Characterization of Li-doped WO₃ nanowires and their enhanced electrocatalytic oxidation of ascorbic acid

Abstract

Li-doped WO₃ nanowires have been hydrothermally prepared and characterized mainly via spectroscopic methods. Both the hexagonal structure distortion and morphology evolution induced by Li doping reveal a lattice expansion of about 0.07 Å. Also, the residence of oxygen vacancy and the enlargement of external surface areas positively correlate with the narrowing of energy band. Subsequently, the electrocatalytic oxidation of ascorbic acid using a Li-doped WO₃ film-coated electrode performs a 13% and 21% negative shift of the oxidation overpotential compared with a WO₃ film-coated electrode and a bare glassy carbon electrode, respectively. A preliminary mechanism has been proposed on the basis of relevant model analyses.

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1. Introduction

As an indispensable ingesta that participates in many human physiological biochemical reactions, the detection of ascorbic acid (AA) in human body fluid is of significance in assessing the status of health. An electrochemical sensor adopting a variety of chemically-modified electrodes renders a progressive technology for overcoming the disadvantages of high overpotential, poor reproducibility, low selectivity and sensitivity on conventional electrodes.^1–4 Thus, much interest has been focused on the use of mediators and modified electrodes to catalyze the oxidation of AA.^5,6

It has been well recognized that oxide-based semiconductors display excellent electrocatalytic activity due to their unique physicochemical properties.^7–14 However, the concentration of free electrons in metal oxide semiconductors is low, i.e., ∼10²¹ electrons per m⁻³.¹⁴ A valid approach to enhancing the electrocatalytic performance is metal doping that achieves a significant raise in free electron concentration exemplified by Nb-doped TiO₂/carbon composite and Li-doped tantalum oxide,^15–17 wherein the electron conductivity and electrocatalytic activity of both improve.

Tungsten oxide (WO₃) is a versatile wide band gap metal oxide that finds extensive application in electrochromic, photochromic and photocatalytic devices.^18–25 Besides, WO₃ can be utilized as an electrode modifier due to its high chemical stability and considerable semiconductivity. Thus, the electrocatalytic property of WO₃ in the morphology of nanorods, nanowires and its aggregates has been highlighted.²⁶ Compared with pristine WO₃, metal-doping can further improve the electrochemical properties of WO₃. As exemplified, Bathe et al. showed that the cyclic stability, charge storage capacity, and reversibility could be improved by addition of Nb₂O₅ to WO₃ films.²⁷ Also, Ti-doped WO₃ film has improved electrical conductivity and reaction kinetics.²⁸ However, the effect of doping on the electroactivity of WO₃ is rarely studied. We propose that development of a highly efficient Li-doped WO₃ electrocatalyst and its application in the electrochemical detection of ascorbic acid in biological systems will be of interest to a broad range of material scientists, as Li ions doping can affect the oxygen vacancies, energy band gap and surface area of WO₃.

In our previous work, we successfully prepared Ta-doped tungsten oxide nanowires and studied their electrocatalytic activity for hydrogen evolution reaction.²⁹ In this paper, Li-doped WO₃ film has been fabricated and homogenously deposited on a glassy carbon electrode. A test of its electrocatalytical oxidation of AA has been carried out to investigate the effect of experimental parameters of cyclic voltammetry in order to explore the mechanism.

2. Experimental

2.1. Chemical reagents and synthesis of Li-doped WO₃ nanowires

Sodium tungstate dihydrate (NaWO₄·2H₂O, analytical grade) was purchased from Kelong Company, lithium nitrate (LiNO₃, 99.9%, analytical grade) was purchased from Sigma-Aldrich, and ammonium sulfate ((NH₄)₂SO₄) was received from Tianjin Chemical Reagents Corporation (China). All other chemicals were purchased from the Chengdou Chemical Reagents Corporation (China).

The synthetic method of Li-doped WO₃ is as follows: 2 g of Na₂WO₄·2H₂O and LiNO₃ (at varying Li/W mole ratios of 0, 0.01, 0.03, 0.05, 0.07, and 0.1) was dissolved in 45 mL of deionized water under stirring at room temperature, then 5 mL of 3 mol L⁻¹ HCl solution was added to the above solution under continuous stirring until tungstenic acid was heavily precipitated, and finally 30 mL of 0.5 mol L⁻¹ (NH₄)₂SO₄ solution was added to this solution, which was then transferred to a Teflon-lined autoclave with a capacity of 100 mL. Hydrothermal treatments were carried out at 200 °C for 24 h. After that, the autoclave was allowed to cool naturally. The final products were collected, washed with deionized water and ethanol several times, and dried in air at 80 °C. The Li-doped WO₃ nanowires were finally obtained.

2.2. Characterization of Li-doped WO₃ nanowires

Powder X-ray diffraction (PXRD) analysis was conducted to characterize the crystalline phase of the Li-doped WO₃ samples on a X'Pert PRO PANalytical diffractometer (Almelo, Netherlands) with Cu Kα radiation (λ 0.15406 nm), operating at 40 kV and 45 mA. The size and shape of the samples were observed on a field-emission scanning electron microscope (FESEM, Philips XL30 FEG, Eindhoven, Netherlands) and transmission electron microscope (TEM, JEM200CX, 120 kV). X-ray photoelectron spectroscopy (XPS) was performed on a RBD upgraded PHI-5000C ESCA system (PerkinElmer) using an Mg-monochromatic X-ray at a power of 25 W and an X-ray-beam diameter of 10 mm, and a pass energy of 29.35 eV. The binding energy was calibrated using the C1s hydrocarbon peak at 284.8 eV. UV-vis diffuse reflectance spectroscopy was carried out on an UV-vis spectrophotometer (Lambda 850, PerkinElmer, USA), equipped with an integrating sphere and a BaSO₄ reference. Nitrogen adsorption and desorption isotherms were measured at 77 K with a Beckman Coulter SA 3100 surface area analyzer. To determine the surface area, the Brunauer–Emmett–Teller (BET) method was used. The doped amount of lithium in WO₃ was measured with inductively coupled plasma (VISTA-MPX CCD Simultaneous ICP-OES, Varian, USA).

2.3. Electrochemical measurements

The Li-doped WO₃ nanowires were further characterized using cyclic voltammetry (CV) in 0.1 mol L⁻¹ KCl containing 0.4 mmol L⁻¹ AA as the electrolyte solution. Prior to the electrochemical measurements, high-purity Ar₂ was bubbled in the above solution for 20 min. During the experiments, high-purity Ar₂ was continually bubbled at the surface of the electrolyte solution. The fabrication of the Li-doped WO₃ film- and pure WO₃ film-coated electrodes is as follows: 20 mg of Li-doped WO₃ or WO₃ powder was dispersed ultrasonically in a mixture of 1.0 mL of ethanol and 8 μL of nafion solution to obtain a suspension. The suspension was spread on the glassy carbon electrode (GCE), and dried for 10 min at 25 °C. This film-coated GCE was used as the working electrode. A Pt coil and an Ag/AgCl electrode (3 mol L⁻¹ KCl) were used as the counter and reference electrodes, respectively. The CV measurements were carried out on a CHI 660C electrochemical workstation (Shanghai Chenhua Instrument Factory, China) at room temperature (22 ± 2 °C).

3. Results and discussion

3.1. Structure and morphology of the pristine and Li-doped WO₃

The XRD patterns of pristine and Li-doped WO₃ presented in Fig. 1 demonstrate a hexagonal phase of good crystallinity with lattice constants a = 7.299 Å and c = 3.899 Å (JCPDS Card no. 33-1387). No lithium-bearing impurities were found and the diffraction peak of (001) facet shifts to lower degree as the lithium doping amount increases. The observations indicate that the lithium ion has been incorporated into the WO₃ lattice and therefore induced local distortions as a result of a slight lattice expansion.

Fig. 1 XRD patterns of the Li-doped WO₃ nanowires at varying Li/W molar ratios of (a)–(f) 0, 0.01, 0.03, 0.05, 0.07, and 0.10, respectively.

Both pristine and Li-doped WO₃ illustrated in Fig. 2 unfold coral-like tangled nanowires forming accessible hierarchical pores. The diameter of those nanowires is in the range of 20 to 30 nm while the length of Li-doped nanowires is roughly longer than that of the pristine nanowires. The high-resolution transmission electron microscopy presented in Fig. 2c and f indicated a marked fringe with the spacing of 0.375, 0.382 nm for the pristine and doped, respectively. Since the labeled fringe corresponds to the (001) facet, the lattice spacing expands 0.07 Å according to the XRD analysis results. Both crystal structure and morphology representations imply that a lattice defect induced by the substitution of W⁵⁺/W⁶⁺ with Li⁺ proceeds to cause local lattice distortion and possibly incurs more exposed oxygen vacancies.

Fig. 2 (a) SEM, (b) TEM and (c) HRTEM images of the pristine WO₃ nanowires. (d) SEM, (e) TEM and (f) HRTEM images of Li-doped WO₃ nanowires at a Li/W molar ratio of 0.05.

3.2. Oxygen vacancies, energy band gap and surface area of the pristine and Li-doped WO₃

A survey of the oxide-based electrocatalyst revealed that the oxygen vacancies, energy band level, and surface textures of the electrode materials regulate their electrocatalytic performance.³⁰ In this part, the overall XPS spectra of Li-doped WO₃ presented in Fig. 3a demonstrates the residence of W, O, and Li elements, the contents of which identify with the composition of the hydrothermal mixture. The O1s XPS spectra of pristine and Li-doped WO₃ illustrated by Fig. 3b and c feature an asymmetrical peak. It has been deconvoluted into two separate peaks using Gaussian distributions,³¹ which are lattice O²⁻ at 530.30 eV and surface OH⁻ at 531.10 eV. The intensity of the hydroxyl group is significantly higher for the Li-doped WO₃ compared with the pristine WO₃. Although the OH⁻ resides as a minor oxygen vacancy, it is beneficial for inhibiting the electron-hole recombination process, thereby improving the electron transmission efficiency,^32,33 further enhancing the electrocatalytic activities of WO₃. Moreover, the real ratios of Li/W in Li-doped WO₃ samples were measured by ICP-OES (see ESI, Table 1†).

Fig. 3 XPS spectra of (a) Li-doped WO₃ (Li/W = 0.05). XPS O1s spectra of (b) undoped and (c) doped WO₃.

The energy band level has been assessed via conventional UV-vis spectroscopy. As illustrated in Fig. 4, the maximum reflectance shifts to higher wavelength after Li doping. The band gap energy (E_g) of the semiconductor can be determined accordingly:

αhv = A(hv − E_g)ⁿ (1)

where α is the absorption coefficient, hν is the incident photon energy, A is a constant, and n is either 2 or 1/2 for direct and indirect transitions, respectively.^34,35 It is known that WO₃ crystal is an indirect-gap semiconductor, thus a n value of 2 is assumed here. The band gaps determined by the linear extrapolation presented in the inset are approximately 3.05 and 2.88 eV for the pristine and Li-doped WO₃ (Li/W mole ratio of 0.05), respectively. The narrowing of Li-doped WO₃ energy band gap could be interpreted in the two mentioned aspects: (1) the local lattice distortion possibly changes the electronic structure and carrier density of WO₃, which results in the improvement of its electrocatalytic activity;^14,17 (2) the oxygen vacancies incur a defect band with a low energy.²⁹

Fig. 4 UV-vis reflectance spectra of undoped and Li-doped WO₃ (Li/W = 0.05). The inset is a plot of (αhν)^1/2 as a function of hν for the undoped and doped WO₃ samples.

The active surface area of the nanoparticles that is also crucial to determining the electrocatalytic property was evaluated. The nitrogen adsorption–desorption isotherms of the pristine and Li-doped WO₃ nanowires are shown in Fig. 5. The BET surface areas of the pristine and Li-doped WO₃ nanowires were determined as 19.42 and 74.92 m² g⁻¹, respectively. As observed, WO₃ featured an enhanced surface area following doping with Li ions, indicative of the presence of a larger number of active sites that would be beneficial towards improving the electrocatalytic activity of the doped sample.

Fig. 5 Nitrogen sorption isotherm curves of undoped WO₃ and Li-doped WO₃ (Li/W = 0.05).

3.3. Electrocatalytic activity of the Li-doped WO₃ film-coated electrode

In this section, the electrocatalytic experimental parameters including the type of working electrode, the concentration of AA and the scan rate have been adopted to evaluate the electrocatalytic activity of the Li-doped WO₃ film-coated electrode. The cyclic voltammogram illustrated by Fig. 6 left displays an anodic peak (E_p) at 0.57 V (Curve a), 0.52 V (Curve c), 0.45 V (Curve b) for the bare GCE, WO₃ film-coated electrode and Li-doped WO₃ film-coated electrode, respectively. Thus, there is a negative shift of 0.12 V of E_p on the Li-doped WO₃ film-coated electrode. The negative shift of the anodic peak position indicates that the Li-doped WO₃ film-coated electrode has excellent electrocatalytic activity toward AA oxidation. Meanwhile, the Li doping amount has been optimized as a Li/W molar ratio of 0.05 since its anodic current is the maximum among those different doping ratios (see Fig. 6 right).

Fig. 6 (Left) Cyclic voltammograms of 4 mmol L⁻¹ AA adopting (a) bare GCE, (b) Li-doped WO₃, and (c) WO₃ electrode in 0.1 mol L⁻¹ KCl solution. (Right) Variation of anodic current as a function of the Li/W molar ratios.

The Li-doped WO₃ film-coated electrode with the optimum doping amount is selected and the effect of AA concentration and scan rate on the anodic current have been assessed in Fig. 7. Fig. 7a shows the cyclic voltammograms of AA at varying concentrations in 0.1 mol L⁻¹ KCl solution using the Li-doped WO₃ film-coated electrode. As observed, the oxidation peak current increased with increasing AA concentrations. The inset of Fig. 7a shows a linear relationship between the anodic peak current and AA concentration in the range of 2–8 mmol L⁻¹, with a correlation coefficient of 0.998. At a fixed potential of 0.5 V the catalytic current observed was also linear dependent on the AA concentration in the range 0.01–4 mmol L⁻¹ and the detection limit was 0.04 mmol L⁻¹. Furthermore, the as-prepared Li-doped WO₃ modified electrode is compared with other modified electrodes on aspects such as linear range and detection limit, which are shown in Table 1; this information demonstrates that the sensor behaves well with a wide linear range and low detection limit towards the oxidation of AA.

Fig. 7 The concentrations of AA studied are (a)–(d) 2, 4, 6, and 8 mmol L⁻¹, respectively. The different potential scan rates of (a)–(f) 10, 30, 50, 70, 100 and 150 mV s⁻¹, respectively.

Table 1 Comparison of electrochemical parameters of the Li-doped WO₃ modified GCE with those of other modified electrodes reported in the literature

Modified electrode	Linear range (μmol L^−1)	Detection limit (μmol L^−1)	Reference
Li-doped Ta2O5/GCE	5000–12 000	—	17
Li doped Bi2O3/CNT/GCE	20–5000	50	36
Fe3O4@Au-s-Fc/GS-chitosan/GCE	6–350	5	37
MgO nanobelts/GCE	25–150	0.2	38
Nano-Cu/PSA III/GCE	0.3–730	0.15	39
Nano-CuO/GCE	0.1–3100	0.095	40
Nano-NiO/GCE	50–3500	860	41
Li-doped WO3/GCE	2000–8000	40	This work

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The cyclic voltammograms of the Li-doped film-coated electrode at various scan rates in the presence of 4 mmol L⁻¹ AA in 0.1 mol L⁻¹ KCl (Fig. 7b) reveal that the catalytic effect of the Li-doped film-coated electrode appears at the higher scan rates because of the considerably higher catalytic reaction rate. Moreover, the catalytic oxidation peak potential shifts to more positive values with increasing scan rate, indicative of a kinetics limitation in the reaction between the redox sites of Li-doped WO₃ film and AA. This result also proves that the electrochemical oxide reaction of AA is an irreversible process. The inset of Fig. 7b shows the linear correlation between the anodic peak currents of AA and the square-root of scan rates, indicative of a diffusion controlled electrode process.

In order to explore the diffusion mechanism, two models have been adopted to determine the rate-limiting step of the process, which are the scan rate-normalized current (I/v^1/2) as a function of scan rate (v) and E_p as a function of logarithm of the scan rate (log v). The fitting results are illustrated in Fig. 8. The shape of the curve in Fig. 8a is typical of an electrochemical–chemical (EC) process.⁴² To obtain information on the rate determining step, the following function involving E_p and logarithm of the scan rates (log v) for an irreversible diffusion-controlled process was assessed:⁴³

where K is a constant, α is the transfer coefficient, n_α is the number of electrons transferred, v is the scan rate, and b is the Tafel slope. Fig. 8b shows the variation of E_p with log v. The slope of the E_p–log v plot was determined as 0.067 and a Tafel slope of 0.134 was obtained. The value of 2.303RT/αn_αF is equivalent to the Tafel slope. The value of the transfer coefficient was then determined to be 0.429. Another method that is used for calculating the transfer coefficient (α) was also employed, using the following equation for an irreversible system:^17,44where n_a is the number of electrons transferred during the electrochemical process. The transfer coefficients obtained from the above fitting are in good agreement and indicate that the electrocatalytic oxidation of AA is an electrochemical–chemical (EC) process, simultaneously controlled by diffusion of solution AA and cross-exchange through the Li-doped WO₃ film.

Fig. 8 (a) Plot of the anodic peak current (I/ν^1/2) as a function of scan rate (ν). (b) Dependence of the peak potential E_p on log ν for the oxidation of AA on Li-doped WO₃ film-coated electrode.

4. Conclusions

Creating an electron deficit by substituting some of the cations of the host oxide with cations having a lower valence is a versatile approach to tailor catalysts. Herein, the insertion of Li⁺ into the WO₃ causes a lattice distortion, surface area enlargement and oxygen vacancy formation. The integrated effect of those structure determinants facilitates the oxidation of AA on the electrocatalyst surface, enhancing its electrocatalytic performance. The anodic peak potential of AA shifted from 0.57 V (versus Ag|AgCl) on a bare GCE to 0.45 V on the lithium–WO₃ film coated electrode. Despite the preliminary results demonstrating that the Li-doped WO₃ film-coated electrode is a promising electrochemical sensor for the detection of AA, the hydrothermal synthesis factors dominating the crystal structure and morphology, the advanced characterizations combined with quantum computation, and electrocatalytic experiments carried out in complex simulant bearing AA are in need of in-depth investigation for the development of a highly selective, sensitive and effective electrochemical detector.

Acknowledgements

This work was financially supported by the Institute of Nuclear Chemistry and Physics, China Academy of Engineering Physics.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07575g

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