Visible Light Photoelectrocatalytic Degradation of Rhodamine B Using Ti / TiO 2-NiO Photoanode

The method of Ti/TiO2-NiO photoelectrode prepared by using sol-gel method continued by calcination process was introduced. The prepared TiO2-NiO film was observed with XRD and TEM. The anatase-rutile TiO2 was mainly on the prepared TiO2-NiO composite surface electrode. In addition to NiO, the composite also formed NiTiO3 that increased with increasing calcination temperature. Photoelectrocatalytic degradation of Rhodamine B (RB) using this electrode was investigated, and anodic potential and pH were optimized. RB degradation was investigated under different conditions, and it showed that photoelectrocatalytic degradation could achieve efficient and complete mineralization of organic pollutant. Through comparison of the photoelectrocatalytic oxidation using the Ti/TiO2-NiO electrode operated by single photoanode with the Ti/TiO2-NiO electrode operated by several photoanode, it was found that the photoelectrocatalytic efficiency of that by series photoanodes was higher. Additionally, photoelectrocatalytic system was performed at the several different photoelectrodes, which verified the higher photocatalytic activity compared with the single photoelectrode.


Introduction
Titanium dioxide (TiO 2 ) was widely used in photocatalytic degradation of the organic pollutants that could not be degraded easily such as dyes because of its favorable physical, chemical and photocatalytic properties and high stability [1]- [8].This photocatalytic method was based on the reactive properties of photogenerated electron-hole pairs.They were generated in the semiconductor (TiO 2 ) particles under irradiation at suitable wavelengths (λ ≤ 400 nm).These electrons and holes could also recombine.Since the hole was a powerful oxidizing agent, it could decompose contaminants adsorbed on the TiO 2 surface.Meanwhile, it could also oxidize water to produce hydroxyl radicals that could decompose organic pollutants in water [9]- [12].There were many reports on photoelectrocatalytic degradation of organic pollutants by using TiO 2 electrodes which were prepared by coating the surfaces of electrically conducting sub-strates (ITO, Ti) with TiO 2 film.In this method, positive potential was applied on the working electrode (Ti/TiO 2 electrode), which could inhibit the recombination of electrons and holes and enhance the efficiency of photocatalytic degradation of organic compounds [5] [6] [13]- [18].
In order to improve the photoelectrocatalytic capability of TiO 2 film, preparation of TiO 2 film was extensively studied.TiO 2 films were often prepared by using either sol-gel method [19]- [25], pray pyrolysis [26] or sputtering [27].Metal oxides, such as CuO, Cu 2 O, Fe 2 O 3 , WO 3 , MoO 3 , ZnO, NiO, SnO 2 , and so on, have been considered for band-gap engineering of TiO 2 as these oxides have compatible processing strategies with TiO 2 [28]- [37].Among these oxides, low band-gap CuO or NiO is used as sensitizers to use visible radiation, whereas other large band-gap oxides (e.g., ZnO, SnO 2 ) are coupled with TiO 2 for extrinsic trapping of photogenerated charge carriers to enhance photoactivity.Among these, coupling TiO 2 with NiO still attracts less attention.The band gaps of SnO 2 and TiO 2 are 3.80 and 3.2 eV [38], respectively, and the CB edge of NiO is 0.5 V above that of TiO 2 [39].When the two semiconductor particles are coupled, the CB of NiO acts as a sink for photogenerated electrons.Since the photogenerated holes move in the opposite direction, they accumulate in the VB of the TiO 2 particle, which increases the efficiency of charge separation.
In this paper, Ti/TiO 2 -NiO electrode was prepared by using sol-gel method.The crystalline structure and surface morphology of TiO 2 film were evaluated by using X-ray diffraction (XRD) and TEM.The effect of crystalline size and structure was discussed.The photoelectrocatalytic ability of this electrode for degradation of Rhodamine B (RB) (Figure 1) and the effect of anodic potential and pH on this photoelectrocatalytic reaction were investigated.The photoelectrocatalytic oxidation using the Ti/TiO 2 -NiO electrode calcinated by furnace was compared.Additionally, photoelectrocatalytic oxidation measurements were performed at the variously counter electrodes of Ti (commercial), Ti/TiO 2 , which indicated the higher photocatalytic activity of the Ti/TiO 2 electrode further.
The mechanism of the RB photodegradation may include the following possible steps [40]: 2 RB TiO (e ) 2 rhodamine products RB OH RB OH The cationic dye radical of RB is degraded into carbon dioxide, water, and mineral acids through a rhodamine intermediate.The rhodamine intermediate could be detected by UV-VIS spectroscopy from approximately 240 nm to 360 nm.In principle, the degradation process of RB decreases the absorption peak at 543 nm, but increases absorption peaks at approximately 240 -360 nm.Qu et al. (1998) [28] reported that OOH • and OH • are necessary for the N-de-ethylation of RB, which, in turn, is necessary for complete degradation of the dye.

Reagent
Titanium (99.7%, in area 10 cm × 3 cm) was purchased from the Far East Ti Equipment Co., Shanghai, China.RB, titanium tetraisopropoxide (TTIP), ethanol, acetic acid, nickel chloride and other chemical reagents were purchased from E. Merck (Indonesia).All reagents were of analytical reagent grade quality.All solutions were prepared with doubly distilled water.

Device
X-ray data were collected by using a D8ADVANCE X-ray diffractometer (Bruker axs Co., Germany) based on CuKα radiation.The 2θ (two-theta) angle of the diffractometer was stepped from 10˚ to 80˚ by 0.03˚ increments.Transmission Electron microscope (TEM) was obtained by a JOEL JEM 1400 (Japan) with an in-column energy filter.Electrochemical experiments were performed with a Potentiostat (Jiangsu Electroanalytical Co., China).The two electrode system consisted of a Ti/TiO 2 -NiO electrode as the working electrode, electrode and a Ti/TiO 2 as the counter electrode.The radiation source was a halogen lamp (300 W, Osram).

TiO 2 Film Preparation
The TiO 2 film has been synthesized by procedures that have been published previously [40].A 10 mL solution of TTIP was hydrolysed in the 100 mL acetic acid glacial solution and mixed under vigorous stirring in an ice water-bath (10˚C -15˚C) until a clear yellow solution of TiO 2 nanocrystals was formed.The solution was heated at 90˚C for 2 h when it became a gel, which was then placed in an oven at 150˚C for 24 h to undergo an aging process.Next, the TiO 2 xerogel was ground and pulverized into a fine powder and calcined in a muffle furnace at 400˚C for 2 h at a heating rate of 10˚C•min −1 .Then, ethanol was added to the TiO 2 powder before dip-coating to the Ti mesh.The TiO 2 -coated Ti mesh was then dried at 100˚C for 5 min.The coating and drying treatments was repeated three times.

TiO 2 -NiO Films Preparation
1.145 grams of synthesized xerogel TiO 2 (previous procedure) was added in 0.81 grams of NiNO 3 •6H 2 O that has dissolved in 25 ml of distilled water.The mixed solution was stirred with magnetic stirrer to fuse.Then, the uniform mixture was dried at 110˚C for 3 h.To improve the crystalline TiO 2 -NiO after the green suspension was formed, it was calcinated at various temperature of 150˚C, 300˚C, 400˚C, 500˚C, 600˚C, and 700˚C for 4 hours at 10˚C/min.The crystalline TiO 2 -NiO composite was superimposed onto a SnO 2 F conductive glass and was dried at 110˚C for 3 h.The coating and drying treatments was repeated three times.

Photoelectrodegradation Experiment
In order to investigate the photoelectrocatalytic (PEC) activity of the prepared sensitized TiO 2 thin film, a series of degradation experiments of Rhodamine B in aqueous solutions were performed.The initial concentration of Rhodamine B was 5 mg•L −1 , with 0.05 mol•L −1 of NaCl used as the supporting electrolyte.The total volume of the solution was 10 mL.Experiments were carried out in an optical quality quartz cell equipped with a twoelectrode system.A copper wire as well as Ti plate was used as the counter electrode.The front of the working electrode was irradiated with a 300 W visible light lamp (Halogen lamp, Osram) and the progress of the photoelectrocatalytic degradation was recorded at defined intervals by UV-VIS spectroscopy.The PEC degradation of RB was performed at voltages of 1.0 V to +10.0 V and at various pH values.

The Characteristics of TiO 2 Film
The TiO 2 films prepared by sol-gel from titanium tetraisopropoxide precursor were characterized by XRD, as shown in Figure 2. The present work, the sol-gel method was carried out under acidic conditions with acetic acid as solvent.The use of acetic acid as the solvent, as well as activating the formation of Ti(OCOCH 3 )(O i Pr) 2 complexes, permits control of both the degree of condensation and the oligomerisation and induces preferential crystallisation of TiO 2 in the anatase phase.In the first case, a spherical and relatively monodisperse aggregates of nanocrystallites could be obtained.The growth kinetics of the aggregates is determined by the stability of the colloid.The TiO 2 particles formed are relatively stable and a white suspension is gradually formed due to the precipitation of large aggregates.
However, under the acetic acid conditions using the Ti(O i Pr) 4 precursor, the complexes formation was occur as described in the equation below [40].
In further hydrolysis and condensation steps, the Ti(OCOCH 3 )(O i Pr) 2 product will produce nanotitania.In this research, nano-TiO 2 was prepared from stock solutions of 110 mL solution containing titanium tetraisopropoxide and acetic acid solution in a volume ratio of 1:10 in a water bath (10˚C -15˚C).The anatase of TiO 2 was obtained after an annealing process at 400˚C (annealing rate of 10˚C min).).This result suggested that the nano-TiO 2 powder was irregularly polycrystalline.The XDR pattern shows that only the anatase phase is formed.
In addition, TEM was used to further examine the crystallite/particle size, crystallinity, and morphology of the samples.Clear spherical and non-homogeneous structures can be seen in Figure 3(a), with the average size of the primary particle approximately 10 nm, which is in good agreement with the value determined by XRD.The particle of TiO 2 in the anatase phase has a mostly spherical morphology (Figure 2(a)).The area selected for electron diffraction pattern analysis (SAED), confirming that the TiO 2 nanoparticles are highly crystalline in the anatase phase, in good agreement with the standard JCPDS # 782-484 and XRD result.

The Characteristics of TiO 2 -NiO Film
The composite of TiO 2 -NiO was obtained after an annealing process at 700˚C (annealing rate of 10˚C min).TEM was used to further examine the crystallite/particle size, and morphology of the TiO 2 -NiO composites samples.Clear spherical and non-homogeneous structures can be seen in Figure 5(a), with the average size of   the primary particle approximately 20 -30 nm, which is in good agreement with the value determined by XRD.The particle of TiO 2 in the anatase phase has a mostly spherical morphology (Figure 5(a)).The area selected for electron diffraction pattern analysis (SAED) (Figure 5(b)), confirming that the TiO 2 -NiO nanoparticles are also highly crystalline in the anatase phase, rutile phase and TiNiO 3 .
While the part of photoanoda TiO 2 -NiO composite shows that TiO 2 -NiO materials were deposited on the Ti contained on porous amorphous silica (Figure 6).The conductivity character of those composite is shown in   were affecting TiO 2 crystal structure transformation.The addition of these dopants was found to suppress the transformation of anatase into rutile (compared with no addition of dopand, Table 1).The occurrence of these structural transformations will change the photoactivity of TiO 2 and further explained in TiO 2 -NiO composite activity for photodegradation of Rhodamine B.

Influence of pH
This experiment was carried out at different pH values of 2.0, 4.0, 6.0, 8.0 and 10.0, respectively, in order to investigate the process of RB photoelectrocatalytic degradation.The experimental results were shown in Figure 9.
It demonstrated that the efficiency of the photoelectrode on the photoelectrocatalytic degradation of RB was  improved with the increase in pH value and the rate constant of the photoelectrocatalytic degradation of RB was also increased at the same time.The results showed that the photoelectrocatalytic activity of the photoelectrode was increased due to the increase in pH value.The study of the effect of pH value on the efficiency of photoelectrocatalytic degradation of organic dye was valuable but complicated.Firstly, pH value had significant effect on the absorption behavior of the organic dye on the catalyst surface.The point of zero charge of the TiO 2 that was the catalyst was at pH 6.4.Thus, the TiO 2 surface was positively charged in the media with pH less than 6.4, whereas it was negatively charged under the conditions with the pH greater than 6.4.The value of pH could also influence the charge carried by the molecule [40], the molecule of RB was cationic form at lower than 4. When the pH value was less on the catalyst surface became difficult because of an electrostatic repulsive force.When the pH value was greater than 4, the molecule of RB was in the zwitterionic form and a certain part of the molecule was attracted by the catalyst surface due to the electrostatic attraction [16].As a result, the efficiency of the photoelectrocatalytic degradation of RB was relatively low with the pH less than 4 while the efficiency was increased with the pH greater than 4. Secondly, the change of pH had an important effect on modifying the position of the TiO 2 conduction band (60 mV per pH unit) [35].Thus, the oxidizing ability of photogenerated holes was raised, i.e. the hydroxyl radical production was facilitated in the oxidation of water (or hydroxide ions) by photogenerated holes.The more alkaline, the more readily water (or hydroxide ions) underwent oxidation to generate hydroxyl radicals on the catalyst surface.Therefore, the photoelectrocatalytic degradation of RB was more efficient due to the increase in pH value.The pH value of 10 was selected in subsequent experiments.

Degradation of RB under Different Applied Potential Bias Conditions by the Proposed Photoelectrode
When the potential was 3.0 V, the rate of photoelectron transport process was lower and controlled the overall PEC oxidation.Thus, the photocurrent increased within the potential of 3.0 -5.0 V. Once the applied bias exceeded 5.0 V, the interfacial oxidation, which was slower than the photoelectron transport, became the rate-determining step of the overall process.Under this condition the photocurrent saturated gradually.The anodic potential was an important parameter in the process of photoelectrocatalytic degradation of RB.The effect of applied potential was determined by using a series of potentials.To study the key factors affecting the photodegradation of RB, a series of tests were executed under different experimental conditions in which the deduction of Vis absorbance was estimated.After 5 min degradation (batch system), about 96.3% of RB was removed.Photoelectrocatalytic (PEC) experiment 1 was carried out under halogen lamp irradiation using the Ti/TiO 2 -NiO photoelectrode with a potential voltage of +10.0 V.It was obvious that the degradation rate was very fast.But their applied potential voltages make the electrode surface damage rapidly.Furthermore, the continuous systems were done with applied potential bias of 4, 5, and 6 volts.The results are shown in Table 2.However, RB could not undergo complete degradation to produce CO 2 and H 2 O, as a result, the intermediates were produced during the process [40].Therefore, PEC method showed high complete mineralization activity in the RB degradation reaction.Figure 10 shows no prominent peak in the UV region around 370 nm as an indication of an intermediate peak of rhodamine.

Compared the Photoelectrocatalytic Oxidation Using Several Photoelectrodes
Compared the photoelectrocatalytic oxidation using the Ti/TiO 2 electrode by batch system as well as by single photoanode (Table 2), the photoelectrocatalytic oxidation process using integrated photoanode can enhance the RB degradation, respectively, as shown in Table 3.The experimental results also demonstrated that the reaction rate of RB degradation using several electrode by continuous system was higher than that by single photoanode.

Conclusion
In this paper, the method of preparation Ti/TiO 2 photoelectrode was firstly presented.The anatase TiO 2 was mainly on the prepared electrode surface.Photoanode of the TiO 2 -NiO composite synthesized by sol-gel method showed that the photoelectrocatalytic degradation ran very well.Photoelectrocatalytic degradation of RB using this electrode was investigated, and the operating conditions were optimized.pH and applied bias voltage affected the rate of photoelectrocatalytic degradation of Rhodamine B. By the comparison of the photoelectrocatalytic oxidation using the Ti/TiO 2 NiO electrode operated by single photoanode and the TiO 2 NiO electrode operated by several photoanode, it was found that the photoelectrocatalytic efficiency of that by series photoanodes was higher.Additionally, photoelectrocatalytic system was performed at the several different photoelectrodes, which verified the higher photocatalytic activity of the single system-treated electrode further.

Figure 10 .
Figure 10.Electronic spectra of Rhodamine B before and after photoelectrocatalytic degradation process at a variation applied voltage.

Table 1 .
Anatase to rutile ratio (A/R) of the synthesized TiO 2 and composite TiO 2 -NiO.

Table 2 .
Degradation of RB (%) under different experimental conditions using Ti/TiO 2 -NiO photoanode with a 300 watts halogen lamp irradiation.