Topography changes of rhodium electrodes induced by the application of fast periodic potential routines

The surface structure of polycrystalline rho-dium electrodes in contact with aqueous sulfuric acid was modiﬁed by chemical etching with hot concentrated acid or by applying fast square waves with an upper potential equal to 1.55 V and a lower potential within the –0.75 V to –0.35 V range. Polycrystalline rhodium and chemical etched electrodes were characterized by voltammetry, Cu underpotential deposition (upd) and X-ray diﬀraction. For electrofaceted surfaces were used voltammetry, Cu upd and SEM, revealing that two modiﬁed rhodium electrodes exhibit similar voltam-metric characteristics as those found for Rh(111) and Rh(110) single crystals, and a third surface with an equal distribution of (110) and (111) planes. In addition, the upd of Cu on those surfaces corroborated the existence of those crystallographic planes. SEM micrographs show surface structures with a high density of terraces and steps. A mechanism of faceting is proposed.


Introduction
The electrochemical faceting of noble metals is a wellestablished procedure for the development of modified surfaces [1,2,3,4,5]. Nevertheless, not only massive bulk metals can be modified by this procedure since also crystallites of noble metals have been deposited on a carbon substrate, resulting in a surface with a preferred crystallographic orientation, as deduced from SEM, STM and X-ray diffraction studies [6].
Most of the work related to electrofaceting of noble metals has been performed on platinum [7,8,9,10,11,12,13] and gold [14,15], and a mechanism for their formation has been furnished. Several processes of electrodissolution and electrocrystallization are often involved in electrofaceting, which occur with a progressive transformation that leads to the formation of stepped surfaces showing a preferred crystallographic orientation [10,16,17]. The electrofaceted electrodes have been characterized by cyclic voltammetry [7,13,16,17], metal underpotential deposition (upd) [4,9], scanning electron microscopy (SEM) [18] and scanning tunneling microscopy (STM) [4,19]. The application of fast periodic potential routines to platinum electrodes results in two stable electrofaceted surfaces which have been named as Pt(100)-like and Pt(111)-like surfaces, taking into account that the voltammograms run in aqueous sulfuric acid resemble those of Pt(100) and Pt(111) single crystals subjected to several hundred potential cycles within the potential domain of the thermodynamic stability of water [20]. Thus, these surfaces exhibit an extensive change in morphology induced by oxygen adsorption, revealing the formation of a highly disordered topography, which in turn also displays a preferred crystallographic orientation. The comparison with the voltammetric behaviour of different platinum single crystals within the 0.05-1.45 V potential range allowed us to recognize that Pt(100)like and Pt(111)-like surfaces resemble those of Pt(310) and Pt(320), respectively [21]. Moreover, Pt(310) and Pt(320) surfaces are obtained also when repetitive potential scans are applied to other single crystals, suggesting that these surfaces exhibit crystallographic planes with an energetically favourable configuration [22,23].
In contrast to platinum and gold, the literature dealing with the electrofaceting of rhodium surfaces is scarce. Thus, the articles involve mainly the application of fast periodic triangular potential perturbations in aqueous acid media within the range of thermodynamic stability of water, leading to the development of highly stepped surfaces with preferred crystallographic orientations [24,25].
The aim of this work is to obtain rhodium faceted surfaces either through chemical etching or by electrochemical methods, applying new routines developed in the laboratory which involve fast periodic square wave potential perturbations where both the cathodic and the anodic switching potentials are located out of the potential range of the thermodynamic stability of water. These surfaces were further characterized.

Experimental
The experiments were performed in a three-body electrochemical cell, consisting of a 30 mL main chamber joined through a Luggin-Haber capillary to a reservoir for the reference electrode, and through a fritted porous plaque to another reservoir for the counter electrode.
Rhodium wires (0.1 and 0.05 mm in diameter, Goodfellow, 99.98%) soldered into soft glass tubes were used as working electrodes, cleaned in hot concentrated H 2 SO 4 and rinsed thoroughly with Millipore-MilliQ water. These electrodes were subjected to a potentiodynamic cycling from 0.05 V to 1.20 V in aqueous 1 M H 2 SO 4 (supporting electrolyte) at v=0.05 V s -1 , until stabilized voltammograms were obtained. The real surface area of the working electrodes (0.017 cm 2 and 0.18 cm 2 ) was measured according to the usual procedures [26]. The counter electrode was a rhodium sheet (1 cm 2 geometric area, Goodfellow, 99.98%), and the reference electrode was a reversible hydrogen electrode (RHE). The potentials in the text are referred to the RHE scale. The electrolytic solutions were freed from dissolved oxygen by bubbling pure nitrogen (AGA, 99.998%) for 30 min before each experiment and over the solution during the experiment.
Potential routines used for the development of electrofaceted surfaces involved square waves with a lower switching potential in the -0.75 V £ E l £ -0.35 V range and an upper switching potential E u =1.55 V, applied at a frequency 5 kHz £ f £ 35 kHz for a time t.
Cu upd experiments were performed in aqueous 10 -4 M CuSO 4 in the supporting electrolyte at v=0.02 V s -1 .
X-ray diffraction measurements were carried out using a Seifert-Scintag PADII diffractometer. The scans were performed with CuKa radiation (k=1.5418 Å ), from 2h=380 to 720 in steps of 0.040, 3 s/step. SEM micrographs were obtained with a JEOL scanning electron microscope, model JSM-5900 LV.

Voltammetric behaviour of pc-Rh
The voltammetic behaviour of polycrystalline rhodium (pc-Rh) in aqueous 1 M H 2 SO 4 is shown in Fig. 1a. The hydrogen-electrosorption potential domain extends from 0.05 V to 0.30 V, where an anodic current peak at 0.10 V with a shoulder at 0.12 V is observed. The cathodic potential region exhibits a current peak at 0.08 V with a shoulder at 0.11 V, whereas the hydrogen evolution reaction begins at E=0.04 V. After graphical deconvolution of the H-electrodesorption potential region, two anodic current peaks located at 0.098 V and 0.134 V can be seen (Fig. 1b). On the other hand, the oxygen-electroadsorption region starts at 0.52 V and extends up to the anodic switching potential E sa =1.2 V, where a wide current contribution with a maximum located at 0.73 V is observed. Reversing the potential scan, oxygen electrodesorption is accomplished, partially overlapping the H-electroadsorption region. The potential of the O-electrodesorption current peak shifts towards negative values for E sa >1.2 V [27,28,29]. The voltammograms of pc-Rh in aqueous 10 -4 M Cu-SO 4 +1 M H 2 SO 4 , run from E sa =0.9 V down to E sc =0.15 V (Fig. 1c), exhibit in the negative-going potential scan several current contributions, namely, a current peak at ca. 0.6 V due to O-electrodesorption and another one at 0.3 V corresponding to upd of Cu, whereas at E=0.2 V bulk Cu electrodeposition takes place. In the positive-going potential scan a current peak located at 0.26 V corresponding to electrodissolution of bulk Cu is noticeable, whereas current contributions observed from 0.45 V to 0.70 V involve the electrodesorption of Cu upd from different crystallographic planes [16,25]. The potential region of Cu upd was graphically deconvoluted (Fig. 1d), resulting for E=0.45 V in three current contributions, namely peak I at 0.48 V, peak II at 0.53 V and peak III at 0.58 V. Current peak II corresponds to the (110) plane, while current peaks I and III to the (100) plane [30]. Current peak III has been also associated with the (111) plane [30]. Consequently, taking into account that from Cu upd results only (110) and (100) planes can be unequivocally assigned, X-ray diffraction analysis of pc-Rh was performed to resolve the uncertainty about the existence of the (111) planes (Fig. 1e). The results corroborate the absence of (111) planes and therefore allowed us to ascribe current peak III to the (100) planes.

Chemically etched Rh surface
When a pc-Rh electrode is immersed in boiling concentrated H 2 SO 4 for 3 min and subsequently quenched in oxygen-free ultrapure water at room temperature, a voltammetric response different from the original one is obtained (Fig. 2a, dashed line). Thus, within the Helectrosorption potential domain an anodic current peak at 0.11 V and a cathodic current peak at 0.06 V are observed. The voltammogram corresponding to Cu upd on this modified Rh surface (Fig. 2a, solid line) reveals the presence of peak III at 0.58 V as the main contribution, indicating the prevalence of (111) planes [30]. Furthermore, a comparison of voltammograms of the Rh-modified surface with that of pc-Rh reveals that the H-electrosorption current peaks show an enhanced asymmetry with a potential peak difference DE p =0.05 V, denoting a higher irreversibility. Consequently, H-electrosorption was studied at different sweep rates (v) (Fig. 2b, c), concluding that for the Helectrodesorption process the current peak increases linearly with ln v whereas the peak potential is independent of v, denoting a reversible behaviour of the adsorbed species. On the other hand, the H-electroadsorption process exhibits also a linear increase of the current peak with v, but the potential peak changes linearly with lnv, suggesting an irreversible process. The irreversibility observed for this electrofaceted surface is in agreement with that reported for Rh(111) single crystals in aqueous 10 -4 M H 2 SO 4 +0.1 M HClO 4 [31,32].

General features
Potential routines employed to achieve electrofaceted surfaces involved repetitive square wave signals, characterized by E u =1.55 V, -0.75 V £ E l £ -0.35 V and a frequency 5 kHz £ f £ 35 kHz. The parameters of the potential routines are gathered in Table 1, where the potential difference, W=E u -E l , was also taken into account. Voltammograms run after applying routines I to III show that three electrofaceted surfaces, Rh-I, Rh-II and Rh-III, are obtained (Fig. 3). The main features of the electrofaceted surfaces are related to the increase in height of the H-electrodesorption current peaks together with their sharpening as compared to those of pc-Rh (Fig. 3b). On the other hand, a greater cathodic charge density related to O-containing species for the three surfaces as compared with the original one is calculated. For Rh-I and Rh-III, new cathodic current contributions located at 0.73 V and 0.80 V are observed. It should be emphasized that after a continuous potential cycling within the 0.05-1.1 V range, the Rh-I and Rh-II electrodes are unchanged. However, Rh-III showed a progressive restructuring, reproducing finally a voltammetric profile resembling that of pc-Rh. Furthermore, the development of electrofaceting was followed through F, the ratio between the height of the H-electrodesorption current peak of the electrofaceted surface and the original one [16]. It has been found that a plot of F as a function of the applied frequency showed a maximum at 35 kHz (Fig. 4).

H-electrosorption region
The main differences in the H-electrosorption voltammetric profiles after the electrofaceting process are related to peak potentials, which in turn are helpful for the identification of the crystallographic planes of the surface. These values, together with those corresponding to pc-Rh and to chemically etched Rh surfaces, are assembled in Table 2. Furthermore, it should be pointed out that the degree of reversibility for the H-electrosorption reaction increases in the order Rh-I>pc-Rh>Rh-II [16]. Both the peak potential values and the degree of reversibility are in accordance with those already reported for Rh single crystals. Thus, the voltammetric behaviour of Rh-I and Rh-II electrodes resembles those of Rh(111) and Rh(110), respectively [30, 31,32,33,34,35]. The application of potential routine III leads to complex voltammetric behaviour in the H-electrosorption potential region as two superimposed but still distinguishable current peaks are observed, as is shown in the graphical deconvolution depicted in Fig. 5. Two redox couples are defined, i.e. the first at 0.104 V/0.078 V and the second at 0.097 V/ 0.087 V, which can be related to those redox couples observed for Rh-I and Rh-II, respectively (Table 2). Consequently, Rh-III can be described as a surface exhibiting an equivalent distribution of (110) and (111) crystallographic planes.

O-electrosorption region
The O-electrosorption on electrofaceted Rh was voltammetrically investigated by applying a gradual increase of E sa from 0.8 V to 1.3 V (Fig. 6a). It can be seen that O-electroadsorption extends from 0.55 V to 1.3 V, showing a broad current peak at ca. 0.75 V, whereas on reversing the potential scan a main cathodic current peak located at 0.43 V (Ic) and two new cathodic current peaks at 0.73 V (IIc) and 0.80 V (IIIc) are observed. These two cathodic peaks can be attributed to electrochemical processes taking place on the electrofaceted surface in the positive-going potential scan for E sa =1.0 V and E sa =1.3 V, respectively. It should be noticed that current peaks IIc and IIIc reported here have been observed only with Rh-I and Rh-III electrodes, both surfaces exhibiting (111) planes. In fact, on pc-Rh and Rh-II for which (111) planes are absent, no additional cathodic current peaks were observed after the application of the potential routines, concluding that  oxygen species associated with current peaks located at 0.73 V and 0.80 V are preferentially formed on (111) planes. In order to obtain a deeper comprehension of Oelectroadsorption/electrodesorption processes, the O region was graphically deconvoluted by means of gaussian contributions. Consequently, three anodic and three cathodic curves (Ia, IIa, IIIa and Ic, IIc, IIIc) reproduce the voltammogram obtained for E sa =1.3 V (Fig. 6b, solid lines)

SEM characterization of electrofaceted Rh surface
The surface topography of Rh-I and Rh-II were examined by SEM (Fig. 7). The application of routine I to pc-Rh (Fig. 7a) yields a surface with a high density of steps, terraces and kinks, revealing that the new topography contains high-index Miller planes (Fig. 7b). Thus, the micrographs show the formation of faceted grains of different sizes with well-defined borders forming angles in the 30-120°range. Besides, smaller faceted grains and a relatively large number of grains approaching faces with triangular shapes can be observed. The same features concerning the degree of surface faceting was observed also for Rh-II electrodes, showing topography with wide terraces and square-like steps (Fig. 7c). These surface topographies are similar to those reported for other electrofaceted metal surfaces [18].  [35,36,37,38]. Besides, the preparation of Rh single crystals faced some obstacles, because the cooling atmospheres used after the application of the flame annealing procedure [35,39] are limited to H 2 +Ar [35,36,37,39] and N 2 +Ar [38], taking into account that the surface is prone to oxidative restructuring [40]. Furthermore, the X-ray diffraction technique is of restricted application for the analysis of the electrofaceted metal surfaces since it informs about both the surface and the bulk of the metal. For this reason, in this work it was used only to characterize pc-Rh, because this electrode satisfies the condition required for the Xray diffraction analysis concerning the distribution of crystalline planes. The results obtained for pc-Rh (Fig. 1e) exhibit peaks corresponding to (111), (200) and (220) planes in different intensity proportions than those expected for powder samples. The intensity of the (111) peak is very low and the one corresponding to (220) is the highest one, giving strong evidence for the prevalence of the (110) plane.  On the other hand, for Rh-electrofaceted electrodes for which the depth of these surfaces involves a few monolayers and their structure is different to that of bulk metal, the application of X-ray diffraction analysis was ruled out. Therefore, the characterization of electrochemically modified Rh surfaces was mainly achieved employing cyclic voltammetry, Cu upd and SEM. In addition, graphical deconvolution of the voltammetric profiles has assisted in the identification of the different current contributions. The latter method has been successfully applied for Pt single crystals [41,42,43], and the fitting procedure using lorentzian curves has been shown to be satisfactory for the study of the H-electrosorption process on Pt [4].

Chemically etched Rh and electrofaceted Rh surfaces
The chemical etching of the pc-Rh surface in hot concentrated sulfuric acid led not only to a significant increase of the (111) planes, but also to the smoothing of the surface. The latter can be deduced from the width of the H-electrodesorption peak, which is distinctive for isoenergetic surface sites [44,45].
In addition, the voltammetric features of electrofaceted Rh surfaces are rather different to those found for etched Rh surfaces. Thus, for the Rh-I electrode, which also exhibits a predominance of (111) planes, the Helectrodesorption peak is sharper than that of etched Rh surfaces, confirming the presence of stepped surfaces [17]. Furthermore, the peak potential is shifted towards more negative values, which indicates a decrease of the Rh-H ad mean binding energy, and consequently an easier electrodesorption of H adatoms. This fact could be explained by considering that for this surface con-figuration the H atoms are adsorbed on steps and kinks, favouring a decrease of lateral interactions [46]. SEM micrographs have confirmed this stepped topography, where the surfaces are composed of a combination of high-index Miller Rh single crystals.
In accordance with the voltammetric behaviour of Rh(111) single crystals [30,35], both (111) planes containing surfaces studied, i.e. Rh chemically etched and Rh-I surfaces, show a certain irreversibility in the Helectrosorption process. Moreover, as reported for Rh(110) single crystals [30], a higher reversibility for Rh-II electrodes is observed.
Previous results reported on Rh electrofaceting [24,25] were devoted only to the H-electrosorption region, while the O-electrosorption region was disregarded. In the present work, particular attention was paid to the Oelectrosorption potential domain because, in contrast to pc-Rh, the electrofaceted electrodes exhibit greater anodic charge densities related to O-containing species formed at E>0.8 V, which are also responsible for the new cathodic current peaks IIc and IIIc. The O-con- taining electrosorbed species would be formed at the surface defects, protecting these sites against the surface diffusion of Rh atoms and hence conferring higher stability to the electrofaceted surfaces. Thus, it is well known from surface diffusion studies, especially those concerning the mobility of surface atoms, either in vacuo or at metal|aqueous solution interfaces, that surface diffusion coefficients of metals, D s , are affected by the applied potential, the electrolyte composition and the adsorbed species, producing a decrease of D s [47,48,49].
Therefore, the solid surface cannot be taken as immutable anymore, but on the contrary, the so-called adsorbate-induced surface restructuring takes place [50]. The surface reconstruction is a function of the strength of the adsorbate-substrate interactions, as well as of the extent of the surface coverage with the interacting substances.
It should be noticed that there exists an adsorptioninduced restructuring of the solid surfaces taking part in heterogeneous reactions, which is a consequence of the outward relaxation of surface atoms interacting with species in the gas phase or in an electrolyte [51]. Such a process implies a decrease in the strength of the interactions between surface and bulk atoms in the solid, with the consequence of an enhanced surface mobility of the corresponding species.

Mechanism of the electrofaceting process
According to the mechanism already proposed for the development of electrofaceted surfaces [52], the first step is related to the oxidation of Rh atoms. The standard potential for the different ionic species that could be formed are 0.7, 0.6 and 0.6 V for the redox couples Rh/ Rh 3+ , Rh/Rh 2+ and Rh/Rh + , respectively [53]. Although these data are given under thermodynamic conditions, it can be used as a first approximation to postulate that the formation of Rh + , Rh 2+ and Rh 3+ could be favoured, although the Rh 2+ species is unstable in aqueous solutions.
Within the anodic half-cycle the first stage expected to occur implies the interaction between Rh and the aqueous electrolyte: where OH species rearrange [54] at the surface while a fraction of the Rh atoms enter into the solution, undergoing metal electrodissolution according to: A subsequent reaction is accomplished: Moreover, at E u the following reaction can take place: The formation of O-containing surface chemical species, Rh(OH) 3 and RhO(OH), was established through XPS and electrochemical studies [55,56]. Otherwise, during the cathodic half-cycle, electrocrystallization of Rh 3+ ions produced in the anodic halfcycle takes place: and also the electroreduction of the O-containing species: Then, the overall process involved in one cycle can be described as electrooxidation/electroreduction reactions at the metal surface promoted by the periodic perturbing potential, resulting in a modified surface depicted as Rh*-containing atoms. The occurrence of reactions (1) and (2) implies, in addition to the surface processes, the participation of mass transport in solution [57,58]. In the absence of diffusing species in solution, the application of the potential routines for values of E u and E l lying above and below E r , the equilibrium potential value for the Rh/ Rh 3+ redox couple, means that during the anodic halfcycle the electrode surface becomes a source of Rh 3+ ions going into the solution. The maximum concentration gradient is then established between the electrode surface and the bulk of the solution through a distance d a , i.e. the corresponding boundary layer thickness. On the other hand, Rh 3+ ion concentration at the electrode surface falls rapidly to zero during the cathodic halfcycle, provided the response of the Rh/Rh 3+ redox couple is very fast. In this case, the concentration gradient of Rh 3+ ions is established through the diffusional boundary layer thickness d c , which is much smaller than d a [57,58].
Moreover, according to thermodynamic data the hydrogen evolution reaction should take place during the cathodic half-cycle. However, the occurrence of fast processes, as those related to the Rh/Rh 3+ redox couple, are favoured by the high frequency employed, as it implies a short time spent at E l . Furthermore, the influence of E l can be better rationalized in terms of the factor W which is associated with the local electric field at the interface through the potential difference [E u -E l ], determining the electric driven force acting on the ions produced during the electrodissolution step. Taking into account the values of f and W, the ions produced during the anodic half-cycle remain at the interface, whereas, later on, owing to the high local electric field, they are driven to the surface during the cathodic half-cycle. Thus, the local electric field determines the speed at which these ions return to the surface, crystallizing either on low-energy (111) or on high-energy (110) surface sites (Table 1).
On the other hand, the voltammetric behavior of the electrofaceted metal can be explained by taking into account the restructured Rh* surface. Hence, the following oxidation sequence can take place during the positive-going potential scan up to E=1.2 V: Reversing the potential scan, Rh*(OH) 3 is electroreduced: In the negative-going potential scan there should be also considered the new cathodic current peaks IIc and IIIc, observed on faceted Rh-I and Rh-III and related to the O-electrodesorption process. Previous studies concerning the interaction of oxygen with Rh single crystals have shown that, on high-index Miller surfaces of Rh involving (111) planes on terraces, reconstruction leading to a reversible formation of double steps takes place [40]. Although this reconstruction has been observed for O-adsorption on the metal|gas interphase, it can be achieved when potential routines are applied to Rh|electrolyte interfaces. Thus, the double step formation is accomplished when a small amount of oxygen is adsorbed on the surface, about 0.1 ML, which is also expected to occur for the restructuring provoked by the fast periodic potential routine. Besides, the excess of Oatom adsorption takes place within the potential domain where peaks IIc and IIIc are observed and for which a charge of about 0.34 ML is involved, suggesting also the restructuring of pc-Rh.

Conclusions
The polycrystalline rhodium surface presents two energetically favourable sites for hydrogen electrosorption, exhibiting (110) and (100) symmetries. The topography of polycrystalline rhodium is modified by chemical etching with boiling concentrated sulfuric acid, yielding a smooth surface with predominance of (111) planes.
The main consequence of this procedure is the redistribution of the original (110) sites, resulting from the surface diffusion of rhodium atoms to lower isoenergetic sites. The application to polycrystalline rhodium electrodes of square wave potential routines, with the upper and lower potential limits located out of the thermodynamic stability of water, yields electrofaceted surfaces with a preferential surface orientation similar to those of highindex Miller planes containing mainly (110) and (111) basal crystallographic planes. The process of electrofaceting involves a sequence of electrooxidation/electroreduction reactions at the metal surface, resulting in three rhodium-modified surfaces. The potential difference W=E u -E l induces a local electric field responsible for the reactions that take place in the anodic hemicycle, resulting in metal ions that enter into the solution and subsequently, in the cathodic hemicycle, return to the surface, crystallizing either on (111) or (110) surface sites.