Fabrication of nanostructured Pd thin films using aerosol-assisted

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Fabrication of nanostructured Pd thin films using aerosol-assisted chemical vapor deposition for the nonenzymatic electrochemical detection of H2O2 Muhammad Ali Ehsan, Md. Mahedi Hasan, Tamanna Islam, Md. Delwar Hossain, Md. Abdul Aziz, and A J Saleh Ahammad ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00131 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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ACS Applied Electronic Materials

Fabrication of nanostructured Pd thin films using aerosol-assisted chemical vapor deposition for the nonenzymatic electrochemical detection of H2O2 Muhammad Ali Ehsana,1, Md. Mahedi Hasanb,1, Tamanna Islamb,1, Md. Delwar Hossain b, Md. Abdul Aziza and A. J. Saleh Ahammadb,* aCenter

of Research Excellence in Nanotechnology, King Fahd University of Petroleum and

Minerals, Dhahran 31261, Saudi Arabia. bDepartment

1These

of Chemistry, Jagannath University, Dhaka 1100, Bangladesh.

authors contributed equally to this work.

*Corresponding author Tel: +880 2 9583794 Fax: +880 2 7113713 E-mail address: [email protected]

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ABSTRACT An improved and facile aerosol-assisted chemical vapor deposition (AACVD) process for the production of palladium thin film on indium tin oxide (PdNP-ITO) electrodes was described and applied for the electrochemical detection of hydrogen peroxide (H2O2). The detailed characterization of the films by X-ray diffraction (XRD), scanning electron microscopy/energydispersive X-ray (SEM/EDX) spectroscopy, and X-ray photoelectron spectroscopy (XPS) analysis proved the high crystallinity and phase purity of the nanosized metallic palladium films without the evolution of any elemental impurities from the precursor compound. The as-prepared electrodes were used for nonenzymatic amperometric H2O2 detection via electrochemical reduction. The LOD was 40.8 nM with a high sensitivity of 760.84 μA/μM cm2. From the experimental scan rate variation analysis, the reduction of H2O2 on the PdNP-ITO electrode surface was determined to be adsorption controlled. For this process of adsorption, we calculated the number of electrons involved during adsorption (n), the charge transfer coefficient (α) and, finally, the rate constant (ks). The process of adsorption of H2O2 on each of the characteristic metallic planes was further studied via Monte Carlo simulations (MCSs). We accounted for both molecular O2 and H2O during the simulations to understand the effects of oxygen and solvent on adsorption since all experiments were conducted in an air-saturated solution. Several numbers of the adsorbate-metal substrate configurations were obtained for the simulations on each crystalline Pd plane, confirming the firm adsorption of H2O2 in the presence of O2 and H2O. Based on analysis of the results from the electroanalytical procedures and MCSs, possible reduction reaction mechanism pathways were proposed. Keywords: AACVD synthesis; Nanostructured palladium; Nonenzymatic detection; Hydrogen peroxide; Monte Carlo simulations.


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1. INTRODUCTION H2O2, the simplest of the peroxide family, plays a crucial role in many industrial, pharmaceutical and biological processes.1,2 In biological systems, hydrogen peroxides are produced as a shortlived species mainly with the help of antioxidant superoxide dismutase through disproportionation of superoxide and when uric acid is produced by xanthine degradation of hypoxanthine through the xanthine pathway.2,3 Hydrogen peroxide is a well-known oxidizing agent that can oxidize DNA, membrane lipids, proteins and thus is highly toxic.4 An abnormal level of H2O2 is often used as an important biomarker for studying Parkinson’s disease, tumors, cancer, cardiovascular diseases and several other diseases.1,5 The normal level of H2O2 in the physiological system is in the nanomolar (10–100 nM) range.5 Hence, the importance of a highly sensitive and selective H2O2 sensor cannot be overstated. Electroanalytical techniques (EATs) are fast, cheap, sensitive and highly selective compared to commonly used techniques for detecting H2O2 species such as fluorescence, colorimetry and chemiluminescence.6-10 One of the major problems of EATs for the sensitive and selective detection of analytes is that conventional electrodes are usually not capable of facilitating effective electron transfer to induce the redox process in many biological analytes, including H2O2.6,7,11-13 Therefore, complex and enzymebased modifications are often made for H2O2 detection.1,5 These enzymatic sensors suffer from several drawbacks, such as complex modification processes and temperature and pH dependence.1,5,14 Even when simple modifications are made, the detection of H2O2 is always performed in deaerated N2-saturated solutions.1,7 However, in real life, the samples always contain at least a small amount of dissolved O2.15 Therefore, a simple electrode modification process for sensitive and selective detection in aerated solutions is of paramount importance for real-life applications. Understanding the redox processes that occur at the electrode/electrolyte surface is key to the sensitive detection of H2O2. Researchers have continuously worked towards understanding these reaction processes.16-19 EATs such as CV and amperometry can help evaluate several important kinetic and thermodynamic parameters of redox processes through the study of the heterogeneous electron transfer rate constant, charge transfer coefficient and rate-determining step of the reactions.14,17 On the other hand, theoretical studies can shed light on the energy change and even actual physical condition at the electrode/electrolyte interface during such redox processes.18,20 Thus, correlating theory and experiment is an important criterion for evaluating



electrode/electrolyte assemblies during the reduction of H2O2 in the presence of solvent molecules and dissolved oxygen to clearly understand the redox process and for fabricating more efficient electrochemical sensors (ECSs) that can effectively operate under ambient conditions. To address the challenges of high sensitivity and selectivity and to fulfill the need to understand the reaction mechanism processes that occur at the electrode surface during H2O2 reduction, researchers have utilized diverse types of electrode modifiers. Metallic nanomaterials (MNMs) and their oxides, such as gold nanoparticles (AuNPs),1,16 silver NPs (AgNPs),5 platinum NPs (PtNPs),7 lead NPs (PbNPs)7 and palladium NPs (PdNPs),16,17 have been extensively used. While almost all of these materials show good sensitivity and long linear range for H2O2 detection, the PdNPs stand out because of their ability to reduce H2O2 at much lower overpotential than that for all the other MNMs.16,17,20 This ability might provide the chance to detect H2O2 in the presence of molecular oxygen without prior nitrogen treatment of the sample; thus, utilizing PdNP-modified ECSs in real-life applications is possible. Nanostructured palladium is a significant member of the nanomaterial family and is of huge interest due to its fascinating chemical and physical properties.21 Palladium has great scientific and technological value with promising applicability in gas sensors, inorganic membranes for H2 gas separation, electronic devices and supported catalysts for a number of chemical reactions in organic chemistry, such as carbon-carbon bond-forming reactions and Suzuki, Heck, coupling, hydrogenation, and dehydrogenation reactions.21-29 Recently, palladium-based nanomaterials have been dedicated to electrochemical monitoring of biological reagents, such as uric acid, dopamine, ascorbic acid, glucose, and chemicals, including hydrogen peroxide, hydrazine and formaldehyde.30-35 As a result, various specialized methodologies for the preparation of palladium nanoparticles with tunable sizes, shapes, designs and unusual capabilities have been reported and discussed.36 However, electrochemical sensing devices composed of electroactive palladium coatings and thin films can be user-friendly, robust, and cost-effective and can have high sensitivity and selectivity. The major obstacle for the use of Pd films in ECSs is finding a facile, inexpensive and rapid fabrication technique that can produce homogenous thin films with high purity and reproducibility. Although chemical vapor deposition (CVD) is an attractive choice for making such metallic thin films, a serious concern with the CVD preparation of palladium is the availability of appropriate precursors with sufficient volatility and thermal stability. Thus, the generation of


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uniform and homogenous films within short deposition times with higher levels of purity remains a challenge. In the past, much effort was directed to the design and synthesis of specifically tailored organometallic and metal-organic complexes of palladium, but implementation of CVD to develop device-grade films was not very fruitful. For example, allylpalladium(II), Pd(C3H5)2, and allyl methyl palladium(II), (CH3)Pd(C3H5)2, were found to be thermally unstable during the CVD process.37 Films made of cyclopentadienyl allyl palladium(II),

(C5H5)Pd(C3H5),

(C5H2F6O2)Pd(C3H5),

and

allyl

dimethyl

hexafluoroacetylacetonate

palladium(II),

tetramethylethylenediamine

palladium(II),

(CH3)2Pd(C6H16N2), were enriched with carbon.38 Precursor compounds dimethyl trimethyl phosphine palladium(II), (CH3)Pd(C3H9P), and dimethyl triethylphosphine palladium(II), (CH3)2Pd(C6H15P)2, produced elemental impurities such as P and C.38 In all of the above examples and in the case of bis-hexafluoroacetylacetonate palladium(II), Pd(C5H2F6O2)2, hydrogen gas was needed to be supplied with the carrier gas to reduce the Pd(II) ions into Pd(0).37,38 In light of the above concerns, the aim of the current work is three-fold: first, to explore the facile and cost-effective aerosol-assisted CVD (AACVD) method by implementing commercially viable metal-organic precursors to directly obtain palladium thin films on a conducting indium-doped tin oxide substrate; second, to test the fabricated films for electrochemical sensing of H2O2 in an aerated solution; and, finally, to correlate the findings between electrochemical experiments and molecular dynamics methods to understand the redox process. The deposition of palladium films was achieved by employing palladium(II) acetate, Pd(CH3COO)2, as a precursor in AACVD. For the operation of AACVD, the solubility of the precursor in an organic solvent is the one key step, whereas the rigorous requirements of the precursor’s volatility and thermal stability are not as important.39 Once these conditions are met, the AACVD method allows the use of commercially available low-cost precursors, which make the deposition process simple, easy and time saving.40 Finally, a correlation was made between experimental and computational analyses for interpreting the electrode/electrolyte interfacial processes responsible for reducing H2O2 in the presence of solvent and dissolved oxygen molecules.



2. EXPERIMENTAL SECTION 2.1. Materials. ITO (30 Ω/sq) electrodes were purchased from Geomatec, Japan. Palladium(II) acetate, Pd(CH3COO)2, trifluoroacetic acid, hydrogen peroxide, dopamine (DA), glucose, ascorbic acid (AA), toluene and uric acid were all received from Sigma-Aldrich, USA. 2.2. Electrode fabrication process. AACVD was used to grow the palladium thin film electrodes. The design and infrastructure of AACVD are well known.41 A total of 80 mg (0.356 mM) of the precursor palladium(II) acetate, Pd(OAc)2, was suspended in methanol (12 mL) and became clear upon the addition of 0.1 mL of trifluoroacetic acid (CF3COOH). Afterwards, the resultant yellow solution was utilized in AACVD to observe the growth of Pd films. Prior to deposition, the glass substrates (i.e., ITO glass) with dimensions of 1.0 x 2.0 cm2 (W x L) were cleaned with soapy water, acetone and isopropanol and were then air dried. For each of the deposition experiments, the substrate was loaded horizontally inside the reactor tube and heated to a deposition temperature of 425 °C, and the temperature was allowed to come to equilibrium for 10 minutes; finally, the deposition process was started. The aerosol mist from Pd(OAc)2 was generated using a piezoelectric ultrasonic humidifier, and the aerosol was carried to the reactor tube by a stream of compressed air with a flowrate of 120 cm3/min. The deposition experiments were continued for 45 minutes. The waste exhaust of the precursor mist was vented into a fume hood. After deposition, the films were allowed to cool to room temperature under a continuous flow of N2 gas. The resulting film electrodes were uniform, shiny, grayish in color and stable in the open air. The adhesion properties of the palladium thin films were verified by the “Scotch tape test,” and the layers remained intact on the ITO substrate. All samples were synthesized multiple times to determine the repeatability of the process, and the data presented are typical for synthesized thin films. 2.3. Metal plane simulations and computational methods. To understand the individual contributions of Pd crystal planes for the electrochemical reduction of H2O2, we simulated five Pd planes, (111), (200), (220), (311) and (222) (obtained from the XRD analysis shown in Figure 1), using a vacuum slab builder.20 The vacuum slabs were produced with (2x2) periodic unit cells with 6 atomic-layer thicknesses.20,42 Periodic 3D vacuum slabs were generated using 15 Å vacuum space to ensure that the adsorbates did not interact with the surface of the bottom layer of the periodic structures.20,42


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To understand the physical system of the electrode/electrolyte interface, the adsorption process should be studied in the presence of solvent molecules since they play a vital role during the electrochemical redox process.43 For this purpose, we simulated the water molecules (H2O), oxygen molecules (O2) and H2O2 molecules individually using the Materials Studio software. The geometry and energy of the Pd metal planes and molecules were optimized using the Forcite tool since this is used for geometry optimization before carrying out a molecular dynamics simulation.18,44 All optimization tasks were carried out with the Ultrafine quality. The Condensed-phase

Optimized

Molecular

Potentials

for

Atomistic

Simulation

Studies

(COMPASS) force field was utilized during the optimization process because it can simulate the gas and condensed-phase properties of Pd metal and other molecules used in this simulation process with high accuracy.44,45 The possible low-energy adsorption sites of the optimized molecules on the surfaces of the metal planes were determined by carrying out Monte Carlo searches with the help of the Adsorption Locator tool.18 The adsorption processes were studied on all 5 of the crystal planes of the Pd vacuum slabs to determine the sites with the most affinity towards the adsorption of the molecules. Adsorption locator studies were also performed with the Ultrafine quality and COMPASS force field.44,45 We studied the adsorption of individual H2O2 molecules, a mixture of H2O2 with water and a mixture of H2O2, H2O and O2 molecules to determine the effect of each species on the adsorption energies. Here, adsorption locator studies were carried out with a single H2O2 molecule in vacuum and with a 0.2:0.8 molecular ratio of H2O2:H2O and a 0.2:0.6:0.2 molecular ratio of H2O2:H2O:O2 to simulate the deaerated and aerated solvent systems. All resultant low-energy configurations of the Monte Carlo search were again optimized in the Ultrafine settings before further analysis. 2.4. Instrumentation for thin-film electrode and electrochemical analysis. X-ray diffraction (XRD) patterns of the palladium film electrodes were recorded using a Rigaku MiniFlex X-ray diffractometer (Japan) with Cu Kα1 radiation (γ = 0.15416 nm), a tube current of 10 mA, and an accelerating voltage of 30 kV. Scanning electron microscopy images of the film electrodes were analyzed by a field emission scanning electron microscope (FESEM, Lyra3, Tescan, Czech Republic) at an accelerating voltage of 20 kV. The elemental stoichiometry and composition of the film electrodes were investigated by energy-dispersive X-ray (EDX, INCA Energy 200, Oxford Inst.) spectroscopy. X-ray photoelectron spectroscopy (XPS) experiments



were performed in a Thermo Scientific Escalab 250Xi spectrometer equipped with a monochromatic Al Kα (1486.6 eV) X-ray source with a resolution of 0.5 eV. During the XPS characterization, ambient temperature conditions were maintained while the pressure was controlled at 5 × 10-10 mbar. The spectra were referenced with adventitious C 1 s peaks at 284.5 eV. A CHI 660E workstation was used for all electrochemical experiments. A three-electrode system was used for all experiments. Ag/AgCl reference and Pt wire counter electrodes were employed. A Power Sonic 603 system was used for cleaning bare ITO electrodes. 3. RESULTS AND DISCUSSION 3.1. Characterization of the palladium film. Figure 1 indicates the XRD pattern of the palladium thin film deposited on an ITO substrate at 425 °C by implementing a methanolic solution of Pd(OAc)2 in AACVD in air atmosphere. The diffraction peaks originating at 2θ = 40.0°, 46.5°, 68.0°, 81.0° and 86.4° correspond to the reflection planes (111), (200), (220), (311) and (222), respectively. The measured peak position, peak intensity and d-spacing values of the XRD data match well with those of the standard cubic palladium pattern (01-088-2335), suggesting the synthesis of a pure Pd product in the form of a thin film. The product is highly crystalline, and all peaks are well resolved. No crystalline impurities, such as oxide formation or other crystalline phases of palladium, were identified from the XRD pattern. Notably, the deposition experiments were conducted using compressed air as the carrier gas, and no palladium oxide formation occurred. In contrast, previous CVD studies required the addition of H2 to the carrier gas to produce metallic Pd films.46 Additionally, the present XRD pattern resembles that of the reported palladium films grown onto a stainless steel substrate by electrochemical methods.47 Although the precursor Pd(OAc)2 contained oxygen atoms, its thermal decomposition under inert AACVD conditions completely eliminated organic moieties in the form of CO2, and Pd-oxide formation was not observed through the XRD results. No crystalline side products, such as palladium oxide, were formed, and all crystalline peaks were completely in agreement with those of pure Pd. The microstructural properties of the palladium thin film produced by AACVD were investigated by SEM analysis, and the resulting micrograph is shown in Figure 2. A lowresolution image [Figure 2(A)] reveals the complete coverage of the substrate surface with


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spherical Pd particles with clear grain boundaries and without any noticeable cracks and voids. The enlarged view of this image further explores the texture and architecture of these spherical particles. Each spherical object is made up of coalesced nanosized Pd particles that are oriented in the vertical direction of the substrate surface. The formation of such nanoseaurchin-type structures is attributed to the homogeneous and heterogeneous gas-phase reactions occurring between the precursor vapors and the surface of the ITO substrate at 425 °C.48 In AACVD, the thin-film microstructure is mainly developed by these gas-phase reactions, and the balance between homogeneous and heterogeneous reactions is essentially required to obtain a thin film with the appropriate nanodesign; otherwise, film formation with a suitable nanostructure is difficult to accomplish. The balance between gas-phase reactions can be created by changing the experimental conditions, such as temperature, type of substrate and deposition solvent. The mechanism of gas-phase reactions for the growth of such nanostructured thin films has been discussed in detail in our previous papers.48,49 The elemental composition and purity of the Pd films were identified by energy dispersive Xray (EDX) analysis. Figure 2(C) shows the EDX spectrum of the film, and Pd is the only element detected under the microscope, which suggests the high purity of the films without incorporation of any oxygen or carbon impurities. Such impurities were commonly observed in previous cases.38 This observation confirms that our strategy of making films in a compressed air atmosphere is successful and that the carbonaceous matter evolved during the AACVD reactions of the Pd(CH3COO)2 precursor is completely removed in the form of CO2; additionally, pure Pd films were deposited without the formation of any oxide material. The purity of the palladium films was further ascertained by XPS, and a high-resolution XPS scan of palladium 3d is shown in Figure 3. The splitting of palladium 3d peaks at binding energies of 333.6 and 339.0 eV was attributed to Pd 3d5/2 and Pd 3d3/2, respectively. These binding energy values are in agreement with those reported in the literature for pure metallic palladium.50,51 Generally, oxide formation on the palladium surface is indicated by a minor shoulder in the palladium 3d peaks at high binding energies. However, in the present case, the shoulder was absent, suggesting that no oxide formed on the palladium layer. The XPS results further validated the XRD and EDX results for the synthesis of impurity-free palladium thin films.



3.2. CV and EIS characterization. Electrochemical studies through CV and EIS analysis are preferable for assigning electron transfer properties and understanding the effects of modification processes. Figure 4 displays the characteristic CVs (A) and Nyquist (B) plots of both the modified and bare ITO electrodes for 1 mM K3Fe(CN)6 in 1.0 M KCl. The CV plot of the bare ITO electrode shows well-defined peaks for the redox change of ferricyanides, as shown in the inset (b) of Figure 4(A). A different electrochemical behavior was observed for the PdNP-ITO electrode (c). Such a voltammetric signal is often challenging to assign due to the consequent structural changes that take place during the redox transition. M. Fleischmann et al. proposed that although the electron transfer of ferro-/ferricyanides is thought to occur following an outersphere reaction mechanism, such an electron transfer process may be affected by the electrode materials in use and is accompanied by a structural change in the coordination shell.52 Hence, a potential-dependent change in the binding motifs of ferro-/ferricyanides might have occurred at the PdNP-modified electrode due to a bridging ligand formed by the C≡N group.52,53 These interactions might have resulted in the change in the redox potential of Fe(II)/Fe(III) ions in the complex, as seen in the CV data (c) of Figure 4(A). It is worth mentioning that the higher oxidation current observed in the CV data might be due to the high stability of the Fe(III)containing complexes at the PdNP-modified electrode surface, hence leading to greater oxidation of the Fe(II)-containing complex. These observations successfully confirmed that PdNP was deposited over the ITO electrode by the AACVD method. The EIS plot in Figure 4(B) representing the PdNP-ITO electrode shows a large semicircle in the high-frequency region. This observation indicates a strong resistance to charge transfer for the ferro-/ferricyanide system at the modified electrode. The EIS plot for the bare ITO electrode shows a small semicircle comprising a small region. This observation was quite analogous to the CV response. 3.3. Electrochemical behavior of H2O2. Voltammetric technique was used to analyze the performance of the proposed PdNP-ITO electrode for detecting H2O2. Initially, the CVs of a bare ITO electrode were performed for both the blank PBS solution and a 1 mM H2O2 solution in aerated 0.1 M PBS at pH 7 as indicated by (f) and (g) in Figure 4(C) (inset). Later, the CVs of the PdNP-ITO electrode for the same blank solution (a), a 10 µM H2O2 solution (c), a 20 µM H2O2 solution (d) and a 30 µM H2O2 solution (e) in aerated 0.1 M PBS were performed as shown in Figure 4(C). Here, the bare ITO electrode showed only background current without any


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significant feature in the presence of H2O2 within the experimental potential window of +0.4 to 0.3 V. The CV of the aerated blank PBS solution produced a cathodic peak in the reduction wave at the modified electrode at a potential of -0.07 V (a). This peak was mostly due to the electrochemical reduction of molecular oxygen from the air-saturated solution and air-exposed electrodes.16,54-56 Reduction of oxygen is known to occur with the formation of H2O2 as follows: O2 + 2H+ + 2e-  H2O2 H2O2 + 2H+ + 2e-  2H2O Overall, O2 + 4H+ + 4e-  2H2O This observation was confirmed when the CV of the deaerated blank solution was performed (Figure 4(C))[b]. In that case, only background current was observed. However, consecutive CV responses of the aerated blank solutions showed a decreasing trend of this characteristic oxygen reduction peak at the PdNP-ITO electrode, as shown in Figure 4(D). This trend was probably due to a lack of dissolved oxygen species when consecutive CVs were performed in the same solution. A clearly distinguishable current response was observed at the modified ITO electrode surface in the presence of H2O2 (c, d, e) at a potential of approximately 0.1 V during the negative potential ramp from +0.4 to -0.3 V. This cathodic current signal was due to the electrochemical reduction of H2O2 at the PdNP-ITO electrode surface. This reduction process involved transferring two electrons to H2O2 from the PdNP-ITO electrode following the reactions below.16,54 H2O2 + H+ + e-  OHads + H2O (adsorption) H+ + OHads + e-  H2O Overall, H2O2 + 2H+ + 2e-  2H2O The above reactions demonstrated that the adsorption of H2O2 at the modified electrode surface took place prior to the direct reduction of H2O2, followed by proton transfer to the adsorbed –OH. Such adsorption of H2O2 on the modified electrode surface was further confirmed by computational analysis. It was clear from the CVs of Figure 4(C)[c, d, e] and (D) that the



oxygen peak current was much lower than the H2O2 reduction current and became indistinguishable when peroxide analytes were added to the PBS solutions. 3.4. The effect of pH on the current signal. The pH of a solution often determines the stability of both the analytes present in the solution and the electrode system in use.6,54 Figure 5(A) shows the CVs obtained for the reduction of H2O2 under different pH conditions. Figure 5(B) shows that the peak current increases from pH 4 to a maximum at pH 6. Afterwards, the current decreases slightly for pH 7; and at more basic solutions, the current decreases steeply. The reason for this phenomenon may be that H2O2 is more stable in acidic solutions than in basic solutions.54 In an alkaline medium, the decomposition of H2O2 dominates and occurs in the following manner: H2O2  H2O + 1/2O2.54 Metal nanoparticles are also able to catalyze such a reaction process. Because it is likely that the electrochemical signal was for the reduction of adsorbed peroxide on the PdNPs, such decomposition might have been responsible for the lowering of the peak current at basic solutions. The pH dependence of H2O2 reduction can also be seen from the pH vs. peak potential plot [Figure 5(B)]. The peak potential shifted to a slightly more negative potential with the increase in the alkalinity of the solution medium, revealing the pH dependence of the H2O2 electrochemical reduction process. In this case, the slope value of the linear fit for potential vs. pH was 22.7 mV/pH. This value is very different from the usual value of 59 mV/pH. The reason for this very low value of peak potential shift might be related to the deactivation of binding sites with the adsorption of the H2O2 species and the reduction of molecular oxygen at the PdNP-ITO surface during consecutive CVs of the pH variation study.16,56 The detection of H2O2 is important for several physiological systems that conventionally operate near a neutral pH condition. Additionally, because the reduction current signal was sufficiently high for pH 7.0, hereafter, the solution pH was kept neutral for the rest of the experiments unless stated otherwise. 3.5. Scan rate optimization and kinetic parameter evaluation. Electrochemical reactions are usually controlled by mass transfer (diffusion-controlled in the absence of migration and convection) or adsorption. The rate-limiting factor can be determined through a scan rate variation study in a straightforward way. We performed a scan rate variation study for the 30 µM H2O2 solution from 5  700 mV/s [Figure 6(A)]. The calibration plot shown in Figure 6(B) demonstrates an increasing trend of the reduction peak with increasing scan rates. This observed


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good linear fit of Jpc with ν stipulates that the H2O2 reduction at the PdNP-ITO-modified electrode must have followed through an adsorption-controlled process. The rate-controlling factor was further investigated by plotting log Jpc against log ν [Figure 6(C)]. The slope of the linear fit was found to be approximately 0.6. This value of 0.6 also indicates that H2O2 reduction followed an adsorption-controlled pathway rather than a diffusioncontrolled pathway.57,58 From the relation shown in Figure 6(D), it can be seen that the reduction peak potential shifted to a negative potential with the logarithmic value of ν. The value of the regression coefficient (0.99) suggests a good linear fit for Epc vs. log ν, while the negative potential shift with the increasing scan rate further confirms the irreversible nature of the electrochemical reduction of H2O2 at the PdNP-ITO electrode surface.14,58 For an adsorption-controlled process, the number of electrons transferred can be evaluated using the Laviron equation: Jpc = nFQν/4RT.58 Here, the number of electrons utilized during reduction is given by n, Q is the charge density, and the rest of the terms have their familiar connotations. The value of n was found to be 0.92, i.e., The adsorption of H2O2 during reduction occurred via the 1e- transfer process, and the overall reduction reaction involved the transfer of two electrons. Hence, the possible adsorption step reaction at the PdNP-ITO electrode will be Pd + H2O2 + H+ + e+ → Pd−OHads + H2O.59 The rate constant for electron transfer (ks) and charge transfer probability (α) were determined from the intercept and slope (S) of the Epc vs. log ν plot of Figure 6(D) using the following equations: S = 2.303RT/(1-α)nF and I = [2.303RT/(1-α)nF] [log (1-α)nF/TRks] + E°. The value of α was found to be 0.7859≈0.79, meaning that the transition state favored the product state much more than the reactant state.14,58 This value of α is consistent with our previous assumption that the reduction process is irreversible. The electron transfer rate constant ks was calculated to be 1.98 s-1. This value is close to two, implying the high electron-transfer capability of the PdNPs through the ITO electrode; thus, facile reduction of H2O2 is facilitated. Because reduction is adsorption-controlled, the concentration of electroactive PdNPs could be determined from Faraday’s law: Q = nFAΓ.58 Here, A is the geometric surface area of the electrode and Γ is the representation of the electroactive PdNPs at the surface of the modified electrode, while the others have the same meanings as mentioned before. Therefore, the



electroactive concentration of PdNPs at the PdNP-ITO electrode was estimated to be 2.40e-9 mol/cm2. This value indicates the deposition of the PdNPs on the ITO surface through the AACVD method and their high reactivity towards the electrochemical reduction of H2O2. 3.6. Study of adsorption processes with Monte Carlo simulations. We investigated the adsorption of H2O2 on each of the simulated planes of pure Pd metal. Prior to the investigation of adsorption, two issues (effects of the solvent and dissolved oxygen) were taken under consideration. Electrochemical analysis confirmed that molecular oxygen from the aerated solution could undergo reduction on the Pd metal. Hence, to understand how adsorption proceeds in the presence of molecular oxygen and to understand the effect of H2O, we accounted for all three adsorbates on each crystal plane during the simulation. Table 1 displays the energy changes for the adsorption of species over the top layer of each plane. The energies are shown for three systems containing different adsorbates, namely, H2O2, H2O2/H2O and H2O2/H2O/O2. The total energy was the sum of the rigid adsorption, deformation and the energies of adsorbate species. The rigid adsorption energy is the energy required when an unrelaxed adsorbate (no geometry optimization of the compound) is adsorbed on the substrate (metal). The deformation energy indicates the energy released for adsorbed molecules relaxing on the substrate after adsorption. The adsorption energy is the sum of both the deformation and rigid adsorption energies and refers to the energy needed for the adsorption of a relaxed adsorbate on the metal substrate. The term dEad/dNi energy refers to the energy required for the desorption of a single molecule from the substrate.18 The value of the dEad/dNi energy is crucial for recognizing the stable adsorption of adsorbates on the substrate. A large desorption energy (H2O2: dEad/dNi) indicates that a high amount of energy would be required to separate the adsorbed H2O2 from the metal. On the other hand, small values of H2O/O2: dEad/dNi energies related to the desorption of H2O and O2 demonstrate a less stable adsorption of H2O and O2. Therefore, configurations with simultaneously high values of H2O2: dEad/dNi and low values of H2O/O2: dEad/dNi indicate strong adsorption of H2O2 in the presence of H2O and O2 in H2O2/H2O and H2O2/H2O/O2 systems. The simulation on the Pd (111) plane resulted in approximately 51 and 78 configurations of the H2O2/H2O and H2O2/H2O/O2 systems, respectively. The simulation on the other planes, namely, (200), (220), (222) and (311), resulted in 54 and 59 configurations for Pd (200), 65 and


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50 configurations for Pd (220), 53 and 74 configurations for Pd (222), and 61 and 60 configurations for Pd (311) for the H2O2/H2O and H2O2/H2O/O2 systems. The simulations on Pd (111), Pd (200) and Pd (222) resulted in a higher number of simulated configurations with relatively high desorption (H2O2: dEad/dNi) energy values for both the H2O2/H2O and H2O2/H2O /O2 systems. These observations confirmed a more favorable adsorption of H2O2 on the respective planes. In Table 1, configurations related to the highest and lowest desorption energies for H2O2 (H2O2: dEad/dNi) are shown. Notably, the system with only H2O2 showed a positive total energy for adsorption on each simulated plane with a value of H2O2: dEad/dNi energy that was much lower than those of other systems, indicating relatively low adsorption. This observation confirmed that condensed-phase adsorption is favorable on Pd metal surfaces. The regions of adsorption on different adsorption sites in each lattice plane are shown in Figure 7. The green spots over each of the simulated Pd planes indicate a favorable region for adsorption, while the red spots define improbable regions for adsorption. Figure 8 displays the configurations of the adsorption of different molecules (H2O2, H2O and O2) on the top layer of each simulated Pd structure with the highest value of (H2O2: dEad/dNi) energy, as shown in Table 1. Here, it was observed that in the presence of both the solvent and molecular oxygen, H2O2 showed a strong preference for various adsorption sites on each simulated crystal plane. Although H2O showed a preference for adsorption sites in both systems, it could not obscure the adsorption of H2O2. However, O2 did not show any tendency of adsorption in the presence of H2O and H2O2 on each plane except on the Pd (220) plane, where O2 tended to be adsorbed along with H2O2. 3.7. Binding energy calculations. The energy for adsorption (Eads) was calculated for the most stable (first configuration) and the least stable (last configuration) configurations obtained for the H2O2/H2O/O2 system. Notably, the numbers of adsorbate-metal substrate configurations obtained for the various planes were 78 for Pd (111), 59 for Pd (200), 50 for Pd (220), 74 for Pd (222) and 60 for Pd (311) for the H2O2/H2O/O2 system, as mentioned earlier. Although having a high desorption energy indicates a high degree of adsorption, the stability of the configurations varies depending upon the value of the total energy. Here, the adsorption energy (Eads) for configurations with the highest and least total energy were calculated for each plane for only the H2O2/H2O/O2 system using the following formula.60 Eads = EPd (jkl)-H2O2-H2O-O2 − EPd(jkl) – EH2O2 – EH2O – EO2



where EPd(jkl)-H2O2-H2O-O2 = total energy of the configuration EPd(jkl) = energy of the crystal plane Pd (jkl) EH2O2, EH2O, EO2 = energy of each isolated molecule (H2O2, H2O or O2) The values of Eads were calculated to be -2.9 eV and -2.04 for the high- and low-energy configurations, respectively, for the (111) plane. The values are -2.96 and -2.06 eV for the (200) plane, -2.86 and -2.24 eV for the (220) plane, -2.92 and -2 eV for the (222) plane and -2.9 eV and -2.25 eV for the (311) plane. The Eads values obtained for each plane demonstrated that the process of adsorption on each plane in the H2O2/H2O/O2 system was chemisorption.20,60 3.8. Probable reaction mechanism. Here, we proposed a reaction pathway for the reduction of H2O2 on the PdNP-ITO-modified electrode based on computational and experimental observations. From the computational studies, we found that H2O2 was chemisorbed on the Pd planes. The equilibrium distances, such as Pd-O, Pd-H and O-H, were measured after geometry optimization of the configurations having the highest desorption energy (H2O2: dEad/dNi) on each simulated crystalline plane for both the H2O2/H2O and H2O2/H2O/O2 systems. These values are tabulated in Table 2. During chemisorption equilibrium, the Pd-O/Pd-H distances varied from 2.8-3.0 Å and from 2.6-3.2 Å, whereas the O-H bond length increased from a normal value of 0.956 Å to approximately 0.974-0.979 Å. These observations revealed that chemisorption of H2O2 on the crystal planes might have occurred via O-O/O-H bond breakage, which resulted in surface adsorption of the OHads and OOHads species.20 From the scan rate analysis, we observed that the adsorption process of the OHads species occurred via transferring a single electron from the electrode to H2O2 under the applied potential. As the adsorption studies showed the possibility of O-H bond breaking and Pd-Oads formation, there might have been OOHads formation during the surface adsorption of H2O2 species. Therefore, proton-assisted association might have produced water via the transfer of another electron to the adsorbed OHads from the electrode. Proton transfer from OOHads to the adsorbed OH might have produced O2 as another byproduct. These two events might have occurred during the reduction of H2O2 on PdNP-ITO. Hence, based on the experimental and computational analysis, we propose that the reduction reaction of H2O2 on the PdNP-ITO electrode primarily followed the O–O bond-breaking pathway through the adsorption


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of OH species along with a limited amount of oxygen production following the second pathway. The possible reaction pathways are schematically represented in scheme 1. 3.9. Amperometric analysis of H2O2 at PdNP-ITO. We investigated the characteristic I-t behavior of the PdNP-ITO electrode for the quantitative analysis of H2O2 using a sensitive amperometric technique. Figure 9 shows the I-t curve of a PdNP-ITO electrode, which demonstrates a stepwise increase in the current responses for each addition of H2O2 in a 0.1 M PBS solution under stirring conditions. Dropwise addition of H2O2 was performed after each 50 s time interval, and the mixture was made homogeneous through continuous stirring. The initial potential was chosen at 0.0 V because at that potential, the electrode gave the highest chronoamperometric current response, as shown in Figure 9(A) (inset). Here, it can be seen that the amperometric responses were linear within the concentration range of 100–1800 nM, as shown in the calibration graph of Figure 9(B). The regression equation for the straight line was Jpeak (μA/cm2) = (-9.31e-6 [μA/ cm2]) + (760.84 [μA/ μM cm2]*[H2O2] [μM]). The sensitivity was found from the slope = 760.84 μA/ μM cm2. The limit of detection for H2O2 was calculated to be 40.8 nM using the formula ([3 x standard error]/slope).61 The performance of the proposed sensor in comparison with that of other established sensors is tabulated in Table 3. 3.10. Interference study. The presence of many coexisting species always introduces the risk of interference while using sensors to detect a particular analyte. Electrochemical sensors are no exception to this phenomenon. The electronic signals of several biomolecules are close to the reduction potential of H2O2 species. Some of these analytes were tested in the present sensor to determine whether they could interfere with the detection of H2O2. For this purpose, we used 0.2 μM H2O2 and 0.1 mM interfering species, namely dopamine (DA), uric acid (UA), glucose (GL) and ascorbic acid (AA). Figure 10 shows that the DA, UA and GL barely gave any interference signal at 0.0 V even when present in excessive amounts. AA gave a small peak-like signal that had almost the same size as that of the charging current signal; most importantly, AA did not interfere with the signal of H2O2 reduction on the PdNP-ITO electrode. 3.11. Stability test. The H2O2 electrochemical sensor was tested for experimental stability and reproducibility with the help of amperometric and CV techniques. Figure 11(A) shows the experimental stability data for a PdNP-ITO electrode in the presence and absence of H2O2 in the solution. A high concentration of H2O2 was used to check whether the electrode could remain responsive and operational in such a solution for a long time. The sensor exhibited reasonably



stable signals for both the 0.1 M PBS and 500 nM H2O2 solutions up to 2500 s. The PdNP-ITO electrodes were tested for storage stability. For this purpose, the electrodes were stored in an ambient environment for approximately 14 days after being used. After this time, the electrodes retained almost 95% of their initial signal. This finding suggested that the PdNP-ITO electrodes have good experimental and storage stabilities. The reproducibility of the modified ITO electrodes was evaluated with four different electrodes. A solution of 10 μM H2O2 was used for this test. Figure 11(B) shows the peak current signals of the four electrodes obtained from their CV analysis. The four electrodes showed almost identical responses towards the detection of H2O2. This observation suggested that the modified electrodes have almost similar features, and the AACVD technique can be used for producing large quantities of PdNP-ITO electrodes. 4. CONCLUSIONS An efficient and commercially available precursor, palladium(II) acetate, is used in aerosolassisted CVD to generate good crystalline metallic palladium thin films with high purity. This improved, facile and inexpensive synthetic strategy has proven very effective compared to the conventional CVD routes, which essentially required custom-made precursors and resulted in films that were impure and unsuitable for electrochemical applications. The completely characterized palladium film electrodes have been potentially applied for the electrochemical analysis of hydrogen peroxide. The electrochemical reduction mechanism of hydrogen peroxide was further analyzed considering the effects of both the solvent and molecular oxygen based on molecular dynamics simulation using Monte Carlo searches, which yielded similar results as those of the experimental analysis. The rate constant involved during electron transfer was calculated to be 1.98 s-1. The amperometric technique was utilized for the sensitive detection of H2O2. The LOD value was 40.8 nM with a linear range from 100 nM to 1.8 μM. The sensor also showed good reproducibility and stability along with high selectivity. These experimental findings suggest that PdNP-ITO electrodes with their ease of mass production through the AACVD process could be effectively utilized for H2O2 detection in real-life scenarios.


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Table 1. Energy parameters for different adsorbate-metal substrate configurations on five crystalline planes obtained for H2O2, H2O2 /H2O and H2O2 /H2O /O2 systems through adsorption locator simulations. For H2O2 /H2O and H2O2 /H2O /O2 systems, the configurations with the highest and lowest H2O2: dEad/dNi values are shown.



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Table 2. Intermolecular distances namely Pd-O, Pd-H and O-H over each simulated Pd crystal plane obtained after geometry optimization of adsorbate-metal substrate configuration having the highest H2O2: dEad/dNi energy for H2O2 /H2O and H2O2 /H2O /O2 systems (data analyzed from Figure 10). Simulated Pd

Pd-O distance

Pd-H distance

O-H bond length

plane

(Å)

(Å)

after adsorption

System

(Å) Pd (111) Pd (200) Pd (220) Pd (222) Pd (311)

3.03

2.65

0.978

H2O2 /H2O

2.858

2.624

0.976

H2O2 /H2O /O2

2.854

2.833

0.979

H2O2 /H2O

2.953

2.737

0.974

H2O2 /H2O /O2

2.954

2.708

0.978

H2O2 /H2O

2.874

3.249

0.975

H2O2 /H2O /O2

3.014

2.586

0.979

H2O2 /H2O

2.839

2.905

0.976

H2O2 /H2O /O2

3.157

3.089

0.979

H2O2 /H2O

2.876

3.158

0.974

H2O2 /H2O /O2


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Table 3. Comparison of the PdNP-ITO electrode for H2O2 sensing with other reported nonenzymatic H2O2 sensors. Modified electrode

Potential

Linear

Detection

Sensitivity

Referenc

(V)

range

limit

(µA/ μM

e

(µM)

(µM)

cm2)

AuNPs−N-GQDsa

−0.3

0.25−13327

0.12

0.186

[1]

Pd-NPs/BGFs/GCEb

−0.17

4–13,500

1.5

0.115

[33]

3DGN/PtNPc

0.45

0.167–7.486

0.125

−

[62]

Ni-Bi/CCd

−0.25

100−500

0.85

18.32

[63]

CdO/MWCNTse

−1.2

0.5–200

0.1

–

[64]

PtNPs-CDs/IL-GO/GCEf

−0.08

1−900

0.1

−

[65]

PdNP-CNF/CPEg

−0.2

0.2–20,000

0.2

4.15

[66]

PdNP-ITO

0.0

0.1−1.8

0.04

760.84

This work

a

AuNPs−N-GQDs: Au nanoparticles on nitrogen-doped graphene quantum dots

b

Pd-NPs/BGFs/GCE: Pd nanoparticles decorated bilayer graphenefilms on GCE

c 3DGN/PtNP:

three-dimensional graphene networks/Pt nanoparticle

d

Ni-Bi/CC: Nickel-borate nanoarray supported on carbon cloth

e

CdO/MWCNTs: Cd oxide electrodeposited multiwall carbon nanotubes on GCE

f

PtNPs-CDs/IL-GO/GCE: Pt nanoparticles-carbon quantum dots/ionic liquid functionalized

graphene oxide on GCE g

PdNP-CNF/CPE: palladinumnanoparticles-carbonnanofibers/carbonpasteelectrode



Scheme1. Probable reaction pathway for electrochemical reduction of H2O2 on PdNP-ITO electrode based on computational and experimental analysis.


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Figure 1. XRD pattern of Pd-thin film grown on ITO substrate at 425 ˚C from methanolic solution of Pd(OAc)2 in AACVD using compressed air as carrier gas.



Figure 2. FESEM surface images of palladium thin film grown on ITO substrate at 425 ˚C recorded at (A) Low (B) High resolutions, respectively. (C) EDX spectrum recorded from palladium thin film prepared on ITO substrate at 425 ˚C.


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Figure 3: High resolution XPS study of palladium thin film showing binding energies for the Pd(0) state



Figure 4. (A) CVs of 1 mM K3Fe(CN)6 in 1M KCl obtained at ITO (b) and PdNP-ITO (c) electrodes and blank 1.0 M KCl obtained at PdNP-ITO (a). Inset showing magnified CVs (a, b). (B) Nyquist plot of the same 1 mM K3Fe(CN)6 taken at bare ITO (a) and PdNP-ITO (b) electrodes with Randle’s circuit. Magnified (a) is shown in the inset of Figure (B). (C) CVs of PdNP-ITO electrode obtained for aerated 0.1 M PBS solution (pH 7) (a), deaerated 0.1 M PBS (pH 7) (b), H2O2 of concentrations of 10 µM (c), 20 µM (d) and 30 µM (e) in aerated 0.1 M PBS solution (pH 7). Inset shows CVs of bare ITO electrode for aerated blank 0.1 M PBS solution (f) and 1 mM H2O2 (g) in PBS of pH 7. (D) Consecutive CV responses of aerated blank solution at PdNP-ITO electrode.


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Figure 5. pH variation (pH: 4-9) CVs for 30 µM H2O2 in 0.1 M PBS solution of the PdNP-ITO electrode (A). Cathodic peak current (to the left) after baseline correction and peak potential (to the right) vs. the pH of the solution from the CVs (B).



Figure 6. Effect of scan rate variation: 5 to 700 (mV/s) (ar) at PdNP-ITO for 30 µM H2O2 in PBS of pH 7.0 through CV analysis (A) Calibration plot of Jpa (mA/cm2) vs. scan rate (mV/s). (B)The log Jpc vs. log ν plot. (C) The relation of peak potential shift with logarithm of scan rate. (D) The error bars of (B) and (D) indicates the deviation of y-axis values with respect to the linear fit (in percentage).


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Figure 7. The various regions of adsorptions on each of the crystalline Pd planes for three different systems with top and side views. Green spots- most probable and red spots- least probable regions for adsorption.



Figure 8. Preferable adsorption sites of H2O2 for the most stable H2O2 adsorption configurations (data from table 1) analyzed for H2O2 /H2O and H2O2 /H2O /O2 systems with top and side views.


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Figure 9. (A) Amperometric I-t curve of PdNP-ITO electrode obtained at 0.0 V potential for successive addition of H2O2 from 100–1800 nM. The inset is showing the chronoamperometric current response vs potential graph. (B) The calibration graph (Jpeak vs V) obtained for I-t curve with regression equation and coefficient (Statistical analysis from 4 different experiments were used for the error bars).



Figure 10. Studying the effect of dopamine (DA), uric acid (UA), glucose (GL) and ascorbic acid (AA) of 0.1 mM concentration on the PdNP-ITO electrode during detection of 0.2 μM H2O2 in 0.1 M PBS using chronoamperometry at 0.0 V.


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Figure 11. The amperometric response of the same PdNP-ITO electrode for 500 nM H2O2 and 0.1 M PBS for ≈ 2500 s (A). The peak currents of CV responses for 10 μM H2O2 at four different PdNP-ITO electrodes (B).



TOC


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Fabrication of nanostructured Pd thin films using aerosol-assisted

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