSnO2 nanotubes. Photocurrent response of SnO2 nanotubes

le at ScienceDirect

nd Compounds

sevier .com/locate / ja lcom

son, many researchers have investigated ZnOSnO composite as

mechanism has been investigated. These results open up a new

the ZnOSnO2 precursor solution. After vigorously stirring at room temperature

these randomly oriented nanotubes have rough surfaces after cal-

d Cofor 3 h, the electrospinning was performed at a direct voltage of 13.5 kV, a distanceof 18 cm between the needle tip and collector, and a feed rate of 0.6 ml/h. Thenthe as-electrospun polymer composite bers were annealed at 500 C for 2 h in air.

2.2. Fabrication of nanotubes modied electrodes

Nickel (Ni) foams were used as the skeletons of nanotubes. Firstly, Ni foams(Changsha Lyrun New Material Co. Ltd., China) were cut into a size of10 15 mm2. The Ni foams were ultrasonically cleaned sequentially in HCl aqueoussolution (volume ratio of 10%), acetone, and deionized water for 15 min each. Sec-ondly, 3 mg as-prepared nanotubes, acetylene black, and Polyvinylidene Fluoride(PVDF) were mixed with a mass ratio of 8:1:1, together with a certain amount ofN-methyl pyrrolidone (NMP). The prepared Ni foams were brushed with the abovemixture with a coating area of 1 cm2, and dried at 120 C for 3 h in nitrogen atmo-sphere. Finally, a copper wire as electrical contact was connected to the Ni foamwith silver paste to decrease contact resistance. SnO2 and ZnOSnO2 nanotubesmodied Ni foams were marked as the electrode 1 and electrode 2, respectively.

2.3. Apparatus and measurements

The morphologies, crystal structures, and chemical characterizations of nano-tubes were analyzed by a high-resolution transmission electron microscope(HRTEM, Tecnai G2F30, FEI, USA) and an X-ray diffraction instrument (XRD, PhilipsX0pert pro MPD the Netherlands). The characteristics of luminescence wererecorded by a spectrometer (JobinYvon LabRAM HR800 UV, YGA) with a wave-length of 325 nm line of 50 mWHeCd laser as the excitation source. Cyclic voltam-metry (CV) curves were obtained with an electrochemical workstation (RST5200,Zhengzhou Shiruisi Technology Co. Ltd., China). A three-electrode system was usedin the measurements, with the electrode 1 or the electrode 2 as the workingperspective for semiconductor nano-composite application inPEC biosensors.

2. Experiments and methods

2.1. Preparation of nanotubes

All the chemicals were of analytical grade without further purication, andaqueous solutions were prepared with ultrapure water. ZnOSnO2 and SnO2 nano-tubes were prepared by electrospinning and subsequent calcination. To obtain theZn/Sn atomic ratio of approximate 1:1 in ZnOSnO2 samples, the precursorsolution of ZnOSnO2 nanotubes was prepared with 1.1 g zinc acetate dihydrate(Zn(CH3COO)22H2O, Tianjin Chemical Corp., China) and 1.1 g tin dichloride dihy-drate (SnCl22H2O, Tianjin Chemical Corp., China) dissolved in a mixture of 2.2 gethanol and 2.2 g N, N-dimethyl formamide (DMF) in a vessel stirred for 1 h. Subse-quently, 0.4 g of polyvinyl pyrrolidone (PVP, Sigma Aldrich, MW 1,300,000) wasadded to the above solution. The precursor solution of pure SnO2 nanotubes wasprepared with 2.2 g SnCl22H2O, all others chemical agents were the same as of2

NO2, HCHO gas sensors, photocatalysis and PEC degradation oforganics [2730].

Herein, we have explored ZnOSnO2 and SnO2 nanotubesmodied electrodes on the performances of PEC non-enzymaticbiosensor towards the response of H2O2. To the authors knowl-edge, this is the rst report to employ semiconductor compositenanostructure on PEC enzyme-free detecting for H2O2. Resultsindicate ZnOSnO2 nanotubes possess larger amount of oxygenvacancies (VO) and better electron transfer than SnO2 nanotubes.ZnOSnO2 nanotubes modied electrode has more efcient sensi-tivity than SnO2 nanotubes modied electrode, and the detectionapplications in optoelectronic, sensormetrics, and biomedical eld[18,2326]. The difference of work function between ZnO and SnO2will result in the moving of negatively charged carriers of theZnOSnO2 composite from SnO2 to ZnO until their Fermi levelsare balanced. When the ZnOSnO2 composite is excited byultraviolet (UV) light, photo-generated electrons and holes can beefciently separated by the rearranged energy band structure[15,20]. These coupled oxide semiconductor material have specialproperties of conductance variability and sensitivity. For this rea-

374 X. An et al. / Journal of Alloys anelectrode, a platinum electrode as the auxiliary electrode, and a saturated calomelelectrode (SCE) as the reference electrode. The supporting electrolyte was 0.1 Mphosphate buffered solution (PBS) with a PH value of 7.0.3.2. Optical properties of nanotubes

Pure SnO2 nanotubes show a wide emission peak in the PL spec-trum, while ZnOSnO2 nanotubes show enhanced UV and visibleemission bands centered at 382, 572 and 620 nm in the PL spec-trum as shown in Fig. 2. Without controversy the UV emission ofZnOSnO2 nanotubes is related to the near-band-gap emission ofZnO nanostructure [33]. Previous results have demonstrated thatsingly ionized oxygen vacancy (VO) of ZnO nanostructure can actas the green luminescent center [34,35]. Therefore, the PL emissionpeak centered at 572 can be attributed to the VO of ZnO nanocrys-tals in ZnOSnO2 composite nanotubes. In addition, it has beenproved that the emission peak at about 610 nm originates fromthe defect electronic state formed by surface oxygen vacancies(SVO) of SnO2 [36]. The Zn2SnO4 appearing in ZnOSnO2 nanotubescan generate defects especially SVO [37]. In other words, bothgreen emission and yellow emission originate from VO ofZnOSnO2 composite nanotubes. The visible light emission inten-sity of ZnOSnO2 nanotubes is 14 times higher than that of SnO2nanotubes implying abundant SVO in ZnOSnO2 nanotubes.Furthermore, considerable researches have shown that SVO insemiconductor oxides can act as active sites and produce trappedstates helping inhibit electronhole pair recombination [20,37,38].

3.3. PEC measurements

In order to investigate the application of these two kinds ofnanotubes in PEC biosensing system, the photoelectrocatalyticbehaviors of the nanotubes modied electrodes towards the elec-trochemical reduction of H2O2 were studied using CV. Various CVcination at 500 C due to the decomposition of PVP. Furthermore,the average diameter of SnO2 nanotubes is smaller than that ofZnOSnO2 nanotubes. The typical selected area electron diffraction(SAED) patterns of nanotubes are displayed in Fig. 1b and d. It canbe seen that the SnO2 nanotube is polycrystalline with a rutilestructure (Fig. 1b), and the ZnOSnO2 nanotube is composed ofpolycrystalline hexagonal wurtzite ZnO and rutile SnO2 (Fig. 1d).Energy-dispersive X-ray spectroscopy (EDS) analyses (Fig. 1e) con-rms that the ZnOSnO2 nanotube composite is composed of Zn,Sn, and O, and the Zn/Sn atomic ratio is about 0.913:1, which isclose to the theoretical ratio of 1:1; the other elements of C andCu are from the Cu micro-grid. These results demonstrate thatthe electrospinning method can prepare samples with controllablecomponents. XRD spectra of the nanotubes (Fig. 1f) conrm theSAED characterization. As shown in the XRD spectrum of SnO2nanotubes, the most prominent peaks can be assigned to the(110), (101), (200), and (211) lattice planes of rutile SnO2 (JCPDSNo. 41-1445). Whereas, the extra peaks in the XRD spectrum ofZnOSnO2 nanotubes can be ascribed to the (100), (002), and(101) lattice planes of wurtzite ZnO (JCPDS No. 65-3411) and thecomposite phase of Zn2SnO4 (JCPDS No. 14-0381). Moreover, thenanotubes formation mechanism is based on the phase separationduring preparation process, which has been intensively investi-gated in previous researches [26,31,32].3. Results and discussion

3.1. Characterization of morphology and structure

Fig. 1a and c shows representative TEM images of the annealedSnO2 and ZnOSnO2 nanotubes, respectively. It is clearly seen that

mpounds 614 (2014) 373378curves were recorded at different scan speeds in a 24.4 lM H2O2solution in dark and UV irradiation conditions, respectively. Asshown in Fig. 3, the dark current and photocurrent of the two

Fig. 1. TEM images of the (a) SnO2, (c) ZnOSnO2 nanotubes. The selected area electron dX-ray spectroscopy of the ZnOSnO2 nanotube and (f) XRD patterns of SnO2 and ZnOS

Fig. 2. PL spectra of SnO2 and ZnOSnO2 nanotubes.

X. An et al. / Journal of Alloys and Compounds 614 (2014) 373378 375electrodes gradually decrease as the scan speed increases. The t-ting plots of the electrode 1 show both linear relationship in darkand under UV irradiation (Fig. 3a and b), while the dark currentand photocurrent of the electrode 2 are exponentially proportionalto the scan speed (Fig. 3c and d). Moreover, it is worth notingthat the dark current and photocurrent of the electrode 2 are muchhigher than those of the electrode 1, suggesting ZnOSnO2nanotubes have better electron transfer property than pure SnO2nanotubes [39,40].

In order to understand the reaction mechanism at electrodesurfaces, the effect of H2O2 concentration on the currents of twoelectrodes were evaluated. Fig. 4a and b shows the dark currentand photocurrent of the electrode 1 with varied H2O2 concentra-tions at a scan rate of 50 mV/s. When H2O2 concentrationincreases, the dark current decreases and the photocurrentincreases. This could be explained by the fact that the dissolvedO2 and H2O2 can capture electrons at electrode surface in the darkcondition. (Eqs. (1) and (2)) [37,4144]:

iffraction (SAED) patterns of (b) SnO2, (d) ZnOSnO2 nanotube, (e) energy-dispersivenO2 nanotubes.

Fig. 3. CV curves of the modied electrode recorded at different scan speeds in PBS solution (PH = 7.0) containing 24.4 lM H2O2. (a) SnO2 in dark, (b) SnO2 under UVirradiation, (c) ZnOSnO2 in dark and (d) ZnOSnO2 under UV irradiation.

Fig. 4. CV curves of modied electrode with different H2O2 concentrations in PBS solution (PH = 7.0) recorded at 50 mV/s with (a) SnO2 in dark, (b) SnO2 under UV irradiation,(c) ZnOSnO2 in dark and (d) ZnOSnO2 under UV irradiation.

376 X. An et al. / Journal of Alloys and Compounds 614 (2014) 373378

O2 e O2 1

H2O2 e OHHO 2

3H2O2 2h 2HO2 2H2O 3Although H2O2 is a strong electron accepter, it is much weaker

than O2 to trap electrons on SnO2 nanotubes surface under UVirradiation. Therefore the presence of dissolved O2 may hinderH2O2 from trapping electrons to some extent and captures elec-trons, while H2O2 acts as a sacricial electron donor to scavengeholes (Eq. (3)) [16,41]. As depicted in Fig. 5a, SnO2 nanotubes cangenerate electronhole pairs under UV irradiation. Hence, electronhole recombination is inhibited and the photocurrent could beenhanced.

O2 h O2 4Fig. 4c shows that the dark current of the electrode 2 decreases

exponentially at the beginning and then reaches saturation, whichcan be attributed to the competitive reactions of the adsorptiondissolved O2 and oxidation H2O2 (Eqs. (1) and (2)) on the surfaceof the electrode 2.

When the electrode 2 was irradiated by UV light, photo-generated electrons transfer can be controlled by the energy levelstructure and the electric potential. Energy band structurearrangement of the ZnOSnO2 composite nanotubes cause photo-generated electrons to assemble at the surface of SnO2 nanocrystal-lites, and photo-generated holes gather at the surface of ZnO. Theresulting photocurrent of the electrode 2 increases approximatelylinearly as a function of H2O2 concentration, as shown in Fig. 4d.

trons and holes are captured by the dissolved O2 and H2O2, respec-

282.

X. An et al. / Journal of Alloys and CoHence, ZnOSnO2 nanotubes electrode possesses wider rangesensing capability in H2O2 under UV radiation than in dark.Fig. 5. Schematic illustrations showing the reactions on the surface of (a) SnO2nanotubes and (b) ZnOSnO2 nanotubes modied electrodes under UV irradiation.[11] Z. Yue, W. Zhang, C. Wang, G. Liu, W. Niu, Mater. Lett. 74 (2012) 180182.[12] K. Chen, X. Feng, R. Hu, Y. Li, K. Xie, Y. Li, H. Gu, J. Alloys Comp. 554 (2013)

7279.[13] C. Cao, C. Hu, W. Shen, S. Wang, J. Wang, Y. Tian, J. Alloys Comp. 550 (2013)

137143.[14] M. Liu, R. Liu, W. Chen, Biosens. Bioelectron. 45 (2013) 206212.[15] W. Wang, L. Bao, J. Lei, W. Tu, H. Ju, Anal. Chim. Acta 744 (2012) 3338.[16] Y. An, L. Tang, X. Jiang, H. Chen, M. Yang, L. Jin, S. Zhang, C. Wang, W. Zhang,

Chem. Eur. J. 16 (2010) 1443914446.[17] X. Zhang, L. Li, X. Peng, R. Chen, K. Huo, P.K. Chu, Electrochim. Acta 108 (2013)

491496.[18] S. Dhobale, P. Joshee, G. Deore, S.L. Laware, S.N. Kale, Appl. Phys. Lett. 98 (2011)

07370410737043.[19] T. Wang, H. Zhu, J. Zhuo, Z. Zhu, P. Papakonstantinou, G. Lubarsky, J. Lin, M. Li,

Anal. Chem. 85 (2013) 1028910295.[20] H. Hongtao, T. Shouqin, X. Jing, X. Zhong, Z. Dawen, C. Di, S. Guozhen,This project was supported by the National Natural ScienceFoundation of China (Grant Nos. 61176058 and 51302122), theChina Postdoctoral Science Foundation (Grant No. 2012M512046),and the Program for Changjiang Scholars and Innovative ResearchTeam in University (Grant No. IRT1251).

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4. Conclusions

UV light induced PEC biosensors for H2O2 detection have beendeveloped based on SnO2 nanotubes and ZnOSnO2 compositenanotubes, respectively. ZnOSnO2 nanotubes possess moreabundant surface oxygen vacancies and better electron transferproperty than pure SnO2 nanotubes, resulting in better PEC sensi-tivity. Dark current responses to varied H2O2 concentrations oftwo kinds of electrodes are considered as a competition of electroncapture between the dissolved O2 and H2O2 at surface oxygenvacancies of nanotubes. For SnO2 nanotubes, the UV excited elec-

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378 X. An et al. / Journal of Alloys and Compounds 614 (2014) 373378

Enhanced photoelectrochemical sensor based on ZnOSnO2 composite nanotubes1 Introduction2 Experiments and methods2.1 Preparation of nanotubes2.2 Fabrication of nanotubes modified electrodes2.3 Apparatus and measurements

3 Results and discussion3.1 Characterization of morphology and structure3.2 Optical properties of nanotubes3.3 PEC measurements

4 ConclusionsAcknowledgementsReferences