Visible-light-driven heterojunction photocatalysts based on g-C3N4 decorated La2Ti2O7: Effective transportation of photogenerated carriers in this heterostructure

Accepted Manuscript Visible-light-driven heterojunction photocatalysts based on gC3N4 decorated La2Ti2O7: Effective transportation of photogenerated carriers in this heterostructure

Fangzhi Wang, Wenjun Li, Shaonan Gu, Hongda Li, Xintong Liu, Chaojun Ren PII: DOI: Reference:

S1566-7367(17)30126-7 doi: 10.1016/j.catcom.2017.04.004 CATCOM 4989

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

2 December 2016 25 February 2017 2 April 2017

Please cite this article as: Fangzhi Wang, Wenjun Li, Shaonan Gu, Hongda Li, Xintong Liu, Chaojun Ren , Visible-light-driven heterojunction photocatalysts based on g-C3N4 decorated La2Ti2O7: Effective transportation of photogenerated carriers in this heterostructure. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.04.004

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ACCEPTED MANUSCRIPT Visible-light-driven heterojunction photocatalysts based on g-C3N4 decorated La2Ti2O7: effective transportation of photogenerated carriers in this heterostructure

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Fangzhi Wang, Wenjun Li,* Shaonan Gu, Hongda Li, Xintong Liu, Chaojun Ren

Beijing Key Laboratory for Science and Application of Functional Molecular and

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Crystalline Materials, University of Science and Technology Beijing, Beijing 100083, China

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Corresponding Author

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*E-mail: [email protected]

ACCEPTED MANUSCRIPT Abstract Lanthanide titanate (La2Ti2O7) with strong reduction-oxidation ability has been regarded as a new promising photocatalyst. To boost its photocatalytic activity, g-C3N4/La2Ti2O7 composites were designed and constructed. The results reveal that La2Ti2O7 nanosheets were well attached to the surface of g-C3N4 and the composites had good visible-light absorption properties. Significantly,

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compared to pure La2Ti2O7 and g-C3N4, the composites exhibited outstanding photocatalytic performance under visible-light irradiation. The enhanced photocatalytic activity could be derived

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from the effective transportation of photogenerated electrons from g-C3N4 to La2Ti2O7. The

photodegradation reaction.

g-C3N4/La2Ti2O7;

Nanocomposites;

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Photocatalysis.

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Effective

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Keywords:

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electrons with high reduction ability can generate more active species to participate in the

transportation;

Visible

light;

ACCEPTED MANUSCRIPT 1. Introduction Semiconductor-based photocatalysis has been regarded as one of the most appealing methods for environmental remediation and solar energy conversion [1]. Presently, perovskite-type lanthanide titanate (La2Ti2O7) with layered structure has been confirmed to serve as a new promising photocatalyst [2,3]. La2Ti2O7 possess a strong reduction-oxidation ability. However, its limited

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visible light response (the bang gap is ca. 3.8 eV) often leads to poor photocatalytic performance, and thus restricts its popularization. The formation of heterojunction is one of attractive and

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efficient strategies for enhancing quantum yield and photocatalytic performance [4]. For instance, Ao et al. have reported BiOI/La2Ti2O7 [5] and BiOBr/La2Ti2O7 [6] composite photocatalyst, which

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both show high-efficiency in the photocatalytic process. In addition, our research group have recently reported FeWO4@ZnWO4/ZnO heterojunction photocatalyst with superior visible light

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photocatalytic performance [7]. Therefore, coupling La2Ti2O7 with other semiconductors to form heterojunction is a good expectation for boosting the photocatalytic activity.

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Graphitic carbon nitride (g-C3N4), a metal-free semiconductor with layered structure, has aroused great concern in water decontamination and producing hydrogen and oxygen via water

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splitting [8,9]. Remarkably, recent results show that g-C3N4-based heterojunction exhibited an

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enhanced photocatalytic performance [10-12]. For example, g-C3N4/SiO2 [13], g-C3N4/ZnAl2O4 [14], and g-C3N4/TiO2 [15] heterojunction photocatalysts have been researched; these photocatalysts all display superior degradation efficiency. Recently, Zhang et al. have studied

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g-C3N4 hybridized N-doped La2Ti2O7 composites [16], and found that the composites exhibits high photocatalytic H2 evolution and MO degradation. What is more, for g-C3N4/La2Ti2O7

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heterojunction system, the band edges of g-C3N4 (ECB = −1.4 eV, EVB = 1.3 eV) [17] can match well with La2Ti2O7 (ECB = −0.39 eV, EVB = 3.33 eV) [6]. As a result, photogenerated electrons can easily transfer from g-C3N4 to La2Ti2O7. These electrons with high reducibility can produce more active species. Thus, inspired by the above analyses, it is a realizable tactic to fabricate g-C3N4/La2Ti2O7 heterojunction in achieving high-efficiency photocatalytic activity. In this work, La2Ti2O7 and g-C3N4 were synthesized via hydrothermal and heat treatment method, respectively. Subsequently, g-C3N4/La2Ti2O7 composites were successfully fabricated using a facile wet-impregnation method. Compared with the pure La2Ti2O7 and g-C3N4, the g-C3N4/La2Ti2O7 composites showed enhanced activity on the photodegradation of MB under

ACCEPTED MANUSCRIPT visible-light irradiation. The outstanding photocatalytic activity could be derived from the effective separation of photogenerated carriers. Moreover, the reasonable enhancement mechanism of C3N4/La2Ti2O7 composites photocatalytic performance was also investigated. This work could expose the nature of the C3N4/La2Ti2O7 composites about improving the

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photocatalytic activity of La2Ti2O7.

2. Experimental Section

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2.1. Synthesis

La2Ti2O7 nanosheets were synthesized using a hydrothermal method. La(NO3)3·6H2O (10 mmol)

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and Ti(SO4)2 (10 mmol) were dissolved in 60 mL of Milli-Q water. Afterwards, 20 mL of NaOH aqueous solution containing 0.1 mol NaOH was added to the above solution under vigorous

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stirring. After 10 min, the obtained precursor was transferred into a 100 mL Teflon-lined stainless steel autoclave, then maintained at 200 °C for 24 h. Subsequently, the white precipitate was

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collected by filtration, washed with Milli-Q water and ethanol several times, and then dried at 80 °C overnight.

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The g-C3N4 powders were prepared by heating melamine. Briefly, 10 g of melamine was placed

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in an alumina crucible with a cover, heated in a muffle furnace at a rate of 2 °C min−1, then maintained at 550 °C for 4 h. The yellow products were collected and milled into powder for further use.

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The g-C3N4/La2Ti2O7 composites were synthesized by using a wet-impregnation method. Typically, an appropriate amount of g-C3N4 and La2Ti2O7 were dispersed into methanol,

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respectively, and both dispersed under sonication for 30 min. Subsequently, the above two solutions were mixed and stirred for 24 h. After volatilization of the methanol, the powder was dried at 100 °C for 12 h. The composites were named as CLa α:β (α:β is assigned to the mass ratio of g-C3N4/La2Ti2O7).

2.2. Characterization of photocatalysts The crystalline phases were obtained by using X-ray diffraction (XRD) (D/MAX-RB; Rigaku, Japan) and their morphologies by scanning electron microscopy (SEM) (S-4800; Hitachi, Japan) equipped with an energy-dispersive X-ray spectrometer (EDX) and transmission electron

ACCEPTED MANUSCRIPT microscopy (TEM) (F-20; FEI, USA). The surface area were determined by nitrogen adsorption at 77.3 K by a Quadrasorb SI gas sorption analyzer (Quantachrome, USA). The UV–vis diffuse reflectance spectra (DRS) were recorded by a UV–vis spectrophotometer (T9s; Persee, China) equipped with an integrating sphere. BaSO4 was used as the reference.

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2.3. Photocatalytic experiments The photocatalytic activities were evaluated by degradation of methylene blue (MB) under

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visible light irradiation (λ ≥ 420 nm, 400 W Xe lamp). Samples (40 mg) were dispersed into 40 mL MB solution (10 mg/L). Prior to visible light irradiation, the reaction suspensions were

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vigorously stirred for 1 h to establish an adsorption–desorption equilibrium. At a given time interval, 4 mL suspensions from each sample were collected and then centrifuged. The

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concentration of MB were monitored by a UV–vis spectrophotometer (T9s; Persee, China) at 664

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nm which is the maximum absorption wavelength.

2.4. Photoelectrochemical measurements

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Photoelectrochemical measurements part was described in Supplementary information.

3. Results and discussion

3.1. Structure and morphology

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Fig. 1 displays the XRD patterns of as-prepared pure g-C3N4, La2Ti2O7, and g-C3N4/La2Ti2O7 composites. The characteristic diffraction peaks of pure La2Ti2O7 and g-C3N4 can be indexed to

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the monoclinic La2Ti2O7 (JCPDS No. 28-0517) [5] and the hexagonal phase of g-C3N4 (JCPDS No. 87-1526) [18]. For g-C3N4/La2Ti2O7 composites, all diffraction peaks were in good agreement with the monoclinic La2Ti2O7. It should be pointed that the diffraction peak of g-C3N4 in (0 0 2) lattice plane is so close to the diffraction peak of La2Ti2O7 in (-1 1 2) lattice plane. So it is difficult to identify the diffraction peaks of g-C3N4 crystalline phase in g-C3N4/La2Ti2O7 composites. Purity of g-C3N4, La2Ti2O7, and CLa 0.5:1 was investigated by EDX analysis. From Fig. S1, pure g-C3N4 is composed of C and N elements, and pure La2Ti2O7 consists of O, La, and Ti elements. As for CLa 0.5:1 composite, C, N, O, La, and Ti signals can be obviously observed in the spectrum. This finding gives evidence that the as-prepared photocatalyst was g-C3N4/La2Ti2O7

ACCEPTED MANUSCRIPT composite. The morphologies of pure g-C3N4, La2Ti2O7, and g-C3N4/La2Ti2O7 composites were obtained by SEM images. It can be found that g-C3N4 (Fig. S2A) were irregular particles consisting of smooth layer structures [19] and La2Ti2O7 (Fig. S2B) presented nanosheet-like morphologies. From Fig. 2A and Fig. S2C-E, for all g-C3N4/La2Ti2O7 composites, the bulk g-C3N4 and

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nanosheet-like La2Ti2O7 can be observed obviously and they were intimately combined to form C3N4/La2Ti2O7 heterojunctions.

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The more detailed microstructure of g-C3N4/La2Ti2O7 composites was further investigated by TEM and HRTEM. As displayed in Fig. S2F, the La2Ti2O7 nanosheets were closely attached to the

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surface of g-C3N4. HRTEM image (Fig. 2B) of the sample shows that the lattice distance of 0.28 nm corresponded well to the (0 2 0) lattice plane of monoclinic La2Ti2O7. A clear interface

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appeared between La2Ti2O7 and g-C3N4, demonstrating the formation of g-C3N4/La2Ti2O7 heterojunction by wet-impregnation process, which will promote the separation efficiency of

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photogenerated carrier.

Fig. S3 shows the N2 adsorption-desorption curves of g-C3N4, La2Ti2O7, and CLa 0.5:1. The

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samples have typical IV isotherms with H1 hysteresis, suggesting that the samples are mesoporous.

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The comparison of the specific BET surface area were listed in Table S1. The results showed that there was a close connect between adsorption-desorption ability and BET surface area; both of the surface area and adsorption-desorption ability of CLa 0.5:1 composite sample are presented

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between that of g-C3N4 and La2Ti2O7. The findings were consistent with the SEM observation, in

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which heterojunction structure were formed by bulk g-C3N4 and nanosheet La2Ti2O7.

3.2. Optical properties Fig. 3A displays the UV–vis DRS of the samples. The La2Ti2O7 exhibited UV-light absorption only, while the g-C3N4 presented an absorption edge at about 450 nm, which were consistent with the reported results [6,20]. After loading the g-C3N4 on the La2Ti2O7 nanosheets, the optical absorption edge of g-C3N4/La2Ti2O7 composites was evidently red-shifted compared to La2Ti2O7, and the extent of red-shifting was enhanced with the increase of loaded-g-C3N4 amount; this is likely to attribute to the heterojunction structure between the La2Ti2O7 and g-C3N4. The band gap energies (Eg) of the photocatalysts can be calculated by the following formula: ahν = A(hν–Eg)n/2,

ACCEPTED MANUSCRIPT where a, h, v, Eg, and A are the absorption coefficient, Planck’s constant, light frequency, band gap energy, and a constant, respectively. Among them, n depends on the type of optical transition of semiconductors (n = 1 for direct transition and n = 4 for indirect transition). The n of g-C3N4 and La2Ti2O7 were both 1[5,10]. As shown in Fig. S4, the band gap energies of as-prepared samples were estimated from the plots of (ahv)2 versus hv. Therefore, the Eg of the g-C3N4 and La2Ti2O7

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were estimated to be 2.70 and 3.72 eV, respectively. In addition, as for composites samples of CLa 0.2:1, CLa 0.5:1, CLa 1:1, and CLa 1:2, the Eg were found to be 2.73, 2.74, 2.76 and 2.77,

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respectively (Table S1). The results further suggested that g-C3N4/La2Ti2O7 composites can be

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excited by visible-light.

3.3. Photocatalytic performances of the samples

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Fig. 3B exhibits the photocatalytic activities of g-C3N4, La2Ti2O7, and g-C3N4/La2Ti2O7 composites for photodegradation of MB under visible-light irradiation. After 120 min of

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visible-light irradiation, the photodegradation of MB was neglectable with La2Ti2O7, and very slow in the presence of g-C3N4. The g-C3N4/La2Ti2O7 photocatalysts manifested much higher

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photocatalytic activities than that of pure g-C3N4 and La2Ti2O7. It is noteworthy that the sample of

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CLa 0.5:1 showed the highest photodegradation efficiency. However, excess g-C3N4 content in g-C3N4/La2Ti2O7 composites could lead a reduction of the photocatalytic performance. This might attribute to the fact that high content of g-C3N4 is not conducive to the formation of the effective

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heterojunction. In addition, excess g-C3N4 might act as the recombination center of photoinduced carrier. For comparison, mechanical mixed CLa 0.5:1 was also used for photodegrading the MB,

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which presented obviously lower activity than that of CLa 0.5:1 composites. This result indicated that the heterojunction structure between g-C3N4 and La2Ti2O7 played a crucial role in boosting the photocatalytic performance. Fig. S5 shows the temporal absorption spectral changes of the photodegradation of MB aqueous solution over CLa 0.5:1. It can be seen that with prolonged irradiation time, the intensity of the characteristic peak at 664 nm decreased gradually, suggesting the structure of MB molecules was decomposed. To test the stability of g-C3N4/La2Ti2O7 composites, the cycling experiments of MB photodegradation was carried out over CLa 0.5:1 and the results were shown in Fig. S6. The photocatalytic performance of CLa 0.5:1 showed almost no decrease even after four cycles,

ACCEPTED MANUSCRIPT indicating that g-C3N4/La2Ti2O7 composites possess excellent photocatalytic stability.

3.4. Photocatalytic reaction mechanism In order to analyze the photocatalysis mechanism of the g-C3N4/La2Ti2O7 photocatalysts, radical and hole trapping experiments [21] were used to investigate the roles of the active species (Fig. 3C). The photocatalytic performance of g-C3N4/La2Ti2O7 was almost invariable after the addition

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of IPA (a quencher of •OH), suggesting that •OH was not formed in the degradation of MB.

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However, when BQ (a quencher of •O2–) or Na2C2O4 (a quencher of h+) was added, the photocatalytic efficiency was obviously suppressed, which indicated the •O2– and h+ serve as the

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major active species in the g-C3N4/La2Ti2O7 photocatalytic process.

To further understand the separation and transportation of photogenerated carriers in the

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g-C3N4/La2Ti2O7 heterostructure, the photocurrent responses test was performed [22]. Fig. S7 displays the photocurrent responses of La2Ti2O7, g-C3N4, and CLa 0.5:1 in the light and dark. It

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can be clearly observed that CLa 0.5:1 had a higher photocurrent density compared with that of pure La2Ti2O7 and g-C3N4. This finding suggested more effective separation and transportation of

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photocatalytic performance.

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photogenerated electron–hole pairs in the g-C3N4/La2Ti2O7 heterostructure, which will boost the

Based on the above analytical results, a reasonable mechanism was proposed to illustrate the enhanced photocatalytic degradation using g-C3N4/La2Ti2O7 heterojunction in Fig. 4. The CB and

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VB potentials of g-C3N4 are −1.4 and 1.3 eV [6,17], and those of La2Ti2O7 are −0.39 and 3.33 eV, respectively, which are beneficial for efficient carriers separation after formation of

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g-C3N4/La2Ti2O7 heterojunction. When irradiated with visible-light, g-C3N4 can be activated to generate electron–hole pairs. The photogenerated electrons in the CB of g-C3N4 will easily transfer to the CB of La2Ti2O7, and the photogenerated holes still stay in the VB of g-C3N4. The electrons with high reducibility can be trapped by the oxygen molecules to produce •O2– radicals that with strong oxidation ability to degrade the organic pollutant, while the holes can directly oxidize reactants. Therefore, it can result in highly efficient separation of photogenereted carrier, and thus lead to high-efficiency visible light photocatalytic activity.

4. Conclusions

ACCEPTED MANUSCRIPT The heterojunction photocatalysts g-C3N4/La2Ti2O7 were successfully fabricated via a facile wet-impregnation method. The g-C3N4/La2Ti2O7 displayed significantly enhanced photocatalytic performance compared to that of pure g-C3N4 and La2Ti2O7 under visible-light irradiation. The quenching experiment indicated the •O2– and h+ were the major active species in the photocatalytic reaction system. Furthermore, the formation of heterojunction between g-C3N4 and La2Ti2O7 can

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contribute to the effective separation of photogenerated electron-hole pairs, leading to a high

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photocatalytic performance.

Acknowledgment

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Science Foundation of China (Grant No. 21271022).

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We gratefully acknowledge the financial support provided by the Project of the National Natural

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Figures captions:

Figure 1. (A) XRD patterns of different samples. Figure 2. SEM and HRTEM images of CLa 0.5:1.

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Figure 3. (A) DRS spectra of different samples; (B) Photodegradation of MB with different photocatalysts under visible-light irradiation (λ≥420 nm); (C) Influence of various scavengers

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on the photocatalytic performance of CLa 0.5:1. Figure 4. Schematic illustration of the proposed charge transfer in the g-C3N4/La2Ti2O7 heterostructure during MB degradation process under visible-light irradiation.

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ACCEPTED MANUSCRIPT Graphical abstract The enhanced photocatalytic activity is derived from the effective transportation of

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photogenerated carriers in g-C3N4/La2Ti2O7 heterostructure.

ACCEPTED MANUSCRIPT Highlights The g-C3N4/La2Ti2O7 heterojunctions were prepared by a wet-impregnation method.

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The composites exhibited superior photocatalytic activity.

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The reasonable mechanism of the enhanced photodegradation efficiency was investigated.

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