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Pd Doped WO Nanostructures as Potential Glucose Sensor with Insight from Electronic Structure Simulations Rajeswari Ponnusamy, Brahmananda Chakraborty, and Chandra Sekhar Rout J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b11642 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 19, 2018

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The Journal of Physical Chemistry

Pd doped WO3 Nanostructures as Potential Glucose Sensor with Insight from Electronic Structure Simulations Rajeswari Ponnusamy, [a], Brahmananda Chakraborty,* [b] Chandra Sekhar Rout *[a], [c] a

School of Basic Sciences, Indian Institute of Technology, Bhubaneswar, Odisha-751013, India b High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India c Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Ramanagaram, Bangalore-562112, India Email: [email protected] Email: [email protected], [email protected]

ABSTRACT: Herein, we report the results of crystal structure dependent non-enzymatic glucose sensing properties of tungsten oxide (WO3) and Pd doped WO3 nanostructures. WO3 nanomaterials with orthorhombic, monoclinic and mixed (ortho+monoclinic) phases were harvested by a facile hydrothermal route by varying the reaction time and subsequent annealing processes. Electrocatalytic activity tests of WO3 samples revealed three-fold oxidation peak current enhancement in monoclinic Pd-doped WO3 nanobricks assembly as compared to the orthorhombic WO3 microspheres. Moreover, Pd-doped WO3 showed higher glucose sensing performance in terms of detection sensitivities of 11.4 µA µM-1 cm-2 (linear range: 5-55 µM) and 5.6 µA µM-1 cm-2 (linear range: 65-375 µM). We have also performed Density Functional Theory (DFT) simulations for the monoclinic WO3 and Pd-doped WO3 to investigate the charge transfer and bonding mechanism of glucose on WO3 and Pd-doped WO3 surface. As the binding energy of glucose is higher in the case of Pd-doped WO3 as compared to bare WO3, it becomes more conducting due to enhancement of density of states 1

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near Fermi level, theoretically, we can predict that Pd-doped WO3 exhibit a better charge transfer media compared to bare WO3 resulting enhanced glucose sensing performance which qualitatively support our experimental data. Hence, our experimental data and theoretical insight from electronic structure simulations conclude that Pd doped monoclinic WO3 is a potential material for the fabrication of real time glucose sensors.

1. Introduction Inorganic semiconductors, especially nanostructured II-VI group metal oxides have gained greater interest among the scientific community as well as in technical personnel due to their pleasing characteristics. In particular, oxide semiconductors are considered as promising materials in the fields like optoelectronics, electrochemistry, catalysis, sensing and biosciences etc.1, 2 In this direction, tailoring of dimension, morphology and crystal structure are found to be the critical parameters that determine the performance of the nanomaterials. Thus, a vast variety of nanostructures starting from zero-dimensional quantum dots to three dimensional hierarchical structures have been synthesized via a number of physical and chemical techniques.3 Hydrothermal process is the most preferable method amongst all due to its easy accessible experimental parameters such as reaction temperature, reaction duration and pressure of reaction environment.4 Crystallographic phase transformation also plays an important role in tuning the properties of nanomaterials for their applications in catalysis, self cleaning devices, corrosion protecting devices and so on.5 Currently, doping of noble metals (Au, Pt and Pd) and their decoration on the surface of various metal oxides are being investigated in electrochemical studies and demonstrated as proficient materials for the detection of biomolecular species. Pd has been frequently considered as an efficient material in electrocatalysis due to its excellent conductivity and is 2


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also affordable. Yi et al. decorated Pd nanoparticles by electrodeposition onto the nanoporous gold wires for sensitive detection of dopamine.6 Hassan et al. reported the amperometric sensing behaviour of SnO2 doped with Pd, In and Ag for detection of MgSO4.7H2O salt and observed significant enhancement in sensitivity for the Pd:SnO2.7 Pd nanoparticles also deposited electrochemically on electrochemically reduced chemically modified graphene oxide by Hossain et al. to use in bimolecular sensing (H2O2 and glucose).8 Beyond this, Mohit et al. explored the high selective detection of extremely low concentration hydrogen by Pd-ZnO nanorods.9 Further, to understand the enhanced sensing performance upon Pd doping and the charge transfer mechanism between glucose and Pd, DFT simulations have been performed by KK Naik et al.10 Jena et al. demonstrated that Au nanoparticles selfassembled on a silicate network can efficiently oxidize the glucose in the absence of any enzymes and redox mediators.11 Among the n-type semiconductors, WO3 possess broad range of applications in flat panel displays, smart windows, gas sensors, temperature sensors etc., owing to its outstanding physiochemical properties like electrochromism, optochromism and gasochromism.12, 13 With the high stability in acidic medium and tunable energy bandgaps (< 3.0 eV), WO3 nanomaterials have been used as the visible light photocatalyst and photoeletrodes.14,

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Recently, WO3 thin film based planar perovskite solar cell with the highest conversion efficiency of 7.18 % has been reported by Zhang et al.16 Also, conversion reaction in WO3 during Li+, Na+, and Ca2+ ions insertion has been systematically studied by He et al. for its application in secondary-ion batteries.17 PEG assisted γ-WO3 nanoparticles synthesized by microwave irradiation showed enhanced electrocatalytic detection activity towards L-Dopa.18 Also, largely enhanced photocurrent response is reported by Danyang et al. for the γ-WO3 decorated with ultrathin graphite like carbon nitride (C3N4) modified photoelectrochmeical 3



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glucose sensor.19 Ma et al. proposed that three dimensional graphene network @WO3 can detect H2O2, ascorbic acid and dopamine as a colorimetric and electrochemical sensors.20 However, electrochemical glucose sensing behaviour of noble metal doped WO3 and their comparative study with different polymorphs has not been reported yet. In this work, WO3 nanostructures with different polymorphs are synthesized hydrothermally by varying the synthesis conditions (reaction time) and successive thermal treatment (at 500º C). Then, the change in crystal structure is focused for the investigation as it is influencing the sensitivity of WO3 during enzyme-free glucose sensing. Further, electrochemical properties of Pd-doped WO3 have been systemically studied by cyclic voltammetry and chronoamperometry measurements and the results are discussed in detail. We have also presented theoretical data from Density Functional theory (DFT) simulations for monoclinic WO3 and Pd-doped WO3 which provide insight from orbital interactions and charge transfer between p orbital of bonded O of glucose and metal d orbitals.

2. Experimental section 2.1 Synthesis and characterization Sodium tungstate dehydrate (Na2WO4. 2H2O) and palladium (II) chloride (PdCl2) were purchased from Sigma-Aldrich and used as received. WO3 of different phases and Pd doped WO3 were prepared via a facile hydrothermal route. First, 40 ml of tungstate solution was prepared in triplicate by dissolving sodium tungstate in deionized (DI) water. 1M H2SO4 solution was dropped in to the two tungstate solutions to adjust the pH to 2 and then sonicated for 15 minutes. The solutions were then transferred to 50 ml teflon reactors and kept at 473 K for 24 and 48 hours respectively. Pd doping was achieved from the third solution by adding 5 wt% of PdCl2 followed by the addition of H2SO4 solution and 4


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sonication. The resultant solution taken in a teflon-lined stainless steel autoclave was kept in an oven and the temperature was maintained at 473 K for 48 hrs. After the hydrothermal reaction, all the reactors were cooled down to room temperature naturally and the precipitates were washed with DI water and ethanol by filtration and dried in air. The pure and Pd-doped WO3 samples collected after the hydrothermal reaction for 48 hrs were annealed at 773 K for 1 hr. The as prepared samples with reaction time of 24 hrs and 48 hrs were named as WO3-24 and WO3-48 BA respectively. The annealed samples were named as WO3- 48 AA and WO3Pd AA for 48 hrs reacted WO3 and WO3-Pd respectively. The crystal structure and phase of the prepared WO3 materials were confirmed by Xray diffraction (XRD, Bruker D8 Advanced diffractometer with Cu Kα X-ray source). The morphology changes and the chemical components were analyzed by Field Emission, Scanning Electron Microscope (FESEM, MERLIN compact with a GEMINI I electron column, Zeiss Pvt. Ltd.) and Energy Dispersive X-ray Spectroscopy (EDAX). Electrochemical measurements were carried out using a PG262A potentiostat/galvanostat (Technoscience Ltd., Bengaluru) at room temperature. A three electrode electrochemical cell containing Ag/AgCl as reference electrode and Pt wire as auxiliary electrode was employed for cyclic voltammetric (CV) studies. To prepare the working electrodes, 2 mg of prepared WO3 powder dispersed (ultra-sonication for 20 min) in ethanol was drop-casted on precleaned Ni foam and crimped. 0.1 M NaOH solution was used as supporting electrolyte for all the measurements performed in the present study.

3. Results and Discussion 3. 1 Structural characterization

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XRD patterns of the as-prepared and annealed WO3 samples are presented in Figure 1A and B. The sample obtained after 24 hrs of reaction exhibits two sets of diffraction peaks corresponding to orthorhombic WO3.0.33 H2O (JCPDS No. 35-0270) and monoclinic WO3 phases (JCPDS No. 43-1035). By increasing the reaction time to 48 hrs, the characteristic diffraction peaks of orthorhombic phase are found alone which implies the complete conversion of mixed phase into single α-WO3. In literature, similar kind of phase transformation has been observed during the hydrothermal synthesis of WO3 which depends on pH of the initial precursor solution and heat treatment.4, 21 However, the changes in phase observed in the present study could be related to the extension of reaction duration. In this case, heat treatment of the α-WO3 leads to the stable monoclinic WO3. Similarly, in the case of Pd-doped WO3 sample, all the diffraction peaks can be indexed to the monoclinic WO3. The XRD pattern of as prepared Pd-doped WO3 (Figure S1) revealed the domination of orthorhombic crystal phase over the monoclinic phase. Morphological investigations of the samples are performed by the FESEM analysis and are presented in Figure 2. Low and high magnification micrographs of the as-prepared WO3 (48 hrs) as shown in Figure 2A and B confirm the successful formation of urchin-like microspheres with the diameter in the 2-3 µm range. It is also noted that, a single microsphere structure is assembled by several irregular shaped nanorods having diameter of ~ 50 nm and length of ~2 µm. Else, WO3- Pd AA structure consists of irregular nanobricks as building blocks which are of the size ~20-30 nm. Careful examination on each brick reveals the mesoporous nature of the surface. Possible chemical reactions involved in the growth of WO3 crystals and the formation of WO3 nanostructures are discussed below: Na2WO4 .2 H 2O + 2 HCl → H 2WO4 + 2 NaCl + 2 H 2O

(1)

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H 2WO4 → WO3 + H 2O

(2)

At first, sodium tungstate dissociate into tungsten ions, sodium ions and OH- ions in water. At this stage, addition of HCl helps in hydrolysis of W ions to form unstable tungstic acid (H2WO4) and also the formation of sodium chloride by-product. During the hydrothermal reaction at 200° C, hydrolysis of H2WO4 occurs and the WO3 nuclei begin to grow as presented in eq. 2. Simultaneously, nanorod-like structures yielded from the WO3 nuclei. With the continuation of the reaction, individual rod-like structures grow longer and then self-assembled to form microsphere-like morphology. All these typical steps of the nanobricks assembled microsphere formation are represented schematically in Scheme 1. Elemental composition of WO3-Pd AA was investigated by EDAX analysis and is shown in Figure 3A which confirms the presence of W, O and Pd. Elemental mapping of the elements O, W and Pd were carried out on a single nanobrick assembled microsphere of WO3-Pd AA and it is shown in the inset of Figure 3A. Uniform distribution of Pd throughout the WO3 sample is confirmed from the elemental mapping and is shown in Figures 3B-D.

3. 2 Phase dependent electrocatalytic glucose sensing of WO3 To assess the effect of structure, phase and doping of WO3 on the biosensing performance, electrochemical tests were performed. Corresponding cyclic voltammetric profiles in the applied potential range of 0 to +0.65 V in 0.1 M NaOH solution are provided in Figure 4A. In the NaOH solution without glucose molecule, a pair of well-defined anodic and cathodic peaks were observed for all the prepared materials. The oxidation peak currents of samples (WO3-48 AA and WO3-Pd AA) with pure monoclinic crystal structure are 1.2 and 1.42 mA at the oxidation peak potential of +0.45 V. On the other hand, 2.5 fold decrease in the anodic peak current (0.61 mA) was observed for the pure orthorhombic phase WO3. However, sample containing mixed ortho- and monoclinic phases exhibit 1.5 fold increase in 7



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peak current than the single orthorhombic phase. In addition, there is a clear anodic peak potential shift from +0.45 V to +0.61 V for the monoclinic to orthorhombic phase change (Figure S2). These results clearly demonstrate the possibility of enhanced electron transfer kinetics of the WO3-Pd AA monoclinic phase towards the glucose oxidation. Current response profiles in presence of different concentrations of glucose (100-1000 µM) were recorded for WO3-48 AA and WO3-Pd AA and are provided in Figure 4B and C. Figure 4D shows comparative current response of WO3-48 AA and WO3-Pd AA against different concentration of glucose which also confirms the significant increase in electrocatalytic activity of the monoclinic WO3 upon Pd doping. CV curves at different glucose concentrations were also recorded for the WO3 samples prepared by 48 hrs of hydrothermal reaction and are given in Figure S3. It is clearly revealed that the orthorhombic phase shows very less electron transfer kinetics towards glucose oxidation as is evident from the obtained data with the current increment of less than 1 mA even for 1000 µM glucose addition. Whereas, monoclinic phase of the samples (WO3-48 AA and WO3-Pd AA) showed more than 3.5 mA enhancement in the response current which confirmed the enhanced glucose sensing properties of monoclinic WO3. Further, the CV response curves of WO3-Pd AA are recorded for different scan rates varying from 5 to 100 mVs-1 in 0.1 M NaOH solution and presented in Figure 4E. Derived calibration plots of anodic and cathodic peak current versus square root of scan rates (Figure 4F) shows the linear dependent with the square root of scan rate and reveals the diffusion-controlled electrochemical process over the surface of the WO3-Pd AA material. The possible electrocatalytic mechanism for the redox reaction on the WO3 surface towards glucose oxidation can be expressed as follows: 20 WO3 + OH − + glu cos e(C6 H12O6 ) → H xWO3 + gluconolactone(C6 H10O6 ) + H 2O + e − (3)

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Since the evaluation of analytical parameters like sensitivity, linear range and response time are crucial for the development of real time glucose sensors, amperometric response of WO3-24, WO3-48 BA, WO3-48 AA and WO3-Pd AA modified electrodes are carried out by successive addition of different concentrations of glucose in the solution. Current versus time (I-t) measurements are carried out in 0.1 M NaOH solution with the applied potential of +0.45 V under continuous stirring conditions. In Figure 5A, the change in response shows step-like behaviour for different glucose concentrations. Figure 5B shows the derived calibration curves of the peak current for successive addition of glucose. As seen from both the response curve and calibration plot, it is clear that WO3-Pd AA possesses enhanced current response as compared to other WO3 materials. Calibration curves of all the samples exhibit two linear ranges of 5-55 µM and 65 µM - 0.375 mM with a fast steady state response time of 4s. However, the highest sensitivities of 11400 µA mM-1 cm-2 and 5060 µA mM-1 cm-2 are achieved for the WO3-Pd AA with a lower detection limit of 4.2 µM. The excellent electrocatalytic performance of the WO3-Pd nanostructure aroused from the following synergistic effects : i) availability of more surface active sites resulting from the mesoporous brick like structure for the effective adsorption of more glucose molecules, ii) enhanced electrical conductivity upon Pd doping that supports the improved electron-transfer kinetics. Absence of obvious redox peaks from the Pd in Figure 4 reveals the combined rather than individual role of Pd in the electrochemical reactions of WO3, iii) monoclinic structural feature also facilitates easy diffusion pathways for the glucose molecules in the electrolyte to accelerate the rapid electron transfer. Mechanism of electrocatalytic oxidation of glucose into glucanolactone by the WO3-Pd AA nanostructure is schematically represented in Scheme 2. The superior electrocatalytic sensing performance observed for the Pd-doped WO3 is comparable with the reported metal oxide microsphere-based non-enzymatic glucose sensors such as Co3O4 microspheres with nanofibers, 3D NiO hollow sphere/reduced graphene oxide 9



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composite, hierarchically assembled ZnO nanosheets microspheres, Pt decorated Carbon hollow spheres (Table 1) and other metal oxides. 22-30 To analyze the influence of Pd doping concentration on the glucose oxidation behaviour of WO3, two different (2 (WO3-Pd2 AA) and 10 (WO3-Pd10 AA) wt.%) Pd doped WO3 samples were prepared and their monoclinic phase is verified by XRD (Figure S4). To understand further about the electron transfer kinetics, electrochemical impedance spectroscopy (EIS) measurements were carried out for WO3-48 AA and WO3 with different concentrations of Pd (2, 5 and 10 wt. %) in 0.1 M NaOH solution at 5 mV/s. The corresponding Nyquist plots showing the variation of real and imaginary parts of impedance (Z' and Z") in the frequency range of 0.01 Hz-100 kHz are represented in Figure S5 A. The low and intermediate frequency regions of the semicircle is fitted using Randles equivalent circuit and the circuit model is given as inset which includes the elements series resistance (Rs), charge transfer resistance (Rct) and electric double layer capacitance (Cdl). Higher Rct value of WO3-Pd2 AA may consequence from the very low concentration of Pd doping. Comparing the EIS semicircles (Figure S5 B) of all, WO3-Pd AA is having much smaller diameter with a low charge transfer resistance of 10.83 kΩ. These results signify that the WO3-Pd AA has a lower internal resistance and hence exhibits faster electron transfer kinetics which is also evident from its superior electrochemical activity during the oxidation of glucose. It is also supported by the CV curves obtained (Figure S6 A and B) in a 0.1 M KCl solution containing 5.0 mM Fe(CN)63-/Fe(CN)64-.

3. 3 Selectivity and stability tests During the real time analysis, co-existence of other oxidative species like ascorbic acid (AA), Lactic acid (LA), uric acid (UA), dopamine (DA), maltose (ML) present in the biological sample could easily influence the accurate detection of glucose. Hence the anti 10


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interference ability of the WO3-Pd AA was tested at the applied potential of +0.45 V in 0.1 M NaOH solution. In human blood, the physiological availability of UA, DA, AA, LA and MA is much lower than the concentration of glucose.26 Hence the amperometric response was tested against the addition of 0.05 M of each interfering species and 0.5 M glucose and the response curve is depicted in Figure S7. Upon glucose addition, Pd doped WO3 showed remarkable increase in current while no significant enhancement was found during the addition of other interfering molecules. The above result confirms the excellent selectivity of the prepared sensor towards glucose detection even in the presence of interferents. To demonstrate the long-term stability of the prepared sensor, current response was studied by chronoamperometric measurement at +0.45 V with 20 µM of glucose. Only 5 % loss in current was found during 3600 s cycle which infers the excellent long-term stability of the Pd doped WO3 glucose sensor.

4. Theoretical Section To qualitatively support our experimental observations and get theoretical insight on orbital interactions and charge transfer mechanism, which play a crucial role in glucose sensing we have performed DFT simulations on monoclinic WO3 and Pd doped WO3 surface.

4.1 Computational Details We have performed spin-polarized calculations by employing the Density Functional Theory based projector augmented wave (PAW) method as implemented in the VASP code.31-34 PAW based pseudo-potentials35 have been used for W, Pd, O, C and H with PW91 as the exchange-correlation functional. The cutoff energy is taken as 600 eV and the brillioun zone is sampled with a Monkhorst pack36 mesh of 9x9x1 and 8x8x8 k-points for surface and bulk geometries respectively. The convergence criterion of 0.01 eV/Å for Hellmann11



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Feynman forces and 10-6 eV for total energy were considered throughout the simulations. The initial structure of α-glucose was optimized in a cubic cell of lattice parameter of 20 Å which is sufficient enough to avoid any periodic interactions, as α-glucose is a molecule. For αglucose molecule, the brillioun zone was sampled using only gamma points.

4.2 Interactions of Pd on WO3 Surface First we have relaxed the monoclinic phase of bulk WO3 and the DFT computed lattice parameters are a= 7.35, b=7.60 and c=7.76 Å, agrees reasonably well with the experimental value37 of a= 7.33, b=7.56 and c=7.73 Å and computed reference value38 of a= 7.30, b=7.54 and c=7.69 Å. In WO6 octahedral, the simulated maximum and minimum W-O bond length in x, y and z directions are 1.89-1.915, 2.09-1.803 and 2.16-1.77 Å respectively. The computed bond length are matching nicely with the previous DFT computed values of 1.865-1.937, 2.087-1.785 and 2.166-1.759 Å in x, y and z directions respectively reported by Ryan Chatten et al.39 The band gap of monoclinic WO3 comes out to be 1.42 eV using GGA exchange correlation functional which agrees with the previous DFT computed value of 1.225 (LDA) and 1.713 (GGA) reported by Xiao Han and Xiaohong.38 But the theoretically predicted band gap is lower than the experimental value of 2.75 eV,40 which may be due to discontinuity of the exchange–correlation potential with respect to the particle number that is already incorporated into the generalised Kohn–Sham single-particle Eigen values.41 The reasonable matching of lattice parameters, W-O bond length and band gap (with reported DFT data) gives us confidence regarding the validity of the simulations techniques and procedures. From the relaxed bulk structure, we have constructed (001) surface of monoclinic WO3 and allowed it to relax. O-terminated (001) surface of WO3 has been considered here as 12


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it has better sensing properties as reported by Xiao Han and Xiaohong Yin42 in the context of NO2 sensing on WO3. On the (001) surface of WO3 we have introduced one Pd atom around 2.5 Å above the surface and geometry optimization was performed. Figure 6A presents the relaxed structure of Pd doped on WO3 (001) surface. Pd is bonded on WO3 (001) surface and makes bond with O atom with a bond length of 2.10 Å. The binding energy of Pd on (001) surface of WO3 was computed using the formula Eb ( Pd ) = E (WO3 + Pd ) − E (WO3 ) − E ( Pd )

(3)

where E(WO3) is the energy of WO3 surface, E(Pd) is the energy of the single Pd atom and E(WO3+Pd) is the energy of Pd doped on (001) surface of WO3. Pd is getting bonded with a binding 1.82 eV. Figure 7A presents the total density of states of (001) surface of WO3 (lower panel) and Pd doped on (001) surface of WO3 (upper panel). We can see that the density of states near Fermi level has been enhanced when Pd is doped on WO3 (001) surface. This may be due to charge transfer from d and s orbitals of Pd to WO3.The enhancement of density of states near Fermi level signify that in presence of Pd, the conductivity and charge transfer capability of WO3 increases.

4.3 Interactions of Glucose Molecule on WO3 and Pd doped WO3 Surface Next, we introduce α-glucose molecule on WO3 (001) surface and Pd doped WO3 (001) surface and allow the systems to relax. Figure 6B displays the relaxed structure of glucose attached on Pd doped WO3 (001) surface. The binding energy of glucose molecule on WO3 (001) surface is of 1.12 eV and the O of glucose molecule makes a bond with W of WO3 with bond length of 2.39 Å. In presence of Pd, the binding energy of glucose molecule on Pd doped WO3 (001) surface is increased to 1.32 eV and the bond length between the O of 13



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glucose molecule and Pd is 2.18 Å. So it is clear that in presence of Pd, glucose molecule is bonded more strongly implying higher charge transfer from O of glucose. Figure 7B depicts the total density of states of glucose molecule attached on (001) surface of WO3 (lower panel) and glucose on Pd doped on WO3 (upper panel). We can notice the enhanced Density of States near Fermi level when glucose is attached to Pd doped WO3 compared to bare WO3. Figure 8 presents the PDOS of p orbital of the O atom of the glucose when it is in glucose molecule (upper panel), attached to WO3 (001) surface (middle panel) and attached to Pd+ WO3 (001) surface (bottom panel). For O atom in glucose molecule, there appear states at and around Fermi level (upper panel). When glucose is attached to WO3 (001) surface and Pd doped WO3 (001) surface, we can notice that some of the states near Fermi level are missing and most of the occupied states are now located deep in the valance band implying charge transfer from the top of the valance band. It is clear from Figure 8 that charge transfer from O of glucose molecule is more in presence of Pd. Figure 9 presents the Partial Density of States for s and d orbitals of Pd for Pd doped WO3 (001) surface (lower panel) and glucose attached on Pd doped WO3 (001) surface (lower panel). When glucose is attached on Pd+WO3, the intensity of DOS for Pd d orbital near Fermi level increases signifying the charge transfer from O of glucose to Pd. As the binding energy of glucose is more on Pd doped WO3 (001) surface compared to bare WO3 (001) surface, Pd doped WO3 (001) surface becomes more conducting and there is more charge transfer from O of glucose to Pd doped WO3 (001) surface, we can infer that Pd doped WO3 exhibit a better charge transfer media compared to bare WO3 resulting enhanced glucose sensitivity in consistency with our experimental data.

5. Conclusions 14


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Non-enzymatic electrochemical glucose sensing properties of hydrothermally synthesized polymorphs of WO3 nanomaterials were successfully demonstrated in this study. As a sensing material for glucose detection, monoclinic Pd doped WO3 showed excellent sensing performance (sensitivity: 11.4 µA µM-1 cm-2) compared with orthorhombic and mixed phases. Enhanced sensing performance observed for the Pd doped WO3 could be attributed from the following factors: i) mesoporous brick like structure provides more active sites for the adsorption of glucose molecules, ii) integrated contribution of Pd on the electrocatalytic properties of WO3 leads to the enhanced electron transfer kinetics, iii) monoclinic phase of WO3 provided preferred penetrable pathways for the diffusion of glucose to adsorb on WO3 surface which in turn advanced the electron transfer rate. Considering the high sensitivity, good anti-interference ability and longer life cycles all together, prepared material acquire prospects in the glucose sensor device fabrication. From electronic structure simulations it can be inferred that glucose molecule interacts strongly on WO3 and Pd doped WO3 surface due to charge transfer from O p orbital of glucose and gets oxidized. In presence of Pd, as WO3 surface becomes more conducting and the charge transfer from O of glucose molecule gets enhanced, we can conclude that Pd doped WO3 exhibit a superior charge transfer media compared to bare WO3 resulting enhanced glucose sensitivity and hence support our experimental data. ASSOCIATED CONTENT

Supporting Information. XRD patterns of as-prepared Pd doped WO3 and annealed (2, 10 wt.%) Pd doped WO3. CV response curves of 48 hrs reacted as prepared WO3 for different concentrations of glucose. EIS spectra and anti-interference study plot. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION 15



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Corresponding Author [email protected] (BC); [email protected], [email protected] (CSR).

Acknowledgements Dr. C.S. Rout would like to thank DST (Government of India) for the Ramanujan fellowship (Grant No. SR/S2/RJN-21/2012). This work was supported by the DST-SERB Fast-track Young scientist (Grant No. SB/FTP/PS-065/2013), UGC-UKIERI thematic awards (Grant No. UGC-2013-14/005) and BRNS-DAE, (Grant No. 37(3)/14/48/2014-BRNS/1502). Dr. B. Chakraborty would like to thank Dr. N.K.Sahoo for support and encouragement. Dr. B. Chakraborty would also like to thank the staff of BARC computer division for supercomputing facility. Dr. RP sincerely thanks DST-SERB for the National Post-doctoral Fellowship (Grant No. PDF/2016/001002).

References 1. Zhuiykov, S. Nanostructured Semiconductor Oxides for the Next Generation of Electronics and Functional Devices; Woodhead Publishing Limited: Cambridge, UK, 2014. 2. Koch, C.C.; Ovidko, I. A.; Seal, S.; Veprek, S. Structural Nanocrystalline Materials: Fundamentals and Applications; Cambridge University Press: Cambridge, UK, 2007. 3. Cao, G.; Wang, Y. Nanostructures and Nanomaterials, 2nd edition; Synthesis, Properties and Applications; World Scientific Publishing Co. Pte. Ltd.: Singapore, 2011. 4. Zhang, H.; Li, Y.; Duan, G.; Liu, G.; Cai, W. Tungsten Oxide Nanostructures Based on Laser Ablation in Water and a Hydrothermal Route. CrystEngComm. 2014, 16, 2491-2498.

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5. Mosquera, A. A.; Albella, J. M.; Navarro, V.; Bhattacharyya, D.; Endrino, J. L. Effect of Silver on the Phase Transition and Wettability of Titanium Oxide Films. Sci. Rep.

2016, 6, 32171. 6. Yi, X.; Wu, Y.; Tan, G.; Yu, P.; Zhou, L.; Zhou, Z.; Chen, J.; Wang, Z.; Pang, J.; Ning, C. Palladium Nanoparticles Entrapped in a Self-supporting Nanoporous Gold Wire as Sensitive Dopamine Biosensor. Sci. Rep. 2017, 7, 7941. 7. Hassan, Majeed,

M. A.

A.

M.;

M.

A.;

Hateef, Al-Jabiry,

A. A.

A.; J.

M.;

Jameel, S.; Hussian, H. A. R. A. Amperometric Biosensor of SnO2 Thin Film Modified by Pd, In and Ag Nanostructure Synthesized by CSP method. Appl. Nanosci. 2014, 4, 927–934. 8. Hossain, M. F.; Park, J. Y. Palladium Nanoparticles on Electrochemically Reduced Chemically

Modified

Graphene

oxide

for

Non-enzymatic

Bimolecular

Sensing.

RSC Adv. 2013, 3, 16109-16115. 9. Kumar, M.; Bhati, V. S.; Ranwa, S.; Singh, J.; Kumar, M. Pd/ZnO Nanorods based Sensor for Highly Selective Detection of Extremely Low Concentration Hydrogen. Sci. Rep. 2017, 7, 236. 10. Naik, K. K.; Gangan, A.; Chakraborty, B.; Nayak, S. K.; Rout, C. S. Enhanced Nonenzymatic Nanosheets:

Glucose-Sensing Experimental

and

Properties DFT

of

Electrodeposited

Investigations.

ACS

NiCo2O4–Pd Appl.

Mater.

Interfaces, 2017, 9, 23894–23903. 11. Jena, B. K.; Raj, C. R. Enzyme-Free Amperometric Sensing of Glucose by Using Gold Nanoparticles. Chem. Eur. J. 2006, 12, 2702-2708. 12. El-Nahass, M. M.; Saadeldin, M. M.; Ali, H. A. M.; Zaghllol, M. Electrochromic Properties of Amorphous and Crystalline WO3 Thin Films Prepared by Thermal Evaporation Technique. Mater. Sci. Semicond. Process. 2015, 29, 201-205. 13. Chang, M.-T.; Chou, L.-J.; Chueh, Y.-L.; Lee, Y.-C.; Hsieh, C.-H.; Chen, L.-J.; Lan, Y.-W.; Chen, C.-D. Nitrogen-doped Tungsten Oxide Nanowires: Low-temperature Synthesis on Si, and Electrical, Optical, and Field-Emission Properties. Small 2007, 3, 658-664. 17



Page 18 of 28

14. Dong, P.; Yang, B.; Liu, C.; Xu, F.; Xi, X.; Houa G.; Shao, R. Highly Enhanced Photocatalytic Activity of WO3 Thin Films Loaded with Pt–Ag Bimetallic Alloy Nanoparticles. RSC Adv. 2017, 7, 947-956. 15. Liu, C.; Yang, Y.; Li, W.; Li, J.; Li, Y.; Shi, Q.; Chen, Q. Highly Efficient Photoelectrochemical Hydrogen Generation using ZnxBi2S3+x Sensitized Plate-like WO3 Photoelectrodes. ACS Appl. Mater. Interfaces, 2015, 7, 10763–10770. 16. Jincheng, Z.; Chengwu, S.; Junjun, C.; Chao, Y.; Ni, W.; Mao, W. Pyrolysis Preparation of WO3 Thin Films using Ammonium Metatungstate DMF/Water Solution for Efficient Compact Layers in Planar Perovskite Solar Cells. J. Semicond.

2016, 37, 033002. 17. He, Y.; Gu, M.; Xiao, H.; Luo, L.; Shao, Y.; Gao, F.; Du, Y.; Mao, S. X.; Wang, C. Atomistic Conversion Reaction Mechanism of WO3 in Secondary Ion Batteries of Li, Na, and Ca. Angew. Chem. Int. Ed. 2016, 55, 6244 –6247. 18. Hariharan, V.; Radhakrishnan, S.; Parthibavarman, M.; Dhilipkumar, R.; Sekar, C. Synthesis of Polyethylene Glycol (PEG) Assisted Tungsten Oxide (WO3) Nanoparticles for L-dopa Bio-sensing Applications. Talanta 2011, 85, 2166-2174. 19. Chen, D.; Jiang, D.; Du, X.; Zhou, L.; Huang, L.; Qian, J.; Liu, Q.; Hao, N.; Li, Y.; Wang, K. Engineering Efficient Charge Transfer Based on Ultrathin Graphite-like Carbon Nitride/WO3 Semiconductor Nanoheterostructures for Fabrication of Highperformances Non-enzymatic Photo Electrochemical Glucose Sensor. Electrochemica Acta 2016, 215, 305-312. 20. Ma, Y.; Zhao, M.; Cai, B.; Wang, W.; Yea, Z.; Huang, J. 3D Graphene Network@WO3 Nanowire Composites: A Multifunctional Colorimetric and Electrochemical Biosensing Platform. Chem. Commun. 2014, 50, 11135-11138. 21. Kalanur, S. S.; Hwang, Y. J.; Chae, S. Y.; Joo, O. S. Facile Growth of Aligned WO3 Nanorods on FTO Substrate for Enhanced Photoanodic Water Oxidation Activity. J. Mater. Chem. A 2013, 1, 3479-3488.

18


Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


22. Guo, C.; Zhang, X.; Huo, H.; Xu, C.; Han, X. Co3O4 Microspheres with Free-standing Nanofibers for High Performance Non-enzymatic Glucose Sensor. Analyst 2013, 138, 6727-6731. 23. Luhana, C.; Bo, X. J.; Ju, J.; Guo, L. P. A Novel Enzymatic Glucose Sensor Based on Pt Nanoparticles-Decorated Hollow Carbon Spheres-modified Glassy Carbon Electrode. J Nanopart Res. 2012, 14, 1158. 24. Ahmad, R.; Tripathy, N.; Khan, M. Y.; Bhat, K. S.; Ahn, M. S; Khang, G.; Hahn, Y. B. Hierarchically Assembled ZnO Nanosheets Microspheres for Enhanced Glucose Sensing Performances. Ceram. Inter. 2016, 42, 13464-13469. 25. Huang, W.; Ding, S.; Chen, Y.; Hao, W.; Lai, X.; Peng, J.; Tu, J.; Cao Y.; Li, X. 3D NiO Hollow Sphere/Reduced Graphene Oxide Composite for High Performance Glucose Biosensor. Sci. Rep. 2017, 7, 5220. 26. Luo, X.; Huang, M.; Bie, L.; He, D.; Zhang, Y.; Jiang, P. CuCo2O4 Nanowire Arrays Supported on Carbon Cloth as an Efficient 3D Binder-free Electrode for NonEzymatic Glucose Sensing. RSC Adv. 2017, 7, 23093-23101. 27. Krishnan, S. K.; Prokhorov, E.; Bahena, D.; Esparza, R.; Meyyappan, M. ChitosanCovered

Pd@Pt

Core-Shell

Nanocubes

for

Direct

Electron

Transfer in Electrochemical Enzymatic Glucose Biosensor. ACS Omega 2017, 2, 1896-1904. 28. Huang, M.; Luo, X.; He, D.; Jiang, P. Hierarchical Co(OH)2 Nanotube Arrays Grown on Carbon Cloth for use in Non-enzymatic Glucose Sensing. Anal. Methods 2017, 9, 5903-5909. 29. Lv,

J.;

Kong,

C.;

Hu,

X.;

Zhang,

X.;

Liu,

K.;

Yang,

S.;

Bi,

J.;

Liu, X.; Meng, G.; Li, J.; Yang, Z.; Yang, S. Zinc ion Mediated Synthesis of Cuprous Oxide

Crystals

for

Non-enzymatic

Glucose

Detection.

J. Mater. Chem. B 2017, 5, 8686-8694. 30. Yan, X.; Gu, Y.; Li, C.; Zheng, B.; Li, Y.; Zhang, T.; Zhang, Z.; Yang, M. Nonenzymatic Glucose Sensor based on the CuS Nanoflakes-Reduced Graphene Oxide Nanocomposite. Anal. Methods 2018, 10, 381-388. 19



Page 20 of 28

31. Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B

1993, 47, 558. 32. Kresse, G.; Hafner, J. Ab Initio Molecular-dynamics Simulation of the Liquid-metalamorphous Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 1425114269. 33. Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab initio total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. 34. Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. 35. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 36. Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. 37. Vogt, T.; Woodward, P. M.; Hunter, B. A. The High-Temperature Phases of WO3. J. Solid State Chem. 1999, 144, 209. 38. Han, X.; Yin, X. Density Functional Theory Study of the NO2-Sensing Mechanism on a WO3 (001) Surface: The Role of Surface Oxygen Vacancies in the Formation of NO and NO3. Molecular Physics 2016, 114, 3546. 39. Chatten, R.; Chadwick, A.V.; Rougier, A.; Lindan, P. J. D. The Oxygen Vacancy in Crystal Phases of WO3. J. Phys. Chem. B 2005, 109, 3146-3156. 40. González-Borrero, P. P.; Sato, F. Medina, A. N.; Baesso, M. L.; Bento, A. C.; Baldissera, G.; Persson, C.; Niklasson, G. A.; Granqvist, C. G.; da Silva, A. F. Optical Band-gap Determination of Nanostructured WO3 Film. Appl. Phys. Lett.

2010, 96, 061909. 41. Seidl, A.; Görling, A.; Vogl, P.; Majewski, J. A.; Levy, M. Generalized Kohn-Sham Schemes and the Band-gap Problem. Phys .Rev. B 1996, 53, 3764.

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42. Han, X.; Yin, X. Density Functional Theory study of the NO2-sensing Mechanism on a WO3 (001) surface: the Role of Surface Oxygen Vacancies in the Formation of NO and NO3. Molecular Physics 2016, 114, 3546.

Figure 1. A) XRD patterns of as-prepared WO3 with hydrothermal reaction time of a) 24 and b) 48 hrs. B) XRD patterns of WO3- 48 and WO3- Pd after annealing. JCPDS cards of orthorhombic and monoclinic phases of WO3 are plotted in the bottom.

21



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Figure 2. A, C) low and B, D) high magnification FESEM micrographs of WO3-48 and WO3- Pd AA respectively.

Scheme 1. Schematic representation of steps involved in the formation of WO3 nanobricks/microspheres. A) Dehydrolysis of tungstic acid for the formation of WO3 nuclei, B)

growth

of

WO3

nanorods

and

C)

self-assembly

of

nanorods

to

form

nanobricks/microspheres. 22


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Figure 3. A) EDAX of WO3- Pd AA with inset showing single microsphere used for the elemental mapping. B-D) elemental mapping of O, Pd and W respectively.

Figure 4. A) Comparative CV curves of monoclinic WO3 and WO3- Pd AA electrodes, CV curves of B) WO3-48 AA and C) WO3- Pd AA in presence of different concentrations of glucose, D) variation of anodic peak current of WO3-48 AA and WO3- Pd AA with different glucose concentration, E) CVs of WO3-Pd AA at different scan rates in 0.1 M glucose, F) variation of anodic and cathodic peak currents versus square root of scan rate. 23



Page 24 of 28

Figure 5. A) Amperometric responses of different WO3 nanostructures at 0.45 V applied potential in 0.1 M NaOH with the successive addition of various doses of glucose, B) calibration plot of current versus different glucose doses.

Scheme 2. Schematic representation of glucose oxidation mechanism on Pd doped WO3 surface.

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Figure 6. DFT optimized structures of (a) Pd doped on WO3 (001) surface, (b) glucose molecule attached on Pd doped WO3 (001) surface; green, purple, blue, yellow and red represent W, O, H, C and Pd respectively.

Figure 7. Total Density of States of (a) WO3 (001) surface, (b) Pd doped on WO3 (001) surface; Fermi level is shown in pink line. 25



Page 26 of 28

Figure 8. Partial density of states of oxygen p orbital of glucose in isolated Glucose (upper panel), when bonded to WO3 (001) surface (middle panel) and when bonded to Pd+WO3 (001) surface (lower panel).

Figure 9. Partial density of states for s and d orbital of Pd for Pd doped WO3 (001) surface (lower panel) and glucose attached on Pd doped WO3 (001) surface (lower panel).

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Table 1. Comparison of glucose sensing performance of reported metal oxides and present work.

Electrode

Linear range

Sensitivity

Reference

Co3O4 MSs with NFs

0.005-12 mM

1440 µA mM-1 cm-2

22

Pt/HCSs

0.04-0.82

4.10 µA mM-1

23

ZnO NSs MSs

0.05-23 mM

210.8 µA mM-1 cm-2

24

3D NiO HSs/RGO composite

0.009-1.12 mM

2.04 mA mM-1 cm-2

25

CuCo2O4 NWAs/CC

0.001 to 0.93 mM

3.93 mA mM-1 cm-2

26

CS/Pd@Pt NC/GOx

1-6 mM

6.82 µA mM-1 cm-2

27

Co(OH)2 NTAs/CC

1-600 µM

2.77 mA mM-1 cm-2

28

Hierarchical Cu2O

Upto 2 mM

3076 µA mM-1 cm-2

29

rGO/CuS NFs

1 to 2000 µM

53.5 µA mM-1 cm-2

30

ERCGO/PdNPs

0.05–10 mM

15.14 µA mM-1 cm-2

8

WO3- 24

5-55, 65-375 µM

7.1, 3.26 µA µM-1 cm-2

This work

WO3- 48 BA

65-375 µM

2.26 µA µM-1 cm-2

This work

WO3- 48 AA

5-55, 65-375 µM

9.56, 3.76 µA µM-1 cm-2

This work

WO3- Pd AA

5-55, 65-375 µM

11.4, 5.6 µA µM-1 cm-2

This work

27



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