J Mater Sci C H E M I C A L R routes O U T E S T Oto M Amaterials T E R I AL S Chemical

Novel Bi12TiO20/g-C3N4 composite with enhanced photocatalytic performance through Z-scheme mechanism Wang Yu1 1

, Lei Zou1

, Haoran Wang1

, and Xiong Wang1,*

School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

Received: 5 January 2018

ABSTRACT

Accepted: 7 April 2018

Novel Bi12TiO20/g-C3N4 composite was successfully prepared with Bi12TiO20 nanoparticles embedded within the fluffy crumpled g-C3N4 nanosheets. Bi12 TiO20/g-C3N4 composites exhibit superior photoactivity and stability. As compared with g-C3N4 and Bi12TiO20, the photocatalytic efficiency of Bi12TiO20/gC3N4 is effectively enhanced about 1.8- and 4.9-fold, respectively. Based on the trapping experiment, OH and O2- radicals are the dominant reactive oxygen species involved in the photocatalytic process. The proposed Z-scheme mechanism of charge transfer markedly promotes the carriers’ migration and separation, leading to the enhanced photocatalytic performance.

Ó

Springer Science+Business

Media, LLC, part of Springer Nature 2018

Introduction Semiconductor photocatalysis has been considered a potential technology for solving the increasingly serious environmental pollution and global energy crisis due to its ability to harvest solar energy to degrade organic contaminants and produce hydrogen from water [1, 2]. Recently, bismuth titanates, including Bi12TiO20, Bi2Ti2O7, and Bi4Ti3O12, have drawn much attention for their excellent physical properties. Bi12TiO20 with sillenite structure is believed to be a promising candidate as visible lightdriven photocatalyst [3]. It consists of two structural units of BiO5 polyhedra and TiO4 tetrahedra, which contribute to its high photocatalytic activity [4]. Zhou et al. prepared Bi12TiO20 by a solid-state reaction of

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https://doi.org/10.1007/s10853-018-2311-7

Bi2O3 and TiO2 powders and investigated the photocatalytic activity for decomposing methanol under visible light irradiation [5]. The excellent photocatalytic performance of Bi12TiO20 was also reported for degradation of methyl orange and phenol under both ultraviolet and visible light illumination [6, 7]. However, the photoactivity is still inhibited due to the rapid recombination of photoexcited electron/hole pairs and thus to a low quantum efficiency. Constructing heterostructure with different semiconductors is an effective approach to promote the photocatalytic activity. Suitable composite photocatalyst can boost the light harvesting or prolong the lifetime of photogenerated carriers or both, and hence, the photocatalytic properties are much improved [8, 9]. Currently, some candidate materials were selected to couple with Bi12TiO20, such as TiO2

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[10], graphene [11], and Bi2WO6 [12], to further improve the photocatalytic performance. Since the pioneering work of Wang et al. [13], graphitic carbon nitride (g-C3N4) serving as visible light photocatalyst has attracted more and more attention. g-C3N4, constituted by layers of two-dimensional (2D) counterparts, has many fascinating features including non-toxic, high chemical stability, low cost, suitable band structure, and easy preparation [14–17]. g-C3N4 sheets can be readily prepared by pyrolysis of nitrogen-abundant organic precursors, such as urea, cyanimide, dicyandiamide, and thiourea. Many efforts have been undertaken to improve its photocatalytic performance such as element doping, heterojunction formation, and surface modification. In this paper, due to the matched band gaps between Bi12TiO20 and g-C3N4, we report the synthesis of Bi12TiO20/g-C3N4 heterostructures through an efficient ultrasonication strategy. The photocatalytic activity of Bi12TiO20/g-C3N4 composite was evaluated by the degradation of organic dye under visible light irradiation. Compared with pure Bi12 TiO20, the photocatalytic performance of Bi12TiO20/gC3N4 is substantially improved, indicating that the heterostructure is beneficial to light harvesting and the subsequent separation of photogenerated carriers. According to the results, a Z-scheme photocatalytic mechanism is proposed.

Experimental Synthesis and characterization All the chemicals were of analytical purity and were used as received without further purification. Bi12 TiO20 was synthesized via a sol–gel method using bismuth nitrate pentahydrate (Bi(NO3)55H2O) and titanium butoxide ((C4H9O)4Ti) as Bi and Ti sources. In a typical procedure, Bi(NO3)55H2O (6 mmol) was fully dissolved in 30 mL of ethylene glycol under stirring. Meanwhile, a stoichiometric amount of (C4 H9O)4Ti was dissolved into H2O2 (5 mL). Citric acid (19.5 mmol) as chelating agent was added to both the solutions. Then, the pH values of the two solutions were adjusted to 11 with NH3H2O, respectively. The resulting transparent solutions were mixed together, and after stirring for 1 h, the mixture was heated in a boiling water bath to get a gel. The gel was then dried

at 140 °C in a blast oven to form a fluffy precursor. Finally, the precursor was calcined in a muffle furnace at 550 °C for 1 h, and the yellow Bi12TiO20 powders were achieved. The g-C3N4 nanosheets were prepared through a two-step calcination [18]. Firstly, bulk g-C3N4 was prepared by annealing 10 g melamine at 520 °C for 4 h with a ramping speed of 5 °C min-1 in a crucible with a cover. The g-C3N4 nanosheets were then prepared by a long-time thermally treating bulk g-C3N4 for 6 h with a slow ramping of 2 °C min-1 under air atmosphere. The Bi12TiO20/g-C3N4 composites were obtained by an ultrasonic method. Bi12TiO20 (120 mg) and g-C3N4 nanosheets were dispersed in ethanol, and the mixture was ultrasonicated at ambient temperature for 4 h and then magnetically stirred overnight. The products were ultimately collected by drying at 70 °C for 4 h. By adjusting the amount of g-C3N4, we obtained a series of composites with various Bi12 TiO20/g-C3N4 weight ratios (3.0, 1.5, 0.7, 0.5, and 0.3), denoted as 3.0BTO/CN, 1.5BTO/CN, 0.7BTO/CN, 0.5BTO/CN, and 0.3BTO/CN, respectively. The crystal structures of the as-prepared samples were characterized on a Bruker D8 X-ray diffractometer with Cu Ka radiation. The morphologies were observed by a field emission scanning electron microscope (FESEM, FEI Quanta 250F) and a transmission electron microscope (TEM, JEOL JEM-2000). The specific surface area was determined with the Brunauer–Emmett–Teller method (V-Sorb 2800). X-ray photoelectron spectroscopy (XPS) was conducted on a Perkin-Elmer PHI-1600 ESCA spectrometer using Mg Ka X-ray source and calibrated with C 1s (binding energy at 284.6 eV). The UV–Vis diffuse reflectance spectra (DRS) of the samples were determined on an UV–Vis spectrophotometer (UV2450, Shimadzu) equipped with an integrating sphere assembly, and BaSO4 was used as a reflectance standard. The photoluminescence (PL) spectra of the as-prepared samples were measured using an Agilent G9800A fluorescence spectrophotometer with an excitation wavelength 350 nm.

Photocatalytic performance Photocatalytic activity was evaluated by the degradation of rhodamine B (RhB) under visible light. All experiments were carried out in a photochemical reactor using a 500 W Xe lamp with a cutoff filter

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([ 420 nm) as the light source at room temperature. In each experiment, 15 mg of powders was added to 10 mL RhB solution (5 9 10-3 g L-1). The suspension was kept in the dark under magnetically stirring for 30 min to ensure adsorption–desorption equilibrium before the lamp was turned on. A certain amount of the mixture was sampled and centrifuged every 30 min, and the supernatant was analyzed by a UV– Vis spectrophotometer (UV-2450, Shimadzu). The photocatalytic recycling experiment was carried out in the same photochemical reactor. The process of each run was similar to the above. After the photocatalytic reaction within 120 min, the catalyst powders were immediately separated from the suspension, washed, and dried at 80 °C for the next runs. The degradation efficiency (DE) was calculated as follow: DE ð%Þ ¼ ðC0  CÞ=C0  100%;

ð1Þ

where C0 is the concentration of RhB after adsorption process and C is the time-dependent concentration of dye upon irradiation.

Results and discussion Microstructure analysis The XRD patterns of the as-prepared samples are shown in Fig. 1. All the diffraction peaks for Bi12 TiO20 are readily assigned to cubic phase sillenite structure (JCPDS No.: 34-0097). No other phases such

Figure 1 XRD patterns of Bi12TiO20, g-C3N4, and BTO/CN composites.

as bismuth oxides and TiO2 can be detected. For the g-C3N4 sample, the two dominant diffraction peaks located at 13.1° and 27.3° are associated with the inplane trigonal N linkage of tri-s-triazine motifs (See Fig. S1 in Supplementary Information) and the periodic stacking of layers for conjugated aromatic systems, respectively [19]. The intensity of (002) remains high because of the restacking of 2D sheets. Observed from the XRD patterns of Bi12TiO20/g-C3N4 composites, it is evident that the introduction of g-C3N4 does not change the phase structure of Bi12TiO20 and all deflections can be indexed to cubic Bi12TiO20. The characteristic peaks from g-C3N4 vanish due to its relatively low diffraction intensity. The morphological features of Bi12TiO20, g-C3N4, and Bi12TiO20/g-C3N4 composite were examined with SEM and TEM. As we can see from Fig. 2a, the pure Bi12TiO20 sample possesses a loosened structure consisting of aggregated nanoparticles with a mean particle size less than 100 nm. Figure 2b shows a typical SEM image of the 2D g-C3N4 nanosheets. To decrease the overall surface energy, highly curved wrinkles can be clearly observed. Figure 2c and d reveals the morphology of 0.5BTO/CN composite. The fluffy crumpled clusters of g-C3N4 sheets are embedded with the Bi12TiO20 particles to form a composite. The tight contact between g-C3N4 nanosheets and Bi12TiO20 with the surface area increased from 12 (Bi12TiO20) to 85 m2/g (0.5BTO/ CN) favors the transfer of the photoinduced carriers, thus benefiting the photocatalytic process. The energy dispersive X-ray (EDX) elemental mapping (Fig. 2e) confirms the composite consisting of C, N, Bi, Ti, and O elements. More detailed morphological information about 0.5BTO/CN was examined by TEM. 2D sheet-like structure with wrinkles can be clearly observed from Fig. 3a and b. The BTO nanoparticles are inlaid in the transparent g-C3N4 nanosheets. The sharp interface between Bi12TiO20 and g-C3N4 evidences the successful construction of BTO/CN heterojunction (Fig. 3c). Further, the lattice fringes reveal a spacing of 0.325 nm, corresponding to the (310) facet of Bi12TiO20. XPS measurement was employed to determine the surface chemical composition of 0.5BTO/CN. Figure 4a shows the Bi 4f XPS core spectrum, and the peaks centered at 163.2 and 157.9 eV are assigned to Bi 4f5/2 and Bi 4f7/2, suggesting the Bi3? in the Bi12 TiO20 crystal structure. The Ti 2p peaks at 466.2 and

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Figure 2 Typical SEM images of a Bi12TiO20, b g-C3N4, and c, d 0.5BTO/CN composite. e EDX elemental mapping of 0.5BTO/CN.

458.5 eV, respectively, correspond to Ti 2p3/2 and Ti 2p1/2 of Ti4? (Fig. 4b). Figure 4c displays the C 1s spectrum, and the peaks located at 287.9 and 284. 2 eV can be ascribed to be N–C=N carbon and graphitic C–C carbon, respectively [20]. The N 1s peak is broad and asymmetric (Fig. 4d) and can be deconvoluted into three components at 401.2, 399.5, and 398.6 eV. The two peaks at 401.2 and 399.5 eV can be attributed to H–N–(C)2 and N–(C)3 groups of g-C3N4. The main peak at 398.6 eV is ascribed to C - N=C group. The peak at 404 eV for the p-excitations cannot be observed, indicating that the g-C3N4 nanosheets are coupled with Bi12TiO20 through p-electrons of CN heterocycles [21].

Optical properties and photocatalytic activities Figure 5 shows the UV–Vis diffuse reflectance spectra (DRS) of the as-prepared samples. The band edge absorptions of pure Bi12TiO20 and pristine g-C3N4 abruptly increase at approximately 460 and 440 nm, respectively, which are ascribed to the intrinsic interband transition absorptions. After the incorporation of Bi12TiO20 into g-C3N4, the absorption edge shifts gradually toward longer wavelength with the particle loading. The extended light-response range will be conducive to the visible light harvesting and the improvement of quantum efficiency, leading to the enhancement of photocatalytic performance for BTO/CN composites. The optical absorption near the band edge can be described by the Tauc’s equation as follow (Eq. 2) [22]:

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Figure 3 a, b TEM and c HRTEM images of 0.5BTO/CN. Inset shows the ED pattern.

ahm ¼ B hm  Eg


n=2

;

ð2Þ

where a, m, Eg, and B are absorption coefficient, light frequency, band gap, and a constant, respectively. According to their transition types, the values of n equal to 1 and 4 for Bi12TiO20 and g-C3N4, respectively. The corresponding Tauc’s plots are displayed in the inset of Fig. 5. Extrapolating the linear relation, the band gap energies of Bi12TiO20 and g-C3N4 are determined to be 2.72 and 2.82 eV, respectively, in agreement with the previous reports [23, 24]. As a representative model pollutant, RhB was chosen to evaluate the photocatalytic performance of the as-prepared samples. The visible light-driven degradation efficiencies of Bi12TiO20, g-C3N4, and Bi12TiO20/g-C3N4 composites with different loadings are shown in Fig. 6a. In the absence of photocatalyst, the RhB concentration rarely varies within the duration of irradiation, meaning that RhB is stable under visible light illumination and the self-photolysis can be ignored. Meanwhile, both pure Bi12TiO20 and

pristine g-C3N4 also deliver poor photocatalytic activity with degradation efficiencies of only 35% and 67%, respectively, which may result from the fast recombination of photoexcited charge carriers. Upon hybridization between Bi12TiO20 and g-C3N4, the photodegradation efficiency gradually increases with increasing the g-C3N4 content. The improvement of photoactivity with the introducing of g-C3N4 sheets may be attributed to the forming of heterojunction as well as the enhanced adsorption capacity. The highest activity for RhB photodegradation can be obtained on the 0.5BTO/CN composite, and RhB is nearly completely decomposed within 120 min under visible light illumination. However, further increasing g-C3N4 results into a slight decline in photoactivity (0.3BTO/CN). It may be due to the aggregation of g-C3N4 leading to the decreased specific surface area, which can obviously be observed from the SEM images (Fig. S3). Besides, the excessive nanosheets can shield the light reaching Bi12TiO20. The kinetics of RhB degradation over various photocatalysts was

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Figure 4 XPS core spectra of 0.5BTO/CN composite. a Bi 4f, b Ti 2p, c C 1s, and d N 1s.

Figure 5 UV–Vis diffuse reflection spectra of Bi12TiO20, g-C3N4, and BTO/CN composites. Inset shows the corresponding Tauc’s plots.

examined by applying the Langmuir–Hinshelwood model [25, 26]. The kinetics curves for RhB degradation are presented in Fig. 6b. The lnC0/C * t plots yield linear relationships, indicating that the photocatalytic degradations of RhB over the as-prepared samples follow pseudo-first-order kinetics [27]. The apparent rate constants k for RhB degradation can be estimated by linear fitting. As compared with those of g-C3N4 (k = 9.83 9 10-3 min-1) and Bi12TiO20 (k = 3.65 9 10-3 min-1), the photodegradation efficiency of 0.5BTO/CN (k = 17.87 9 10-3 min-1) is much enhanced about 1.8- and 4.9-fold, respectively. The value of k for RhB over 0.5BTO/CN is comparable to those of Bi12TiO20/TiO2 hierarchical heterostructure (0.0166 min-1) [10] and PANI/Bi12 TiO20 (0.018 min-1) [23].

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Figure 6 a Visible light photocatalytic degradation of RhB over different photocatalysts. b Kinetic linear simulation curves of RhB photocatalytic degradation.

From the absorption spectral evolution of RhB in the presence of 0.5BTO/CN (Fig. S2a), it is notably to find that the characteristic absorption at 553 nm gradually shifts toward shorter wavelength with increasing irradiation time. In the degradation process, RhB undergoes a stepwise N-deethylation and a synchronously cleavage of the conjugated chromophore structure [28, 29]. The obvious blue shift manifests that N-deethylation is the predominant procedure during the RhB photodegradation over Bi12TiO20/g-C3N4. To examine the stability of Bi12TiO20/g-C3N4 composites, the recycle experiment of 0.5BTO/CN was conducted (as shown in Fig. S2b). After four cycles, the photoactivity declines slowly, partly caused by the inevitable catalyst loss during the recovery. The stable crystalline structures of the composites play a vital role in the sustainable photocatalytic property.

Photocatalytic mechanism

electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, Ee is the energy of the free electrons on the hydrogen scale (4.5 eV), and Eg is the band gap energy of the semiconductor. The X values for Bi12 TiO20 and g-C3N4 are determined to be 6.19 and 4.76 eV, respectively. The conduction band (CB) and valence band (VB) potentials of Bi12TiO20 are calculated to be 0.34 and 3.05 eV (vs NHE), respectively. Besides, the ECB and EVB values of g-C3N4 are estimated to be -1.14 and 1.67 eV (vs NHE), respectively. Both the CB and VB levels of Bi12TiO20 are more positive than those of g-C3N4. To further reveal the main reactive oxygen species (ROSs) directly involved in the photodegradation over BTO/CN, the radical trapping experiment was performed. Silver nitrate (AN), benzoquinone (BQ), isopropyl alcohol (IPA), and ammonium oxalate (AO) were used as scavenger agents for electron,  OH, O2-, and h?, respectively. As shown in Fig. 7, all the scavengers can partially inhibit the

The band edge position is an intrinsic property of semiconductor and closely related to its photocatalytic activity. The band potentials of the as-prepared Bi12TiO20 and g-C3N4 were determined to explore the mechanism. For a semiconductor, the edge potential can be calculated using the empirical equations (Eqs. 3, 4) [30–32]: EVB ¼ X  Ee þ 0:5Eg

ð3Þ

ECB ¼ EVB  Eg ;

ð4Þ

where EVB and ECB are the valence band and conduction band edge potentials, respectively. X is the

Figure 7 Visible light photodegradation of RhB over 0.5BTO/ CN in the presence of different scavengers.

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photocatalytic activity of 0.5BTO/CN. Among them, the photodegradation efficiencies are greatly suppressed in the presence of IPA and BQ, indicating that OH and O2- radicals are the dominant ROSs in the photocatalytic process. Upon the visible light illumination, both Bi12TiO20 and g-C3N4 are simultaneously photoexcited to generate e-/h? pairs (Eqs. 5, 6). If the photoinduced charge carriers transfer through a traditional heterojunction model, the photogenerated electrons will migrate from the CB level of g-C3N4 to that of Bi12 TiO20, and the holes from the VB of Bi12TiO20 to the VB of g-C3N4. Although the e-/h? pairs are efficiently separated, due to the more positive CB level of Bi12TiO20 (0.34 eV vs NHE), the CB electrons cannot reduce the adsorbed O2 into O2- (E0(O2-/ O2) = - 0.28 eV) [33], which is not in accord with the radical trapping experiment. Based on the above results, the formation of Zscheme photocatalytic structure between Bi12TiO20 and g-C3N4 is proposed, and the schematic illustration is displayed in Fig. 8. The CB electrons of Bi12 TiO20 directly migrate to recombine with the VB holes of g-C3N4 (Eq. 7). The accumulated electrons in the g-C3N4 CB (eCB(CN)) with strong reduction ability (-1.14 eV vs NHE) will react with O2 to produce O2(Eq. 8). However, the photoexcited holes in the VB of Bi12TiO20 (h? VB(BTO)) with strong oxidation ability (3.05 eV vs NHE) can oxide H2O/OH- to form OH (E0(OH/H2O) = 2.27 eV) [34, 35] or directly oxide RhB molecules (Eqs. 9–11). To verifying the transfer of the photoexcited charge carriers, the photoluminescence (PL) measurement was conducted at room temperature. As we know, the PL emission originates from the energy

Figure 9 Room temperature PL spectra of g-C3N4, 0.3BTO/CN, and 0.5BTO/CN composites.

dissipation in the carrier recombination process, and the PL intensity depends on the recombination rate [36]. As exhibited in Fig. 9, the pure g-C3N4 displays the strongest emission at ca. 435 nm, in line with its band gap energy. Comparatively, the emission of 0.5BTO/CN is weakest among them, which is consistent with the photocatalysis results, indicating that the coupling of Bi12TiO20 with g-C3N4 can effectively suppress the recombination of photoexcited electron/hole pairs. It also implies that there exists an optimal interaction between Bi12TiO20 and g-C3N4 for the best photocatalytic performance [37]. According to the results and discussion, OH and  O2- radicals as the predominant ROSs, together with h? VB(BTO), boost the photocatalytic process. The Zscheme mechanism of charge transfer markedly promotes the carriers’ migration and separation, leading to the enhanced photocatalytic performance. The mechanism can be described with Eqs. (5)–(12) below.  Bi12 TiO20 þ hm ! hþ VB ðBTOÞ þ eCB ðBTOÞ

ð5Þ

 g-C3 N4 þ hm ! hþ VB ðCNÞ þ eCB ðCNÞ

ð6Þ

þ e CB ðBTOÞ þ hVB ðCNÞ ! recombination

ð7Þ

  e CB ðCNÞ þ O2 ! O2 ðCNÞ

ð8Þ

hþ VB ðBTOÞ hþ VB ðBTOÞ



ð9Þ



ð10Þ

þ H2 O ! HO ðBTOÞ 

þ OH ! HO ðBTOÞ

hþ VB ðBTOÞ þ RhB ! degradation prducts ðminorÞ Figure 8 Schematic illustration for the separation and transport of electron/hole pairs in the Bi12TiO20/g-C3N4 composite.

ð11Þ

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HO ðBTOÞ=  O2
ðCNÞ þ RhB ! degradation prducts ð12Þ major

[5]

Conclusions [6]

In summary, the visible light-driven Bi12TiO20/gC3N4 composite photocatalyst was successfully synthesized through an ultrasonic route with superior photoactivity and stability. The 0.5 BTO/CN composite shows the highest photocatalytic activity among the as-prepared samples. The photocatalytic efficiency is enhanced by about 1.8 and 4.9 times as compared with g-C3N4 and Bi12TiO20, respectively.  OH and O2- radicals are validated as the dominant ROSs involved in the photocatalytic process, and the Z-scheme mechanism is proposed to expound the highly efficient charge separation and the superior photocatalytic activity. This work can provide a new insight into the formation of the Zscheme photocatalysts.

[7]

[8]

[9]

[10]

Acknowledgements [11]

This work was financially supported by NSFC (21001064) and the Natural Science Foundation of Jiangsu Province (BK2010487). [12]

Electronic supplementary material: The online version of this article (https://doi.org/10.1007/ s10853-018-2311-7) contains supplementary material, which is available to authorized users.

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