Synthesis of mesoporous SiO₂/Cu₂O–graphene nanocomposites and their highly efficient photocatalytic performance for dye pollutants

Abstract

A mesoporous SiO₂/Cu₂O–graphene composite, a novel material, was successfully synthesized using a self-assembly method with tetraethyl orthosilicate (TEOS). During the reaction, Cu₂O and silica nanoparticles were loaded on the graphene sheets. The mesoporous structure, morphology, pore diameter, pore volume, and surface area of the mesoporous SiO₂/Cu₂O–graphene composite photocatalysts were obtained via X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, transmission electron microscopy (TEM), Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), FT-IR spectra, nitrogen adsorption/desorption isotherms using the BET and BJH method and photocurrent analyses. The photocatalytic degradation of rhodamine B (RhB), methylene blue trihydrate (MB), and reactive black B (RBB) in an aqueous solution under visible light irradiation was observed via UV spectrophotometry after measuring the decrease in their concentrations. The mesoporous structure of silica nanoparticles with a large surface plays a major role in the increased photodegradation as well as in the decomposition by the catalysts. Through recycling experiments, we conclude that the mesoporous SiO₂/Cu₂O–graphene composite had good stability during photocatalysis under visible light irradiation. The mesoporous SiO₂/Cu₂O–graphene nanocomposite is expected to become a candidate material for photodegradation with excellent performance.

---

1. Introduction

Graphene has been successfully decorated with numerous inorganic materials such as Au, ZnO, TiO₂, and Cu salts through various methods, leading to nanocomposites with a photocatalytic activity that is useful in the degradation of organic dyes.^1–4 Of these, graphene-based composite materials have attracted a significant amount of attention because recent studies have shown their applicability in electronics, photocatalytic systems, and photovoltaic devices.^5–7 The graphene surface decorated with nanomaterials can overcome the aggregation of individual graphene sheets and of the nanomaterials themselves.^8–10 With the outstanding development of science and technology, some new advances on graphene-based photocatalyst have been presented. Ting Xiong et al. demonstrated the important role of reduced graphene oxide (RGO) in UV-induced photocatalysis of RGO-based nanocomposites.¹¹ Yuhan Li et al. proposed enhancing the photocatalytic activity of bulk g-C₃N₄ by introducing mesoporous structures and hybridizing with graphene.¹²

Cu₂O is a p-type semiconductor oxide that is considered to have potential for use in catalytic systems, fuel cells, and solar cells.^13,14 Cu₂O has the advantage of a narrow band gap energy (2.00 eV) and a suitable energy level position that make it a promising inorganic material for use in photodegradation under visible light.^15–17 In addition, Cu₂O is inexpensive and non-toxic, with a high chemical durability that is particularly manifested in its unsurpassed optical decomposition, and as such is a promising candidate for scientists to prepare highly applicable nanocomposites.¹⁸ For example, Chen Kunfeng et al. reported on a Cu₂O nanocrystal and Cu₂O–graphene composite paper for use as lithium-ion battery anode materials.¹⁹ Zhang et al. synthesized a Cu₂O–graphene using the solvothermal method and also discussed the use of Cu₂O–graphene in an electrochemical sensor for dopamine.²⁰

Recently, silica nanoparticles (SiO₂) have become popular materials with potential applications in numerous fields, including ceramics, catalysts, pharmaceutical products, electronic packaging, photonic and chemical–mechanical polishing due to their ordered mesostructure, large surface area, flexible pore size, and thermal stability.^21–23 Due to such advantages, silica nanoparticles (SiO₂) have been considered to have be photocatalyst with higher efficiency than TiO₂.^20,24,25 In addition, the improvement in the pore diameter, pore volume and surface area of the silica nanoparticles has attracted a significant amount of attention in various fields of science. Fengyuan Zhang²⁰ observed that surfactants will improve the pore size and the distribution of the silica nanoparticles.²⁶ The combination of graphene oxide and silica nanoparticles exhibits many outstanding properties when compared with bare graphene oxide and silica nanoparticles,²⁷ and coating silica nanoparticles with many different kinds of inorganic materials has been reported to result in unique properties with many uses.^28,29 With their different band gap energy levels, Cu₂O and SiO₂ particles can be combined to produce a composite with a highly efficient photocatalysis of dye pollutants under visible light irradiation.^30–32 Nevertheless, the synthesis of mesoporous SiO₂/Cu₂O–graphene composites through a facile method, such as the self-assembly method, and their effective use in photocatalytic systems has not been demonstrated yet.

In this study, a mesoporous SiO₂/Cu₂O–graphene composite was prepared using a self-assembly method. A facile route was implemented to synthesize the mesoporous SiO₂/Cu₂O–graphene using tetraethyl orthosilicate (TEOS) as the silica precursor at a pH of 9.5–10. The dispersion medium consisted of a mixture of water and ethanol. Cetyltrimethylammonium bromide (CTAB) was used as the structure creator, and the silica mesoparticles formed after hydrolysis. The structure and morphology of the mesoporous SiO₂/Cu₂O–graphene composites were characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, transmission electron microscopy (TEM), Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis-DRS), X-ray photoelectron spectroscopy (XPS). The FT-IR spectra were also recorded as evidence to confirm the structure of the survey composites. The pore diameter, pore volume and surface area of the mesoporous SiO₂/Cu₂O–graphene composites were determined via nitrogen adsorption/desorption isotherms using the BET and BJH method. Photodegradation experiments were then conducted with rhodamine B (RhB), methylene blue trihydrate (MB), and reactive black B (RBB) organic dyes in an aqueous solution under visible light irradiation. The recycling experiments were carried out for five repeated cycles to investigate the stability of the photocatalysts.

2. Experimental

2.1 Reagents

Graphene oxide was prepared in the laboratory from natural graphite using Hummer–Offerman's method and was used to form the composites. Copper(II) acetate (Cu(CH₃COO)₂·H₂O, 95%), rhodamine B (RhB, C₂₈H₃₁ClN₂O₃), and methylene blue trihydrate (MB, C₁₈H₁₈ClN₃S·3H₂O) were purchased from Samchun Pure Chemicals Co. Ltd, Korea. Reactive Black B 195% (RBB) was purchased from JAY Chemical Industries Limited, India. Polyvinylpyrrolidone (PVP) K-30 was purchased from Junsei Chemical Co., Ltd, Japan. Ethanol (C₂H₅OH, 95%) was purchased from Duskan Pure Chemicals Co. Ltd, Korea. Tetraethyl orthosilicate (TEOS, 99%) was purchased from Aldrich Chemistry, Germany. Cetyltrimethylammonium bromide (CTAB, C₁₉H₄₂BrN, 99%) and ammonium hydroxide (NH₄OH), and ethylene glycol (C₂H₆O₂, 99%) were purchased from Daejung Chemicals Co. Ltd, Korea. Urea (CO(NH₂)₂) was purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification, and all experiments were carried out using distilled water.

2.2 Synthesis nanocomposites

2.2.1 Synthesis of the Cu₂O nanoparticles. 0.32 g Cu(CH₃COO)₂·H₂O was dissolved in 50 mL distilled water and 8 mL of ethylene glycol, then magnetic stirred for 30 min. Then, 0.24 g CO(NH₂)₂ was added into the above solution and keep stirring for 30 min to form a transparent solution. After a hydrothermal reaction occurred at 180 °C for 10 hours, the temperature of the mixture was brought down to ambient temperature, and the mixture was filtered using Whatman filter paper (Φ = 110 mm). The obtained solid was collected and washed with distilled water 3 times and ethanol 95% twice. After drying under vacuum at 70 °C for 3 hours, the obtained samples were collected for characterization.

2.2.2 Synthesis of the Cu₂O–graphene nanocomposite. 0.005 mol Cu(CH₃COO)₂·H₂O was dissolved in 50 mL distilled water, then heated to 60 °C with magnetic stirring for 30 min to form part A. Sonication of the graphene oxide (0.01 g) in 10 mL of 95% ethanol was conducted for 30 min to form part B. Part A was mixed with part B and 0.1 g PVP, with continued vigorous stirring for 1 hour at 80 °C. The pH value was adjusted with NaOH 1 M until it reached pH 9 and was maintained for 1 hour. After a hydrothermal reaction occurred at 180 °C during 10 hours, graphene oxide was reduced to a graphene nanosheet with the deposition of Cu₂O compounds on the surface to form the Cu₂O–graphene nanocomposite. Afterward, the temperature of the mixture was brought down to ambient temperature, and the mixture was filtered using Whatman filter paper (Φ = 110 mm). The obtained solid was collected and washed with distilled water 3 times and ethanol 95% twice. The Cu₂O–graphene nanocomposites were obtained after drying in a vacuum at 90 °C for 24 h. After drying under vacuum at 105 °C for 24 h, the achieved powder was calcined at room temperature to 550 °C for 8 hours followed by heating at 550 °C for 6 hours.

2.2.3 Synthesis mesoporous SiO₂/Cu₂O–graphene nanocomposite. 0.36 g CTAB was dissolved in 57 mL distilled water, then stirred with magnetic stirring for 30 min to form part C. Part D was obtained via ultrasonication of the Cu₂O–graphene nanocomposite (0.4 g) in 20 mL distilled water for 30 min. Separately, 4 mL TEOS, 80 mL ethanol, and 5 mL NH₃ 25% were mixed under vigorous stirring to form part E. Part C, D, and E were mixed together with continued vigorous stirring for 6 hours at room temperature. Eventually, the pH value was adjusted with NH₄OH 25% until reaching pH 9.5–10 and was then maintained for 1 hour. The above dispersion was transferred to an autoclave for a hydrothermal reaction at 100 °C for 24 hours. Afterward, the temperature of the mixture was reduced to ambient temperature, and the mixture was filtered using a Whatman filter paper (Φ = 110 mm). The product was washed with distilled water 3 times and 95% ethanol twice. After drying under vacuum at 105 °C for 24 h, the achieved powder was calcined at room temperature to 550 °C for 8 hours followed by heating at 550 °C for 6 hours.

With the catalytic base (pH = 9.5–10), the hydrolysis reaction of TEOS easily forms Si(OH)₄ with OH⁻ ions that can directly penetrate toward the Si atoms and replace every part of the –OC₂H₅ group in the TEOS molecule. Therefore, condensing the ≡Si(OH) group of silica gel made up with structural hierarchical priorities for all sides, resulting in nanosilica material that often formed spherical sructures.^23,33–38 The preparation procedure is shown in Scheme 1.

Scheme 1 Synthesis of mesoporous SiO₂/Cu₂O–graphene by a simple self-assembly method.

2.3 Characterization

X-ray diffraction (XRD, Shimadzu XD-D1) was used to determine the crystallinity with monochromatic high-intensity CuKα radiation (λ = 1.5406 Å). The shape and structure of the nanomaterial surface was analyzed at a high-resolution using SEM (JSM-5600 JEOL, Japan). UV-vis diffuse reflectance spectra (DRS) analysis was conducted using a UV-vis spectrophotometer (Neosys-2000) by using BaSO₄ as reference at room temperature and converted from reflection to absorbance by the Kubelka–Munk method. Transmission electron microscopy (TEM) was also used to examine the size and distribution of the titanium and iron particles deposited on the fullerene surface of various samples. High Resolution Transmission Electron Microscopy (HRTEM, JEOL, JEM-2010, Japan) was used to observe the surface state and structure of the survey composites at an acceleration voltage of 200 kV. The TEM specimens were prepared by placing a few drops of sample solution on a carbon grid. The XPS analysis was performed using a VG Scientific ESCALAB250 XPS system equipped with a monochromated Al Kα X-ray source (hν = 1486.6 eV) with charge compensation. The Raman spectra of the prepared samples was observed using a spectrometer (Jasco Model Name NRS-3100) with an excitation laser wavelength of 532.06 nm. Nitrogen adsorption/desorption isotherms studies were prepared using a Micromeritics ASAP 2020M + C operating at 77 K using the Brunauer–Emmett–Teller (BET) method to determine the surface area. The survey samples were degassed at 150 °C for 4 h prior to the surface area measurements, whereas the pore size distribution was calculated according to the Barret–Joyner–Halenda (BJH) method. Photoelectrochemical measurements were performed using a self-made photoelectrochemical system installed a 250 W halogen lamp as the irradiation source. The photocurrent measurement was performed by a computer-controlled Versa-STAT-3 electrochemical analyzer. A survey composite modified photoelectrode with an active area of 1 cm² was used as the working electrode, and a Pt wire and saturated Ag/AgCl were used as the counter and reference electrodes, respectively. All the photocurrent measurements were conducted by dipping the survey composite modified photoelectrode into a mixture of 0.1 M KCl and 0.5 M TEA at a constant potential of 0 V vs. Ag/AgCl. A UV/Vis spectrophotometer (Optizen POP, Mecasys, Korea) was used for the photodegradation experiments.

2.4 Photocatalytic activity

For the typical photocatalytic test carried out at room temperature, the mesoporous SiO₂/Cu₂O–graphene nanocomposite used in this study was found to be 0.05 g, dissolved in 100 mL RhB solution (200 ppm). Prior to the irradiation, the above mixture solution was maintained in a dark box for 2 hours to establish the adsorption/desorption equilibrium of organic dyes. Next, the solution was irradiated with visible light radiation (λ ≥ 420 nm). The first sample was withdrawn at the end of the dark adsorption period before switching on the light to determine the RhB concentration in the solution after dark adsorption. The starting point (t = 0) of the reaction was defined as the point where the concentration of RhB solution was recorded as c₀. Afterward, these samples were extracted from the solution mixtures in the reactor at regular intervals of 30, 60, 90, 120, and 180 min. The powders were dispersed using a centrifuge machine (10 000 ppm/15 min) before the analysis. The dye concentration in the solution was measured as a function of the irradiation time. The photodegradation of the MB and RBB solution proceeded following a similar process.

A UV-spectrophotometer (Opizen POP, Korea) was used to analyze the photodegradation of the dye solutions in terms of the concentration (c). A spectrophotometric analysis was performed on each sample of the dye solutions at regular time intervals to obtain the absorbance spectrum. The spectral range was investigated at λ_max = 554 nm, 665 nm, 591 nm, respectively, for RhB, MB, and RBB by using a calibration curve since no reaction occurred with the absorption of products at these wavelengths. The degradation capacity (η%) was calculated as

η(%) = (1 − c/c₀) × 100

The photodegradation of organic dyes was also observed for RhB, MB, and RBB with (a) Cu₂O–graphene, (b) Cu₂O–graphene–TiO₂, (c) mesoporous SiO₂/Cu₂O–graphene and (d) Cu₂O particles, following the same procedure as that mentioned above.

Next, recycling experiments were carried out for five repeated cycles to investigate the stability of the photocatalytic performance. After each cycle, the catalyst was centrifuged, washed with ethanol, deionised water, and dried before reuse for the next experiment.

3. Results and discussion

3.1 Characterization

An X-ray diffraction (XRD) analysis was conducted with the composites to obtain the component crystalline phase information and assess the purity and crystalline size of the nanocomposite. The XRD results of the mesoporous SiO₂/Cu₂O–graphene calcined at 550 °C is shown in Fig. 1. All diffraction peaks were clear, and the purity of the mesoporous SiO₂/Cu₂O–graphene was expressed through the presence of two types of main diffraction peaks with Cu₂O and SiO₂ signals, without any unexpected peaks. Because of the dominant effect of silica, the presence of SiO₂ particles is seen through the broad diffraction peak at 2θ of 22.96°.^39,40 Next, according to the XRD results shown in Fig. 1, the Cu₂O signal was confirmed by the clear distribution of the peaks obtained according to the Miller index planes corresponding to (110), (111), (200), (220) and (311) (JCDS 78-2076), respectively.^41,42 On the other hand, no graphene peaks were recorded in the XRD patterns of the nanocomposites, indicating that the immobilization of SiO₂ particles onto the surface of the graphene sheets disturbs the ordered structure of most graphene sheets.^43,44 Looking over Fig. 1, the obtained composites were almost perfect with a single phase, high crystallinity, and high purity due to the absence of unexpected peaks and sharper diffraction peaks. The signal of these single peaks in mesoporous SiO₂/Cu₂O–graphene confirms the development of Cu₂O and SiO₂ on the graphene sheets.

Fig. 1 XRD patterns of the mesoporous SiO₂/Cu₂O–graphene composite.

The morphology of the mesoporous SiO₂/Cu₂O–graphene sample was also assessed during the initial evaluation via SEM, and the results are presented in Fig. S1 (ESI†). The SEM images also indicate that both fine Cu₂O and SiO₂ nanoparticles uniformly covered the graphene surface. The morphological features of the SiO₂ particles included a small size with spherical shapes and good particle dispersion. This indicates an increased photocatalytic activity with the mesoporous SiO₂/Cu₂O–graphene nanocomposite. With this morphogenesis and sustainable link, the ability to switch the charge between Cu₂O and SiO₂ as well as switch the charge among Cu₂O, SiO₂, and graphene increased. Subsequently, an increased photocatalytic activity with the mesoporous SiO₂/Cu₂O–graphene nanocomposite can be observed.

TEM images were taken for the mesoporous SiO₂/Cu₂O–graphene nanocomposites to further investigate the structure and provide clear images of the dispersion and morphology, as shown in Fig. 2. Fig. 2(a) shows the existence of quantum dots with an irregular shape, which indicates that the Cu₂O nanoparticles covered the surface of the graphene sheets. In Fig. 2(b), smaller Cu₂O nanoparticles with a size around 5–10 ppm can be seen to be uniformly distributed throughout the surface of the small TiO₂ rods with an average size of about 150–200 nm, as well as on the graphene surface. The typical morphologies of the mesoporous SiO₂/Cu₂O–graphene nanomaterials were confirmed in Fig. 2(c and d). It was clear that the spherical silica nanoparticles with a pore diameter smaller than 2 μm were successfully hanged on the graphene surface. In this case, the graphene nanosheets play a major role in a facile template for the TEOS hydrolysis.³⁷ The results of HR-TEM in Fig. 2(e and f) show the detailed structure of the mesoporous SiO₂/Cu₂O–graphene nanomaterials. Besides the amorphous in nature of the silica particles were clearly seen, the single-crystallinity of Cu₂O nanoparticles were found to exist on the spherical shapes surface of the mesoporous SiO₂ with the pure black microsphere-like shape which implied that the particles is solid and not core–shell or inner hollow structure.⁴⁵ The formation of the mesoporous SiO₂ and Cu₂O nanoparticles on the graphene surface was also observed in Fig. 2(f). As a result, the moving charge from SiO₂ to Cu₂O will easily occur and help limit the recombination of the photogenerated electron–holes, releasing the necessary energy for the photodegradation reaction. The mesoporous silica nanoparticles were also promised to provide a more exposed surface for the photocatalytic activity. The above results indicate that the mesoporous SiO₂/Cu₂O–graphene nanomaterial will become a potential candidate due to its photocatalytic activity. The TEM results matched the results from SEM, confirming that the experimental conditions and processes were adequate.

Fig. 2 TEM image of the Cu₂O–graphene (a), Cu₂O–graphene–TiO₂ (b) and mesoporous SiO₂/Cu₂O–graphene (c and d) composites and HR-TEM image of mesoporous SiO₂/Cu₂O–graphene (e and f).

The Raman spectra were measured to consolidate the structural information of the mesoporous SiO₂/Cu₂O–graphene nanocomposite. The Raman spectra of Cu₂O–graphene and mesoporous SiO₂/Cu₂O–graphene nanocomposites were presented in Fig. 3. The existence of the D and G bands in the Raman results is evidence of the presence of carbon in the survey nanomaterials. The D and G bands of the Cu₂O–graphene composite appeared at 1340 cm⁻¹ and 1590 cm⁻¹, respectively, while the D band at 1350 cm⁻¹ and G band at 1600 cm⁻¹ of the mesoporous SiO₂/Cu₂O–graphene nanocomposite was confirmed, as shown in Fig. 3. The D band shifted from 1340 to 1350 cm⁻¹, indicating changes in the chemical bond between Cu₂O–graphene and mesoporous SiO₂/Cu₂O–graphene nanocomposite. In the graphene oxide-based samples, the D band is an ordinary feature of sp³ defects, and the G band provides information on the in-plane vibration of the sp² bonded carbons.⁴⁶ The intensity ratio (I_D/I_G) reflects the order of defects in the graphene oxide or graphene. It strongly depends on the extent of the disorder in the graphitic material.⁴⁷ Following Fig. 3, the calculated I_D/I_G ratio of the Cu₂O–graphene composite was found to be 0.960, while the I_D/I_G ratio of mesoporous SiO₂/Cu₂O–graphene was 0.897. The decrease in the I_D/I_G ratio indicates an increase in the number of graphene layers or a partial reduction in the graphene oxide into graphene, which is consistent with the reports.⁴⁸ In other words, the decrease in I_D/I_G proved that sp² carbon domains were repaired, but a certain amount of sp³ or twisted carbon atoms as defects still existed.⁴⁹ Furthermore, the lower I_D/I_G ratio shows a better defect repair mechanism.⁵⁰

Fig. 3 Raman spectra of the mesoporous SiO₂/Cu₂O–graphene and Cu₂O–graphene nanocomposites.

The typical FT-IR spectrum of the survey materials was taken as shown in Fig. 4(a). Considering the FT-IR spectrum of the Cu₂O–graphene nanocomposites, it shows three main peaks that can be attributed to oxygen-containing groups. The peaks centered at 1110, 1400, and 1560 cm⁻¹ assisted the C–O vibrations from the alkoxy groups, C–OH stretching vibrations, and C=O stretching peak of the conjugated ketone, respectively.^51,52 After a self-assembly reaction with TEOS at pH 9.5–10, the FI-IR spectrum of mesoporous SiO₂/Cu₂O–graphene nanocomposite displayed new bands, confirming that the silica nanoparticles covered the graphene surface. The bands at 805 and 1080 cm⁻¹ were attributed to Si–O–Si symmetric stretching, and the stretching vibration of the Si–O–Si band, respectively.⁵³ In contrast with the disappearance of the C=O groups, the above evidence suggests that the typical carboxyl group was altered into Si–O–C bonds.³⁷ When compared to the FT-IR spectrum of the Cu₂O–graphene nanocomposite between 1300 and 1600 cm⁻¹, it was easy to see the absence of the characteristic peaks for oxygen-containing groups, which indicated that the oxygen-containing groups were unfixed.³⁹ In other words, all FT-IR spectrum results concluded that mesoporous SiO₂ were produced on the surface of the graphene sheets. As the XPS results show in Fig. 4(b and c), the surface bonding state of the survey nanomaterial was identified as containing Si included Si 2p, and Si 2s at 106 eV, and 155 eV, respectively. The C 1s spectra of the composites were presented in the region from 282.5–294.7 eV, indicating the presence of non-oxygenated C (C=C/C–C) in aromatic rings (284.5 eV), as well as the C in C–O–C (286.3 eV), where the epoxy or hydroxyl group (C–O) is at 286.7 eV and C=O or the carboxyl is at 288.1 eV.^54,55 The presence of the O 1s signal of the nanocomposites is due to absorbed oxygen.

Fig. 4 FT-IR spectra of the Cu₂O–graphene and mesoporous SiO₂/Cu₂O–graphene composites and XPS results of the mesoporous SiO₂/Cu₂O–graphene composites.

The results of the Kubelka–Munk transformation from the UV-vis diffuse reflectance spectra of the mesoporous SiO₂/Cu₂O–graphene composite are displayed in Fig. 5. The band gap energy value was achieved at the point at which the straight line approaching the curve intersects the horizontal axis. The band gap energy values of the mesoporous SiO₂/Cu₂O–graphene composite correspond to 1.80 eV, which is lower than that of the Cu₂O–graphene composite.⁵⁶ This phenomenon promises an increase in the catalytic activity of the mesoporous SiO₂/Cu₂O–graphene composite.

Fig. 5 hν versus [(k/s)hν]² graph of the mesoporous SiO₂/Cu₂O–graphene composite.

The surface properties and the results of the porosity analysis of the composites are exhibited in Fig. 6, and the corresponding textural parameters are displayed in Table 1. The nitrogen adsorption/desorption isotherm results of the Cu₂O–graphene, Cu₂O–graphene–TiO₂ composites clearly exhibit a similar type II curve. The pore size calculated using the BJH method was about 24.15 and 11.29 nm. The BET surface area of the Cu₂O–graphene and Cu₂O–graphene–TiO₂ composites was 10.74 and 41.55 m² g⁻¹, respectively. According to the hysteresis loop in the relative pressure region around 0.45–0.98, the nitrogen adsorption/desorption isotherms showed that the SiO₂/Cu₂O–graphene nanocomposite exhibited a similar type IV curve. In other words, the SiO₂/Cu₂O–graphene nanocomposite existed with a mesoporous structure. After the self-assembly method with TEOS, the surface area of the SiO₂/Cu₂O–graphene inhibited a big difference in the Cu₂O–graphene, Cu₂O–graphene–TiO₂ composites. Due to the mesoporous structure, the SiO₂/Cu₂O–graphene showed a nanopore size of 4.19 nm and a very large surface area with BET results at about 287.49 m² g⁻¹. Besides, the total pore volume of SiO₂/Cu₂O–graphene was 66.05 cm³ g⁻¹, which is 33 times more than the total pore volume of Cu₂O–graphene (2.47 cm³ g⁻¹). With an increase in the surface area and decrease in the pore size, as well as the large total pore volume, the SiO₂/Cu₂O–graphene is expected to have a high photocatalytic activity.

Fig. 6 The nitrogen adsorption/desorption isotherms (above) and pore size distributions (below) for the Cu₂O–graphene, Cu₂O–graphene–TiO₂ and mesoporous SiO₂/Cu₂O–graphene composites.

Table 1 Micropore structure parameters of the Cu₂O–graphene, Cu₂O–graphene–TiO₂ and mesoporous SiO₂/Cu₂O–graphene composites

Sample name	BET (m^2 g^−1)	Total pore volume (cm^3 (STP) g^−1)	Average pore size (nm)
Cu2O–graphene	10.74	2.47	24.15
Cu2O–graphene–TiO2	41.55	9.55	11.29
SiO2/Cu2O–graphene	287.49	66.05	4.19

---

Fig. 7 displays the photocurrent response of the ITO/as-prepared nanocomposites based on time under visible light irradiation, which was carried out for five times at 20 s intervals. It was obviously observed that no current was observed in the dark, which provides the evidence of no photoinduced charge separation occurs. The as-synthesized Cu₂O–graphene presented the high photocurrent response (∼98.0 μA cm⁻²) during repeating on–off illumination cycles, while the low photocurrent density was ∼20.0 μA cm⁻² for SiO₂–graphene nanocomposite. Besides, the mesoporous SiO₂ and Cu₂O–graphene combination with a mass ratio at 100 : 5 reached two times enhanced photocurrent density (∼40.0 μA cm⁻²) compared to SiO₂–graphene nanocomposite. The improved photocurrent of the mesoporous SiO₂/Cu₂O–graphene implied an enhanced the photoinduced electron–hole separation at the p–n junction of the SiO₂/Cu₂O hybrid, while the holes were scavenged by the TEA, and graphene plays a role as a charge transfer medium.^57,58 As the results, the electrons were transported to the ITO electrode, resulting in photocurrent generation. Because of the reduced interaction between as-prepared survey nanocomposites and ITO under visible light irradiation, the photocurrent density decreased over time.⁵⁹ On the other hand, the porous network structure also contributes to the enhanced transition of photogenerated carriers.⁶⁰

Fig. 7 Photocurrent measurement with Cu₂O–graphene, mesoporous SiO₂/Cu₂O–graphene and SiO₂–graphene modified photoelectrodes.

3.2 Photodegradation

The organic dye degradation progressed in two stages. First, the adsorption/desorption equilibrium of organic dyes was established in a dark box for 2 hours. Then, the photocatalytic degradation experiment of the dye solutions was conducted under visible light irradiation with different irradiation times (from 0 to 180 min). The effects of the different organic dyes are given below.

3.2.1 Degradation of rhodamine B (RhB). Fig. 8 shows the photocatalytic degradation of the RhB aqueous solution in the present of the Cu₂O–graphene, Cu₂O–graphene–TiO₂, mesoporous SiO₂/Cu₂O–graphene composites and Cu₂O at room temperature. As shown in Fig. 8, in the RhB case, the adsorption effect of the mesoporous SiO₂/Cu₂O–graphene was maximized. A longer time resulted in a decrease in the maxima of the absorption intensity in the visible region (around 555 nm). The mesoporous SiO₂/Cu₂O–graphene nanocomposite expressed the best photodegradation, with a value of around 89.27%, which is a big difference when compared to that of Cu₂O–graphene–TiO₂ (41.26%). The most significant difference in the photocatalytic effect of the Cu₂O–graphene (9.50%) and Cu₂O (4.35%) compared to the mesoporous SiO₂/Cu₂O–graphene can also be seen. We noticed that the mesoporous SiO₂/Cu₂O–graphene composite provided the best degradation for the RhB solution.

Fig. 8 Degradation efficiency of different composites with RhB under visible light. The concentration of RhB was 200 ppm. The amount of composites were 0.05 g.

3.2.2 Degradation of methylene blue (MB). The results of the photodegradation of the survey composites with MB were investigated under the same experimental conditions shown in Fig. 9. The MB case also showed that the adsorption effect of the mesoporous SiO₂/Cu₂O–graphene is better than that of the Cu₂O–graphene, Cu₂O–graphene–TiO₂, and Cu₂O after being maintained for 2 hours in the dark. The effect of the photodegradation on mesoporous SiO₂/Cu₂O–graphene far exceeds that of both Cu₂O–graphene, Cu₂O–graphene–TiO₂, and Cu₂O with 68.62, 12.08%, 12.46%, and 3.63% respectively.

Fig. 9 Degradation efficiency of different composites with MB under visible light. The concentration of MB was 200 ppm. The amount of composites were 0.05 g.

3.2.3 Degradation of reactive black B (RBB). Fig. 10 shows the degradation results of the Cu₂O–graphene, Cu₂O–graphene–TiO₂ and mesoporous SiO₂/Cu₂O–graphene composites and Cu₂O under visible light irradiation with the RBB solution. The results shown in Fig. 10 indicate that the degradation efficiency decreased as the irradiation time increased. All survey composites achieved a 60% removal for the RBB solution. After 180 min under visible light irradiation, the mesoporous SiO₂/Cu₂O–graphene retained the highest photodegradation effect of 84.07%. Meanwhile, the photodegradation results for Cu₂O–graphene, Cu₂O–graphene–TiO₂, and Cu₂O were 67.98%, 74.51%, and 10.94% respectively. The above results indicate that the mesoporous SiO₂/Cu₂O–graphene has potential as a material with high photodegradation activity.

Fig. 10 Degradation efficiency of different composites with RBB under visible light. The concentration of RBB was 200 ppm. The amount of composites were 0.05 g.

Photocatalytic reactions with different photocatalysts can be expressed using the Langmuir–Hinshelwood model.⁶¹ The photocatalytic degradation of organic dyes containing different photocatalysts obeys pseudo-first-order kinetics with respect to the concentration of the organic dyes.

−dc/dt = k_appc (I)

The integration of (I) (with the restriction of c = c₀ at t = 0, with c₀ as the initial concentration in the bulk solution after dark adsorption and t the reaction time) will lead to the following expected relation:

−ln(c/c₀) = k_appt (II)

where c and c₀ are the reactant concentration at time t = t and t = 0, respectively, k_app and t are the apparent reaction rate constant and time, respectively. According to (II), a plot of −ln(c/c₀) versus t will yield a slope of k_app. The results are shown in Fig. S2 (ESI†).

To investigate the stability of the photocatalytic performance, the mesoporous SiO₂/Cu₂O–graphene was used to degrade the RhB dye in five repeated cycles, and the results are shown in Fig. S3 (ESI†). As shown in Fig. S3(a),† the photocatalytic activity of the mesoporous SiO₂/Cu₂O–graphene still exhibited good results after five recycling runs. This confirmed that the mesoporous SiO₂/Cu₂O–graphene had a high stability under visible light irradiation. In spite of the lower diffraction peak intensity, we can see three types of main diffraction peaks indexed to Cu₂O, and SiO₂ signals still remain in the XRD patterns in Fig. S3(b).† The TEM image from Fig. S3(c)† also presented the original structure of the mesoporous SiO₂/Cu₂O–graphene nanocomposite after five cycling photocatalysis process. Therefore, we can conclude that the mesoporous SiO₂/Cu₂O–graphene composite has the good stability during photocatalysis.

Besides the narrowing of the band gap energy, the excellent photocatalytic activity of the mesoporous SiO₂/Cu₂O–graphene is explained to be a result of the pore diameter, pore volume and surface area. Graphene oxide and silica nanoparticles provided a large surface for photodegradation, and in the photodegradation experiments, we can see that the adsorption capacities of the mesoporous SiO₂/Cu₂O–graphene are higher than those for Cu₂O, Cu₂O–graphene and Cu₂O–graphene–TiO₂ mainly due to the large surface area of the mesoporous silica particles.⁶² In our case, the BET of mesoporous SiO₂/Cu₂O–graphene was more than 7–28 times that for the BET results of the Cu₂O–graphene and Cu₂O–graphene–TiO₂ composites. With these outstanding properties, the adsorption effects of mesoporous SiO₂/Cu₂O–graphene produced the best adsorption effects for the RhB, MB, and RBB solution.

Graphene oxide also plays the role of an electron acceptor, photosensitizer, and the adsorbent to efficiently enhance the photodegradation of organic dyes.^63,64 The presence of the high density of oxygen-containing functional groups, such as hydroxyl groups, epoxy, and carboxyl at the edge or on the large surface area of graphene oxide provides the more space for ionic/electro interaction between dye molecules and the aromatic rings of graphene oxide sheets, and from that, the better adsorptivity can be achieved.^65,66 Besides, we can not ignore the role of an electron acceptor of graphene oxide which participates in facilitating electron transfer process from SiO₂ and Cu₂O to join the reduction reaction as well as enhancing charge separation and therefore, enhancing the photodegradation.^67,68

On the other hand, the mesoporous structure of the silica nanoparticles plays a major role in the increased photodegradation effect as well as the surface of graphene sheets. In this case, the mesoporous structure allows a greater amount of guest species over the catalysts, and the reaction will not be limited to the photocatalytic surface. For the large surface area, the active site population and accessibility to the active sites increase, and the photocatalytic activity will thus increase.⁶⁹

The photocatalytic activity of the mesoporous SiO₂/Cu₂O–graphene composite reflects two factors that influence the degradation rate: the adsorption capacity due to the mesoporous structure and the decomposition effect through catalysis. The valence band of the SiO₂ is higher than that of Cu₂O. Therefore, the photogenerated electrons of SiO₂ can switch to the valence band of Cu₂O and then shift to the graphene surface to join the reduction reaction (e⁻_CB). The photogenerated holes of Cu₂O can also switch the graphene surface to participate in oxidation reactions (h⁺_VB). With this mechanism, the reduced electron–hole recombination leads to an increase in the catalytic ability. The photogenerated electron–hole shifts to the surface and interacts with substances, such as the hydroxyl group and oxygen, where the adsorption creates free radicals on the surface of the semiconductor.^70,71

4. Conclusions

The results of the experiments above indicate that we successfully synthesized mesoporous SiO₂/Cu₂O–graphene composites by combining TEOS and Cu₂O–graphene with CTBA at pH 9.5–10 and calcined at 550 °C. The XRD investigation of the photocatalyst showed the presence of silica nanoparticles due to broad diffraction peaks as well as a cubic Cu₂O phase. SEM and TEM images suggested that both Cu₂O and SiO₂ nanostructures were successfully loaded onto the transparent graphene sheets. Furthermore, the spherical silica nanoparticles with pore diameter as well as the quantum dot-sized Cu₂O are promising materials that can enhance the photocatalytic activity of mesoporous SiO₂/Cu₂O–graphene composites. The Raman and DRS spectrum also confirm the structures of the mesoporous SiO₂/Cu₂O–graphene composite, and the FT-IR spectrum shows the structural differences between the Cu₂O–graphene and mesoporous SiO₂/Cu₂O–graphene composite through the appearance of a new characteristic band and the removal of oxygen-containing groups. The nitrogen adsorption/desorption isotherm provided information regarding the pore diameter, pore volume, and surface area. The photocurrent analyses also provided the evidence of the enhanced photocatalytic activity of the mesoporous SiO₂/Cu₂O–graphene composite.

The mesoporous structure of the silica nanoparticles plays a major role in increasing the photodegradation effect as well as the surface of the graphene sheets. The results of this study thus show that the material is capable of the photocatalytic degradation of different organic dyes under visible light irradiation. The results of the photodegradation suggest that the mesoporous SiO₂/Cu₂O–graphene composite is much more effective photocatalyst than both Cu₂O–graphene, Cu₂O–graphene–TiO₂ composites and Cu₂O when under the same experimental conditions.

References

1. Y. He, N. Zhang, L. Zhang, Q. Gong, M. Yi, W. Wang, H. Qiu and J. Gao, Mater. Res. Bull., 2014, 51, 397–401.
2. F. Xu, Y. Yuan, D. Wu, M. Zhao, Z. Gao and K. Jiang, Mater. Res. Bull., 2013, 48, 2066.
3. Q. Zhou, Y. Zhong, X. Chen, X. J. Huang and Y. C. Wu, Mater. Res. Bull., 2014, 51, 244.
4. A. Safavi, F. A. Mahyari and M. Tohidi, Mater. Res. Bull., 2013, 48, 3399.
5. L. Zhu, Z. D. Meng, M. L. Chen, F. J. Zhang, J. G. Choi, J. Y. Park and W. C. Oh, J. Photocatal. Sci., 2010, 1, 69.
6. H. Zhang, X. Lv, Y. Li, Y. Wang and J. Li, ACS Nano, 2010, 4, 380–386.
7. S. R. Kim, M. K. Parvez and M. Chhowalla, Chem. Phys. Lett., 2009, 483, 124–127.
8. D. C. T. Nguyen, K.-Y. Cho and W.-C. Oh, Appl. Surf. Sci., 2017, 412, 252–261.
9. G. Williams, B. Seger and P. V. Kamat, ACS Nano, 2008, 2(7), 1487–1491.
10. J. Li, K. Yu, K. Qian, H. Cao, X. Lu and J. Sun, Nanoscale Res. Lett., 2014, 9, 172–180.
11. T. Xiong, F. Dong, Y. Zhou, M. Fu and W.-K. Ho, J. Colloid Interface Sci., 2015, 447, 16–24.
12. Y. Li, Y. Sun, F. Dong and W.-K. Ho, J. Colloid Interface Sci., 2014, 436, 29–36.
13. P. E. de Jongh, D. Vanmaekkelbergh and J. J. Kelly, Chem. Commun., 1999, 1069–1070.
14. B. Rai, Sol. Cells, 1988, 25, 265–271.
15. M. Hara, T. Kondo, M. Komoda, S. Ikeda, K. Shinohara, A. Tanaka, J. N. Kondo and K. Domen, Chem. Commun., 1998, 3, 357–358.
16. K. Domen, J. N. Kondo, M. Hara and T. Takata, Bull. Chem. Soc. Jpn., 2000, 73, 1307–1331.
17. L. Huang, F. Peng, H. Yu and H. J. Wang, Solid State Sci., 2009, 11, 129–138.
18. Z. Gao, J. Liu, F. Xu, D. Wu, Z. Wu and K. Jiang, Solid State Sci., 2012, 14, 276–280.
19. C. Kunfeng, L. Jun and X. Dongfeng, Energy and Environment Focus, 2012, 1, 50–67.
20. F. Zhang, Y. Li, Y.-e. Gu, Z. Wang and C. Wang, Microchim. Acta, 2011, 173, 103–109.
21. Y. C. Hsu, Y. T. Hsu, H. Y. Hsu and C. M. Yang, Chem. Mater., 2007, 19, 1120–1126.
22. M. Tiemann, Chem. Mater., 2008, 20, 961–971.
23. N. Venkatathri, Bull. Mater. Sci., 2007, 30, 615–617.
24. Y. J. Acosta-Silva, R. Nava, V. Hernández-Morales, S. A. Macías-Sánchez, M. L. Gómez-Herrera and B. Pawelec, Appl. Catal., B, 2011, 110, 108–117.
25. T. Kamegawa, D. Yamahana and H. Yamashita, J. Phys. Chem. C, 2010, 114, 15049–15053.
26. H. T. Phuong, D. T. Ngo, N. D. Toan, T. T. Son, N. T. Ngoc Bich, N. H. Anh, N. L. Anh and P. H. Trang, Petrovietnam, 2016, 9, 24–33.
27. H. Song, L. Zhang, C. He, Y. Qu, Y. Tian and Y. Lv, J. Mater. Chem., 2011, 21, 5972–5977.
28. Z. Y. Wang, Q. F. Lu, M. G. Kong and L. D. Zhang, Chem.–Eur. J., 2007, 13, 1463–1670.
29. Y. Li, X. L. Zhang, R. Qiu, R. Qiao and Y. S. Kang, J. Phys. Chem. C, 2007, 111, 10747–10750.
30. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J. M. Tarascon, Nature, 2000, 407, 496–499.
31. Y. Mao, T. Park, F. Zhang and H. Zhou, Small, 2007, 7, 1122–1139.
32. Z. Wang, Y. Du, F. Zhang, Z. Zheng, X. Zhang, Q. Feng and C. Wang, Mater. Chem. Phys., 2013, 140, 373–381.
33. S. Giri, Synthesis and characterization of zirconia coated silica nanoparticles for catalytic reactions, National Institute of Technology, Rourkela, India, 2008.
34. J. Knipping, H. Wiggers, B. Rellinghaus, P. Roth, D. Konjhodzic and C. Meier, J. Nanosci. Nanotechnol., 2004, 4(8), 1039–1044.
35. L. Hao, X. Gong, S. Xuan, H. Zhang, X. Gong, W. Jiang and Z. Chen, Appl. Surf. Sci., 2006, 252(24), 8724–8733.
36. T. Sugama and B. Lipford, J. Mater. Sci., 1997, 32(13), 3523–3534.
37. Q. Sun, P. J. Kooyman, J. G. Grossmann, P. H. H. Bomans, P. M. Frederik, P. C. M. M. Magusin, T. P. M. Beelen, R. A. Van Santen and N. A. J. M. Sommerdijk, Adv. Mater., 2006, 15(13), 1097–1100.
38. S. H. Zhang, I. Laurer and K. K. Unger, Adv. Mater., 1997, 9, 254.
39. X. M. Guo, X. G. Liu, B. S. Xu and T. Dou, Colloids Surf., A, 2009, 345, 141–146.
40. W. L. Zhang and H. J. Choi, Langmuir, 2012, 28(17), 7055–7062.
41. Joint Committee on Powder Diffraction Standards, Diffraction Data File, No. 5–666, ICDD International Center for Diffraction Data (formerly JCPDS), Pennsylvania, USA, 1991.
42. F. Zhang, Y. Li, Y.-e Gu, Z. Wang and C. Wang, Microchim. Acta, 2011, 173, 103–109.
43. Q. Liu, J. B. Shi, T. Wang, L. X. Zeng and G. B. Jiang, Angew. Chem., Int. Ed., 2011, 50, 5913–5917.
44. X. Zhou and T. Shi, Appl. Surf. Sci., 2012, 259, 566–573.
45. L. Chen, Y. Zhang, P. Zhu, F. Zhou, W. Zeng, D. D. Lu, R. Sun and C. Wong, Sci. Rep., 2015, 5, 9672.
46. J. Zhang, Z. Xiong and X. S. Zhao, J. Mater. Chem., 2011, 21, 3634–3640.
47. J. Shen, B. Yan, M. Shi, H. Ma, N. Li and M. Ye, J. Mater. Chem., 2011, 21, 3415–3421.
48. K. Kudin, B. Ozbas, H. Schniepp, R. Prud'homme, I. Aksay and R. Car, Nano Lett., 2008, 8, 36.
49. X. Zhou and T. Shi, Appl. Surf. Sci., 2012, 259, 566–573.
50. K. Ullah, S. Ye, S. B. Jo, L. Zhu, K. Y. Cho and W. C. Oh, Ultrason. Sonochem., 2014, 21(5), 1849–1857.
51. S. J. Park, J. H. An, R. D. Piner, I. Jung, D. X. Yang, A. Velamakanni, S. B. T. Nguyen and R. S. Ruoff, Chem. Mater., 2008, 20, 6592–6594.
52. A. V. Murugan, T. Muraliganth and A. Manthiram, Chem. Mater., 2009, 21, 5004–5006.
53. W. G. Leng, M. Chen, S. X. Zhou and L. M. Wu, Langmuir, 2010, 26, 14271–14275.
54. Y. Qi, J. R. Eskelsen, U. Mazur and K. W. Hipps, Langmuir, 2012, 28, 3489–3493.
55. Y.-W. Hsu, T.-K. Hsu, C.-L. Sun, Y.-T. Nien, N.-W. Pu and M.-D. Ger, Electrochim. Acta, 2012, 82, 152–157.
56. A. A. Dubale, W.-N. Su, A. G. Tamirat, C.-J. Pan, B. A. Aragaw, H.-M. Chen, C.-H. Chen and B.-J. Hwang, J. Mater. Chem. A., 2014, 2, 18383–18397.
57. Y. Wu, D. Chu, P. Yang, Y. Du and C. Lu, Catal. Sci. Technol., 2015, 5, 3375–3382.
58. A. Ali and W. C. Oh, Sci. Rep., 2017, 7(1), 1867.
59. Q. Hao, X. Niu, C. Nie, S. Hao, J. Ge, W. Zou, D. chen and W. Yao, Phys. Chem. Chem. Phys., 2016, 18, 31410–31418.
60. M. Zhang, W. Luo, Z. Wei, W. Jiang, D. Liu and Y. Zhu, Appl. Catal., B, 2016, 194, 105–110.
61. Y. Li, X. Li and J. Li, Water Res., 2006, 40, 1119.
62. T. Jiang, A. S. Poyraz, A. Iyer, Y. Zhang, Z. Luo, W. Zhong, R. Miao, A. M. El-Sawy, C. J. Guild, Y. Sun, D. A. Kriz and S. L. Suib, J. Phys. Chem. C, 2015, 119(19), 10454–10468.
63. F. Dong, Z. Y. Wang, Y. J. Sun, W. K. Ho and H. D. Zhang, J. Colloid Interface Sci., 2013, 401, 70–79.
64. T.-D. Nguyen-Phan, V. H. Pham, E. W. Shin, H.-D. Pham, S. Kim, J. S. Chung, E. J. Kim and S. H. Hur, Chem. Eng. J., 2011, 170, 226–232.
65. G. Williams, B. Seger and P. V. Kamat, ACS Nano, 2008, 2, 1487–1491.
66. I. V. Lightcap, T. H. Kosel and P. V. Kamat, Nano Lett., 2010, 10, 577–583.
67. S. Min and G. Lu, J. Phys. Chem. C, 2012, 116(48), 25415–25424.
68. Y. J. Sun, W. D. Zhang, T. Xiong, Z. W. Zhao, F. Dong, R. Q. Wang and W. K. Ho, J. Colloid Interface Sci., 2014, 418, 317–323.
69. K. Li, T. Chen, L. Yan, Y. Dai, Z. Huang, J. Xiong, D. Song, Y. Lv and Z. Zeng, Colloids Surf., A, 2013, 422, 90–99.
70. R. Raghav, P. Aggarwal and S. Srivastava, AIP Conf. Proc., 2016, 1724, 020078.
71. J. P. Espinós, J. Morales, A. Barranco, A. Caballero, J. P. Holgado and A. R. González-Elipe, J. Phys. Chem. B, 2002, 106, 6921–6929.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra03526h

---