Enhanced Photocatalytic Degradation of Rhodamine B Using C/Fe Co-Doped Titanium Dioxide Coated on Activated Carbon

Abstract

Carbon and iron co-doped titanium dioxide catalyst coated on activated carbon (Fe-C-TiO₂/AC) was successfully synthesized using the sol-gel method, followed by hydrothermal treatment. Commercial activated carbon was treated by HNO₃ prior to being coated by the as-synthesized catalyst. The composite was characterized by XPS, XRD, UV-Vis spectrophotometry, IR, TEM, HR-TEM, and BET. The performance of the supported catalysts was evaluated in the degradation of rhodamine B (RhB) in the solution under visible-light irradiation. The results showed that, with the appropriate amount of activated carbon, prepared Fe-C-TiO₂/AC catalysts exhibited higher catalytic activities and Fe-C-TiO₂/AC system showed the best performance. The photocatalytic degradation efficiency of Fe-C-TiO₂/AC was enhanced due to the synergistic effect between AC (adsorption effect) and Fe-C-TiO₂ (photocatalysis effect). This facilitated the photocatalytic degradation of RhB by Fe-C-TiO₂.

1. Introduction

TiO₂ photocatalyst has been widely used in the wastewater treatment and other environmental remediation because of its high efficiency, nontoxicity, propitious recycle ability, and low cost. Besides, the final products of the photocatalysis in the advanced oxidation process are CO₂, H₂O, inorganic ions, and minerals that usually have insignificant impacts on environments [1, 2]. However, the application of TiO₂ is limited because the TiO₂ with band gap ∼3.2 eV is only activated under UV radiation and it is difficult to recycle. Therefore, modification of catalyst is crucial to enhance its photocatalytic activity. To present, catalytic efficiency of TiO₂ was significantly improved by being doped with metals such as V, Cr, W, and Fe or with metal oxides like ZnO, CuO, and so on for expanding the absorption edge to the visible region [3–6]. Among these metals, TiO₂ doped with Fe is in progress because Fe can replace the positions of Ti⁴⁺ in the crystalline, reducing the band gap energy, which is conducive to the activation in the visible light [3, 4, 7]. Moreover, Fe ions act as traps to incarcerate electrons and hinder the recombination of electron/hole pairs, resulting in the increase in catalysts’ performance. In addition, the addition of some nonmetal elements such as N, C, S, P, and halogens into TiO₂ structure can also narrow the band gap of TiO₂, reinforcing the photocatalytic activity of the catalyst [2, 8]. Some research reported that transition metals/(N, C, S) co-doped into TiO₂ would greatly facilitate the photocatalytic performance of the corresponding catalysts under visible irradiation conditions, which was attributed to the synergistic effects between the doped C and N atoms and lower band gap energy [1, 3]. TiO₂-based photocatalysts are usually employed either as slurry or on support. In slurry condition, a big challenge lies in the separation and recovery of the small-sized particles after treatment of pollutants, which limits the application of the photocatalyst in real conditions. The utilization of support immobilization such as glass fibers, glass beads, or steel has also been employed. However, they have issue due to adhesion force between the photocatalyst surface and the support [9]. Recently, there are increasing interests in a synergetic approach. Different materials are usually designed to provide a high surface area to support the catalyst [8, 10]. These materials share some common characteristics: binding to the catalyst, nondestroying catalyst, high surface area, and high affinity to the adsorption of pollutant molecules [10]. Activated carbon is one of the cost-effective synergist with not only clutching the photo-agents and free radicals (HO^⋅) but also adsorbing pollutant molecules on the photochemical centers of the catalyst apart from its mechanical stability [10]. There are surprisingly quite a few papers using this method, and there is no published work on C/Fe co-doped titanium dioxide coated on activated carbon that is reported in this manuscript. Hence, in the present work, TiO₂ co-doped iron and carbon coated on activated carbon that was treated by HNO₃ was prepared by sol-gel followed by solvothermal method. The characterization of the catalyst was conducted by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), infrared spectroscopy (IR), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), ultraviolet-visible (UV-Vis) spectroscopy, and Brunauer–Emmett–Teller (BET) surface area measurement. The catalytic activity of the catalyst was examined by the degradation of RhB dye.

2. Experimental

2.1. Catalyst Preparation

2.1.1. Chemicals

TIOT (tetraisopropyl orthotitanate 98%), nitric acid (HNO₃ 68%), ethyl alcohol (C₂H₅OH 99.7%), iron (III) nitrate (Fe(NO₃)₃.9H₂O pure), and rhodamine B (C₂₈H₃₁ClN₂O₃) were purchased from Sigma-Aldrich. Activated carbon Tra Bac (AC—particle sizes from 0.075 mm to 4.75 mm, surface area BET 928 m²/g) was obtained with the aid of a TriStar 3000 V6.07 A.

2.1.2. Preparation of Fe-C-TiO₂ Coated on Activated Carbon Pretreated by HNO₃ (Fe-C-TiO₂/AC)

(i)Treatment of carbon with HNO₃: The coal is ground to a fine powder 0.16 mm before being washed with water and boiled for 2 hours to eliminate gases such as O₂, CO₂, and SO₂. Next, extracted carbon was immersed into HNO₃ 12M and stirred for 3 hours at room temperature and then soaked for 24 hours. Finally, activated carbon (AC) was washed several times with distilled water and then dried for 3 hours at 100°C.(ii)Preparation of Fe-C-TiO₂/AC: 6 mL of TIOT was added into 34 mL of ethylic alcohol to make solution A. Solution B was prepared from 17 mL of ethanol, 0.4 mL of nitric acid (68%), 1.6 mL distilled water, 48.2 mg of Fe(NO₃)₃.9H₂O, and 0.2 g of AC. The solution A was dropped into solution B under slow stir for 14 hours at room temperature and left for gelation for 2 days before being autoclaved in a Teflon vessel for 10 hours at 180^°C. The resulted powder was washed and dried at 100^°C for 24 hours to obtain the Fe-C-TiO₂/AC catalyst.

2.2. Characterization of As-Synthesized Photocatalyst

The crystal structure of catalyst was determined by XRD (D8-Advance 5005). Transmission electron microscope (TEM, JEOL JEM-1010 electron Microscope) and high-resolution transmission electron microscopy (HR-TEM, Hitachi H-9000 NAR, Japan Advanced Institute of Science and Technology) were used to investigate the particle size and morphology of the samples. XPS spectra of the prepared samples were measured by an X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra—Frederick Seitz Materials Research laboratory-University of Illinois, Urbana-Champaign, USA). The absorbance was conducted by UV-Vis spectroscopy (Tasco-V670 photospectrometer). Functional groups were identified by IR spectroscopy (IR prestige 21). Nitrogen isothermal adsorption (Brunauer–Emmett–Teller (BET)) was determined by TriStar 3000 V6.07 A. Rhodamine B (RhB) concentrations were determined by a UV-Vis spectroscopy at the wavelength of 553 nm (the absorption maximum wavelength of RhB).

2.3. Photocatalytic Performance of As-Synthesized Photocatalysts

The catalytic activity of prepared materials was examined by the degradation of RhB solution (20 mg/L) under visible-light irradiation. The compact light (36W) was used instead of solar light with a range of wavelengths from 400 to 700 nm. An appropriate amount of catalyst (1–3 g/L) was added into 100 mL of RhB solution in a 250 mL beaker. The mixture was mixed at a constant rate in the dark for 30 minutes to ensure the desorption/adsorption equilibrium before being irradiated. After certain periods of time, the mixtures were sampled to determine the RhB concentrations by the UV-Vis spectroscopy.

3. Results and Discussion

3.1. Characterization of Synthesized Photocatalysts

Overall XPS spectra of principal elements Ti 2p, O 1s, C 1s, N 1s, and Fe 2p in synthesized photocatalysts are presented in Figure 1(a). The high-resolution scan over Ti 2p, O 1s, C 1s, N 1s, and Fe 2p spectral regions is shown in Figures 1(b)–1(f), respectively. The Ti 2p spectrum consists of peaks located at 459.1 eV and 464.8 eV, indicating the existence of Ti (IV) in TiO₂ component [1]. This suggests that the doping of iron and carbon then attached to the AC carrier does not alter the chemical state of TiO₂. This result is consistent with the XRD results (Figure 2). O 1s spectrum consists of the main peak at 529.9 eV and shoulder peak at 531.9 eV. These peaks correspond to Ti-O bond and O-H groups in TiO₂ [8]. This hydrogen hydroxyl group is useful for adsorption of organic substances or it can capture photogenic holes to form HO^• free radicals that increase photocatalytic activity of photocatalysts.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 1 

XPS spectra of Fe-C-TiO₂/AC (a), Ti 2p (b), O 1s (c), C 1s (d), N 1s (e), and Fe 2p (f).

Figure 2 

XRD patterns of Fe-C-TiO₂/AC, Fe-C-TiO₂, and TiO₂.

The presence of peaks at 711.1 eV and 723.1 eV is attributed to Fe^III 2p_3/2 and Fe^III 2p_1/2; it is possible to predict the presence of iron in Fe³⁺ oxidation state. In addition, there is also the binding energy value at 709.2 eV corresponding to Fe^II 2p_3/2, which characterizes the existence of Fe²⁺ ions, possibly in FeO [5]. Coexistence of Fe³⁺ and Fe²⁺ can enhance the photocatalytic efficiency based on the following reactions:in which Fe³⁺ acts as a conductive photonic electron trap to prevent recombination of electrons and holes, while Fe²⁺ provides electrons to adsorb oxygen on the catalytic surface and helps the charge transfer on the surface faster [1, 5].

Typical peaks of C are shown in Figure 1(d). The main peak with energy value at 284.3 eV is assigned to carbon graphite. The presence of 285.9 eV and 288.4 eV peaks is associated with C-O and C=O bonds of catalytic surface carbonates [11]. The existence of this carbonate radical increases the sensitivity for catalysis, because of the involvement of carbon-containing functional groups in activated carbon [1]. A peak at 282.5 eV corresponds to the Ti-C bond in the nanocrystalline catalyst attached to AC, and this link proves that part of the carbon is involved in replacing the oxygen in the network TiO₂ anatase crystals. The XPS analysis confirmed that iron and carbon had successfully entered the TiO₂ network and also showed the successful binding of catalyst to activated carbon activated by HNO₃.

The XRD patterns (Figure 2) show that Fe-C-TiO₂ after being coated on activated carbon exhibits the typical peaks referring to the anatase form [6]. The phase composition of nanoscale particles is preserved compared with the original materials (Figure 3).

(a)

(b)

Figure 3 

TEM images of Fe-C-TiO₂/AC (a) and HR-TEM of Fe-C-TiO₂/AC (b).

The TEM images of the Fe-C-TiO₂/AC sample (Figure 3(a)) show that the catalyst system contains very fine particles, with typical size of 5 nm, evenly distributed on the surface of activated carbon. The result of HR-TEM (Figure 3(b)) confirms the Fe-C-TiO₂ catalyst binding on AC is based on black spots covered by catalytic spheres [10].

Proposed mechanism of TiO₂ coating process on AC is shown in Figure 4. After AC was pretreated with HNO₃, carbon supplied more hydroxyl and acid groups on the surface, which bond to OH⁻ and H⁺ ions of catalyst to enhance the adhesiveness.

Figure 4 

Proposed mechanism of TiO₂ coating process on AC.

Table 1 shows that the surface areas of pristine AC and AC carrying catalysts measured by BET are different. The reason is that during the treatment of AC, HNO₃ is a strong oxidant, leading to changes in porous structure, which might reduce the number of small capillaries and increase the number of large ones, therefore reducing the surface area.

Table 1 

Physical properties of AC, Fe-C-TiO₂, and Fe-C-TiO₂/AC.

IR spectra of pristine TiO₂, Fe-C-TiO₂, and Fe-C-TiO₂/AC are presented in Figure 5. Peak at 1436 cm⁻¹ refers to the -COO-Ti vibration. The peaks at 551 cm⁻¹ and 502–505 cm⁻¹ are assigned to the bonds of Ti-O and Ti-O-C, respectively.

Figure 5 

IR spectra of Fe-C-TiO₂/AC, Fe-C-TiO₂, and TiO₂.

The UV-Vis spectra of the samples are displayed in Figure 6. The results show that by coating Fe-C-TiO₂ on AC, the absorption band of Fe-C-TiO₂/AC was significantly expanded to the visible region in comparison with the absorption band of pristine TiO₂ and Fe-C-TiO₂. Indeed, as expected, the main absorption edge of TiO₂ was estimated to be about 398 nm (2.96 eV) due to its intrinsic band gap absorption. The onset of the absorption spectrum of Fe-C-TiO₂ was shifted to the visible region because of the Fe and C dopants while Fe-C-TiO₂/AC presented main absorption spectrum at the region above 480 nm. This result suggests that the Fe-C-TiO₂/AC catalyst is more active under visible light.

Figure 6 

UV-Vis spectra of pure AC, Fe-C-TiO₂/AC, Fe-C-TiO₂, and TiO₂.

From the UV-Vis spectra of pristine TiO₂, Fe-C-TiO₂, and Fe-C-TiO₂/AC (Figure 7), the band gap energies estimated from the intercept of the tangents to the plots by using Kubelka–Munk method were 2.96, 2.17, and 1.63 eV for pristine TiO₂, Fe-C-TiO₂, and Fe-C-TiO₂/AC, respectively.

Figure 7 

Plot of [F(R)·hν]^1/2 vs energy (eV) for pure AC, Fe-C-TiO₂/AC, Fe-C-TiO₂, and TiO₂.

3.2. Catalytic Activity of Fe-C-TiO₂/AC in the Degradation of RhB under Visible Light

The comparison of photocatalytic performance of Fe-C-TiO₂/AC, Fe-C-TiO₂, and TiO₂ with that of pure AC in the degradation of RhB under visible-light irradiation with the same catalyst load (1.6 g/L) is presented in Figure 8. AC adsorbs RhB and reaches the equilibrium after 60 min of reaction, which can be proved by unchanged RhB concentration over time. The results also indicate that Fe-C-TiO₂/AC is the best photocatalyst in degradation of RhB in solutions with more than 99% of RhB removal after 90 min while pristine TiO₂ exhibits the lowest performance (only some 40% of RhB is removed after the same reaction time). Pure AC and Fe-C-TiO₂ present photocatalytic efficiency in between pristine TiO₂ and Fe-C-TiO₂/AC. This illustrates the synergy between adsorption capacity of AC and the improvement of Fe-C-TiO₂ compared with pristine TiO₂.

Figure 8 

Comparison of photocatalytic activity of Fe-C-TiO₂/AC, Fe-C-TiO₂, and TiO₂ with that of pure AC.

Figure 9 shows that the optimal catalyst load was 1.6 mg/L in the degradation of RhB in solutions while more dilute or concentrated catalyst concentrations will result in lower levels of RhB degradation. In fact, more than 99% of RhB was decomposed after 90 min of visible-light irradiation with a catalyst load of 1.6 mg/L while these figures for the catalyst loads at 1, 1.2, 1.4, 1.8, and 2.5 were, respectively, 40%, 50%, 53%, 78%, and 62%. This can be explained by the fact that the higher quantity of catalyst load may cause the light absorption hindrance, decreasing the efficiency of RhB degradation, whereas the lower catalyst load would lead to lower active sites ready for the degradation of the pollutant.

Figure 9 

Optimization of catalyst load in the degradation of RhB by Fe-C-TiO₂/AC.

The photocatalytic degradation of RhB by the Fe-C-TiO₂/AC system can be explained by the following mechanism proposal (Figure 10): (1) when hν ≥ (E_C − E_V), then electrons would be excited in the valence band of TiO₂ by the process: TiO₂ + hν (UV) ⟶ TiO₂ (e_CB− + h_VB+); (2) when (E_C–E’_V) ≤ hν < (E_C − E_V), electrons can be excited from the Fe-C-TiO₂/AC energy level by the following process: Fe-C-TiO₂/C + hν (visible) ⟶ Fe-C-TiO₂/AC (e_CB− + h_VB+); and (3) when (E’_V − E_V) ≤ h < (E_C − E_W), electrons would be excited and moved from the valence band of TiO₂ to the Fe-C-TiO₂/AC energy level by the process: TiO₂ + Fe-C-TiO₂/C + hν (visible) ⟶ Fe-C-TiO₂/AC (e_CB−) + TiO₂ (h_VB+). The departed electrons and holes subsequently migrate to the surface of the catalysts and react with adsorbed H₂O and O₂ molecules, forming HO^⋅ and O₂^⋅, respectively. HO^⋅ and O₂^⋅ radicals are mainly responsible for the degradation of RhB in solution [12].

Figure 10 

Schematic mechanism of photocatalytic degradation of RhB by Fe-C-TiO₂/AC (adopted from [12]).

The catalytic stability in RhB decomposition under visible-light irradiation was studied. After each decomposition cycle (90 min), the catalyst was centrifuged and washed with distilled water and then used for further treatment of RhB in solution. The results show that the catalyst exhibited good photocatalytic activity after 5 cycles, as shown in Figure 11. The ability to decompose RhB over 5 cycles remained high (>87%). This research has proved that Fe-C-TiO₂/AC is a highly durable, economically suitable, and practical catalyst.

Figure 11 

Photocatalytic recycling of Fe-C-TiO₂/AC in RhB decomposition: first cycle (a); second cycle (b); third cycle (c); fourth cycle (d); fifth cycle (e).

4. Conclusions

The Fe-C-TiO₂ catalyst was successfully carried on the AC treated by HNO₃. The nature of the catalyst remained to be unchanged regarding phase composition, particle sizes, and structure. Furthermore, the large surface area of AC adsorbs more organic molecules, leading to enhance the RhB degradation in solutions. The catalyst after being deposited on AC carriers activated with HNO₃ has better photocatalytic activity than activated carbon without catalysts. The Fe-C-TiO₂/AC catalyst also presented better photocatalytic performance in RhB degradation compared with pristine TiO₂ and with Fe-C-TiO₂. The optimal catalyst load for RhB decomposition (initial concentration of 20 mg/L) was found to be of 1.6 g/L. The catalysts with larger size favor the catalyst separation from solution and recycling. The Fe-C-TiO₂ carried on AC sample is a potential catalyst in degradation of toxic organic compounds under visible-light irradiation.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work was partly supported by the team project of VLIR-UOS with code number ZEIN2016PR431. The authors would like to thank the RoHan project for the utilization of instruments.