An eco-friendly synthesized mesoporous-silica particle combined with WSe2-graphene-TiO2 by self-assembled method for photocatalytic dye decomposition and hydrogen production

To address the limitations of titanium dioxide (TiO2) and expand the applicability of the photocatalytic activity of TiO2,WSe2 and silica, an eco-friendly, self-assembled method for combining a silica precursor with a WSe2-graphene-TiO2 composite with cetyltrimethylammonium bromide (CTAB) as surface active agents is proposed. Firstly, for the main target, the photocatalytic degradation of organic dye solutions with different initial pH levels and catalyst dosages under visible light irradiation was surveyed. The as-synthesized sample exhibited highly efficient photocatalytic effects for the treatment of the SO dye solution in the optimal conditions of this study, which included a solution with a pH level of 11 and 0.05-gram dosage of the catalyst. Secondly, previous photocatalytic hydrogen production studies reported markedly better outcomes with SiO2/WSe2-graphene-TiO2 than with the binary WSe2-graphene and ternary WSe2-graphene-TiO2 composites under ambient conditions with and without 20% methanol sacrificing reagents. The SiO2/WSe2-graphene-TiO2 composite is promising to become a potential candidate for photocatalytic performance that performs excellently as well as offer an efficient heterosystem for hydrogen production.

ScIEnTIfIc REPORTS | (2018) 8:12759 | DOI: 10.1038/s41598-018-31188-w dichalcogenides) source was investigated to help overcome some of the limitations of most materials such as only displaying photocatalytic properties when activated with ultraviolet radiation (for a large band gap materials case) or exhibiting a fast recombination phenomenon (for a small band gap semiconductor case) [14][15][16][17] .
Among the TMDCs family, tungsten diselenide (WSe 2 ) is a semiconductor with a suitable band gap of approximately 1.2 eV for bulk which is a potential photocatalyst under visible light [18][19][20] . As mentioned previously, due to the fast electron-hole recombination, WSe 2 has not yet led to a breakthrough for photocatalytic performance 21 . Thus, the combination of WSe 2 , graphene nanosheets and other suitable catalysis candidates can likely enhance photocatalytic properties. Ruishen Meng et al. reported the design of graphene-like gallium nitride and WS 2 / WSe 2 nanocomposites for photocatalyst applications. The results demonstrated that the photocatalyst performance improved due to the prevention of the electron-hole pairs from recombination as well as the suitability of the two heterostructures' band alignment 22 . Bo Yu et al. presented the enhanced photocatalytic properties of graphene modified few-layered WSe 2 nanosheets 16 . Furthermore, many reports have presented on the combination of graphene oxide and TiO 2 . Therefore, WSe 2 , with its narrow band gap, was selected for pairing with TiO 2 to address the limitations of TiO 2 and expand the applicability of the photocatalytic activity of both TiO 2 and WSe 2 .
For some time, photocatalysts that are supported by materials with large pore sizes and high porosity has attracted keen interest 23 . Due to its great advantages such as a large, specific surface area, large pore volume, and uniform and adjustable nano pore size, mesoporous silica is a promising material source for photocatalytic activity 24,25 . Furthermore, mesoporous materials, such as mesoporous silica, have been used as supports for different metal oxide nanoparticles that enhance the catalytic performances over their non-supported analogues [26][27][28][29] . Xavier Collard et al. synthesized the novel mesoporous ZnO/SiO 2 composites for the photodegradation of organic dyes. Materials with a mesoporous structure that possessed a high surface area and a narrow pore size distribution exhibited the effective photodegradation of rhodamine B with good results 30 .
In this work, a SiO 2 /WSe 2 -graphene-TiO 2 nanocomposite was synthesized using a simple method by using the silica precursor (tetraethyl orthosilicate-TEOS) at a pH level of 9.5-10, with cetyltrimethylammonium bromide (CTAB) as surface-active agents. The as-obtained composites were characterized via XRD, nitrogen adsorption/ desorption isotherms, SEM, TEM, SAED, Raman spectroscopy, UV-vis DRS, XPS and PL. Furthermore, the photodegradation experiments under visible light irradiation were then proceeded with aqueous solutions of organic dyes with different initial pH levels and catalyst dosages. The recycling experiments were surveyed to investigate the photocatalyst stability. The photocatalytic hydrogen production studies of the as-synthesized nanocomposites were tested with an aqueous solution containing 20% methanol as the sacrificial reagent.
According to the results in Fig. 2, WG, WGT and SWGT composites exhibited a similar type-II curve. Results of BET surface area analysis technique and the pore size and total pore volume, which were obtained from nitrogen adsorption/desorption isotherms of the different survey composites (see Table 1). According to the BET method, the surface area was about 4.87 and 7.14 m 2 /g for the WG and WGT composites, respectively. Simultaneously, the surface area of the SWGT was calculated as 30.67 m 2 /g, which was the highest specific surface area among all the survey samples as well as more than 5 and 7 times that of the WG and WGT composites, respectively. In parallel with the highest specific surface area, SWGT also exhibited a much stronger structure with the highest total pore volume of 7.05 cm 3 /g. The total pore volumes of WG and WGT composites were 1.12, and 1.64 cm 3 /g, respectively, which were about 7 times less than that of the SWGT composite. Moreover, there was  Figure 3 provides the typical images as well as the shape and structure of the survey samples, which were analyzed using the SEM method. As SEM results of the WG composite in Fig. 3(a) suggest, the fine WSe 2 particles existed with a rod-like morphology that was approximately 1-10 µm in length. The fine WSe 2 particles still exhibited the rod-like morphology well in the WGT composite near the presence of the irregular-shaped anatase TiO 2 particles, as shown in Fig. 3(b). As expected, the as-prepared SWGT exhibited the SiO 2 nanoparticles with small-sized spherical shapes and good particle dispersion, as seen in Fig. 3(c,d). More importantly, the highlight was the presence of the micro porous holes on the surface of the round SiO 2 . This phenomenon indicated increased photocatalytic activity with the SWGT composite due to their high contact area with the pollutant organic dyes.
TEM images were recorded, and the results are provided in Fig. 4. According to the TEM images, the typical morphologies of SiO 2 , WSe 2 , TiO 2 , and graphene were confirmed. As displayed in Fig. 4(a), a combination of WSe 2 and TiO 2 nanoparticles, were darker in color and decorated onto the surface of the silica. Moreover, the graphene layers clearly presented as transparent nanosheets (see Fig. 4(a)). To collect additional information, the WSe 2 nanoparticles were exhibited as rectangle shape blocks, wherein the diameter of the particles was observed to be in the range of (25-36) nm. Specifically, we observed some spherical TiO 2 particles of uniform sizes in the nanocomposite (see Fig. 4(b)), which formed a mixture with the rectangle-shaped blocks of WSe 2 and covered the surface of the silica and the graphene sheets. From the achieved SAED patterns in Fig. 4(c), the d spacing values of 0.28, and 0.25 nm attributed to the Debye-Sherrer rings of (100) and (103) planes of WSe 2 can be obtained 39 . On the other aspect, the single-phase anatase phase of TiO 2 can be obtained on SAED pattern in Fig. 4(d), presenting the d spacing values corresponding to the (200), (105), (204), and (220) lattice planes 40 . The silica nanoparticles that were displayed as gray layers provided a large plate structure for the anchoring of the WSe 2 and TiO 2 nanoparticles.
According to XPS analysis in Fig. 5(a), the surface bonding state of SWGT composite demonstrated the presence of W, Se, Si, C, Ti, and O elements. Following Fig. 5(b), the existence of the C-C (284.9 eV, aromatic rings), the C=O (287.7 eV) and O-C=O (289.1 eV) can be seen 41,42 . Furthermore, the peak at (531.9 and 533.5) eV was assigned to the absorbed oxygen which provided the presence of the O 1 s signal after a self-assembled reaction, as shown in Fig. 5(c). Moreover, the binding energy located at (37.9 and 40.0) eV that were ascribed to W 4f as displayed in Fig. 5(d) 43 . The presence of Se 3d, Ti 2p, and Si 2p was also identified in the XPS results, as shown in    The Raman spectroscopy of the SWGT composite is depicted in Fig. 6. The characteristic peaks of the WSe 2 and TiO 2 were achieved on the Raman spectra, as seen in Fig. 6(a). The obtained TiO 2 signals were strong and displayed high intensity and located at the range shift around (150, 392, 516, and 638) cm −1 which were related to the E g (1), B 1g (1), A 1g + B 1g (2), and E g (2) modes of anatase TiO 2 and confirmed the presence of TiO 2 on the last obtained composite [47][48][49] . A difference in the location of the TiO 2 peaks of the WGT and SWGT composites are provided in Fig. 6(a). After the calcination temperature condition (550 °C), the main peak at ~150 cm −1 of the TiO 2 in the SWGT composite was blue shifted when compared to the WGT composite. Due to the chemical method for preparing the SWGT composite and calcination treatment, there may be oxygen defects and phonon confinement, which can lead to this kind of frequency shift along with size effect 50 . The results demonstrate that as-synthesized SWGT was successfully synthesized after a self-assembled reaction of the WGT composite with TEOS. The WSe 2 signal was obtained in the region of 247 cm −1 (E 1 2g band) which presented the signal of a single-layer WSe 2 51 . Due to the dominant effect of silica, the low characteristic WSe 2 peak overlapped. Therefore, we did not achieve the WSe 2 signal in the SWGT Raman spectroscopy result. This outcome can be explained by the small initial amount of WSe 2 source in the composite. According to Fig. 6(b), the existence of graphene in the SWGT composite can be confirmed by D and G bands. In the current case, D and G bands were located at (1350-1640) cm −1 , respectively. Specifically, in our survey SWGT composite, the existence of SiO 2 nanoparticles on the graphene surface can be led to disturb the low-intensity peak of the D-and G-band signals [52][53][54] .
The UV-vis diffuse reflectance spectrum of the WG, WGT, and SWGT composites is provided in Fig. 7. The first observation results were a maximum absorption wavelength in the visible region within a range of (400-500) nm, which corresponded to the shift of electrons from the conduction to the valence band. Following the results in Figs. 7(a-c), the absorption edge of the SWGT composite shifted to a higher wavelength than the WG composite, and red-shift absorption occurred. This phenomenon leads to improve the photocatalytic performance of SWGT composite for not only the photodegradation activity but also photocatalytic hydrogen evolution. The band gap energy that was obtained from the Kubelka-Munk transformation from the UV-vis diffuse reflection data is displayed as an insert picture in Figs. 7(a-c). The calculated band gap energies were around (2.8-3.1) eV; the samples of WG, WGT, and SWGT were (2.64, 3.44, and 2.56) eV, respectively. These results suggested that the band gap energy values of the SWGT composite corresponded to 2.56 eV, which is lower than that of both the WG and WGT composites.
PL spectra of SWGT composite with an excitation wavelength of 325 nm is presented in Fig. 8. According to the PL result, SWGT material showed the wide PL signal and a strong peak at the range of 490 to 550 nm which are related to excitonic PL mainly obtained from surface oxygen vacancies and defects of the semiconductor nanoparticles 55 . The achieved SWGT composite exhibited a greatly influenced by the intensity and response Photodegradation. Survey the effect of different organic dyes. The photocatalytic activity over the SWGT composite was tested to evaluate the photocatalytic behavior of the SWGT composite at room temperature and typical atmospheric pressure for the degradation of SO, RhB, and MB as candidate sorbents from the cationic and anionic dye groups (MO, and TBBU) in aqueous solutions, as shown in Fig. 9.
Reviewing all photodegradation results in Fig. 9(a), the WGT nanocomposite exhibited good photodegradation results for degradation of MB organic dye solution. In the case of MB dye solution, the WGT composite has reached approximately 44.25% dye removal which is higher than that of SO and RhB was (42.43, and 36.39)%, respectively. In the case of anionic dyes, under presence of the WGT nanocomposite, degradation rate of TBBU organic dye revealed higher results than MO dye solution which reached approximately (12.95, and 6.42)% dye removal, respectively.
The photocatalytic degradation in Fig. 9(b) demonstrates that both the adsorption and the photodegradation effects in the presence of the SWGT composite were maximized and achieved the highest adsorption capability at about 94.19%. After 5 hours of the photocatalytic activity, the SWGT composite still demonstrated the best decolorization capability with 97.94% removal of the SO dye solution. In the case of cationic dye group, the SWGT composite also exhibited good decolorization capability for the RhB and MB solution cases with a final removal of organic dye at (61.03, and 80.06)%, respectively. On the contrary, the SWGT composite did not present good photocatalytic activity for the anionic dye group (MO, and TBBU). In the case of MO solutions, the SWGT composite exhibited low photocatalytic activity at 15.55%, which was lower than those of the cationic organic dyes. After 180 minutes under visible light irradiation, the SWGT composite displayed a photodegradation effect of 18.92% for the TBBU solution. Overall, the aforementioned results indicated that the SWGT composite is a good photocatalyst candidate for the degradation of cationic organic dyes with high photodegradation activity that far exceeds that of the anionic organic dyes.
A plot of k app is presented in Fig. 10 and Table 2. The SO degradation rate constant for the SWGT composite was 5.2 × 10 −3 min −1 , which was a better result than that of the other cationic-anionic organic dyes such as 3.0 × 10 −4 , 9.0 × 10 −4 , 9.0 × 10 −4 , and 6.0 × 10 −4 min −1 corresponding to RhB, MB, MO, and TBBU, respectively. The SWGT composite is therefore a new potential material for photocatalyst activity. Survey of the effect of solution pH and dosage of catalyst. The SWGT composite retained the highest absorption capacity for the SO solution. This outcome was the reason for which we chose the SO solution to survey the effect of solution pH levels and the amount of composites in this study's experiments. In the adsorption process, the pH factor exhibited an important role 3,60 . The pH value was tested, while keeping other parameters constant, at various initial pH levels between the range of 3-11 where the addition of the required amounts of 0.1 mol/L of NaOH or HCl solution was used to adjust the pH values. It clearly demonstrated the difference in the photocatalytic activity results of the pH solution in the presence of SWGT composite for the removal of SO organic dye. The photocatalytic activity results displayed in Fig. 11(a) demonstrate that the SO dye removal increased from 62.07% to 98.05% with the increase of the pH values from 3 to 11. The photodegradation effect was not more influenced in the pH ranges of 7 to 11; but, it far exceeded for the acid solution because the safranine O is a cationic (positive charge containing) dye 61 .
The photocatalytic degradation of the aqueous solution of SO was processed with different amounts that ranged between 0.03 to 0.05 g of the prepared SWGT composite in order to survey the effects of the initial amount of nanocomposite while keeping another parameters constant. With the decrease in the initial amount of catalyst, the degradation efficiency of the SO solution decreased. The decolorization capacity of the 0.05 g SWGT composite had the best result with 97.94% removal of dye after 5 hours for the photocatalytic activity. The photocatalytic activity results in Fig. 11(b) show that the dye removal decreased from 97.94% to 60.92% with the decrease in the dosage amount from 0.05 to 0.03 g in the SO solution. The interactive surface of the photocatalyst, as well as the fixed volume of the dye solution, decreased with the decreasing dosage amount of the photocatalyst; subsequently, the decolorization capacity of the composite decreased.
Photocatalytic hydrogen production studies. For photocatalytic hydrogen evolution, the SWGT composite has a semiconductor role that converts sunlight energy into chemical energy under ambient conditions with and without sacrificial atmospheric pressure at room temperature [62][63][64][65][66] . In this experiment, methanol was used as the sacrificial reagent that further enhanced the catalytic activity of the semiconductor by providing electrons to consume the photogenerated holes which led the recombination time of the semiconductor to increase. The hydrogen evolution results for the SWGT composite with and without 20% methanol sacrificing reagents under visible light irradiation are provided in Fig. 12. According to the hydrogen evolution results among the survey composites (Fig. 12), the highest photocatalytic H 2 evolution rate was observed when methanol was used as the sacrificial reagent in the presence of the SWGT composite. It achieved the H 2 evolution rate of 2.004 mmol per 11 hours. However, the photocatalytic H 2 evolution rate of the SWGT composite also reached a high value in pure water at 1.718 mmol per 11 hours. For the SWGT case, the presence of 20% methanol sacrificing reagents did not lead to a significant difference in the photocatalytic H 2 evolution rates. It is a promising candidate for being a semiconductor in a highly active photocatalyst as well as obtaining great application and photocatalytic activity from a pure aqueous solution. Figure 12 displays the H 2 evolution rate for WG and WGT photocatalysts. The H 2 evolution rate of the WG composite from aqueous solution without the sacrificial reagent was 0.818 mmol per 11 hours; while it was 1.309 mmol per 11 hours in 20% methanol sacrificing reagents. About the WGT composite, the H 2 evolution rate was 0.859 mmol per 11 hours and 1.472 mmol per 11 hours in the pure water and methanol aqueous solutions, respectively. According to the aforementioned results, the SWGT photocatalyst achieved the best hydrogen evolution rate, which is more than 2 times higher in both the pure water and methanol aqueous solutions. The above comparison implied a great photocatalytic hydrogen evolution rate than the ternary photocatalyst (WGT) and binary photocatalyst (WG).

Conclusions
A SiO 2 /WSe 2 -graphene-TiO 2 composite can be synthesized by using a simple self-assembly process. The main diffraction peaks of SiO 2 /WSe 2 -graphene-TiO 2 composite was well identified by the SiO 2 , WSe 2 and TiO 2 signals that were investigated by XRD patterns. Nitrogen adsorption/desorption isotherms provided evidence that the SiO 2 /WSe 2 -graphene-TiO 2 composite had not only the highest specific surface area but also exhibited a much stronger structure with the highest total pore volume than all other samples. Furthermore, the presence of the micro porous holes on the surface of the round SiO 2 as well as WSe 2 and TiO 2 nanostructures were achieved through SEM and TEM imagery. The Raman, DRS and XPS data provided more information regarding the structure of the SiO 2 /WSe 2 -graphene-TiO 2 composite.
The photodegradation experiments indicated that SiO 2 /WSe 2 -graphene-TiO 2 composite is a good photocatalyst candidate for the degradation of cationic organic dyes with high photodegradation activity that far exceeds that of the anionic organic dyes. The optimal conditions for this study included a solution with a pH level of 11 and catalyst dosage 0.05 g for the SO solution case. The SiO 2 /WSe 2 -graphene-TiO 2 photocatalyst achieved the best hydrogen evolution rate than the ternary photocatalyst (WGT) and binary photocatalyst (WG). The results of the characterization and the photodegradation suggested that SiO 2 /WSe 2 -graphene-TiO 2 material is a promising material for the photodegradation of organic dyes as well as can facilitate the development of an efficient heterosystem for hydrogen production under visible light irradiation. was dissolved in 20 mL of distilled water, then dropped into 50 mL HNO 3 0.5 M in a three-necked flask (100 mL) and heated to 120 °C with magnetic stirring to eliminate the H 2 O and O 2 . Separately, a selenium salt was obtained by adding a combination of 0.01 mol anhydrous sodium sulfite (Na 2 SO 3 ) and 0.004 mol crude selenium (Se) powder to 200 ml of ethylene glycol. A hydrothermal process at 180 °C for 36 hours was processed with both solutions. After washing step with 95% ethanol and distilled water, the solid was dried under a vacuum at 105 °C for 1 day to obtain the WSe 2 material.

Reagents
Synthesis of the WSe2 -graphene nanocomposite. Graphene oxide (0.2 g) in 100 ml of ethylene glycol was ultrasonicated for a half hour (Ultrasonic Processor, VCX 750, 500-Watt, Korea, Power 500-Watt, frequency 20KHz, Amplitude 50%, low intensity). The achieved WSe 2 powder was mixed at equal volumetric ratios of 1:1    Synthesis of the WSe 2 -graphene-TiO 2 nanocomposite. 0.2 g graphene oxide was sonicated in 100 ml ethylene glycol for 30 minutes, followed by adding 0.1 g TiO 2 nanopowder and 0.1 g the achieved WSe 2 powder (0. Characterization. An X-ray diffraction was recorded using Shimadzu XD-D1. SEM was recorded using JSM-5600 JEOL, Japan. A DRS analysis was obtained by UV-vis spectrophotometry (Neosys-2000). TEM and selected-area electron diffraction (SAED) patterns were also used to investigate the size and distribution of the nanoparticles deposited on the graphene surface of the various samples. XPS analysis was observed using a VG Scientific ESCALAB250. Raman spectra can be obtained by a Jasco Model Name NRS-3100 spectrometry. Nitrogen adsorption/desorption isotherms studies were investigated by a Micromeritics ASAP 2020 M nvC operating at 77 K, the surface area was calculated by using the Brunauer-Emmett-Teller (BET) method. and the pore size distribution was calculated according to the Barrett-Joyner-Halenda (BJH) method. The photodegradation experiments were analyzed by a UV-spectrophotometry (Opizen POP, Korea). The photoluminescence (PL) spectra were recorded by a fluorescence spectrophotometer (F−4500, Hitachi, Japan) for an excitation wavelength of 325 nm at room temperature.
Photocatalytic activity. The photodegradation experiment was processed under ambient conditions atmospheric pressure at room temperature without any sacrificial. Generally, 0.05 g SiO 2 /WSe 2 -graphene-TiO 2 nanocomposite was dissolved in a 100-ml organic dye solution. The visible light source was made from an 8-watt lamp (Fawoo, Lumidas-H, Korea, λ ≥ 420 nm) with a filter (Kenko Zeta, transmittance m)90%). Firstly, a mixture solution of nanocomposite and organic dyes was kept without any light source for 120 min. The first sample was taken out at the end of 120 min kept in a dark box. The c 0 is the concentration of dye solution at the starting point (t = 0). After that, other samples were taken out from the mixture solution each 30 min. Then, the powders were removed by using a centrifuge machine. The photocatalytic degradation of SO, RhB, MB, MO, and TBBU solutions tested after the above process with concentrations of 1 × 10 −4 , 5 × 10 −4 , 5 × 10 −4 , 5 × 10 −4 and 1.25 × 10 −4 mol/l, respectively. On the other aspect, the effects of different initial pH levels (3)(4)(5)(6)(7)(8)(9)(10)(11) and catalyst dosages (0.03-0.05 g) were surveyed while keeping another parameter constant following by the photodegradation test. The effects of the above factors were expressed through the percent of dye removal. By using a UV-spectrophotometry, the concentration c the dye solutions can be obtained. The spectral range was surveyed at λ max = 520, 554, 665, 465 and 349 nm for SO, RhB, MB, MO and TBBU, respectively. The degradation capacity (η%) was calculated as (1): o Photocatalytic hydrogen evolution system. Using the typical photocatalytic test conducted under ambient conditions with and without sacrificial atmospheric pressure, the SiO 2 /WSe 2 -graphene-TiO 2 nanocomposite was 0.1 g, dissolved in a 200-ml solution. The solution of 20% methanol was used as a sacrificial reagent. The visible light source was made from an 8-watt lamp (Fawoo, Lumidas-H, Korea, λ ≥ 420 nm) with a filter (Kenko Zeta, transmittance >90%) to prevent any radiation below 410 nm and to ensure that the photocatalytic activity was conducted under visible light for 10 hours at 20 cm from the glass reactor. The amount of hydrogen gas evolved was measured at 25 °C with the atmospheric condition by a gas chromatograph (GC7900, Thermal conductivity detector), a molecular sieve 5A column. The nitrogen gas was used as the carrier gas.
Data availability. The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.