Methyl Violet Degradation Using Photocatalytic and Photoelectrocatalytic Processes Over Graphite / PbTiO 3 Composite

Photocatalytic and photoelectrocatalytic degradation of methyl violet dye using Graphite/PbTiO3 composites has been conducted. The purposes of this research were to examine photocatalytic and photoelectrocatalytic degradation of methyl violet using Graphite/PbTiO3 composite. Synthesis of Graphite/PbTiO3 composite was successfully performed via sol-gel method by mixing graphite powder, titanium tetra isopropoxide precursor solution (TTIP) and Pb(NO3)2. The Graphite/PbTiO3 composites were characterized using X-Ray Diffraction (XRD), Fourier Transform-Infra Red (FT-IR), and Scanning Electron Microscopy (SEM). The XRD diffractogram and IR spectrum of Graphite/PbTiO3 composite revealed all characteristic peak of graphite and PbTiO3. Photocatalytic degradation process showed that Graphite/PbTiO3 composite with ratio 1/1 decreased concentrations of methyl violet up to 92.20 %. While photoelectrocatalytic degradation processed for 30 minutes at neutral pH and 10 V voltage degraded the methyl violet until 94 %. However, the photoelectrocatalysis is still not significance to improve methyl violet degradation compared with photocatalysis. Copyright © 2018 BCREC Group. All rights reserved


Introduction
Textile industrial wastewater containing synthetic dyes, having chromophore groups, is extremely harmful to the environment, especially the river ecosystem in Indonesia.Methyl violet dye, a class of triphenilmethane, is one of commonly dyes intensively used in nylon, wool, silk, and cotton dying process.Due to its com-plex structure, methyl violet is stable and difficult to degrade.Based on literature review, it was found that aniline contained in this compound is toxic, mutagenic and carcinogenic [1].These chemicals trigger tumors in several species of fish [2].Thus, it is necessary to handle this harmful substances accumulated in wastewater for sustainability.
Photocatalysts semiconductor have been utilized by some researchers on wastewater treatment to degrade the dye through photocatalytic degradation, electrocatalytic degradation and photo-electrocatalytic degradation.Senthilkumaar et al. [3] has successfully performed photocatalytic degradation of methyl violet using nanocrystalline TiO2 with 125 W mercury lamp for 60 minutes.Another experiment was also performed by Gholami et al. [4] utilizing TiO2 composed with SiO2 as photocatalyst in dye removal with outstanding result compared to TiO2 or SiO2 single materials.The photocatalytic process, unfortunately was less effective in treating a high concentration of wastewater since their low adsorption of the photocatalyst resulted low reaction rate and required a long time [5].Therefore, to enhance the TiO2 effectiveness, it needs some modifications such as formation of new compounds derived TiO2, adsorbent addition or electrical energy addition.
Formation of material derived from TiO2, so called PbTiO3, is one of modification conducted on preparation of TiO2 photocatalyst materials.PbTiO3, also known as perovskite oxide, is recently received great attention since its efficiency in photocatalytic activities with high quantum yields [6].Moreover, Pb 2+ was able to adsorb visible light because of its smaller band gap.Thus, this metal oxide, having Eg of 3.0 eV lower than TiO2 (3.2 eV), can be utilized as an efficient visible-light photocatalyst for various purposes [7,8].
Addition of Adsorbent like graphite will increase the effectiveness of photocatalyst semiconductor and photocatalytic or photoelectrocatalytic degradation processes.Economically, graphite is abundant and cheap and readily available in waste batteries.However, utilizing of graphite extracted from batteries as photocatalyst composed with PbTiO3 is still not yet studied.In this study, we combined a graphite from batteries as adsorbent and PbTiO3 catalyst carrier to enhance photocatalytic or photo-electrocatalytic degradation of dye.Moreover, graphite from batteries was reported having similar physical properties to graphite electrode [5].
With the assistance of electrical energy, the transfer of electrons from valence band to conduction band flowing in the electrode rod was easier to achieve and produce a hole (h + ) in the valence band belonging to photocatalyst material.Separation of electrons and holes was better, so electrons recombination reaction on the photocatalyst can be minimized.The flow of electrons in the electrode will also oxidize the dye solution without activating photocatalyst.Consequently, the dye's oxidation-reduction (redox) reactions will go faster.Therefore, the electric current is expected to improve effective-ness of degradation.Thus, the study of methyl violet degradation methods using composite electrode Graphite/PbTiO3 as modification of existing research is needed and has not been studied.Therefore this research focused on photocatalytic and photoelectrocatalytic degradation of methyl violet using Graphite/PbTiO3 composite.

Synthesis of PbTiO3
The PbTiO3 compound was synthesized via sol-gel method with mole ratio 1:1 of Pb(NO3)2 powder and TTIP precursors.A total of 3 mL TTIP was hydrolyzed with 30 mL of acetic acid in the water bath (±14 °C).After obtaining a white viscous solution, then it was continuously stirred and heated at 90 °C to form a white gel.Then, 1.19 g of Pb(NO3)2 powders was added into the solution while stirring.After cooled at room temperature, the obtained gel was dried at 150 °C for ±24 hours.The formed composite was then annealed by varying calcination temperature at 300, 400, 500 and 600 °C with heating rate 10 °C/min to optimize the PbTiO3 crystalline phase.

Synthesis and characterization of graphite/PbTiO3 electrode composites
Graphite/PbTiO3 composite was prepared through sol-gel method using purified graphite powder, Pb(NO3)2 powder, and TTIP.The graphite used in this research was purified from batteries pasta.The purification was carried out by soaking 10 g of graphite powder in 100 mL of 0.1 M HNO3 for 24 hours.The powder was then filtered and neutralized using distilled water and dried at room temperature.The purified graphite powder was then sieved with 100 mesh size.Synthesis procedure of graphite/PbTiO3 composites was same as the PbTiO3 synthesis procedure.After obtaining a white gel, a result of TTIP hydrolysis, 1.19 g Pb(NO3)2 powders and 1.6 g of graphite powder was introduced into the gel solution.The formed gel was then dried at 150 °C for ±24 hours and then annealed at optimum calcination temperature of PbTiO3 crystalline phase formation.In this study was also varied the graphite/PbTiO3 ratio by 1/3, 1/2, 1/1, 2/1, and 3/1 (w/w).
Fabrication of electrode was carried by mixing the Graphite/PbTiO3 composite and silica glue with 4:1 ratio (w/w).It was fabricated in electrode mold with 0.5 cm of diameter and 2.5 cm of length and pressed with 1000 psi.Then, the electrode rod was dried at 120 °C for 6 hours.Graphite/PbTiO3 composite was characterized using FT-IR spectrometer, XRD and SEM-EDX.FTIR characterization was recorded at wavenumber range 500-4000 cm -1 , whereas the XRD was noted at 2θ range 20-80°.

Photocatalytic degradation procedure of methyl violet dye
Photocatalytic degradation process of methyl violet dye was conducted by adding 0.3 g composite powder into 30 ml of methyl violet dye solution 5 ppm.The 9 watt UV lamp was positioned 12 cm above the sample.The solution was stirred for 30 minutes.The photocatalysts employed in this study were graphite, PbTiO3 and Graphite/PbTiO3 composites at ratio 1/3, 1/2, 1/1, 2/1, and 3/1 (w/w).

Photo-electrocatalytic degradation procedures of methyl violet dye
A photo-electrocatalytic degradation was performed in 20 mL solution of methyl violet 5 ppm.Then the work scheme as presented in Figure 1 was assembled by placing graphite electrodes as the cathode and Graphite/PbTiO3 composite as the anode with 2 cm for distance.The 9 watt UV lamp was located 12 cm above the samples.The solution was stirred while flowed with each variation of the power supply voltage by 7.5; 10, and 12.5 V for 30 minutes.Then, the absorbance of the solution was measured using UV-Vis spectrophotometer both before and after the photo-electrocatalytic degradation process.The pH was also varied in this study by conditioning the methyl violet solution at pH = 3 (acid), pH = 7 (neutral), and pH = 10 (alkaline).

Synthesis of PbTiO3
Synthesis of PbTiO3 was performed by combining sol-gel method on the TiO2 synthesis according to previous research [9,10] and directly added with Pb(NO3)2 after achieving TiO2 sol-gel phase.The produced PbTiO3 compounds was in the form of yellow powder.Approximately, there were five steps on PbTiO3 synthesis.The first stage was the forming TiO2 suspension by the presence of acetic acid.When the environment system reached 90 °C, TiO2 sol-gel was formed.In the same time, Pb(NO3)2 was introduced into the system to produce PbTiO3 sol-gel.The PbTiO3 powder was obtained after annealed PbTiO3 xerogel at temperature 300-600 °C generated by drying PbTiO3 sol-gel.
The final products, PbTiO3 powder, were then characterized by XRD which its diffractogram pattern is shown in Figure 2 37.8565° and 39.6192° dominated with high intensity at PbTiO3 XRD diffractogram annealed at 300 °C.These peaks indicated the typical peaks belong to Pb(NO3)2 according to JCPDS No. 36-1462.In this stage, there are no observed the peaks of TiO2, PbO, or even PbTiO3 crystals indicating the crystal was not yet formed.However, by annealing the sample at 400 °C, the PbTiO3 compound was successfully formed which it was proofed by the presence of high intensity at 2θ of 31.4805°,32.4616°, and 39.2179° corresponding to JCPDS No. 74-2495.The synthesized PbTiO3 annealed at 400 °C has high degree of crystallinity with the crystal size 39.98 and yield by 90.16% compared to samples annealed at 300, 500 and 600 °C.The rutile phase of TiO2 oxide formed at the synthesized product at 500 °C and 600 °C indicated by the presence of 2θ of 28.1395°, 36.6100°,39.8133°, 41.400°, 54.2616° (Figure 2c), and 54.2155° (Figure 2d) according to JCPDS No. 87-0710.This confirmed that annealing at 500 °C and 600 °C caused the decreasing of PbTiO3 crystallinity if compared to the product annealed at 400 °C.The high annealing temperature triggered fast of the formation reaction rate.Unfortunately, by this condition, the structuring and crystal formation become lower.

Characterization of graphite/PbTiO3 composites
Synthesis of Graphite/PbTiO3 was carried out in the same way as PbTiO3 synthesis pathway.The introducing of graphite together with Pb(NO3)2 was performed after achieving TiO2 sol-gel form.XRD diffractogram of Graphite/ PbTiO3 composite shows peaks at 2θ = 26.5076°which is the characteristic peaks of graphite and peaks at 2θ = 31.8382°;2θ = 39.2949°;2θ = 46.3154°;2θ = 57.1402°correspond to PbTiO3 (Figure 3).It can be pointed that the addition of graphite into PbTiO3 system affecting of the PbTiO3 crystallinity.The broad and low intensity of Graphite/PbTiO3 peaks compared to PbTiO3 outlined that the crystallinity decreased influenced by amorphous phase of graphite.
Ratio of graphite/PbTiO3 influenced the crystallinity of the resulted product.Based on our previous report [10], the graphite/PbTiO3 diffractograms outlined in Figure 4 showed that the composites constructed with graphite/ PbTiO3 ratio 1/1 having highest crystallinity than other composition.The addition of graphite with high concentration decreased the composites crystallinity due to the amorphous phase of graphite dominating in the system.
Infrared spectrum of the precursor materials and each graphite/PbTiO3 composition are presented in Figure 5 and 6.In the TiO2 IR spectrum (Figure 5) appears absorbance bands around 3409.33, 1630.88, and 609.5-420.5 cm -1 which they are TiO2 characteristic absorption.A peak at 3409.33 cm -1 is the O-H stretching vibration while a peak at 1630.88 cm -1 is the O-H bending vibration binding to Ti.A broad adsorption band around 609.5-420.5 cm -1 is the characteristic vibration band belonging to O-Ti-O.In the Figure 5a and 5b revealed the peaks differences at around 600-1000 cm -1 between TiO2 and PbTiO3 indicating the entering of Pb into the TiO2 structure.In the Figure 6 was observed the vibration around 3439.23 cm -1 indicating O-H stretching vibration of  graphite.Moreover, it was also found a peak at 1323.22 cm -1 typically Pb-O vibrations.The Pb-O peak intensity increased by increasing of PbTiO3 ratio to graphite.
Scanning Electron Microscopy (SEM) was utilized to evaluate material surface morphological structure, so that the differences of morphological structure between graphite and graphite/PbTiO3 can be microscopically observed.Growing of PbTiO3 in the graphite surfaces was detected by comparing morphological structure of graphite (Figure 7a) with graphite/PbTiO3 (Figure 7b).The growth of PbTiO3 in the graphite surfaces was shown by the presence of white layers attaching on the graphite surfaces thus the graphite pores was covered and the resulted composites to be denser than the origin graphite.

Photocatalytic degradation
Photocatalytic degradation (photodegradation) is one of methods to decrease the methyl violet dye concentration aided by photons (light).The applied light source was 9 Watt UV lamps and performed for 30 minutes in a black box reactor.The Photodegradation of methyl violet dye by various Graphite/PbTiO3 ratio was aimed to determine the optimal mass ratio based on its ability to degrade the methyl violet dye observed from the decline in the value of the absorbance at a wavelength of 586 nm.
In Figure 8 can be observed percentage of methyl violet dye degradation with initial concentration of 5 ppm processed for 30 minutes.Based on the experimental results, the optimum photodegradation was achieved at Graphite/PbTiO3 ratio 1/1.This is consistent with results revealed by XRD data with best crystallinity at Graphite/PbTiO3 ratio 1:1.In addition, this ratio balanced the process between the PbTiO3 photocatalytic activity and dye adsorption by graphite.
Chemically, photocatalytic degradation process is initiated by reduction-oxidation reaction mechanism where the hole + at valence band reacts with adsorbed water in the material surfaces to form a hydroxyl radical.Additionally, electrons of conduction band could reduce the adsorbed oxygen to generate a hydroxyl radical [11], as presented in equation 1-5.
OH· + substrat (dye) → product (5) However, if there are an electron (e -cb) involved to react with oxygen, then the reaction follows the Equation 6-10.The TiO2 irradiated with energy that more than its bandgap energy (> 3.2 eV) will created an electron in conduction band and hole at valence band.The created electron will reacted with O2 adsorbed in Ti(III) sites and reduced it to be superoxide radical anion (O2· -).The formed hole oxidized either organic molecules or OH -ions, and the H2O molecules adsorbed in TiO2 surfaces become radical OH -.Together, both hydroxyl radical anion and O2· -known as oxidizing agent on photocatalytic process [12].

Photo-electrocatalytic degradation
Photo-electrocatalytic degradation combined the photodegradation and electrodegradation methods.With the assistance of light energy and electricity are expected more effective process to degrade the dye.Graphite/PbTiO3 composite act as electrodes on the positive pole (anode).Using the KCl in photoelectrodegradation system serves as a medium taking place quickly to degrade the dye.
Degradation of methyl violet performed because of the electrocatalytic and photocatalytic processes.By placing graphite electrode in catode and the graphite/PbTiO3 composite in anode, the possible reaction probably occurred at each electrodes following equation 11-21.The outside potential given to the system is not only can separating electrons on conductive band from oxidation process, but also reducing the electron-hole recombination and separating the oxidation and reduction sides [13].TiO2 is also electrochemically oxidized forming electron (e -cb) and hole (h + vb).The hole (h + vb) could initiated the oxidation reaction and in other side the e -cb initiated reduction reaction in the semiconductor surfaces [14].The occurring reduction-oxidation reaction mechanism is that h + vb oxidized water or hydroxyl group of methyl violet adsorbed in the material surfaces to form a hydroxyl radical (OH·).However, in the same times, e -cb reduced the adsorbed oxygen to form superoxide and hydroperoxide radical anions [11].Photo-electrodegradation was performed by irradiating 9 watt UV lamp at voltage variation by 7.5, 10, and 12.5 V. Providing positive potential on photo-electrodegradation process, it resulted electrons generated by the excitation of TiO2 with a faster flow rate thus the photode-gradation of methyl violet occurred.The percentage degradation of methyl violet dye concentration in the 30 minutes can be seen in Figure 9.
Generally, the addition of electrical charge enhanced the dye degradation to form a simple compound.However, based on this experiments, the best applied electrical charge was achieved at 10 V with degradation value up to 94%.Since, the addition of 12.5 V could be attributed the electron flow in electrode becoming faster thus the ion moving from anode to catode also faster.Consequently, the anode ability to adsorb the dyes molecules was limited due to it cannot balancing the ion flow rate in the solution.Moreover, the result of applied high electrical charge produced more gasses just like Cl2 and H2 in bulky solution which in turn the surrounding electrode prevents degradation process of dyes by Graphite/PbTiO3 composites.Therefore, the photoelectrondegradation is still not significant to improve methyl violet degradation.But the photoelectrocatalytic may important and significantly improve the degradation when the dyes concentration is higher.

Effect of pH on photoelectrocatalytic degradation of methyl violet
On of condition should be noticed influencing the photoelectron-degradation of methyl violet, is pH condition.In this study, we carried out photoelectron-degradation process at three pH condition, i.e. 3 (acid), 7 (neutral), and 11 (base).The testing was conducted at variation of times with electrical charge by 10 V.The degradation trend of methyl violet toward pH condition was showed in Figure 10.In the initial stage, methyl violet showed higher degradation achieved at basic condition (pH = 11).However, at the equilibrium state, the best condition is in the neutral condition (pH =7) with degradation percentage by 94.00 %.At the neutral condition, the dyes will be degraded into anion and cationic substances.However, by decreasing the pH (acid condition), the presence of H + undergoes chemically reaction with nitrogen atom in methyl violet chemical structure by substituting methyl group [15], as illustrated in Figure 11.In this stage, by the formation of new structure of methyl violet containing N atom bounded with two hydrogen atom to form -NH2, made the methyl violet harder to interact with anodes since it tend to be more partial positive (δ + ) than before causing the chromophore group to further away from the anode due to the charge interpolation.Consequently, degradation of methyl violet at acid condition was lower compared to others.Another reason also came from the presence of Cl2 gas generated from HCl.The presence of the Cl2 gas will coated the electrode surfaces preventing the degradation process.
At the basic condition, since the methyl violet contain aromatic rings having resonance effect, the OH -substances (represented as basic condition), chemically interacted with cationic site at chromophore group.The oxygen atom on -OH group is electronegative and it has pair of free electrons which able to fill the electron void at the anode.Therefore, the ability to degrade the methyl violet at basic condition was higher than at the acid.Moreover, the basic condition also predisposed the interaction between dyes molecules and graphite electrode surfaces.By increasing of anionic group number in the electrode surfaces, the cation from chromophore group will be easily to attach in the graphite electrode surfaces.Subsequently, the degradation ability of electrodes was also enhanced.
Qualitative analysis of methyl violet degradation using UV-Vis spectrophotometer was carried out to proofing the degradation of methyl violet structure into simple chemical substances.Figure 12 reveals the UV-Vis spectra of methyl violet before and after degradation recorded from 200-800 nm.The maximum wavelength of methyl violet at 586 nm after degradation decreased indicating the decreasing of methyl violet concentration in solution.Moreover, it was also observed that after degradation process at all pH condition appeared new absorbance peak around 200-350 nm.It indicated that the methyl violet structure degraded or transformed into simple chemical structure.

Conclusion
The composition of the mass ratio between graphite and PbTiO3 of Graphite/PbTiO3 composite electrode influenced the formed crystals structure and its ability to degrade the dye.By using photodegradation method, the effectiveness of Graphite/PbTiO3 composite performance was achieved at a ratio of 1/1 degrading methyl violet until 92.20 %.Meanwhile, effectiveness of photoelectrocatalyst of Graphite/PbTiO3 composite electrode was accomplished at 10 V and neutral pH (pH = 7) with degradation percentages reaches up to 94.00 %.However, the photoelectrodegradation is still not significant to improve methyl violet degradation.support in this project.

Figure 11 .Figure 9 .Figure 10 .
Figure 11.Methyl violet chemical structure at acid and basic condition

Figure 12 .
UV-Vis absorbance spectra of methyl violet before and after degradation