The Grain Size Influence on Boron Doped Diamond Sensitivity for Nitrate Electrochemical Reduction

Abstract

The excessive presence of nitrate ions (NO 3 -) in water mainly caused by the fertilizing in agriculture process may promote dangerous ecological complications [1]. Considering different methodologies to detect and to destruct nitrate ions, the electrochemical process appears as a promising and cheaper technique [2]. Due to its singular electrochemical properties, boron doped diamond electrodes have shown good efficiency for eletrolyses and nitrate reduction [3]. None the less, the diamond properties may depend on the different experimental conditions such as doping level and sp 2 /sp 3 ratio that influence the film grain size and also its crystallographic orientation. This work presents the electrochemical behavior for nitrate reduction using heavily doped diamond electrodes produced at different growth times that represents an evolution in the film grain size. These different morphologies may determine the better physical-chemical characteristic for electroanalytical applications.The films were prepared by hot filament CVD technique using the standard mixture of H 2 /CH 4 with an additional H 2 flux passing through a bubbler containing a suitable concentration of B 2 O 3 dissolved in methanol to assure films with acceptor concentration around 10 21 cm -3 . The growth times were 5, 7 and 10 h corresponding to samples A,B and C, respectively. Diamond quality, morphology and crystallinity were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy and x-ray diffraction techniques. Electrochemical measurements were carried out from a Potentiostat Autolab PGSTAT 32 using solutions of 0,1 mol/L de Na 2 SO 4 , 0,1 mol/L KNO 3 e 0,1 mol/L Na 2 SO 4 + 0,1 mol/L KNO 3 .The SEM and AFM analyses of diamond films showed the grain size increase as a function of the growth time, which are present in Fig. 1 for samples A and C. The films A and C showed heterogeneity due to the competition among their crystallographic planes. The electrochemical response for nitrate reduction was obtained from cyclic voltammetry comparing the results for the three samples without and with nitrate addition.The influence of the electrolyte composition may be observed in Fig.2. The hydrogen evolution is kept at the same potential region for the three electrodes without nitrate addition. Otherwise, in Fig 2b with the nitrate presence, a negative shift in the potential region is observed mainly for electrode grown during 5 h. This behavior is attributed to the morphologic difference among such samples. As the grain size decreases the sp 2 content at the grain boundary increases promoting an easier diamond oxidation leading to an electrode surface that blocks the active surface sites.

Electrocatalytic reduction of nitrate – a step towards a sustainable nitrogen cycle

“…However, Matsushima et al found that although the BDD film grown for 5 h had a smaller grain size and larger electrode surface area than films grown for 10 h, the BDD films grown for 10 h with the larger grain size showed a higher sensitivity for nitrate reduction. 276 This behaviour can be explained by the fact that the increased grain size favours the decrease of the sp 2 bonded C amount at its grain boundary, which could more stably tune the BDD surface to the oxidation process and prevent the blocking of the active surface sites. Garcia-Segura et al summarised that the sp 3 (diamond)/sp 2 (GF) ratio has a significant effect on the electrochemical properties of BDD electrodes.…”

The Dependence of Oxidation Parameters and Dyes’ Molecular Structures on Microstructure of Boron-Doped Diamond in Electrochemical Oxidation Process of Dye Wastewater

“…39 On the other hand, the large grain size of the BDD 18 electrode increased the surface roughness and the electro-catalytic active area of the electrode surface, thereby increasing the degradation rate of BDD. 40 Moreover, higher surface boron concentration enhanced the conductivity of BDD 18 . 38 Based on the above analysis, with the deposition time increasing, the changes of the microstructure of BDD films are conducive to the improvement of electrochemical performance.…”