LOCAL PARAMETERS OF THE SURFACE ROUGHNESS OF ELECTROCHEMICALLY DEALLOYED Ag-Pd ALLOYS

Local characteristics of surface roughness of the polycrystalline Ag-Pd solid solutions (4 and 8 at. % Pd) which underwent potentiostatic electrochemical dealloying in an acidic nitrate aqueous solution were determined by scanning electron and atomic force microscopy. A qualitative analysis of the SEM and AFM micrographs demonstrated substantial morphological roughening of the surface of Ag-Pd alloys. This results from the formation of hollows and cracks caused by selective leaching of electronegative silver and recrystallization of electropositive palladium into its highly-developed phase. A quantitative analysis of the AFM-data allowed us to determine the numerical values of the main local roughness characteristics of anodically modifi ed alloys. The negative value of the coeffi cient of surface asymmetry indicates the formation of deep cavities during the dealloying process. The recorded mean roughness generally depends on the AFM-scanning zone, but it stabilizes if the scan zone exceeds several tens of micrometers. The roughness of dealloyed Ag-Pd systems is of microand nano-size and increases with anodic potential and electric charge of the electrochemical modifi cation of both studied Ag4Pd and Ag8Pd alloys. This effect can be accounted for by a signifi cant rise in the anodic dissolution rate of silver from the alloys in the overcritical region of polarization, resulting in the formation of deeper surface defects. An increase in the average height of surface irregularities with modifi cation time conforms to the square root law, confi rming the non-stationary mass transfer kinetics of the selective dissolution process. Variation of the anodic potential and electric charge makes it possible to fi nd the optimal conditions for the electrochemical synthesis of electrode Ag-Pd materials with given values of arithmetic mean and root-mean-square roughness. The assumption that harmonic sinusoidal function represents the Ag-Pd surface microprofi le proved the linear dependence of the roughness factor on dealloying time.


INTRODUCTION
Anodic dissolution and corrosion of alloys often becomes selective [1][2][3] due to signifi cant difference of components in thermodynamic and electrochemical properties.Selective dissolution (SD) leads to the formation of a non-equilibrium surface layer, whose chemical, phase and microstructural state is changed compared with the bulk of the alloy [4].Especially significant changes can be appeared if A-B alloy undergoes anodic dissolution in the overcritical region of potentials and charges, when, along with ionization of electronegative component A, recrystallization of noble metal B into its own nanoporous or dispersed phase takes place [5][6][7][8][9][10][11].As a consequence, the dealloying process can be used to synthesize electrode materials characterized by a morphologically highly developed surface.Indeed, the corrosive etching of the active metal in a chemically aggressive solution makes it possible to obtain fi ne powders and monolithic spongy metals, for example, gold [5,7,10], silver [7], and palladium [6,7,11].
The disadvantage of free corrosion dealloying is the uncontrolled pore size of the synthesized material.On the contrary, using electrochemical anodic SD it is possible to regulate the morphological properties and chemical composition of the surface layer of a dealloyed material [8][9][10], and to obtain nanoporous materials with a highly developed surface structure.Moreover, deep electrochemical etching allows to synthesize materials that do not require additional stabilization in a matrix [10,11].
The highly developed surface layer forming at SD of an alloy is characterized by an increased area of the electrode / electrolyte interface.The problem of determining the local and integral morphological parameters of the surface of anodically modifi ed alloys is actual both in the kinetic description of the electrochemical processes occurring on it, and in substantiating the possibility of practical use of materials synthesized by the method of selective dissolution.
There are two measures of the surface area of a real solid electrode [12], namely the true surface area (S real ) and geometric one (S geom ); their ratio is equal to the roughness factor f r [13]: f r = S real /S geom .
(1) Unfortunately, there is no universal method for determining S real and f r , because their values depend on the measurement method, on the theoretical model on which this method is based, and on the conditions of its application [14].In addition, S real and f r are the integral parameters of the electrode surface and do not allow describing the local geometric characteristics of the surface.To determine the latter, scanning electron microscopy (SEM) and atomic force microscopy (AFM) methods are used.The arithmetic average roughness R a , the mean square roughness R q and the radial wavelength L a make it possible to suffi ciently characterize the microrelief of the surface of an alloy subjected to SD.The parameter R a is defi ned as the average deviation of all points of the AFM image from the mid-plane, and R q -as the average value of the measured deviations from the mid-plane.The quantity L a serves as a parameter of the periodicity of the surface structure and, in fact, is the average distance between adjacent irregularities [15].
The purpose of this work is the microscopic determination of the local characteristics of a highly developed surface of Ag-Pd alloys formed by anodic selective dissolution in an acidic nitrate aqueous solution in the overcritical region of anodic potentials and electric charges.

EXPERIMENTAL
Polycrystalline homogeneous alloys of the Ag-Pd system with bulk atomic fraction of palladium 4 and 8 at.% obtained by direct alloying of the components according to the procedure described in [16], were selected for the study.Making electrodes, the alloys were cut, polished, and placed in a frame of polymerized epoxy resin.Preparation of the working surface of the electrodes included stripping on sanding paper, polishing on chamois with an aqueous MgO suspen-sion to a mirror fi nish, degreasing with ethyl alcohol, and washing twice with bidistilled water.
Anodic selective dissolution of Ag-Pd alloys was carried out at room temperature in an unmixed and deaerated with argon solution of 0.1 M KNO 3 + 0.001 M HNO 3 + 0.001 M AgNO 3 under potentiostatic polarization conditions using the IPC-Pro-L electrochemical complex in a standard three-electrode cell with non-separated cathode and anode spaces.The auxiliary electrode was Pt.The silver chloride reference electrode was located in a separate vessel and connected to the cell by an electrolytic bridge fi lled with a saturated solution of ammonium nitrate, with a Luggin capillary.The solution was prepared on bidistilled water using salts of reagent and analytical grade, as well as a pre-standardized solution of acid.The potentials E are given relative to the standard hydrogen electrode scale, the specifi c charge q is calculated per unit of the true initial surface area of the Ag-Pd electrode, the roughness factor of which is determined by a combined electrochemical method [17].
The critical potentials and charges corresponding to the start of the surface development of the systems under study, evaluated by the chronoamperometry method according to [18], are listed in the table.The morphological development of the surface layer of theAg4Pd alloy was carried out at a constant overcritical potential E mod > E cr .The Ag8Pd alloy was subjected to anodic modifi cation with variable values of both electric charge q mod > q cr and electrode potential E mod > E cr .
Local data on the surface microrelief of Ag-Pd electrodes were obtained on JSM-6510LV electron microscope (JEOL Ltd) and on Solver P47PRO atomic force microscope (NSG 20 cantilever, made of silicon and coated with gold) in a semicontact (intermittent-contact) scanning mode of a sample.The surface study of the samples was carried out at fi ve different points of a scanning area of mesoscopic (65×65 μm 2 , 10×10 μm 2 ) and microscopic (2×2 μm 2 , 1×1 μm 2 ) size.The local parameters of the surface microrelief were found from the data of digital and statistical processing of the obtained AFM images in the Gwyddion 2.30 soft.The results of the SEM and AFM studies were obtained in the Voronezh State University Center for collective use of scientifi c equipment (CKPNO VGU).

RESULTS AND DISCUSSION
Micrographs of the surface of the samples obtained by the SEM method confi rm the substantial morphological development of the surface layer of Ag-Pd alloys after their overcritical selective dissolution (Fig. 1).Figures 2 and 3 show the AFM images and surface microprofi les of the studied Ag-Pd alloys.Along with the texture caused by the preparation of the samples for the experiment, there are additional cavities formed directly during the process of electrochemical dissolution of silver from the Ag-Pd alloy.
The advantage of the AFM method in comparison with scanning electron microscopy is the ability to determine the numerical values of the local roughness characteristics of anodically modifi ed alloys.For instance, the analysis showed that the coeffi cient of asymmetry, which characterizes the skewness of the distribution of the microprofi le [15], takes mostly negative values.This indicates that not protrusions predominate on the surface of the anodically modifi ed alloy, but hollows formed, apparently, during the selective dissolution of silver and recrystallization of palladium.
It turned out that the microscopically detected root-mean-square roughness R q increases with the size of the scanning zone (L xy ) if R q > L xy .However, under the condition L xy > 50 μm, this parameter reaches an almost constant value (Fig. 4).This is consistent with the literature data on the ambiguity of the results of determining morphological characteristics by microscopic methods, if the characteristic length of the scan area does not exceed the average size of the irregularities of the surface under study [15,19,20].
At the same time, regardless of the value of L xy , an increase in nanometer roughness with an increase in the value of charge q mod is observed for both Ag4Pd and Ag8Pd alloys (Fig. 5).Using the example of Ag8Pd system, it was also found that the parameters R a and R q increase with increasing excess of the potential E mod of the anodic modifi cation over the critical value E cr .The observed effect can be explained by a noticeable increase in the rate of the dealloying process at E > E cr , q > q cr [1,2,4] and, as a result, by the formation of deeper extended structural defects in the surface layer of selectively dissolving Ag-Pd alloys.Thus, varying the values of anode potential E mod and charge q mod passed through the system allows to obtain electrode materials with a given degree of surface development, manifested in the appearance of channels and cracks of nano-and micrometer scale, which are absent before the onset of anodic polarization.
Along with the arithmetic mean and rootmean-square roughness, we determined the radial wavelength of the microrelief L a , and estimated the dependence of the surface development factor on time f r (t).We used the simplifying assumption that the microprofi le of the surface of Ag-Pd electrodes can be described by a harmonic sinusoidal function.It was taken into account that if the roughness values R a are signifi cantly less than the average distance between the irregularities L a , then the following expression can be used the dependence of the roughness factor on the time of anodic selective dissolution of the alloy [21]: The analysis showed that the average height of surface irregularities increases with time of anodic modifi cation according to the square root law (Fig. 6), i.e.R t t a ( ) ª ◊ const .This indirectly confi rms the data on the diffusion control of the process of selective dissolution of Ag-Pd alloys in an acidic nitrate medium, previously obtained by non-stationary electrochemical methods [16].Moreover, the average wavelength L a of a microprofi le takes a constant value, and L a >> R a .Taking into account the totality of the analysis data, the dependence of the surface development degree on time can be described by a linear function as follows: This function coincides with the formula for the transient roughness factor of selectively dissolving studied Ag-Pd-alloys which we obtained earlier [22] using the voltammetric method.This indicates a correlation between the integral and local characteristics of the surface roughness of the studied electrode systems, which were found independently using non-stationary electrochemical and microscopic methods.

CONCLUSION
The surface roughening of Ag-Pd alloys during dealloying in nitrate aqueous solution is confi rmed by SEM and AFM methods.It was shown that the local roughness of anodically modifi ed alloys increas- Modifi cation modes: (1) -E mod = 888 mV, q mod = 28 mC/cm 2 , (2) -E mod = 888 mV, q mod = 56 mC/cm 2 , (3) -E mod = 888 mV, q mod = 113 mC/cm 2 , ( 4) -E mod = 773 mV, q mod = 16 mC/cm 2 , ( 5) -E mod = 823 mV, q mod = 58 mC/cm 2 , ( 6) -E mod = 903 mV, q mod = 486 mC/cm R a , m es with the overcritical potential and charge of anodic dissolution for both studied Ag4Pd and Ag8Pd alloys.Using the results of the work it is possible to optimize the conditions of the electrochemical dealloying synthesis of new electrode materials with given integral and local characteristics of the surface, such as the real surface area, arithmetic mean and rootmean-square roughness, as well as radial wavelength of the microrelief.