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TiO2 nanosheets
Author’s Accepted Manuscript High performance of electrocatalytic oxidation and determination of Hydrazine based on Pt nanoparticles/TiO2 nanosheets Xiaoyu Yue, Wenxiu Yang, Miao Xu, Xiangjian Liu, Jianbo Jia www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(15)30225-3 http://dx.doi.org/10.1016/j.talanta.2015.08.002 TAL15862
To appear in: Talanta Received date: 1 June 2015 Revised date: 27 July 2015 Accepted date: 2 August 2015 Cite this article as: Xiaoyu Yue, Wenxiu Yang, Miao Xu, Xiangjian Liu and Jianbo Jia, High performance of electrocatalytic oxidation and determination of Hydrazine based on Pt nanoparticles/TiO2 nanosheets, Talanta, http://dx.doi.org/10.1016/j.talanta.2015.08.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High Performance of Electrocatalytic Oxidation and Determination of Hydrazine Based on Pt Nanoparticles/TiO2 Nanosheets Xiaoyu Yue a,b, Wenxiu Yang a,b, Miao Xu a,b, Xiangjian Liua,b, Jianbo Jia a a) State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China b) University of Chinese Academy of Sciences, Beijing 100049, P. R. China
Abstract: In this work, highly dispersed Pt nanoparticles (PtNPs) were deposited on the surface of the TiO2 nanosheets (TiO2NSs) by the photodeposition method in the presence of methanol as holes scavengers. The results indicated that PtNPs were distributed on TiO2NSs successfully with the diameter of ca. 5-9 nm. The electrochemical experiments such as electrochemical impedance spectroscopy and cyclic voltammetry were used to study the electrochemical properties of the PtNPs/TiO2NSs modified glassy carbon electrode (GCE). The as-prepared PtNPs/TiO2NSs/GCE presents excellent electrocatalytic activity for hydrazine. The sensor can be used to determine hydrazine at low potential with a wide linear range, high sensitivity, and fast response time. Moreover, the sensor exhibits good selectivity and reproducibility. In addition, the recoveries were 100.1-105.3% for hydrazine in the tap water, indicating the PtNPs/TiO2NSs/GCE should be a promising sensor for the determination of hydrazine in real samples.
Keywords: Pt nanoparticles; TiO2 nanosheets; Photodeposition; Electrocatalytic activity; Hydrazine sensor 1. Introduction Nowadays, hydrazine is widely used in many fields, such as antioxidants, insecticides, plant growth regulators, explosives, corrosion inhibitors and catalyst in fuel for
Corresponding author. Tel. & Fax: +86-431-85262251. E-mail address: [email protected] (J. Jia).
aircrafts and satellites [1-3]. Although hydrazine possesses significant applications, it is still a hepatoxic substance, neurotoxin and carcinogen [4]. Therefore, it is practically necessary to explore an effective and simple sensing method to detect hydrazine for environmental and biological aspects [5]. Since TiO2 was found to be a chemically stable, low cost, environment-friendly, and efficient hydrogen production photocatalyst in 1972 [6], it has been widely used for different kinds of applications, such as photocatalysis [7], solar cells [8], and sensors [9]. Recently, many novel TiO2 nanomaterials with new composition and structure have been used for various sensors [10]. Among them, two-dimensional TiO2 nanosheet (TiO2NS) is particularly interesting owing to the following reasons: 1) especially ultra-thin TiO2NS provides large interaction area; 2) it can serve as hosting material to load guest functional nanomaterials and the as-prepared nanocomposite structure has advantages of both components [11]. In addition, the TiO2NSs demonstrated better electrochemical performances than TiO2 nanotubes and anatase nanoparticles [12]. Moreover, anatase TiO2 with highly reactive facet (001) has attracted a lot of attention due to its high surface energy [13]. Recently, the properties of the strong metal support interaction (SMSI) between Pt and metal oxides (WO3 [14], SiO2 [15], and TiO2 [16]) were explored widely. Especially, the SMSI found between Pt and TiO2 can drastically change the electronic structure by charge transfer, thereby affecting the activity and durability of the catalysts [17]. Moreover, Pt deposited on TiO2 can act as an electron trap, which is facilitating the capture of electrons. Therefore, the Pt/TiO2 nanomaterials were used in many fields, such as photocatalysis [18], electrocatalysis [19], and sensor [20]. In this work, TiO2NSs were used as the catalyst support, and Pt nanoparticles (PtNPs) were deposited onto the TiO2NSs by photoreduction. The PtNPs/TiO2NSs show great
performances of electrocatalytic oxidation of hydrazine at low potential with a wide linear range, a high sensitivity and the fast response, which can be attributed to the support ultra-thin anatase TiO2NSs, the small-sized and high dispersed PtNPs on TiO2NSs, and the SMSI between Pt and TiO2NSs.
2. Experimental 2.1 Chemicals and Apparatuses Ti(OBu)4 and H2PtCl6.6H2O were obtained from National Chemical Reagent Company (Shanghai, China). Nafion (5 wt %) were purchased from Sigma-Aldrich. Methanol, isopropyl alcohol, absolute ethanol, hydrofluoric acid, potassium chloride, sodium chloride, nickel acetate, cobalt acetate, ferric nitrate, zinc chloride, sodium dihydrogen phosphate, dibasic sodium phosphate, D-glucose, and ascorbic acid were of analytical grade from Beijing Chemical Reagent Company (China) without further purification. Ultrapure water from Water Purifier (Sichuan Water Purifier Co., Ltd., China) was used in all the experiments. Transmission electron microscopy (TEM) was performed on a HITACHI H-600 Analytical TEM with an accelerating voltage of 100 kV. X-ray diffraction (XRD) data were recorded using model D8 ADVANCE (BRUKER, Cu Kα radiation, Ɩ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurement was carried out on an ESCALABMKII X-ray photoelectron spectrometer (VG Scientific, UK). The light of photodeposition came from PLS-SXE300 light (Beijing Perfect Light Technology Co., Ltd.). The electrochemical impedance spectroscopy (EIS) measurement was tested on a Zahner Zennium electrochemical workstation (ZAHNER-Elektrik GmbH & Co. KG, Germany). Cyclic voltammetry (CV) and chronoamperometry were measured by a standard three-electrode cell using a CHI 832B electrochemical workstation (CH
Instruments, Shanghai). A modified glassy carbon electrode (GCE) with a diameter of 3.0 mm was served as the working electrode, while a Pt foil and a KCl saturated Ag/AgCl electrode were used as counter and reference electrodes, respectively. 2.2 Synthesis of TiO2NSs and PtNPs/TiO2NSs. The TiO2NSs were prepared according to reported method with minor modified [21]. In a typical procedure, 1.50 mL of hydrofluoric acid solution (40 wt %) was dropped to 12.50 mL of Ti(OBu)4 under strong stirring. After stirring for 2 h, the solution was transferred into a 50 mL Teflon tube and kept at 180 ºC for 21 h. After being cooled to room temperature, the gained samples were separated by the high-speed centrifugation and washed with ethanol and ultrapure water for several times, then dried at 80 ºC in an electric oven. The PtNPs/TiO2NSs were prepared by the photodeposition technique in the presence of methanol as holes scavengers. The 10 mL suspension containing TiO2NSs (0.1000 g), H2PtCl6.6H2O (4.60 mM), and methanol (5.0 mL) was irradiated by light from 300 W Xe lamp under strong stirring at room temperature. Finally, the prepared products were separated by centrifugation, rinsed with ultrapure water, and dried at 80 ºC. 2.3 Preparation of the modified electrode Before the experiment, the GCE was polished carefully with 0.3 and 0.05 m alumina slurries washed with water, and followed by sonication in ethanol and ultrapure water successively. A suspension of PtNPs/TiO2NSs was prepared by mixing 5.0 mg of PtNPs/TiO2NSs in 2.0 mL of Nafion (5 wt %), isopropyl alcohol and water (v/v/v = 0.075:2:20) with sonication. Then, an aliquot of 10 L of the suspension was dropped on the GCE with a microsyringe. The modified electrodes were dried under an infrared lamp before use. For comparison, the TiO2NSs modified GCE (TiO2NSs/GCE) was prepared according to the same procedure. Tap water was
collected from the laboratory. The sample was diluted with 0.20 M isometric phosphate buffered saline (PBS, pH 7.0) before measurements. All the electrochemical tests were carried out at room temperature.
3. Results and discussion 3.1 Surface characterization The morphology and structure of the as-prepared products were characterized by TEM. Fig. 1 shows the TEM images of TiO2NSs (a) and PtNPs/TiO2NSs (b). The results indicate that the TiO2NSs are the rectangular nanosheets with the side length of ca. 30-50 nm. Moreover, after comparing the morphologies of TiO2NSs and the PtNPs/TiO2NSs, it can be observed that PtNPs with the diameter of about 5-9 nm were dispersed on the surface of TiO2NSs, as given in Fig. 1b. Fig. 1 The XRD patterns of TiO2NSs and PtNPs/TiO2NSs can be used to confirm their crystal structures. As exhibited in Fig. 2, both of them show the diffraction patterns of anatase TiO2 (JCPDS no. 21-1272), agreeing with the reported result [21], and there is only anatase TiO2 that can be observed in both TiO2NSs and PtNPs/TiO2NSs. Moreover, the peaks appeared at 39.7°, 46.2°, 67.4°, and 81.2° in PtNPs/TiO2 NSs are the characteristic peaks of platinum, which can be assigned to the (111), (200), (220), and (311) planes of face-centered cubic Pt, respectively [22]. Pt (311) is not clearly observed due to the strongly overlap with the peak of the anatase TiO2 at 82.6°. The grain size of PtNPs was calculated by Scherrer’s equation: d(Å) = Kλ/βcosθ, where K is a coefficient (0.9), λ is the wavelength of X-ray used (0.154 nm), β is the full-width half maximum of respective diffraction peak (rad), θ is the angle at the position of peak maximum (rad) [23]. The average size of PtNPs is ca. 9.6 nm based on Pt (111),
which is basically the same as the result of TEM. Fig. 2 XPS is carried out to analyze the chemical state of species present in the samples. Fig. 3a displays the XPS spectra of TiO2NSs and PtNPs/TiO2NSs. It reveals the presence of oxygen, titanium, and carbon in both samples. The carbon came from the testing process of XPS [24]. Compared with TiO2NSs, the Pt 4f7/2 and Pt 4f5/2 can be observed clearly in PtNPs/TiO2NSs indicating that the PtNPs was stabilized on TiO2NSs. The Pt 4f spectrum of PtNPs/TiO2NSs in Fig. 3b contains two peaks at 69.9 and 73.2 eV, which were attributed to Pt 4f7/2 and Pt 4f5/2 of metallic Pt [25]. It is known that the Pt 4f7/2 binding energy of Pt0 is around 71.2 eV. The significantly shift of the Pt 4f may be caused by the electron transfer from TiO2NSs to PtNPs, which is due to the SMSI [26]. Fig. 3 3.2 Electrochemical characterization EIS can be used to measure the impedance value of the electrode surface during the process of frequency variation, which can reflect the properties of the electrode interface including the electrolyte resistance (Rs), the capacity of the electric double layer (Cd), Warburg impedance (Zw), and the surface charge-transfer resistance (Rct) [27]. Generally, the smaller semicircle arc in the high-frequency region indicates the smaller charge transfer resistance and faster interfacial charge transfer. The Nyquist diagrams and the equivalent circuit were given in Fig. 4. It was clear that the TiO2NSs/GCE owned a very large electrochemical resistance. However, the radius of semicircle arc of the PtNPs/TiO2NSs/GCE is much smaller than that of TiO2NSs/GCE, which displays a smaller resistance, suggesting the PtNPs were successfully immobilized on the surface of TiO2NSs, and implied the faster reaction [28].
Therefore, the PtNPs/TiO2NSs/GCE has faster electron transfer and should be benefit for enhancing the electrocatalytic oxidation of hydrazine. Fig. 4 3.3 Electroanalytical determination of hydrazine Cyclic voltammetry was used to investigate the electrochemical performances of hydrazine on the modified electrodes. Fig. 5 shows the CV curves of the TiO2NSs/GCE and PtNPs/TiO2NSs/GCE in the absence and presence of hydrazine (2.0 mM) in 0.1 M PBS (pH 7.0). From Fig. 5a, it was observed that there was no obvious oxidation peak at TiO2NSs/GCE, which demonstrated that TiO2NSs have no clearly catalytic property for hydrazine. However, for the PtNPs/TiO2NSs/GCE, the CV curves changed with an obvious oxidation peak of hydrazine at ca. 0.3 V, which was lower than that of the TiO2-Pt nanofibers/GCE (0.55 V) [20]. The improvement of the electrocatalytic activity of hydrazine can be attributed to the strongly adherent, highly dispersed and small-sized PtNPs on the TiO2NSs, which had the SMSI with TiO2NSs [26]. Fig. 5 The influence of the scan rate on oxidation current of hydrazine at the PtNPs/TiO2NSs/GCE was displayed in Fig. 6. The oxidation peak current (Ipa) increases linearly with the square root of the scan rate from 5 to 160 mV s−1, and the regression equation is Ipa = 13.611 × v1/2 - 27.975 with R2 = 0.994 (Fig. 6b), indicating that the oxidation process is controlled by diffusion. Moreover, the oxidation potential (Epa) simultaneously shifts to positive values with a linear correlation between the peak potential and the logarithm of scan rate, and the regression equation is Epa = 0.0931 × log (v) + 0.1466 with R2 = 0.996 (Fig. 6c). The Tafel slope is b = 0.059/(1−α)nα (α, the transfer coefficient; nα, the number of electrons involved in the
rate-determining) [29]. It was calculated that the nα is equal to 1, illustrating that the rate-limiting step is one-electron transfer process. Fig. 6 Fig. 7 shows the chronoamperometric curve of the PtNPs/TiO2NSs/GCE on successive injection of hydrazine at 0.3 V in a N2-saturated 0.1 M PBS (pH 7.0). The sensor exhibits wide linear ranges of 20-900 and 900-2100 M with the sensitivity of 187.4 and 100.0 A mM-1cm-2, respectively. The linear range of the as-proposed sensor is wider than those of the TiO2-Pt nanofibers/GCE [20] and ordered mesoporous carbons-polydopamine/Pt [30], and the sensitivity is higher than that of ZnO nanorods electrode (44.24 A mM-1 cm-2) [31]. The limit of detection is found to be 2.0 M with a signal-to-noise (S/N) ratio of 3. Fig. 7 The effect of substances that might interfere with the response of the modified electrode was studied. The current obtained for each interfering substance in the presence of 40 M N2H4 was compared with that of 40 M N2H4, and this ratio was used as the criterion for the selectivity of the sensor. There are no observable interference in the presence of the same concentration of K+, Na+, Zn2+, Ni2+, Co2+, Fe3+, glucose, and 3-fold NO3-, 3-fold Cl−, 4-fold CH3COO−. However, equivalent concentration of Cu2+ and ascorbic acid interfere obviously (40.2% and 30.3% increase, respectively). The reproducibility and repeatability were studied as well. The electrode-to-electrode reproducibility was investigated from the response to 0.20 mM N2H4 on 4 different electrodes, yielding a relative standard deviation (RSD) of 5.24%. The repeatability of one sensor was estimated with a RSD of 3.66% for 4 successive tests. The results implied that the PtNPs/TiO2NSs sensor had good great reproducibility and repeatability. In addition, to verify the practical use of the
PtNPs/TiO2NSs sensor, the recovery of hydrazine in the tap water was examined. The recoveries of 100.1-105.3% with RSD of 3.28% (n = 4) were obtained, indicating the PtNPs/TiO2NSs/GCE should be a promising sensor for the effective determination of hydrazine.
4. Conclusion In conclusion, the novel PtNPs/TiO2NSs nanomaterials were prepared by the photoreduction method successfully. The PtNPs/TiO2NSs sensor exhibits great performances for electrochemical oxidation and determination towards hydrazine at low potential. The results indicated that the support ultra-thin anatase TiO2NSs, the dispersed and small-sized PtNPs, and the SMSI between PtNPs and TiO2NSs were significant for the sensor, which had a wide linear range and high sensitivity for hydrazine. In addition, the PtNPs/TiO2NSs sensor showed good anti-interference ability,
reproducibility,
and
repeatability.
The
excellent
performances
of
electrocatalytic oxidation and determination of hydrazine make the PtNPs/TiO2NSs promising for the future application in electrochemical sensors and biosensors. Moreover, the materials also may broaden the application of TiO2 nanomaterials in electroanalysis.
Acknowledgments We acknowledge the Ministry of Science and Technology of China (No. 2013YQ170585).
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Captions Fig. 1 TEM images of (a) TiO2NSs and (b) PtNPs/TiO2NSs. Fig. 2 XRD patterns of TiO2NSs and PtNPs/TiO2NSs. Fig. 3 The XPS spectra of (a) TiO2NSs and PtNPs/TiO2NSs, and (b) Pt 4f region of PtNPs/TiO2NSs. Fig. 4 The EIS for (a) TiO2NSs/GCE and (b) PtNPs/TiO2NSs/GCE in 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 M KCl solution. Inset: the equivalent circuit. Fig. 5 The CV curves of (a) TiO2NSs/GCE and (b) PtNPs/TiO2NSs/GCE in the absence and presence of hydrazine (2.0 mM) in 0.1 M PBS (pH 7.0) at a scan rate of 50 mV s−1. Fig. 6 (a) CV curves of hydrazine oxidation at PtNPs/TiO2NSs/GCE in 0.1 M 25 mL PBS (pH 7.0) containing 2.0 mM hydrazine at different scan rates (5, 10, 20, 40, 60, 80, 100, 120, 140, and 160 mV s−1). Inset: (b) the relationship between the oxidation peak current (Ipa) and the square root of the scan rate, (c) the relationship between the oxidation peak potential (Epa) and log (v). Fig. 7 Chronoamperometric curves of the PtNPs/TiO2NTs/GCE on successive injection of hydrazine at 0.3 V in a N2-saturated 0.1 M PBS (pH 7.0). Inset: the
calibration curve of N2H4.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Graphical abstract
Highlights: 1. The dispersed and small-sized PtNPs were deposited onto TiO2NSs by photoreduction. 2. The PtNPs/TiO2NSs sensor shows excellent electrocatalytic oxidation of hydrazine at low potential. 3. The PtNPs/TiO2NSs sensor exhibits a wide linear range and high sensitivity for hydrazine.
PII: DOI: Reference:
S0039-9140(15)30225-3 http://dx.doi.org/10.1016/j.talanta.2015.08.002 TAL15862
To appear in: Talanta Received date: 1 June 2015 Revised date: 27 July 2015 Accepted date: 2 August 2015 Cite this article as: Xiaoyu Yue, Wenxiu Yang, Miao Xu, Xiangjian Liu and Jianbo Jia, High performance of electrocatalytic oxidation and determination of Hydrazine based on Pt nanoparticles/TiO2 nanosheets, Talanta, http://dx.doi.org/10.1016/j.talanta.2015.08.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High Performance of Electrocatalytic Oxidation and Determination of Hydrazine Based on Pt Nanoparticles/TiO2 Nanosheets Xiaoyu Yue a,b, Wenxiu Yang a,b, Miao Xu a,b, Xiangjian Liua,b, Jianbo Jia a a) State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China b) University of Chinese Academy of Sciences, Beijing 100049, P. R. China
Abstract: In this work, highly dispersed Pt nanoparticles (PtNPs) were deposited on the surface of the TiO2 nanosheets (TiO2NSs) by the photodeposition method in the presence of methanol as holes scavengers. The results indicated that PtNPs were distributed on TiO2NSs successfully with the diameter of ca. 5-9 nm. The electrochemical experiments such as electrochemical impedance spectroscopy and cyclic voltammetry were used to study the electrochemical properties of the PtNPs/TiO2NSs modified glassy carbon electrode (GCE). The as-prepared PtNPs/TiO2NSs/GCE presents excellent electrocatalytic activity for hydrazine. The sensor can be used to determine hydrazine at low potential with a wide linear range, high sensitivity, and fast response time. Moreover, the sensor exhibits good selectivity and reproducibility. In addition, the recoveries were 100.1-105.3% for hydrazine in the tap water, indicating the PtNPs/TiO2NSs/GCE should be a promising sensor for the determination of hydrazine in real samples.
Keywords: Pt nanoparticles; TiO2 nanosheets; Photodeposition; Electrocatalytic activity; Hydrazine sensor 1. Introduction Nowadays, hydrazine is widely used in many fields, such as antioxidants, insecticides, plant growth regulators, explosives, corrosion inhibitors and catalyst in fuel for
Corresponding author. Tel. & Fax: +86-431-85262251. E-mail address: [email protected] (J. Jia).
aircrafts and satellites [1-3]. Although hydrazine possesses significant applications, it is still a hepatoxic substance, neurotoxin and carcinogen [4]. Therefore, it is practically necessary to explore an effective and simple sensing method to detect hydrazine for environmental and biological aspects [5]. Since TiO2 was found to be a chemically stable, low cost, environment-friendly, and efficient hydrogen production photocatalyst in 1972 [6], it has been widely used for different kinds of applications, such as photocatalysis [7], solar cells [8], and sensors [9]. Recently, many novel TiO2 nanomaterials with new composition and structure have been used for various sensors [10]. Among them, two-dimensional TiO2 nanosheet (TiO2NS) is particularly interesting owing to the following reasons: 1) especially ultra-thin TiO2NS provides large interaction area; 2) it can serve as hosting material to load guest functional nanomaterials and the as-prepared nanocomposite structure has advantages of both components [11]. In addition, the TiO2NSs demonstrated better electrochemical performances than TiO2 nanotubes and anatase nanoparticles [12]. Moreover, anatase TiO2 with highly reactive facet (001) has attracted a lot of attention due to its high surface energy [13]. Recently, the properties of the strong metal support interaction (SMSI) between Pt and metal oxides (WO3 [14], SiO2 [15], and TiO2 [16]) were explored widely. Especially, the SMSI found between Pt and TiO2 can drastically change the electronic structure by charge transfer, thereby affecting the activity and durability of the catalysts [17]. Moreover, Pt deposited on TiO2 can act as an electron trap, which is facilitating the capture of electrons. Therefore, the Pt/TiO2 nanomaterials were used in many fields, such as photocatalysis [18], electrocatalysis [19], and sensor [20]. In this work, TiO2NSs were used as the catalyst support, and Pt nanoparticles (PtNPs) were deposited onto the TiO2NSs by photoreduction. The PtNPs/TiO2NSs show great
performances of electrocatalytic oxidation of hydrazine at low potential with a wide linear range, a high sensitivity and the fast response, which can be attributed to the support ultra-thin anatase TiO2NSs, the small-sized and high dispersed PtNPs on TiO2NSs, and the SMSI between Pt and TiO2NSs.
2. Experimental 2.1 Chemicals and Apparatuses Ti(OBu)4 and H2PtCl6.6H2O were obtained from National Chemical Reagent Company (Shanghai, China). Nafion (5 wt %) were purchased from Sigma-Aldrich. Methanol, isopropyl alcohol, absolute ethanol, hydrofluoric acid, potassium chloride, sodium chloride, nickel acetate, cobalt acetate, ferric nitrate, zinc chloride, sodium dihydrogen phosphate, dibasic sodium phosphate, D-glucose, and ascorbic acid were of analytical grade from Beijing Chemical Reagent Company (China) without further purification. Ultrapure water from Water Purifier (Sichuan Water Purifier Co., Ltd., China) was used in all the experiments. Transmission electron microscopy (TEM) was performed on a HITACHI H-600 Analytical TEM with an accelerating voltage of 100 kV. X-ray diffraction (XRD) data were recorded using model D8 ADVANCE (BRUKER, Cu Kα radiation, Ɩ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurement was carried out on an ESCALABMKII X-ray photoelectron spectrometer (VG Scientific, UK). The light of photodeposition came from PLS-SXE300 light (Beijing Perfect Light Technology Co., Ltd.). The electrochemical impedance spectroscopy (EIS) measurement was tested on a Zahner Zennium electrochemical workstation (ZAHNER-Elektrik GmbH & Co. KG, Germany). Cyclic voltammetry (CV) and chronoamperometry were measured by a standard three-electrode cell using a CHI 832B electrochemical workstation (CH
Instruments, Shanghai). A modified glassy carbon electrode (GCE) with a diameter of 3.0 mm was served as the working electrode, while a Pt foil and a KCl saturated Ag/AgCl electrode were used as counter and reference electrodes, respectively. 2.2 Synthesis of TiO2NSs and PtNPs/TiO2NSs. The TiO2NSs were prepared according to reported method with minor modified [21]. In a typical procedure, 1.50 mL of hydrofluoric acid solution (40 wt %) was dropped to 12.50 mL of Ti(OBu)4 under strong stirring. After stirring for 2 h, the solution was transferred into a 50 mL Teflon tube and kept at 180 ºC for 21 h. After being cooled to room temperature, the gained samples were separated by the high-speed centrifugation and washed with ethanol and ultrapure water for several times, then dried at 80 ºC in an electric oven. The PtNPs/TiO2NSs were prepared by the photodeposition technique in the presence of methanol as holes scavengers. The 10 mL suspension containing TiO2NSs (0.1000 g), H2PtCl6.6H2O (4.60 mM), and methanol (5.0 mL) was irradiated by light from 300 W Xe lamp under strong stirring at room temperature. Finally, the prepared products were separated by centrifugation, rinsed with ultrapure water, and dried at 80 ºC. 2.3 Preparation of the modified electrode Before the experiment, the GCE was polished carefully with 0.3 and 0.05 m alumina slurries washed with water, and followed by sonication in ethanol and ultrapure water successively. A suspension of PtNPs/TiO2NSs was prepared by mixing 5.0 mg of PtNPs/TiO2NSs in 2.0 mL of Nafion (5 wt %), isopropyl alcohol and water (v/v/v = 0.075:2:20) with sonication. Then, an aliquot of 10 L of the suspension was dropped on the GCE with a microsyringe. The modified electrodes were dried under an infrared lamp before use. For comparison, the TiO2NSs modified GCE (TiO2NSs/GCE) was prepared according to the same procedure. Tap water was
collected from the laboratory. The sample was diluted with 0.20 M isometric phosphate buffered saline (PBS, pH 7.0) before measurements. All the electrochemical tests were carried out at room temperature.
3. Results and discussion 3.1 Surface characterization The morphology and structure of the as-prepared products were characterized by TEM. Fig. 1 shows the TEM images of TiO2NSs (a) and PtNPs/TiO2NSs (b). The results indicate that the TiO2NSs are the rectangular nanosheets with the side length of ca. 30-50 nm. Moreover, after comparing the morphologies of TiO2NSs and the PtNPs/TiO2NSs, it can be observed that PtNPs with the diameter of about 5-9 nm were dispersed on the surface of TiO2NSs, as given in Fig. 1b. Fig. 1 The XRD patterns of TiO2NSs and PtNPs/TiO2NSs can be used to confirm their crystal structures. As exhibited in Fig. 2, both of them show the diffraction patterns of anatase TiO2 (JCPDS no. 21-1272), agreeing with the reported result [21], and there is only anatase TiO2 that can be observed in both TiO2NSs and PtNPs/TiO2NSs. Moreover, the peaks appeared at 39.7°, 46.2°, 67.4°, and 81.2° in PtNPs/TiO2 NSs are the characteristic peaks of platinum, which can be assigned to the (111), (200), (220), and (311) planes of face-centered cubic Pt, respectively [22]. Pt (311) is not clearly observed due to the strongly overlap with the peak of the anatase TiO2 at 82.6°. The grain size of PtNPs was calculated by Scherrer’s equation: d(Å) = Kλ/βcosθ, where K is a coefficient (0.9), λ is the wavelength of X-ray used (0.154 nm), β is the full-width half maximum of respective diffraction peak (rad), θ is the angle at the position of peak maximum (rad) [23]. The average size of PtNPs is ca. 9.6 nm based on Pt (111),
which is basically the same as the result of TEM. Fig. 2 XPS is carried out to analyze the chemical state of species present in the samples. Fig. 3a displays the XPS spectra of TiO2NSs and PtNPs/TiO2NSs. It reveals the presence of oxygen, titanium, and carbon in both samples. The carbon came from the testing process of XPS [24]. Compared with TiO2NSs, the Pt 4f7/2 and Pt 4f5/2 can be observed clearly in PtNPs/TiO2NSs indicating that the PtNPs was stabilized on TiO2NSs. The Pt 4f spectrum of PtNPs/TiO2NSs in Fig. 3b contains two peaks at 69.9 and 73.2 eV, which were attributed to Pt 4f7/2 and Pt 4f5/2 of metallic Pt [25]. It is known that the Pt 4f7/2 binding energy of Pt0 is around 71.2 eV. The significantly shift of the Pt 4f may be caused by the electron transfer from TiO2NSs to PtNPs, which is due to the SMSI [26]. Fig. 3 3.2 Electrochemical characterization EIS can be used to measure the impedance value of the electrode surface during the process of frequency variation, which can reflect the properties of the electrode interface including the electrolyte resistance (Rs), the capacity of the electric double layer (Cd), Warburg impedance (Zw), and the surface charge-transfer resistance (Rct) [27]. Generally, the smaller semicircle arc in the high-frequency region indicates the smaller charge transfer resistance and faster interfacial charge transfer. The Nyquist diagrams and the equivalent circuit were given in Fig. 4. It was clear that the TiO2NSs/GCE owned a very large electrochemical resistance. However, the radius of semicircle arc of the PtNPs/TiO2NSs/GCE is much smaller than that of TiO2NSs/GCE, which displays a smaller resistance, suggesting the PtNPs were successfully immobilized on the surface of TiO2NSs, and implied the faster reaction [28].
Therefore, the PtNPs/TiO2NSs/GCE has faster electron transfer and should be benefit for enhancing the electrocatalytic oxidation of hydrazine. Fig. 4 3.3 Electroanalytical determination of hydrazine Cyclic voltammetry was used to investigate the electrochemical performances of hydrazine on the modified electrodes. Fig. 5 shows the CV curves of the TiO2NSs/GCE and PtNPs/TiO2NSs/GCE in the absence and presence of hydrazine (2.0 mM) in 0.1 M PBS (pH 7.0). From Fig. 5a, it was observed that there was no obvious oxidation peak at TiO2NSs/GCE, which demonstrated that TiO2NSs have no clearly catalytic property for hydrazine. However, for the PtNPs/TiO2NSs/GCE, the CV curves changed with an obvious oxidation peak of hydrazine at ca. 0.3 V, which was lower than that of the TiO2-Pt nanofibers/GCE (0.55 V) [20]. The improvement of the electrocatalytic activity of hydrazine can be attributed to the strongly adherent, highly dispersed and small-sized PtNPs on the TiO2NSs, which had the SMSI with TiO2NSs [26]. Fig. 5 The influence of the scan rate on oxidation current of hydrazine at the PtNPs/TiO2NSs/GCE was displayed in Fig. 6. The oxidation peak current (Ipa) increases linearly with the square root of the scan rate from 5 to 160 mV s−1, and the regression equation is Ipa = 13.611 × v1/2 - 27.975 with R2 = 0.994 (Fig. 6b), indicating that the oxidation process is controlled by diffusion. Moreover, the oxidation potential (Epa) simultaneously shifts to positive values with a linear correlation between the peak potential and the logarithm of scan rate, and the regression equation is Epa = 0.0931 × log (v) + 0.1466 with R2 = 0.996 (Fig. 6c). The Tafel slope is b = 0.059/(1−α)nα (α, the transfer coefficient; nα, the number of electrons involved in the
rate-determining) [29]. It was calculated that the nα is equal to 1, illustrating that the rate-limiting step is one-electron transfer process. Fig. 6 Fig. 7 shows the chronoamperometric curve of the PtNPs/TiO2NSs/GCE on successive injection of hydrazine at 0.3 V in a N2-saturated 0.1 M PBS (pH 7.0). The sensor exhibits wide linear ranges of 20-900 and 900-2100 M with the sensitivity of 187.4 and 100.0 A mM-1cm-2, respectively. The linear range of the as-proposed sensor is wider than those of the TiO2-Pt nanofibers/GCE [20] and ordered mesoporous carbons-polydopamine/Pt [30], and the sensitivity is higher than that of ZnO nanorods electrode (44.24 A mM-1 cm-2) [31]. The limit of detection is found to be 2.0 M with a signal-to-noise (S/N) ratio of 3. Fig. 7 The effect of substances that might interfere with the response of the modified electrode was studied. The current obtained for each interfering substance in the presence of 40 M N2H4 was compared with that of 40 M N2H4, and this ratio was used as the criterion for the selectivity of the sensor. There are no observable interference in the presence of the same concentration of K+, Na+, Zn2+, Ni2+, Co2+, Fe3+, glucose, and 3-fold NO3-, 3-fold Cl−, 4-fold CH3COO−. However, equivalent concentration of Cu2+ and ascorbic acid interfere obviously (40.2% and 30.3% increase, respectively). The reproducibility and repeatability were studied as well. The electrode-to-electrode reproducibility was investigated from the response to 0.20 mM N2H4 on 4 different electrodes, yielding a relative standard deviation (RSD) of 5.24%. The repeatability of one sensor was estimated with a RSD of 3.66% for 4 successive tests. The results implied that the PtNPs/TiO2NSs sensor had good great reproducibility and repeatability. In addition, to verify the practical use of the
PtNPs/TiO2NSs sensor, the recovery of hydrazine in the tap water was examined. The recoveries of 100.1-105.3% with RSD of 3.28% (n = 4) were obtained, indicating the PtNPs/TiO2NSs/GCE should be a promising sensor for the effective determination of hydrazine.
4. Conclusion In conclusion, the novel PtNPs/TiO2NSs nanomaterials were prepared by the photoreduction method successfully. The PtNPs/TiO2NSs sensor exhibits great performances for electrochemical oxidation and determination towards hydrazine at low potential. The results indicated that the support ultra-thin anatase TiO2NSs, the dispersed and small-sized PtNPs, and the SMSI between PtNPs and TiO2NSs were significant for the sensor, which had a wide linear range and high sensitivity for hydrazine. In addition, the PtNPs/TiO2NSs sensor showed good anti-interference ability,
reproducibility,
and
repeatability.
The
excellent
performances
of
electrocatalytic oxidation and determination of hydrazine make the PtNPs/TiO2NSs promising for the future application in electrochemical sensors and biosensors. Moreover, the materials also may broaden the application of TiO2 nanomaterials in electroanalysis.
Acknowledgments We acknowledge the Ministry of Science and Technology of China (No. 2013YQ170585).
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Captions Fig. 1 TEM images of (a) TiO2NSs and (b) PtNPs/TiO2NSs. Fig. 2 XRD patterns of TiO2NSs and PtNPs/TiO2NSs. Fig. 3 The XPS spectra of (a) TiO2NSs and PtNPs/TiO2NSs, and (b) Pt 4f region of PtNPs/TiO2NSs. Fig. 4 The EIS for (a) TiO2NSs/GCE and (b) PtNPs/TiO2NSs/GCE in 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 M KCl solution. Inset: the equivalent circuit. Fig. 5 The CV curves of (a) TiO2NSs/GCE and (b) PtNPs/TiO2NSs/GCE in the absence and presence of hydrazine (2.0 mM) in 0.1 M PBS (pH 7.0) at a scan rate of 50 mV s−1. Fig. 6 (a) CV curves of hydrazine oxidation at PtNPs/TiO2NSs/GCE in 0.1 M 25 mL PBS (pH 7.0) containing 2.0 mM hydrazine at different scan rates (5, 10, 20, 40, 60, 80, 100, 120, 140, and 160 mV s−1). Inset: (b) the relationship between the oxidation peak current (Ipa) and the square root of the scan rate, (c) the relationship between the oxidation peak potential (Epa) and log (v). Fig. 7 Chronoamperometric curves of the PtNPs/TiO2NTs/GCE on successive injection of hydrazine at 0.3 V in a N2-saturated 0.1 M PBS (pH 7.0). Inset: the
calibration curve of N2H4.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Graphical abstract
Highlights: 1. The dispersed and small-sized PtNPs were deposited onto TiO2NSs by photoreduction. 2. The PtNPs/TiO2NSs sensor shows excellent electrocatalytic oxidation of hydrazine at low potential. 3. The PtNPs/TiO2NSs sensor exhibits a wide linear range and high sensitivity for hydrazine.