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

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. e composite was characterized by XPS, XRD, UV-Vis spectrophotometry, IR, TEM, HRTEM, and BET. e performance of the supported catalysts was evaluated in the degradation of rhodamine B (RhB) in the solution under visible-light irradiation. e 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. e 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). is facilitated the photocatalytic degradation of RhB by Fe-C-TiO2.


Introduction
TiO 2 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 CO 2 , H 2 O, inorganic ions, and minerals that usually have insignificant impacts on environments [1,2].However, the application of TiO 2 is limited because the TiO 2 with band gap ∼3.2 eV is only activated under UV radiation and it is difficult to recycle.erefore, modification of catalyst is crucial to enhance its photocatalytic activity.To present, catalytic efficiency of TiO 2 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][4][5][6].Among these metals, TiO 2 doped with Fe is in progress because Fe can replace the positions of Ti 4+ 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 TiO 2 structure can also narrow the band gap of TiO 2 , reinforcing the photocatalytic activity of the catalyst [2,8].Some research reported that transition metals/(N, C, S) co-doped into TiO 2 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].TiO 2 -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.e 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].ese 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].ere are surprisingly quite a few papers using this method, and there is no published work on C/Fe codoped titanium dioxide coated on activated carbon that is reported in this manuscript.Hence, in the present work, TiO 2 co-doped iron and carbon coated on activated carbon that was treated by HNO 3 was prepared by sol-gel followed by solvothermal method.e 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.e catalytic activity of the catalyst was examined by the degradation of RhB dye.e resulted powder was washed and dried at 100 °C for 24 hours to obtain the Fe-C-TiO 2 /AC catalyst.

Characterization of As-Synthesized Photocatalyst.
e 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).
e 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).

Photocatalytic Performance of As-Synthesized
Photocatalysts.
e catalytic activity of prepared materials was examined by the degradation of RhB solution (20 mg/L) under visible-light irradiation.e 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.
e 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.

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).e 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.e Ti 2p spectrum consists of peaks located at 459.1 eV and 464.8 eV, indicating the existence of Ti (IV) in TiO 2 component [1]. is suggests that the doping of iron and carbon then attached to the AC carrier does not alter the chemical state of TiO 2 .is 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.ese peaks correspond to Ti-O bond and O-H groups in TiO 2 [8]. is 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.
e 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 Fe 3+ 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 Fe 2+ ions, possibly in FeO [5].Coexistence of Fe 3+ and Fe 2+   2 Journal of Chemistry  can enhance the photocatalytic efficiency based on the following reactions: in which Fe 3+ acts as a conductive photonic electron trap to prevent recombination of electrons and holes, while Fe 2+ 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).e main peak with energy value at 284.3 eV is assigned to carbon graphite.e presence of 285.9 eV and 288.4 eV peaks is associated with C-O and C�O bonds of catalytic surface carbonates [11].
e 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 TiO 2 anatase crystals.e XPS analysis confirmed that iron and carbon had successfully entered the TiO 2 network and also showed the successful binding of catalyst to activated carbon activated by HNO 3 .
e XRD patterns (Figure 2) show that Fe-C-TiO 2 after being coated on activated carbon exhibits the typical peaks referring to the anatase form [6]. e phase composition of nanoscale particles is preserved compared with the original materials (Figure 3).e TEM images of the Fe-C-TiO 2 /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.
e result of HR-TEM (Figure 3(b)) confirms the Fe-C-TiO 2 catalyst binding on AC is based on black spots covered by catalytic spheres [10].
Proposed mechanism of TiO 2 coating process on AC is shown in Figure 4.After AC was pretreated with HNO 3 , carbon supplied more hydroxyl and acid groups on the surface, which bond to OH − and H + ions of catalyst to enhance the adhesiveness.
Table 1 shows that the surface areas of pristine AC and AC carrying catalysts measured by BET are different.e reason is that during the treatment of AC, HNO 3 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.
IR spectra of pristine TiO 2 , Fe-C-TiO 2 , and Fe-C-TiO 2 / AC are presented in Figure 5. Peak at 1436 cm −1 refers to the -COO-Ti vibration.e peaks at 551 cm −1 and 502-505 cm −1 are assigned to the bonds of Ti-O and Ti-O-C, respectively.
e UV-Vis spectra of the samples are displayed in Figure 6. e results show that by coating Fe-C-TiO 2 on AC, the absorption band of Fe-C-TiO 2 /AC was significantly expanded to the visible region in comparison with the absorption band of pristine TiO 2 and Fe-C-TiO 2 .Indeed, as expected, the main absorption edge of TiO 2 was estimated to be about 398 nm (2.96 eV) due to its intrinsic band gap absorption.e onset of the absorption spectrum of Fe-C-TiO 2 was shifted to the visible region because of the Fe and C dopants while Fe-C-TiO 2 /AC presented main absorption spectrum at the region above 480 nm. is result suggests that the Fe-C-TiO 2 /AC catalyst is more active under visible light.
From the UV-Vis spectra of pristine TiO 2 , Fe-C-TiO 2 , and Fe-C-TiO 2 /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 TiO 2 , Fe-C-TiO 2 , and Fe-C-TiO 2 /AC, respectively.

Catalytic Activity of Fe-C-TiO 2 /AC in the Degradation of RhB under Visible Light.
e comparison of photocatalytic performance of Fe-C-TiO 2 /AC, Fe-C-TiO 2 , and TiO 2 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.e results also indicate that Fe-C-TiO 2 /AC is the best photocatalyst in degradation of RhB in solutions with more than 99% of RhB removal after 90 min while pristine TiO 2 exhibits the lowest performance (only some 40% of RhB is removed after the same reaction time).Pure AC and Fe-C-TiO 2 present photocatalytic efficiency in between pristine TiO 2 and Fe-C-TiO 2 /AC. is illustrates the synergy between adsorption capacity of AC and the improvement of Fe-C-TiO 2 compared with pristine TiO 2 .
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%.
is 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.
e photocatalytic degradation of RhB by the Fe-C-TiO 2 /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 TiO 2 by the process: TiO 2 + hν (UV) ⟶ TiO 2 (e CB − + h VB +); (2) when • radicals are mainly responsible for the degradation of RhB in solution [12]. 4 Journal of Chemistry e catalytic stability in RhB decomposition under visiblelight 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.e results show that the catalyst exhibited good photocatalytic activity after 5 cycles, as shown in Figure 11.e ability to decompose RhB over 5 cycles remained high (>87%).
is research has proved that Fe-C-TiO 2 /AC is a highly durable, economically suitable, and practical catalyst.

Conclusions
e Fe-C-TiO 2 catalyst was successfully carried on the AC treated by HNO 3 .e 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.e catalyst after being deposited on AC carriers activated with HNO 3 has better photocatalytic activity than activated carbon without catalysts.e Fe-C-TiO 2 /AC catalyst also presented better photocatalytic performance in RhB degradation compared with pristine TiO 2 and with Fe-C-TiO 2 .
e optimal catalyst load for RhB
electrons can be excited from the Fe-C-TiO 2 /AC energy level by the following process: Fe-C-TiO 2 /C + hν (visible) ⟶ Fe-C-TiO 2 /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 TiO 2 to the Fe-C-TiO 2 /AC energy level by the process: TiO 2 + Fe-C-TiO 2 / C + hν (visible) ⟶ Fe-C-TiO 2 /AC (e CB −) + TiO 2 (h VB +). e departed electrons and holes subsequently migrate to the surface of the catalysts and react with adsorbed H 2 O and O 2 molecules, forming HO • and O 2 • , respectively.HO • and O 2