1 Introduction

Photocatalytic technology over semiconductor is an ideal way to solve the energy problem and the environment issue using abundant solar energy [1]. In the past 40 years, a large number of studies were communicated for the photocatalysis of pollutant degradation, hydrogen production, water splitting and CO2 reduction over various photocatalysts [2,3,4,5,6]. Up to now, the photocatalysis of pollutant degradation is one of the widely attentive researches, and various hypotheses have been suggested. According to the reported studies of photodegradtion [7,8,9], it is generally accepted that the electron–hole pairs in semiconductor are generated by illuminating light with enough energy, and the photogenerated carriers then react with dissolved oxygen or water molecules to emerge with reactive oxidizing species including hydroxyl radicals (•OH) and superoxide radicals (•O2−) [10]. Meanwhile, the photogenerated holes could decompose pollutants into small molecules [11]. In the degradation studies of organic dyes [12,13,14], it cannot be ignored that the excess negative charges inside the conduction band by the dye excitation are removed by molecular oxygen forming superoxide radicals, while the proton from the dye molecule is separated generating a double bond [15, 16]. Hence, it is concluded that pollutant molecules are not only oxidized by the photogenerated holes and reactive oxidizing radicals, but also offer electrons for catalysts to form reactive oxygen species in photodegradation process.

The degradation photocatalysis, as a multistep and complex process, was affected by diversified factors including the redox ability and transfer of the photogenerated carriers and the situation of catalytic solution [17,18,19]. Among them, the influence of the hydrion in catalytic solution on the surface state of photocatalysts cannot be ignored in photocatalytic degradation [20,21,22]. For instance, Merka. et al. [23] reported that adjusting the pH value of the solution changed the surface states of Pb3Nb4O13 photocatalysts, the RhB photodegradation process and efficiency. Shun et. al. [24] proved that the degradation of RhB was greatly enhanced in the presence of acid, and the predominant reactive oxygen species of superoxide is responsible for the efficient degradation of RhB. Although the reported study has proven that the acid effect plays a positive role for photocatalysis of RhB degradation [25,26,27], the relation between the mechanism of RhB degradation and the hydrion in solution needs a detailed understanding. So far, two different mechanisms of RhB photodegradation are discussed, including the successive deethylation of the four ethyl groups and the direct degradation of the chromophoric system [28]. All degradation mechanisms of RhB were impacted by numerous conditions, such as the redox ability of photogenerated carriers, the hydrion concentration and the kinds and content of oxidizing species [23]. The formation of active oxidation radicals and species in solution was influenced by the energy band position of photocatalysts and hydrion concentration [29, 30]. Therefore, it is still urgently required to study the effect of the hydrion in solution on the interaction of the dye and the semiconductor and the formation of reactive oxidation species and the intermediates in RhB degradation.

The large surface area, appropriate band gap and chemical stability of the graphitic carbon nitride (g-C3N4) have drawn enormous attention in the photocatalysis fields [31]. Meanwhile the photodegradation performance of g-C3N4 was affected by the hydrion on the catalytic surface or in solution, depending on the previous reports [30, 32]. In the present work, the influence of pH value on the photocatalytic activity of RhB degradation over g-C3N4 catalyst was investigated. The rate of RhB degradation can be accelerated with decreasing pH values. The reactive oxidation species and intermediates in the RhB degradation process at different pH values were discovered using ESR, HPLC, LC/MS and MS measurement. We found that two decomposition processes were existed in photocatalytic RhB degradation with different acidity. The possible photocatalytic mechanism of RhB degradation over g-C3N4 was proposed on the basis of the experimental results.

2 Experimental

2.1 Photocatalyst preparation

All the reagents were of analytical grade and were used without further purification. The g-C3N4 was prepared by calcining method [33]. In the typical synthesis, 15 g urea powder was put into an alumina crucible with a cover. Then it was treated at 600 °C for 1 h with the heating rate of 10 °C/min. and the powder was collected after HNO3 solution (0.5 mol/L) washing. Then an yellow powder was obtained at 550 °C for 1 h. Finally, the fluffy sample (g-C3N4) was washed by HNO3 solution (0.1 mol/L) and ethyl alcohol and then dried at 80 °C. g-C3N4-B sample was obtained from urea by a one-step calcination at 550 °C for 2 h.

2.2 Characterization

X-ray diffraction (XRD) patterns were measured using an X’Pert PRO X-ray powder diffractometer (PANalytical, the Netherlands) with Cu-Kα radiation (λ = 1.5418 Å). The FT-IR spectra were recorded on a Nicolet Nexus spectrometer on samples embedded in KBr pellets. X-ray photoelectron spectroscopy (XPS) analysis was performed on a XSAM 800 X-ray photoelectron spectroscope (Kratos, UK) and the binding energy was calibrated on the reference C 1 s peak at 284.3 eV. UV–Vis diffuse reflectance spectrum (DRS) was obtained using a Cary 5000 UV–Vis spectrometer (Agilent Technologies, USA), using BaSO4 as reference. The electron spin resonance (ESR) spectra were recorded at the X band using a Bruker EMX-10/12 spectrometer with a 100-kHz magnetic field modulation at a microwave power level of 19.9 mW.

2.3 Photocatalysis of RhB degradation

The photocatalytic tests of RhB degradation over g-C3N4 were carried out under visible-light irradiation at room temperature. For a typical photocatalytic experiment, 0.05 g of catalyst was suspended in 500 mL RhB aqueous solution (10 mg L−1) with constant stirring. Prior to the irradiation, the suspensions were magnetically stirred in the dark for 1 h to ensure the adsorption equilibrium between catalyst and dye. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used to adjust the original pH values of the system. During the irradiation, 5 mL sample was taken out and centrifuged at given intervals. The RhB concentration was detected spectrophotometry with 552 nm wavelength. The influence of the acidity on the photocatalytic activity was studied with 300 W Xe light irradiation. The sunlight-driven photocatalytic activity was measured from Jun. 8 to Aug. 2 2019 at our lab (N35°16′59′′ E 113°55′43′′) in Xinxiang City, Henan Province, China. The sunlight light intensity reached 62.2 ~ 90.4 mW/cm [2]. The RhB photosensitization was carried out as the above similar method using Xe lamp light. The reactive oxidation species during degradation process were determined by means of scavengers and ESR technique. The concentration of RhB and intermediates were determined by high-performance liquid chromatography (HPLC) (Agilent 1100, USA) using a C18 column (200 mm × 4.6 mm, 5 μm, Agilent, USA). The identification of intermediate was conducted by LC/MS/MS (Agilient 1290LC-6540 accurate mass Q-TOF, USA) equipped with an electrospray ionization (ESI) source. The intermediates were also detected by a Thermo DSQII Trace gas chromatography interfaced with a Polaris Q ion trap mass spectrometer (GC/MS, Thermo Fisher Scientific, USA). The experiment process is detailed in Supplementary Material S1, S2 and S3.

3 Results and discussion

3.1 Characterization of g-C3N4

The XRD patterns of the investigated carbon nitride are shown in Fig. S1. All diffraction peaks of the as-prepared g-C3N4 sample were indexed as the phase of g-C3N4 (JCPDS 00–87-1526) [34], suggesting their high purity. The mainly peaks at 13.0° and 27.5° were corresponding to the typical (0 0 1) and (0 0 2) crystal planes, respectively. [35] To further explore the structure of g-C3N4 sample, the FT-IR spectrum was measured. As shown in Fig. S2, several strong bands in the region of 1200–1650 cm−1 were found corresponding to the typical stretching modes of C–N heterocycles [36], and the broad peaks at around 3000–3480 cm−1 revealed the existence of incompletely condensed secondary and primary amines [37]. Additionally, the characteristic peak at 810 cm−1 was observed, which belonged to breathing mode of the triazine units [37]. XPS measurement was conducted to identify the chemical composition and bonding configuration of the as-fabricated products. In Fig. S3, the sample was mainly composed of C, N and O elements. The high-resolution XPS spectra were deconvoluted by Gaussian-Lorenzian analysis method. For C 1 s spectrum (Fig. 1a), three peaks at 284.7, 287.8 and 285.9 eV were observed, which were attributed to sp2 C–C bonds, sp2 hybridized carbon in N-containing aromatic ring (N–C=N) and the C−NH2 species on the g-C3N4, respectively [38, 39]. In Fig. 1b, four fitted peaks were observed at around 398.1, 399.0, 400.3 and 403.9 eV, respectively. These peaks were regarded as the sp2 hybridized nitrogen involved in triazine rings (C–N=C), the tertiary nitrogen N–(C)3 groups, the free amino groups (C-N–H), and π-excitations, respectively [40]. The O 1 s peak (Fig. S4) was deconvoluted into two components at 531.9 and 533.4 eV, which were related to the adsorbed H2O molecules and hydroxyl groups on the surface, respectively [38, 41].

Fig. 1
figure 1

XPS high-resolution spectra of a C 1 s and b N 1 s of the g-C3N4 sample

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The UV–Vis diffuse reflectance spectrum of g-C3N4 was illustrated in Fig. S5. The sample exhibited an absorption edge at about 470 nm, which indicated that g-C3N4 possessed response ability for visible light. Considering g-C3N4 to be an indirect semiconductor [42, 43], the band gap (Eg) was calculated to be about 2.75 eV.

3.2 Photocatalytic performance of RhB degradation

The photodegradation activities were evaluated under visible light irradiation (λ > 420 nm, Xe lamp light). In Fig. 2a, the self-degradation effect was ignored under visible light illumination. After 2 h visible-light irradiation, the efficiency for RhB degradation reached 92.0% over g-C3N4 sample, which was distantly higher than that of g-C3N4-B sample. Based on the N2 adsorption measurement, the surface area of g-C3N4 reached 86 m2/g, which was higher than that of g-C3N4-B (51 m2/g). It meant that the double calcination promoted the surface area of photocatalyst. With decreasing pH value in the system, the RhB photodegradation rate increased gradually (Fig. 2b). In the acidic solution (pH 4.3), the efficiency of RhB degradation reached 99.1% after 15 min of illumination. Based on the above photocatalytic condition, the degradation efficiency still reached above 95% after the 8th cycling test (Fig. 2c). According to the XRD and XPS results of used sample (Fisg. S6 and S7), no obvious changes of the crystal structure and chemical state on the surface were discovered. In order to quantitatively understand the reaction kinetics of the RhB degradation, the photocatalytic process of RhB degradation followed the pseudo-first order kinetics model in our experimental condition (Fig. 2d) [23]. According to the apparent first-order linear transform, the pseudo-first order rate constant k was calculated and listed in Table S1. The result indicated a rather good correlation to the pseudo-first-order reaction kinetics (R > 0.99). The degradation rate decreased obviously with the increase of the pH values, and the rate constant k value in acidic system (pH 4.3) was about 8.6 times as much as that in neutral system (pH 7.0). It was implied that the hydrogen ion was a crucial factor in the photocatalytic degradation. Several existence forms of RhB molecules were observed in different solutions [44], and existence of RhB existed in a zwitterionic form in this study (pH 4 ~ 10). Considering the zero proton condition (ZPC) point of g-C3N4 (Fig. S8), the g-C3N4 sample showed superior adsorption ability toward RhB molecules in the solution with a high pH value (> 7.32) [45]. In addition, the adsorption process of the g-C3N4 material toward RhB was characterized by the Langmuir adsorption isotherm model (Fig. S9). The adsorption ability for RhB molecules in the solution of pH 10.0 reached 30.30 μmol/g. However, the excessive RhB molecules on the g-C3N4 surface inhibited the formation of oxygen radicals, which decreased the photodegradation efficiency. Hence, there was no certain correlation between the increase of photocatalytic performance and RhB adsorption ability over g-C3N4 photocatalyst.

Fig. 2
figure 2

Photocatalytic activity of RhB degradation over g-C3N4 using Xe lamp light(a blank test at pH 7.0, b catalytic system with different pH values, c cycling test at pH 4.3) and first-order kinetics plots for RhB photodegradation by g-C3N4 photocatalysts with different solution d

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The photocatalytic performance in acidic system (pH 4.3) was measured under natural sunlight and the results were shown in Fig. 3. After 30 min of sunlight illumination, the efficiency of RhB degradation reached about 95%. It was notable that the photocatalytic activity still maintained above 85% in the 4th cycling under sunlight.

Fig. 3
figure 3

Photocatalytic activity of RhB degradation over g-C3N4 in acidic system under sunlight

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The influence of variety anions in real water system on the photocatalytic activity was not ignored. The degradation activities with multiple additional cations were measured. As shown in Fig. 4, the photocatalytic performance of RhB degradation in acidic system never decreased with cation disturbance. It was notable that the degradation efficiency reached 85% after natural sunlight irradiation in acidic solution containing mixed cations. Hence, the acidic system plays a positive effect on photocatalytic activity for RhB degradation over g-C3N4 material.

Fig. 4
figure 4

Photocatalytic activity of RhB degradation over g-C3N4 in acidic solution (pH < 4.0) after 20 min irradiation(300 W Xe light) with different added metal-ion (0.05 mol/L). (*Added K(I), Na(I), Ca(II), Fe(III), Mg(II), Al(III), Zn(II) and Pb(II) after 30 min of sunlight irradiation)

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3.3 Photocatalytic process of RhB degradation

Figure 5 displayed the light absorption spectra of catalytic system during RhB degradation. The solution absorbance at pH values of 4.3 and 7.0 all decreased rapidly. The shift of the absorbance peak was observed at acidic system (pH 4.3) only if the decomposition was nearly completed (inset of Fig. 5a). Yet, a distinct shift (about 50 nm) occurred at pH 7.0 (inset of Fig. 5b). It implied that the hydrion in solution influenced the RhB degradation rate and decomposition process

Fig. 5
figure 5

Absorption spectra for RhB degradation in different systems (a pH 4.3 and b pH 7.0, the inset images: the changes of the maximum absorption wavelength)

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To further investigate the process of RhB degradation, HPLC measurements were carried out, and the results were presented in Fig. 6. It was noted that other peaks, except for RhB molecule, were detected before irradiation both at pH 4.3 and pH 7.0, indicating that a small portion of RhB was degraded. After visible light irradiation (Xe lamp light), the signal intensity of RhB molecule remarkably declined with prolonging irradiation time. However, four new peaks appeared, which were on behalf of N-de-ethylated intermediates. The intensities of additional peaks were enhanced and then diminished gradually, which indicated the decomposition of dye structure. Considering the results of ESI-TIC/MS measurement (Figs. S10 and S11), the peaks of a (tR 11.397 min), b (tR 8.568 min), c (tR 6.552 min), d (tR 5.074 min) and e (tR 4.076 min) were assigned to RhB, N,N,N’-tri-ethylated rhodamine (DER), N,N-di-ethylated rhodamine (DR), N-ethylated rhodamine (ER) and Rhodamine (R), respectively. Based on the above analysis, the depletion of the RhB absorbance was ascribed to the destruction of the conjugated structure. The hypsochromic shift of the absorbance peak position was due to the formation of a series of N-de-ethylated intermediates in a stepwise manner [21]. Therefore, during photodegradation process, RhB de-ethylation brought the wavelength change of its major absorption peak to move toward the blue region, which ranged from 552 nm (RhB) to 539 nm (DER), 522 nm (DR), 510 nm (ER) and 498 nm (R), respectively. According to the RhB degradation process in acidic system, it was found that the de-ethylation of RhB molecule was competitive with its cleavage [46, 47]. However, due to the distinct position shift of the absorbed peak in Fig. 5, the de-ethylation process played a main effect on the RhB degradation in neutral system.

Fig. 6
figure 6

HPLC spectra of RhB at different time of irradiation at pH 4.3 (a) and pH 7 (b)

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Based on the LC/MS spectra (Fig. S12), two chromophore cleavage intermediates, including N-(2,4-dihydroxycyclohexa-2,5-dien-1-ylidene)-N-ethylethanaminium (m/z = 182.1176) and 4-(ethylamino) benzene-1,3-diol (m/z = 153.0790), were identified after 5 min of irradiation in acidic system, while these intermediates were not detected in neutral solution. The results indicated that the de-ethylation process and chromophore cleavage happened simultaneously in the solution of pH 4.3, which enhanced the degradation efficiency. Combined with the results of reactive species, a large number of •OH in acidic system was beneficial to hydroxylation on chromophore and then lead to its cleavage. Besides, some other intermediates, such as organic acids, alkanol, ester and amine, were also identified by GC/MS and listed in Table S2. The benzene rings and 2,6-di-tert-butylphenol were produced from chromophoric cleavage in initial stages. Other detected intermediates were mostly chain compounds, which were derived from the cleavage of the xanthene ring in RhB molecule and N-de-ethylated intermediate structure by deethylation, hydroxylation and carboxylation [48]. These products were further oxidized to tetradecanoic acid, palmitic acid, stearic acid, 2,2-dihydroxyethyl palmitate, 2,3-dihydroxypropyl stearate, ethanamine, propan-1-amine, carbonic acid and propane-1,2,3-triol, which were further mineralized into CO2, H2O and NO3− [49]. On the basis of MS result (Fig. S12), the fragmentation pathway was proposed during photodegradation process. As shown in Scheme 1, the RhB molecule was degraded by the hydroxylation and de-ethylation process in the acidic solution (pH 4.3). For the neutral solution (pH 7.0), RhB molecule was decomposed by only de-ethylation reaction, and the degradation intermediates were further resolved into small molecules by deoxygenation and hydroxylation. The hydroxylation process in the acidic system promoted the photocatalytic efficiency of RhB degradation.

Scheme 1.
scheme 1

Schematic diagram for RhB degradation over g-C3N4 photocatalysts

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3.4 Photocatalytic mechanism of RhB degradation

Photocatalytic mechanism is an important factor to further enhance the degradation efficiency. The photosensitization effect and direct electron–hole oxidation are vital for the photocatalysis of dye degradation. Since the as-prepared g-C3N4 only responded to the light energy with smaller wavelength (λ < 460 nm), a single wavelength filter of 500 nm was adopted to eliminate the photo-absorption of g-C3N4, exploring the photosensitization driven. As shown in Fig. 7, the degradation efficiency of RhB under 485 ~ 515 nm light irradiation still presented a slight decrease. Therefore, the degradation process was driven by RhB excitation.

Fig. 7
figure 7

Photocatalytic activity of RhB degradation over g-C3N4 under different light wavelength ranges with the same light intensity

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Figure 8 displayed the effect of different scavengers on the photodegradation activities. In the case of neutral system (Fig. 8a, b), the degradation efficiency decreased markedly with excess addition of KI. It was indicated that the hVB+ and/or •OH species were important in the process of RhB oxidation. However, the addition of EDTA-2Na, as the scavenger of hVB+, did not distinctly inhibit the photocatalytic activity of RhB degradation. Furthermore, the addition of BQ suppressed the RhB degradation, which implied that O2−• was also the reactive specie in the degradation process. When the pH value of the solution was controlled to 4.7, similar results were achieved with addition of various scavengers (Fig. 8c, d). The above results suggested that •OH and O2−• radicals had significant effect on the process of RhB degradation.

Fig. 8
figure 8

Photocatalytic activity of RhB degradation over g-C3N4 in different solutions with pH 7.0 (a, b) and 4.3 (c, d)

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It was known that the VB edge potential of g-C3N4 (+ 1.57 eV) was negative than the standard redox potential of •OH/OH− (+ 2.38 eV vs. NHE) [50, 51]. Although •OH radicals were not formed by photogenerated holes in valence band of g-C3N4, RhB molecules were effectively decomposed, owing to the redox potentials of RhB and RhB* (0.95 and − 1.42 V vs. NHE). [22] Meanwhile, according to the calculated band gap (Eg) of g-C3N4, the CB edge potential was determined to be − 1.18 V (vs. NHE). The electrons from the excited RhB* were transferred to g-C3N4 providing a favorable driving force for the electron injection. In other words, the injected electrons caused the RhB photosensitization over g-C3N4 under 500 nm light irradiation. Additionally, the CB potential was more negative than the standard redox potential of O2/O2−• (− 0.33 V vs NHE) [46]. A large quantity of O2−• species were produced from dissolved oxygen by the electrons in CB of g-C3N4. It was notable that O2−• radicals were apt to react with protons to form various active oxidizing species including the peroxy radical (HOO•) and hydrogen peroxide (H2O2), especially in acidic system. Therefore, the increasing content of hydrion promoted the photocatalytic performance of RhB degradation.

ESR characterization was employed to further investigate the effect of •OH and O2−•, and the results were shown in Fig. 9. DMPO-containing aqueous suspension of g-C3N4 displayed an obvious 1:2:2:1 quartet signal and a 1:1:1:1:1:1 sixfold signal after irradiation (Xe lamp light) which were the characteristics of DMPO-OH• and DMPO-O2−• [52, 53], respectively. It was indicated that OH• and O2−• were formed under irradiation.

Fig. 9
figure 9

ESR spectra for the DMPO-containing aqueous suspensions of g-C3N4 in solutions with different pH values after 10 min of visible light irradiation

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In the acidic system (pH 4.3), the characteristic peaks of DMPO-OH• were observed, while the peaks assigned to DMPO-O2−• nearly disappeared. It was possible that abundant protons in the acidic system reacted smoothly with O2−• to generate a series of species including HOO• and •OH, [54] as shown in Eq. (1)–(4). Therefore, a large quantity of •OH was produced, and •OH radicals became the main active specie in acidic system (pH 4.3). Based on the band gap structures of g-C3N4 and RhB, combining the effects of scavengers and ESR analysis, the possible mechanism was proposed for the photodegradation of RhB over g-C3N4 in solutions with different pH values as follows:

$${\text{O}}_{{2}} + {\text{ e }} \to {\text{ O}}_{{2}}^{ - } \bullet$$
(1)
$${\text{O}}_{{2}}^{ - } \bullet \, + {\text{ H}}^{ + } \to {\text{ HOO }} \bullet$$
(2)
$${\text{2 HOO}} \bullet \, \to {\text{ O}}_{{2}} + {\text{ H}}_{{2}} {\text{O}}_{{2}}$$
(3)
$${\text{H}}_{{2}} {\text{O}}_{{2}} + {\text{ O}}_{{2}}^{ - } \bullet \, \to {\text{ OH}}^{ - } + \, \bullet {\text{OH }} + {\text{ O}}_{{2}}$$
(4)
$${\text{g}} - {\text{C}}_{{3}} {\text{N}}_{{4}} + hv(\lambda > { 4}00{\text{ nm}}) \, \to e_{{{\text{CB}}}}^{ - } + h_{{{\text{VB}}}}^{ + } \left( {{\text{g}} - {\text{C}}_{{3}} {\text{N}}_{{4}} } \right)$$
(5)
$${\text{RhB}} + hv(\lambda > { 4}00{\text{ nm}}) \, \to {\text{ RhB}}^{\ast +} + {\text{ e}}^{ - } \left( {{\text{RhB}}} \right)$$
(6)
$$e^{ - } \left( {{\text{RhB}}} \right) \, \to e_{{{\text{CB}}}}^{ - } \left( {{\text{g}} - {\text{C}}_{{3}} {\text{N}}_{{4}} } \right)$$
(7)
$$e_{{{\text{CB}}}}^{ - } + {\text{ O}}_{{2}} \to {\text{ g}} - {\text{C}}_{{3}} {\text{N}}_{{4}} + {\text{ O}}_{{2}}^{ - } \bullet$$
(8)
$${\text{pH }} = { 7},{\text{ RhB}}^{\ast + } /{\text{RhB }} + \, \bullet {\text{OH}}/{\text{O}}_{{2}}^{ - } \bullet /h_{{{\text{VB}}}}^{ + } \to {\text{ degradation products}}$$
(9)
$${\text{pH }} \le { 4}.{3},{\text{ O}}_{{2}}^{ - } \bullet \, + {\text{ H}}^{ + } \to {\text{ HOO}} \bullet$$
(10)
$${\text{2 HOO}} \bullet \, + {\text{O}}_{{2}}^{ - } \bullet \, \to {\text{ OH}}^{ - } + \, \bullet {\text{OH }} + {\text{ 2 O}}_{{2}}$$
(11)
$${\text{RhB}}^{\ast + } /{\text{RhB }} + \, \bullet {\text{OH}}/h_{{{\text{VB}}}}^{ + } \to {\text{ degradation products}}$$
(12)

4 Conclusions

In general, hydrion played a key role for photocatalytic RhB degradation over g-C3N4. The photocatalytic activity of g-C3N4 was increased gradually with decreasing pH values of catalytic solution, and the efficiency of RhB degradation reached nearly 95.0% within 20 min after 8 cycles in acidic solution (pH < 4.3). ·OH was induced by the reaction between hydrion and O2−· radical, and the RhB degradation occurred simultaneously via the successive deethylation of four ethyl groups and the direct degradation of the chromophoric system. Various oxidizing species and intermediates in the above two degradation processes were discovered by ESR, GC/MS and LC/MS measurements. In addition, the photosensitization efficiently promoted the photocatalytic activity. The investigation of RhB degradation system could establish a foundation for deeply understanding the degradation process of organic dyes.