Enhanced Photodegradation of Rhodamine B Using Visible-Light Sensitive N-TiO₂/rGO Composite

Abstract

Rhodamine B (RhB) is extensively used for dyeing purposes, and cannot be completely removed using traditional water treatment technologies. Here, we report for the first time the photodegradation of RhB using nitrogen-doped titanium dioxide (N-TiO₂) on reduced graphene oxide (rGO) composite (N-TiO₂/rGO). The work primarily highlights the synergistic effect of the incorporation of N-TiO₂ and rGO and its kinetic study for the photodegradation of RhB. The N-TiO₂/rGO composite was synthesized by dispersing titanium(IV) isopropoxide and urea, followed by annealing treatment via the hydrothermal method with rGO. Scanning electron microscopy (SEM) images illustrated that N-TiO₂ particles with an irregular round shape and white color were dispersed onto the rGO surface. X-Ray diffraction (XRD) patterns revealed that N-TiO₂/rGO composite showed an anatase phase of TiO₂ with a diffraction peak of 2θ = 25.622°. The gas sorption analysis (GSA) showed that N-TiO₂/rGO had surface area, pore volume, and pore size of 53.393 m²/g, 0.096 cc/g, and 3.588 nm, respectively. The thermogravimetric-differential thermal analysis (TG-DTA) showed an anatase phase of TiO₂ that appeared at a temperature of 200–500 °C, with a weight loss of 2.50%. According to the ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) study, TiO₂, N-TiO₂, and N-TiO₂/rGO had band gap energies of 3.25, 2.95, and 2.86 eV, respectively. The highest photodegradation of RhB was obtained at the optimum condition in pH 2 with a photocatalyst mass of 20 mg and an irradiation time of 90 min. The photocatalytic activity of N-TiO₂/rGO using visible light showed a higher percentage of photodegradation at 78.29%, compared to 44.08% under UV light. The kinetic study of the photodegradation of RhB using N-TiO₂/rGO followed the pseudo-second-order model.

1. Introduction

A significant environmental problem is the pollution caused by harmful organic dyes used in the textile industries; these industries are growing as a result of consumer demand, and fashion trends are continuously shifting over time. The textile industry does not produce significant amounts of solid waste, though liquid waste in the form of dyes is produced in large volumes; these dyes contaminate the waterways (rivers and sewers [1,2,3]). Depending upon the industrial application, dyes can be classified into various classes, including simple dyes, acidic dyes, vat dyes, disperse dyes, basic dyes, azo dyes, reactive dyes, and solvent dyes [4].

RhB is one of the most widely used cationic xanthene dyes in the textiles and foodstuffs; it also plays a crucial role in the dyeing process in the paint, leather, and paper industries. RhB exposure, however, has several negative impacts on humans. RhB can cause skin, eye, and gastrointestinal irritation, as well as liver cancer when it is directly exposed or consumed. As a result of the impact of a significant amount of organic waste, particularly textile dyes, researchers are concentrating on how to deal with the environmental liquid waste generated by the textile sector. As a wastewater treatment technology, organic, chemical, and physical, as well as merging the three processes, have all been employed [5,6,7].

A variety of conventional techniques, most notably electrodialysis, membrane filtration, precipitation, adsorption, electrochemical reduction, and electro-deionization, were frequently used in the past to remove contaminants from wastewater. However, these procedures use a significant amount of energy and may be made more challenging by the transfer of contaminants across different fluids, distinct wastes, and by-products produced during the treatment of wastewater. Photocatalysis, a simple and green technology process using mild conditions, is the preferred process due to its effectiveness at degrading organic pollutants contained in wastewater into carbon dioxide, water, or other small molecules, and reducing or oxidizing inorganic pollutants into harmless substances [8,9].

TiO₂ is frequently employed for the photodegradation of many organic contaminants, including heavy metals, pesticides, dyes, and phenols for water purification [10,11]. Several modifications of TiO₂, performed by combining TiO₂ with metals, non-metals, or semiconductors, were conducted to produce TiO₂ with high photocatalytic activity. The modification of TiO₂ with carbon materials is one of the most promising researched techniques for the preparation of highly active photocatalysts since the electron-hole pairs recombination is inhibited, thus enhancing the performance of photocatalysts under the visible light. Carbon materials represent an intriguing class of carbon precursors that can be employed in TiO₂ modification due to their distinctive porous structure, electrical characteristics, and substantial adsorptive capacity. Heterogeneous catalysis extensively uses conventional carbon materials, including graphene-based materials. Numerous preparation methods for TiO₂/rGO photocatalysts have been published in the literature, including simple mixing, sol-gel, and hydrothermal or solvothermal [12,13,14,15,16,17,18].

Non-metal doping is another approach used to increase the photocatalytic activity of TiO₂. Non-metal dopant has advantages when compared to metal dopant; the resulting catalyst is more affordable, has higher photostability, and emits less pollutants [19,20]. Doping TiO₂ with N atoms modifies the electronic structure, where N 2p energy level is generated close to the valence band, leading to a decrease in the band gap of TiO₂. As a result, the light absorption of the semiconductor is extended to visible light. Moreover, N atoms can form metastable centers and suppress the recombination of charge carriers in N-TiO₂ due to their precise balance, small ionization energy, and an atomic radius similar to O atoms. However, electron-hole pairs photogenerated with N-TiO₂ under visible light have a limited time to inhibit recombination [21,22]. This limitation can be overcome by supporting N-TiO₂ on carbon materials such as reduced graphene oxide (rGO). rGO provides increased excitability formation and a reduced recombination rate [23]. Thus, the photocatalytic performance of the overall reaction also be increased due to the presence of a synergistic effect. Therefore, a composite of N-TiO₂/rGO may exhibit efficient and sustainable photocatalytic material under visible light.

To the best of our knowledge, however, there has never been a work that discusses the use of N-TiO₂/rGO as a photocatalyst for the degradation of RhB. In this present study, we hypothesize that N-TiO₂/rGO can produce significantly reactive oxygen species so that the degradation efficiency of RhB dyestuffs can be advanced. The photocatalytic performance and kinetics of N-TiO₂/rGO under optimal conditions were systematically investigated. RhB is often considered as one of the main dye pollutants in textile industry waste, which is difficult to degrade by conventional treatment. Thus, RhB was used as a model pollutant in evaluating the photocatalytic performance.

2. Materials and Methods

2.1. Materials

Graphite powder, potassium permanganate (KMnO₄ 99%), hydrogen peroxide (H₂O₂ 80%), sulfuric acid (H₂SO₄ 97%), hydrazine (N₂H₄ 60%), sodium nitrate (NaNO₃ 99%), nitric acid (HNO₃ 99%), titanium(IV) isopropoxide (Ti[OCH(CH₃)₂]₄ 98%), urea (CH₄N₂O 99%), hydrochloric acid (HCl 37%), n-propanol (C₃H₇OH 99%), and ethanol (C₂H₅OH 99%) were obtained from Merck.

2.2. Material Synthesis

2.2.1. Synthesis of rGO

A total of 1 g of graphite was oxidized with 0.5 g of NaNO₃, 3 g of KMnO₄, and 23 mL of H₂SO₄ at a temperature below 20 °C. Then, the mixture was stirred at 40 °C for 3 h. A total of 50 mL of distilled water was slowly added and stirred at 50 °C for 1 h, followed by the addition of 50 mL distilled water and 5 mL H₂O₂. Next, the solution was filtered and washed with HCl, and rinsed until pH was neutral. The precipitate obtained was dried at 80 °C for 4 h to obtain graphite oxide (GO) [24]. Then, 1.2 g of solid GO in 400 mL H₂O was sonicated for 3 h, then combined with 0.4 mL of N₂H₄ as a reducing agent and stirred for 3 h at 70 °C. The precipitate was filtered and dried at 90 °C for 1 h to obtain rGO.

2.2.2. Synthesis of N-TiO₂

A solution of C₃H₇OH and Ti[OCH(CH₃)₂]₄ with a ratio of 95:5 v/v was diluted with HNO₃ until it reached pH 2. The mixture was stirred for 12 h at room temperature. Then, 100 mL of CH₄N₂O 1 M solution was added and underwent sonification for 2 h. The solution was filtered and dried at 100 °C for 5 h and calcined at 450 °C for 3 h to obtain N-TiO₂ [25]. Then, 40 mg of rGO was dissolved with 80 mL of aqueous and 40 mL of C₂H₅OH and sonicated for 30 min.

2.2.3. Synthesis of N-TiO₂-rGO

A total of 400 mg of N-TiO₂ was added to the solution and stirred for 2 h. The solution was then transferred to an autoclave and heated in the oven at 120 °C for 3 h. The solution was transferred into a beaker glass and stirred under UV light (T5 UV light 8 W) for 2 h. Then, the solution was filtered and rinsed with distilled water. The precipitate was dried at 70 °C for 12 h to obtain N-TiO₂/rGO. Stages of the synthesis method of N-TiO₂/rGO are schematically shown in Figure 1.

2.3. Material Characterization

The crystalline phase of the material was determined using an XRD (Shimadzu XRD-7000, Japan), the surface morphology and element content of the material were analyzed using a SEM-EDX (Hitachi S-4800, Japan), and the textural properties of the material were determined using a GSA (Quantachrome Novatouch LX-4, Austria). To detect variations in material weight as a function of temperature or time, a TG-DTA (Hitachi STA-200RV, Japan) was employed. To find the band gap energy of the material, a UV-Vis DRS (Shimadzu UV-2401 PC, Japan) was used.

2.4. Study of Photocatalytic Activity

The determination of the optimum pH was carried out at various pH solutions of 2, 3, 4, 5, 6, 7, 8, 9, and 10 with the addition of 20 mg N-TiO₂/rGO photocatalyst in 10 ppm RhB solution for 60 min under UV light. Determination of the optimum photocatalyst mass used variations of 10, 20, and 30 mg at the optimum pH with an irradiation time of 60 min under UV light (T5 UV light 8 Watt). The results of the optimum pH and photocatalyst mass were used for variations in irradiation time of 0, 30, 60, 90, and 120 min. A UV-Vis spectrophotometer was then used to analyze the filtrate at the optimum wavelength of 554 nm [26].

3. Results and Discussion

3.1. SEM-EDX Characterization

Figure 2 depicts the SEM images of different morphology of materials. Graphite shows large flakes and an irregular surface structure. After being oxidized, the graphite turned into GO with a more regular surface due to the agglomeration of the graphite sheets so that it became thicker. Thus, the GO had wrinkles at the edges of the material; this was caused by the presence of angle tension between the carbon atom bonds so that wrinkles were formed by stabilization of the atomic arrangement [27]. Furthermore, the morphology of the rGO material became completely irregular due to thermal reduction; this resulted in smaller particles and thinner sheet exfoliation when compared to GO by chemical exfoliation [28]. N-TiO₂ had white granules with an uneven spherical form, and small round particles were sticking to the surface. After the modification, N-TiO₂/rGO had a more regular shape when compared to N-TiO₂; small white particles of N-TiO₂ were dispersed onto the rGO surface [29].

The presence of elemental composition of material can be ascertained by conducting EDX.

Table 1. Elemental composition of materials.

Sample	Element Percentage (wt%)
	C	O	N	Ti
Graphite	100	-	-	-
GO	60.42	39.58	-	-
rGO	83.17	16.83	-	-
N-TiO2	5.01	48.36	1.85	44.78
N-TiO2/rGO	27.88	37.94	0.61	33.57

lists the C element of graphite with a percentage of 100 wt%; GO materials were observed for the appearance of O, indicating that graphite was successfully oxidized. The decrease in the mass percentage of the O element in rGO was caused by the reduction process of groups containing oxides. N-TiO₂ contained N element with a percentage of 1.85 wt%, so it could be seen that the N dopant was successfully doped on the TiO₂ material. The lower percentage of Ti element in N-TiO₂/rGO indicates that some TiO₂ enter the interlayers of rGO.

3.2. XRD Characterization

The diffractogram patterns of material recorded at diffraction angle range 2θ = 2–80° and Cu Kα radiation (λ = 1.5406 Å) are shown in Figure 3 and Figure 4, respectively. The diffractogram pattern of pure graphite material shows characteristic sharp peaks with high intensity at 2θ = 26.669° (111), with a distance between layers of 0.334 nm, according to Joint Committee on Powder Diffraction Standards (JCPDS) data No. 01-075-2078.

GO material show the low intensity of the reflection peak at 2θ = 10.651° (111), with a spacing between layers of 0.339 nm, and a small peak at 2θ = 26.274° (111), with a spacing between layers of 0.339 nm, according to JCPDS data No. 01-082-2261. The crystallinity of the material decreases as the peak intensity decreases. The intercalation of oxide groups, consisting of carbonyl, epoxy, carboxyl, and hydroxyl groups at the basic plane of graphite, was produced by an oxidation process that increased the space between the layers. In addition, these groups also contributed to hydrophilicity of atomic layers [30].

The diffractogram pattern of the rGO material shows a low intensity broad peak at 2θ = 26.024° (002), with a distance between layers of 0.342 nm. Functional groups containing oxygen were removed efficiently as shown by the reduced peak intensity at 2θ = 10.651° (111) and the reduced distance between the rGO layers. The wide peaks of the rGO material are due to its amorphous nature.

The diffractogram pattern of the N-TiO₂ material shows sharp peaks at 2θ = 25.582°, 38.262°, 48.266°, 54.214°, 55.154°, 62.531°, and 75.438°; meanwhile, the N-TiO₂/rGO material shows peaks at 2θ = 25.622°, 26.364°, 38.187°, 48.306°, 54.344°, 55.144°, 62.7014°, and 75.288°. The diffractograms of the TiO₂-based materials had peaks corresponding to plane reflections (101), (112), (200), (105), (211), (213), and (215). These peaks were ascribed to the anatase phase of TiO₂ in accordance with JCPDS data No. 00-021-1272. The structure and phase of TiO₂ did not change in the presence of rGO in the composite. No typical diffraction peak of rGO was found in N-TiO₂/rGO; this was due to the low rGO ratio in the synthesis process and the possibility of a new peak being covered by the TiO₂ diffraction peak at 25.622° (111) [31].

The Debye–Scherrer equation shown in Equation (1) can be used to calculate the size of a crystal shown in Equation (1), where D indicates the size of crystal, $\lambda$ indicates wavelength of X-ray (1.5406 Å), β denotes Full Width at Half Maximum (FWHM), K represents Scherrer constant (0.9), and θ indicates Bragg diffraction angle [32].

Table 2. Average crystal size of materials.

Sample	Distance between Layers (nm)	D (nm)	Average D (nm)
Graphite	0.334	15.801	15.801
	0.830	6.999
GO	0.339	7.416	7.208
rGO	0.342	4.160	6.438
	0.348	9.641
	0.235	9.261
N-TiO2	0.188	9.296	13.215
N-TiO2/rGO	0.169	10.304	12.818
	0.166	11.792
	0.148	11.926
	0.126	13.558

presents crystal size data. The average crystal size of graphite, GO, rGO, N-TiO₂, and N-TiO₂/rGO were 15.801, 7.208, 6.438, 13.215, and 12.818 nm, respectively.

$$D=K\lambda /\beta \cos \theta$$

(1)

The crystal size of graphite decreased after the oxidation and reduction processes. The incorporation of N-TiO₂ and rGO caused a decrease in crystal size from 13.215 nm to 12.818 nm. The modification of rGO suppressed the phase transition and exhibited the sintering of TiO₂ crystallites. A smaller crystal size indicated better photocatalytic activity due to its stronger redox ability [12,33]. In general, crystal size influences photocatalytic performance, but it should be noted that photocatalytic activity is not only affected by crystal size. Other factors that can affect photocatalytic performance are crystallinity, surface area, pore size, energy band gap, electronic material properties, and so on. Although rGO has the smallest size, it is amorphous; this means that its potential for photocatalytic activity is lower [34]. N-TiO₂/rGO has good photocatalytic performance due to its small size and crystalline nature.

3.3. GSA Characterization

Table 3. GSA analysis results.

Sample	Surface Area (m2/g)	Pore Volume (cc/g)	Pore Size (nm)
Graphite	3.951	0.014	7.144
GO	216.313	0.565	5.225
rGO	83.407	0.103	2.469
N-TiO2	60.502	0.109	3.602
N-TiO2/rGO	53.393	0.096	3.588

shows the textural characteristic of materials. Graphite, GO, rGO, N-TiO₂, and N-TiO₂/rGO had pore sizes of 7.144, 5.225, 2.469, 3.602, and 3.588 nm, respectively. All materials exhibited as mesoporous material in the range of 2–50 nm. The surface area of rGO reduced when GO was transformed into rGO. The sonification-based exfoliation of GO into reduced graphene oxide was one of the reasons contributing to this decrease. The most efficient method for almost completely exfoliating GO was discovered to be sonication, although it severely damaged the graphene sheet, causing a change in surface size from micron to nano [35].

In comparison to N-TiO₂, N-TiO₂/rGO composite has less surface area and porosity. A lower pore volume was generated due to a decrease caused by the significant accumulation of N-TiO₂ particles on the rGO surface; consequently, the surface area was reduced. When the particle size was reduced, the surface area to pore volume ratio increased, which was a sign that the ratio of surface area to pore volume was becoming increasingly relevant. The decrease in surface area and pore volume in N-TiO₂/rGO was caused by N-TiO₂ occupying the surface of rGO [36].

3.4. TG-DTA Characterization

Figure 5 shows the results of TG-DTA characterization to measure weight loss from materials as a function of temperature or time. The TG-DTA analysis shows that graphite did not exhibit any decomposition or change in mass. In the GO, there was a weight loss of 17.50% at a temperature of less than 180 °C, which indicates the evaporation of free water in the graphite material. The second stage of decomposition was possibly the partial release of large amounts of oxygen-containing functional groups, shown to occur with a weight loss of 74.30% in the range of 100–200 °C. GO material showed good thermal stability at temperatures above 200 °C. In the rGO, there was a weight loss of 5.92% at a temperature of 100 °C which indicates the presence of water evaporation; further weight loss occurred at a temperature of 100–200 °C with a weight loss of 6.79%. A temperature above 200 °C indicates good thermal stability in the rGO.

Figure 5 demonstrates the three steps of weight reduction in N-TiO₂ as the temperature increases. First, at a temperature of 100 °C, caused by the loss of residual water and ethanol by 2.94%. Second, at a temperature of 100–200 °C, a weight loss of 1.06% was due to the decomposition of organic matter. Third, at a temperature of 200–450 °C, caused by the formation of an anatase crystalline phase of an amorphous form of TiO₂, with a weight reduction of 0.39%. The N-TiO₂/rGO material shows a weight loss of 2.25% at a temperature 200 °C due to a loss of water and solvent residue. At a temperature of 200–500 °C, a second stage of decomposition occurred, indicating a decrease in weight by 2.50%. The last stage of decomposition, at a temperature of 780–900 °C with a weight loss of 0.60%, was due to the change of the anatase phase of TiO₂ into the rutile phase.

3.5. UV-Vis DRS Characterization

The purpose of UV-Vis DRS characterization is to ascertain the band gap energy of material. The Kubelka–Munk function F(R) equation and the Tauc plot can be used to calculate the band gap energy of photocatalyst as shown in Equation (2), where n is a constant for the optical transition type and has a value of 2 for indirect transitions [37].

(F(R) × hv) 1/n = C (hv − Eg)

(2)

Figure 6 shows that the band gap energy of TiO₂ can be reduced by the addition of N dopant, from 3.25 to 2.95 eV, while the incorporation of rGO and N-TiO₂ can also reduce the band gap energy to 2.86 eV. It indicates that the modification of TiO₂ using N and rGO can successfully reduce the band gap energy of TiO₂, which has a significant impact on the performance of photocatalyst.

3.6. Effect of pH

RhB shows the degradation rate at pH range between 2–10 with the most optimum degradation percentage of 61.87% at pH 2 as presented in Figure 7. RhB is more easily de-graded at acidic pH than at alkaline pH; in aqueous solvent, RhB is a negatively charged dye. The positively charged surface of TiO₂ in acidic conditions causes more RhB to be absorbed on the surface of N-TiO₂/rGO and reacts with free radicals [38].

3.7. Effect of Photocatalyst Mass

The photodegradation of RhB at various photocatalyst masses of 10, 20, and 30 mg is shown in Figure 8. The use of a photocatalyst mass of 20 mg at optimum pH 2 resulted in the highest degradation percentage (53.66%). It is obvious that the photocatalyst concentration must be optimal. The enhanced photocatalytic activity was obtained when the photocatalyst mass was increased from 10 mg to 20 mg by increasing the active sites on the photocatalyst surface. However, a higher dose of photocatalyst will block the active sites and limit the light to entering the N-TiO₂/rGO surface, thus causing a decrease in the formation of several hydroxyl radicals that may take part in the actual degradation.

3.8. Effect of Irradiation Time

The photodegradation of RhB at various irradiation times between 0 and 120 min is shown in Figure 9. During the photocatalytic reaction for 90 min, the highest percentage of RhB degradation (44.08%) was obtained at the optimum pH and photocatalyst mass. The irradiation time is an important indicator to determine the interaction between N-TiO₂/rGO and light to produce hydroxyl radicals for the photodegradation of RhB [39]. A longer irradiation time increases the photon energy hitting the surface of photocatalyst, causing the generation of more hydroxyl radicals, thus increasing the photocatalytic activity until it reaches optimum condition [40].

3.9. Effect of Light Irradiation

Figure 10 shows that the photodegradation of RhB using N-TiO₂/rGO led to a higher percentage of degradation (78.29%) under visible light, compared to 44.08% under UV light with the same optimum condition (pH of 2, photocatalyst mass of 20 mg, irradiation time of 90 min). It indicates that N-TiO₂/rGO can absorb visible light energy more optimally, thus producing more radicals to degrade RhB. The modification of N-TiO₂ material with rGO can suppress electron hole recombination during the reaction to increase photodegradation of RhB.

Figure 11 shows the photodegradation mechanism of RhB using N-TiO₂/rGO. When N-TiO₂/rGO is exposed to light, electrons in the valence band (VB) are transported to the conduction band (CB), thus producing electrons in the CB and holes in the VB. Doping TiO₂ with N atoms changes the electronic structure, leading to the formation of new energy gap states. The energy level of N 2p is generated above the O 2p; this causes the band gap of TiO₂ to decrease from 3.25 eV to 2.95 eV so that the absorption of light extends to visible light. The CB and VB potentials can be calculated using Equations (3) and (4), where E_e is the free electron energy (4.50 eV), X is the absolute electronegativity, and E_g is the band gap of semiconductor. The X value for TiO₂ is 5.81 eV [41,42].

E_VB = X − E_e + 0.5 E_g

(3)

E_CB = E_VB − E_g

(4)

The VB of N-TiO₂ (2.94 eV) shows a lower level than the highest occupied molecular orbital (HOMO) of rGO (1.98 eV), and the CB of N-TiO₂ (−0.32 eV) has a lower level than the lowest unoccupied molecular orbital (LUMO) of rGO (−0.6 eV). rGO acts as an electron acceptor to prevent electron-hole recombination, where electrons excited from the VB to the CB will be captured by LUMO of rGO. Under visible light, rGO also acts as a photosensitizer so that electrons excited from the HOMO to the LUMO will be transferred to the CB of N-TiO₂ to improve the efficiency of charge separation and photocatalytic activity [43]. The electrons will react with oxygen to generate superoxide radicals, while the holes will react with hydroxyl ions and water to form very reactive hydroxyl radicals. As a result, RhB dyes will react with hydroxyl and superoxide radicals to form CO₂ and H₂O [44,45,46,47,48,49,50,51,52,53,54,55,56,57]. Photodegradation mechanism of RhB shown in Equations (5)–(10).

N-TiO₂/rGO + hv → N-TiO₂/rGO (e⁻) + N-TiO₂/rGO (h⁺)

(5)

rGO + (e⁻) → rGO (e⁻)

(6)

h⁺ + OH⁻ → •OH

(7)

h⁺ + H₂O → •OH + H⁺

(8)

e⁻ + O₂ → •O₂⁻

(9)

N-TiO₂/rGO (•OH or •O₂⁻) + RhB → CO₂ + H₂O

(10)

Some synergistic research works for comparison with this study are summarized in

Table 4. The photodegradation of RhB in this study compared with previous studies.

Photocatalyst Materials	Light	Dye Concentration	Photocatalyst Mass	Irradiation Time	Photodegradation	Ref.
Cr-doped TiO2	Hg lamp 500 W	10 mg/L	0.02 g	90 min	98%	[48]
PbCrO4/TiO2	Tungsten 200 W	1 × 10−5 M	0.5 g	600 min	70%	[49]
Ag2CrO4	LED 5 × 24 W	10 mg/L	0.1 g	35 min	73%	[50]
Cu-TiO2	LED 15 W	15 mg/L	0.05 g	360 min	65%	[51]
Bi2WO6	Xenon 500 W	1 × 10−5 M	0.5 g	60 min	19%	[52]
Ag2C2O4/BiMoO6	Xenon	1 × 10−5 M	0.2 g	100 min	78.84%	[53]
CNT-TiO2	UV	10 mg/L	10 g	20 min	85%	[54]
Fe-TiO2/rGO	Solar simulator	20 mg/L	0.6 g	120 min	91%	[55]
SiO2/C-TiO2	UV	20 mg/L	0.1 g	60 min	61.7%	[56]
Cu-TiO2	Natural light	10 mg/L	0.02 g	120 min	97%	[57]
N-TiO2/rGO	Xenon	10 mg/L	0.02 g	90 min	78.29%	This Study

. It is determined that under specific experimental circumstances, the N-TiO₂/rGO composite is an excellent photocatalyst for the degradation of RhB because of its low price, high efficiency, and good performance. Under visible light, N-TiO₂/rGO has a short irradiation time with a high degradation efficiency when compared to other photocatalysts. Some earlier literature suggests photocatalysts contain metallic charges, such as chromium (Cr), silver (Ag), bismuth (Bi), iron (Fe), and copper (Cu). Apart from the fact that many metals are expensive and toxic, the presence of metals in photocatalysts can result in photo-corrosion during the photocatalytic processes under visible light [58].

3.10. Kinetic Study

The kinetic study of photodegradation of RhB are analyzed using the Lagergren rate equation [59]. Within the pseudo-first-order kinetic model shown in Equation (11), qe₁ (mg/g) represents the quantity of degraded RhB according to the unit mass of photocatalysts at equilibrium time and k₁ (min⁻¹) represents the Lagergren rate constant. In the pseudo-second-order kinetic model shown in Equation (12), qe₂ (mg/g) represents the amounts of RhB degraded at the equilibrium time and k₂ (g/mg min) represents the rate constant. In addition, q_t represents the amounts of RhB degraded per unit mass of the photocatalyst at time, where t (min) represents contact time.

$$\log (qe-qt)=\log qe-\frac{\mathrm{kt}}{2.303} \left(\mathrm{Pseudo-first-order} \mathrm{kinetic} \mathrm{equation}\right)$$

(11)

$$\frac{\mathrm{t}}{qt}=\frac{1}{\mathrm{k}q{e}^2}+\frac{\mathrm{t}}{qe} \left(\mathrm{Pseudo-second-order} \mathrm{kinetic} \mathrm{equation}\right)$$

(12)

The kinetic study was conducted with 10 mg/L initial concentrations of RhB and 0.02 g amounts of N-TiO₂/rGO. The log(qe − qt) vs t plot slope is used to calculate the values of qe₁ and k₁.

Table 5. Kinetic parameters on the photodegradation of RhB.

Pseudo-First-Order Kinetic Model			Pseudo-Second-Order Kinetic Model
qe1	k1	R2	qe2	k2	R2
1.490992463 mg/g	0.000047 min−1	0.4194	10.96417229 mg/g	0.4287318 g/mg·min	0.9999

shows the kinetic parameters on the photodegradation of RhB using N-TiO₂/rGO. Based on the result, the qe values are not consistent with the experimental data. However, qe₂ and k₂ values have been calculated from the plots of t/q vs. t. The theoretical qe₂ value is in accordance with the experimental data (R² = 0.9999). It indicates that the photodegradation of RhB using N-TiO₂/rGO followed the pseudo-second-order kinetic model.

4. Conclusions

N-TiO₂/rGO was successfully synthesized by the hydrothermal method. The characterization data indicated that N-TiO₂ was doped and dispersed onto the rGO surface. N-TiO₂/rGO was found to be highly crystalline of anatase TiO₂. The modification of TiO₂ with N and rGO reduced the band gap of the photocatalyst from 3.25 to 2.86 eV. N-TiO₂/rGO had surface area, pore volume, and pore size of 53.393 m²/g, 0.096 cc/g, and 3.588 nm, respectively. The anatase phase of TiO₂ appeared at 200–500 °C with a weight loss of 2.50%. The photodegradation of RhB using N-TiO₂/rGO followed a pseudo-second-order kinetic model with the highest photodegradation percentage of 78.29% under visible light at the optimum condition in pH of 2 with a photocatalyst mass of 20 mg for 90 min.