Overpotential Deposition Research Articles

Initial stages of silver electrodeposition on single crystal electrodes from ionic liquids

We present a comprehensive study on the initial stages of silver electrodeposition on Pt (111), Au (111), and Au (100) single crystal surfaces in low-viscosity ionic liquids (ILs) containing dicyanamide anions: 1-butyl-1-methylpyrrolidinium dicyanamide [BMP][DCA] and 1-butyl-3-methylimidazolium dicyanamide [BMIm][DCA]. Electrochemical methods in combination with in situ scanning tunneling microscopy (STM) and ex situ atomic force microscopy (AFM) are employed to explore the Ag underpotential (upd) and overpotential (opd) deposition processes as well as the stability of the single crystal electrode surfaces in the absence of Ag+ ions. The substrate material is shown to significantly affect the mechanism of Ag deposit nucleation and growth in the ILs. While no Ag upd is detected on Pt (111), the formation of a Ag upd monolayer on a Au (111) electrode in both ILs is clearly visualized by in situ STM. The Ag adlayer formation on the Au electrodes in the underpotential regime facilitates Ag opd, which starts on Au (111) and Au (100) at much less negative potentials than on Pt (111). There is an excellent agreement between the electrochemical (voltammetry and chronoamperometry), AFM and STM data, demonstrating the nucleation and growth of individual Ag crystallites on Pt (111) according to the Volmer–Weber mechanism and the layer–by–layer growth of Ag deposit on Au (111) and Au (100). Only at high overpotentials, the Ag growth on the gold electrodes switches to the Stranski-Krastanov mode involving the appearance of 3D crystallites.

Voltammetric Study of Tin Electrodeposition on Polycrystalline Gold from Sulfuric and Methanesulfonic Acid

In this work, we have studied tin electrodeposition on polycrystalline gold electrodes from two different supporting electrolytes: sulfuric acid (SA) and methanesulfonic acid (MSA), both of them commonly used in the industry. This work aims to understand the effect of the different electrolyte anions on the deposition process. We show at least three different tin deposition mechanisms on gold: irreversible adsorption, underpotential deposition, and overpotential (bulk) deposition. Underpotential deposition leads to the formation of a layer of tin in SA and MSA with a coverage around ML (monolayer) and ML, respectively. The UPD Sn layer is however somewhat uncharacteristic as it is associated with island formation and surface alloying. Cyclic voltammograms in an extended potential range showed five distinct peaks: two cathodic peaks associated with tin underpotential and overpotential deposition, and three main anodic peaks, corresponding to the oxidation of the bulk Sn, of the AuSn intermetallic layer, and of the adsorbed Sn(II) to Sn(IV). Both voltammetric and rotating disk electrode measurements show that the kinetics of tin electrodeposition in MSA is slower than in SA, which we ascribe to Sn-MSA complex formation in solution. Slow Sn deposition in MSA promotes AuSn formation, in contrast to SA in which bulk tin deposition is more prominent. Complete Levich-type mass transport control of tin deposition in SA and MSA was only reached at low scan rate due to concurrent HER on the uncovered gold surface during the deposition process at higher scan rates. An unexpected surface-confined passivation process is observed in both electrolytes.

Effect of Temperature on Li Electrodeposition Behavior in Room-Temperature Ionic Liquids Comprising Quaternary Ammonium Cation

Secondary batteries with Li metal anodes have attracted considerable research attention as power sources for electric vehicles. However, the widespread application of these batteries is hindered by short circuiting due to Li dendrite formation, which highlights the need for a more detailed mechanistic understanding of this process. Typical Li-metal-based secondary batteries contain organic solvents, which are unstable under harsh cathodic conditions (i.e., at the Li redox potential), and therefore, decompose to form surface films. Conversely, room-temperature ionic liquids comprising quaternary ammonium cations (QA-RTILs) are very stable under these conditions and are thus considered promising alternatives to the commonly used carbonate solvents, as has been demonstrated in our previous studies. Herein, we investigate the effect of temperature on Li electrodeposition behavior in a selected QA-RTIL, revealing that this behavior is mainly determined based on the deposition overpotential and the Li diffusion coefficient.

Understanding the Barrier Layer Formed via Adding BTAH in Copper Film Electrodeposition

The influence of surface adsorption of benzotriazole (BTAH) and of chloride ions (Cl−) on the kinetics of copper electrodeposition/dissolution in copper sulfate solutions and on copper deposit characteristics have been investigated using electrochemical quartz crystal microbalance (EQCM) combined with cyclic voltammetry (CV). The addition of BTAH alone increases the overpotential of copper deposition, whereas a Cu(I)BTA complex forms at potentials higher than 0.08 V (vs. SCE) accompanied with the occurrence of copper anodic dissolution. With simultaneous addition of BTAH and Cl−, surface adsorption of Cl− competes with that of BTAH during the initial stage of copper nucleation. Different cuprous reaction intermediates form in the examined potential range −0.4 to 0.3 V (vs. SCE), which partly eliminate the favorable effect of BTAH on the deposited copper. A BTAH-containing adsorbed layer formed on the matte side of electrodeposited copper film in the presence of BTAH with or without Cl−, exhibiting a barrier surface property and an improved corrosion resistance compared with the copper film electrodeposited in the electrolyte without addition of BTAH.

Electrochemically induced highly ion conductive porous scaffolds to stabilize lithium deposition for lithium metal anodes

The electrochemically-induced lithiophilic Li–Zn alloy scaffold with high ionic conductivity, together with the Li2O passivated surface, can reduce the nucleation overpotential of Li deposition, enhance the Li+ ions diffusion and guide the homogeneous nucleation of Li, and thus suppressing the lithium dendrite growth.

Electrochemical aspects of tin electrodeposition on copper in acid solutions

The electrodeposition of tin on copper electrodes was studied in sulphuric acid solutions in the 283–338 K range by employing electrochemical techniques, including electrochemical quartz crystal microbalance, and scanning electron microscopy imaging. The voltammetric profiles show the presence of both, underpotential and overpotential electrodeposition of tin. While the first process (upd) involves surface controlled reactions, the latter (opd) is under mass transport control. SEM imaging shows the formation of nanoparticles under upd conditions and larger ones in the opd potential region. Analysis of chronoamperometric response indicates an instantaneous nucleation and 2D growth coupled to an adsorption process during the early stages of underpotential deposition. Conversely, data obtained in the overpotential deposition range is consistent with the occurrence of a progressive nucleation and 3D growth under diffusion control. Electrochemical impedance experiments are characterised by Nyquist plots that exhibit two capacitive time constants and a single capacitive time constant followed by a Warburg contribution in the upd and opd potential ranges, respectively. Rotating ring-disc voltammograms display current peaks in the corresponding ring profiles associated with the formation of adsorbates on the disc during the underpotential deposition. From rotating disc data the diffusion coefficient of Sn(II) ions at the different temperatures and the corresponding diffusion activation energy were determined. A reaction scheme for the electrochemical behaviour of tin on copper electrodes in the range between the threshold potentials for the copper electrodissolution and the hydrogen evolution reactions is proposed. Mass transport of soluble species, formation of weakly and strongly adsorbed species on copper and the diffusion of tin atoms into the copper lattice are considered.

Low-Loading of Pt Nanoparticles on 3D Carbon Foam Support for Highly Active and Stable Hydrogen Production.

Minimizing Pt loading is essential for designing cost-effective water electrolyzers and fuel cell systems. Recently, three-dimensional macroporous open-pore electroactive supports have been widely regarded as promising architectures to lower loading amounts of Pt because of its large surface area, easy electrolyte access to Pt sites, and superior gas diffusion properties to accelerate diffusion of H2 bubbles from the Pt surface. However, studies to date have mainly focused on Pt loading on Ni-based 3D open pore supports which are prone to corrosion in highly acidic and alkaline conditions. Here, we investigate electrodeposition of Pt nanoparticles in low-loading amounts on commercially available, inexpensive, 3D carbon foam (CF) support and benchmark their activity and stability for electrolytic hydrogen production. We first elucidate the effect of deposition potential on the Pt nanoparticle size, density and subsequently its coverage on 3D CF. Analysis of the Pt deposit using scanning electron microscopy images reveal that for a given deposition charge density, the particle density increases (with cubic power) and particle size decreases (linearly) with deposition overpotential. A deposition potential of −0.4 V vs. standard calomel electrode (SCE) provided the highest Pt nanoparticle coverage on 3D CF surface. Different loading amounts of Pt (0.0075–0.1 mgPt/cm2) was then deposited on CF at −0.4 V vs. SCE and subsequently studied for its hydrogen evolution reaction (HER) activity in acidic 1M H2SO4 electrolyte. The Pt/CF catalyst with loading amounts as low as 0.06 mgPt/cm2 (10-fold lower than state-of-the-art commercial electrodes) demonstrated a mass activity of 2.6 ampere per milligram Pt at 200 mV overpotential, nearly 6-fold greater than the commercial Pt/C catalyst tested under similar conditions. The 3D architectured electrode also demonstrated excellent stability, showing <7% loss in activity after 60 h of constant current water electrolysis at 100 mA/cm2.

Dendrite-free lithium electrode cycling via controlled nucleation in low LiPF6 concentration electrolytes

Lithium metal electrodes are not widely used in rechargeable batteries as dendritic lithium growth and electrolyte reactions raise serious stability and safety concerns. In this study, we show that reproducible two-dimensional lithium deposition can be realized using a lithium salt concentration of 0.020 M, an added supporting salt, and a short lithium nucleation pulse. This approach, which is common in electrodeposition of metals, increases the lithium nuclei density on the electrode surface and decreases the extent of Li+ migration favoring dendritic lithium growth. Contrary to common belief, ascribing the dendrite problem to heterogeneous lithium nucleation due to an unstable solid electrolyte interphase layer, we show that the main lithium deposition problem stems from the difficulty to obtain two-dimensional deposition at the low lithium deposition overpotentials encountered in conventional high-lithium concentration electrolytes. The present results hence clearly demonstrate that two-dimensional lithium deposition can be realized in lithium-metal-based batteries.

Oxygen-rich carbon nanotube networks for enhanced lithium metal anode

Developing safe and efficient lithium (Li)-metal anode is promising for the next-generation rechargeable batteries, such as Li–S and Li–O2 batteries. The design of lithium plating host with unique nanostructure and lithiophilic surface has been proven to be one of the most efficient strategies to control Li plating/stripping process and inhibit lithium dendrite growth. Here we first report the fabrication of well-defined oxygen-rich carbon nanotube (O-CNT) network via a facile and scalable ultrafast “barbecue” approach. Remarkably, the as-obtained three-dimensional (3D) O-CNT network electrode can serve as a promising Li-metal host with a low Li deposition overpotential and a flat voltage profile at various current densities, as well as long-term durability. The DFT calculation and experimental results further prove that the ketonic (C=O) group on CNT surface acts as efficient lithiophilic site to guide Li-metal nucleation and growth. In addition, the 3D porous network structure of O-CNT network with high conductivity facilitates electron and Li ions transport. Thus, this work provides a new strategy to construct high-performed Li-metal anodes toward high-performance Li-based batteries.

Study of photoelectrochemical conductivity mechanism and electrochemical impedance spectroscopy of bulk CuInTe2 – Electrolyte interface

CuInTe2 (CIT) thin films were electrochemically deposited in an aqueous electrolyte on fluorine doped tin oxide (FTO) coated glass substrates for potentials ranging from −0.6 V to −0.9 V at pH 4. Films were annealed at 400 °C for 15 min in air ambient. Both as-deposited and annealed layers were characterized by various characterization techniques. The conductivity type of the CIT layers was studied by photoelectrochemical (PEC) response and solid–electrolyte interface by impedance spectroscopy. As-deposited samples confirmed amorphous nature of CIT in contrast to that of polycrystalline films obtained upon annealing. Three prominent reflections (1 1 2), (2 2 0)/(2 0 4) and (3 1 2)/(1 1 6) of chalcopyrite CIT were obtained upon annealing. The optical band gap values ∼0.90 eV and 1.08 eV and ∼0.88 eV and 1.01 eV were estimated for as-deposited and annealed CIT layers deposited at −0.7 V and −0.8 V respectively. The most prominent A1 mode observed at 123 cm−1 in the Raman spectra of chalcopyrite CIT was shifted towards lower wavelengths due to the development of tensile strain. Very compact, well adhesive and void free globular layers were deposited at pH 4. The indium content was found to be increased with increasing the deposition potential which agrees well with the overpotential deposition of indium. Photoelectrochemical (PEC) study confirms the growth of p-type CIT layers. The negative resistance at higher frequency domain (3 MHz to 10 kHz) from electrochemical impedance spectroscopy (EIS) confirms the power/energy giving nature of CIT/electrolyte interface at higher frequency and diffusion mechanism of ionic species dominate at lower frequency region.

Anion effects on the interfacial alloying in successively electrodeposited Cu and Au ultrathin films

The alloying between successively deposited ultrathin (∼10 nm) films of Cu and Au is explored. Au is deposited on Cu films via conventional overpotential deposition (OPD) and mediated methods in chloride containing electrolyte (CCE), and via OPD in thiosulfate containing electrolyte (TCE). It is determined via electrochemical means that Au deposition on Cu results in alloy formation in all cases, justified by the exothermic enthalpy of Au-Cu mixing. Deposition in TCE results in more homogeneous, multi-phase alloy formation evidenced by positively shifted Cu stripping behavior. This is attributed to the thiosulfate adsorption limiting Au and Cu mobility thus yielding the formation of continuous and smaller-grained over-layers. In contrast, the enhanced mobility of Au ad-atoms in CCE facilitates 3D growth, evidenced by SEM to yield larger grains with well-defined facets and limited alloying. It was shown via XPS that alloys deposited from TCE exhibit a concentration gradient from the center of films to the exteriors and that S is present in the as-deposited films. In addition, following Cu stripping, it is shown that a variety of nanoporous and S-free Au structures can be formed with variable ligament and facet size, ultimately providing opportunity for catalyst design on the nanoscale.

A study on the effect of inter-electrode gap and pulse voltage on current density in electrochemical additive manufacturing

Electrochemical Additive Manufacturing—a novel non-thermal metal additive manufacturing process offers some advantages over traditional energy beam-based layered manufacturing processes, which have several inherent limitations such as thermal damages, residual stress, and part size restrictions. In this process, the principle of localized electrochemical deposition of metals is combined with the additive manufacturing procedure to manufacture metal parts at room temperature directly from computer-aided design files. The focus of research work presented in this paper is on the modeling of the current density produced in the electrochemical deposition system based on the Fick’s law of diffusion and electrode kinetics using the Butler–Volmer equation. The current densities involved in the micro-electrochemical additive manufacturing on nickel have been found to be several orders of magnitude higher than the standard electroplating studies, making it a very high overpotential deposition. The model developed in this work was used to study the effects of applied potential, pulse duty cycle, inter-electrode gap, and concentration on the current density and the transient diffusion layer thickness. The model predicted that lower inter-electrode gaps facilitated higher current density leading to faster deposition rates. The pulse voltage was found to produce higher current densities during the pulse-on time. The modeled current density values were validated with experimental results using the in-house-built electrochemical additive manufacturing setup.

Reducing lithium deposition overpotential with silver nanocrystals anchored on graphene aerogel.

Li metal as an anode for high-energy-density batteries is actively pursued due to its high specific capacity and ultralow electrochemical potential. Unfortunately, Li dendrite growth might induce a short circuit creating safety hazards that limit the practical applications of Li metal anode batteries. Herein, a novel anode of graphene aerogel (GA) decorated with silver nanocrystals (AgNCs@GA) is reported for effective suppression of lithium dendrite growth and improvement in coulombic efficiency at various current densities. This improved performance is attributed to AgNCs. This loaded AgNCs with high Li affinity serve as Li deposition sites, which deeply reduce the overpotential of Li nucleation and electrodeposition. Therefore, it successfully realizes stable Li deposition/stripping processes with enhanced coulombic efficiency at various current densities and areal capacities. The pre-lithiated AgNCs@GA is evaluated as an anode in a Li battery and demonstrates remarkable performance in comparison with a commercial lithium foil.

Effect of Concentration on the Electrochemistry and Speciation of the Magnesium Aluminum Chloride Complex Electrolyte Solution.

Magnesium batteries offer an opportunity to use naturally abundant Mg and achieve large volumetric capacities reaching over four times that of conventional Li-based intercalation anodes. High volumetric capacity is enabled by the use of a Mg metal anode in which charge is stored via electrodeposition and stripping processes, however, electrolytes that support efficient Mg electrodeposition and stripping are few and are often prepared from highly reactive compounds. One interesting electrolyte solution that supports Mg deposition and stripping without the use of highly reactive reagents is the magnesium aluminum chloride complex (MACC) electrolyte. The MACC exhibits high Coulombic efficiencies and low deposition overpotentials following an electrolytic conditioning protocol that stabilizes species necessary for such behavior. Here, we discuss the effect of the MgCl2 and AlCl3 concentrations on the deposition overpotential, current density, and the conditioning process. Higher concentrations of MACC exhibit enhanced Mg electrodeposition current density and much faster conditioning. An increase in the salt concentrations causes a shift in the complex equilibria involving both cations. The conditioning process is strongly dependent on the concentration suggesting that the electrolyte is activated through a change in speciation of electrolyte complexes and is not simply due to the annihilation of electrolyte impurities. Additionally, the presence of the [Mg2(μ-Cl)3·6THF]+ in the electrolyte solution is again confirmed through careful analysis of experimental Raman spectra coupled with simulation and direct observation of the complex in sonic spray ionization mass spectrometry. Importantly, we suggest that the ∼210 cm-1 mode commonly observed in the Raman spectra of many Mg electrolytes is indicative of the C3v symmetric [Mg2(μ-Cl)3·6THF]+. The 210 cm-1 mode is present in many electrolytes containing MgCl2, so its assignment is of broad interest to the Mg electrolyte community.

Electrochemical Synthesis of Nanoporous Platinum Nanoparticles Using Laser Pulse Heating: Application to Methanol Oxidation

Nanoporous platinum nanoparticles (NPs) have been proposed as promising electrocatalytic materials. Routes to produce them typically consist of chemical synthesis or selective dissolution of one component of a two-component mix. Here we show that by employing a pulsed laser heating approach during electrodeposition, whereby the electrode/electrolyte interface is continually heated and cooled, NPs with a nanoporous structure can be grown directly on the electrode (boron-doped diamond) surface. Transmission electron microscopy shows the NPs to be composed of loosely packed aggregates of much smaller crystalline particles of size 2–5 nm, with the porosity increasing with increasing deposition overpotential. In contrast, electrodeposition at room temperature (RT) results in particles which show a considerably more compact morphology and fewer higher index crystal facets, as revealed by electron diffraction techniques. Pulsed heating also offers a route toward controlling the monodispersity of the electrodeposited NPs. When applied to the oxidation of methanol, the laser-heated NPs show considerably higher catalytic current densities in comparison to RT-deposited particles. The highest catalytic activity is observed for the most porous NPs produced at the highest overpotential. Interestingly, the ratio of the forward oxidative current to the backward current is highest for those particles deposited under laser-heated conditions but with the smallest overpotential. This suggests that the most catalytically active NPs may also encourage binding of residual adsorbed carbon monoxide and that a compromise must be reached.

Metal Underpotential Deposition to Quantify Defects in 2D Materials

We demonstrate how metal UPD may find use as a general tool to determine the collective defect area in hybrids between 2D materials (graphene, hexagonal boron nitride, etc.) and various substrate metals. By investigating copper UPD on a monolayer of hexagonal boron nitride (h-BN) on Rh(111), we explore how this process can be used to quantify the defects in the h-BN monolayer which form during its chemical vapor deposition. In addition, the UPD signature allows assessing the potential window of the h-BN/metal hybrid, which is important to explore its functionality under ambient and electrochemical conditions. Importantly, UPD itself does not alter the defect area on repeated cycling. Overpotential deposition, on the other hand, is shown to have significant consequences on the defect area. We show that this non-innocent Cu electrodeposition involves intercalation originating at initial defects, causing irreversible delamination of the h-BN layer; this effect therefore may be used for 2D material nanoengineering.

Improving halide-containing magnesium-ion electrolyte performance via sterically hindered alkoxide ligands

While homoleptic magnesium dialkoxides (MgR2, R = alkoxide) have shown promise as precursors for magnesium-ion electrolytes, the effect of ligand steric bulk on the performance of electrolytes based on these compounds is not fully understood. Increasing steric hindrance, studied via R groups with additional phenyl moieties, produces electrolytes with sequentially lower deposition overpotentials (less than −90 mV), higher purity Mg deposits (ca. 100% Mg), and lower overall cell impedances. The two largest alkoxide ligands show consistent cycling behavior and low stripping and plating overpotentials over 200 constant-current plating/stripping cycles. A deep-red visual change and the presence of large solubilized magnesium particulates above 450 nm in size is observed in an electrolyte containing magnesium bis(triphenylmethoxide) and aluminum chloride in contact with an abraded magnesium anode. Further morphological and impedance characterization show that this electrolyte system rapidly activates the magnesium metal anode surface to produce low overpotentials and, as such, is a candidate for further investigation.

Copper underpotential deposition on boron nitride nanomesh

The boron nitride nanomesh is a corrugated monolayer of hexagonal boron nitride (h-BN) on Rh(111), which so far has been studied mostly under ultrahigh vacuum conditions. Here, we investigate how copper underpotential deposition (upd) can be used to quantify defects in the boron nitride monolayer and to assess the potential window of the nanomesh, which is important to explore its functionality under ambient and electrochemical conditions. In dilute sulfuric acid, the potential window of h-BN/Rh(111) is close to 1volt, i.e. larger than that of the Rh substrate, and is limited by molecular hydrogen evolution on the negative and by oxidative removal on the positive side. From copper upd on pristine h-BN/Rh(111) wafer samples, we estimate a collective defect fraction on the order of 0.08–0.7% of the geometric area, which may arise from line and point defects in the h-BN layer that are created during its chemical vapour deposition. Overpotential deposition (opd) is demonstrated to have significant consequences on the defect area. We hypothesise that this non-innocent Cu electrodeposition involves intercalation originating at initial defects, causing irreversible delamination of the h-BN layer; this effect may be used for 2D material nanoengineering. On the relevant timescale, upd itself does not alter the defect area on repeated cycling; therefore, metal upd may find use as a general tool to determine the collective defect area in hybrids between 2D materials and various substrate metals.

Electrodeposition of Zn–Ni–P compositionally modulated multilayer coatings: An attempt to deposit Ni–P and Zn–Ni alloys from a single bath

The effects of bath composition and deposition variables on the electrodeposition of Zn–Ni–P alloys were studied in order to develop a single bath for deposition of Ni–P/Zn–Ni compositionally modulated multilayer coatings (CMMCs). The basis for development of the bath was a large increase in the Ni deposition rate compared to that of Zn at low deposition overpotentials combined with the impossibility of codeposition of Zn with P. EDS analysis demonstrated that the deposits obtained from the Zn–Ni–P bath at low overpotentials were practically all Ni–P, while the alloy deposited at high overpotentials was mainly Zn–Ni with around 3.2wt% P content.

Exploration of Novel Magnesium Battery Electrolytes based on Inorganic Salts

The theoretical volumetric capacity of Mg-ion batteries is far higher than state-of-the-art Li-ion batteries (3833mAh/cm3 for Mg vs. 850 mAh/cm3for Li). The development of suitable Mg-conductive electrolytic solutions has been a major barrier to Mg-ion technology: at present there is a need for liquids with wide anodic stability windows, whose interfaces allow plating and stripping of Mg with high coulombic efficiency and low deposition overpotential. We have produced a number of Mg-conductive electrolytes by direct reaction of inorganic chloride salts with suspensions of Mg powder in nonaqueous solvents. The Mg-carrying cation in these electrolytes is presumably similar to the one that provides conductivity in the recently discovered magnesium aluminum chloride complex (MACC) electrolyte [1,2], but there is apparently no metal-complex anion, and no need for an auxiliary Lewis acid during synthesis. The chemically synthesized Mg-conductive electrolyte formed by reacting ZnCl2 with Mg in tetrahydrofuran is stable up to 3.0 V vs. Mg/Mg2+, and produces an ionic conductivity of 0.24 mS/cm at room temperature. Cyclic voltammetry shows reversible Mg plating and stripping on platinum. The deposition overpotential of freshly synthesized electrolyte is below 200mV, and the coulombic efficiency of plating and stripping approaches 100%. Over a period of a week, the coulombic efficiency of plating/stripping only decays by 5%; the deposition overpotential increases more substantially with aging, however. Electrochemical Impedance Spectroscopy (EIS) was used to analyze the transport properties of the solutions. Other properties, such as composition and structure of electrolytic solutions, the morphology of Mg deposits, and thermal stability will be also discussed to explore the precise mechanism by which this and Mg electrolytes formed by other chloride salts form, as well as their feasibility for future magnesium battery applications. Barile, Christopher J. et al. J. Phys. Chem. C 118.48 (2014): 27623-27630 Vardar, Gulin, et al. Chem. Mater. 28.21 (2016): 7629-7637 Figure 1