Improved SERS performance of single-crystalline TiO2 nanosheet arrays with coexposed {001} and {101} facets decorated with Ag nanoparticles

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Improved SERS performance of single-crystalline TiO2 nanosheet arrays with coexposed {001} and {101} facets decorated with Ag nanoparticles Lei Yang a,b,∗ , Weihua Wang b , Haiyan Jiang c , Qianghua Zhang b , Huihui Shan b , Miao Zhang a , Kerong Zhu a , Jianguo Lv d , Gang He a , Zhaoqi Sun a,∗∗ a

School of Physics & Material Science, Anhui University, Hefei 230601, PR China Institute of Applied Physics, AOA, Hefei 230031, PR China c Hefei University of Technology, Hefei 230009, PR China d School of Electronic & Information Engineering, Hefei Normal University, Hefei 230601, PR China b

a r t i c l e

i n f o

Article history: Received 13 January 2016 Received in revised form 25 September 2016 Accepted 27 September 2016 Available online xxx Keywords: TiO2 nanosheet SERS {001} Facets charge-transfer

a b s t r a c t In this paper, single-crystalline TiO2 nanosheet (TNS) arrays with dominant {001} facets decorated with Ag nanoparticles were synthesized on fluorine-doped tin oxide (FTO) substrate by a simple hydrothermal method and a magnetron sputtering method. Using Rhodamine 6G (R6G) as the probe molecule, we investigated the sensitivity, reversibility and uniformity of surface-enhanced Raman scattering (SERS) substrates. It is noted that the TNS with 10 s Ag-sputtering achieves the highest Raman signals, and the intensity of the SERS spectra on TNS/Ag has a significant enhancement compared with Ag film. The excellent SERS performance is attributed to synergistic effect of electromagnetic mechanism (EM) and charge-transfer (CT) enhancement mechanism. Furthermore, the band bending of the adjacent region between {001} and {101} facets, provides an additional CT from Ag to TNS, which would result in strong enhancement of the SERS signal. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Surface-enhanced Raman scattering (SERS) has drawn increasing attention due to its wide application in medicine, biology, and molecular science due to its capability of detecting moleculelevel information on the surface of SERS-active substrates [1–4]. In the early days, most of studies about SERS has been limited to noble metal nanostructures (Au, Ag, and Cu) by taking advantage of the electromagnetic mechanism (EM) [5–7]. Under laser irradiation, electrons on the rough noble metal surface are excited to form localized surface plasmon resonance (LSPR) effect, which causes the enhancement of Raman signal of the analytes [8,9]. However, for noble metal nanostructures, due to their expensive in fabrication, instability and poor biocompatibility, which limit their utilization in SERS application. Recently, SERS signals on the surface of semiconductor materials, such as TiO2 , Cu2 O, ZnO, ZnS

∗ Corresponding author at: School of Physics & Material Science, Anhui University, Hefei 230601, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (L. Yang), [email protected] (Z. Sun).

have also been reported [10–13]. Especially, semiconductor/metal composites have received growing interest due to their stronger Raman enhancement which associated with tunable LSPR of metallic nanostructures, induced by a charge-transfer (CT) mechanism at the semiconductor/metal interface [14–17]. TiO2 is an important n-type semiconductor material in solar energy conversion, catalysis and health-oriented applications due to its non-toxicity, chemical stability, and low cost [18–20]. In particular, single-crystalline TiO2 nanosheets (TNS) with high percent {001} facets attract more attention due to their distinctive chemical and physical properties [21,22]. However, so far, SERS substrate study based on TNS with dominant {001} facets structure is rarely reported. Herein, TNS films with coexposed {001} and {101} facets decorated with Ag nanoparticles were synthesized on fluorinedoped tin oxide (FTO) substrate by a simple hydrothermal method and a magnetron sputtering method. Using Rhodamine 6G (R6G) as the probe molecule, we have investigated relationship between SERS activity of TNS/Ag nanocomposites and Ag content. We were not able to detect the SERS signal of R6G from the bare TNS film surface, meanwhile, it is to be noted that the intensity of the SERS spectra on TNS/Ag has a significant enhancement compared with Ag film. The origin of the enhancement on TNS/Ag composites may

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be ascribed to synergistic effect of EM and CT enhancement mechanism. In addition, Grätzel et al. showed that the interfacial charge separation at PbS/TiO2 made of dominant {001} facets is five times faster than that on dominant {101} facets. Majima et al. investigate the crystal-face dependence of interfacial CT reactions on individual anatase TiO2 crystals [23]. In this system, TNS films with coexposed {001} and {101} facets decorated with Ag nanoparticles were served as SERS-active substrates, the different band edge positions of {001} and {101} facets combined with Ag nanoparticles is expected to be valuable for the enhancement of the SERS signal. 2. Experimental 2.1. Preparation of TNS/Ag films All major reagents are analytical grade and are used as received without further purification. TNS films were prepared according to a modified hydrothermal method [24]. In detail, 13 mL hydrochloric acid and 17 mL deionized water were mixed in a 50 mL beaker, followed by addition of 0.5 mL of tetrabutyl titanate. The mixture was stirred for 10 min, followed by addition of 0.25 g ammonium hexafluorotitanate, and stirred for another 30 min. Then the solution was transferred into the autoclave for hydrothermal treatment at 160 ◦ C for 12 h. In the Teflon-lined cylindrical autoclave there was a piece of FTO with the conductive side facing up. After the autoclave was cooled to room temperature, the FTO substrates was washed with deionized water and then dried at 80 ◦ C. The obtained TNS array films were annealed at 500 ◦ C for 2 h before the deposition of silver. Ag particles were deposited onto the TNS array films by DC magnetron sputtering (MSP-3200) using a Ag target (99.99% purity) at room temperature, the base pressure was 6 × 10−4 Pa, and the sputtering power was 50 W. During deposition, Ar flow rate was kept at 30 sccm, the working pressure was kept at 1 Pa. The sputtering time was 3, 5, 10, 20 s, and the obtained TNS/Ag array films were denoted as TNS/Ag-3, TNS/Ag-5, TNS/Ag-10, and TNS/Ag-20, respectively. 2.2. Characterization The surface morphologies of the TNS/Ag array films were characterized by field-emission scanning electron microscope (FESEM, Hitachi, S4800) and high-resolution transmission electron microscopy (HRTEM, JEM-2100). Microstructures of the TNS/Ag were examined by X-ray diffraction (XRD, MAC, M18XHF) with Cu K␣ radiation. The absorption spectra of the TNS/Ag films were measured on an ultraviolet-visible spectrophotometer (UV-2550, Shimadzu). 2.3. SERS experiments Raman spectra and confocal Raman mapping were recorded by micro-Raman spectroscope system (Renishaw PLC in Via-Reflex). A 532 nm laser was used as the excitation source. The laser spot size was about 1 ␮m in diameter and the laser power was 0.15 mW. The data acquisition time was 10 s. For each sample, the Raman spectra measurement was repeated three times at different places. For SERS studies, R6G solution with different concentrations was used for SERS measurement. First, the SERS substrates were immersed into R6G solution for 30 min in order to reache an adsorption/desorption equilibrium, then the SERS substrates were rinsed with deionized water and dried under ambient condition before SERS measurement. For recyclability of the fabricated SERS substrates, according to the SERS measurement results, the TNS/Ag10 and the 10−5 M R6G solution were selected to evaluate the recyclable property. After the SERS evaluation of TNS/Ag-10, the

substrate was immersed in deionized water and irradiated with a 36 W high-pressure mercury lamp (main wavelength: 365 nm) for 30 min, and the distance between them was 2 cm. Then the sample was rinsed with deionized water and dried at 80 ◦ C. To ensure recyclable SERS detection, the “detection−cleaning” process was repeated three cycles. 3. Results and discussion 3.1. SEM images Fig. 1 shows the SEM images of the TNS/Ag array films. The TNS growing on the substrate exhibit the regular tetragonal sheetstructured end planes with about 1800 nm in length and 160 nm in thickness. SEM cross-section view in Fig. 1(b) shows that the TNS grow slightly obliquely on the FTO substrate. According to the Wulff construction [25], the two exposed tetragonal surfaces are identified as {001} facets, and the small side surfaces are {101} facets of the anatase single crystal [26,27]. Thus, the percentage of exposed {001} facets can be derived according the calculation method [28,29], this value is statistically around 88%. The morphologies of TNS with different Ag-sputtering times are shown in Fig. 1(c)–(f). When the sputtering time is 3 s, numerous Ag particles are distributed on the TNS randomly. With increasing the sputtering time to 5 s, the Ag particles become larger, and the distances among them get smaller. For the Ag-sputtering time is 10 s, the surface coverage increases and the some Ag particles get in close contact with each other. It is obvious that the sizes of Ag particles increase and the distances among them are smaller with increasing sputtering time, however, after 20 s of sputtering, the surface is entirely covered with Ag film. Particle size distribution histograms (Fig. S1) show that the mean sizes of Ag particles are 15.2, 36.3, and 50.0 nm and Ag coverage of TNS are 12.2%, 48.6%, and 87.2% for TNS/Ag-3, TNS/Ag-5, and TNS/Ag-10, respectively. 3.2. XRD Fig. 2 shows the XRD patterns of TNS with different Agsputtering times. Three main peaks located at 2␪ = 25.2◦ , 37.8◦ , 48.1◦ can be attributed to anatase (101), (004) and (200) crystal face (tetragonal, I41/amd, JCPDS no. 21-1272). Meanwhile, It is worthwhile to note that all the diffraction peaks of TNS were assigned to pure anatase phase [30,31]. Compared with the general method that preparing TiO2 [32,33], an obvious intensity increasing of the (004) diffraction peak appear, indicating the large area of {001} facets exposed [24]. The rhombus marked peaks located at 2␪ = 38.1◦ , 44.1◦ , 64.5◦ and 77.4◦ are the (111), (200), (220) and (311) planes of silver [34]. It is noted that when the sputtering time is less than 10 s, the diffraction peaks of Ag are not easily observed, which may due to the low degree of crystallinity and uniform distribution of Ag particles [35]. For TNS/Ag-20, the diffraction peak of TiO2 at (004) slightly intensified, It may related to overlapping of (111) plane of Ag. 3.3. TEM and EDS The microstructures of the TNS were further investigated by TEM. Fig. 3(a) shows that the samples consisted of well-defined rectangular sheet-shaped structure. The inset of Fig. 3(a) is a SAED pattern of the TNS. The SAED pattern can be indexed as the [001] zone axis diffraction, which indicates that the top and bottom surfaces of the TNS are {001} facets. the diffraction spots in SAED pattern combine with XRD patterns further indicate that the rectangular sheet-shaped TiO2 is single-crystalline anatase. The energy dispersive X-ray spectroscopy (EDS) indicates that the TNS/Ag-5 is

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Fig 1. SEM images of TNS (a) top view (b) cross-section view. SEM images of TNS after Ag-sputtering for (c) 3 s, (d) 5 s, (e) 10 s, (f) 20 s.

Fig. 2. XRD patterns of TNS after Ag-sputtering for different times.

composed of Ti, O, Sn, and Ag elements. Ti, O, and Ag elements come from TNS/Ag-5, Sn peaks come from the FTO substrate. 3.4. UV–vis absorption spectra Fig. 4 represents the UV–vis absorption spectra of TNS with different Ag-sputtering times. The bare TNS only shows a strong absorption in wavelength smaller than 400 nm, which is associated with the band gap of TiO2 . The TNS/Ag films display enhanced

visible light absorption, which is attributed to the SPR of Ag particles [36]. The peak wavelength of the SPR is relative to the particle size, shape and surrounding environment [37,38]. With the sputtering time increasing from 3 to 10 s, the mean size of Ag particles increases from 15 to 50 nm, and their SPR peaks red shift from 560 to 715 nm, indicating the strong dipole-dipole interaction between neighboring Ag particles and the inhomogeneity in size and shapes of Ag nanoparticles [39,40]. However, after 20 s of sputtering, The spectra of TNS/Ag-20 shows a the broad peak at 500 nm, from SEM images it can be seen that Ag particles contacted with each other and form a Ag film, the different extinction feature of the TNS/Ag20 should be attributed to the shape and dielectric environment effects of Ag particles, which could possibly affect SERS performance. In particular, a bigger Ag particles will cause closer gaps, and these narrow gaps would lead to SERS “hot spots” [41,42]. Furthermore, Shan et al. proposed that the position of plasmon absorption may change due to the interaction between Ag and semiconductor nanocrystals [14]. The plasmon absorption peak ␭ of Ag is represented by the following equation  = [42 c 2 meff ε0 /Ne2 ]

1⁄2

(1)

where meff and N are the effective mass of the free electron and the electron density of the metal Ag. Under visible light irradiation, the hot electrons transfer from the Ag particles to the TiO2 due to the LSPR, leading to a decrease of electron density of Ag and a red shift of the plasmon absorption [43]. Thus, the shift of the plasmon may be attributed to the synergistic effect of Ag particles size, shape and dielectric environment. It can be noticed that TNS/Ag-20

Fig. 3. TEM image (a), and EDS spectra of TNS/Ag-5(b). The inset of (a) is a SAED pattern of the anatase single-crystalline TiO2 nanosheet.

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Fig. 6(a) presents the typical SERS spectra of R6G with different concentrations from 1.0 × 10−5 to 1.0 × 10−9 M on TNS/Ag-10, and the SERS signals decrease with the diluting of the R6G aqueous solutions. The characteristic Raman peaks of R6G remained clearly observable even with a solution of 1.0 × 10−9 M. This indicates that TNS/Ag as SERS substrates have high sensitivity. The sensitivity of the TNS/Ag-10 substrate is calculated by the SERS enhancement factor (EF) [40,56]: EF = (

Fig. 4. UV–vis absorption spectra of the samples with Ag-sputtering for different times.

shows the lowest absorption compared with other TNS/Ag, which can be attributed to the coverage of connected Ag layer that has a high reflection in the optical region [36,44]. Besides, Cho et al. have reported that the change of the Ag layer from isolated nanoparticles to connected island films leads to a change of SP mode from localized SP (LSP) to propagating SP (PSP) [45]. With the increasing of Ag coverage, the cracks between the connected Ag particles and the LSP around the Ag clusters decrease result in decreased absorption [46]. Given that the TNS phase is the same, a decrease in absorbance at around 330–360 nm and the absorption edge redshift of TNS/Ag-20 might be related to the interaction between Ag film and TNS. In addition, the spectra of TNS/Ag-20 shows a sharp rise at about 320 nm, which corresponds to the onset of interband absorption threshold energy h = 3.9 eV (318 nm) of Ag [47]. 3.5. SERS detection of R6G dye The sensitivity of the SERS substrates was examined by using R6G as the probe molecules. Before SERS detection, the TNS with different Ag-sputtering time were immersed in R6G solution for 30 min, and then washed with deionized water and dried under ambient condition. Fig. 5(a) shows the SERS spectra of R6G (10−6 M) adsorbed on TNS with different Ag-sputtering times. The characteristic peaks at 613, 773, 1126, 1183, 1308, 1360, 1505 and 1650 cm−1 are assigned to R6G [48,49]. It is evident that the SERS signals of R6G increase with the sputtering time increasing from 3 to 10 s, and the TNS/Ag-10 achieves the highest SERS signals. Fig. 5(b) demonstrates the SERS intensity at 1360 cm−1 of R6G adsorbed on different SERS substrates. The interpretations for the highest SERS signals of the TNS/Ag-10 can be explained as follows. First, as shown in SEM images, the surface of TNS/Ag-10 is covered with closepacked Ag nanoparticles, which generate a high density of hotspots [50]. Second, previous studies have shown that SERS intensity would increase with increasing the metal particle size, and the particle size at around 60 nm possesses the strongest enhancement [39]. The mean size of Ag particles is 50 nm for TNS/Ag-10, which is the nearest to this size. Third, for tilted nanosheet arrays, the incident light normal to the substrates can induce a stronger electric field compared with vertically aligned metallic nanostructure [51,52]. Finally, for high energy {001} facet, it possesses 100% unsaturated 5-fold-coordinated titanium atoms (Ti5c ), and exhibits higher activity, thus showing a stronger physicochemical interaction with the foreign atoms and leading to the enhancement of SERS intensity [53–55].

ISERS Nsolid )( ) Isolid NSERS

(2)

ISERS and Isolid are the Raman signal intensities at 1360 cm−1 of the 1.0 × 10−6 M R6G adsorbed on TNS/Ag-10 substrate and solid R6G on glass, respectively. NSERS and Nsolid are the numbers of probe molecules on SERS substrates and solid sample, respectively. Fig. 6(b) shows the spectra comparison result of the SERS substrate with that of the non-SERS substrate. Assuming the adsorbent molecules are monolayer distributed on the surface of SERS substrates, the numbers of adsorbent molecules can be estimated by NSERS = (NA CV/Ssub )Slaser

(3)

where NA is Avogadro constant, C is the molar concentration of the solution (1.0 × 10−6 M), V is the volume of the droplet (2 ␮L), Ssub is the solution dispersed area on substrate (5 mm in diameter), and Slaser is the size of the laser spot (1 ␮m in diameter) [56]. The numbers of probe molecules in the solid sample can be calculated by Nsolid = Slaser dNA /M

(4)

where d is penetration depth (about 2 ␮m), ␳ is density of solid R6G (0.79 g/cm3 ), and M is molar mass of R6G (479 g/mol). According to Formulas (3) and (4), Nsolid /NSERS has a value of 3.24 × 104 . Taking the value of ISERS /Isolid (9.64) into account, the EF is estimated to be about 3.0 × 105 . This EF value is comparable to that of composites of noble metal and semiconductor material [57,58]. Compared with pure metallic nanostructures, semiconductor/metal structured materials show a good reversibility. Especially, for TiO2 nanosheet arrays, the high percentage of {001} facets will facilitate to accomplish high photocatalytic activity, thus increase the efficiency of reversible SERS behavior [59,60]. After SERS detection, the TNS/Ag was immersed in deionized water and irradiated with a high-pressure mercury lamp for a certain time. As shown in Fig. 7(a), the SERS signals of R6G became very weak, and it almost disappeared after 60 min of irradiation. This indicates that TNS/Ag arrays as SERS substrates also provide a perfect photodegradation performance. Fig. 7(b) shows the results of the reversible SERS behavior of the TNS/Ag-10 with 3 repeated treatments. Apparently, the enhancement performance is still good, indicating that the TNS/Ag is feasible to be used as a repeatable SERS active substrate. In addition to the high sensitivity and good reversibility, the reproducibility of SERS signal is also crucial for SERS performance. To evaluate the uniformity of the TNS/Ag substrates, SERS spectra of R6G at 1360 cm−1 by a 2D point by point mapping mode on a 20 × 20 ␮m area was collected. As shown in Fig. 8(a), SERS spectra of 10−6 M R6G obtained from different spots maintain a fine stability due to the regular nanosheet structure, which is capable of avoiding random field distribution as nanoparticles. Furthermore, as shown in Fig. 8(b), twenty different locations were randomly collected on the TNS/Ag-10 substrate, the relative standard deviation (RSD) of the SERS intensity at 1360 cm−1 is about 13.4%. The above results clearly demonstrated the excellent uniformity and reproducibility of the TNS/Ag SERS substrates. Fig. 9(a) compares the SERS spectra of R6G adsorbed on a TNS/Ag-10, a glass/Ag-10 and a bare TNS substrate. It is obvious that on the bare TNS substrate there was no SERS signal, whereas a certain enhancement in SERS spectra on the Ag

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Fig. 5. (a) The SERS spectra of R6G (10−6 M) adsorbed on TNS with different Ag-sputtering times. (b) The SERS intensity at 1360 cm−1 of R6G adsorbed on different samples.

Fig. 6. (a) SERS spectra of R6G solution at various concentrations on the TNS/Ag-10. (b) SERS spectra of 10−6 M R6G on the TNS/Ag-10 and solid R6G on glass.

Fig. 7. (a) Raman spectra of R6G solution (1 × 10−5 M) adsorbed on the TNS/Ag-10 before and after UV irradiated for a certain time. (b) Reversible SERS behavior of the TNS/Ag-10 substrate with 3 repeated treatments.

substrate. In addition, an obviously enhanced signal was obtained from TNS/Ag-10 surface compared with the glass/Ag-10 substrate. Since deposition time was fixed as 10 s for both substrates, the

amounts of Ag nanoparticles should be less varied. This suggests that the SERS enhancement is not due to the Ag or TiO2 materials individually but occurs only on the combined TiO2 /Ag nanocom-

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Fig. 8. (a) SERS mapping of R6G at 1360 cm−1 on a TNS/Ag substrate. (b) SERS spectra of R6G on twenty spots of the substrate.

Fig. 9. (a) SERS spectra of R6G on TNS/Ag-10, a Ag film and a bare TNS film. (b) Schematic of electron transfer in TNS/Ag under visible light irradiation.

posite. It is well known that there are two main SERS enhancement mechanisms: EM and CT mechanism. The EM is based on excitation of the SPR of the noble metal. On the rough Ag film surface, where R6G are adsorbed, the SPR would lead to a local electromagnetic field enhancement and result in the increasement of SERS signal, such as SERS spectra on the glass/Ag-10 film. When Ag nanoparticles deposited on the TNS, the transfer of electrons from Ag to TiO2 occurs due to different work function [61,62], which corresponds to CT between Ag and TiO2 . As shown in Fig. S2, TNS/Ag-10 shows a lower fluorescence intensity compared with TNS. It has reported that the emission of fluorescence and Raman spectra are opposite. The quenching of fluorescence can be ascribed to the CT between Ag and TNS [14,63]. The charge redistribution results in the negatively charged TiO2 and positively charged Ag. The junction region of opposite charged TiO2 and Ag formed a higher local electromagnetic field, and may induce a larger Raman enhancement. Thus, the highest SERS enhancement for TNS/Ag-10 arrays may be described to Ag nanoparticles EM combined with TiO2 /Ag interface CT enhancement mechanism. In addition, previous reports indicated that the interaction between the {001} and {101} facets can drive electron and hole transfer to different crystal facets [53,64], and demonstrated that {001} and {101} facets are oxidative and reductive sites, respectively. As shown in Fig. 9(b), the band bending of the adjacent region between {001} and {101} facets, makes the electrons easily transfer from {001} to {101} facets, therefore

provides an additional CT from Ag to TNS. Therefore, for anatase single crystal TNS decorated with Ag nanoparticles, the band bending at the junction between {001} and {101} facets. which would also responsible for SERS enhancement.

4. Conclusions In summary, TNS films with coexposed {001} and {101} facets decorated with Ag nanoparticles were synthesized on FTO substrate by a simple hydrothermal method and a magnetron sputtering method. The SERS activity of TNS/Ag was characterized by using R6G as the probe analyte. The results show that the TNS/Ag-10 possessed high sensitivity, good reversibility and uniformity. The excellent SERS performance is attributed to synergistic effect of EM and CT enhancement mechanism: (i) the high density of hot spots generated by the Ag nanoparticles deposited on oriented two-dimensional TNS arrays; (ii) the strong electromagnetic field owing to the charge transfer between Ag and TNS; (iii) additional charge transfer from Ag to TNS due to band bending between {001} and {101} facets. In addition, due to SPR effect of the Ag nanoparticles and high activity {001} facets, such a stable TNS/Ag composite structure can be applied in sensors, environmental science, biomedicine and so on.

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Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 51472003, 51272001, 51576208, 91326101 and 51572002), the National Key Basic Research Program (2013CB632705), National Magnetic Confinement Fusion Science Program of China (Nos. 2013GB113004, 2015GB15007 and 2015GB120006), the Doctor Scientific Research Fund and Cooperative Innovation Research Center for Weak Signal-Detecting Materials and Devices Integration of Anhui University. The authors would like to thank Yonglong Zhuang and Zhongqing Lin of the Experimental Technology Center of Anhui University, for the electron microscope test and discussion.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.09.162.

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Biographies Lei Yang received his M.S. degree in School of Physics and Material Science, Anhui University China. He is currently working toward Ph.D. degree with Prof. Sun at Anhui University. His current research interests include 2D materials and nanomaterials based sensors. Weihua Wang received his Ph.D. degree in Chinese Academy of Science, China. He is currently working as a professor at Army Officer Academy. Haiyan Jiang is working for her Ph.D. degree in Hefei University of Technology, China. Her research interests are metallic materials and their applications in sensors. Qianghua Zhang received his M.S. degree in Army Officer Academy. His current research interests is focused on optoelectronic devices. Huihui Shan received her M.S. degree in School of Physics and Material Science, Anhui University China. Her research interests are synthesis of nanomaterials and their applications in sensors. Miao Zhang received her Ph.D. degree in School of Physics and Material Science, Anhui University, China. Her research interests are in the electrochemical fabrication, characterization and application of a wide range of materials including nanostructured semiconducting materials, metal oxides materials. Kerong Zhu received his Ph.D. degree in Nanjing University, China. He is currently working as a Senior Experimentalist at Anhui University. He research interests is Raman spectroscopy and its application in nanomaterials science. Jianguo Lv received his Ph.D. degree in School of Physics and Material Science, Anhui University, China. He is currently working as a Professor at Hefei Normal University. He research interests is nanotechnology, materials science, electronics. Gang He received his Ph.D. degree in Chinese Academy of Science, China. His research interests include low-dimensional nanomaterials, nanoelectronics and nanofabrication. Zhaoqi Sun is a professor at School of Physics and Material Science, Anhui University China. His research interests include low-dimensional nanomaterials, solar cells and semiconducting materials for optical and electronic applications.

Please cite this article in press as: L. Yang, et al., Improved SERS performance of single-crystalline TiO2 nanosheet arrays with coexposed {001} and {101} facets decorated with Ag nanoparticles, Sens. Actuators B: Chem. (2016), http://dx.doi.org/10.1016/j.snb.2016.09.162