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Enhanced Photocatalytic Activities in g-CN via Hybridization with Bi-Fe-Nb-containing Ferroelectric Pyrochlore Xiaofeng Yin, Xiaoning Li, Wen Gu, Fangfang Wang, Yijun Zou, Shujie Sun, Zhengping Fu, and Yalin Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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ACS Applied Materials & Interfaces

Enhanced Photocatalytic Activities in g-C3N4 via Hybridization with Bi-Fe-Nb-containing Ferroelectric Pyrochlore Xiaofeng Yin,a Xiaoning Li,b Wen Gu,a Fangfang Wang,a Yijun Zou,a Shujie Sun,a Zhengping Fua,c*and Yalin Lua,b,c,d*

a

Department of Materials Science and Engineering, CAS Key Laboratory of

Materials for Energy Conversion, University of Science and Technology of China, Hefei 230026, P. R. China b c

National Synchrotron Radiation Laboratory, Hefei 230029, P. R. China

Synergetic Innovation Center of Quantum Information and Quantum Physics &

Hefei National Laboratory for Physical Sciences at Microscale, Hefei 230026, P. R. China d

Laser Optics Research Center, US Air Force Academy, Colorado 80840, USA

Corresponding Authors *E-mail: [email protected]; [email protected] Address: Hefei National Laboratory for Physical Sciences at Microscale, Hefei 230026, P. R. China

Tel: +86-551-63603194

ACS Paragon Plus Environment


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Abstract Ferroelectricity may promote the photocatalytic performance because carrier separation efficiency can be effectively improved by the internal electrostatic field caused by spontaneous polarization. Heterostructures which combine ferroelectric materials with other semiconductor materials can be further advantageous to the photocatalysis process. In this work, the Bi1.65Fe1.16Nb1.12O7 was hybridized with g-C3N4 via a facile low temperature method. The results of high resolution transmission electron microscopy (HRTEM) confirmed that the tight interface were formed

between

g-C3N4

(g-C3N4)-(Bi1.65Fe1.16Nb1.12O7)

and

Bi1.65Fe1.16Nb1.12O7,

heterojunction

more

superior

which visible

gave light

photocatalytic performance. The degradation of Rhodamine B (RhB) by the optimized (g-C3N4)0.5-(Bi1.65Fe1.16Nb1.12O7)0.5 under visible light was almost 3.3 times higher than that of monomer Bi1.65Fe1.16Nb1.12O7 and 7.4 times of g-C3N4, respectively. The (g-C3N4)0.5-(Bi1.65Fe1.16Nb1.12O7)0.5 sample also showed the highest photocurrent in the photoelectrochemical tests. To further verify the benefit of the built-in electric field on the photocatalysis performance, Bi2FeNbO7 with higher spontaneous polarization was also synthesized and hybridized with g-C3N4. Both Bi2FeNbO7 and (g-C3N4)0.5-(Bi2FeNbO7)0.5

exhibited

better

photocatalytic

activities

than

Bi1.65Fe1.16Nb1.12O7 and (g-C3N4)0.5- (Bi1.65Fe1.16Nb1.12O7)0.5, although the latter ones had stronger visible-light absorbance. This implies the great promising prospects for applying ferroelectric materials on solar energy harvest.

Keywords: g-C3N4, Bi1.65Fe1.16Nb1.12O7, heterojunction, ferroelectricity, visible-light, photocatalysis

1 Introduction Semiconductor-based

photocatalytic

redox

technologies

are

very

solutions to solve environmental and energy problems, because they can easily exploit and use the natural abundant solar energy.1-2 Over the past few decades, titanium dioxide has become the most studied semiconductor photocatalytic


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material.3-6 However, it shows poor response to visible-light originating from its wide band gap and depressed photocatalytic performance owing to rapid recombination of the photogenerated carriers. Therefore, it is still very urgent to seek highly efficient visible-light-active photocatalyst. In recent years, the use of two kinds of semiconductor photocatalytic materials with matching energy band to build heterojunction is an effective way to achieve high efficiency photocatalytic performance.7-13 The heterojunction photocatalyst formed by two narrow bandgap semiconductors can absorb more visible light energy than conventional photocatalyst. Moreover, in the case of well-matched band-structure coupling semiconductor photocatalyst, the carriers have longer lifetime due to facilitated charge transferring and separation. Among the various allotropes of carbonitrides, graphite phase carbon nitride (g-C3N4) is considered to be the most stable phase at room environment. Under visible light conditions, the g-C3N4 shows moderate capacity of hydrogen production and degradation of organic pollutants.14-20 Unlike TiO2, which only absorbs ultraviolet light, g-C3N4 exhibits a higher visible light absorption because of its relatively small bandgap of 2.7-2.8 eV. Compared to many common photocatalyst of sulfide and oxynitride semiconductor, g-C3N4 is more thermal and chemical stable due to its aromatic C-N heterocycles and strong van der Waals interactions between the layers. What’s more, g-C3N4 can not only be feasibly prepared at low cost because of its earth-abundant elements: carbon and nitrogen, but also that its properties can be adjusted by simple strategies. More importantly, the polymeric nature of g-C3N4 assures enough flexibility of the structure, which can serve as a host matrix of excellent compatibility to various inorganic nanoparticles. The above-mentioned features are beneficial to fabricate g-C3N4-based heterojunction materials. Until to now, many g-C3N4-based heterojunction materials were synthesized.21-24 For example,

(g-C3N4-TiO2),25

(g-C3N4-Bi2WO6),26

(g-C3N4-ZnO),27

(g-C3N4-SrTiO3),28 (g-C3N4-CdS),29 and (g-C3N4-BiVO4)30etc. However, new strategies should be exploited because the simple heterojunctions can only be



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very limited to enhance the separation efficiency of photogenerated electron hole pairs. Ferroelectric pyrochlore A2B2O7 materials, in which carriers separation are enhanced by the built-in electric field created by spontaneous polarization, was recently proposed to improve the photocatalytic property.31-33 When Fe is incorporated into pyrochlore A2B2O7, the localized 3d orbitals of Fe will induce interband states, and the couple between interband states with conduction or valence band will decrease the band gap into visible-light range. Furthermore, the insert of Fe can bring magnetic properties. Therefore, the ferroelectric pyrochlore A2B2O7 family with Fe may be a good candidate to design highly efficient heterojunction photocatalyst which benefit from the spontaneous polarization and more visible-light absorbance. In this work, the Bi-Fe-Nb-containing ferroelectric pyrochlore was adopted to construct heterojunctions with g-C3N4 for the first time. The stoichiometry ratio Bi1.65Nb1.12Fe1.16O7 (BiFeNbO) was selected because it has the highest Fe content in all Bi-Fe-Nb-containing pyrochlore (BFNO), in which the higher Fe may induce better visible-light response during the photocatalytic degradation processes.34 Compared to single component BiFeNbO or g-C3N4, the (g-C3N4)-(BiFeNbO) heterojunction exhibited better photocatalytic activity during the visible-light photodegradation of rhodamine B and salicylic acid experiments. It can be speculated that (g-C3N4)-(BiFeNbO) exhibited stronger visible light photocatalytic activity which may originate from the efficient separation of photogenerated carriers at the heterojunction interface of the two constituent components. The (g-C3N4)-(BiFeNbO) heterojunction and the associated

photocatalytic

enhancement

mechanism

were

systematically

investigated. In order to clarify the important role of ferroelectric spontaneous polarization

during

the

photocatalytic

reactions,

the

Bi2FeNbO7

and

(g-C3N4)-(Bi2FeNbO7) were also synthesized. Bi2FeNbO7 showed higher spontaneous polarization than BiFeNbO, as expected, Bi2FeNbO7 and (g-C3N4)-(Bi2FeNbO7)

exhibited

better

photodegradation


activity

than

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BiFeNbO

and

(g-C3N4)-(BiFeNbO),

which

reveal

the

importance

of

spontaneous polarization in the photocatalytic reactions. In this case, the electric field created by ferroelectric spontaneous polarization may be more important

than

the

visible-light

absorbance

in

the

enhancement

of

photocatalysis. 2. Experimental section 2.1 Synthesis of the g-C3N4-BFNO (Bi1.65Fe1.16Nb1.12O7 and Bi2FeNbO7) photocatalyst All raw materials used for synthesis are purchased directly from commercial companies without further

purification. The g-C3N4 was

synthesized by heating melamine to 550 ° C for 4 h at normal room atmosphere in the Muffle furnace according to the literature.35 For further use, the light yellow products obtained by calcining was ground into powders. The single-phase pyrochlore Bi1.65Fe1.16Nb1.12O7 and Bi2FeNbO7 nanoparticles were prepared by coprecipitation. Bi(NO3)3•5H2O, Fe(NO3)3•9H2O and NbCl5 were used as starting materials in the synthesis of BFNO nanoparticles. In a typical procedure, 4M dilute nitric acid was used to dissolve the stoichiometric ratio of Bi(NO3)3•5H2O and Fe(NO3)3•9H2O, while hydrochloric acid was used to dissolve the corresponding amount of NbCl5. Subsequently, ammonia solution was added dropwise to the mixed solution of the two above-mentioned solutions under continuous stirring to precipitate the cations until the pH was adjusted to 9. The resulting yellow precipitate was repeatedly washed with deionized water and ethanol to neutral. Finally, the collected sample was calcined at 650 °C for 3 h in the Muffle furnace after drying at 70 °C overnight. To prepare the g-C3N4-BFNO composite, an appropriate amount of g-C3N4 and BFNO were transferred to 30 ml of deionized water followed by constant stirring for one hour to achieve a homogeneous suspension. Then, the suspension was placed in an oven and dried at 70 ° C for 8 hours to remove moisture. After grinding thoroughly, the samples were sintered at 450 ° C for 2



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hours to obtain sufficient energy to form close heterogeneous junctions between g-C3N4 and BFNO, which would have very positive effect on the transfer of photogenerated carriers at the interface. What’s more, the 7.5:2.5, 5:5, 2.5:7.5 mass ratio of g-C3N4-BFNO were prepared according to the above process and marked

as

(g-C3N4)0.75-(BFNO)0.25,

(g-C3N4)0.5-(BFNO)0.5,

and

(g-C3N4)0.25-(BFNO)0.75, respectively. As a reference, the pure BFNO and g-C3N4 samples were also synthesized by the aforementioned method. The preparation flow chart of the g-C3N4-BFNO heterojunction was illustrated in Fig. S1. 2.2 Characterization The Rigaku-TTR III X-ray diffractometer using Cu-Ka radiation was used to test the X-ray diffraction (XRD) patterns of the as-prepared powders. The transmission electron microscopy (TEM, JEM-2010), scanning electron microscopy (SEM, JSM-6700F) and high resolution transmission electron microscopy (HRTEM, JEM-2010) were used to characterize the morphology and microstructure of the samples. The UV-vis spectrophotometer (SOLID3700) was performed to monitor the diffuse reflectance spectra. The LC ferroelectric analyzer (Radiant Technology Product, USA) was used to measure the ferroelectric hysteresis of the BFNO disks compressed from the corresponding powders without calcination. The pessing pressure was 1 MPa and the diameter and thickness of the disk were 5 mm and 0.5 mm, respectively. The disk was coated with silver paste as electrode and the area of the silver paste was 3.14 mm2. The vibrating sample magnetometer (VSM) (EV-7, ADE Co, USA) was performed to test the ferromagnetic properties of the prepared BFNO powders. The decay time spectra was monitored by a spectrophotometer (FLS920, Edinburgh Instruments Ltd.) and the photoluminescence (PL) spectra were texted by a fluorescent spectrophotometer (F-4600, Hitachi Ltd., Japan) equipped with a 200 W Xe lamp as the light source. 2.3 Photocatalytic Tests The photodegradation experiments of Rhodamine B (RhB) and Salicylic acid (SA) at ambient temperature were performed to measure the photocatalytic


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activity of the as-prepared powders. The light source was a 20 W fluorescent lamp and the wavelength was 400-720nm. In general, 50 mg target powders were added in 50 mL 5 mg/L SA and RhB solution, respectively. In order to ensure the adsorption desorption balance between the photocatalytic powders and contaminants, the solution was constantly stirred for 30 minutes without illumination. Then, each 4 ml suspension was taken out in the fixed light period and the particles were removed by centrifugation at high speed. The UV-vis absorption spectrum was used to monitor the variation of maximum absorption peak for each supernatant concentration of RhB and SA. The maximum absorption peak of RhB and SA were 553 nm and 300 nm, respectively. 2.4 Photoelectrochemical Measurements Due to that dye degradation reactions are limited for evaluating the photocatalytic activities, 36 the photoelectrochemical tests were performed in standard three-electrode cell by introducing a photoelectrochemical system (CHI-660E, Shanghai Chenhua, China) with a 0.5 M Na2SO4 electrolyte solution. The prepared g-C3N4, g-C3N4-BFNO and BFNO FTO film electrodes, Ag/AgCl electrode and platinum electrode were used as working electrodes, reference electrode and counter electrode, respectively. The irradiation intensity of the 300-W Xe lamp (Solaredge 700, China) was 10 mW/cm2. The photocurrent value of the samples were recorded with the switching lamp at a 0.0 V bias voltage, while the current density versus applied voltage characteristics were tested from 0.0 V to 1.5 V. Under the setting frequency of 5000 Hz, the Mott-Schottky plots of the g-C3N4 and BFNO were measured under the loading voltage range of 0 V to -1.2 V (vs. Ag/gCl).

3. Results and discussion 3.1 Tests and Characterization of the (g-C3N4)-(BiFeNbO) heterojunctions



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Fig. 1 XRD patterns (g-C3N4)-(BiFeNbO).

and

photographs:

g-C3N4,

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BiFeNbO,

and

The color of the composite particles gradually changed from light yellow to dark brown with the increase ratio of BiFeNbO, which was shown by the inserted photographs in Fig. 1. The Fig. 1 also exhibited the XRD patterns of the (g-C3N4)-(BiFeNbO) heterojunctions and individual g-C3N4 and BiFeNbO. According to the XRD results, the nanoparticles showed good crystallinity and the phase of BiFeNbO didn’t change during the hybridization. The pure BiFeNbO sample displayed very sharp and clear diffraction peaks and these diffraction peaks were very consistent with the standard card. The significant diffraction peaks of 27.47 ° and 13.04 ° in the g-C3N4 sample were corresponded to the (002) and (100) planes of the graphite phase carbonitrides, respectively. In g-C3N4, the (002) plane came from the interlayer stacking and (100) plane was originated from the inter-planar separation.15 When the ratio of g-C3N4 in (g-C3N4)-(BiFeNbO) heterojunctions was less than 0.75, the phase of g-C3N4 was not observed in the XRD patterns.


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Fig. 2 TEM and HRTEM images of the powders: (a) g-C3N4; (b) BiFeNbO; (c) (g-C3N4)0.5–(BiFeNbO)0.5 heterojunction; (d) and (e) magnified (g-C3N4)0.5–(BiFeNbO)0.5 heterojunction; (f) lattice stripes of the BiFeNbO. The TEM and HRTEM were used to observe the microstructure of the powders. A typical wrinkled layered structure was found in the pure g-C3N4 sample (Fig. 2(a)). According to Fig. 2(b), the pure BiFeNbO sample was consisted of many nanospheres with an average diameter of 40 nm and the nanospheres tend to adhere one sphere to another. As for the composited samples, BiFeNbO nanospheres with deeper diffractive contrast were densely attached to the surface of g-C3N4 to form close heterojunction (Fig. 2(c)). The heterojunctions between BiFeNbO nanospheres and g-C3N4 plates were so strong that the ultrasonic treatment prior to the preparation of the TEM sample failed to disengage BiFeNbO nanospheres. Under the magnified TEM multiple (Fig. 2(d) and 2(e)), the g-C3N4 and BiFeNbO phases and the clear and dense interface between them can be observed. Due to indistinct in-plane diffraction (100), the two-dimensional arrangement of pure g-C3N4 was too weak to look for the lattice stripes. Therefore, the clear lattice stripes in the HR-TEM image (Fig. 2(f)) belonged to the crystal BiFeNbO. What’s more, the measured interplanar spacing of 0.3054 nm was consistent with the (222) plane of the



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pyrochlore phase BiFeNbO, which was also corresponded to the XRD pattern (Fig. 1). This high quality heterojunction can promote the transfer of photogenerated carriers in two-phase interfaces to ultimately improve the photocatalytic activity.

Fig. 3 (a) UV-Vis diffuse reflectance spectra of g-C3N4, BiFeNbO and (g-C3N4)-(BiFeNbO) samples; (b) Photocatalytic degradation of RhB under visible-light irradiation: g-C3N4, BiFeNbO, (g-C3N4)-(BiFeNbO) –Mix and (g-C3N4)-(BiFeNbO) heterojunction samples; (c) The K value as a function of component for RhB degradation; (d) Cyclic comparison experiments of (g-C3N4)0.5-(BiFeNbO)0.5 heterojunction. The UV-vis diffuse reflectance spectroscopy was used to record the optical

absorption

spectrum

(Fig.

3(a))

of

g-C3N4,

BiFeNbO

and

(g-C3N4)-(BiFeNbO) heterojunctions. The bandgap of g-C3N4, BiFeNbO and (g-C3N4)-(BiFeNbO) composite can be obtained from the Tauc equation: αhv = A(hv-Eg)n/2, where α, h, v, A and Eg represent the light absorption coefficient,


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Planck constant, light frequency, a constant and bandgap, respectively.37 The n value of direct bandgap semiconductors is 1 and the indirect bandgap semiconductors is 4. In a detail, the bandgap can be obtained by the intersection of the linear portion extension line of (αhv)2 vs (hv) function and the X axis (α=0). The estimated absorption edge of g-C3N4 and BiFeNbO were 443 nm and 506 nm, which coincide with the bandgap of 2.80 eV and 2.45 eV, respectively. With the elevated ratio of g-C3N4, the bandgap (Fig. S3) of the series proportion (g-C3N4)-(BiFeNbO) heterojunctions increased from 2.56 eV to 2.76 eV. When the ratio of g-C3N4 and BiFeNbO was appropriate, the more effective heterojunctions were be formed in (g-C3N4)-(BiFeNbO) which was beneficial to the separation and transmission of photogenerated carriers, subsequently

improving

the

photocatalytic

activity.

Among

all

the

heterojunctions, the (g-C3N4)0.5-(BiFeNbO)0.5 revealed the best photocatalytic activity although the pure BiFeNbO showed more visible-light absorption. In this case, the higher separation efficiency of photogenerated carriers dominated the photocatalytic process. Under the current lab conditions, we investigated the photocatalytic activity of the samples by photodegradation experiments of RhB under a fluorescent lamp (20W). In general, the light source was placed at a distance of 10 cm from the surface of the (g-C3N4)-(BiFeNbO)/RhB suspension. The degradation of RhB was characterized by a change in the intensity of the absorption peak at 553 nm, and the degradation curve was plotted in Fig. 3(b). It was apparent that either the pure g-C3N4 or pure BiFeNbO showed poor photocatalytic activity, which was derived from their high recombination rate of photogenerated carriers. As we can see, the heterojunctions showed a significantly enhanced photodegradation activity compared to the g-C3N4 and BiFeNbO.

The

photocatalytic

activity

first

increased

from

(g-C3N4)0.25-(BiFeNbO)0.75 to (g-C3N4)0.5-(BiFeNbO)0.5 and, then, gradually decreased with the further increase in the proportion of g-C3N4. The results demonstrated that excess g-C3N4 in the composite would create an unsuitable



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ratio between BiFeNbO and g-C3N4, thus reducing the quantity of the effective heterojunctions

which

can

promote

photocatalysis.

The

(g-C3N4)0.5-(BiFeNbO)0.5 exhibited the highest photocatalytic activity, which can degrade 92% of rhodamine B within 4 hours. Moreover, the (g-C3N4)0.5-(BiFeNbO)0.5 exhibited higher photocatalytic activity with a 92% degradation ratio than the (g-C3N4)-(BiFeNbO)-Mix with only 42%, indicating the importance of calcination in the formation of effective heterojunctions. Compared to the

well-performing flower-like Bi5FeTi3O15

with 96%

degradation ratio after four hours under more stronger light source (500 W Xe lamp),38 our samples (92% degradation ratio after four hours with a 20 W fluorescent lamp) showed better visible light photocatalytic activity. Considering the low concentration (5mg/L) of rhodamine B, the pseudo first-order reaction mode was used to imitate the kinetics process of photodegradation (Fig. 3(c)).30 According to the linear regression of ln(C0/C) vs time, the dynamics k values can be calculated. The as-prepared (g-C3N4)0.5-(BiFeNbO)0.5 heterojunction showed highest k value of 0.553 h-1, which was 3.3 and 7.4 times that of the BiFeNbO (0.170 h-1) and g-C3N4 (0.075 h-1), respectively. This result implied that the coexistence of g-C3N4 and BiFeNbO gave rise to synergic effects, which played a crucial role in the enhanced photocatalytic activity. As shown in Fig. S5(a), the (g-C3N4)0.5-(BiFeNbO)0.5 also had highest photocatalytic activity with a 53% degradation ratio of SA after 4 fours irradiation. The Fig. S5(b) represented the dynamic k value of (g-C3N4)0.5-(BiFeNbO)0.5 and the corresponding individuals for the degradation of SA. As expected, the (g-C3N4)0.5-(BiFeNbO)0.5 heterojunction showed the biggest k value of 0.160 h-1, which was approximately 2.7 and 2.0 times that of the BiFeNbO (0.060 h-1) and g-C3N4 (0.080 h-1), respectively. Due to the best photocatalytic activity, (g-C3N4)0.5-(BiFeNbO)0.5 heterojunction was chosen to do the cycle experiments of the photodegradation. Generally speaking, the stability of photocatalyst was vital for the practical applications. To test the cycle stability of as-prepared (g-C3N4)-(BiFeNbO) photocatalyst, four cycle experiments of photodegradation have been done.


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According to the (Fig. 3(d)), the photodegradation rate of RhB still stayed above 87% after four cycles. What’s more, there was not obvious distinction between the XRD partens (Fig. S4) of (g-C3N4)0.5-(BiFeNbO)0.5 before and after the photocatalytic experiment. These results indicated that the as-prepared (g-C3N4)-(BiFeNbO) photocatalyst were photo-stable in the photocatalytic degradation. 3.2 Possible Photocatalytic heterojunction

Mechanism

of


(g-C3N4)-(BiFeNbO)


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Fig. 4 PL decay curves measured at λex = 254 nm and λem = 450 nm for (a) pure g-C3N4; (b) (g-C3N4)0.5-(BiFeNbO)0.5; (c) pure BiFeNbO; (d) Photoluminescence (PL) of g-C3N4 and g-C3N4/BiFeNbO heterojunctions with different mass ratio (λex=365 nm); (e) photocurrent-time curves of g-C3N4,


Page 14 of 25

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(g-C3N4)0.5-(BiFeNbO)0.5 and BiFeNbO; (f) Current density vs applied voltage characteristics with light on and off. The above results reveal that the (g-C3N4)-(BiFeNbO) heterojunctions displayed better visible-light photocatalytic activity than the individual g-C3N4 and BiFeNbO. This may be derived from the accelerated transmission of the photogenerated carriers at the

heterojunction interfaces. In fact, the

recombination rate of photogenerated carriers can be reflected by the lifetime of carriers, which can be reflected by the PL decay spectra shown in Fig. 4(a)-(c). The PL decay curves of the g-C3N4, BiFeNbO and (g-C3N4)0.5-(BiFeNbO)0.5 were recorded by being excited at 254 nm and monitored at 450 nm. The decay curves fit well to a double-exponential function as:

Iሺtሻ = ‫ܣ‬ଵ ݁ ሺି௧/ఛଵሻ + ‫ܣ‬ଶ ݁ ሺି௧/ఛଶሻ + ‫ܫ‬ሺ0ሻ

(1)

Using the formula for τ,

τ = ሺ‫ܣ‬ଵ ߬ଵଶ + ‫ܣ‬ଶ ߬ଶଶ ሻ/ሺ‫ܣ‬ଵ ߬ଵ + ‫ܣ‬ଶ ߬ଶ ሻ

(2)

The PL lifetime decreased in the following order: (g-C3N4)0.5-(BiFeNbO)0.5 > g-C3N4 > BiFeNbO. The longer PL lifetime hinted the lower carriers recombination rate and the higher photocatalytic activity.8 The separation efficiency of the photogenerated carriers was also characterized by the photoluminescence (PL) spectrum because the reduced separation efficiency may mean high PL intensity. The intensity of PL spectrum (Fig. 4(d)) at 475 nm gradually decreased with increase ratio of BiFeNbO in the (g-C3N4)-(BiFeNbO) heterojunctions, implying the effective heterojunctions greatly inhibited the recombination of carriers and improved the photocatalytic performance. It is well known that the photocurrent is generated by the separation of the photogenerated electrons and holes in the semiconductor material, meaning that the stronger photocurrent represents a higher separation efficiency of photogenerated electron hole pairs. The photo-electrochemical properties of the g-C3N4, BiFeNbO and (g-C3N4)0.5-(BiFeNbO)0.5 electrodes were measured in 0.5 M Na2SO4 electrolyte under a 300W Xe lamp with 400 nm filter (λ>400



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Page 16 of 25

nm). In the cycles of several switching lamps, the curves of the transient photocurrent of g-C3N4, BiFeNbO and (g-C3N4)0.5-(BiFeNbO)0.5 electrodes vs time (t) were recorded. As seen in Fig. 4(e), the initial current value in the dark was low and the instantaneous current value was rapidly increased with the light turning on. When the light source was turned off, the current was quickly reduced to the initial value, meaning that the generation of photocurrent mainly came from the irradiation of the light source. The photocurrent value of the (g-C3N4)0.5-(BiFeNbO)0.5, g-C3N4

and BiFeNbO electrodes were 1.20, 0.35

and 0.1 uA/cm2, respectively. They were consistent with the results of PL decays of the corresponding samples. The longer lifetime meant the higher photocurrent.

Compared

with

g-C3N4

and

BiFeNbO,

The

(g-C3N4)0.5-(BiFeNbO)0.5 showed a considerably enhanced photocurrent density,

indicating

that

the

enhanced

photocatalytic

activity

of

(g-C3N4)0.5-(BiFeNbO)0.5 heterojunction did originate from the efficient separation and transport efficiency of the photogenerated carriers at the interface. To

further

study

the

photoelectrocatalyst

performance

of

(g-C3N4)0.5-(BiFeNbO)0.5, the current-voltage (I-V) curve of the samples were measured under darkness and irradiation of visible-light, respectively. According to the Fig. 4(f), the overpotential in the darkness was 1.2 V, while the overpotential was reduced to 0.8 V when the samples was irradiated with visible light. Owing to the aggregation of photogenerated carriers under irradiation, the overpotential of photoelectrocatalyst

decreased

obviously,

which

meant

that

the

(g-C3N4)0.5-(BiFeNbO)0.5 showed possible application in the photoelectrocatalyst field under visible-light in the future.


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Fig. 5 (a) Ferroelectric hysteresis curve of the Bi2FeNbO7 and BiFeNbO nanoparticles measured at electrical field; (b) UV-Vis diffuse reflectance spectra of Bi2FeNbO7, (g-C3N4)0.5-(Bi2FeNbO7)0.5, BiFeNbO and (g-C3N4)0.5-(BiFeNbO)0.5 samples; (c) Photodegradation of RhB of Bi2FeNbO7, (g-C3N4)0.5-(Bi2FeNbO7)0.5, BiFeNbO and (g-C3N4)0.5-(BiFeNbO)0.5 samples; (d) Photocurrent-time curves of electrodes made of Bi2FeNbO7, (g-C3N4)0.5-(Bi2FeNbO7)0.5, BiFeNbO and (g-C3N4)0.5-(BiFeNbO)0.5 samples. All of above consequences showed that the formation of g-C3N4-BiFeNbO heterojunction can effectively enhance the visible-light photocatalytic activities. But what role did ferroelectric spontaneous polarization play in the improved photocatalytic activity? To explore the special role of ferroelectric spontaneous polarization

during

the

photocatalytic

reactions,

the

Bi2FeNbO7

and

(g-C3N4)-(Bi2FeNbO7) were also synthesized. The Fig. S9 indicated that the Bi2FeNbO7 had almost the same XRD patterns with the Bi1.65Nb1.12Fe1.16O7 (BiFeNbO) except a slightly shift of the diffraction peak because of the different iron element



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percentage. Moreover, as described in the Fig. S10, the Bi2FeNbO7 showed similar spherical morphologies with average diameter of forty nanometers compared to the BiFeNbO. The ferroelectric hysteresis curve of the unsintered disks compressed from the corresponding BiFeNbO and Bi2FeNbO7 nanoparticles was shown in Fig. 5(a). An obvious hysteresis loop can be seen in the curve. The remnant polarization (2Pr) increased from 0.35 µC/cm2 (BiFeNbO) to 0.80 µC/cm2 (Bi2FeNbO7), meaning that the Bi2FeNbO7 had stronger ferroelectric spontaneous polarization. Fig. 5(b) showed the optical property of BiFeNbO, Bi2FeNbO7 and the corresponding heterojunctions which were characterized by the UV-vis spectrophotometer. According to Tauc equation, the absorption edge of BiFeNbO sample was estimated at 506 nm (2.45 eV) and the Bi2FeNbO7 sample was 496 nm (2.50 eV). The enhanced visible-light absorbance of BiFeNbO will be beneficial to the photocatalytic reactions. However, as shown in Fig. 5(c), the Bi2FeNbO7 exhibited a better photocatalytic activity with 60% photodegradation ratio of RhB than the 47% photodegradation ratio of BiFeNbO after four hours visible-light irradiation. What’s more, the photodegradation ratio of g-C3N4)0.5-(Bi2FeNbO7)0.5 was 99% while the (g-C3N4)0.5-(BiFeNbO)0.5 was 90%. It may be deduced that the built-in electric field created by ferroelectric spontaneous polarization will be more important than the visible-light absorbance during the photocatalytic reactions in this system. The Fig. 5(d) further demonstrated the above points. In general, the higher photocurrent indicates the higher separation efficiency of photogenerated carriers. The separation efficiency can be greatly improved by strong built-in electric field created by ferroelectric spontaneous polarization. The Bi2FeNbO7 and g-C3N4)0.5-(Bi2FeNbO7)0.5 showed higher photocurrent than BiFeNbO and (g-C3N4)0.5-(BiFeNbO)0.5, although the latter ones had stronger visible-light absorbance. It indicated that the Bi2FeNbO7 and (g-C3N4)0.5-(Bi2FeNbO7)0.5 had bigger ferroelectric spontaneous polarization, which was consistent with the polarization (P)-field (E) loop results. Compared with the visible-light absorbance, the ferroelectric spontaneous polarization dominated the photocatalytic activity in the photocatalytic reactions.


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Fig. 6 Schematic diagram of the behaviour of photo-generated carriers in the (g-C3N4)-(BFNO) heterojunction under visible-light irradiation, IEF represents the internal electric field. The aforementioned experiment results confirmed that the (g-C3N4)-(BFNO) heterojunction can effectively improve their photocatalytic activity because of the high separation efficiency of the photogenerated carriers in the interface. The followed contrastive experiments between BiFeNbO and Bi2FeNbO7 demonstrated that the ferroelectric spontaneous polarization played a more important role than the visible-light absorbance in this ferroelectric pyrochlore system during the photocatalytic reactions. Thus, a possible mechanism for the photocatalytic reactions was proposed as illustrated in Fig. 6. When the photocatalytic reactions system was irradiated by a visible light source, the photogenerated electron-hole pairs will separate and transfer to the surface of nanoparticles. Compared to BiFeNbO, the built-in electric field of Bi2FeNbO7 was stronger, which will promote more photogenerated carriers to arrive at the nanoparticles surface as well as the interface. Therefore, in both Bi2FeNbO7 and its heterojunction (g-C3N4)0.50-(Bi2FeNbO7)0.50, there will be more electrons and holes effectively participating in subsequent photocatalytic reactions, ending up with higher photocatalytic activity. On the other hand, the heterojunction composite displayed enhanced photocatalytic activity compared to the relative individuals. The schematic diagram was drawn in the second part of Fig. 6 with BiFeNbO as the example.



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Page 20 of 25

According to the experiment results, the bandgap of g-C3N4 and BiFeNbO were 2.8 eV and 2.45 eV, respectively. In (g-C3N4)0.5-(BiFeNbO)0.5, both the band matching and high quality heterojunction interface were beneficial for the separation of photogenerated carriers. The flat-band potential of g-C3N4 and BiFeNbO was determined as -1.14 eV and -0.55 eV from the Mott-Schottky plots, respectively. In general, the flat-band potential is located just below the conduction band minimum (CBM).39 The valence band maximum (VBM) of g-C3N4 and BiFeNbO was estimated to be ~1.66 eV and ~1.90 eV using its band gap.

Because

of

the

staggered energy

band and intimate

interfaces,

photogenerated electrons on the CBM of g-C3N4 spontaneously move to the CBM of BiFeNbO, while the holes on the VBM of BiFeNbO freely migrate to the VBM of g-C3N4. During the photocatalytic reaction, the separated electrons and holes were constantly being consumed via redox reactions in the surface active sites. As a result, the pollutants will be degraded continuously.

Conclusions

In summary, a (g-C3N4)-(BiFeNbO) composite photocatalyst was developed by a mild low temperature annealing treatment. After the introduction of ferroelectric BiFeNbO, the (g-C3N4)-(BiFeNbO) photocatalyst showed a moderate visible-light absorption and exhibited improved photodegradation activity of rhodamine B. The degradation of Rhodamine B (RhB) by the optimized (g-C3N4)0.5-(Bi1.65Fe1.16Nb1.12O7)0.5 under visible light was almost 3.3 times higher than that of monomer Bi1.65Fe1.16Nb1.12O7 and 7.4 times of g-C3N4, respectively. This enhancement originating from the improved separation and transmission efficiency of photogenerated carriers in the intimate heterojunction interfaces has been proved by the PL decay spectrum, photocurrent response spectrum and photoluminescence (PL) spectra. The cycling experiment indicated that the (g-C3N4)-(BiFeNbO) heterojunctions photocatalyst had an excellent photocatalytic stability and recyclability. The current-voltage (I-V) curve which was

texted

under

darkness

and

visible-light


indicates

that

the

Page 21 of 25

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(g-C3N4)0.5-(BiFeNbO)0.5 may have potential application in photoelectrocatalyst field in the future. What’s more, the similar ferroelectric pyrochlore Bi2FeNbO7 and (g-C3N4)-(Bi2FeNbO7) were synthesized to explore the special role of ferroelectric spontaneous polarization during the photocatalytic reactions. The Bi2FeNbO7 and (g-C3N4)0.5-(Bi2FeNbO7)0.5 with stronger built-in electric field resulted from ferroelectric spontaneous polarization showed better photocatalytic activity than Bi2FeNbO7 and (g-C3N4)0.5-(BiFeNbO)0.5 with more visible-light absorbance. In this case, the ferroelectric spontaneous polarization dominated the photocatalytic activity in the photocatalytic reactions system.

Supporting Information Available: Formation process of the (g-C3N4)-(BFNO) heterojunctions; SEM images of the powders; Plot of (ahv)2 versus (hv) of the samples; Absorption-desorption equilibrium curve of BiFeNbO sample; Photocatalytic degradation of SA; XRD patterns of (g-C3N4)0.5-(BiFeNbO)0.5 heterojunction before and after photocatalytic reaction; Temperature dependence of the magnetization; Current density vs applied voltage; XRD patterns of BiFeNbO and Bi2FeNbO7 samples; SEM images of Bi2FeNbO7 and BiFeNbO samples; Mott-Schottky plots the as-prepared Bi2FeNbO7 and BiFeNbO samples. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement This work was financially supported by the National Natural Science Foundation of China (51627901), The National Key Research and Development Program of China (2016YFA0401004), Provincial Natural Science Research Project of Anhui Colleges (KJ2014ZD40), Key Research Program of Chinese Academy of Sciences (KGZD-EW-T06), and the External Cooperation Program of BIC, Chinese Academy of Sciences (211134KYSB20130017).



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Enhanced Photocatalytic Activities of gC3 ... - ACS Publications

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