Photoelectrochemical performance of g-C₃N₄/Au/BiPO₄ Z-scheme composites to improve the mineralization property under solar light

Abstract

g-C₃N₄/Au/BiPO₄ as a hierarchical Z-scheme system was prepared through three steps at different reaction temperatures, using thiourea as a precursor to synthesize g-C₃N₄. Their photoelectric activities were also investigated via the degradation of 10 mg L⁻¹ methyl orange (MO). g-C₃N₄/Au/BiPO₄ exhibited remarkably promoted photocatalytic activity and photoelectrochemical property compared with g-C₃N₄, BiPO₄ and g-C₃N₄/BiPO₄. The electrochemical impedance spectra and photocurrent result also proved that efficient charge separation and a better electron transport property were achieved by g-C₃N₄/Au/BiPO₄. Such excellent performances can be attributed to the Z-scheme system which improved the separation efficiency of photo-induced carriers and suppressed the recombination of electron–holes. It is worth pointing out that the metallic Au species not only acted as a solid state electron mediator, but also could absorb photons from incident light and present a surface plasmon resonance effect in this hybridization system.

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1. Introduction

As a dynamic, interdisciplinary field, nanomaterial science is very popular among scientists around the world. In recent years, studies have mainly focused on the photoelectric activities of nanostructured compounds, because of their fascinating optical, electrical and chemical properties, which can be managed by the structure, material and size.^1–3 Particularly interesting is the photoelectrochemical properties of TiO₂ nanotubes (NTs), considering their potential application in photovoltaics,⁴ photoinduced water splitting⁵ and more generally photocatalysis.⁶ The possibility of 1D charges transport along the NTs, as well as orthogonal charge separation at the nanoscale^5,7 and the presence of long-lived electrons⁸ are the most significant photoelectrochemical features of NTs. Photoelectrochemical analysis for semiconductor materials constitutes the most attractive approach towards photocatalysts degradation of organic pollutants.⁹

As π-conjugated materials, graphene-like ultrathin g-C₃N₄ nanosheets has attracted much attention because of the large surface area and fast charge transfer, which exhibit improved performance for photocatalysis and biosensing.^10–15 What's more important, the tri-s-triazine ring structure and the high condensation of g-C₃N₄ make the polymer possess high stability with respect to thermal and chemical attacks.¹⁶ Compared with conventional inorganic semiconductors, g-C₃N₄ is a sustainable, cost-effective and environmentally friendly semiconductor consisting of only carbon and nitrogen, and therefore has attracted extensive interest for its promising applications in the photodegradation of organic pollutants^17–19 the photo-splitting of water²⁰ and oxygen reduction reaction.²¹ Additional, g-C₃N₄ can easily coat on other compounds' surface because of the soft polymer different from the inorganic π-conjugated materials.²² Nevertheless, the photocatalytic efficiency is still poor because of the fast recombination rate of its photo-generated charge carriers. The g-C₃N₄ was used to combine with NiFe to enhance the photocatalytic property.²³ Pany and K. M. Parida²⁴ synthesized N, S–TiO₂/g-C₃N₄ nanocomposite to enhance the specific surface area and the effective charge separation properties of the 10TuT photocatalyst. The Au also used to deposit on the g-C₃N₄ to improved performance.²⁵

Z-Scheme photocatalytic system is more effective than traditional heterojunction and the single semiconductor. Under illumination, photon-generated carrier was stimulated. The photon-generated electrons in the lowest unoccupied molecular orbital of photosystem II transfer to combine with the photon-generated holes in the highest occupied molecular orbital of photosystem I molecular orbital of photosystem I through the electron mediator. The Z transfer way of electrons can keep the oxidative holes in the lower valence band and reductive electrons in the higher conduction band. The recombination rate of electrons and holes reduce and the redox ability was enhanced. The Z-scheme TiO₂–Au–CdS system reported in 2006 is the first example of an all-solid-state PS–C–PS system, which is constructed by the photochemical deposition–precipitation method.²⁶ They put forward the concept of all-solid-state Z-scheme photocatalysts. The resulting TiO₂–Au–CdS system showed higher photocatalytic activity compared with the two-component Au–TiO₂ and TiO₂–CdS systems. The reason is that the presence of Au between TiO₂ and CdS facilitate the separation of photo-generated charge carriers in TiO₂ and CdS. It was came out that the photo-generated electrons in conduction band (CB) of TiO₂ could transfer through the Au layer and recombine with the photo-generated holes left in valance band (VB) of CdS. Compared with the conventional p–n heterojunction photocatalysts, the Z-scheme photocatalytic system can remain the strong reducibility and oxidizability of the both photocatalysts.^27–29 Haili Lin et al. synthesized novel Z-scheme AgI/Ag/AgBr composite and found it displayed excellent photocatalytic activity for the degradation of methyl orange.³⁰ Liang Ding et al. synthesize CdS/Au/TiO₂ Z-scheme photosystem which improved the water-splitting efficiency by 200%.³¹

BiPO₄ is a kind of attractive ultraviolet light activity photocatalyst and it had attracted increasing interest over the past few years. The N-doped BiPO₄ photocatalysts were successfully synthesized via microwave hydrothermal method and it was found that the photocatalytic activity of BiPO₄ was remarkably improved.³² The novel hybrid architectures of BiPO₄ and mesoporous g-C₃N₄ have been synthesized as a composite photocatalyst for environmental application.³³ Novel composite architectures made up of graphitic carbon nitride quantum dots/bismuth phosphate nanocrystals have been synthesized as a visible light-induced photocatalyst for efficient degradation of methyl orange.³⁴ A series of BiPO₄ functionalized reduced graphene oxide (BiPO₄–rGO) nanocomposites were prepared using a one-step solvothermal method. Compared with the pure BiPO₄ nanoparticles, all of the as-prepared BiPO₄–rGO NCs with different starting mass ratios of graphene oxide (GO) to BiPO₄ showed an enhanced PEC response.³⁵

In this paper, we constructed an original Z-scheme g-C₃N₄/Au/BiPO₄ structure and discussed the photoelectrochemical (PEC) performance toward the dye degradation. Compared with other Z-scheme system, in g-C₃N₄/Au/BiPO₄, Au as a good conductor was inserted between g-C₃N₄ and BiPO₄ to form the ohmic contact which can reduce the distance of electrons in Z-scheme. The process of photocatalytic degradation is closely related with the separation and transmission rate of photo-generated charge carriers. The PEC performance of heterostructures is the expression of photo-electricity conversion efficiency. The PEC performance reflects a series of process under light illumination relevant with charge carriers, such as the generating and capturing of photon-generated carrier, direct recombination and indirect recombination and so on. Hence, we can acquire the transmission of photo-generated charge carriers within the material through PEC performance. This process is similar to the subprocedure of photocatalytical reaction. The concentration of photo-generated charge carriers is decided by the competition between the production and recombination of charge carriers. The different reaction processes will result in different carrier lifetime, further the photocatalytic performance is different. For instance, the lifetime of photo-generated charge carriers which are caught by traps then are released and recombined is longer than the lifetime of carriers which directly combine. The photocatalytic performance is decided by the concentration of photo-excited carrier.

In addition, the PEC performance of g-C₃N₄, BiPO₄, g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ are discussed through degradation efficiencies. The effect of Au nanoparticle modification between g-C₃N₄ and BiPO₄ is also investigated.

2. Experiment

2.1 Synthesis of g-C₃N₄/Au/BiPO₄

All reagents for synthesis and analysis were used without any further treatments. BiPO₄ was prepared through a simple one step hydrothermal process. 8 mmol of Bi(NO₃)₃·5H₂O was dissolved in 100 mL distilled water and stirred for 30 min at room temperature, while 8 mmol of Na₃PO₄·12H₂O and 30 mL distilled water were put in a beaker under magnetic stirring for 10 min, then was added into Bi(NO₃)₃ solution dropwise. The final mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 160 °C for 20 h. Afterwards, the products were filtered, washed and dried in vacuum.

Au nanoparticles were deposited on the surface of BiPO₄. The Au precursor was prepared by mixing a 1 mM HAuCl₄·4H₂O aqueous solution (450 mL) with a certain amount of NaOH solution (1 mol L⁻¹) in order to adjust pH to 6. The reaction processes are as follows:

HAuCl₄ + 5NaOH = NaAuO₂ + 4NaCl + 3H₂O (2-1)

2NaAuO₂ + 3HCHO + NaOH = 2Au + 3HCOONa + 2H₂O (2-2)

10 g of BiPO₄ was added into the Au precursor solution and stirred for 3 h under 70 °C. The solution was deposited for 5 min. The samples were filtered, washed, and then dried in an oven at 80 °C for 24 h in vacuum.

3.6 g of thiourea was added into 100 mL ethanol, the solution was sonicated for 3 h. The result mixture was transferred into an oven to dry at 60 °C for a whole night. The prepared was finely grinded and calcined in 550 °C for 2 h at the heating rate of 15 °C min⁻¹. The g-C₃N₄ dispersed in ethanol and sonicated for 1 h to layer. 0.48 g of Au/BiPO₄ was added into the ethanol contain g-C₃N₄, followed by vigorous stirring for 30 min. The product was finely calcined in muffle furnace on 550 °C for 5 h at the heating rate of 15 °C min⁻¹. The Au/BiPO₄ was coated by g-C₃N₄.

2.2 Analytical characterization

The crystalline phase of the products were characterized by X-ray diffraction (XRD, D/max-2200pc, Japan) on 40 kV and 40 mA with Cu Kα (λ = 0.15418 nm). UV-visible absorbance spectra were obtained for the dry-pressed disk samples with a UV-visible spectrophotometer (Lambda-950, Perkin Elmer Corporation, America). The photoelectric activities are investigated via the degradation of 10 mg L⁻¹ methyl orange (MO). After degradation, the concentration of pollutants in the liquid supernatant are tested with the UV-visible spectrophotometer. TEM images were conducted on a FEI Tecnai G2 F20 S-TWIN transmission electron microscope operated at 200 kV. All electrochemical experiments were performed with a CHI660D electrochemical workstation (CH Instruments, Shanghai, China). 50 mg catalysts was suspended in 2 mL Nafion aqueous solution (1 wt%), the mixtures were ultrasonically scattered for 15 min to form homogeneous solution. Then, 0.1 mL solution was dropped on the fluorine doped tin oxide (FTO) glass (2 × 4 cm). After evaporation of the water in air, the catalysts were attached onto the surface of FTO glass as the working electrode, a platinum wire as the counter electrode, an Ag/AgCl/3 M KCl as the reference electrode. X-ray photoelectron spectroscopy (XPS) analyses were carried out on ESCALAB 250 Xi spectrometer. The binding energy of C 1s used to calibrate the charging effect. The light source was a 300 W Xe lamp was employed for the simulated sunlight irradiation.

3. Result and discussion

3.1 XRD analysis

The XRD patterns of as-prepared powder g-C₃N₄/Au/BiPO₄ composites are shown in Fig. 1. It can be seen the basic diffraction peak of g-C₃N₄ is at around 27.8°, it is well indexed as (002) diffraction planes (JCPDS 87-1526).³⁶ As compared with the pure g-C₃N₄, for the g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ composites, diffraction peaks of the BiPO₄ had been detected, which in accordance with BiPO₄ (JCPDS 89-0287). However, the diffraction peaks of pure BiPO₄ and Au/BiPO₄ are matched well with BiPO₄ (JCPDS 80-0209). During the process of coating g-C₃N₄ on the surface of BiPO₄ and Au/BiPO₄, reaction temperatures rise to 550 °C, the crystal forms of BiPO₄ and its crystallinity was changed. This transformation between the two kinds of crystal of BiPO₄ do not destroy the relationship between crystal structure and topological structure, only change the orientation of oxide polyhedron which consist of atoms of Bi and P. While the diffraction peaks of the Au was obviously detected at approximate 38.2° and 44.4° which could be assigned to (111) and (200) crystal phases of Au (JCPDS 65-2870). For the g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ composites, only BiPO₄ and Au diffraction peaks were found. The characteristic peaks of BiPO₄ (27.13°) and g-C₃N₄ (27.4°) were too close to distinguish. The lattice constants of (200) crystal plane of BiPO₄ and (002) crystal plane of g-C₃N₄ are similar, which helps to reduce the force of surface coating when g-C₃N₄ cladding BiPO₄, make it easy to form the coating structure.

Fig. 1 XRD patterns of pure g-C₃N₄, as well as of the g-C₃N₄/BiPO₄, Au/BiPO₄ and g-C₃N₄/Au/BiPO₄ composite.

3.2 TEM analysis

The TEM images revealed the morphology and microstructure of the sample. Fig. 2 showed the pictures of g-C₃N₄ dispersed in ethanol before and after ultrasonic treatment. By means of ultrasonic processing, lumpy g-C₃N₄ was fully exfoliated into individual layers.

Fig. 2 TEM of g-C₃N₄ dispersed in ethanol (a) before and (b) after ultrasonic treatment.

The single-layer of g-C₃N₄ has higher surface energy. The specific surface area of the nanorods of BiPO₄ was small, the surface energy was lower. So the platelike g-C₃N₄ can easily adsorb on the surface of nanorod BiPO₄ spontaneously. It can be seen from Fig. 2(b) that the thickness of g-C₃N₄ is about 3 nm.

In Fig. 3(a), BiPO₄ exhibits nanorod-like structure, the length of the BiPO₄ nanorod was about 150 ± 50 nm, the width of the BiPO₄ nanorod was about 30 ± 5 nm. The morphology of BiPO₄ was uniform, and it was inclined to agglomerate. In Fig. 3(b), the Au nanoparticles uniformly distribute on the surface of BiPO₄. In Fig. 3(c), the g-C₃N₄ formed an obvious and continuous coating layer on the surface of Au/BiPO₄ and the surface of this coating was smooth. It was confirmed that the capping layer was not only the simply physical adsorption, but also the chemical adsorption on the surface of Au/BiPO₄. These all make the coating layer existed steadily. The thickness of this exterior coating-layer was about 3–4 nm. Fig. 3(d) is the HR-TEM image of g-C₃N₄/Au/BiPO₄, by carefully measuring the lattice parameters and comparing with the data in JCPDS, the lattice spacing around 0.193 nm and 0.237 nm observed in this image agree well with the (301) of BiPO₄ (JCPDS 89-0287) and (111) of Au (JCPDS 65-2870). Due to the weak crystallization property of g-C₃N₄ on the edge of g-C₃N₄/Au/BiPO₄ composite, the lattice fringe was not clear to be seen.

Fig. 3 TEM of (a) BiPO₄; (b) Au/BiPO₄; (c) g-C₃N₄/Au/BiPO₄; and HR-TEM image of (d) g-C₃N₄/Au/BiPO₄.

3.3 UV-vis absorption spectra

The UV-vis absorption spectra of composites were shown in Fig. 4. As is well-known, the optical absorption of a semiconductor is closely related to its electronic structure. The absorption wavelength edge of BiPO₄ is about 310 nm in the UV region. According to the reports³⁷ about the band structure of oxysalts which contain Bi³⁺, the electrons jumping from O 2p orbit to Bi 6p orbit causes the electronic stimulated transition of BiPO₄ at 320 nm absorption edges. It can be obviously seen that the absorption edge of pure g-C₃N₄ occurred at about 530 nm. However, as the introduction of Au, BiPO₄ exhibited an extra absorption peak at the visible region (about 540 nm), which could be attributed to the localized surface plasmon resonance (LSPR) effect of the Au nanoparticles (regarded as a process of electron fluctuation).³⁸ The electrons in Au 6sp band could obtain energy and transfer to the higher energy level in its orbital equipped with the similar gaseous behavior of the fluctuation in the stimulation of external light.³⁹ After the decoration of g-C₃N₄, the absorption rate of g-C₃N₄/BiPO₄ increased to visible light region, but the reflectivity of visible light reduced. Compared with g-C₃N₄/BiPO₄, stronger absorption of g-C₃N₄/Au/BiPO₄ in the visible region at wavelengths longer than 500 nm was observed. Some studies^40,41 proved that a slight red shift appeared, because the energy gap of sample was changed and the valence band constitution narrowed after compounding with other composites. These demonstrated the g-C₃N₄ existed on the surface of BiPO₄ in the form of heterojunction but not the simple physical touch.

Fig. 4 UV-vis absorption spectra of BiPO₄; g-C₃N₄; g-C₃N₄/BiPO₄; Au/BiPO₄ and g-C₃N₄/Au/BiPO₄.

3.4 Characterization of XPS

It can be known from the previous analysis that g-C₃N₄ compounding Au/BiPO₄ was not the physical adsorption process but act through chemical bond which make sure that the g-C₃N₄ existing on the surface of Au/BiPO₄ steadily. In Fig. 5, the chemical valence state and elementary composition of the g-C₃N₄/Au/BiPO₄ composite was characterized by X-ray photoelectron spectroscopy (XPS). Fig. 5(a) showed the XPS spectra of g-C₃N₄/Au/BiPO₄. In Fig. 5(b), the C 1s peaks at 284.9 and 288.8–289.1 eV were assigned to adventitious carbon and C–H bonds and to C=O bonds, respectively.⁴² In Fig. 5(c), XPS spectrum of N 1s can be deconvoluted into three peaks centered at 398.5 eV, 399.7 eV and 401.1 eV. The 398.5 eV peak is usually assigned to sp²-hybridized nitrogen (C–N=C) dominated in the g-C₃N₄. The other two weak peaks correspond to sp³ C–N bonds and amino groups with a hydrogen atom (C–N–H), respectively.⁴³ Fig. 5(d) showed the XPS spectrum of Au 4f with the characteristic peaks at 84.1 eV and 87.7 eV of Au 0, confirming the existence of Au 0 in the composite.^38,44 Compared with the characteristic peak of Bi in BiPO₄ at 159.31 eV and 164.64 eV,⁴⁵ the characteristic peak of Bi in g-C₃N₄/Au/BiPO₄ shifts toward large binding energy to 160.05 eV and 165.29 eV in Fig. 5(e). This proved the electron density of Bi decreased and the valence state elevated after compounding.

Fig. 5 XPS survey spectrum of g-C₃N₄/Au/BiPO₄ (a) and its high-resolution XPS spectra: (b) C 1s, (c) N 1s, (d) Au 4f, (e) Bi 4f and (f) P 2p.

3.5 Electrochemical studies

Fig. 6 showed the transient photocurrent response of g-C₃N₄, BiPO₄, g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ samples over several on–off cycles of Xe light irradiation. In the process of photocatalyst working, the separation and migration of electrons and holes was the key factor to decide the photocatalytic efficiency. By measuring the intensity of photocurrent, we can know the details of the separation and migration of electrons and holes of photocatalyst. Along with the light switched, all samples revealed fast photocurrent response and each response were even. Under Xe light irradiation, the photocurrent intensity of g-C₃N₄/Au/BiPO₄ was the strongest in all sample, it is about 3 times of BiPO₄, 2 times of pure g-C₃N₄ and 1.2 times of g-C₃N₄/BiPO₄. After compounding with g-C₃N₄, the photocurrent of BiPO₄ enhanced greatly. The introduction of Au into BiPO₄ caused an increase in the photocurrent under Xe light irradiation, indicating that the LSPR effect of Au nanoparticles induced by Xe light make a big difference on the interface charge transfer. The light of Xe included ultraviolet and visible light. When Xe light irradiation, the separation of photoelectrons from vacancies in g-C₃N₄ and BiPO₄ were stimulated, the Au nanoparticles also provided ohmic contact to g-C₃N₄ and BiPO₄ which facilitated the transmission rate electrons and holes.

Fig. 6 Photocurrent response of g-C₃N₄; BiPO₄, g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄.

Electrochemical impedance spectra (EIS) measurements are conducted to investigate the charge transfer resistance and the separation efficiency of the photoinduced charge carriers.⁴⁶ The EIS spectra of the pure g-C₃N₄, BiPO₄, g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ composites were measured to investigate their charge transfer capability under illumination. Alternating current impedance spectroscopy of the four samples is all sicircle shape, it indicates that the transmission of surface charge is the rate-determining step of the reaction. Under the illumination of Xe light, compared with the pure g-C₃N₄ and BiPO₄, the EIS Nyquist plots of g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ exhibit much smaller semicircles in Fig. 7, indicating a significant decrease of the charge transfer resistance in the g-C₃N₄/Au/BiPO₄ composites. The LSPR effect of Au nanoparticles induced by visible region light could provide electrons to BiPO₄, which also meant that the photo-generated electrons transfer from Au surface to BiPO₄. By the introduction of g-C₃N₄, the concentration of carrier increased, significant changes have taken place in the electrode surface, the resistance of interface reaction decreased. To BiPO₄ and g-C₃N₄, the enhancement caused by the impact of recombination was mutual. It can be speculated that the transfer of photo-induced carrier occurred between BiPO₄ and g-C₃N₄. The result agreed with the consequence of photocurrent. In order to get the accurate information, the ZsimpWin analysis software was used to simulate the EIS Nyquist plot. The CDC code, such as R(Q(R(RC))), could well explain the behavior of electrode process under illumination.⁴⁷ In Fig. 7, the equivalent circuit includes solution resistance (R_L), electrolytic resistance (R₁), charge-transfer resistance (R₂), space charge capacitance (C) and electrochemical double-layer capacitance (Q). We fitted the equivalent circuit for 3 times, the value of R₂ is about 343 kΩ. The fitting results are showed in Table 1.

Fig. 7 EIS Nyquist plots of g-C₃N₄; BiPO₄; g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ composites.

Table 1 Z-Fit equivalent circuit data of g-C₃N₄/Au/BiPO₄ films for 3 times fitting

Sample	RL (Ω)	Q (×10^−5 s^n)	n	R1 (×10^4 Ω)	C (×10^−5 F)	R2 (×10^5 Ω)
1st	17.64	3.832	0.9406	4.259	1.291	3.438
2nd	17.82	3.414	0.9613	4.281	1.085	3.431
3rd	17.82	3.414	0.9613	4.279	1.085	3.430

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The PEC activity of each sample was investigated by measuring the relationship of the current–voltage (I–V) with the film immersed in 0.1 M Na₂SO₄ solution under simulated solar irradiation. Fig. 8 shows the current–voltage (I–V) characteristics of pure g-C₃N₄, BiPO₄, g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ composites. The current density recorded under illumination showed steep increase above 0.62 V. Apparent current density was observed due to the coating of g-C₃N₄, the photocurrent density obtained from g-C₃N₄/Au/BiPO₄ is higher than other samples under illumination. The g-C₃N₄/Au/BiPO₄ photoanode generated a photocurrent density of 150 μA cm⁻² at 1.1 V vs. Ag/AgCl in a 0.1 M Na₂SO₄ solution, which is 2 times higher of the anode made of g-C₃N₄/BiPO₄ and 4 times higher than that of the BiPO₄ anode. In the potential ranges from 0.62 to 1.3 V, the hysteresis cycles were observed. The role of Au particles was to mediate the electron transfer from BiPO₄ to g-C₃N₄ and to prevent the recombination of electron–hole pairs in BiPO₄.⁴⁸ Compared pure BiPO₄, the enhancement could be associated to charge accumulation at the g-C₃N₄/Au/BiPO₄ electrode interface due to some capacitive properties of the g-C₃N₄ (ref. 49) and the electrons transfer of Au.

Fig. 8 I–V characteristics of pure g-C₃N₄; BiPO₄; g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ under simulated solar irradiation.

3.6 Photocatalytic activity of the samples

In Fig. 9, the photocatalytic behaviors of the as-prepared g-C₃N₄, BiPO₄, g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ crystals are explored for the degradation of MO dye under simulated sunlight irradiation. With irradiation for 160 min, the degradation efficiencies of g-C₃N₄, BiPO₄, g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ were about 14%, 42%, 74% and 88%. The g-C₃N₄/Au/BiPO₄ shows the highest photocatalytic activities in all samples. The weak crystallization property of g-C₃N₄ reduced the conduction of carriers in it and g-C₃N₄ can absorb a part of ultraviolet ray, these all reduce the light utilization efficiency of g-C₃N₄. After loading g-C₃N₄ on the surface of BiPO₄, electron–hole pairs were stimulated, the electrons flow to BiPO₄ and holes flow to g-C₃N₄ under simulated sunlight irradiation. The electrons and holes are separated effectively, thus catalytic activity of g-C₃N₄/BiPO₄ is improved. On the condition that Au loading at the g-C₃N₄/BiPO₄ interface, there are benefits of both ways. First was facilitating the photoinduced electrons transfer from the inside to the surface in g-C₃N₄; second was accelerating the electron passing from g-C₃N₄ to BiPO₄.⁵⁰ The observed first-order rate constants of the degradation reaction (K_obs) are showing Fig. 10.

Fig. 9 Photocatalytic degradation of MO of pure g-C₃N₄; BiPO₄; g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄.

Fig. 10 The degradation rate constants of MO on different samples.

The rate constant for g-C₃N₄, BiPO₄, g-C₃N₄/BiPO₄ and g-C₃N₄/Au/BiPO₄ nanocomposite are 3.7 × 10⁻⁴, 3.5 × 10⁻³, 6.9 × 10⁻³ and 1.1 × 10⁻² min⁻¹, respectively. Hence, the photocatalytic activity of g-C₃N₄/Au/BiPO₄ nanocomposite was about 3 and 1.6 times than those of BiPO₄ and g-C₃N₄/BiPO₄, respectively.

3.7 Mechanism of enhanced photoelectrochemical properties

Through the research of photoelectric performance, we can reveal the transport process of carriers. This process is closely related to photocatalytic process. The migration ability of photo-generated carriers can be detected via the photocurrent, whose rate is relative to the recombination rate of electron–hole pairs in the photocatalyst.⁵¹ Endo et al. showed that the increase of migration velocity of the electron in CB would lead to easy migration of photo-generated carriers to the surface and result in oxidation–reduction reaction.⁵² Under simulated sunlight irradiation, the photocurrent of the g-C₃N₄/Au/BiPO₄ is the highest among all sample, it could effectively promote the separation of photo-generated carriers, which is consistent with the results of the EIS spectra.

Fig. 11 is the proposed mechanism for Z-scheme charge-carrier transfer process in the g-C₃N₄/Au/BiPO₄ photocatalyst. The depositing Au existed between the g-C₃N₄ and BiPO₄ can participate in the transition process of photo-generated charge carrier. Under the simulated sunlight irradiation, the weak reductive photoelectrons excited from BiPO₄ could be transferred to Au very fast. Moreover, the weak holes excited from g-C₃N₄ could also be quickly transferred to Au. At the same time, the strong reductive photoelectrons would be left on the conduction band of g-C₃N₄, and the strong oxidizing holes would be left on the valence band of BiPO₄. These photo-generated electrons can be involved in the H⁺ to ˙OH reduction process and the holes might be involved in the oxidation reaction of translating OH⁻ into ˙OH which is the main reactive species during the process of degradation MO. According to the Z-scheme charge-carrier transfer process, the oxidation/reduction ability of photo-generated holes and electrons can be enhanced, and the lifetime of photoholes from BiPO₄ and photoelectrons from g-C₃N₄ can be increased, thus accelerate the separation and transmission of the photo-generated carrier and then enhance the PEC ability⁵³ which is in accordance with the electrochemical test results.

Fig. 11 (a) Charge transfer illustrative scheme for the photoelectrochemical measurements; (b) the band diagram values of g-C₃N₄/Au/BiPO₄ as the working electrode.

3.8 Photocatalytic mechanism research

There are several key factors influencing the photocatalytic capability of a semiconductor, including the absorption capacity of the incident photons, the photo excitation efficiency of electron–hole pairs, the migration and separation rate of carriers, the redox capability of excited-state electrons and holes, as well as the nature of its surface/interface chemistry, which included the surface energy and chemisorption properties of the photocatalyst.⁵⁴

Due to the existence of over-potential, electrons of g-C₃N₄ generated after optical excitation can't transfer to H⁺ rapidly. This problem can be solved through depositing Au paired between the g-C₃N₄ and BiPO₄. Because of the Au–H bonds formed between Au and g-C₃N₄ which is advantageous to the transfer of electrons. In the reaction system, the oxygen united with the photo-induced electrons to form ˙O²⁻. Due to the effect of vugular electrostatic interaction, ˙O²⁻ absorb on the surface of heterocompound g-C₃N₄/Au/BiPO₄ photocatalyst. The adsorption capacity of pollutant weaken after oxidize by ˙O²⁻, then the pollutant separate from the surface of g-C₃N₄/Au/BiPO₄.

The g-C₃N₄ that synthesized in lab shows poor crystallinity and poor dispersion in water. It can also absorb blue and violet light in visible region, thus reduced the utilization of light. After coating by g-C₃N₄, the specific surface area of g-C₃N₄/BiPO₄ increased and helped the MO contact with the catalyst adequately. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of g-C₃N₄ were more negative than the conduction band and valence band of BiPO₄. Under the simulated sunlight irradiation, both g-C₃N₄ and BiPO₄ were motivated. The photo-induced electron transfer from the HOMO of g-C₃N₄ to the conduction band of BiPO₄. The photo-generated holes on valence band of BiPO₄ can transfer to the lowest unoccupied molecular orbital LUMO of g-C₃N₄. As an excellent transport material, g-C₃N₄ helped the holes transfer to the surface of hybrid catalyst. The organic pollutant can be oxidative degradation directly.

We can comprehend the mechanism of the remarkably enhanced photocatalytic performance of the g-C₃N₄/Au/BiPO₄ photocatalysts according to the following aspects. Firstly, the g-C₃N₄ formed on the surface of Au/BiPO₄ and the absorption of visible light in solar light was enhanced remarkably due to the surface plasmonic resonance effect. Secondly, acting as a sink for the photo-generated electrons, the addition of Au enhanced the charge separation between electrons and holes in order to improve the photocatalytic activity. Thirdly, in Fig. 11(b), an all-solid-state Z-scheme photocatalytic system was constructed by employing conductor Au as the electron mediator. The insertion of Au conductor between g-C₃N₄ and BiPO₄ formed the known ohmic contact with low contact resistance. In the meantime, the strong reductive photoelectrons were left on the conduction band of g-C₃N₄ and the strong oxidizing holes were on the valence band of BiPO₄. Compared with the g-C₃N₄/BiPO₄, the ability of left electrons and holes in the Z-scheme systems are stronger.

4. Conclusions

The g-C₃N₄/Au/BiPO₄ as a hierarchical Z-scheme system was prepared in three steps: (1) BiPO₄ was synthesized by hydrothermal process; (2) Au nanoparticles were deposited on the surface of BiPO₄ through chemical bath deposition; (3) the g-C₃N₄ coated Au/BiPO₄ through sonicating and vigorous stirring. Consequently, g-C₃N₄/Au/BiPO₄ showed higher photocurrents and photocatalytic activity. So excellent performance could be attributed to the Z-scheme system, which improve the separation efficiency of photoinduced carriers and it is beneficial to suppress recombination of electron–holes.

Acknowledgements

Natural Science Foundation of Shaanxi (2015JM5213).

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