Construction of 2D/0D/2D face-to-face contact g-C3N4@Au@Bi4Ti3O12 heterojunction photocatalysts for promising dye degradation CURRENT STATUS: POSTED

Herein a new type of 2D/0D/2D face-to-face contact g-C 3 N 4 12 photocatalysts have been developed. The ternary composite photocatalysts are constructed by coupling Bi 4 Ti 3 O 12 nanosheets with g-C 3 N 4 nanosheets face-to-face and sandwiching Au nanoparticles between Bi 4 Ti 3 O 12 and g-C 3 N 4 nanosheets. The as-prepared g-C 3 N 4 @Au@Bi 4 Ti 3 O 12 composite photocatalysts were systematically investigated by various characterization techniques including XRD, UV–vis DRS, FTIR, SEM, TEM and XPS. The degradation experiments were carried out by removing rhodamine B (RhB) from water under simulated sunlight. It is found that the g-C 3 N 4 @Au@Bi 4 Ti 3 O 12 composite photocatalysts exhibit much enhanced photodegradation performance when compared with bare Bi 4 Ti 3 O 12 and g-C 3 N 4 nanosheets, and moreover they exhibit excellent photocatalytic stability in recycling dye degradation. The underlying photodegradation mechanism of the g-C 3 N 4 @Au@Bi 4 Ti 3 O 12 composite photocatalysts was systematically investigated and discussed. 3 N 4 nanosheets face-to-face and sandwiching Au nanoparticles between them, 2D/0D/2D face-to-face contact g-C 3 N 4 @Au@Bi 4 Ti 3 O 12 heterojunction photocatalysts have been prepared. Photocatalytic experiments suggest a good activity of the as-prepared ternary composite photocatalysts for photocatalytically degrading RhB under simulated sunlight irradiation. At 120 min of photoreaction, the 10%CN@4.2%Au@BTO composite photocatalyst is demonstrated to


Introduction
Water, being an important element of life, is getting polluted ever increasingly due to the generation of huge amount of wastewater annually from chemical industries. Organic dyes (e.g., rhodamine B (RhB)) are majorly included in the industrial effluent, which impose a potential threat to our health and survival due to their high water solubility, chemical stability, non-biodegradability and carcinogenicity. In recent years, semiconductor-based photocatalysis has gained increasing interest in water remediation [1][2][3][4]. This technology stands out due to its capability of utilizing solar radiation to decompose the harmful organic dyes. The sunlight-induced conduction band (CB) electrons (e − ) and valence band (VB) holes (h + ) in photocatalysts possess the reduction/oxidation capabilities are the basic reactive species, which take part in the direct or indirect redox reactions to cause the dye decomposition. Nevertheless, the photodegradation activities of most semiconductor photocatalysts are limited due to the easy recombination of photoexcited e − /h + pairs. Efficient suppression of the photoexcited electron/hole recombination has become one of most important strategies to improve the photodegradation performances of semiconductors [5][6][7][8][9][10][11].
Noble metal and carbon nanomaterials, attracting extensive attention owing to their outstanding physical properties and broad application prospects [12][13][14][15][16][17][18][19][20], have been extensively employed to modify the photocatalysts in order to improve their photodegradation performances [21][22][23][24][25][26]. The main enhanced photocatalytic mechanism is that these nanomaterials can serve as good electron sinks to trap the photoproduced electrons, thus leading to the decreased combination of e − /h + pairs. units, has received much recent attention as an important semiconductor photocatalyst for photodegradation of organic pollutants [36][37][38][39][40][41]. Due to its special layer structure, Bi 4 Ti 3 O 12 exhibits a high anisotropy of photocatalytic properties. In particular, the (010) facet is expected to have a high photocatalytic activity because the photoproduced electrons and hole are readily separated and migrate to the (010) facet driving by the polarization electric field (along the [010] direction) [42]. This was confirmed by the observation of extremely high photodegradation activity of twodimensional (2D) Bi 4 Ti 3 O 12 nanosheets with nearly 100% exposed (010) facet [35]. Graphite-like carbon nitride (g-C 3 N 4 , CN) is famously known to be a metal-free polymeric semiconductor, showing a promising visible-light-responsive photoactivity [38]. Due to its simple preparation and excellent thermal/chemical stability, g-C 3 N 4 is interesting for the application in organic dye photodegradation.
Herein g-C 3 N 4 and Bi 4 Ti 3 O 12 nanosheets have been coupled together face-to-face via a hydrothermal route. The derived 2D/2D g-C 3 N 4 @Bi 4 Ti 3 O 12 heterojunctions have a maximum contact area and are beneficial to the carrier transfer and separation. Furthermore, zero-dimensional (0D) Au nanoparticles have been sandwiched between g-C 3 N 4 and Bi 4 Ti 3 O 12 nanosheets to construct 2D/0D/2D g- All chemical reagents (analytical grade) used in the present experiments were directly supplied by chemical industries without further purification. A simple pyrolysis of melamine was used to fabricate g-C 3 N 4 nanosheets. Typically, 5 g of melamine was put in a corundum boat, semi-closed with a cover, and then calcinated in a tube furnace (520 °C, 4 h). The calcinated product was ground and obtained as final g-C 3 N 4 nanosheets.
A hydrothermal method as elaborated in the literature was employed to synthesize Bi 4 Ti 3 O 12 nanosheets [35]. A stoichiometric amount of bismuth nitrate pentahydrate (Bi(NO 3 ) 3 ·5H 2 O, 1.9402 g) was dissolved in 10% (v/v) dilute nitric acid solution. To the Bi(NO 3 ) 3 solution was then slowly added with titanium tetrachloride solution (0.5691 g TiCl 4 + 20 mL deionized water) and sodium hydroxide solution (4.8 g NaOH + 40 mL deionized water). The resultant mixture was loaded into a Teflon-lined autoclave and subjected to the hydrothermal reaction at 200 °C for 24 h. After that, the precipitate was gathered as Bi 4 Ti 3 O 12 nanosheets, which were washed with deionized water/absolute ethanol and drying at 60 o C for 12 h.

Fabrication of CN@BTO composites
During the above hydrothermal preparation process of Bi 4 Ti 3 O 12 nanosheets, stoichiometric g-C 3 N 4 nanosheets were uniformly dispersed in the precursor mixture solution, which results in the preparation of CN@BTO composites. The hydrothermal reaction temperature/time and sample collection/washing/drying process were performed under the same conditions. By adding different amounts of g-C 3 N 4 nanosheets in the precursor mixture solution, several CN@BTO composite samples with different g-C 3 N 4 mass fractions were prepared (i.e., 5%CN@BTO, 10%CN@BTO and 15%CN@BTO).

Fabrication of CN@Au@BTO composites
The CN@Au@BTO composites were prepared using the following two processes. The first process was to decorate Au nanoparticles on the surface of g-C 3 N 4 nanosheets via a photocatalytic reduction route. Stoichiometric ammonium oxalate (AO, 0.025 g) and g-C 3 N 4 nanosheets (0.13 g) were put in deionized water (80 mL), followed by ultrasonic dispersion (30 min) and magnetic stirring (1 h).
Subsequently, HAuCl 4 aqueous solution (0.029 mol L − 1 , 1 mL) was added to the above suspension, and then irradiated by a 15 W low-pressure mercury lamp for 30 min. The product, i.e. CN@Au composite, was rinsed with deionized water/absolute ethanol and subjected to drying at 60 °C for 12 h. In the second step, the as-prepared CN@Au composite was loaded in the precursor mixture solution that was used for the preparation of Bi 4 Ti 3 O 12 nanosheets as described in 2.1 section. The subsequent hydrothermal treatment process followed the same procedure for the Bi 4 Ti 3 O 12 preparation. The composite derived according to this procedure was designated as 10%CN@4.2%Au@BTO, where g-C 3 N 4 and Au occupy a mass fraction of ~ 10% and ~ 4.2%, respectively. Figure 1 depicts the preparation process of CN@BTO and CN@Au@BTO composite photocatalysts.

Sample characterization methods
X-ray powder diffraction (XRD) measurements were carried out by using a D8 Advance X-ray diffractometer (λ Cu-kα = 0.15406 nm). A TU-1901 double beam spectrophotometer was applied for the analysis of ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). Fourier transform infrared (FTIR) spectra were collected on a Spectrum Two FTIR spectrophotometer. A JSM-6701F field-emission scanning electron microscope was employed for the scanning electron microscopy (SEM) observations. Transmission electron microscopy (TEM) investigations were carried out by means of a JEM-1200EX field-emission transmission electron microscope. X-ray photoelectron spectroscopy (XPS) was recorded on a PHI-5702 multi-functional X-ray photoelectron spectrometer.

Photocatalytic testing process
Simulated-sunlight driven photodecomposition of RhB was used to evaluate the photoactivities of the as-prepared photocatalysts. The initial RhB concentration was C 0 = 5 mg L − 1 and the photocatalyst dosage was C photocatalyst = 1000 mg L − 1 . The photoreactor (capacity: 200 mL) was filled with 100 mL of RhB solution together with 100 mg of the photocatalyst. The adsorption experiment was performed by placing the photoreactor in the dark for 30 min, during which a magnetic stirring was applied. After that, turn on the light (a 200-W xenon lamp emitting simulated sunlight) to start the photocatalytic experiment. The change of the RhB concentration during the photocatalysis was determined by testing its absorbance at λ RhB = 554 nm by using a UV-vis spectrophotometer. The photocatalyst was removed from the reaction solution by centrifugation testing the absorbance of the reaction solution.

Results And Discussion
XRD patterns of g-C 3 N 4 , Bi 4 Ti 3 O 12 and 10%CN@4.2%Au@BTO were recorded to determine their crystalline structures, as displayed in Fig. 2. On the XRD pattern of g-C 3 N 4 , two diffraction peaks are observed at 13.17° and 27.40°, which correspond to the in-plane structural packing motif of tri-striazine units (i.e., (100) facet) and the inter-layer stacking of conjugated aromatic system (i.e., (002) facet), respectively [47]. This diffraction feature suggests that g-C 3 N 4 nanosheets are obtained. The diffraction peaks of Bi 4 Ti 3 O 12 , matching well with the diffraction data of PDF#35-0795, imply the formation of pure Bi 4 Ti 3 O 12 orthorhombic phase (cell: 0.545 × 3.282 × 0.541 nm 3 ). The XRD pattern of 10%CN@4.2%Au@BTO is very similar to that of bare Bi 4 Ti 3 O 12 , indicating no structural change of the orthorhombic Bi 4 Ti 3 O 12 in the composite. The XRD pattern of the composite presents no diffraction peaks assignable to g-C 3 N 4 nanosheets and Au nanoparticles, which is possibly due to the low diffraction intensities of g-C 3 N 4 and low content of Au.
It is necessary to characterize the light-absorption characteristics of nanomaterials because they are highly related to their physical properties [48][49][50]. UV-vis DRS measurements were carried out to determine the optical absorption properties of Bi 4 Ti 3 O 12 , g-C 3 N 4 , 10%CN@BTO and 10%CN@4.2%Au@BTO. As shown in Fig. 3(a), Bi 4 Ti 3 O 12 , g-C 3 N 4 and 10%CN@BTO have a poor visiblelight absorption in the wavelength region λ > 450 nm. In contrast, the 10%CN@4.2%Au@BTO composite with the introduction of Au nanoparticles manifests a relatively higher visible-light absorption, which is attributed to the strong light absorption of Au nanoparticles in the visible-light region. The absorption peak observed at around 550 nm for the 10%CN@4.2%Au@BTO composite can be characterized as the plasmon resonance peak of Au nanoparticles [35]. structure [35]. The peaks at 1096 and 1403 cm − 1 could originate from the symmetric and antisymmetric stretching vibrations of CO 3 2− groups introduced on the surface of Bi 4 Ti 3 O 12 during the hydrothermal synthesis process, respectively [52]. The absorption peak located at 1642 cm − 1 is induced by the H-O bending vibration of water molecules [53]. On the FTIR spectrum of g-C 3 N 4 ( Fig. 4(b)), the characteristic absorption peaks of g-C  Energy-dispersive X-ray spectroscopy (EDS) spectrum was collected from the 10%CN@4.2%Au@BTO composite ( Fig. 7(a)), which clearly shows the inclusion of C/N/O/Bi/Ti/O/Au species in the composite.
The former two peaks correspond to the binding energies of Ti 4+ -2p 3/2 and Ti 4+ -2p 1/2 , respectively, whereas the third peak can be ascribed to Bi-4d 3/2 binding energy [35,56]. The O-1 s XPS spectrum ( Fig. 8(c)) reveals two kinds of oxygen species, i.e., the crystal lattice oxygen of Bi 4 Ti 3 O 12 (529.9 eV) and chemisorbed oxygen species on the sample (531.9 eV) [35,57,58]. The C-1 s XPS spectrum ( Fig. 8(d)) presents three peaks located at 284.8, 288.0 and 282.6 eV, which are characterized as the carbon existing in the instrument, sp 2 -hybridized carbon in g-C 3 N 4 , and metal carbides, respectively [47]. On the N-1 s XPS spectrum (Fig. 8(e)), the binding energy peak at 397.5 eV is ascribed to sp 2hybridized nitrogen (C = N − C) resulting from g-C 3 N 4 [47]. The strong peak at 392.2 eV could be ascribed to the formation of metal nitrides. The observed binding energies at 84.1 (Au-4f 7/2 ) and 87.8 eV (Au-4f 5/2 ) on the Au-4f XPS spectrum (Fig. 8(f)) are indicative of the existence of metallic Au nanoparticles in the composite [35]. is demonstrated to be the optimal composite photocatalyst, over which the degradation percentage of RhB reaches 85.3% after 120 min photoreaction. Furthermore, by sandwiching Au nanoparticles between Bi 4 Ti 3 O 12 and g-C 3 N 4 nanosheets, a more promising ternary 10%CN@4.2%Au@BTO composite photocatalyst is achieved, which causes 94.4% of RhB to be photodegraded. As organic dyes are generally "tricky" in their photocatalytic degradation process, the exact photodegradation mechanism needs to be further investigated [60].
The photodegradation performances of the samples are further elucidated from the kinetic viewpoint.
As displayed in Fig. 9(b), the degradation kinetic plots of RhB conform perfectly to the pseudo-firstorder kinetic equation Ln(C t /C 0 ) = − k app t [61] due to their good linear behavior with R 2 larger than 0.99. The apparent first-order reaction rate constant k app can be employed for the quantitative comparison between the photodegradation performances of the photocatalysts. According to the values of k app as displayed in Fig. 9(b), it is concluded that the 10%CN@BTO composite possesses a photodegradation activity ~ 1.8 and ~ 3.9 times over that of bare Bi 4 Ti 3 O 12 and g-C 3 N 4 , respectively; whereas the photodegradation activity of the ternary 10%CN@4.2%Au@BTO is increased by 2.3 and 5.0 times compared with that of bare Bi 4 Ti 3 O 12 and g-C 3 N 4 , respectively.
To examine the reusability of 10%CN@4.2%Au@BTO for photocatalytic degradation of RhB, the photocatalyst was collected by centrifugation after the photodegradation experiment and recovered with deionized water rinsing. The next photodegradation experiment was carried out under the same procedure by loading the recovered 10%CN@4.2%Au@BTO in fresh RhB solution. To balance the minor loss of the photocatalyst after each run, fresh photocatalyst was added. As seen in Fig. 9(c), the photodegradation percentage of RhB within 120 min reaction slightly decreases from 94.4% at the 1st cycle to 90.1% at the 4th cycle, implying the degradation percentage of RhB undergoes only a minor loss (3.3%). The recycling photocatalytic experiment clearly demonstrates an excellent stability of the 10%CN@4.2%Au@BTO composite photocatalyst for repeatedly degrading organic dyes.
In the 10%CN@4.2%Au@BTO photodegradation system, the active species including hydroxyl (•OH) radicals, superoxide (•O 2 − ) radicals and photoexcited holes were determined by active species trapping experiments as described in the literature [62]. Ethanol (scavenger for •OH, 5 mL), benzoquinone (BQ, scavenger for •O 2 − , 0.1 mmol) and ammonium oxalate (AO, scavenger for h + , 0.1 mmol) were separately added in the photoreaction solution to examine their effects on the RhB degradation. As shown in Fig. 9(d), the addition of ethanol has a minor effect on the photodegradation of RhB, suggesting a very small role of •OH in the photodegradation process. The dye degradation is obviously inhibited by the introduction of BQ or AO, confirming that •O 2 − and h + are the main reactive species causing the dye degradation. In particular, the photoexcited h + plays the largest role in the photocatalysis due to the highest suppression efficiency by AO. To quantitatively determine the role of the reactive species in the photodegradation process, the trapping experiments using more quenchers are necessary [63].
It is noted that Bi 4 Ti 3 O 12 is an intrinsic n-type semiconductor (E g = 3.13 eV) and g-C 3 N 4 behaves as an intrinsic p-type semiconductor (E g = 2.83 eV) [35]. The CB/VB potentials of Bi 4 Ti 3 O 12 and g-C 3 N 4 are theoretically estimated, by using the method elaborated in the literature [64,65], to be − 0.19/+2.94 and − 1.19/+1.64 V vs NHE (normal hydrogen electrode), respectively, as schematically depicted in Fig. 10. When Bi 4 Ti 3 O 12 nanosheets, Au nanoparticles and g-C 3 N 4 nanosheets are coupled to form 2D/0D/2D face-to-face contact CN@Au@BTO heterojunctions, electrons will diffuse from ntype Bi 4 Ti 3 O 12 to p-type g-C 3 N 4 , and conversely, holes will diffuse from p-type g-C 3 N 4 to n-type            Schematic illustration of the photocatalytic mechanism of the g-C3N4@Au@Bi4Ti3O12 heterojunction photocatalysts.

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