tz-Bi0.92Gd0.08VO4

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Preparation of Self-Assembled Spherical g-C3N4/tz-Bi0.92Gd0.08VO4 Heterojunctions and Their Mineralization Properties Chengcheng Zhao, Guoqiang Tan, Jing Huang, Wei Yang, Huijun Ren, and Ao Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06501 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 13, 2015

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ACS Applied Materials & Interfaces

Preparation

of

Self-Assembled

Spherical

g-C3N4/tz-Bi0.92Gd0.08VO4

Heterojunctions and Their Mineralization Properties

Chengcheng Zhao, Guoqiang Tan*, Jing Huang, Wei Yang, Huijun Ren, Ao Xia

School of Materials Science and Engineering, Shaanxi University of Science & Technology, Xi’an 710021, China *Email address:[email protected] (G.Q. Tan)

ABSTRACT: A novel kind of spherical g-C3N4/tz-Bi0.92Gd0.08VO4 heterojuctions are prepared via a simple microwave hydrothermal method. The sphere self-assembled mechanism and the heterojunction photocatalytic mechanism are mainly studied in this article. The results show that the added g-C3N4 sheets first anchor on ms-BiVO4 surfaces and then polymerize to form the coating layers, generating steric effect which is competing with Gd3+ induction effect to affect the crystal transformation (from ms-BiVO4 to tz-BiVO4) and its growth during those processes. Afterward, independent coated structures are further polymerized and assembled into g-C3N4/tz-Bi0.92Gd0.08VO4 spheres. Because ECB (-0.95V) of g-C3N4 is more negative than ECB (-0.05V) of tz-BiVO4, the photoelectrons of g-C3N4 can be transferred into tz-BiVO4 surfaces through the heterojunction structure, so as to promote the separation rate of electron-hole pairs. In general, the adding of g-C3N4 can introduce hydroxyl groups to catch the photoholes and can inject electrons to react with dissolved oxygen to boost the production of active groups. Depending on such an orderly cooperation, g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunction catalysts exhibit high and stable mineralization properties. KEYWORDS: spheres; g-C3N4/tz-Bi0.92Gd0.08VO4; self-assembled; energy potential structure; mineralization mechanism

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1. INTRODUCTION Since 1972, when Fujishima and Honda found the photocatalytic water splitting property of n-type semiconductor TiO21, we have entered a new age of heterogeneous photocatalysis2-4, especially with a rapid development in recent years. Because the photocatalysis technology is of strategic importance to the serious environmental pollution and the global energy shortage, as the times require, plenty of new-type photocatalysts emerge to drive photocatalytic reactions. The BiVO4 we have studied is just one important candidate of them. Generally, ms-BiVO4 is the most efficient one among the three crystal forms. However, according to our previous study, the tz-BiVO4 containing different rare earth elements, especially Gd, have surprisingly higher photocatalytic properties than that of pure ms-BiVO45-6.We therefore continue to further study this kind of tz-BiVO4 and try to promote the mineralization properties under UV light. In order to improve the carriers’ separation efficiency and reaction stability in complex photocatalytic systems, various modified approaches have been developed 7-9, among which heterojuction have attracted great attention. Up to now, many successful examples, such as CuO/BiVO4, Bi2WO6/BiVO4, TiO2/BiVO4, CeO2/BiVO4, InVO4/BiVO4 and Co3O4/BiVO410-15 have been reported to exhibit greatly improved photoactivity. But as far as we know, researches about heterojuctions with BiVO4 and organic materials are relatively unexplored. Among the many candidates, graphitic carbon nitride (g-C3N4) as a metal-free, chemically stable and nontoxic substance and especially with its narrow band gap (Eg=2.7eV) is supposed to be a good photocatalyst. In addition, according to previous reports, the bands of ms-BiVO4 are well matched with those of g-C3N4, which will be helpful for the separation and transportation of the photogenerated electron-hole pairs. Therefore, we studied a novel kind of heterojuction composed of g-C3N4 and tz-BiVO4 containing a certain amount of Gd. There are several methods for


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preparing g-C3N4, including CVD, PVD, several methods based on them (plasma-CVD, HF-CVD) and pyrolyzing precursors16-18. Among them, pyrolysis precursor is the most economical one without complex route. Based on many reports, melamine is recognized to be a stable, low-cost and easily operated precursor19-20. During the pyrolysis process, it will generate some intermediate product-melem (C6N7(NH2)3) which can also be further polymerized during the post-heat treatment21. In a common route, 520-600℃ is considered to be a viable temperature range22. In this article melamine is to be pyrolysed at 600℃ to get a high crystallinity of g-C3N4. To date, some studies have been reported about g-C3N4/ms-BiVO423-24. However, the heterojuction composed of g-C3N4 and tz-BiVO4 and the growth mechanism about how g-C3N4 wrap around tz-BiVO4 grains have not been discussed yet. Therefore, in this article, growth mechanism, band potentials of heterojuction structure and their influences on photogenerating, separating and transferring of carriers were mainly studied. All the samples were synthesized via a microwave hydrothermal method which could shorten synthesis time and heat the precursor solution evenly. In addition, we introduced some electrochemistry means to assist the study of heterojuction structure and mineralization mechanism. 2. EXPERIMENTAL SECTION 2.1. Preparation 2.1.1. Powder Samples All the reagents were of analytical grade and were used without any further purification in this article. The g-C3N4 was prepared by calcining melamine directly at 600 ℃ for 3h in a semi-closed system to prevent sublimation of melamine. Finally, 2.5g g-C3N4 was got after calcining 10g melamine, with the production yield of 25%. The obtained yellow blocks were then grinded into fine powders.



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The following was the synthetic route of g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions: First, Bi(VO3)3H2O and NH4VO3 were dissolved in deionized water at room temperature (RT) and 80℃ respectively. Then the two solutions were mixed together, adding 5mL 5mol/L NaOH. After that, g-C3N4 (BiVO4﹕ g-C3N4=100﹕x, x=0, 5, 10, 15, 20, 30, mole ratio) and Gd(NO3)36H2O (Bi﹕ Gd=0.92﹕0.08, mole ratio) were orderly added into the mixture with strong magnetic stirring at RT to get the precursor. The microwave hydrothermal treatments were carried out in teflon-lined autoclaves with the packing ratio of 55-60%, maintaining at 200℃ for 40min. The precipitates were collected and washed with deionized water and ethanol respectively for several times, and then dried at 80℃ for 12h to get the final powder products. The synthesized g-C3N4/tz-Bi0.92Gd0.08VO4 samples are abbreviated as 0-CN, 5-CN, 10-CN, 15-CN, 20-CN and 30-CN according to their own g-C3N4 adding contents. 2.1.2. Film Electrodes A total of 0.1g as synthesized sample was dissolved into a mixed solution of 1mL anhydrous ethanol and 0.1mL acetyl acetone with stirring and ultrasonic dispersing to get a stable suspension. Then the suspension was spread out on a1.5∗2cm2 FTO glass substrate for 3 times by a spin-coating method. After liquid volatilization, the substrate was calcined at 150ºC for 3h to get the film electrode. 2.2. Characterization The phase composition of the samples was characterized by a powder X-ray diffraction (XRD, D/max-2200PC, Rigaku Japan; CuKa, λ=0.15406nm, 40kV, 40mA). Energy dispersive X-ray spectrometry (EDS) was used for chemical analysis. Fourier transform infrared spectroscopy (FT-IR, VERTE70, Bruker, German) was employed to analyse the structure. The morphologies and microcharacterizations were observed on a field emission scanning electron microscopy

(FE-SEM, S4800)

and a transmission


electron

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microscopy (TEM, FEI TECNAI G2F20 S-TWIN, USA). The zeta potential was measured by Nano Particle size and zeta potential analyzer (NAMO-ZS, Malvern, UK). The UV-vis diffuse reflectance spectra (DRS, Cary 5000, Agilent, USA) were used in the wavelength range of 200-800nm to study the absorption range. 2.3. Photocatalytic Analyses Rhodamine B (RhB) was used as degradation target under UV light irradiation of a 300W Hg lamp, which was conducted in an XPA-7 photochemical reactor (Xujiang Machine Factory, Nanjing, China). A total of 0.05g photocatalyst was added into 50 mL RhB solution (5×10−6 mol/L) each time, and then the solution was stirred in darkness for 30min before illumination to achieve an adsorption-desorption equilibrium

between

photocatalyst and RhB molecules. The concentrations of RhB supernatants after centrifuged were analyzed using a UV-vis spectrophotometer (SP-756P) in a certain time interval and the total organic carbon (TOC) contents of them were measured by a total organic carbon analyzer (TOC, Liqui TOC II, Elementar, German). Different content of benzoquinone (BQ) (1mmoL), ethylene diamine tetraacetic acid disodium salt (EDTA-2Na) and (1mmoL) tert–butyl alcohol (TBA) (0.3mL) were added into the degradation system to catch O2•-, h+ and •OH respectively. 2.4. Electrochemical Analyses Electrochemical analyses were performed on an electrochemical workstation (CHI660E, China), using a standard three-electrode cell with platinum as a counter electrode and the saturated Ag/AgCl electrode as a reference electrode. The g-C3N4/tz-Bi0.92Gd0.08VO4 film electrodes were suspended in the 0.1mol/L Na2SO4 aqueous electrolyte as working electrodes. Photocurrent curves were recorded by an amperometric method under intermittent irradiation. Electrochemical impedance spectra (EIS) were obtained in the frequency range of 0.01-100000Hz and then interpreted by a



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nonlinear least-squares fitting procedure using a commercial software (ZsimpWin). Mott-Schottky plots were carried out at 1000Hz and scanned from anodic to cathodic. An Hg lamp (300W) was used as a light source for UV light irradiation in photoelectrochemical analyses. 3. RESULTS AND DISCUSSION 3.1.Self-Assembled Mechanism of g-C3N4/tz-Bi0.92Gd0.08VO4 Spheres 3.1.1. Phase and Structural Analyses The X-ray powder diffraction was employed for crystal structure determination as shown in Figure 1. Figure 1a is the XRD diffraction pattern of g-C3N4 prepared through calcining melamine. It is indicative that the diffraction peak at 2θ=27.72°has a right shift of 0.2° compared with other reports, which is a characteristic interlayer stacking peak of aromatic systems22 and is commonly indexed as (002) plane with an interlayer spacing of 0.322nm. It is likely the result of rapid growth process of g-C3N4, getting thinner sheets, leaving residual stress on the surfaces of g-C3N4, causing lattice distortion and decreasing crystal planes spacing. In addition, some weak convex peaks appear at 2θ=13.18°, 2θ=21.98° and 2θ=26.51° respectively, indexed of (001), (111) and (002) planes of melamine (C3N3H6) (JCPDS NO. 24-1923) and of some associated crystalline intermediates-melem21,

25

. These intermediates

will be further polymerized into g-C3N4 during the post microwave hydrothermal environment21. As

is

clearly

showed

in

Figure

1b,

most

peaks

of

all

the

g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions can be indexed to tz-BiVO4 (JCPDS NO.14-0133), implying the phase transformation from ms-BiVO4 into tz-BiVO4 under the induction of Gd3+. With the addition of g-C3N4, the peak of g-C3N4 appears

at

2θ=27.74°


(green

dashed

heterojunctions

are

line),

indicating

synthesized

the

successfully.

However, this peak disappears unexpectedly after the content of 10%, while the peak of ms-BiVO4 (JCPDS NO.14-0688) appears at 2θ=28.82° (red


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dashed line) and then wears off with the g-C3N4 content increasing. This is because the g-C3N4 sheets which are made up of s-triazine/tri-s-triazine rings connected by terminal N atoms26 begin to anchor on the ms-BiVO4 crystal nucleus surfaces gradually before phase transformation. In this process, the generated steric effects might impede the transformation from ms-BiVO4 to tz-BiVO427-28, giving rise to the appearance of the weak ms-BiVO4 peak. With g-C3N4 increasing to 15%-20%, most of the g-C3N4have coated on the tz-BiVO4 or the untransformed ms-BiVO4 crystals and then are wrapped in the agglomerates, so that a very small proportion of residual independent g-C3N4 sheets cannot have sensible diffraction peaks. Figure 1c is the EDS spectrum of 20-CN and it can be seen that the composite contains C, N, O, Bi, V and Gd elements, further proving that the heterojunctions can be synthesized successfully through this method. In order to obtain more information of the cell parameters in detail, Rietveld refinement was conducted on the original XRD data, as is listed in Table. 1. The space groups of tz-BiVO4 and ms-BiVO4 are I41/amd:2 and C2/c:c3 relatively. We can see that there are 88% tz-BiVO4 and 12% g-C3N4 in 5-CN, and the cell parameters of tz-BiVO4 are almost the same as those of 0-CN. The contents of tz-BiVO4, g-C3N4 and ms-BiVO4 in 10-CN are 85%, 2% and 12% respectively, and the cell parameters of tz-BiVO4 have an obvious change while the grain sizes change a little. When the g-C3N4 content reaches above 15%, only tz-BiVO4 and ms-BiVO4 exist in the heterojunctions. The cell parameters of tz-BiVO4 basically have no any change while the grain size is decreased.

From

that,


it

can

be

heterojunctions

further are

proved

synthesized

that

the

successfully.

Moreover, the increasing g-C3N4 can cause remarkable lattice distortion of tz-BiVO4 crystals. Figure 2 shows the FT-IR spectra of 20-CN, 0-CN and pure g-C3N4. All the three have wide absorption bands at 3400cm-1 and 1600cm-1 related to the



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stretching vibration and bending vibration of OH-, suggesting the presence of residual adsorption water on the sample surfaces29. In the spectrum of g-C3N4, the band at 812 cm-1 is the characteristic breathing mode of the triazine units22, the wide one at 1200-1600cm-1 corresponds to the typical stretching modes of C-N or C=N in heterocycles, and the band at 3200cm-1accords with the stretching vibration mode of N-H which is the residual amino (C-NH2 or 2C-NH) with incomplete polymerization after the calcining process30,31. In the spectrum of 0-CN (the same as typical tz-BiVO4), the strong absorption band at 751cm-1 is assigned to υ1(VO4) and υ3(VO4), and the band at 812cm-1 is observed as CO2 derived bands32. We can see that the main peaks of 0-CN and g-C3N4 appear in 20-CN through comparison, implying the g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions are synthesized successfully. Additionally, the disappearance of the peak at 812cm-1 is due to the further polymerization and consumption of the triazine units during the microwave process while that at 3200cm-1 confirms that the residual amino groups of melamine are removed simultaneously, further promoting the formation and reaction of g-C3N4. FE-SEM, TEM, HRTEM and SAED images of all the samples are shown in Figure 3 to exhibit the morphology characteristics. Before compositing, the two independent substances show regular shapes. As we can see in Figure 3a-b, g-C3N4 is composed of nanosheets (5-9nm thick) and some mixing particles. The sheets have very clear lattice arrangement (Figure 3b upper part), and the spacing between adjacent lattices is 0.326nm, corresponding to the (002) plane of g-C3N4. The spacing of the particles (Figure 3b bottom part) in two directions are corresponding to (002) and (111) planes of melamine, indicating its incomplete pyrolysis. In Figure 3c-d, 0-CN has uniformly distributed prismatic nanobars (1-1.5µm long, 0.1-0.2µm wide, 0.1-0.2µm thick). The spacing between adjacent lattices is 0.362nm, corresponding to the (200) plane of tz-BiVO4.The regular and bright electron diffraction spots suggest that the detected tz-BiVO4 is of single crystal.


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As the compositing process proceeding, photocatalysts make significant changes in morphology. From 5-CN (Figure 3e), the nanobars are no longer of the regular prismatic structure but a little bending. Then in 10-CN (Figure 3f-g), the nanobars are coated with the small particles evenly and spherical structures consisting of a small amount of short bars begin to emerge. In the further observation of the coated part, its lattice arrangement can be seen clearly and the interplanar distance is 0.326nm, corresponding to the (002) plane of g-C3N4, which suggests that the adding g-C3N4 will gradually adsorb on tz-BiVO4 to form coating structures and then agglomerate together. The additional lattice fringe of 0.387nm is possibly the intermediate. In 15-CN (Figure 3h), the above assembled process is further advanced and spheres (2.5-6µm diameter) are distributed uniformly. While with the g-C3N4 increasing to 20%, the dimension of the spheres (Figure 3i-k) has an unexpected decrease (0.5-2.5µm diameter). Some rough transparent protuberances appear on the sphere surfaces. The lattice arrangement results are corresponded to the (002) plane of g-C3N4 and the (200) plane of tz-BiVO4, further confirming it a heterojunction structure of g-C3N4 and tz-BiVO4 which is of single crystal. In 30-CN (Figure 3l), the size of spherical structures with dense and smooth surfaces increase again. 3.1.2. Self-Assembled Mechanism Analyses Herein, according to the above analyses and the zeta-potential change, a deduction about the growth process is shown as follows (Figure 4): first, Bi(NO3)3, Gd(NO3)3 and NH4VO3 are dissolved in deionized water, hydrolyzing, so as to form ms-BiVO4 and t-GdVO4 grains under the effects of surface energy in the solution5, and then transforming into tz-Bi0.92Gd0.08VO4 nanobars (Eq. 1 to Eq. 4). The melamine is pyrolyzed to produce g-C3N4 sheets accompanied by a few intermediate particles which can further polymerize in the microwave hydrothermal environment (Eq. 5 to Eq. 6). When g-C3N4 are added into the precursor, the ms-BiVO4 are anchored by them and then



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transformed into tz-BiVO4 under the induction of t-GdVO4 (Eq. 7). Due to the introduction of hydroxyl groups, g-C3N4 leaves reverse ion adsorption on ms-BiVO4 surfaces, and the zeta potential of the solution is changed from 40.21mV (0-CN) to -32.08mV (5-CN). Because the induction effect of t-GdVO4 on phase transformation is disturbed by the steric effect of g-C3N4, the phase transition and crystal growth of ms-BiVO4 are set back to a certain degree, giving rise to the appearance of ms-BiVO4 in Figure 1 in Section 3.1.1. With the g-C3N4 content increasing, the anchoring and polymerization of g-C3N4 on ms-BiVO4 keep on going, and the scale lessening coated structure gradually self-assemble into spheres (Eq. 8). In the meantime, the hydroxyl groups decrease due to the polymerization, leading to the decrease of zeta potential from -32.08mV (5-CN) to -29mV (15-CN). A little different from the above reaction, the polymerization in 20-CN between g-C3N4 and coating layer and that between layers lag behind the adsorption, implying the Eq. 9does not reach a balance. Based on this, more hydroxyl groups than before are surprisingly exposed, resulting in the sudden increase of zeta potential to -37.21mV. As to 30-CN, Eq. 9 reaches the final balance, the spheres are dense and the zeta potential decrease again to -30.42mV. It can be concluded that the amount of exposed hydroxyl groups, which is of significant influence to their photocatalytic effect, is closely related with their own morphology and adding content of g-C3N4.

Bi(VO3) 3 + H2O → BiONO3 + 2H+ + 2NO 3− −

BiONO3 + VO3 → ms - BiVO4 (amorphous) + NO3

(Eq.1) −

Gd(NO3 )3 + VO3− → GdVO4 (crystalline) + NO3− ion GdVO 4 + ms - BiVO 4 (amorphous ) phase  transit  → tz - Bi 0.92 Gd 0.08 VO 4

(Eq.2) (Eq.3) (Eq.4)

C 3 N 3 H 6 heat  → g - C 3 N 4 (sheets) + intermedia tes (particles )

(Eq.5)

post - heat treatment intermediates (particles)    → g − C3 N 4(sheets)

(Eq.6)


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GdVO4 + ms - BiVO4 + g - C3 N 4 + g - C3 N 4 /ms - BiVO4 → g - C3 N 4 /ms - BiVO4 + g - C3 N 4 /tz - Bi0.92Gd 0.08VO4 (Eq.7)

g - C3 N4 /tz - Bi0.92Gd0.08VO4 + g - C3 N4 → −[g - C3 N4 ]n − tz - Bi0.92Gd0.08VO4 (Eq.8) − [g - C3 N 4 ]n − tz - Bi0.92Gd0.08VO4 + g - C3 N 4 → − [[g - C3 N 4 ]n − tz - Bi0.92Gd 0.08VO4 − g - C3 N 4 ]n −

(Eq.9)

In conclusion, self-assembled mechanism of g-C3N4/tz-Bi0.92Gd0.08VO4 spheres could be described as following (inset of Figure 4): g-C3N4 first anchor on ms-BiVO4 surfaces and then polymerize during the crystal transformation and growth process, forming coating layers. After that, independent coated structures will further aggregate and assemble into g-C3N4/tz-Bi0.92Gd0.08VO4 spheres. 3.2.

Mineralization

Mechanism

of


Heterojunctions 3.2.1. Photocatalytic Response and Mineralization Research At present, a general method is to measure the photocatalytic property by monitoring the optical absorption change at 554nm. Calculation and data-fitting then can be conducted on the base of the absorption value as shown in Figure 5a. All the g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions exhibit higher degradation rate than that of the pure one (0-CN). Among them, 20-CN performs the most prominent decolorizing efficiency and a UV-degradation rate of 94% after 20min, which is 39.97% higher than that of 0-CN. The TOC change is of great significance to study the thorough mineralization degree of RhB macromolecules during the photodegradation process. As shown in Figure 5b, the determined TOC values are calculated into TOC removal rates. We can see that the TOC removal of RhB solution without any photocatalyst is scarcely changed while those of solution over the as-synthesized g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions have remarkable ascent within 20min under the UV irradiation, suggesting their efficient



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mineralizationproperties33. The TOC removal rate of 20-CN reaches the highest 75% which is an excellent result in the field of photocatalysis. With the g-C3N4 content increasing, the mineralization efficiencies exhibit a rise first and then a decline, illustrating that there is an appropriate composite ratio between g-C3N4 and tz-BiVO4. The main peaks with no offset in the UV-visible absorption spectra of 20-CN (upper inset of Figure 5a) indicate that the catalyst can quickly crack the RhB conjugate rings rather than stepwise remove ethyl34-36. Moreover, the kinetic curves of RhB photodegradation processes over g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions are plotted (bottom inset of Figure 5a, the same color represents the same heterojunction). Obviously, 20-CN has the highest degradation rate of 0.13938. These results are entirely corresponded with those in Figure 5b. In order to detect the roles of active species during the photocatalytic process, BQ, EDTA-2Na and TBA were used as active species scavengers of O2•-, h+ and •OH respectively (inset of Figure 5b). It’s obvious that the photocatalytic activities are reduced by 83.5%, 1.15% and 50.05% respectively with BQ, TBA andEDTA-2Na. Thus, it could be inferred that the O2 • - serves as the main role, • OH effects sub-mainly while h+ contributes a little during the degradation reaction. In order to study the stability of the catalyst, recycle experiment was conducted with the presence of 20-CN for five times. As shown in Figure 5c, we can see that during the 5 cycle tests, degradation rates after every 20min run are 94%, 91%, 89%, 88% and 87% respectively, totally dropped by 7%, indicating only a fraction of active species are consumed during the degradation process37. The XRD patterns before and after the cycle tests are compared (Figure 5d), we can see the phase is unchanged, that is to say the catalyst crystal structures are not damaged. The results show that the prepared g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions can produce photoinduced species with high activity and good use stability. Electrochemical

tests

are

available

means


to

research

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inducing-separating-transporting behaviors of charge carriers to further study and explain the mineralization processes. As shown in Figure 6, all the catalysts have an effective photocurrent response as soon as the irradiation exerting for several times. The 20-CN has an outstanding current density which is corresponded with the mineralization property. It can be elucidated that the heterojunction can produce a larger number of charge carriers, migrating effectively to generate the current. Additionally, the currents of the heterojunction films, which are different from those of 0-CN, do not directly rise to the top or fall to the bottom at the beginning or end of the irradiation, but slowly changing. That may be the result of the surface defects’ capturing or the over-thickness’

delaying

to

electronstransferring38-40.

In

general,

the

compositing of g-C3N4 and tz-BiVO4 promotes the light response intensity and the electron-hole pair separation rate, which is helpful to the mineralization. The EIS Nyquist plots (Figure 7) of the g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunction films can be accomplished by a ZsimpWin software. The used circuit model is R(Q(R(RC))) (Figure 7a), including solution resistance (RL), charge transfer resistance (R) which is the main research object, space charge capacitance (C) and electrochemical double-layer capacitance (Q). It can be seen that the fitting curve (full line) can well match with the measured curve (dotted line), supporting it a valid circuit models. In the darkness, the radiuses are generally large, indicating a large resistance to allow only a few charge-transmissions41. When light up, the radiuses are noticeably reduced, among which 20-CN has the largest decrease (Figure 7e). Obviously, electron-hole pair separation rates and carrier migration rates have been greatly improved42. The specified fit values of circuit components are listed in Table. 2. When light off, 0-CN essentially has the lower R value of 4.80×106 Ω than the other, and when light up, R values for all samples are significantly reduced, among which 20-CN has the largest decrease from 1.8×106 Ω to 0.28×106 Ω. It can be



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Page 14 of 34

inferred to know its excellent charge separation and migration potential to benefit the degradation, which is totally in accordance with current-voltage test and mineralization properties. 3.2.2. Heterojunction Mineralization Mechanism Analyses In order to get the heterojunction absorption range and the band gap, UV-vis diffuse reflectance spectra (DRS) were conducted as Figure 8. According to the spectra, all the samples show strong absorptions in UV range, and thus favoring their UV degradation properties. The band gaps are estimated by Eq. (10).43

αhν = A(hν - E g ) n/2

(Eq.10)

The n values of tz-BiVO4 (direct-gap semiconductor)44 and g-C3N4 (indirect-gap semiconductor)45 are 1 and 4 respectively, taking αhν1/2 and αhν2 as the function of hν (inset of Figure 5). With the help of data fitting, it is easy to get the Eg values of tz-BiVO4 (2.87eV) and g-C3N4 (2.7eV) which are virtually in accordance with the values reported before. We can also notice that there is an absorption tail of 15-CN, 20-CN and 30-CN at 440-510nm, which means an extension absorption in the visible range, and maybe the result of mixing phases in the composites46 or surface defects47. Combined with the photodegradation results, it can be confirmed that heterojunction structure can have an impact on the intrinsic optical absorption capability of photocatalysts. Figure

9

shows


the

Mott-Schottky

heterojunction

films

(MS) with

linear

curves

of

segments

representing a depleted state of the majority carrier in the space charge region. The positive slopes demonstrate they are n-type semiconductors. Unlike other samples, there are two linear segments in 15-CN, indicating there exists more than one space charge region. The series stacking in those two regions bring about the MS orientation change48. The x-intercept of the linear tangent is a flat band potential (EFB) and is always approximately used as a conduction


Page 15 of 34

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band potential (ECB). Valence band potential (EVB) can therefore be calculated based on (ECB) and Eg described before, as listed in the inset table of Figure 9. According to the above inferential calculation, energy band structure of heterojunction photocatalysts and mineralization mechanism can be described as the following Scheme 1: under UV irradiation, both of the two phases produce photoelectron-hole pairs. Because ECB (-0.95V) of g-C3N4 is more negative than ECB (-0.05V) of tz-BiVO4, the photoelectrons of g-C3N4 can be transferred into tz-BiVO4 surfaces through the heterojunction structure. Meanwhile, EVB (2.82V) of tz-BiVO4 is more positive than EVB (1.75V) of g-C3N4, so that the photoholes of tz-BiVO4 can transfer into g-C3N4 surfaces same as photoelectrons. In this way, photoelectron-hole pairs are efficiently separated. The photoelectron-hole pairs transferred to surfaces will then create reduction-oxidation reactions respectively, producing a large amount of superoxide radicals and hydroxyl radicals, which can directly degrade and mineralize the adsorbed RhB molecules. Throughout the mineralization process, on the one hand, the adding of g-C3N4 can introduce hydroxyl groups to catch the photoholes to produce •OH groups; on the other hand, it can inject electrons to tz-BiVO4, which can react with dissolved oxygen and boost the production of

O2•- groups. Depending on such an orderly cooperation,

g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunction catalysts exhibit high and stable mineralization properties. 4. CONCLUSION The g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions are synthesized at 200℃ for 40min under microwave hydrothermal conditions, forming self-assembled spheres. Due to the steric effects of g-C3N4 sheets, the phase transition and growth are effected and then result in grain size decrease. Meanwhile, polymerization occurs between g-C3N4 units, eventually making them self-assemble into spherical g-C3N4 coated with tz-BiVO4 heterojunctions. The



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Page 16 of 34

heterojunctions are n-type semiconductors with excellent UV-light response. The introduction of g-C3N4 can form mass hydroxyl groups to capture photoholes and inject large amounts of photoelectrons into tz-BiVO4, so as to effectively

promote

the

separation

and

mobility

of

photogenerated

electron-hole pairs and generate active groups to rapidly mineralize RhB molecules. On account of different compositing proportions and agglomeration degrees, the degradation properties are different from each other and 20% is ultimately regarded as a favourable proportion. It has been verified that the as-prepared heterojunctions are also of good circulation stabilities with good application potential in the long run. ACKNOWLEDGMENTS This work is supported by the Project of the National Natural Science Foundation of China (Grant No. 51172135); State-Level College Students’ Innovation and Entrepreneurship Training Program for Local Colleges and Universities (201310708003); the Academic Leaders Funding Scheme of Shaanxi University of Science & Technology (2013XSD06); Doctorate Scientific Research Initial Fund Program of Shaanxi University of Science & Technology (BJ4-13); the Graduate Innovation Fund of Shaanxi University of Science and Technology (SUST-A04).


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Ettringite

[Ca3Al(OH)6]2(SO4)3⋅26H2O. Cement. Concrete. Res. 2004, 34, 869-881. (29) Beneventit, P.; Capelletti, R.; Kovács, L.; PéterÁ, M. A. M. L, Ugozzoli, F. FTIR Spectroscopy of OH Stretching Modes in BSO, BGO and BTO Sillenites. J. Phys.: Condens. Mater. 1994, 6, 6329-6344.



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Figure1.

(a)

XRD

pattern

of

g-C3N4,

(b)XRD

Page 22 of 34

patterns

g-C3N4/tz-Bi0.92Gd0.08VO4heterojunctions and (c) EDS spectrum of 20-CN.


of

Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure2. FT-IR spectra of g-C3N4, 0-CN and 20-CN.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure3. SEM, TEM, HRTEM and SAED images of g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions ((a-b) g-C3N4,(c-d) 0-CN, (e) 5-CN, (f-g) 10-CN, (h) 15-CN, (i-k) 20-CN and (l) 30-CN, (t-BVO and CN are short for tz-BiVO4 and g-C3N4).


Page 24 of 34

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Figure4. Zeta-potential of g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions and morphological evolution mechanism associated with that change.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure5.

Mineralization

performance

with


heterojunctions under UV light: (a) Degradation rate of RhB (The inset images show the degradation process and reaction rate constant), (b) TOC removal of RhB with the presence of g-C3N4/tz-Bi0.92Gd0.08VO4 samples (The inset image show the active species detecting results), (c) Recycle experiments of 20-CN and (d) XRD patterns of 20-CN before and after the recycle experiments.


Page 26 of 34

Page 27 of 34

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Figure6. Amperometric i-t curves of g-C3N4/tz-Bi0.92Gd0.08VO4 films.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure7. EIS Nyquist plots (dot) and Z-fit equivalent circuit (curve) of g-C3N4/tz-Bi0.92Gd0.08VO4 films: (a) 0-CN, (b) 5-CN, (c) 10-CN, (d) 15-CN, (e) 20-CN and (f) 30-CN.


Page 28 of 34

Page 29 of 34

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Figure

8.

UV-visible

reflectance

diffusion

g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions.


spectra

(DRS)

of


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure9. Mott-Schottky plots of g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions (1000HZ).


Page 30 of 34

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme

1.

Proposed

mineralization

g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions.


mechanism

over


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

Table 1. Rietveld refined structural parameters of g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunctions (t-BVO, m-BVO and CN are short for tz-BiVO4, ms-BiVO4 and g-C3N4 respectively). Error Coefficient Sample

Symmetry Rw

Rwnb

Rb

0-CN

17.17

16.14

9.60

5-CN

13.64

24.04

8.18

10-CN

13.26

11.34

8.79

15-CN

13.92

13.24

9.42

20-CN

19.49

17.91

14.9 6

30-CN

11.93

10.38

9.60

t-BVO:100% t-BVO:88% (CN:12%) t-BVO:85% (CN:2%, m-BVO:12%) t-BVO: 89% (m-BVO:10%) t-BVO:95% (m-BVO:4%) t-BVO:98% (m-BVO:2%)

Space Group

Lattice Parameters

Crystallite Size/nm

a/Å

b/Å

c/Å

I41/amd:2

7.285

7.284

6.446

33.00

I41/amd:2

7.286

7.286

6.447

45.73

I41/amd:2

5.292

11.574

5.153

46.90

I41/amd:2

5.166

11.639

5.120

39.50

I41/amd:2

5.167

11.634

5.116

39.30

I41/amd:2

5.164

11.626

5.120

38.30


Page 33 of 34

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Table 2. Z-fit equivalent circuit data of g-C3N4/tz-Bi0.92Gd0.08VO4 films. CDC code

RL(Ω)

0-CN(Dark)

R(×106 Ω)

C(×10-5 F)

Q(×10-5 Ssecn)

n

R value

Error Coefficient

164.6

4.80

3.45%

2.801

18.690

0.7519

0-CN(UV) 5-CN(Dark) 5-CN(UV) 10-CN(Dark) 10-CN(UV) 15-CN(Dark) 15-CN(UV) 20-CN(Dark) 20-CN(UV) 30-CN(Dark)

163.8 149.5 151.1 146.8 144.9 141.5 136.7 125.6 124.9 186.9

0.66 2.9 0.44 1.95 0.33 1.89 0.34 1.80 0.28 4.00

4.13% 8.82% 3.17% 9.02% 6.62% 9.37% 2.39% 4.34% 2.12% 9.46%

2.947 3.383 3.252 3.458 3.283 3.754 6.421 3.365 2.913 7.379

16.710 4.764 4.909 8.489 10.720 4.157 8.013 11.600 27.460 2.317

0.7581 0.8380 0.8342 0.8120 0.7925 0.8120 0.7893 0.8360 0.7310 0.9083

30-CN(UV)

206.5

0.69

4.07%

3.396

3.984

0.8765



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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tz-Bi0.92Gd0.08VO4

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