Citation

Zhijun Luo, Lingling Qu, Jianzhong Jia, Jing Wang, Suying Jiang, Zhiren Wu, Xiangyang Wu. TiO₂/EDTA-rich carbon composites: Synthesis, characterization and visible-light-driven photocatalytic reduction of Cr(Ⅵ)[J]. Chin. Chem. Lett., 2018, 29(3): 547-550  

Zhijun Luo^a,b,c, Lingling Qu^a, Jianzhong Jia^a, Jing Wang^a, Suying Jiang^b,c, Zhiren Wu^a,b,c,1, Xiangyang Wu^a,1    

With the development of steel manufacturing, leather–tanning, dye and paint, mobile hexavalent chromium (Ⅵ) compounds have been the major metal ions hazardous to human beings, due to their high toxicity and carcinogenic properties [1, 2]. However, trivalent chromium Cr(Ⅲ) is considered a low toxicity, necessary trace element in human nutrition. Since Cr(Ⅲ) is mostly immobile, which can be precipitated down from aqueous solution by the form of Cr(OH)₃ [3, 4], the conversion of Cr(Ⅵ) to Cr(OH)₃ in neutral and alkaline solutions is considered as an efficient strategy for the removal of mobile chromium (Ⅵ) compounds. Compared with conventional chemical reduction methods, photocatalytic reduction of Cr(Ⅵ) using the semiconductor shows many advantages, such as high efficiency, low cost, green and mild reaction conditions [5, 6].

As one of the most promising photocatalyst, titanium dioxide (TiO₂) has been successfully used in the pollutant degradation, removal of heavy metal ions and air purification [7, 8]. Considering the restriction of UV light, people have been employed a variety of strategies to improve the photocatalytic performance of TiO₂ by extending the excitation wavelength to visible light, such as metal/ non-metal doping, semiconductor coupling and surface modification via organic materials [9-13]. Recently TiO₂ modified with ligand-to-metal charge transfer (LMCT) complex has been another important strategy to extend the optical response of TiO₂ to visible light region [14]. However, the visible light sensitization mechanism based on LMCT complex is different from the conventional dye sensitization. In the LMCT process, the electron is photoexcited directly from the ground state adsorbate (without involving the excited state of the adsorbate) to the conduction band (CB) of TiO₂. However, in a typical dye sensitization, the photochemcial process of dye sensitization is initiated by the HOMO-LUMO photoexcitation of dye molecules which are preadsorbed onto the TiO₂ surface, followed by electron transfer from the excited dye to the CB of TiO₂ [15]. Generally, the LMCT complexes on TiO₂ can be induced by chemisorption (through a bond formation) or physisorption in aqueous solution. These adsorption process, especially the physisorption, often makes the LMCT complexes on the surface of TiO₂ unstable [14, 16].

Ligands play crucial roles in the characteristics of LMCT complexes, such as binding mode, optical response, and charge transfer efficiency. Carboxylate was often used to fabricate the LMCT complexes on the surface of TiO₂ and shown excellent visible light photocatalytic activities. Ethylenediaminetetraacetic acid (EDTA) with high carboxyl group content and strong coordination ability is an excellent ligand for the fabrication of LMCT complexes on the surface of TiO₂ [14]. In the present study, a facile low temperature carbonization process was developed to fabricate the TiO₂/EDTA-rich carbon composites (TiO₂/EDTA-RC). Due to the low carbonization temperature, abundant EDTA can be remained in carbon sheet rather than thermal decomposition. Carbon is insoluble in water, which makes the TiO₂/EDTA-RC more stable in aqueous solution. The unique structure of TiO₂/EDTA-RC give it advantages over the conventional TiO₂-EDTA complexes which were fabricated via the EDTA adsorption on the surface of TiO₂ in the aqueous solution. Carbon sheet of TiO₂/EDTA-RC acts as a supporter, TiO₂ nanoparticles and EDTA can homogeneously disperse into the carbon sheet supporter and form the TiO₂-EDTA complexes. TiO₂/EDTA-RC exhibits marked absorption of visible light and excellent photoreduction of Cr(Ⅵ) activity via LMCT process.

The XRD pattern of TiO₂/EDTA-RC is shown in Fig. 1a. Two diffraction peaks located at the 2θ values of 43.8° and 48.7° can be indexed to the monoclinic titanate H₂Ti₃O₇ (JCPDS card No. 36-654). Clearly, the further evidence is required to ascertain the crystallographic structure of TiO₂/EDTA-RC. Fig. 1c presents the Raman spectrum of TiO₂/EDTA-RC. The bands at around 391 cm^-1, and 630 cm^-1 are typical bands of the anatase phase of TiO₂, and around 153 cm^-1, 192 cm^-1, and 270 cm^-1 could be from the H₂Ti₃O₇, which are in good agreement with the Raman spectrum of H₂Ti₃O₇ obtained by Lin et al. and Zheng et al. [17, 18]. The results demonstrate that there are two different TiO₂ crystalline phases, anatase and monoclinic titanate H₂Ti₃O₇ in TiO₂/EDTA-RC. Moreover, the broad D and G bands around 1361 and 1576 cm^-1 are expected to correspond to the amorphous carbons. The large I_D/I_G value (0.9) reveals the low degree of graphitization of the EDTA-RC in TiO₂/EDTA-RC. In addition, we found no obvious diffraction peak for the carbon in XRD pattern (Fig. 1a), which also suggests the EDTA-rich carbon is amorphous carbon. The composition of TiO₂/EDTA-RC is further analyzed by the TG experiment. Fig. 1d shows TG plots for the TiO₂/EDTA-RC with heat treatment at 5 ℃/min in air. The powders exhibit about 12.11 wt% weight loss below 190 ℃ due to the release of adsorbed water. The major weight loss take place between and 620 ℃ and the weight loss is about 61.48 wt%. From the above results of TG, we can conclude that the mass percentage of TiO₂ in TiO₂/EDTA-RC is about 26.41 wt%.

Figure 1

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Fig. 1. (a) XRD pattern, (b) optical photograph, (c) Raman spectra, and (d) thermogravimetric (TG) curve of TiO2/EDTA-RC

SEM and TEM were performed to examine the morphology and structure of the TiO₂/EDTA-RC. From the low-magnification SEM image (Figs. 2a & b), we can see that the TiO₂/EDTA-RC are mainly composed of many 2-D sheets of about 1 mm thick. TiO₂ nanoparticles with a diameter about 10 nm are uniformly distributed in carbon sheet, which is beneficial for the synthesis of EDTA-TiO₂ complex. Furthermore, The small particle size (about 10 nm), the low mass percentage (26.41 wt%) and the EDTA-RC coating of TiO₂ may be responsible for the broad and weak XRD peaks of TiO₂ (Fig. 1a).

Figure 2

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Fig. 2. (a, b) SEM images, (c, d) SEM image and the corresponding EDS mapping data for Ti (e, f) TEM images of TiO2/EDTA-RC

Fig. 3a shows the FT-IR spectrum of pure EDTA and TiO₂/EDTARC. For pure EDTA, two pair typical bands of carboxylic group, two asymmetric bands (1674 and 1628 cm^-1) and two symmetric bands (1475 and 1395 cm^-1), can be observed clearly [19, 20]. However, for TiO₂/EDTA-RC, two pair bands are drifted. Two asymmetric bands shift from 1674 and 1628 cm^-1 to 1695 and 1584 cm^-1, respectively. Two symmetric bands shift from 1475 and 1395 cm^-1 to 1452 and 1380 cm^-1, respectively. Compared with the pure EDTA, the shift of typical bands of carboxylic group for TiO₂/EDTA-RC indicates that the carboxylate group of EDTA is successfully coordinated with the TiO₂ and form the EDTA-TO₂ complex [19, 20]. In addition, the IR spectra of TiO₂/EDTA-RC shows a intensive broad band around 600 cm^-1 which is attributed to the Ti-O-Ti bonds stretching modes [21]. The wide band at approximately 3000–3700 cm^-1 reveals the existence of large numbers of hydroxyl groups [22]. To investigate the surface composition and chemical state of TiO₂/EDTA-RC, X-ray photoelectron spectroscopy (XPS) was carried out. Fig. 3b shows the different XPS spectral regions corresponding to the different elements for the TiO₂/EDTARC. The XPS spectrum indicates the binding energies for C 1s, O 1s, N 1s, Na 1s, Ti 2s, Ti 2p_3/2, Ti 3s, and Ti 3p of the TiO₂/EDTA-RC. The partial XPS data of C 1s (Fig. 3c) can be deconvoluted into C－C at about 284.5 eV, C－O at about 285.3 eV and C＝O at about 287.7 eV [23, 24]. In partial XPS spectrum of Ti 2p (Fig. 3d), the peaks for Ti 2p_3/2 and Ti 2p_1/2 appeared at 458.6 and 464.3 eV, which imply the formation of TiO₂ in the TiO₂/EDTA-RC [25]. The peak for N 1s (Fig. 3e) is derived from the EDTA [26]. The FT-IR and XPS data demonstrated that abundant EDTA incorporated in carbon (EDTArich carbon) at a low carbonization temperature (300 ℃), even without the protection of inert gas. Carboxylate from EDTA-RC are successfully coordinated with the positive titanol group of TiO₂ and induce the EDTA-TiO₂ complex formation.

Figure 3

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Fig. 3. (a) FT-IR spectra, (b) XPS survey spectra, (c) C s high-resoltion XPS spectra, (d) Ti 2p high-resoltion XPS spectra, and (e) N s high-resoltion XPS spectra of TiO2/ EDTA-RC

The carboxyl group content of TiO₂/EDTA-RC is an important factor for obtaining more EDTA-TiO₂ complex. The carboxyl group content of TiO₂/EDTA-RC were determined by chemical titration method. The carboxyl group content of TiO₂/EDTA-RC can reach 2.53 mmol/g which is higher than that (0.422 mmol/g) of carbon nanotubes treated with hot H₂SO₄/HNO₃ [27]. Due to large numbers of carboxyl group on the surface of TiO₂/EDTA-RC, the zeta potential values of TiO₂/EDTA-RC are more negative than pure TiO₂. The surface charge measured as zeta potential is presented in Fig. S1 in Supporting information for the pure TiO₂ and TiO₂/EDTARC at different pH values. The pH of zero point charge (pH_PZC) of pure TiO₂ and TiO₂/EDTA-RC are 6.7 and 3.2, respectively. According to the previous report, H₂CrO₄ and HCrO₄^- are the main form at pH less than 4.0 [28]. When the pH less than 3.2, negative charged Cr(Ⅵ) ions can be easily adsorbed on the positive charged surface of TiO₂/EDTA-RC by electrostatic attraction and ion exchange, which will benefit the subsequent photocatalytic reduction of Cr(Ⅵ). As shown in Fig. 4a, with the initial pH value increase from 2.0 to 7.0, the Cr(Ⅵ) reduction rate is significantly depressed which demonstrates the effect of initial pH on the photocatalytic reduction Cr(Ⅵ) kinetic rate by TiO₂/EDTA-RC. At pH 2.0, almost 98% Cr(Ⅵ) can be reduced under visible light irradiation (λ > 420 nm), while the activity for Cr(Ⅵ) conversion is negligible when initial pH value is high than pH 4.0.

Figure 4

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Fig. 4. (a) the effect of initial pH value on photoreduction of Cr(Ⅵ) by TiO2/EDTARC; (b) comparison of the photocatalytic performance with no photocatalyst, pure TiO2, TiO2/C, TiO2/EDTA-RC, and F-TiO2/EDTA-RC; (c) DRS spectra of EDTA, pure TiO2, TiO2/C, F-TiO2/EDTA-RC and TiO2/EDTA-RC; (d)Proposed photocatalytic mechanism of TiO2/EDTA-RC under visible light irradiation

To distinguish the main factor in TiO₂/EDTA-RC for the excellent visible-light-driven photocatalytic activities, pure TiO₂, TiO₂/ EDTA-RC, TiO₂/C (without EDTA), and F-TiO₂/EDTA-RC were used as photocatalysts to reduce Cr(Ⅵ). Fig. 4b shows the dark adsorption of Cr(Ⅵ) and the photocatalytic conversion of Cr(Ⅵ) removal over different photocatalysts. Cr(Ⅵ) can be adsorbed on the surface of pure TiO₂, TiO₂/EDTA-RC, TiO₂/C, and F-TiO₂/ EDTA-RC. For TiO₂, TiO₂/C (without EDTA), and TiO₂/EDTA-RC, the adsorption/desorption equilibrium are achieved rapidly (< 15 min). However, F-TiO₂/EDTA-RC need more time to reach equilibrium about 30 min. The surface fluorides of TiO₂/EDTA-RC may lower the electrostatic attraction between the anionic chromate specie (HCrO₄^-) and F-TiO₂/EDTA-RC at pH less than 4.0. TiO₂/EDTA-RC exhibit excellent activity of photocatalytic reduction Cr(Ⅵ) under visible light irradiation (λ > 420 nm). As shown in Fig. 4b, for TiO₂/EDTA-RC more than 98% of 0.2 mmol/L of Cr(Ⅵ) is reduced after 2 h under visible light irradiation, while pure TiO₂ and TiO₂/C in the absence of EDTA are almost inactive. In addition, the activity of photocatalytic reduction Cr(Ⅵ) was significant decreased after the fluorination of TiO₂/EDTA-RC. As is well known, NaF can favor the formation of the complex with TiO₂ by replacing its surface hydroxyl groups, which the surface hydroxyl group (Ti-OH) of TiO₂ is substituted by the surface fluoride group (Ti-F) and restrains other surface complex formation [20, 29]. Therefore, when NaF prohibit the formation of TiO₂-EDTA during the synthesis of TiO₂/EDTA-RC, the visible light photoreduction of Cr(Ⅵ) is dramatically depressed due to The lack of TiO₂-EDTA complex of F-TiO₂/EDTA-RC. Obviously, the formation of the TiO₂-EDTA complexes is the most importance for the visible light activation. This is similar to the previously reported observations [20]. The photocatalytic reduction Cr(Ⅵ) activities of TiO₂/EDTA-RC synthesized by different EDTA dosages were also evaluated. Fig. S2 display the photocatalytic reduction ratio of Cr(Ⅵ) of TiO₂/EDTA-RC synthesized by different EDTA dosages. When the EDTA dosages increase from 0 to 1 g, the reduction ratio of Cr(Ⅵ) steeply increased to 85%. When the EDTA dosages increase from 1 g to 2 g, the reduction ratio of Cr(Ⅵ) increase slowly and the maximal reduction ratio of Cr(Ⅵ) 98% is reached at 2 g EDTA dosage. In case the EDTA dosages exceed 2 g, there is no marked increase for the reduction ratio of Cr(Ⅵ).

To get possible photocatalytic reduction of Cr(Ⅵ) mechanism of TiO₂/EDTA-RC, optical absorption of pure EDTA, pure TiO₂, TiO₂/C (without EDTA), TiO₂/EDTA-RC and F-TiO₂/EDTA-RC have been investigated using UV-visible diffuse reflection spectra (as shown in Fig. 4c). It can be easily seen that EDTA functionalized TiO₂ (TiO₂/EDTA-RC and F-TiO₂/EDTA-RC) show the obvious enhancement in the visible light absorption compared with TiO₂/C, although pure EDTA and pure TiO₂ have no absorption in the visible-light region. TiO₂/EDTA-RC exhibits more excellent absorption than F-TiO₂/EDTA-RC, which is consistent with their visible-light-driven photocatalytic performance of F-TiO₂/EDTARC (Fig. 4b). These results indicate EDTA can improve the photocatalytic activity of the complex by extending its light absorption in the visible-light region. For TiO₂/EDTA-RC, the enhanced visible light photocatalytic reduction of Cr(Ⅵ) should be attributed to the LMCT complex formation of TiO₂-EDTA complexes which extends the absorption into the visible light region. Meanwhile, carbon support plays the important role in the performance of TiO₂/EDTA-RC by avoiding the aggregation and minimizing the size of TiO₂ simultaneously.

On the basis of the above results, the visible-light photocatalytic mechanism of TiO₂/EDTA-RC can be schematically described in Fig. 4d. Considering that rich EDTA coordinated strongly with TiO₂ in carbon and formed TiO₂-EDTA complexes on the surface of TiO₂, the visible light irradiation can directly excite an electron from the ground state of the TiO₂-EDTA complexes to TiO₂ CB. The CB electrons excited through the LMCT mechanism can be fast trapped by Cr(Ⅵ) as electron acceptors and Cr(Ⅵ) is reduced to Cr(Ⅲ).

In summary, we have successfully synthesized a novel photocatalyst TiO₂/EDTA-RC and demonstrated its enhanced photocatalytic activity for the reduction of Cr(Ⅵ) under visible light irradiation (λ > 420 nm). It was observed that more than 98% of 0.2 mmol/L of Cr(Ⅵ) is reduced after 2 h of visible light irradiation by using TiO₂/EDTA-RC as photocatalyst. TiO₂/EDTA-RC possess abundant TiO₂-EDTA complexes and unique structure. Through the contrast experiment of pure TiO₂, TiO₂/EDTA-RC, TiO₂/C, and FTiO₂/EDTA-RC, it is demonstrate that abundant TiO₂-EDTA complexes of TiO₂/EDTA-RC are responsible for the excellent visible light absorption properties and photoreduction activities of Cr(Ⅵ). In addition, the unique structure of TiO₂/EDTA-RC, which TiO₂ nanoparticles (about 10 nm) and EDTA uniformly disperse into the carbon sheet supporter, is benefit to the visible-light photocatalytic reduction of Cr(Ⅵ). This structure of TiO₂/EDTA-RC can avoid the aggregation of TiO₂ nanoparticles in the aqueous solution and provide more photocatalytic reaction point for the reduction of Cr(Ⅵ).

We thank Dr. Xiazhang Li in Changzhou University for his help in TEM characterization. This work was financially supported by the Natural Science Foundation of Jiangsu Province (No. BK20130485), (No. BK20130485), Highly Qualified Professional Initial Funding of Jiangsu University (No. 10JDG120), and Jiangsu Collaborative Innovation Center of Technology and Material of Water Treatment.

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.09.025.