3.1 The XRD analysis of rGO/egTiO2 composites
Figure 1a shows the XRD pattern of rGO/equaixial geometry of TiO2 composite with different mass ratio. All the diffracted peaks are coincidence with the standard XRD pattern of anatase crystal TiO2 and tetragonal system (JCPDS, No.21-1272). There is no intensity increase of rGO peak at about 2θ = 25° was observed, while the peak at 2θ = 9.44° for GO was has disappeared. The disappeared peak of rGO was attributed to a lot of particles available on the surface rGO composite caused by the compounding with TiO2, it is surface cannot sustain the original shape and the original stack-up rules, thus the peak of rGO cannot appear. The XRD of pure rGO is prepared with the same method, as shown in Fig. 1(b), there is a wide peak at about 2θ = 25°, which proves that it has been restored. Observing several XRD patterns of rGO/equaixial geometry TiO2 composites with different ratios, it is found that with the increase of rGO content, the crystal surface strength of composites has gradually declined, which was the most obvious by observing the peak corresponding to the strongest peak (101). The compound with the lowest rGO content is 2.6%, which has the highest XRD intensity. There are double peaks at around 2θ = 55° is the most obvious and complete, indicating that it has high crystalline, and also when the content of rGO is relatively high, it will slightly affect the crystallinity of TiO2 crystals. The XRD pattern preliminary confirmed that the two are successful compounding with each other. In the XRD pattern, there are no other redundant peaks except TiO2 peaks, which proves the samples are relatively pure.
3.2 The morphology observation of rGO/egTiO2 composites under SEM
The SEM image of rGO/equaixial geometry TiO2 composites with different mass ratios as shown in Fig. 2. In order to get a better comparison, the SEM image of pure rGO and TiO2 was added. Figure 2(a), is the SEM images of the pure rGO sample. Figure 2(b, c, d, e, and f) successively show the SEM images of rGO/equaixial geometry TiO2 composite samples with mass ratios of 12.7%, 6.4%, 4.3%, 3.2% and 2.6%. Figure 2(g and h) are the SEM images of pure equaixial geometry TiO2 particles with different magnification ratios. It can be seen clearly after compounding with each other rGO lamellas covered by a lot of TiO2 particles to observe the presence of rGO. Some bare areas on the rGO surface are chosen, thus the corresponding rough particle density maybe not be observed in the picture with proportion, which will be seen in TEM images. Although there will be aggregations of particles, it can be observed from Fig. 2 that TiO2 is equably dispersed on rGO layers, make the dispersion more equable, which indicates that the aggregations of TiO2 particles were prevented after compounding with rGO. The recombination of TiO2 particles on rGO layers also prevented the accumulation of rGO layers, the two interacted with each other. Figure 2(e) is the SEM images of rGO and TiO2 with a ratio of 3.2%, which is a little different with other recombination, rGO layers wrap TiO2 particles, but only one side is recombined, it is probably that because the oxygen-containing groups of rGO just oxidized one side, so causing one side recombination. SEM image has confirmed again that the two have been successfully combining, which is consistent with the XRD results.
3.3 The TEM morphology and the EDX diagram of rGO/egTiO2 composites
TEM image of rGO/equaixial geometry TiO2 composites with different mass ratios as display in Fig. 3, for better comparison, the TEM images of pure rGO and TiO2 were added. Figure 3(a) is a TEM image of a pure rGO. Figure 3(b, c, d, e, and f) are TEM images of rGO/equaixial geometry TiO2 composite samples with different magnification ratios of GO and TiO2, whose mass ratios are respectively 12.7%, 6.4%, 4.3%, 3.2% and 2.6%. Figure 3(g) is a TEM representation of TiO2 particles in pure equaixial geometry. Figure 3 (h) is an energy dispersive X-Ray spectroscopy of the rGO/equaixial geometry TiO2 composite sample with a mass ratio of 4.3%, with a lowerest proportion of rGO the average density of TiO2 is bigger, the particles are closer, but it can be observed that the edges of rGO layers in figures from 12.7% until the proportion is 2.6%, TiO2 particles are completely covered rGO layers. Its edges cannot be observed, the large TiO2 particles gathering together can only be observed. The aggregation of TiO2 particles in this period is different from the previous spontaneous aggregation of particles that is because, after the hydrothermal reaction, there was some binding force between TiO2 and rGO that caused particles to grow together with the lamella. In Fig. 3(b), it can be observed that growing many TiO2 particles on rGO lamella of micron-level very equable, whose average size of crystal particles can only reach 16.5nm. There will be relatively more particle aggregations in fold areas, which is the normal phenomenon. However, in the surrounding areas of rGO and TiO2 composites, there is no other scattering and single TiO2 particles, the difference in particles density between the rGO layers and copper screen used in tests could further indicate that there is a certain strength interaction between TiO2 particles and rGO particles, and the two cannot be separated by ultrasonic treatment. This kind of combination is unlike the mixture merely at the physical level. The results have proved again that the recombination of the two is successful, which is consistent with the results of XRD and SEM, which confirmed that the super-molecule has been successfully prepared by the hydrothermal method and solvothermal method. Besides, Fig. 3(h) has provided supplementary data for the results, the elemental analysis of the EDX image shows that the elements composed by composites are Ti, O, and C, which also supplemented the results of XRD.
3.4 The UV-Vis diffuse reflectance spectra of rGO/egTiO2 composites
The optical properties of photocatalysts are assessed using UV-visible reflectance spectroscopy and band-gap energy. Figure 4 shows the UV-vis diffuse reflection spectrum of P25, equaixial geometry TiO2 composites with composite ratios of 2.6%. It can be seen from the figure that P25 and equaixial geometry TiO2 have relatively strong absorption in the ultraviolet region, the absorption edge of them is approximately 370 nm. While none in the visible region. However, rGO/equaixial geometry TiO2 composites exhibited a certain intensity of absorption in the visible light region, which indicates that the absorption intensity of the samples in the visible light region has been improved after recombination. Moreover, it can be observed that qualitative red-shift towards higher wavelengths at the absorption edges of composites. The findings show that the dominance of rGO, which has excellent conductivity and π-conjugation, can significantly boost photocatalytic efficiency in this system. This mechanism is that graphene and semiconductor will produce chemical effects of a certain degree, C element may be doped in, such as carbon doping, forming a doping level, which can widen or narrowing the semiconductor to expand the response to visible light. We can calculate the band-gap energy of P25 and equaixial geometry TiO2 according to Fig. 4, because TiO2 is the indirect band-gap semiconductor (Kurniawan et al. 2020), therefore, according to the Eq. (2):
Since the absorption coefficient is in proportion to the absorbance A, here the (Ahv) 1/2 is replaced with the (αhv) 1/2. Where α is referred to as the absorption coefficient, h is referred to as the Planck constant (J s−1), the incident light frequency (Hz), the proportional coefficient is C, and prohibited band-gap energy is Eg. Hence the Eg of P25 is 2.78eV and the Eg of TiO2 equaixial geometry is 3.05eV. But higher band-gap anatase has a better oxidizing ability, meaning the efficiency of photo-catalysis is better than P25.
3.5 The Raman shift spectrum and Electrochemical impedance spectra of rGO/egTiO2 composites
The Raman spectrum of nanocomposite TiO2, rGO, and rGO/equaixial geometry TiO2 is shown in Fig. 5(a). Four characteristic bands appear at 0-800 cm− 1 in the TiO2 spectrum at 145 cm− 1, 385 cm− 1, 506 cm− 1, and 640 cm− 1, corresponding to the main vibration modes of the anatase phase, E1 g, B1 g, A1 g, and Eg, respectively (Guo et al. 2019, Mathpal et al. 2013). The characteristics of the disordered carbon and graphic carbon bands display a simple, subtle small change in free carbon of the pyrolysis temperature of rGO/equaixial geometry TiO2. Two distinctive bands appear in the rGO spectrum at 1324 and 1606 cm− 1, referring to D (disordered carbon) and G (graphic carbon) bands, respectively (Xu &Cheng 2013). Because of edge and internal structural defects (sp3 defects), the D band is due to the disruption of the symmetrical hexagonal graphitic lattice. On the other hand, the characteristics of all sp2 carbon forms (C-C bonds) constitute the G band (Garrafa-Gálvez et al. 2019). According to the Gaussian-Lorentzian fitting were calculated, the intensity ratio (ID/IG) of rGO/equaixial geometry TiO2 (0.83), less than the intensity ration of rGO (0.84), This is because of the reduced sp2 domain in the planes of carbon atoms, suggesting a large number of defects on the rGO sheets due to the TiO2 formation (Prabhu et al. 2017).
The photocurrent calculation was conducted to research the interfacial charge transfer dynamics, the higher the photocurrent value, the more effectively splitting the photogenerated electron-hole pairs (Wang et al. 2015b). Figure 5(b, c) is a representation of rGO, geometry TiO2, and rGO/equaixial geometry TiO2, transient photocurrent responses. For each light turned on, a sudden and uniform photocurrent increase was clearly observed, and then the photocurrent decreased to a constant value as soon as the light was turned off. Furthermore, the photocurrent for rGO/equaixial geometry TiO2 was far higher than that of rGO and geometry TiO2, indicating that under visible light irradiation as shown in Fig. 5b, and under solar radiation, as shown in Fig. 5c, and more efficient charge separation of electron-hole pairs in rGO/equaixial geometry TiO2. As a result, this can be deduced that hybridization improves the separation and transfer efficiency of photoinduced electrons and holes, resulting in an increase in photocurrent density. This result is consistent with the results of the degradation curves in Fig. 7.
In addition, electrochemical impedance spectra (EIS) calculation was conducted to obtain a deeper understanding of the charge transfer properties of rGO, geometry TiO2, and rGO/equaixial geometry TiO2. In the EIS of rGO/equaixial geometry TiO2 spectrum, the arc size radius was smaller than that of rGO and geometry TiO2, as seen in Fig. 6(d, e). This suggests that the rGO/equaixial geometry TiO2 had a lower resistance, and the transition speed of electrons of the composite was higher than the others, largely due to the heterostructured construction between rGO and geometry TiO2.
3.6 Nitrogen absorption − desorption isotherms and pore size of rGO/egTiO2 composite
As an essential parameter for evaluating the photocatalytic activity of the nanostructure is the surface area. BJH and BET techniques are associated with pore size distribution and real surface area measurements. The isotherms of N2 adsorption-desorption were calculated at 200 ºC for rGO, equaixial geometry TiO2, and rGO/equaixial geometry TiO2, and the isotherms are shown in Fig. 6. According to the IUPAC classification (Khan et al. 2019, Yu et al. 2018), isotherms are classified into three types as type IV of mesoporous nature. The specific surface area of rGO/equaixial geometry TiO2 composite was 115.80 m2/g, while the average pore size was 2.822 nm, as seen in Fig. 6(a), larger than that of surface area (3.32 m2/g), (16.38 m2/g) and pore size (28.184 nm), (17.22 nm) of rGO and equaixial geometry TiO2, respectively as seen in Fig. 6(b, c). The findings suggest that the rGO/equaixial geometry TiO2 composite BET surface area is higher than that of rGO, and equaixial geometry TiO2. The broad heterogeneous photocatalyst surface area can provide more surface-active adsorption sites for reactant molecules, making photocatalytic process more efficient.
3.6 The photocatalytic performance of rGO/egTiO2 composite on organic dye degradation
After confirming the successful recombination between equaixial geometry TiO2 and rGO, we also need to simulate test its photo-catalysis performance, stimulate pollutions but still choose the organic dye stuffs RhB and MB, since they are both refractory contaminants that are commonly found in textile wastewater
Firstly, we tested the degradation curve figures of rGO/equaixial geometry TiO2 composites under ultraviolet rays with different composite ratios to RhB by using mercury lamps of 370W, and compared with single equaixial geometry TiO2 and single rGO at the same time, as showed in Fig. 7 is their whole degradation process on RhB, which includes dark absorption and photo-catalysis degradation. There will also be degradation occurring in dyestuffs in blank test tubes under ultraviolet light, they were be degraded by about 20% in 60 minutes. Secondly, observe the process of dark absorption, we found that the proportion of the rGO composites increased, the dyestuffs absorbed in the process of its dark absorption were also increased until the proportion of rGO reached 12.7%. The absorption process can make the absorbance of dyestuffs decrease 50%, but the single rGO could directly absorb nearly 85%, which indicates the absorption effect of rGO on dyestuffs is very strong, it could be attributed to its shape which is the very thin lamellas with relatively large specific surface areas. It can be seen from Fig. 7 (a) that whether it is single rGO or single equaixial geometry TiO2, its degradation rates are relatively low, but the degradation rate of other composites with various ratios have increased, which are different. Directly, when the rGO proportion reaches 12.7% and 2.6%, the degradation rate is the fastest. Which degraded almost all the dyestuffs for RhB 97.5% in 60 minutes. The difference is that when the rGO ratio reaches 12.7% the absorption is relatively strong, the absorption of the rGO composites with the ratio of 2.6% is relatively less strong than the former, but it can degrade dyestuffs with higher concentration to the same level, so we think the overall degradation of the rGO composites with the ratio of 2.6% is better. Figure 7(b) showed the degraded UV-vis absorption spectrum of rGO/equaixial geometry TiO2 composites with the mass ratio of 2.6% of rGO: TiO2 to RhB, whose absorption on dye stuffs is not very intense, but the degradation ratio is very excellent, the final results of degradation showed that its absorption peaks of all the wavelengths are almost zero. Afterward, under the same conditions, we tested the degradation curve figures of the rGO/equaixial geometry TiO2 composites with different composite ratios on MB and compared them with single equaixial geometry TiO2 and single rGO at the same time, as showed in Fig. 7(c) is the whole degradation process on MB. Its rules of the dark absorption process are consistent with that of RhB. Single rGO absorbed 88% dyestuffs in one hour. The degradation rates of composites with different ratios are relatively close, but the degradation rate of the rGO composite with the ratio of 6.4% is the fastest, which degraded almost all the dyestuffs for MB around 97% in 60 minutes. Therefore, Fig. 7(d) showed a UV-vis absorption spectrum of rGO/equaixial geometry TiO2 composites with a mass ratio of 6.4% of rGO: TiO2 on the degraded MB. It is found that comparing each single component, the photo-catalysis activity of composites increased by analyzing the degradation process of rGO and TiO2, which can be attributed to the cooperative effect of rGO and TiO2, making agglomerate TiO2 dispersed on rGO lamellas, graphene lamellas are also divided. The two shaped hetero junctions, the graphene is used as electron acceptor materials, and the character of its readily electricity conduction can effectively prevent the recombination of photo-electrons and holes, providing better conditions and time of the occurring of oxidization process than before.
To explore the degradation process under sunlight the sample was tested and degradation curve pictures of the rGO/equaixial geometry TiO2 composites with three composite ratios on RhB and MB presented in Fig. 7, under the simulation sunlight via adopting a high-intensity discharge lamp of 800W. We compared them with a single equaixial geometry TiO2 and single rGO at the same time, took samples to test UV-vis absorption spectrum every two hours. We selected the composites with the rGO ratios of 12.7%, 4.3%, and 2.6% for RhB and 12.7%, 6.4%, and 2.6% for MB to degrade. Figure 7 (e, g) is their whole degradation process on RhB and MB. Because TiO2 itself has the character of being sensitive to ultraviolet ray and of being insensitive to visible light, whose degradation process is relatively low under the simulation sunlight, and its degradation rate cannot match the absorption rate, the absorption rate still obeys previous rules. Which almost degraded all the dye stuffs for RhB and MB around 94% and 92%, respectively after 6 hours of degradation. This result verified the results of UV-vis diffuse reflection tests in Fig. 4, whose visible photo-response increased, the photocatalysis got improved. Pure rGO showed the fastest “degradation” rate in the first 2 hours after turning on the lights, then the rate obviously slowed down, it is probably because that its absorption effect can play the role, other than the degradation effect. Figure 7 (f, h) showed the UV-vis absorption spectrum image of the rGO/equaixial geometry TiO2 compounds with the mass ratio of 4.3% of rGO: TiO2 on RhB and 6.4% of rGO: TiO2 on MB, degradation respectively.
We summarized the degradation findings with samples from Photocatalysts, their K app numbers, and R2 values as seen in Table 1. The following Eq. (3) was used to analyze the kinetic photocatalytic degradation of organic dyes:
where the k app is the pseudo-first-order rate constant, co is the initial concentration of dye, and c is the dye concentration after photocatalytic degradation at time t.
Table 1
Photo-catalysts samples, quantities of their K app and R2 values.
Samples
|
Photocatalytic activity %
|
Light sources
|
Times (hr)
|
K app (min-1)
|
R2
|
RhB
|
MB
|
|
|
RhB
|
MB
|
RhB
|
MB
|
rGO
|
85.0
|
88.0
|
UV
|
1
|
0.022
|
0.038
|
0.87
|
0.92
|
egTiO2
|
84.0
|
89.0
|
UV
|
1
|
0.026
|
0.036
|
0.97
|
0.96
|
rGO/egTiO2
|
97.5
|
97.0
|
UV
|
1
|
0.047
|
0.056
|
0.93
|
0.95
|
rGO
|
80.0
|
80.0
|
Sunlight
|
6
|
0.24
|
0.24
|
0.87
|
0.85
|
egTiO2
|
84.0
|
82.0
|
Sunlight
|
6
|
0.26
|
0.25
|
0.95
|
0.95
|
rGO/egTiO2
|
94.0
|
92.0
|
Sunlight
|
6
|
0.36
|
0.35
|
0.96
|
0.97
|
3.7 The photocatalytic mechanism of the rGO/egTiO2 composite
To describe the degradation mechanism of organic dye (MB and RhB) through rGO/equaixial geometry TiO2 composites photo catalysts under UV and solar irradiation are shown in Fig. 8. In MB and RhB degradation, the enhanced performance is mainly due to the function of reducing graphene sheets present in rGO/equaixial geometry TiO2. In the degradation procedure, reducing graphene sheets serve as (1) an absorbent, (2) a charge separator, and (3) a photosensitive. The compound is dual structure, and there exists close interface contact among components. In the process of absorption, rGO has a big specific surface area, which endowed it the relatively good absorption ability, which can increase the opportunities of pollutant molecules and materials to contact with each other.
Under the irradiation of light, the absorption of organic dye is increased by the function of rGO. The exceptional adsorption capability of rGO/equaixial geometry TiO2 was credited to simple physical and chemical adsorption resulting from (π-π) interactions on the rGO sheets between organic dyes and the aromatic domains (Liu et al. 2016). The process of adsorption enhanced the concentration of organic dye molecules over rGO sheets, placing the organic dye closer to the TiO2 photocatalytic surface, which is a requirement for photocatalytic activity improvement.
Under the UV irradiation semiconductor TiO2 will produce the match of light-generated electrons with electron holes. Light-generated electrons (e−) will make simulations from VB to CB, which caused the light-generated electron holes(h+) in the VB as shown in Fig. 8(a). If there is no rGO, the electrons and electron holes irritated by such light irradiation will quickly revert to the ground state due to its instability, which represents the electrons back to VB quickly, which will cause fluorescence emission and low catalytic activity on dye stuffs. However, with the presence of rGO, the interface contacts of the two makes the interface form a hetero junction. Here exists separation region of space charge in hetero junction, the electrons are prone to flow to low Fermi level from high Fermi level to adjust Fermi level;as the calculated work function of graphene is (4.42eV), ECB of anatase TiO2 is about 4.21 eV (Li et al. 2021).
So firstly, graphene can accept the light-generated electrons from TiO2 to prevent the recombination of the match of light-generated and electron holes, which is the first reason. Secondly, the recombination of such relatively strong and interactive forces makes TiO2 crystal particles at nano-level to disperse on the graphene slices at the micron level, which is easy to be gathered, the particles are prone to contact with dyestuffs after they dispersed, and rGO has strong absorption effect, which further improved the contact possibility.
The third reason is that graphene itself has excellent electrical conductivity when the light-generated electrons are transferred to rGO. The rGO could work as the carrier of electron flow to react with O2 in dyestuff to produce O2•−, which will continue to make a series of reactions to produce−OH, the composite ratio of light-generated electron holes that left on VB below, so the free holes in TiO2 react with H2O or OH− to produce O2• − and •OH that have strong oxidization ability, and as such reacts with organic dyes which can degrade to CO2, H2O, and other intermediates. In a word, there exists a synergistic effect, its photo-catalysis performance can be enhanced.
Under solar radiation, the function of rGO is to instantaneously acts as a photosensitizer and a charge separator. The solar spectrum contains only about 3–5% ultraviolet light and 50% visible light (∼2.8 eV), which forms electron-hole pairs in nanoparticles of TiO2, which are not enough to excite TiO2 nanoparticles (approximately 3.2 V) (Pitre et al. 2017).
However, as shown in Fig. 8(b), when the rGO/equaixial geometry TiO2 catalyst is irradiated via solar light, the response is provided via visible light part when the rGO photo-excitation is possible due to a higher energy level than the measured band gap of rGO/equaixial geometry TiO2 composite (3.05 eV), whose function is similar to the photosensitive process of organic dyes. The rGO is photoexcited from (HOMO) level to (LUMO) level. The rGO in the excited state transference electrons into the conduction band of TiO2 and the excited electrons can be trapped by molecular oxygen that is found in the reaction system, where oxygen is activated to form superoxide radicals (O2• −) and other oxidative species for the degradation of organic dyes and converted into CO2, H2O, and other intermediates.
This mechanistic way elucidated the photo-excitation of rGO/equaixial geometry TiO2 under solar irradiation (ultraviolet and visible light), where it is shown that the rGO sheets serve as a charge separator to limit electron-hole pair recombination. But at the same time, it also acts as a photosensitizer, converting the large band gap semiconductor into a visible photo-activity that lets amplified the photodegradation of organic dyes.