Lead

Article pubs.acs.org/JPCC

Elucidating the Cathodic Electrodeposition Mechanism of Lead/Lead Oxide Formation in Nitrate Solutions Lingcong Meng,† Jon Ustarroz,†,§ Mark E. Newton,‡ and Julie V. Macpherson*,† †

Department of Chemistry and ‡Physics, University of Warwick, Coventry, CV4 7AL, U.K. § Vrije Universiteit Brussel (VUB), Research Group Electrochemical and Surface Engineering (SURF), Pleinlaan 2, 1050 Brussels, Belgium S Supporting Information *

ABSTRACT: The production of crystalline lead oxide (PbO) structures, directly on the surface of an electrode in (nitrate) solution, via electrochemical deposition of lead ions (Pb2+), is unequivocally demonstrated and the formation mechanism elucidated. Boron doped diamond electrodes are used as the deposition platform. We show the effect of electrode potential, deposition time, presence of oxygen, and temperature on the formation process. At room temperature, under both deoxygenated and aerated conditions, high-resolution microscopy reveals a predominant nanoparticle (NP) morphology. In contrast, under laser-heated conditions, both NPs and half-hexagon shaped “plates” result. Transmission electron microscopy reveals these “plates” to be crystalline β-PbO. Plate prominence, under heated conditions, increases as the driving potential and deposition time is increased. By deoxygenating the solution and applying a deposition potential such that hydroxide ion (OH−) formation is negligible, only NPs are observed, which, from cyclic voltammetry data, are confirmed to be elemental Pb. We thus propose that Pb NPs and OH− play a crucial role in the PbO formation process. Electrodeposited Pb NPs catalyze OH− generation from either oxygen or nitrate reduction (oxygen reduction occurs at a less negative applied potential than nitrate reduction) driving the formation of lead hydroxide (Pb(OH)2) via a precipitation route. The Pb(OH)2 subsequently dehydrates to PbO, a process significantly accelerated by temperature. Hence, by controlling temperature, potential, and solution conditions, cathodic electrodeposition of Pb2+ can lead to the preferential formation of PbO crystalline structures on the electrode surface.

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reported applications of PbO in electrocatalysis13,14 and energy storage.15 Hence, understanding the electrodeposition mechanism of lead ions (Pb2+) in solution and whether hydroxides/ oxides of the metal can be formed using electrochemical methodologies alone is important for future use of this material. Boron doped diamond (BDD) is an excellent electrode material for electrodeposition studies, as it has an extended cathodic potential window, low background currents, and insensitivity to oxygen reduction.16 Diamond also has an exceptionally high heat diffusivity, a product of a high thermal conductivity of 600 W m−1 K−1 at 300 K,17 a low heat capacity (∼500 J kg−1 K−1), and a high oxidation temperature (∼700 °C) in air.18 Thus, it is also an extremely useful platform for investigating metal deposition under both ambient and elevated temperatures. Although temperature is a very important variable in electrochemical processes, interestingly, the vast majority of literature studies of metal electrodeposition take place at ambient temperature.19−21 In this work, we employ a pulsed IR laser setup with a freestanding BDD macrodisk electrode22,23 to investigate the

INTRODUCTION For metal ions in solution, depending on the conditions of the experiment, it is normally the elemental metal that is cathodically electrodeposited; however, metal hydroxide formation via a precipitation route is also possible via electrochemical means, under suitable driving potentials to produce hydroxide ions (OH−) in sufficient quantities.1,2 While metal hydroxide formation has been shown for the majority of metals, certain metals will dehydrate (a temperature dependent process) from the hydroxide form to the oxide. For example, zinc ions in the presence of electrochemically produced OH− convert to the more stable crystalline zinc oxide form in solution.3,4 In the case of lead, while anodic oxidation techniques are used commonly to produce lead dioxide, PbO25,6 (due to Pb2+ → Pb4+ + 2e−) and cathodic electrodeposition to produce Pb, there is, to the best of our knowledge, only one report on the electrochemical production of lead hydroxide7 and limited discussion on lead oxide, PbO.7−11 For the latter, PbO is thought to form only after cathodically deposited “wet” Pb is removed from solution into air or an oxygen-containing atmosphere,8,9,11 thus disputing formation of the material directly using electrochemical means. Although PbO2 has attracted more attention, particularly given its use as one component of the anode in the lead acid battery,12 there are © 2017 American Chemical Society

Received: January 30, 2017 Revised: February 25, 2017 Published: February 27, 2017 6835

DOI: 10.1021/acs.jpcc.7b00955 J. Phys. Chem. C 2017, 121, 6835−6843

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The Journal of Physical Chemistry C electrodeposition mechanism of Pb2+, in nitrate solution, as a function of temperature, potential, and oxygen presence. The use of a pulsed laser approach, which heats from the rear, i.e., not through solution, is preferred over conventional isothermal approaches such as the water bath, due to improved mass transport control in the system and the ability to reach over 100 °C (in water) without boiling.24 We thus determine whether it is possible to form PbO directly in solution using electrochemical methods, and elucidate the mechanism. Deposition morphologies are analyzed using field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). These measurements are complemented with electrochemical anodic oxidation (stripping) experiments.

mechanically removed using a blade and then dispersed in absolute ethanol (Fisher Scientific, 99.5%), followed by centrifugation for 15 min (Eppendorf, 12,000 rpm). A drop of solution (4 μL) from the bottom of the ethanol solution containing the sedimented deposits was placed onto a lacey carbon TEM grid (Agar Scientific) and left until the ethanol had evaporated. HR-TEM was conducted using a JEM 2100 TEM instrument (JEOL, LaB6 filament, operated at 200 kV) equipped with selected area electron diffraction (SAED). Nonisothermal Laser Heating Setup. An all diamond coplanar macroelectrode (conducting diamond sealed in insulating diamond) was employed, synthesized by Element Six (Harwell, U.K.) using a modified procedure to that employed for the fabrication of all diamond microelectrode arrays.22 The fabrication of the all diamond disk electrode, assessment of electrode quality, and setup for the laser heated cell are described in Supporting Information S2. The ON/OFF period (duty cycle) was fixed as 20 ms/180 ms unless stated otherwise. Electrochemical Measurements. All electrodeposition experiments were carried out in a three-electrode system using an Ivium Compactstat. Cyclic voltammetry (CV) was carried out to determine the reduction potential for Pb deposition under both ambient and laser heated, deoxygenated, and aerated conditions. Differential pulse voltammetry (DPV) was employed during Pb stripping, over the potential range from −1 to 0 V, with DPV conditions of 2 mV potential step, 0.01 s pulse width, and 50 mV pulse amplitude. To achieve a clean BDD surface in between measurements, the electrode was first anodically polarized at +1.4 V vs Ag/AgCl for 600 s and then polished with alumina powder (0.05 μm sized particles, micropolish, Buehler, German) on a water saturated polishing pad (Microcloth, Buehler). This procedure ensured no Pb remained on the electrode and provided a clean and reproducible surface for subsequent measurements.26 COMSOL Modeling. The temperature at the electrode/ electrolyte interface was simulated using COMSOL Multiphysics 4.3b (COMSOL, SE) finite element modeling (FEM) software, using the approach outlined previously25 but with modifications of dimensions to account for the geometry of the all diamond electrode employed (500 μm thickness and 4 mm in diameter) and the laser on/off pulse time. Supporting Information S3a shows FEM simulations of the temperature profile at the BDD/solution interface for the 1st and 25th laser pulses at time t = 20, 100, and 200 ms at a laser power density, Pd, of 1.2 kW cm−2, indicating the rapid temperature change at the electrode/electrolyte interface. Supporting Information S3b shows the corresponding experimentally measured electrode surface temperatures compared with simulation, for a laser heated time period of (i) 10 s and (ii) 300 s (laser pulsing on for 20 ms and off for 180 ms).

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EXPERIMENTAL SECTION Solutions. All reagents were prepared without further purification. All solutions were prepared from Milli-Q water (Millipore Corp., U.S.), with a resistivity of 18.2 MΩ cm at 25 °C. The supporting electrolyte was 0.1 M potassium nitrate (KNO3, Sigma-Aldrich, 99%) unless otherwise stated. Temperature calibration and open circuit potential (OCP) experiments were carried out in a solution containing 0.5 mM potassium ferricyanide (Fe(CN6)3−, Sigma-Aldrich, 99%) and 0.5 mM potassium hexacyanoferrate trihydrate (Fe(CN6)4−, SigmaAldrich, ≥98.5%), as described previously25 and in Supporting Information S1. Electrochemical cell characterization redox solutions contained 1 mM ruthenium hexaamine chloride (Ru(NH3)63+, Acros Organics, 98%). The Pb2+ containing solutions were prepared from lead nitrate (Pb(NO3)2, Aldrich, 99.99%), pH 6.4. Surface Characterization. Microscopic studies were carried out ex situ after electrodeposition. All deoxygenated experiments at ambient temperature (T = 22 ± 1 °C) were carried out in a nitrogen (N2) purged glovebox (Plas Laboratories, Lansing, MI, U.S.), where the solution was purged under N2 gas before transferral to the glovebox. All of the deoxygenated experiments under laser-heated conditions were carried out after purging the solution with continuous N2 gas for 30 min and flowing N2 gas during electrochemical measurements. For all microscopic studies, after electrodeposition, the electrode was rinsed with deoxygenated water and either dried in a N2 purged glovebox or blown dry using a N2 gas line, to avoid any oxygen contamination. It has been previously reported8,9,11 that, once “wet” electrodeposited Pb is dried in an oxygen free atmosphere, the dry Pb is largely unreactive and can be kept in the metallic form for long periods of time. Hence, our drying procedure aims to negate any possible conversion of Pb to PbO upon removal from solution and during the time scale of our microscopy measurements. FE-SEM (Zeiss Supra55VP) was employed to characterize the electrode both before and after electrodeposition. FE-SEM images were recorded using a secondary electron detector at 2 kV to highlight the metal deposits on the BDD surface.26 All AFM images were recorded ex situ in tapping mode using an Enviroscope AFM instrument (Bruker, U.K.). For each deposition, at least three images were recorded in different areas of the surface for both AFM and FE-SEM measurements. EDX spectra were recorded using the EDX unit equipped with the FE-SEM instrument at a working distance of 10 mm and accelerating voltage of 7.5 keV. For high resolution TEM (HR-TEM) measurements, deposits on the BDD surface were

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RESULTS AND DISCUSSION Cyclic Voltammetric Studies of Pb Deposition at Ambient Temperature: Effect of Dissolved Oxygen. Initial CV experiments under ambient temperature, to explore the redox characteristics of Pb2+ reduction and oxidative stripping, were carried out under both aerated and deoxygenated solution conditions. Figure 1a shows CVs for the reduction of 100 μM Pb2+ in deoxygenated (black) and aerated (red) solutions, running negatively from 0 to −1.4 V and back to 0 V. Under deoxygenation, a clear reduction wave is seen, with a half wave reduction potential, E1/2, at −0.76 V and charge, Q , for Pb2+ reduction of 1.49 μC (obtained by integrating between 6836


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The Journal of Physical Chemistry C

Figure 1. (a) CVs for the reduction of Pb2+ at room temperature at the BDD electrode in aerated (red) and deoxygenated (black) 0.1 M KNO3 solution containing 100 μM Pb(NO3)2. The scan rate was 50 mV s−1. (b) DPVs (pulse width 0.01 s, pulse amplitude 50 mV, step size 2 mV) at room temperature after Pb deposition at the BDD electrode in a deoxygenated solution at different deposition potentials: (i) −0.8 V (green); (ii) −1.0 V (blue); (iii) −1.2 V (red); (iv) −1.4 V (black). Inset: The same experiments but under aerated conditions. Pb deposition was carried out in 0.1 M KNO3 solution containing 100 μM Pb(NO3)2 for 30 s.

−0.6 and −1.2 V). On the backward scan, a well-defined stripping peak is observed at ca. −0.52 V, indicating anodic dissolution of Pb from the surface. Q for Pb oxidation is calculated as ∼1.42 μC, which is only slightly smaller than that obtained for the reduction of Pb2+. In contrast, under aerated conditions, the CV is markedly different; most noticeable is the appearance of a large second cathodic wave with E1/2 at ca. −0.87 V, due to the reduction of oxygen (ORR). While minimal sp2 content BDD (as employed herein) is known to be inert toward ORR under these solution pH conditions,27 during the cathodic scan, Pb is first electrodeposited, which can serve as an electrocatalyst for ORR.28 The ORR (eq 1) O2 (aq) + 2H 2O + 4e− → 4OH−(aq)

In all cases, a deposition time of 30 s was employed and the electrode was held at potentials of (i) −0.8 V, (ii) −1.0 V, (iii) −1.2 V, and (iv) −1.4 V. For the aerated case (inset to Figure 1b and Table 1), the stripping current (and the peak area) associated with the removal (oxidation) of electrodeposited Pb on the surface increases with increasing applied potential; however, the peak currents are ca. 3−4 times smaller than those under deoxygenated conditions (Figure 1b and Table 1). By deoxygenating, the same trend of increasing Pb dissolution peak current with applied potential is seen except at −1.4 V, the highest applied potential, where the amount of Pb stripped from the surface is actually smaller than that at both −1.0 and −1.2 V. We believe the observations in Figure 1 result from the generation of OH− from the ORR (eq 1) at freshly deposited Pb deposits, and at higher (more negative than −1.2 V) reductive potentials from the reduction of nitrate ions (eq 2),29 present in excess in the solution.

(1)

consumes dissolved oxygen and produces hydroxide ions (OH−). Also evident is the significantly reduced stripping peak for Pb oxidation (Q = 0.08 μC, ∼18× smaller than that under deoxygenated conditions), suggesting that the amount of Pb that can be anodically dissolved from the surface has been reduced. This was further explored by investigating the size of the Pb differential pulse stripping peak current (and the area under the stripping peak) as a function of electrode driving potential under both aerated and deoxygenated conditions, Figure 1b, for the same solution conditions as Figure 1a. The DPV peak current and area under the peak values are listed in Table 1.

NO3−(aq) + 7H 2O + 8e− → NH4 +(aq) + 10OH−(aq) (2)

This is supported by the CV data in Supporting Information S4 which shows that nitrate reduction on freshly deposited Pb begins at potentials more negative than −1.2 V. It is also likely in this potential region that water reduction contributes to the production of OH−. Electrogeneration of OH− is a common method in the literature to form metal hydroxides at electrode surfaces.1,2 Pb(OH)2 is, however, thought to be unstable, transforming, if it can, via a dehydration mechanism to the more stable PbO form, eq 3.9,30,31

Table 1. Pb DPV Stripping Peak Current (and the Area under the Stripping Peak) as a Function of Electrode Driving Potential after Pb Deposition under Both Aerated and Deoxygenated Conditions deposition potential (V vs Ag/AgCl) −0.8 −1.0 −1.2 −1.4 a

peak current for aerated solution (μA) 0.21 0.53 0.74 0.79

(0.15 (0.33 (0.51 (0.55

× × × ×

10−5)a 10−5) 10−5) 10−5)

Pb(OH)2 → PbO(s) + H 2O

Formation of Pb(OH)2 or PbO means that the amount of free Pb able to be anodically dissolved is now reduced. Thus, under aerated conditions, where OH− generation is possible via eqs 1 and 2 (the extent to which both equations apply depends on the applied electrode potential), less Pb on the surface would be expected than the deoxygenated case, where OH− formation can occur only by eq 2. In Figure 1b, the decreased stripping current (and peak area) under deoxygenated conditions at the highest applied potential, −1.4 V, is attributed to OH− formation via eq 2.

peak current for deoxygenated solution (μA) 0.88 2.39 2.60 1.96

(4.43 (1.03 (1.08 (0.95

× × × ×

(3)

10−6) 10−5) 10−5) 10−5)

Area under the stripping peak. 6837


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The Journal of Physical Chemistry C Cyclic Voltammetric Studies of Pb Deposition under Heated Conditions: Effect of Dissolved Oxygen. CVs for the reduction of Pb2+ under laser heated conditions (1.2 kW cm−2, pulsed laser on for 20 ms and off for 180 ms, corresponding to an initial average electrode/electrolyte interfacial temperature of 72.5 °C, Supporting Information S3b) in both deoxygenated (black) and aerated (red) solutions are shown in Figure 2.

formation of Pb(OH)2 or PbO as a consequence of the electrocatalytic reduction of oxygen, which in turn results in a smaller amount of elemental Pb able to be anodically dissolved. Microscopy Analysis for Deposition under Heated Conditions: Effect of Dissolved Oxygen. Ex situ FE-SEM and TEM measurements were carried out to study the morphology and crystallinity of the Pb deposits on BDD after deposition under both deoxygenated and aerated conditions at elevated temperature and under two different driving potentials (a) −1.0 V vs Ag/AgCl, where only ORR takes place, and (b) −1.4 V vs Ag/AgCl, where both ORR and nitrate ion reduction (eqs 1 and 2) are possible. Elevated temperature conditions were employed to enhance the formation of possible PbO structures. Note, for all experiments, immediately after electrodeposition, the BDD electrode was rinsed with deoxygenated water and then dried immediately in N2 atmosphere. This was to avoid any possible conversion of freshly deposited Pb into PbO upon removal from solution.8 A Pb2+ concentration of 50 μM and deposition time of 300 s were chosen in order to produce sufficient deposits on the surface for analysis. Initial experiments (Figure 3) were carried out under laser heated conditions (Pd of 1.2 kW cm−2, initial surface temperature 72.5 °C, laser on for 20 ms and off for 180 ms) at (a) −1.0 V and (b) −1.4 V in the (i) absence and (ii) presence of dissolved oxygen in solution. In the absence of oxygen and at a potential where OH− formation is minimal, i.e., applied potential of −1 V, Figure 3ai shows that the BDD surface is dominated by Pb NPs with many of them coalescing to reduce surface energy.33 Under these conditions, it is difficult to extract a representative NP density. No preference for deposition on higher doped facets of the polycrystalline BDD surface was observed. In contrast, under aerated conditions and under potential conditions where eq 1 contributes, the FE-SEM image shows a very different surface morphology (Figure 3aii). The BDD surface contains both thin, half hexagonal flat “plate” structures and NPs. The “plates” have a density of 0.4 plates/μm2 (collected from an average of three images) with a base diameter of 0.5−1.1 μm, while the NPs have a density of ca. 8.2 NPs/μm2, much lower than for deoxygenated conditions (Figure 3ai). A few of the “plates” stand vertically from the surface (highlighted by a red circle); however, the majority lie flat. We believe the “plates” grow upward from the surface (vide inf ra) and then fall flat either after reaching a certain size or as a result of the drying process in the N2 atmosphere prior to imaging.34 By driving the deposition potential at −1.4 V, where eq 2 contributes, under deoxygenated conditions, Figure 3bi again shows the presence of “plates” (in contrast to Figure 3ai). The hexagonal “plates” have become more hemispherical in shape and have a density of 0.7 plates/μm2 with a base diameter in the range 0.3−1.1 μm. Pb NPs are also observed with a density of ∼20 NPs/μm2. The “plates” thus appear to correlate with the ability of the electrode to electrogenerate OH− either from ORR (eq 1) and/or nitrate reduction (eq 2). Under aerated conditions, at −1.4 V, where OH− production will be increased due to eqs 1 and 2 both operating, both the density and base diameter of the “plate” structures increase (Figure 3bii) with a density of ca. 1.2 plates/μm2 and base diameter in the range 0.8−2 μm. The “plates” also seem to show 3D growth from the center region of the flat hemisphere. A NP density of ∼25 NPs/μm2 was also observed, similar to that obtained under deoxygenated conditions (Figure 3bi).

Figure 2. CV for the reduction of Pb2+ at 72.5 °C (laser heated condition: 1.2 kW cm−2, pulsed laser on for 20 ms and off for 180 ms) at the BDD electrode in aerated (red) and deoxygenated (black) 100 μM Pb(NO3)2 solution in 0.1 M KNO3. The scan rate was 50 mV s−1.

The CVs were run negatively from 0 to −1 V and back to 0 V (to explore ORR only, eq 1). The pulsed response in the current is due to the pulsed laser switching on and off during heating. Under deoxygenated conditions, during the cathodic scan, a diffusion limited current for the reduction of Pb2+ is obtained (∼2.1 μA). The current magnitude is greatly increased (by nearly an order of magnitude) compared to that under deoxygenated conditions at ambient temperature, indicating an enhancement in the mass transport rate at elevated temperature, as expected. E1/2 is −0.69 V, which shows a positive shift of 70 mV compared to that recorded under deoxygenated ambient temperature conditions. The temperature coefficient for the Pb2+/Pb couple is reported as −0.359 mV K−1,32 which, from a thermodynamic perspective only, indicates a negative shift of the Pb2+ reduction potential at elevated temperature. Therefore, our observation of a positive shift suggests increased ET kinetics for Pb deposition under high temperature dominates over thermodynamic effects. In the anodic sweep, a Pb stripping peak at ca. −0.51 V is observed and the charge passed for Pb oxidation is 3.3 μC which is more than twice the value obtained under deoxygenated ambient temperature. This further confirms that more Pb has been deposited at elevated temperature. Similar to the CV recorded at ambient temperature (under deoxygenated conditions), Q for Pb stripping (3.3 μC) is only slightly smaller than that obtained from Pb2+ reduction (3.5 μC, calculated by integrating between −0.6 and −1.0 V). The emergence of the ORR is again obvious in the CV under heated, aerated conditions, where the ORR current is ca. 4 times larger than that at room temperature. The E1/2 for ORR is ca. −0.73 V (compared to −0.87 V under ambient temperature), indicating that the ET kinetics for ORR is more facile at elevated temperature. The stripping peak for Pb oxidation is again reduced greatly in magnitude under aerated, heated conditions with Q calculated to be 0.13 μC, which is ∼24 times smaller than that obtained under deoxygenated, heated conditions. This points again toward the 6838


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Figure 3. Typical secondary electron FE-SEM images after laser heated deposition at (a) −1 V and (b) −1.4 V vs Ag/AgCl in (i) deoxygenated and (ii) aerated Pb2+ solutions. The red circle in part aii highlights the “standing up” structure of the “plate”. Laser heating conditions: Pd of 1.2 kW cm−2, 72.5 °C, laser on for 20 ms and off for 180 ms. Deposition was carried out in 0.1 M KNO3 containing 50 μM Pb(NO3)2 for 300 s.

surface (red) shows a large C peak at 0.277 keV, whereas the spectrum collected on the plate shows O and Pb peaks at 0.523 and 2.342 keV, respectively, which indicates the presence of both Pb and O in the “plate” structures. TEM analysis of an individual “plate” deposited under the same conditions as Figures 3aii and 4 is shown in Figure 5, where “plate” structures with a base diameter ranging from 0.7 to 1.0 μm were observed. The corresponding selected area electron diffraction (SAED) pattern in Figure 5b shows a clear spot pattern, indicating the single crystalline structure of the “plate”, as evidenced by HR-TEM images in Figure 5c. The calculated d-spacings from the SAED pattern were consistent with standard values (Joint Committee on Powder Diffraction Standards (JCPDS) card, No. 76-1796) for orthorhombic β-PbO. Spots in the SAED pattern were indexed according to the JCPDS card of the orthorhombic β-PbO, showing the (200) and (210) facets. Thus, both EDX and TEM prove unequivocally that these “plates” are crystalline PbO structures. It is not possible to analyze the NPs in this way due to the drifting effect when collecting the EDX spot data, and the very low density and small size of NPs presented in the TEM grid. However, as the stripping peaks in Figures 1 and 2 suggest that some of the deposits must remain in the Pb form, we speculate that the NP structures are likely to be electrodeposited elemental Pb. Note, it was not possible to employ X-ray diffraction on this structure due to the low density of “plates” on the electrode surface. The formation mechanism of the crystalline PbO “plates” is worth investigating further. Figure 6a shows a typical high resolution FE-SEM image of the “plates” formed after electrodeposition at −1 V for 300 s in 50 μM Pb2+ aerated solution under laser heated conditions (Pd of 1.2 kW cm−2, 72.5 °C, laser on for 20 ms and off for 180 ms, Figure 3aii). The “plates” in these images lie flat on the surface, having fallen flat, and three zoomed-in images of individual “plates” are shown in Figure 6a. Clearly evident are hemispherical voids at the base of

In order to further confirm the composition and crystalline structure of the “plates”, EDX and TEM analysis were carried out ex situ after electrodeposition at −1 V for 300 s in 50 μM Pb2+ aerated solution under laser heated conditions (Pd of 1.2 kW cm−2, 72.5 °C, laser on for 20 ms and off for 180 ms), i.e., Figure 3aii. EDX spectra were collected from both the horizontal lying “plates” and the bare BDD surface. As the “plate” structure was not thick enough, the spectrum collected from a single plate was dominated by the carbon signal from the BDD, so overlapping “plates” were chosen for analysis. Figure 4 shows the EDX spectra collected on the overlap region

Figure 4. EDX spectra taken at a spot on the overlap between plate structures (black) and at a spot on the background BDD surface (red). The inset FE-SEM image highlights the spots where the EDX spectra were collected. Spectra and FE-SEM images were collected after electrodeposition at −1 V for 300 s in 50 μM Pb2+ aerated solution under laser heating conditions: Pd of 1.2 kW cm−2, 72.5 °C, laser on for 20 ms and off for 180 ms.

of “plates” (black cross) and the background BDD surface (red cross). The inset FE-SEM image shows the spots for spectra collection. The spectrum from the background BDD 6839


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Figure 5. Typical (a) HR-TEM image and (b) SAED pattern of the “plate” structure. (c) HR-TEM image showing the crystallinity of the “plate” structure. The TEM image and SAED pattern were collected after electrodeposition at −1 V for 300 s in 50 μM Pb2+ aerated solution under laser heating conditions: Pd of 1.2 kW cm−2, 72.5 °C, laser on for 20 ms and off for 180 ms.

Pb and PbO can be formed by direct electrochemical means, at the surface of an electrode. Our observations contrast with the current literature which postulates PbO forms only after removal of “wet” electrodeposited Pb from solution and exposure to an oxygen containing atmosphere,8,9 and therefore does not result from Pb2+ in the presence of electrochemically generated hydroxide ions.8 Under heated conditions, the formation of the β-PbO “plates” is clearly evident. As dehydration is involved, it is not surprising that locally heating the electrode/electrolyte interface expedites the formation of these structures. In contrast, when working at room temperature and even using parameters where OH− formation is exacerbated, i.e., in oxygenated solution and −1.4 V driving potential (0.1 M KNO3 containing 50 μM Pb2+, 300 s deposition time), ex situ AFM images showed only NPs on the BDD surface (Supporting Information S5). The corresponding stripping voltammetry again confirmed Pb dissolution, indicating that these are likely to be NPs in elemental Pb form. To investigate whether it was possible to form “plate” structures under ambient temperature, the deposition time was increased from 300 to 1800 s. Experiments were carried out in both deoxygenated and aerated solutions at −1.4 V, as shown in Figure 7a and b, respectively. Under deoxygenated conditions, both Pb NPs and a few PbO “plates” are present, with a NP density of 22 NPs/μm2 and “plate” density of 0.01 plates/μm2 (base diameter of 1−5 μm). This indicates that “plate” formation is a much slower process under ambient temperature. Under aeration, the density of NPs is approximately the same, ∼23 NPs/μm2, and the “plates” have increased in density ∼0.03 plates/μm2 while retaining a similar base diameter (1−5 μm). Finally, time-dependent microscopic observations of the electrode surface were employed in order to follow the evolution of the morphology change of the structures on the BDD electrode. Deposition was carried out in an aerated 0.1 M KNO3 solution containing 50 μM Pb2+ under laser heated conditions (Pd of 1.2 kW cm−2, 72.5 °C, laser on/off: 20 ms/180 ms) at a deposition of −1.4 V (Figure 3bii), to enhance the formation of the PbO structures, for different times of (a) 10 s, (b) 50 s, (c) 100 s, and (d) 300 s. The resulting ex situ microscopic (FE-SEM and AFM) images are shown in Figure 8. After 10 s, the AFM image in Figure 8aii shows only NPs with a mean height of ca. 4.1 and 10−20 nm in diameter. Note height measurements in AFM are more accurate than lateral measurements, as the latter may overestimate the true particle width, due to tip convolution affects. As the deposition time

Figure 6. (a) Zoomed-in secondary electron FE-SEM images highlighting the “plate” structures. The scale bar in part a is 400 nm. (b) Proposed mechanism for the formation of PbO “plate” structures.

the “plates”, which we believe could highlight the position of Pb NPs responsible for the formation of the PbO crystal. As the PbO crystal is not the same structure as the Pb NPs, the crystal can separate away from the Pb NPs and fall flat on the surface. We thus believe that, at the initial electrodeposition stage, Pb NPs form on the electrode surface, which are capable of catalyzing the turnover of oxygen or nitrate (applied potential dependent process) to form OH−. Those that are suitably spaced, sized, and/or of the correct morphology to promote efficient OH− production act as catalytic and nucleation sites for local Pb(OH)2 formation, due to a precipitation reaction between OH− ions, which diffuse hemispherically away from the NP, and Pb2+ present in solution. The Pb(OH)2 rapidly transforms to PbO, resulting in the “plate” crystal growth observed, as shown schematically in Figure 6b. Interestingly, Pb NPs have been observed at the base of PbO platelets before.11 Our electrochemical (CV and stripping voltammetry) and microscopic data thus support the theory that both elemental 6840


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Figure 7. Typical secondary electron FE-SEM images after room temperature Pb deposition at −1.4 V vs Ag/AgCl in (a) deoxygenated and (b) aerated solution. Deposition was carried out in 0.1 M KNO3 containing 50 μM Pb(NO3)2 for 1800 s.

Figure 8. Typical secondary electron (i) FE-SEM and (ii) AFM images of the BDD surface after laser heated Pb deposition at −1.4 V vs Ag/AgCl in 0.1 M KNO3 solution containing 50 μM Pb(NO3)2 for different times: (a) 10 s, (b) 50 s, (c) 100 s, and (d) 300 s. Parts biii and ciii are the histogram (data collected from an average of three AFM images) of NPs on the BDD surface after electrodeposition for 50 and 100 s, respectively. The insets in parts biii and ciii are the cross-sectional plot of the “plate” structure highlighted in parts bii and cii, respectively. Laser heating conditions: Pd of 1.2 kW cm−2, 72.5 °C, laser on for 20 ms and off for 180 ms.

ambient temperature. The FE-SEM image again shows both the “upright” and flat “plate” structures. The plate density is ca. 0.08 plates/μm2 with a base diameter ranging from 300 to 500 nm.

increases to 50 s, both FE-SEM and AFM images confirm the presence of PbO “plates”, showing the crystallization process operates on a relatively fast time scale compared to that under 6841


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The Journal of Physical Chemistry C A cross-sectional view of the “plate” (highlighted in the AFM image in Figure 8bii) is plotted (inset in Figure 8biii), showing the thickness of the structure is ca. 14.8 nm, and the “plate” surface is relatively flat (1−2 nm roughness). A histogram of the NP distribution in Figure 8biii shows that the NPs have a mean diameter of 4.3 nm, which is similar to the size obtained at the time of 10 s deposition. As the deposition time increases to 100 s, the “plates” increase in size with a base diameter ranging from 500 to 800 nm and a density rising to 0.44 plates/μm2. Some of the “plate” structures retain a similar thickness (ca. 15.1 nm) to those grown for 50 s of deposition (inset in Figure 8ciii, black). However, others show 3D growth from the center of the “plate” structure, as can be clearly seen in the cross-sectional plot (inset in Figure 8ciii, red). A histogram of the NP distribution in Figure 8ciii shows that the mean diameter of NPs is ca. 5.2 nm, and the number of NPs has increased significantly. PbO “plate” formation is dominant after 300 s of deposition, as shown in Figure 8d. In this case, “plates” have a base diameter of 0.8−2 μm, overlapping with each other substantially, and showing significant growth over the surface of the “plates”. The move from a defined half hexagonal structure to hemispherical to 3D growth from the planar surface is interesting. The shape of the plate suggests growth takes place preferentially at the thin edges. We can speculate that once the crystalline structure reaches a certain size it may fall over. Once this happens three-dimensional supply of Pb2+ ions is now constrained and preferential growth can no longer proceed as it did. Instead, nucleation takes place on the surface of the “plate”.

the elemental Pb form and capable of dissolution from the surface at the expected Pb/Pb2+ dissolution potential. The reason why some NPs stay as Pb nuclei whereas others may promote the formation of PbO “plates” is still not fully understood. Importantly, the results presented confirm that the formation of crystalline PbO structures directly on an electrode surface is possible via cathodic electrodeposition of Pb and electrochemical generation of hydroxide. By controlling the temperature, electrode potential, dissolved oxygen content, and deposition time, control of the morphology of the resulting structures can be achieved. Finally, the nanoscale thin crystalline PbO “plate” structures lend themselves perfectly for further investigation using high resolution electrochemical imaging techniques,35 to assess, e.g., electrocatalytic activity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00955. S1: Calibration procedure for measuring the temperature at the BDD/electrolyte interface. S2: (a) All diamond macrodisk electrode fabrication and assessment of electrode quality and (b) setup for laser heating experiments. S3: (a) FEM simulations of the r, z temperature profile for the 1st and 25th pulse at t = 20, 100, and 200 ms at Pd of 1.2 kW cm−2. (b) Experimentally recorded BDD electrode/electrolyte interfacial temperatures under nonisothermal heating conditions. S4: CV data showing the reduction characteristics for freshly deposited Pb (deoxygenated at −0.8 V for 300 s in 100 μM Pb2+) in a 0.1 M KNO3 solution. S5: Shorter time scale (300 s) Pb deposition under ambient, aerated conditions. (PDF)

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CONCLUSIONS A systematic study was undertaken to determine if and under what conditions it was possible to form PbO structures directly on an electrode surface via electrochemical reduction of Pb2+. An all diamond coplanar BDD macrodisk electrode was utilized as the substrate electrode, and experiments were conducted under both ambient temperature and laser heated conditions (initial interfacial temperature = 72.5 °C), in the absence and presence of dissolved oxygen and under different applied potential conditions. Two electrode potentials were employed, −1.0 V, where ORR is possible on freshly deposited Pb, and −1.4 V, where both ORR and nitrate reduction occur. Both electrochemical reduction routes result in hydroxide formation. Under aerated conditions, a smaller Pb stripping peak was obtained than that under deoxygenated conditions. The size of the stripping peak under both conditions increased when laser heating the interface. Under heating, FE-SEM and AFM revealed the presence of both NPs and “plates” on the electrode surface. “Plates” were absent when the solution was deoxygenated and an applied potential of −1 V was employed. Under ambient temperature irrespective of the applied potential, no “plates” were observed, unless significantly longer deposition times were implemented in the presence of oxygen. EDX and TEM studies revealed the “plates” to be crystalline β-PbO. PbO was thus proposed to form directly in solution from freshly deposited Pb NPs acting as catalytic nuclei for the localized production of OH− (the BDD surface itself does not turn over ORR). The OH− in turn reacts with Pb2+ via a precipitation mechanism to form unstable Pb(OH)2 which dehydrates quickly to PbO, a process highly accelerated under laser heated conditions. The fact that Pb stripping was observed from the BDD surface in the presence of NPs and PbO “plates” suggested that at least some (if not all) of the NPs were still in

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jon Ustarroz: 0000-0003-0166-6915 Julie V. Macpherson: 0000-0002-4249-8383 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.V.M. thanks the Royal Society for an Industry Fellowship. L.M. would like to thank the University of Warwick for a Chancellor’s International Scholarship and Dr. Maxim Joseph for COMSOL modelling assistance. We all thank Element Six for synthesizing the all diamond macroelectrode. J.U. acknowledges funding from the Fonds Wetenschappelijk Onderzoek in Flanders (FWO, postdoctoral grant 12I7816N). We thank Dr. James Iacobini (Warwick Chemistry) for setting up the laser cell and Mr. Steve Hindmarsh (Warwick Physics) for the TEM measurements.

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