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Graphene oxide (GO)-TiO2 nanotubes (TNTs) were synthesized by one-step anodization. An as-prepared photocatalyst was characterized by FE-SEM, XRD, AES, PL, and UV-Vis DRS. To determine the effect of synthesis conditions on organics degradation, we prepared the catalyst with different GO concentrations, anodization voltages, and times. Optimum synthesis conditions were GO concentration 0.25 g L−1 and anodization at 48 V for 2 h. Compared with TNTs, GO-TNTs showed a 3.6-fold increase in photocatalytic efficiency under visible light. Recycling was performed to investigate the stability of a GO-TNT catalyst. GO-TNTs is favorable for the separation of charges (e-/h+), promotion of the formation of OH•, h+, and superoxides, which degrade organics.
© 2017 Optical Society of America
1. Introduction
The paper, plastic, dyeing, and textile industries consume color and a large volume of water for dyeing their products. Additionally, these processes produce wastewater that contains dyestuffs, which have a detrimental effect on the environment [1]. Once dyes enter wastewater, they become more stable, and the complex aromatic structures of these materials may interfere with their natural biodegradation processes [2]. Therefore, these dye contaminants must be removed from wastewater before they are released into the environment.
In order to lower the levels of these pollutants in the effluent to an acceptable level, it is necessary to develop an efficient and inexpensive method. Several methods have been applied for the treatment of dye contaminants including chemical precipitation [3], membrane filtration [4], and adsorption [5]. However, in these treatment methods, a further separation technique is required to dispose of the large amounts of residual sludge that they produce.
In recent years, photocatalytic degradation of organic contaminants using semiconductor-based nanomaterials has greatly advanced [6, 7]. TiO2 plays a leading role in photocatalysis because of its availability, stability, non-toxicity, chemical inertness, and efficacy in the oxidation of various organic molecules [8, 9]. TiO2 produces hydroxyl (OH) radicals that have been used to decompose organic contaminants. However, TiO2 shows decreased efficiency under visible light because its band gap energy (~3.2 eV [10]) is activated by ultraviolet (UV) light; solar light is made up of only 5% UV light [11]. Furthermore, TiO2 exhibits high recombination of electrons (e-) and holes (h+). To overcome these drawbacks, different strategies have been used such as metal deposition [12], non-metal doping [13], and coupling with other semiconductors [14].
Recently, research into visible-response photocatalysts has become popular; these investigations have generally focused on materials such as carbon, fullerene, and graphene [15–17]. Graphene has attracted much attention due to its mechanical, electronic, and thermal properties [18]. Furthermore, graphene oxide (GO) has been shown to increase the efficiency of TiO2 under visible light. It increases the adsorption of organics on TiO2 surfaces and reduces the recombination of e- and h+ [19–21].
Anodization is a useful method for modifying TiO2 surfaces and for achieving a nanotubular structure [22, 23]. Many researchers have used GO as a modification material for anodized TiO2 nanotubes (TNTs). This has been completed with a variety of GO coupling strategies such as the simple dipping method [24], vapor thermal treatment [25], and electrochemical deposition [26]. These TNT alterations were achieved using a two-step process: anodization of the TNTs followed by GO coupling. To improve this two-step experimental method, we proposed a one-step synthesis process. This reported synthesis process is highly practical, consumes less energy, and requires a relatively short time for synthesis.
In this study, we synthesized an enhanced visible-light GO-TNTs photocatalyst with a new one-step anodization method. We determined the effects of different GO concentrations, anodization voltages, and anodization times on the degradation of organics. We also found the optimum synthesis conditions necessary to produce a photocatalyst with a strong visible-light response. Recycling of GO-TNTs was also carried out in order to investigate the stability of the catalyst. The synergistic effects of GO coupling were also discussed with the proposed organics removal mechanism.
2. Material and methods
2.1 Chemicals
Graphite powder (99.99%) and Ti foil (99.5%, 2x5 cm2, and a thickness of 0.25 mm) were provided by Alfa Aesar, Korea. The other chemicals used in this study were methylene blue dye (MB) (C16H18CIN3S, 95.0%, Showa, Japan), ethylene glycol (C2H6O2, 94.5%, Daejung, Korea), potassium permanganate (KMnO4, 99.3%, Duksan, Korea), sulfuric acid (H2SO4, 98.1%, Daejung, Korea), ammonium fluoride (NH4F, 97.0%, Junsei, Japan), hydrochloric acid (HCl, >35%, Daejung, Korea), and sodium nitrate (NaNO3, >99.0%, Daejung, Korea). All of these chemicals were of analytical grade and were used without further purification.
2.2 Synthesis of catalyst
2.2.1 Synthesis of graphene oxide
GO was synthesized by a modified Hummer’s method [27]. First, 1 g of graphite powder, 0.5 g of sodium nitrate, and 25 mL of sulfuric acid were mixed. Then, 3 g of potassium permanganate was added and mixed for 12 h at 35 °C. The obtained brown-colored GO solution was washed five times with deionized (DI) water and a 1 M hydrochloric acid before being dried at 60 °C.
2.2.2 Synthesis of GO-TNTs
For synthesis of one-step GO-TNT first, to remove impurities, Ti foil was sonicated in acetone, ethanol, and DI water (for 10 min each) and then dried with N2 gas. The electrolyte solution consisted of ethylene glycol (EG):DI water (94:5 wt%), ammonium fluoride (1 wt%), and different concentrations of GO (0.125 - 1.0 g L−1). Platinum was used as the cathode, and a distance of 2 cm was maintained between the two electrodes. Anodization was achieved at different voltages (40 - 65 V) and different anodization times (1.5 - 3.5 h), as shown in Table 1. Anodization was performed using an N6702A power supply (Agilent Technologies, Spain). After anodization, the Ti foil was washed with DI water and dried with N2 gas. To improve the anatase crystalline structure of TiO2, annealing was performed at 550 °C for 1.5 h in a furnace.
Table 1. Synthesis conditions of the one-step GO-TNTs catalyst with different GO concentrations, anodization voltages, and anodization times.
For synthesis of two-step GO-TNT, firstly TNT was synthesized using electrolyte EG:DI water (94:5 wt%) with ammonium fluoride (1 wt%). Anodization was performed at 48 V for 2 h, then annealing at 550 °C for 1.5 h. In second step GO was attached through electrochemical anodization using GO concentration of 0.25 g L−1 and anodization at 48 V for 1 min, then dried at 60 °C for 12 h.
2.3 Characterization
The surface structure of the GO-TNTs catalyst was analyzed using field emission scanning electron microscopy (FE-SEM, Sigma, Carl Zeiss Co., Germany). The crystalline material phase was identified by X-ray diffraction (XRD, D8-Advance, Bruker-AXS Co., Germany). The atomic composition and surface analysis of the catalyst were measured by Auger electron spectroscopy (AES 700, PHI 700 xi). The excited state of the semiconductor was determined using photoluminescence spectroscopy (Micro Confocal PL, Mono Ra 750i) using excitation wavelength of 325 nm, and the emission was observed from 300 to 800 nm in a single scan mode with He-Cd laser. The band gap energy was identified using UV-Vis diffuse reflectance spectroscopy (Lambda 650S, Perkin Elmer).
2.4 Photocatalytic experiments
The photocatalytic degradation performance of the visible-light GO-TNTs catalyst was used to examine the degradation of MB dye. A 35 mL MB solution with an initial concentration of 5 mg L−1 was used in a laboratory-scale reactor. The glass reactor having inner diameter of 9 cm and height of 5 cm, with visible lamps mounted above the MB solution. The Ti catalyst film size was 2x5 cm2. Visible-light photocatalytic activity was achieved using a fluorescent lamp (32 W, λ = 400-700 nm, Philips Co., Netherlands) at room temperature. The fluorescent lamp luminous efficacy was up to 90 lm w−1 and we have used UV cut off filter to remove wavelength <420 nm The photoelectrocatalytic activity was tested at an external bias of 5 V. First, the experiment was performed in the dark for 20 min to achieve adsorption equilibrium. Later, visible light was supplied for 150 min. The quantity of MB degradation was measured using UV-Vis spectrophotometry (DR3900, HACH, USA) at a wavelength of 664 nm.
A reusability experiment was also carried out for the GO-TNTs catalyst to examine the stability of the visible-light catalyst. The experimental conditions were the same as those used for the photoelectrocatalytic process with an external bias of 5 V. After each cycle, the GO-TNTs catalyst was washed with DI water and dried in an oven at 70 °C for 1 h in order to remove any deposits and impurities from the catalyst surface.
3. Results and discussion
3.1 Characterization
3.1.1 FE-SEM and XRD
Figures 1(a)-1(c) show FE-SEM images of the TNTs and one-step prepared GO-TNTs composite. Uniform and highly-oriented TiO2 nanotubes were achieved by anodization. The TiO2 nanotube diameter was about 50 nm, and the nanotube length was 1,200 nm, as shown in Figs. 1(a) and 1(b), respectively. The optimum photocatalytic degradation results were found at synthesis conditions of GO concentration 0.25 g L−1, anodization voltage 48 V, and anodization time 2 h and the FE-SEM is shown in Fig. 1(c). The FE-SEM image of GO-TNTs show that GO was well adhered to the TiO2 surface and filled the interior of the nanotube surface. This is because nanotube formation and GO coupling occurred at the same time.
Fig. 1 FE-SEM images of: (a) TNTs, (b) cross view, and (c) GO-TNTs, (d) XRD patterns of TNTs and GO-TNTs.
The XRD patterns of TNTs and one-step GO-TNTs composite are shown in Fig. 1(d). The TNT films were annealed at 550 °C for 1.5 h in order to obtain the anatase crystalline structure. The anatase TiO2 diffraction peaks at 25.3°, 37.9°, 48.1°, and 55.0° correspond to the (101), (004), (200), and (105) planes, respectively [28]. The XRD spectrum of the GO-TNTs composite shows diffraction peaks of anatase and Ti phases of TNTs together with GO. Two GO peaks with low intensity were also observed at 26.3° and 41.1° overlapped with Ti peak [26].
3.1.2 Auger electron spectroscopy
The atomic composition and surface mapping of the one-step GO-TNTs prepared via our one-step fabrication process are shown in Fig. 2. GO, as carbon (C), appears as the dark regions in Fig. 2(a). The other elements of Ti and oxygen (O), can be seen in Figs. 2(b) and (c), respectively. Figure 2(d) shows the AES atomic compositions of the one-step GO-TNTs. The atomic compositions of C, O, and Ti were 5.8, 62.0, and 32.2%, respectively. From the AES results, it was confirmed that GO was successfully coupled with the anodized TNT surface.
3.1.3 Photoluminescence and UV-Vis diffuse reflectance spectra
The photoluminescence (PL) spectra of the TNTs and GO-TNTs one-step and two-step composites are shown in Fig. 3(a). The PL data indicate the excited state of the photocatalyst semiconducting material, which was affected by the recombination of photogenerated charges (e- and h+) [29]. Here, a lower PL intensity indicates better charge separation of e- and h+. In Fig. 3(a), a low PL spectra intensity was observed for the GO-TNTs catalyst prepared with our one-step process than other catalysts. This was due to the fact that graphene acts as an electron transport material for TiO2 to reduce recombination of e- and h+. This increased the efficiency of the photocatalytic activity of the GO-TNTs composite under visible light.
Figure 3(b) shows the UV-Vis diffuse reflectance spectroscopy (DRS) results of TNTs and GO-TNTs one-step and two-step composites. Pure TNTs showed absorption in the UV-light region at the band gap energy of anatase TiO2 3.18 eV. Alternatively, the GO-TNT one-step and two-step catalysts showed absorbance within the visible-light region with an approximate band gap energy of 2.87 eV and 2.89 eV, respectively. This result corresponds to the sub-band gap conditions of the TNTs [30]. Compared with pure TNTs, the absorption of the GO-TNTs catalyst in the visible-light region was increased due to the GO coupling. The band gap energy was calculated using Eq. (1):
Here, Eg is the band gap energy in eV, h is Plank’s constant, c is the speed of light, and λ is the wavelength in nm.
3.2 Photocatalytic degradation results
3.2.1 Effect of GO concentration on MB degradation
Figure 4 shows the effect of GO concentration (0.125 - 1.0 g L−1) on the degradation of MB. Here, we fixed the anodization voltage at 48 V and the anodization time at 2 h. The photocatalytic MB degradation was 44, 57, 48, 45, and 41% at GO concentrations of 0.125, 0.25, 0.5, 0.75, and 1.0 g L−1, respectively. We observed that the optimum GO concentration for MB degradation occurred at 0.25 g L−1 under visible light. The photocatalytic results showed that MB degradation was increased with increasing GO concentration up to 0.25 g L−1; at higher concentrations, the degradation performance decreased. The decrease in MB degradation above 0.25 g L−1 was due to excess GO loading, which may have caused the formation of a dense layer of GO on the TiO2 nanotube surface, thereby reducing the photocatalytic efficiency.
Fig. 4 Effect of GO concentration on MB degradation under visible light at a fixed anodization time of 2 h and an anodization voltage of 48 V: (a) C/Co and (b) Ln(C/Co).
3.2.2 Effect of anodization time on MB degradation
To determine the effect of anodization time on MB degradation, the photocatalytic activity was performed at different anodization times (1.5 - 3.5 h), as shown in Fig. 5. Here, we fixed the GO concentration at 0.25 g L−1 and the anodization voltage at 48 V. The photocatalytic MB degradation was 42, 57, 55, 53, and 35% at anodization times of 1.5, 2.0, 2.5, 3.0, and 3.5 h, respectively. We observed that the optimum anodization time for MB degradation was 2 h. The photodegradation results showed that the degradation rate of MB increased with increasing anodization times up to 2 h; for longer anodization times, the degradation rate was reduced. The reduction in MB degradation above 2 h was caused by the fact that longer anodization times can affect the TiO2 nanotube length. This resulted in a reduction of the photocatalytic efficiency in the GO-TNT composite prepared with our one-step process.
Fig. 5 Effect of anodization time on MB degradation under visible light at a fixed GO concentration of 0.25 g L−1 and an anodization voltage of 48 V: (a) C/Co and (b) Ln(C/Co).
3.2.3 Effect of anodization voltage on MB degradation
Figure 6 shows the effect of anodization voltage (40 - 65 V) on MB degradation. Here, we fixed the anodization time at 2 h and the GO concentration at 0.25 g L−1. The photocatalytic MB degradation was 43, 57, 50, 31, and 23% at anodization voltages of 40, 48, 55, 60, and 65 V, respectively. We observed that the optimum anodization voltage for MB degradation was 48 V. The photocatalytic results showed that MB degradation increased with increasing anodization voltage up to 48 V; for higher anodization voltages, MB degradation decreased. The decrease in MB degradation above 48 V was due to the excess anodization voltage, which can affect the TiO2 nanotube diameter, thereby reducing the overall photocatalytic efficiency.
Fig. 6 Effect of anodization voltage on MB degradation under visible light at a fixed GO concentration of 0.25 g L−1 and an anodization time of 2 h: (a) C/Co and (b) Ln(C/Co).
3.2.4 Comparison of photocatalytic results
The comparison of the photocatalytic degradation of MB by TNTs and GO-TNTs one-step and two-step catalysts is shown in Fig. 7. Very little MB degradation was observed when the experiment was conducted using only catalyst or photolysis. This blank experiment confirmed that TiO2 catalyst needs a light source to degrade organics [31]. The visible-light one-step GO-TNT composite showed a higher MB degradation rate compared with two-step GO-TNTs and pure TNTs. The MB degradation was 16, 57 and 41% by TNTs, GO-TNTs one-step and two-step, respectively. The GO-TNT composite prepared with our one-step process (optimum synthesis conditions: GO concentration of 0.25 g L−1 and anodization at 48 V for 2 h) showed a 3.6-fold increase in MB degradation compared to the pure TNTs.
Fig. 7 Photocatalytic degradation of MB dye under visible light irradiation by TNTs and GO-TNTs one-step and two-step: (a) C/Co and (b) Ln(C/Co).
Improved visible-light photocatalytic degradation efficiency of GO-modified TNT composites was also observed in previous studies by Li et al. [32] and Song et al. [24]. They observed enhanced MB degradation of GO-modified TNTs under visible-light irradiation compared with pure TNTs. The reaction rate constant values (k) were calculated according to Eq. (2):
Here, C is the MB concentration at a particular time, Co is the initial MB concentration, k is a pseudo first-order reaction rate constant in min−1, and t is the light irradiation time in min. The summary of the reaction rate constant values and their associated R2 values for TNTs and GO-TNTs is shown in Table 2. The photocatalytic activity of the one-step GO-TNT composite (optimum synthesis conditions: GO concentration of 0.25 g L−1, anodization voltage of 48 V, and anodization time of 2 h) has a higher reaction rate constant value (0.0057 min−1) than any of the other samples. This sample also showed a higher photocatalytic degradation activity (57%) for MB dye.
Table 2. Reaction rate constant values (k) and associated R2 values for TNTs and GO-TNTs.
Here, higher visible-light photocatalytic activity of GO-TNTs was observed for the following reasons. GO coupling may reduce the band gap energy of the TiO2, which enhances the absorption of TNTs under visible light irradiation [24]. GO coupled on TiO2 can trap photogenerated electrons, thereby reducing the recombination of charges e- and h+. Also, the specific surface areas of GO-TNT composites are large, which helps the adsorption of organics onto the photocatalyst surface and provides more accessible active sites for the degradation of organics [32].
3.2.5 Reusability investigation
A reusability investigation was performed to determine the stability of the visible-light GO-TNT catalyst (GO concentration of 0.25 g L−1 and anodization at 48 V for 2 h). The reusability experiment was carried out for five cycles, as shown in Fig. 8. The experimental conditions were the same as those used for the photocatalytic process. The degradation efficiency for the GO-TNT composite prepared with our one-step process was stable, even after four cycles. This catalyst was stable due to the fact that nanotube formation and GO coupling occurred at the same time.
3.2.6 Photocatalysis mechanism of modified TiO2
Here, we propose a possible photocatalysis mechanism for the GO-modified TiO2, as shown in Fig. 9. This photocatalytic degradation mechanism contains these four steps: (1) the visible light irradiation on TiO2 surface improved by the GO, (ii) the excitation of e- from valance band (VB) to the conduction band (CB) of TiO2, and leaving the h+ in VB, (iii) the e- and h+ firstly reacts with H2O, and (iv) then induced the degradation of organics [33].
The organics degradation by the modified photocatalytic process can be explained by Eqs. (3-9). Initially, when exposed to visible-light irradiation, the GO-TiO2 photocatalyst generates e- and h+ [34]. The generated e- from the CB of TiO2 [35] leaving h+ in the VB of TiO2.
The generated h+ on the TiO2 surface reacts with water or hydroxide ions (OH‾) to produce hydroxyl radicals (OH•), and the generated e- reacts with O2 to produce superoxides (O2• ‾).
OH•, O2• ‾, and h+ degrade the organics into the oxidized products [36].
4. Conclusions
An enhanced visible-light GO-TNT photocatalyst was successfully synthesized and confirmed by FE-SEM, XRD, AES, PL, and UV-Vis DRS analysis. The GO-TNT catalyst was prepared by a one-step anodization process. We determined the effect of GO concentration, anodization voltage, and anodization time on the degradation of organics. The optimum synthesis conditions were a GO concentration of 0.25 g L−1 with an anodization voltage of 48 V and an anodization time of 2 h. This sample showed a high photocatalytic activity for organics degradation under visible light. The photocatalytic activity of the GO-TNT composite was 3.6 times higher than that of pure TNTs. Additionally, the recycling results indicated that the GO-TNT composite was stable for many cycles and showed efficient separation of charges (e- and h+). The organics removal was due to the formation of OH radicals, h+, and superoxides. The observed increase in visible-light photocatalytic activity of the GO-TNT was caused by the GO coupling.
Funding
National Research Foundation of Korea (NRF) (NRF-2016R1A2A1A05005388).
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