Introduction

The unceasing development of industries has been leading to the serious pollution of the environment. The contaminated water source has become an urgent issue because various industrial processes and human activities have been releasing organic pollutants which can generate toxicity and carcinogenicity for aqueous environment [1, 2]. Therefore, water treatment has been considered as an effective way to mitigate the water crisis and pollution. Many conventional water treatments such as ion exchange [3], adsorption [4], membrane [5], and biological processes [6] have been used over the past years. However, these conventional treatments can only transfer organic pollutants into other phases or secondary toxic byproducts; hence, it is difficult to meet the increasing criteria of water quality [7]. As a result, the development of alternative technologies has been acquired to solve these issues. By far, photocatalytic technology using semiconductor has drawn much attention of scientists due to its advantages such as complete mineralization, no further waste disposal, low cost, mild temperature, and pressure conditions [8]. Among photocatalytic semiconductors, titanium dioxide (TiO2) is considered as a benchmark catalyst with a wide band gap (3.2 eV), which has been used extensively for years [9,10,11]. Due to its wide band gap, TiO2 is only activated under ultraviolet (UV) light which occupies 3–5% of the whole solar light [12]. Therefore, this is a huge limitation and is economically inefficient for a photocatalytic system.

Recently, bismuth vanadate (BiVO4) has emerged as an ideal visible light-driven photocatalyst which can adsorb effectively the visible light due to the narrow band gap of 2.4 to 2.8 eV. Besides, it also has outstanding properties such as environmentally friendly, resistance to corrosion, high stability, good dispersibility, and inexpensiveness [2, 13, 14]. However, the photocatalytic applications of BiVO4 are limited by the poor charge transportation and the weak surface adsorption [15]. Due to these drawbacks, various strategies have been developed, such as constructing heterostructure system [16, 17], adding bias energy [18, 19] and doping with other materials [20] for enhancing the photocatalytic activity of BiVO4.

The combination of BiVO4 with metal oxide or porous materials has demonstrated the effective improvement of the specific surface area, the crystalline structure, and the photocatalytic activity in recent years [21]. Among these hybrid materials, mesoporous silica (SiO2) has been considered as a promising porous material due to its good properties such as well biocompatibility, large specific surface area, and easy functionalization [22, 23]. Thus, doping SiO2 with BiVO4 fabricates the p–n junction which enhances the separation of electron–hole pairs. Besides, the slow photons generated by SiO2 also enhance the light utilization [24]. As a result, it is expected that the immobilization of SiO2 on the surface of BiVO4 can strengthen effectively the separation of electron–hole pairs and the absorption ability leading to the enhanced photocatalytic activity. In addition to SiO2, graphene oxide (GO) has also been used as an effectively supporting material to disperse and stabilize photocatalyst in photocatalysis. It has many marvelous properties, such as high conductivity, high surface area, high transport mobility, and greater mechanical elasticity, due to the dense atoms and the honeycomb structure composed of sp2-bonded carbon atoms [25]. Furthermore, GO has oxygen containing functional groups which can act as favorable anchoring centers for active species in photocatalytic process. As a result, it helps to prevent the recombination of electron–hole pairs and further improve the photocatalytic activity [26]. However, the researches on the BiVO4/SiO2/GO nanocomposites have been rarely reported. Besides, the conventional methods use multistep approach and strong reducing chemical for the fabrication of nanocomposite.

Therefore, the present work aims to improve the photocatalytic activity of BiVO4 by the addition of SiO2 and GO using a one-step hydrothermal method. Various analytical techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET), were used to clarify the morphology and crystalline structure of BiVO4/SiO2/GO nanocomposites. Furthermore, the dye removal efficiency of nanoparticles was evaluated by measuring the removal percentage of methylene blue (MB) in the batch test under the visible light irradiation.

Methodology

Materials

All chemicals were obtained from Sigma-Aldrich and were used as received without any further purification, including bismuth (III) nitrate pentahydrate [Bi(NO3)3.5H2O], ammonium metavanadate (NH4VO3), graphite flake, tetraethyl orthosilicate (TEOS), potassium permanganate (KMnO4), sulfuric acid (H2SO4), sodium nitrate (NaNO3), hydrochloric acid (HCl), hydrogen peroxide (H2O2), sodium hydroxide (NaOH), methylene blue (C16H18ClN3S), benzoquinone (C6H4O2), terephthalic acid (C8H6O4), and ammonium oxalate (C2H8N2O4).

Preparation of graphene oxide

In the present work, GO was synthesized by using Hummers’ method [27] with a modification from graphite flakes. In this method, 10 g of graphite flake and 5 g NaNO3 was mixed in 600 mL of H2SO4 solution under an ice bath. Then, 60 g of KMnO4 was added slowly into the solution and stirred for 2 h. Subsequently, the mixture was heated at 35 °C for 24 h. After cooling down to room temperature, 1 L of DI water was poured slowly into this mixture and kept at temperature not exceeding 90 °C. In order to eliminate the excess of KMnO4, 5 mL of H2O2 was dropped into the mixture by which the color changes to bright yellow. The resulting mixture was washed repeatedly by the centrifugation with 5% HCl, ethanol, and DI water until the pH was neutral. Finally, the resultant was dried at 80 °C for 24 h to get GO powder.

Preparation of nanocomposites

Typically, 5 mmol of Bi(NO3)3.5H2O and 5 mmol of NH4VO3 were stirred separately in 50 mL of ethanol, while an amount of GO was sonicated in 20 mL ethanol for 30 min. Then, these solutions were mixed together, and followed by adding 1 mL of TEOS and 5 mL of DI water. This mixture was stirred for 3 h at room temperature and adjusted to pH 5 by NaOH 2M before heated at 180 °C for 10 h in a Teflon-lined stainless steel autoclave. Finally, the resultant was washed with ethanol and DI water for few times, and then dried at 80 °C for 24 h.

In comparison, pure BiVO4 and BiVO4/SiO2 were also synthesized in the same procedure without the addition of GO sheets.

Characterization

The morphological structure and crystal phase of the as-prepared nanocomposites were characterized by X-ray powder diffraction (XRD, Philips X’Pert MPD) using Cu Kα (λ = 1.54056 Å) radiation and field emission scanning electron microscopy (SEM, LEO1455VP), and Brunauer–Emmett–Teller (BET) measurements (Adtosorb 1 MP, Quantachrome) were conducted to investigate the surface properties of sample.

Photocatalytic activity

The photocatalytic activity of the as-prepared samples was evaluated by the removal MB in solution. The photocatalytic experiment was carried out as follows: 0.05 g of nanoparticle was added to 50 mL of 10−5 M of MB solution. Initially, the solution was stirred and kept in dark condition for 30 min to reach the adsorption–desorption equilibrium. Afterwards, it was irradiated by two 18-W halogen lamps (Essential MR, Philips, Thailand) for 30 min. The sample was collected every 10 min and measured by UV–Vis spectrophotometer (UV–6100 Double beam spectrophotometer, Mapada) at 664 nm of wavelength.

Results and discussions

Figure 1 presents the phase compositions of GO, pure BiVO4, BiVO4/SiO2, and BiVO4/SiO2/GO nanocomposites, which was analyzed by XRD technique. Generally, it can be seen that pure BiVO4, BiVO4/SiO2, and BiVO4/SiO2/GO nanocomposites show almost the same diffraction peaks corresponding to the monoclinic scheelite of BiVO4 (JCPDS no. 14–0688) which exhibits the highest photocatalytic activity [28]. In the meantime, GO sample is observed mainly at around 11° of 2θ. However, the peak of GO disappeared in the XRD pattern of BiVO4/SiO2/GO nanocomposites, this is because GO is probably reduced to graphene during hydrothermal process and the content of GO is low in the nanocomposite system [13].

Fig. 1
figure 1

XRD patterns of GO, BiVO4, BiVO4/SiO2, and BiVO4/SiO2/GO

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The morphology of as-prepared samples was investigated by SEM analysis as shown in Fig. 2. From Fig. 2a, it can be seen that the GO sheets are an aggregation of thin sheets with distinct edges and wrinkled surface. In the meantime, BiVO4 and BiVO4/SiO2 are formed by irregular particles with the different aggregation and the size of several micrometers. Figure 2d shows the morphology of BiVO4/SiO2/GO nanocomposite where BiVO4 and SiO2 are supported uniformly on the GO sheets, this is because the reduction of GO to graphene during hydrothermal reaction and the functional groups of GO facilitates to the adhesion of BiVO4 and SiO2 on the sheets.

Fig. 2
figure 2

SEM images of GO (a), BiVO4 (b), BiVO4/SiO2 (c), and BiVO4/SiO2/GO (d)

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Figure 3 presents the removal efficiency of MB over nanoparticles under visible light irradiation. For pure BiVO4, 25% of MB was adsorbed under the dark adsorption, and 47% of MB was degraded after 30 min under visible light irradiation.

Fig. 3
figure 3

Photodegradation of MB over BiVO4, BiVO4/SiO2, and BiVO4/SiO2/GO nanocomposites under visible light irradiation

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In the meantime, 50% of MB was adsorbed under the dark adsorption in the case of BiVO4/SiO2 due to the increase of the specific surface area and the pore size as shown in Table 1. The data indicates that the BiVO4/SiO2 has a higher specific surface area than pure BiVO4. Besides, the pore size is also increased from 12.11 of BiVO4 to 12.29 of BiVO4/SiO2; thus, BiVO4/SiO2 has a better adsorption for the degradation of MB than pure BiVO4. This increases the contact of photocatalyst and dye molecules leading to approximately 77% of MB photodegraded over BiVO4/SiO2 under visible light irradiation. Similarly, the adsorptive capacity is significantly increased by the addition of GO nanosheets in the BiVO4/SiO2/GO nanocomposite, this is because the specific surface area of BiVO4/SiO2/GO nanocomposite is 3.5 and 1.7 times higher than that of the pure BiVO4 and the BiVO4/SiO2 respectively, leading to 88% of MB adsorption after only 30 min of dark condition. Moreover, the migration and the separation of photogenerated carriers are also improved by the high conductivity and the mobility of GO nanosheets. Therefore, BiVO4/SiO2/GO nanocomposite exhibited the highest efficiency for MB removal efficiency, where 95% of concentration is removed under 30 min of visible light irradiation. This proves that the photocatalytic activity of BiVO4/SiO2/GO nanocomposite has been improved greatly for the MB dye removal and it can be considered as a promising photocatalyst for further applications.

Table 1 Surface properties of the as-prepared samples
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Conclusions

In this work, BiVO4/SiO2/GO nanocomposites have been successfully synthesized by hydrothermal method. The analytic results showed that the as-prepared samples have the same diffraction peaks of monoclinic scheelite of BiVO4 structure. The BiVO4 and SiO2 were integrated on the GO sheets with the different levels of aggregation and the size of several micrometers. The BiVO4/SiO2/GO nanocomposites exhibited a much higher MB removal efficiency compared with BiVO4 and BiVO4/SiO2 because the specific surface area and the pore size of BiVO4/SiO2/GO were increased significantly by the addition of SiO2 and GO. As a result, nanocomposites can be considered as promising materials for aqueous organic pollutant. In addition, the results in the present work also indicated the very high significant data on the difference between dye removal and dye degradation mechanisms. The detail of this will be discussed in the further work.