TiO2 Nanosheets for Sensitive Electrochemical Sensing of Heavy

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Surface-Electronic-State-Modulated, Single-Crystalline (001) TiO2 Nanosheets for Sensitive Electrochemical Sensing of Heavy-Metal Ions Wen-Yi Zhou, Jinyun Liu, Jieyao Song, Jinjin Li, Jinhuai Liu, and Xing-Jiu Huang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04023 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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Analytical Chemistry

Surface-Electronic-State-Modulated, Single-Crystalline (001) TiO2 Nanosheets for Sensitive Electrochemical Sensing of Heavy-Metal Ions

Wen-Yi Zhou,†,‡,§ Jin-Yun Liu,†,§ Jie-Yao Song,‡ Jin-Jin Li,ǁ Jin-Huai Liu,*,†,‡ and Xing-Jiu Huang*,†,‡

†

Key Laboratory of Environmental Optics and Technology, Institute of Intelligent

Machines, Chinese Academy of Sciences, Hefei 230031, P.R. China

‡

Department of Chemistry, University of Science and Technology of China, Hefei

230026, P.R. China

ǁ

Department of Micro/Nano Electronics, Shanghai Jiao Tong University, Shanghai

200240, P.R. China

§

These two authors contributed equally to this work.

* Correspondence should be addressed to J.H.L. (email: [email protected]) and X.J.H. (email: [email protected]) Tel.: +86-551-65591142; fax: +86-551-65592420. 1

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ABSTRACT Intrinsically low conductivity and poor reactivity restrict many semiconductors from electrochemical detection. Usually, metal- and carbon-based modifications of semiconductors are necessary, making them complex, expensive, and unstable. Here, for the first time, we present a surface-electronic-state-modulation-based concept applied to semiconductors. This concept enables pure semiconductors to be directly available for ultrasensitive electrochemical detection of heavy-metal ions without any modifications. As an example, a defective single-crystalline (001) TiO2 nanosheet exhibits high electrochemical performance toward Hg(II), including a sensitivity of 270.83 µA µM−1 cm−2 and a detection limit of 0.017 µM, which is lower than the safety standard (0.03 µM) of drinking water established by the World Health Organization (WHO). It has been confirmed that the surface oxygen vacancy adsorbs an O2 molecule while the Ti3+ donates an electron, forming the O2.− species that facilitate adsorption of Hg(II) and serve as active sites for electron transfer. These findings not only extend the electrochemical sensing applications of pure semiconductors, but also stimulate new opportunities for investigating atom-level electrochemical behaviors of semiconductors by surface electronic state modulation.

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INTRODUCTION Nanomaterials have been widely used in the electrochemical detection of heavy-metal ions because of their high sensitivities, low detection limits, and rapid responses.1-6 However, electrochemical sensing nanomaterials are commonly limited to conductive metal- and carbon-based systems.4 For transition-metal-oxide-based electrodes, modifications with other materials are usually required due to the poor conductivity and reactivity of such electrodes, which make the sensing systems complex, expensive, and unstable.5,6 In addition, the improved electrochemical performance (including the sensitivity and the limits of detection) is commonly ascribed to the two-step process in which adsorption of analyte onto particles and subsequent desorption and diffusion to the electrode is required.7 This has been experimentally and theoretically demonstrated, while the electrochemical mechanism at an atomic level is still unclear.1,8 Among many transition-metal oxides, TiO2 possesses high potential as an electrochemical sensing material because of its abundance, low cost, and high stability.9-11 However, the wide-bandgap energy (3.2 eV) makes TiO2 more appropriate for photocatalysis.9,12 Pure TiO2 has rarely been considered for electrochemical detection of heavy-metal ions due to its intrinsic low conductivity and poor reactivity,12 while modified materials such as DNA/C/TiO2,13 and Ti/TiO2 nanotube/Au14 composites show high electrochemical performance. Currently, the same as many other semiconductor electrodes, it is still very challenging to directly apply a pure TiO2-based electrode for electrochemical detection of heavy-metal ions. 3



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Recently, a defective TiO2 crystal exposed with a high-energy (001) facet has been reported to be an efficient electrocatalyst for an oxygen reduction reaction.15 Defects are able to modulate the surface-electron state and serve as active sites for redox reactions.16-19 The electrons produced by native defects can be trapped in crystal lattice defect centers,18,20 forming a small polaron,21 which has been confirmed by first-principle calculation22 and electron-paramagnetic-resonance (EPR) study.23 The polaron with an abundance of trapped electrons is expected to serve as a preferentially active site for adsorption and chemical reactions.18,24,25 Here, we report a sensitive electrochemical sensing performance of a defective single-crystalline (001) TiO2-nanosheet toward heavy-metal ions (e.g., Hg(II)). The defective TiO2 nanosheets are modulated by a surface Ti3+ ion and an oxygen vacancy (OV). By using the defective TiO2 nanosheet-modified electrodes, the modification of TiO2 with other materials for electrochemical detection is no longer necessary. The presence of surface Ti3+ ion and OVs is confirmed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, electron spin resonance (ESR), and Fourier-transform infrared (FTIR) spectroscopy. The effect of surface Ti3+ ion and OVs for enhancing the stripping signals is also investigated by adsorption experiments accompanying with the XPS and extended X-ray absorption fine structure (EXAFS) analysis.

EXPERIMENTAL DETAILS TiO2 Nanosheet Preparation. The defective TiO2 nanosheets were prepared via a 4


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modified hydrothermal method.26 Briefly, 25 mL of Ti(OBu)4, 3 mL of concentrated H2SO4, 0.5 mL of HF (40 wt.%) and 2.5 mL of ultrapure water were added into a Teflon autoclave and maintained at 180 oC for 24 h. Here, since HF is a kind of contact poison that is extremely corrosive, it should be handled carefully. After the reaction, the precipitate was filtered and washed with ethanol and distilled water to remove the residual organic solvent, and dried in air at 60 oC. Then, the products were annealed in air at different temperatures (25, 150, 300, 450, 600, and 800 oC) for 2 h at a ramp rate of 5 oC min−1 (the samples were designated as T-1, TiO2-150, T-2, TiO2-450, T-3, and T-4, respectively). TiO2 nanoparticles with the (101) facet enclosed were prepared via a similar hydrothermal method without using HF, and designated sample T-5. Standard anatase TiO2 nanoparticles (5–10 nm) were purchased from Aladdin (Shanghai, China).

Characterization. Morphology and structure were characterized by high-resolution transmission electron microscopy (HRTEM) measurements (Model No. JEM-2010, JEOL, Ltd., Tokyo, Japan). X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert Pro X-ray diffractometer with Cu Kα radiation (1.5418 Å). EXAFS measurements were performed to investigate bulk-phase information. XPS measurements were performed on a ESCALAB MKII spectrometer (VG Scientific, USA) with an Mg Ka X-ray source (1253.6 eV, 120 W) at a constant analyzer. ESR was conducted to analyze the electronic states of Ti and O atoms using a JES-FA 200 X-Band Spectrometer (JEOL, Ltd.). The infrared spectra were recorded between 4000 5



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and 600 cm−1 on a FTIR spectrometer (Magna-IR 750, Nicolet Instrument, Inc., USA). The surface area was measured with a Brunauer-Emmet-Teller (BET) method on a Builder 4200 instrument (Tristar II 3020M, Micromeritics Instrument Corp., USA). Raman measurements were carried out on a Lab RAM HR800 confocal microscope Raman system (Horiba Jobin Yvon, Inc., USA). Thermal analysis was performed by thermal gravimetric analysis-differential scanning calorimetry (using a DTG-60H, TG/DTA Simultaneous Measuring Instrument, SHIMADZU, Japan), at a ramp rate of 10 oC min−1 in air.

Impedance Spectroscopy Measurements. The electrochemical tests were conducted on a CHI660D computer-controlled potentiostat (Chenhua Instruments Co., Shanghai, China), using a conventional three-electrode system, with the modified or bare glassy-carbon electrode (GCE, 3 mm diameter) as a working electrode, Ag/AgCl as a reference electrode, and Pt wire as a counter electrode. Electrochemical impedance spectroscopy (EIS) measurements were taken with an AC voltage amplitude of 5 mV within the frequency range of 105–1 Hz in a solution consisting of 5 mM K3Fe(CN)6 and 0.1 M KCl. Mott-Schottky plots were measured in 0.1 M Na2SO4 solution at a fixed frequency of 1000 Hz in the applied voltage range of 0–0.5 V.

TiO2 Electrode Fabrication. First, 5 mg of TiO2 samples were dispersed in 5 mL of water to form a suspension. Then, 7 µL of the TiO2 nanocrystal suspension were 6


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pipetted onto the surface of a freshly polished glassy-carbon electrode (GCE), and the solvent was evaporated under room temperature conditions to obtain the TiO2-modified GCE.

Electrochemical Measurements. Square wave anodic stripping voltammetry (SWASV) was used for Hg(II) detection under optimal experimental conditions. A deposition potential of −1.4 V was applied for 150 s to the working electrode with stirring. The SWASV responses were recorded between 0 and 0.5 V with a step potential of 4 mV, amplitude of 25 mV, and frequency of 25 Hz. A desorption potential of 0.8 V for 150 s was performed to remove the residual metals under stirring. All experiments were carried out at room temperature, with Ag/AgCl serving as a reference electrode.

Adsorption Measurements. Adsorption experiments were performed using a batch technique. Typically, 10 mg of TiO2 samples and 10 mL of 8.5 µM Hg(II) aqueous solution were added into a vial at room temperature. The vial was then continuously stirred for 24 h, after which the adsorbents were separated by high-speed centrifugation and dried at 60 oC for further XPS and X-ray absorption fine structure (XAFS) analysis. The concentration of Hg(II) remaining in the solution was analyzed using inductively coupled plasma atomic emission spectrophotometry (ICP-AES) (Model ICP 6300 spectrometer, Thermo Fisher Scientific, USA). The pH value of the solution was adjusted to 6.0 by NaAc (0.1 M) and HAc (0.1 M) solutions. 7



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XAFS Analysis. XAFS measurements were conducted at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF). Ti K-edge EXAFS at 4966 eV and X-ray absorption near-edge structure (XANES) measurements were recorded in the transmission mode, while Hg L3-edge EXAFS at 12284 eV was recorded in the fluorescence mode. The acquired EXAFS data were processed according to standard procedures using the ATHENA module implemented in the IFEFFIT software packages. The EXAFS χ(k) spectra were obtained by subtracting the post-edge background from the overall absorption and then normalized with respect to the edge-jump step. For Ti spectroscopy, k2-weighted χ(k) data in the k space ranging from 2 to 10 Å−1 were Fourier-transformed. For Hg spectroscopy, k3-weighted χ(k) data in the k space ranging from 3 to 11 Å−1 were Fourier-transformed to radial structure functions (RSFs) using a Hanning window (dk=1) to separate the EXAFS contributions from different coordination shells. The experimental data was fitted in R space via ARTEMIS (another module implemented in the IFEFFIT software packages). For fitting the experiment data, N was fixed at the crystallographically expected value, and other parameters (△E0, S02, △r, σ2) were free to be refined.

RESULTS AND DISCUSSION Morphology and Structure of TiO2 Nanosheets. TiO2 nanosheets were prepared using a modified hydrothermal method in which 98% concentrated H2SO4 solution was employed as a solvent,26 followed by heat treatment at different temperatures (25, 8


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300, 600, and 800 oC, designated T-1, T-2, T-3, and T-4, respectively). The structure was investigated via HRTEM (Figure 1a and Supporting Information, Figure S1) and XRD (Figure 1d). T-1, T-2 and T-3 exhibit a similar morphology. In contrast, heat treatment at 800 oC resulted in crystal transformation and a morphology change. Using a BET method, the surface area of the TiO2 nanosheets (T-1) was found to be approximately 58.9 cm2 g–1 (Figure S2).

Figure 1. Morphology and structure of the TiO2 nanosheets thermally-treated at different temperatures. a) Overview TEM images. b) HRTEM images. Scale bar: 5 nm. The (001) and (101) facets are marked. c) Compositional curves recorded along the yellow arrows shown in b). Disordered lattice structures of T-1 and T-2 are highlighted by dashed lines. d) XRD patterns. 9



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The (001) crystal facet with a lattice spacing of 0.232 nm can be observed in the upper part of Figure 1b; in the lower part, the (101) crystal facet shows a lattice spacing of 0.353 nm, suggesting that the bottom/top surfaces are enclosed by the (001) facet. The percentage of (001) facet in the TiO2 nanosheets is estimated to be ~68% based on the truncated octahedron structure.26 It is noted that the H2SO4 solution could induce rapid hydrolysis, resulting in abundant native defects in the TiO2 nanosheets.27 A high concentration of H+ is expected to release H2 to reduce Ti4+ to Ti3+ on the surface.28 F− has been widely used for TiO2 facet engineering because it can stabilize the (001) facet energy which has been confirmed experimentally and theoretically.29,30 Disordered lattice fringes induced by defects for T-1 and T-2 are shown in Figure 1b. The (101) facet of T-4 shows a lattice spacing of 0.353 nm. The fast Fourier transform patterns of T-1, T-2, T-3 and T-4 are shown in Figure S3 of the Supporting Information. The defects in TiO2 nanosheets are confirmed by HRTEM images (Figure 1c). For T-3 and T-4, the distances between adjacent lattice fringes are consistent with the standard interplanar spacing, indicating a high crystallinity. In Figure 1d, the T-1, T-2, and T-3 peaks can be indexed to anatase TiO2 phase (JCPDS No. 21-1272), while T-4 annealed at 800 oC can be partially indexed to rutile TiO2 (JCPDS No. 21-1276), suggesting that a rutile phase appears under heat treatment at 800 oC. This is consistent with the results of thermal- and chemical-bond analysis shown in Figures S4 and S5 (see Supporting Information). 10


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Electrochemical Performance of TiO2 Toward Hg(II). The SWASV method was employed for electrochemical detection of Hg(II). Optimized experimental conditions are shown in Figure S6. The effect on the differential electrode active areas is considered, and thus the current density is adopted (Figure S7). In Figure 2, The stripping peaks of Hg(II) are at approximately 2.5 V.

Figure 2. Hg(II) measurement results using a SWASV method. Typical SWASV responses and corresponding linear calibration plots of samples are as follows: a),b) 11



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T-1; c),d) T-2; e),f) T-3; and g),h) T-4 modified GCE toward Hg(II). Electrolyte, 0.1 M NaAc-HAc solution; pH = 6.0; deposition potential, −1.4 V vs Ag/AgCl electrode; deposition time, 150 s; amplitude, 25 mV; increment potential, 4 mV; frequency, 25 Hz. The dashed lines refer to the baselines. The error bars represent the standard deviations of five independent measurements of the same sample.

The peak current densities increase linearly versus the concentration of Hg(II) because of the oxidation of Hg to Hg(II). The sensitivities of T-1, T-2, T-3, and T-4 toward Hg(II) are 270.83, 159.52, 20.51, and 95.81 µA µM−1 cm−2, respectively, which are much better than for a bare GCE (13.75 µA µM−1 cm−2), as shown in Figure 3a. The sensitivities follow the sequence as T-1>T-2>T-4>T-3.

Figure 3. Statistical distribution of electrochemical performance of TiO2 electrodes toward Hg(II). a) Comparison of sensitivities toward Hg(II). Inset illustrates the role of the oxygen vacancy in catalysis. b) Corresponding limits of detection (LOD). The dashed line indicates the concentration limited by the WHO. Data for each sample were extracted from five control experiments.

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Since T-1, T-2, and T-3 exhibit similar morphology and phase, the discrepancies in the sensitivity can be ascribed to the surface defects. Considering the difference in phase structure between T-3 and T-4, the enhanced sensitivity of T-4 might be the result of the anatase and rutile heterostructure. Owing to the higher activity of the (001) crystal facet compared to the (101) facet, T-1 presents a higher sensitivity than TiO2 nanoparticles (T-5), as shown in Figures S8 and S9.9 The LODs of T-1-, T-2-, T-3- and T-4-based electrodes toward Hg(II) are 0.017, 0.024, 0.189, and 0.208 µM, respectively (Figure 2b). The results of the T-1- and T-2-based electrodes meet the requirement of the drinking water safety standard (0.03 µM) determined by the World Health Organization (WHO). This performance also exceeds those of many reported electrodes (Supporting Information, Table S1). Taking the sample T-1/GCE as an example for detecting 1.3 µM Hg(II), we find that the peak current density ranges from 333.3 to 366.9 µA cm−2, indicating a good stability (Figure S10). This can be ascribed to the uniform and stable TiO2-nanosheets film on the electrode surface. In addition, the peak current density of Hg(II) decreases by less than 10% in the presence of 10 µM Cu, Ni, and Zn ions (positive ions) and Cl−, SO42− (anions). The OV catalysis is illustrated in the inset of Figure 2a. Conventionally, Hg(II) is reduced at a constant negative voltage to form Hg(0), and then oxidized at an appropriate voltage.1 However, OV plays a significant role in the catalytic reaction, which might reduce the reaction energy barrier,15 as discussed below.

Surface Electronic State of TiO2 Nanosheets. To investigate the bulk-phase 13



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information of TiO2 nanosheets, the Ti K-edge X-ray absorption spectra of T-1, T-2, T-3, T-4, and the standard anatase TiO2 as a reference were recorded, as shown in Figures 4 and S11. Among T-1, T-2, T-3, T-4, and the standard anatase TiO2 nanoparticles, there is no obvious difference in local coordination and oxidation state of bulk phases because of the similar pre-edge and edge regions, as shown in Figure 4a. From the white line in the figure, we can also see that the curves of T-1, T-2, and T-3 are similar to the standard anatase, suggesting that T-1, T-2, and T-3 are in an anatase phase. Calcination below 600 oC would not result in different phases. A small peak appears in the curve of T-4 (marked by the dashed line), suggesting the rutile phase of T-4, which is in agreement with the previous XRD characterization (Figure 1d) and some reports.31,32 The pre-edge peaks A1, A2, and A3 can be assigned to the 1s-1t1g, 1s-2t2g, and 1s-3eg transitions of Ti in TiO2 crystal.33 Pre-edge peaks Atet might be the evidence of tetrahedral coordination (Figure 4b). Since no peak shifts ~3 eV to a lower energy level, Ti4+ is not reduced to Ti3+ in bulk phase, remaining in a +4 valence state (Figure 4c).34 In the Fourier-transformed Ti K-edge EXAFS results (Figure 4d), the Ti-O bond length becomes longer (no phase correction), which can be deduced from the positive shift of the Ti-O bond peak.33 The fitted data and more detailed bond length information are shown in Figure S12 and Table S2.

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Figure 4. Bulk-phase information of the TiO2 nanosheets. a),b) Ti K-edge XANES spectra, c) first derivative and d) magnitude part of Fourier-transformed k2-weighted χ(k) of Ti K-edge EXAFS spectra for T-1, T-2, T-3, T-4, and anatase TiO2 nanoparticles.

The surface states of TiO2 nanosheets were investigated by XPS, Raman spectra, ESR, and FTIR spectroscopy, respectively (Figure 5 and S13). In Figures 5a-c, the binding energies of Ti 2p and O 1s are 458.7 and 529.9 eV, respectively. A shoulder peak appears at the low energy region (457.5–455.5 eV) of the TiO2 without heat treatment (T-1), which is attributed to Ti3+ (Figure 5b). With an increased annealing temperature, the low-energy shoulder in Ti the 2p spectrum disappears, while the peak positions of Ti 2p and O 1s shift to low energy (Figures 5a and c). This is consistent with the TiO2 treated at 150 and 450 oC (Figures S13a and b). The Ti 2p XPS spectrum of the sample with the (101) facet exposed also confirms the presence of 15



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Ti3+ (Figure S14).

Figure 5. Surface-electronic-state characterizations. a),b),c) XPS, d) Raman spectra, e) ESR, and f) FTIR spectra of the TiO2 nanosheets.

Raman spectra confirm the Ti3+ in TiO2 nanosheets in terms of the positive peak shift (Figure 5d).28 A crystal transformation is shown in a wide Raman spectrum (Figure S13c). In ESR spectra (Figure 5e), the signal at g=2.003 is indexed to an OV.35 The signals of Ti3+ (g=1.990 and 1.982)36 disappear as the annealing temperature rises to 300 oC (T-2). However, ESR signals appear at gT-2>T-3, which is in accordance with the electrochemical sensing performance. Because there is no difference in the morphology and local coordination and oxidation state in bulk for these three samples, it is reasonable to ascribe the electrochemical sensing performance to the catalysis of surface OV concentration. When comparing double integrations of all the defect ESR signals among four samples, the defect concentration follows the sequence: T-1>T-2>T-4>T-3, which is in agreement with the detection performance.

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Figure 6. a) Comparative plots of detection performance and b) normalized double integration of the ESR signal of four samples.

Defect-Dependent Adsorption Capability and Electronic Properties. Since the defects induced by Ti3+ and OVs would impact the adsorption for electrochemical detection, the adsorption capacity toward Hg(II) was measured, and the results are shown in Figure 7a. As the concentration of native defects decreases, which is confirmed by ESR, the adsorption capacity is reduced. In Figure 7b, XPS spectra of Hg 4f5/2 (105.0 eV) and Hg 4f7/2 (100.9 eV) in the TiO2 after Hg(II) adsorption are presented. The adsorption capacity follows the sequence T-1>T-2>T-3>T-4. The adsorption capability of T-1, T-2, and T-3 is in the sequence T-1>T-2>T-3, which agrees with the sensitivities from electrochemical measurements (Figure 3a).

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Figure 7. Native-defects-impacted adsorption capability and the interaction between Hg(II) and TiO2. a) Statistical distribution of adsorption capacity of T-1, T-2, T-3, and T-4 toward Hg(II), extracted from five control experiments. b) XPS spectra of Hg 4f after adsorption. c) Fitted magnitude of k3-weighted Hg L3-edge EXAFS spectra. d) Fourier transforms and their fits of EXAFS spectra with uncorrected phase shift.

As the O2 molecules can be adsorbed onto the surface OVs preferentially,15 surface reduced Ti3+ would transfer an electron to the adsorbed O2 to form a superoxide radical (O2.−) that adsorbs electropositive Hg(II). Hg L3-edge X-ray absorption spectra of Hg/T-1 and Hg/T-3 after adsorption were investigated (Figure S16). Background-subtracted, k3-weighted χ(k) functions and RSFs are obtained using the ATHENA program. Fitting of the EXAFS was conducted through ARTEMIS to obtain quantitative information about coordination numbers (CNs), bond distance (R), static and thermal disorder (σ2), phase shift (△E0), and R-factor, as shown in Table 1. For the first shell (Hg-O), the accuracies of the fit parameters are CN±0.22 and R±0.01 for

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Hg/T-1, CN±0.07 and R±0.01 for Hg/T-3, respectively. For the second shell (Hg-Hg), accuracies are CN±0.44 and R±0.02, and CN±0.15 and R±0.04, respectively. In Figure 7d, the peaks at R≈1.75 and 3.0 Å in RSFs (without phase correction) are indexed to Hg-O and Hg-Hg, respectively.39 As listed in Table 1, the coordination numbers and intensity of Hg-O (1.4) and Hg-Hg (2.8) in Hg/T-1 are higher than in Hg/T-3 (0.6, and 2.4, respectively), suggesting that more O and Hg atoms locate around the central Hg atoms.40

Table 1. EXAFS Analysis on T-1 and T-3 after Hg(II) Adsorption. Sample Hg/T-1 Hg/T-3

Path Hg-O Hg-Hg Hg-O Hg-Hg

CN (NS02) 1.4 2.8 0.6 2.4

R (Å) 2.05 3.08 2.06 3.06

σ2 0.003 0.019 0.005 0.015

△E0 (eV) 15.07 -27.93 15.05 -20.76

R-factor 0.0039 0.0026

New energy levels can be introduced by Ti3+ self-doping below the bottom of conductance band,41,42 which narrows the inherent bandgap and enhances the electrical conductivity (Figure 8a). Mott-Schottky plots were collected using the following equation:15 =

,

(1)

where , NA, e, , , , , , and denote space-charge capacitance, electron carrier density, elementary charge value, vacuum permittivity, relative permittivity, applied potential, flat-band potential, the Boltzmann constant, and temperature, respectively. 20


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Figure 8. Native-defect-induced electronic properties. a) EIS and b) Mott-Schottky plots of TiO2 nanosheets. c) In a perfect crystal (left-hand part), charge is transferred through the whole nanocrystal, while in a defective crystal (right-hand part), the charge is transferred through the surface.

In Figure 8b, the linear slopes of the Mott-Schottky plots are positive, indicating an n-type TiO2 with self-doping of Ti3+ and OVs.43 The negative shift of V among T-1, T-2 and T-3 indicates a positive shift of the Fermi level, which facilitates the charge separation on the semiconductor/electrolyte interface.44 From Eq (1), the smallest slope (T-1) indicates that the highest defect concentration provides the largest electron carrier density, which improves the electrical conductivity and transfer of charge carriers.43 The increased defects not only increase the density of hydroxyl groups on the TiO2 surface, but also enhance the pseudocapacitance which promotes the redox reactions.45 For a perfect TiO2 nanocrystal (left-hand part of Figure 8c), less charge carriers are distributed on the surface. In contrast, on the surface of a defective TiO2 nanosheet (right-hand part of Figure 8c), the excess electrons would be trapped by O2 to form O2.−, accelerating the electron transfer.18,46

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CONCLUSIONS In summary, we have demonstrated a surface-electronic-state modulation on pure semiconductors (e.g. TiO2 nanocrystal) for high-performance electrochemical sensing toward heavy-metal ions. A redox reaction occurs on the surface of TiO2 nanocrystals in which the balance between adsorption and desorption is no longer required. It is significantly different from the common two-step process during stripping measurements by insulating a nanoparticle-modified electrode. Raman spectra confirm the presence of Ti3+ in the modulated TiO2 nanosheets in terms of positive peak shift. In the ESR spectra, the signal at g=2.003 can be indexed to OVs; while the FTIR spectra indicate the presence of OVs and reductive Ti3+. EXAFS spectra confirm that the OV adsorbs an O2 molecule while the Ti3+ donates an electron, forming O2.− species that facilitate adsorption of Hg(II) and serve as active sites for electron transfer. These findings show that the surface-electronic-state modulation could obviate the modification need of many semiconductors with other materials to address the low conductivity and poor reactivity issues.

ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China under awards #21475133, U1532123, 21277146, and 661573334. X.-J. Huang acknowledges the CAS Interdisciplinary Innovation Team of the Chinese Academy of Sciences, China, for financial support.

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SUPPORTING INFORMATION Supporting Information Available: Morphology and the statistical analysis of the size and distribution; Nitrogen adsorption-desorption isotherms; The corresponding fast Fourier transform patterns; DSC analysis; The ratio of F to Ti element of TiO2 samples; Optimal experimental conditions; Calculation of electrically active area of electrodes; Comparison of electrochemical sensitivities; Morphology and structure of TiO2 nanoparticles; Stability and anti-interference ability studies. Normalized Ti K-edge absorption curves; EXAFS analysis; Existence of oxygen vacancy and Ti3+; Ti 2p spectrum; Photographs; Normalized Hg L3-edge absorption curves in the relevantand post-edge regions. Comparison on the electrochemical performance of different electrodes used for Hg(II) detection; Results of EXAFS analysis for TiO2 sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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TOC figure:

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TiO2 Nanosheets for Sensitive Electrochemical Sensing of Heavy

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