Time-dependent growth of the dendritic silver prepared using square wave voltammetry technique for methylene blue photodegradation


 Silver (Ag) particle is a promising photocatalyst material with relatively high catalytic activity and good absorption in the visible light region. A dendritic structure of Ag has been studied in the purpose to enhance photocatalytic activity due to a large surface area and active site number of the metallic Ag particles. In this work, the Ag dendritic structure was synthesized from a surfactant-free electrolyte using the square wave voltammetry technique. The time-dependent growth of the Ag dendrites and their photocatalytic activity on methylene blue (MB) photodegradation are reported. Morphological analysis exhibits the fractal dendritic structure of Ag was found to continuously grow by increasing the deposition time. The Ag dendrites showed a low charge transfer resistance (366.21 Ω) and high specific capacitance (2.09 F/g). A high rate of MB degradation (45.57%) under ultraviolet irradiation indicated that the Ag dendrites produced using this technique are effective for the photocatalytic degradation of MB dye.


INTRODUCTION
Photocatalysis is considered a promising method for degrading organic pollutants and has been widely studied due to its low cost, environmental friendliness, and low risk of generating new toxicants 1, 2 . Among photocatalyst materials, silver nanoparticles have been considered as potential metallic-based photocatalyst due to good absorption capability in the visible light region 3, 4 . In addition, silver is stable, exhibits superior electrocatalytic activity, and has good reproducibility and optical cross--sectional characteristics 5-7 .
Modifying morphology of the nanostructures is a potential approach to obtain high levels of catalytic activity. Numerous synthetic routes have been developed to produce various Ag photocatalyst shapes, including wires 8 , rods 9 , sheets 10 and dendrites. Ag dendrites with multilevel branching structures stand out among other nanostructures due to their large specifi c surface area that provides large active sites at the ends of their branches or between adjacent Ag branches 13 . Among preparation methods, electrodeposition has been utilized to prepare different shapes of metallic nanoparticles 14 . To produce a dendritic structure of Ag, various electrodeposition techniques such as potentiostatic 15, 16 and galvanostatic 17, 18 have been reported. The use of surfactants 19-21 and templates 17, 22 were also investigated to obtain the desired structures. However, preparation of the dendritic structure in an additive-free solution remains challenging.
Square wave voltammetry is another technique that has been reported to be successfully employed for platinum (Pt) nanodendrite preparation. In this technique, a large-amplitude interval as a symmetrical square wave is employed to the working electrode. Moreover, the shape of fi ne Pt dendrites could be controlled by adjusting the pulse potentials 23 . This report describes the advantage of the square wave voltammetry technique for developing the multilevel structure of the metallic particle. In this work, the square wave voltammetry technique was employed in the preparation of Ag dendrite structure from a surfactant-free electrolyte. The growth of the Ag structure was observed at different deposition times. The as-prepared dendrites were then examined to investigate their electrochemical properties and photocatalytic activity. The photocatalytic activity toward methylene blue, which is known for its poor degradation and the cause of health problems, was also studied 24, 25 .

Materials
Silver nitrate (AgNO 3 ) and nitric acid (HNO 3 ) were supplied by PT. Merck Indonesia. The chemicals were analytical grade reagents and used for electrolytes preparation without further purifi cation. Indium tin oxide (ITO) coated polyethylene terephthalate (PET) was purchased from Kintec Company. The substrate was rinsed with ethanol and double-distilled water before electrodeposition.

Synthesis of Ag dendrites
The Ag dendrites were prepared from the electrolytes containing 0.5 mM AgNO 3 and 0.1 mM HNO 3 . The electrodeposition was carried out by square wave voltammetry techniques using an EA 163 eDAQ potentiostat in a three-electrode electrochemical cell. A platinum plate with 1 x 1 cm 2 size and Ag/AgCl were used as the counter and reference electrodes, respectively. Indium tin oxide-coated polyethylene terephthalate (ITO-PET) was used as a substrate for the deposition of Ag. The electrodeposition was conducted at room temperature with upper and lower potentials of 1.75 V (Ag/AgCl) and -0.3 V (Ag/AgCl), respectively. The potential pulse duration was 100 ms, and the deposition times were varied from 1 to 20 minutes.

Characterization
The as-prepared Ag dendrites were analyzed under a scanning electron microscope (FESEM, FEI Quanta 650). The presence of Ag was verifi ed through an energy dispersive X-ray (EDX) spectrometer coupled to the FESEM. The electrochemical properties were examined using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. The cyclic voltammetry measurement was carried out using an EA163 eDAQ potentiostat within the voltage range of −0.9 V to −0.2 V. Furthermore, EIS measurements were conducted by an ERZ100 eDAQ electrochemical impedance analyzer within a frequency range of 1 Hz to 10 kHz. Both CV and EIS measurements were conducted in an electrolyte containing 0.5 M KCl. All measurements were carried out at room temperature.

Photodegradation Assays of Methylene Blue
The photocatalytic properties of the Ag dendrites were studied by examining the photodegradation of MB under ultraviolet (UV) radiation. This photodegradation performance was evaluated by recording the absorption spectrum of MB for 30 min using a GBC Cintra 2020 spectrophotometer. Figure 1 shows SEM micrographs which indicate the evolution of the morphology of Ag dendrites at different deposition times. Figure 1a exhibits the presence of Ag particles on the substrate, which represents the initial growth stage at a deposition time of 1 minute. Ag dendrites with main trunks and secondary branches were found to be formed at the deposition time of 5 min (Fig. 1b). When deposition time increased, the dendrites extended considerably and grew larger with more pronounced high-order branches assembled a dendritic fractal structure (Fig. 1c-d). The formation of fractal structures of Ag dendrites could be associated with the two different applied potentials during sample preparation that controlled using the square wave voltammetry technique. At upper potential (E u ), 1.7 V vs Ag/AgCl, the electric fi eld drove the Ag ions to move toward the working electrode and then adsorbed on the surface. The adsorbed ions were reduced to Ag metals that initiated to form a nucleus of the Ag particle on the active site lied over the ITO surface. The nucleus then grew along the pulse period. When the applied voltage switched to the lower potential (E l ), -300 mV vs Ag/ AgCl, the particles growth ceased due to nonequilibrium conditions as a consequence of more negative voltage than those of Ag reduction potential. A previous study reported that at low potential, the reduced metal atoms on the high surface energy region tend to diffuse to low surface energy area and produced a thermodynamically stable morphology shape 23 . Subsequent nucleus formation that followed by particle growth of Ag recurred when the potential was returned to the upper value. The new nucleus was formed on the previously deposited Ag particles rather than on the bare substrate as a consequence of a low activation overpotential desired for the Ag ions reduction 26, 27 . Hence, this mechanism facilitated the continuous nucleation and particle growth resulted in the fractal structure of dendritic Ag through the oriented attachment process. Figure 2 exhibits the EDX spectrum that revealed the presence of the Ag in the deposit obtained through a square wave voltammetry deposition, while the rest of the elements came from the substrate.

Morphological and structural characterization
The structure of the electrodeposited Ag dendrites was examined using X-ray diff ractometer. The XRD pattern is presented in Figure 3 which exhibited diff raction peaks at 2 = 38.13, 53.67, 64.77, 76.32, and 81.52. These peaks correspond to the (111), (220), (331), and (222) planes, respectively. Compared to the pattern of JCPDS card number 00-004-0783, the measured pattern showed a very high facet (111) that occurred during the pulse period of the lower potential (E l ) as discussed above. Figure 4a shows Nyquist plots obtained from the EIS measurements. The plots indicate the intercept value at high frequencies and the equivalent series resistance (R s ) value, which include electrolyte resistance, inherent resistance, and contact resistance on the interface formed between electrolyte and the electrode surface 30 . It also shows the charge-transfer resistance (R ct ) or Faraday resistance correlated with the intercalation and deintercalation of the ions. Figure 4b presents the R ct value of Ag deposits with different deposition times that shows a downward trend. The nanostructure of Ag dendrites exhibited the lowest charge transfer resistance (as low as 366.2 Ω) at a deposition time of 20 min. Lower R ct values indicated good conductivity due to low internal electrode resistance, which could enhance the electrochemical activity of the material by reducing ion diffusion circuits 31 . Good electrical conductivity indicated faster charge transfer and larger capacitances, thus increasing the photocatalytic activity 32 .

Electrochemical analysis
To further explore the electrochemical performance, the cell-specifi c capacitance of Ag dendrite deposits was evaluated from the cyclic voltammogram using the following equation 33 .
intensity of (111) plane, while a peak of (200) was not observed. This could be attributed to the orientation growth of the deposit. To evaluate the preferred orientation of the structure, texture coeffi cient (TC) was determined from the XRD pattern by the following equation 28, 29 . (1) where I (hkl) is the relative intensity of a plane experimentally measured for the specifi ed plane, Io (hkl) is the standard intensity of the same plane taken from JCPDS card number 00-004-0783, N is the total number of refl ections, and n is the number of diffraction peaks. The calculated values for the observed planes are shown in Table 1 that displays a high texture coeffi cient value of the (111) plane. Since a randomly texture presented by a texture coeffi cient of 1, this result indicates an abundance-oriented crystallite along the (111) plane. The high preferred orientation of the plane could be associated with the diffusion of the reduced Ag to a lower energy   where I is current, m is active material mass, V is potential window, and SR is scan rate. The comparison of the CV curves in terms of scan rates from 10 mV/s to 200 mV/s is shown in Figure 5a. The quasi-rectangular CV curve at a scan rate of 200 mV/s suggested that the electrode behaved as a pseudocapacitor. The calculated capacitances plot in Figure 5b shows that Ag dendrites at deposition time of 20 min delivered the highest specifi c capacitance (2.09 F/g). The specifi c value of larger capacitances was attributed to a larger specifi c surface area of the material 34 . Electrodes with larger surface areas showed enhanced photocatalytic activity due to their large number of active catalyst sites. Figure  5b exhibits a decrease in the specifi c capacitance value at higher scan rates. At low scan rates, the electrolyte might penetrate to the material more thoroughly and increase contact with the electrode surface, resulting in a high capacitance value 35 . Furthermore, at a high scan rate, the contact of ions in the electrolyte became fi nite, which then decreased the capacitance 36 .

Methylene Blue Photodegradation
The photocatalytic activity of Ag dendrites for methylene blue (MB) degradation was investigated under UV light irradiation. Figure 6 shows that the optical absorbance of MB at the main absorption band (centering at 664 nm) rapidly decreased with time, indicating that it was degraded effectively. The percentage of MB degradation calculated using equation 3 is shown in Figure 7. The highest value was obtained at a deposition time of 20 min, and 45.57% of the MB had degraded after 30 min of UV irradiation.
where C o = initial dye concentration and Ct = dye concentration after irradiation time t.
The mechanism underlying the photocatalytic degradation of MB can be understood as the role of active species, namely, superoxide radicals (•O 2− ) and hydroxyl radicals (•OH), which act as strong oxidizing agents for MB. These species are formed when a water molecule is adsorbed onto silver dendrite active sites and dissociated due to the high energy surface, producing radicals 3, 4 . Therefore, the percentage of MB degradation increased when the deposition time of the catalyst increased. This should be attributed to the dendritic growth at longer deposition times (Fig. 1) which can increase the number of active sites to produce radicals. The wider dendritic structures also reduced the charge transfer resistance and increased the specifi c capacitance value which allowed the chemical reaction to take place at the catalyst-electrolyte interface.

CONCLUSION
Ag dendrite was synthesized from a surfactant-free electrolyte and the evolution of the morphology of dendritic structures was tuned by varying the deposition time. The potentials applied using the square wave voltammetry technique facilitate continuous nucleation and particle growth on the preferred (111) plane and thus produce a fractal structure of the Ag dendrites. The dendritic structure was found to grow continuously by increasing the deposition time. The expanded dendrites exhibit a plentiful active site that produced the smallest R ct values due to a decrease in ion diffusion circuits, which indicated faster charge transfer and larger capacitances. The condition resulted in high photocatalytic activity for methylene blue degradation. This work has presented a simple route for the synthesis of Ag dendrites, which may have great potential for photocatalytic reactions.

CONFLICTS OF INTEREST
The authors have declared no confl icts of interest regarding this published work.