Spent Coffee Grounds Derived Carbon Loading C, N Doped TiO2 for Photocatalytic Degradation of Organic Dyes

Titanium dioxide (TiO2) is an ideal photocatalyst candidate due to its high activity, low toxicity and cost, and high chemical stability. However, its practical application in photocatalysis is seriously hindered by the wide band gap energy of TiO2 and the prone recombination of electron-hole pairs. In this study, C, N doped TiO2 were supported on spent coffee grounds-derived carbon (ACG) via in situ formation, which was denoted as C, N–TiO2@ACG. The obtained C, N–TiO2@ACG exhibits increased light absorption efficiency with the band gap energy decreasing from 3.31 eV of TiO2 to 2.34 eV, a higher specific surface area of 145.8 m2/g, and reduced recombination rates attributed to the synergistic effect of a spent coffee grounds-derived carbon substrate and C, N doping. Consequently, the optimal 1:1 C, N–TiO2@ACG delivers considerable photocatalytic activity with degradation efficiencies for methylene blue (MB) reaching 96.9% within 45 min, as well as a high reaction rate of 0.06348 min−1, approximately 4.66 times that of TiO2 (0.01361 min−1). Furthermore, it also demonstrated greatly enhanced photocatalytic efficiency towards methyl orange (MO) in the presence of MB compared with a single MO solution. This work provides a feasible and universal strategy of synchronous introducing nonmetal doping and biomass-derived carbon substrates to promote the photocatalytic performance of TiO2 for the degradation of organic dyes.


Introduction
During recent decades, the environmental pollution issue, especially the water pollution issue, has emerged as an important concern with a significant impact on humans' daily lives [1]. In particular, organic dyes, which possess a complex molecular structure and good chemical stability, are challenging to biodegrade naturally [2,3]. Plentiful technologies including adsorption [4,5], biodegradation [6,7], and membrane [8], etc. have been utilized to separate or remove organic pollutants from wastewater, but cannot degrade them thoroughly and eventually make them enriched in living organisms [9,10]. Therefore, it is critically important to develop a highly efficient and eco-friendly strategy to solve these environmental issues.
In this context, photocatalytic degradation is regarded as an emerging and green technology that converts renewable, clean, and freely available solar energy to chemical energy to degrade pollutions into more benign species without secondary pollution [11][12][13][14][15][16]. As one of the most promising photocatalysts, titanium dioxide (TiO 2 ) has drawn substantial attention due to its high activity, low toxicity and cost, and high chemical stability [11,[16][17][18][19][20]. However, there are two main intrinsic shortcomings of TiO 2 : one is its relatively wide band gap (about 3.2 eV), meaning that only UV light (less than 5% of the total solar spectrum) can be absorbed and efficiently utilized [21]; the other is the high recombination rate of 4 h for activation with the ratio of KOH:coffee grounds = 1:1, followed by being dried at 80 • C for 8 h and carbonized again under the same conditions. Finally, the obtained coffee grounds-derived carbon was washed with deionized water and dried, which is denoted as ACG.

Preparation of C, N-TiO 2 @ACG
As illustrated in Figure 1, first, 10 mL of tetrabutyl titanate solution was added dropwise to 50 mL of absolute ethanol and stirred for 30 min to obtain a homogeneous solution (solution A). Next, 3 mL of deionized water, 3 mL of anhydrous ethanol, and 1 mL of 65% nitric acid solution were mixed thoroughly to produce solution B, which was slowly dropped into solution A. At this time, 1 g of ACG was slowly added to the mixed solution and magnetically stirred for 6 h to obtain the gel. After drying and grinding, the powder was calcined at 450 • C for 2 h in an N 2 atmosphere. The obtained photocatalysts with the mass ratios of TBOT to ACG at 1:0.5, 1:1 and 1:2 were labeled as 1:0.5 C, N-TiO 2 @ACG, 1:1 C, N-TiO 2 @ACG and 1:2 C, N-TiO 2 @ACG, respectively. For comparison, C, N-TiO 2 was also prepared under the same conditions without adding ACG. The commercial nano-TiO 2 was labeled TiO 2 .

Preparation of Coffee Grounds-Derived Carbon
The coffee grounds were smashed and sieved discretely. They were then soaked in 0.2 M dilute hydrochloric acid for 4 h, washed with ethanol and deionized water to neutral, and dried under a vacuum at 60 °C for 24 h. Subsequently, the coffee grounds were precarbonized at 1000 °C for 2 h in an N2 atmosphere, soaked in 60 g/L KOH solution for 4 h for activation with the ratio of KOH: coffee grounds = 1:1, followed by being dried at 80 °C for 8 h and carbonized again under the same conditions. Finally, the obtained coffee grounds-derived carbon was washed with deionized water and dried, which is denoted as ACG.

Preparation of C, N-TiO2@ACG
As illustrated in Figure 1, first, 10 mL of tetrabutyl titanate solution was added dropwise to 50 mL of absolute ethanol and stirred for 30 min to obtain a homogeneous solution (solution A). Next, 3 mL of deionized water, 3 mL of anhydrous ethanol, and 1 mL of 65% nitric acid solution were mixed thoroughly to produce solution B, which was slowly dropped into solution A. At this time, 1 g of ACG was slowly added to the mixed solution and magnetically stirred for 6 h to obtain the gel. After drying and grinding, the powder was calcined at 450 °C for 2 h in an N2 atmosphere. The obtained photocatalysts with the mass ratios of TBOT to ACG at 1:0.5, 1:1 and 1:2 were labeled as 1:0.5 C, N-TiO2@ACG, 1:1 C, N-TiO2@ACG and 1:2 C, N-TiO2@ACG, respectively. For comparison, C, N-TiO2 was also prepared under the same conditions without adding ACG. The commercial nano-TiO2 was labeled TiO2.

Materials Characterizations
The crystal structure was characterized by X-ray diffraction (Shimadzu XRD-7000, Kyoto, Japan) with Cu Kα radiation (λ = 0.15418 nm). The morphologies were observed on a field emission SEM (Hitachi S-450, Japan) with an accelerating voltage of 15 kV. The Raman spectra of the photocatalysts were analyzed using a Raman spectrometer (Shanghai Precision Instrument Co., Ltd. GS1000, Shanghai, China). The Brunauer-Emmett-Teller (BET) technique was used to assess the specific surface areas and porosities of the

Materials Characterizations
The crystal structure was characterized by X-ray diffraction (Shimadzu XRD-7000, Kyoto, Japan) with Cu Kα radiation (λ = 0.15418 nm). The morphologies were observed on a field emission SEM (Hitachi S-450, Japan) with an accelerating voltage of 15 kV. The Raman spectra of the photocatalysts were analyzed using a Raman spectrometer (Shanghai Precision Instrument Co., Ltd. GS1000, Shanghai, China). The Brunauer-Emmett-Teller (BET) technique was used to assess the specific surface areas and porosities of the photocatalysts on a surface area analyzer (MicrotracBel mike2020, Osaka, Japan). The electrochemical impedance spectroscopy (EIS) was tested on an electrochemical analyzer (Chenhua Instruments Company CHI 760B, Shanghai, China) in the frequency range of 0.01-100,000 Hz. The ultraviolet-visible diffuse reflectance spectra (DRS) were obtained with a UV-vis diffuse reflectance spectrophotometer (HITACHI U3310, Oita, Japan). Photoluminescence (PL) spectra of the photocatalysts were conducted using a fluorescence spectrophotometer (RF-6000) at the excitation wavelength of 380 nm.

Photocatalytic Degradation
The photocatalytic degradation reaction was implemented with MB and MO solution as the organic pollutants irradiated vertically by a 300-W xenon lamp (PLS-SXE300/300UV xenon lamp (Perfect Light Co., Beijing, China)). Typically, 100 mg of the photocatalyst was dispersed in 100 mL of 20 mg/L organic pollutant solution (MB or MO solution). Before irradiation, the suspension was stirred for 30 min in the dark to achieve adsorption equilibrium. When degrading the mixed solution of MB and MO, 50 mL of 20 mg/L aqueous solution of MB and MO was mixed and 50 mg of photocatalyst was added for photocatalytic degradation. At regular time intervals (15 min), 1.0 mL suspension was withdrawn and analyzed by UV-vis spectrophotometer. The degradation rate (Dr) of the organic pollutant (MB or MO) is calculated in Equation (1) as follows: where C 0 and C refer to the initial contaminant concentration and the contaminant concentration at the test time, respectively.

Results and Discussion
The XRD patterns shown in Figure  These results indicate that the structure is not changed by the addition of ACG. For ACG, the broad diffraction peaks appearing at 25.6 • and 43.4 • correspond to the (002) and (001) crystal planes of graphitized carbon. For the C, N-TiO 2 @ACG, the XRD patterns are largely consistent with that of TiO 2 . When the ratio of ACG is low, no obvious diffraction peaks of carbon can be observed due to its low crystallinity and its surface covering by TiO 2 particles [20]. With the increase in ACG, the diffraction peaks belonging to carbon gradually emerge and become obvious, as marked in 1:2 C, N-TiO 2 @ACG with a red circle, demonstrating the successful recombination of TiO 2 and ACG.  The morphologies of photocatalysts were characterized by SEM and are shown in Figure 3. The SEM image of C, N-TiO2 in Figure 3a shows that C, N-TiO2 appears as nanosized particles with a slight agglomeration, which is caused by the van der Waals force between particles [59]. As shown in Figures 3b and S1a,b, C, N-TiO2@ACG exhibits a wellconstructed macroscale porous architecture, which is naturally derived from coffee grounds and the different amounts of C, N-TiO2 particles that are uniformly loaded on the surface of the carbon skeleton of the ACG with the change in ratio, as for 1:0.5 C, N-TiO2@ACG, 1:1 C, N-TiO2@ACG and 1:2 C, N-TiO2@ACG; this can be further demon- The Raman spectra of TiO 2 , C, N-TiO 2 and 1:1 C, N-TiO 2 @ACG composite catalysts are shown in Figure 2b. The characteristic peaks at 144, 399, 515 and 639 cm −1 in all samples belong to TiO 2 , which is consistent with the XRD results. Furthermore, two reflections located at 1358 cm −1 and 1603 cm −1 in 1:1 C, N-TiO 2 @ACG are assigned to the D (disordered carbon) and G (graphitized carbon) bands of the carbon from the ACG. The calculated I D /I G intensity ratio was 0.9, confirming the high graphitization degree of the ACG, which means that such highly graphitized carbon possesses excellent electronic conductivity and can enhance the separation efficiency of carriers in photocatalytic reactions. Furthermore, the weaker intensity of the characteristic peaks of carbon is probably associated with the loading of TiO 2 on the ACG.
The morphologies of photocatalysts were characterized by SEM and are shown in Figure 3. The SEM image of C, N-TiO 2 in Figure 3a shows that C, N-TiO 2 appears as nano-sized particles with a slight agglomeration, which is caused by the van der Waals force between particles [59]. As shown in Figure 3b and Figure S1a,b, C, N-TiO 2 @ACG exhibits a well-constructed macroscale porous architecture, which is naturally derived from coffee grounds and the different amounts of C, N-TiO 2 particles that are uniformly loaded on the surface of the carbon skeleton of the ACG with the change in ratio, as for 1:0.5 C, N-TiO 2 @ACG, 1:1 C, N-TiO 2 @ACG and 1:2 C, N-TiO 2 @ACG; this can be further demonstrated by the high-magnification image of 1:1 C, N-TiO 2 @ACG illustrated in the inset of Figure 3b. This unique natural porous structure is conducive to the increase in the specific surface area of the catalyst, enhancing its adsorption of organic pollutants, and thus improving the photocatalytic performance. Element mapping ( Figure 3c) shows that Ti, O, C and N were evenly distributed in C, N-TiO 2 , indicating that C and N elements were successfully doped into TiO 2 . For 1:1 C, N-TiO 2 @ACG, Ti, O, C, and N elements were evenly distributed ( Figure 3d) and specifically, Ti, O, and N were distributed in the same areas, indicating that C, N-TiO 2 was successfully loaded on the porous skeleton of coffee grounds-derived carbon. The morphologies of photocatalysts were characterized by SEM and are shown in Figure 3. The SEM image of C, N-TiO2 in Figure 3a shows that C, N-TiO2 appears as nanosized particles with a slight agglomeration, which is caused by the van der Waals force between particles [59]. As shown in Figures 3b and S1a,b, C, N-TiO2@ACG exhibits a wellconstructed macroscale porous architecture, which is naturally derived from coffee grounds and the different amounts of C, N-TiO2 particles that are uniformly loaded on the surface of the carbon skeleton of the ACG with the change in ratio, as for 1:0.5 C, N-TiO2@ACG, 1:1 C, N-TiO2@ACG and 1:2 C, N-TiO2@ACG; this can be further demonstrated by the high-magnification image of 1:1 C, N-TiO2@ACG illustrated in the inset of Figure 3b. This unique natural porous structure is conducive to the increase in the specific surface area of the catalyst, enhancing its adsorption of organic pollutants, and thus improving the photocatalytic performance. Element mapping ( Figure 3c) shows that Ti, O, C and N were evenly distributed in C, N-TiO2, indicating that C and N elements were successfully doped into TiO2. For 1:1 C, N-TiO2@ACG, Ti, O, C, and N elements were evenly distributed ( Figure 3d) and specifically, Ti, O, and N were distributed in the same areas, indicating that C, N-TiO2 was successfully loaded on the porous skeleton of coffee grounds-derived carbon. Considering that the photocatalytic reaction occurs on the surface of the photocatalysts, the specific surface area (SSA) is a very significant parameter, which will influence the active site of the photocatalyst and its contact with pollutants, thus affecting the photocatalytic degradation efficiency. The N 2 adsorption-desorption isotherms shown in Figure 4 present the fact that TiO 2 and 1:1 C, N-TiO 2 @ACG exhibit type-IV isotherms, implying the existence of mesopores, which can also be directly observed from the inserted pore size distribution diagram. Compared with TiO 2 , 1:1 C, N-TiO 2 @ACG has significantly more micro-and mesopores within 10 nm, which will increase the specific surface area of the catalyst. Accordingly, the calculated SSA of the TiO 2 and 1:1 C, N-TiO 2 @ACG is 78.7 and 145.8 m 2 /g. The loose porous structure and increased SSA of 1:1 C, N-TiO 2 @ACG will facilitate the contact and adsorption, together with the subsequent photocatalytic degradation of organic pollutants.
Considering that the photocatalytic reaction occurs on the surface of the photocatalysts, the specific surface area (SSA) is a very significant parameter, which will influence the active site of the photocatalyst and its contact with pollutants, thus affecting the photocatalytic degradation efficiency. The N2 adsorption-desorption isotherms shown in Figure 4 present the fact that TiO2 and 1:1 C, N-TiO2@ACG exhibit type-IV isotherms, implying the existence of mesopores, which can also be directly observed from the inserted pore size distribution diagram. Compared with TiO2, 1:1 C, N-TiO2@ACG has significantly more micro-and mesopores within 10 nm, which will increase the specific surface area of the catalyst. Accordingly, the calculated SSA of the TiO2 and 1:1 C, N-TiO2@ACG is 78.7 and 145.8 m 2 /g. The loose porous structure and increased SSA of 1:1 C, N-TiO2@ACG will facilitate the contact and adsorption, together with the subsequent photocatalytic degradation of organic pollutants. The DRS were investigated to intuitively explore the light absorption properties and band structure of the photocatalysts and are shown in Figure 5a. TiO2 displays strong UV light absorption but is bare of any visible-light absorption. Notably, C, N-TiO2@ACG exhibits a redshift, showing a broadened spectral response range and indiscriminate absorption in both visible-light and UV-light regions, which is attributed to the C, N doping and its loading on coffee grounds-derived carbon, although the absorption intensity of UV light is inferior to that of TiO2. Figure 5b shows the corresponding band gap energies (Eg) calculated by the Kubelka-Munk method for TiO2 and 1:1 C, N-TiO2@ACG, which are about 3.31 and 2.34 eV, respectively. The greatly reduced Eg is because that C, N co-doping introduces impurity energy levels above the valence band of TiO2, narrows the band gap, and expands the response range of the catalyst to visible light [60]. The DRS were investigated to intuitively explore the light absorption properties and band structure of the photocatalysts and are shown in Figure 5a. TiO 2 displays strong UV light absorption but is bare of any visible-light absorption. Notably, C, N-TiO 2 @ACG exhibits a redshift, showing a broadened spectral response range and indiscriminate absorption in both visible-light and UV-light regions, which is attributed to the C, N doping and its loading on coffee grounds-derived carbon, although the absorption intensity of UV light is inferior to that of TiO 2 . Figure 5b shows the corresponding band gap energies (Eg) calculated by the Kubelka-Munk method for TiO 2 and 1:1 C, N-TiO 2 @ACG, which are about 3.31 and 2.34 eV, respectively. The greatly reduced Eg is because that C, N co-doping introduces impurity energy levels above the valence band of TiO 2 , narrows the band gap, and expands the response range of the catalyst to visible light [60]. EIS was employed to identify the charge transportation dynamics of as-prepared photocatalysts and the obtained results are shown in Figure 6. Among them, 1:1 C, N-TiO2@ACG has the smallest Nyquist semicircle diameter, namely the lowest charge transfer resistance, indicating that the photogenerated electrons can be separated and transferred rapidly and that their recombination can be suppressed, thus increasing the effective carrier number and improving the photocatalytic activity. The PL spectrum shown in Figure 6b demonstrates an emission peak at about 450 nm for both TiO2 and 1:1 C, N-TiO2@ACG, which is consistent with previously reported TiO2-based photocatalysts [11], EIS was employed to identify the charge transportation dynamics of as-prepared photocatalysts and the obtained results are shown in Figure 6. Among them, 1:1 C, N-TiO 2 @ACG has the smallest Nyquist semicircle diameter, namely the lowest charge transfer resistance, indicating that the photogenerated electrons can be separated and transferred rapidly and that their recombination can be suppressed, thus increasing the effective carrier number and improving the photocatalytic activity. The PL spectrum shown in Figure 6b demonstrates an emission peak at about 450 nm for both TiO 2 and 1:1 C, N-TiO 2 @ACG, which is consistent with previously reported TiO 2 -based photocatalysts [11], and the PL intensity of 1:1 C, N-TiO 2 @ACG was distinctly lower than that of TiO 2 , indicating the efficiently promoted separation of photo-generated electron-holes pairs, thereby also foreboding the enhanced photocatalytic activity of 1:1 C, N-TiO 2 @ACG. EIS was employed to identify the charge transportation dynamics of as-prepared photocatalysts and the obtained results are shown in Figure 6. Among them, 1:1 C, N-TiO2@ACG has the smallest Nyquist semicircle diameter, namely the lowest charge transfer resistance, indicating that the photogenerated electrons can be separated and transferred rapidly and that their recombination can be suppressed, thus increasing the effective carrier number and improving the photocatalytic activity. The PL spectrum shown in Figure 6b demonstrates an emission peak at about 450 nm for both TiO2 and 1:1 C, N-TiO2@ACG, which is consistent with previously reported TiO2-based photocatalysts [11], and the PL intensity of 1:1 C, N-TiO2@ACG was distinctly lower than that of TiO2, indicating the efficiently promoted separation of photo-generated electron-holes pairs, thereby also foreboding the enhanced photocatalytic activity of 1:1 C, N-TiO2@ACG. The photocatalytic degradation activities of as-prepared photocatalysts were investigated towards the MB under the stirring rate of 300 rpm (to simulate the actual industrial wastewater treatment), and the results are shown in Figure 7. The C/C0 at the time of 0 demonstrate that with the increase in ACG, the adsorption of the MB increases, directly proving the porosity and absorbability of the ACG. The characteristic absorption peak of the MB is located at the wavelength between 400 and 800 nm. After irradiation for 45 min, the photocatalytic properties follow the order of TiO2 (57.8%) < C, N-TiO2 (65.5%) < 1:0.5 C, N-TiO2@ACG (64.4%) < 1:2 C, N-TiO2@ACG (85.9%) < 1:1 C, N-TiO2@ACG (96.9%) (as shown in Figure 7a). To quantitatively characterize the degradation kinetics of the MB by different photocatalysts, the kinetic plots (ln (C/C0)) as a function of irradiation time were outlined and present a good linear fitting (Figure 7b). k of 1:1 C, N-TiO2@ACG is as high The photocatalytic degradation activities of as-prepared photocatalysts were investigated towards the MB under the stirring rate of 300 rpm (to simulate the actual industrial wastewater treatment), and the results are shown in Figure 7. The C/C 0 at the time of 0 demonstrate that with the increase in ACG, the adsorption of the MB increases, directly proving the porosity and absorbability of the ACG. The characteristic absorption peak of the MB is located at the wavelength between 400 and 800 nm. After irradiation for 45 min, the photocatalytic properties follow the order of TiO 2 (57.8%) < C, N-TiO 2 (65.5%) < 1:0.5 C, N-TiO 2 @ACG (64.4%) < 1:2 C, N-TiO 2 @ACG (85.9%) < 1:1 C, N-TiO 2 @ACG (96.9%) (as shown in Figure 7a). To quantitatively characterize the degradation kinetics of the MB by different photocatalysts, the kinetic plots (ln (C/C 0 )) as a function of irradiation time were outlined and present a good linear fitting (Figure 7b). k of 1:1 C, N-TiO 2 @ACG is as high as 0.06348 min −1 , which is approximately 4.66, 4.08, 2.15 and 2.08 times that of TiO 2 (0.01361 min −1 ), C, N-TiO 2 (0.01557 min −1 ), 1:0.5 C, N-TiO 2 @ACG (0.02947 min −1 ) and 1:2 C, N-TiO 2 @ACG (0.03054 min −1 ). These results indicate that C, N doping can partially enhance the photocatalytic performances, and for introducing the ACG substrate, too much ACG brings in more adsorption but meanwhile fewer photocatalysts, and too much TiO 2 will cause agglomeration, thus affecting the catalytic ability. As a consequence, 1:1 C, N-TiO 2 @ACG demonstrates the optimal photocatalytic degradation capability towards the MB, which is close to or superior to the reported results, as shown in Table 1. (0.01361 min −1 ), C, N-TiO2 (0.01557 min −1 ), 1:0.5 C, N-TiO2@ACG (0.02947 min −1 ) and 1:2 C, N-TiO2@ACG (0.03054 min −1 ). These results indicate that C, N doping can partially enhance the photocatalytic performances, and for introducing the ACG substrate, too much ACG brings in more adsorption but meanwhile fewer photocatalysts, and too much TiO2 will cause agglomeration, thus affecting the catalytic ability. As a consequence, 1:1 C, N-TiO2@ACG demonstrates the optimal photocatalytic degradation capability towards the MB, which is close to or superior to the reported results, as shown in Table 1.

This work
The photocatalytic degradation of 1:1 C, N-TiO2@ACG towards the MO was conducted to test its catalytic ability for different dyes under simulated visible light. As shown in Figure 8a, 1:1 C, N-TiO2@ACG also exhibits good adsorption of the MO, and the degradation rate of the MO was 65% within 40 min. For the mixed solution of the MB and MO, the degradation effect of 1:1 C, N-TiO2@ACG photocatalyst was greatly enhanced, and the degradation rate of the MO already reached 90% at 20 min and more than 93% within 40 min (Figure 8b,c). The photocatalytic degradation rate k for the MO in the mixed solution was 0.09466 min −1 , 9.97 times higher than that (0.00949 min −1 ) of the MO solution ( Figure 8d). This greatly enhanced photocatalytic efficiency towards the MO in the presence of the MB is consistent with previous literature [67,68] and also indicates a possible competition between different dyes during the catalytic procedure and the mutual influence (promotion or suppression) dyes have on each other's degradation rate depending on the specific dye. Meanwhile, 1:1 C, N-TiO2@ACG @ACG also had a good removal effect on the MB from mixed dye solutions, with 92% MB removal in 45 min and a photocatalytic rate of k = 0.03595 min −1 . After 45 min of photocatalytic degradation by 1:1 C, N-TiO2@ACG, the mixed solution became obviously clear (as shown in the inset in Figure   Figure 7. (a) the photocatalytic degradation rate towards MB within 45 min (b) the corresponding fitted photocatalytic kinetic curves (−ln (C/C 0 ) = kt) of photocatalysts.
The photocatalytic degradation of 1:1 C, N-TiO 2 @ACG towards the MO was conducted to test its catalytic ability for different dyes under simulated visible light. As shown in Figure 8a, 1:1 C, N-TiO 2 @ACG also exhibits good adsorption of the MO, and the degradation rate of the MO was 65% within 40 min. For the mixed solution of the MB and MO, the degradation effect of 1:1 C, N-TiO 2 @ACG photocatalyst was greatly enhanced, and the degradation rate of the MO already reached 90% at 20 min and more than 93% within 40 min (Figure 8b,c). The photocatalytic degradation rate k for the MO in the mixed solution was 0.09466 min −1 , 9.97 times higher than that (0.00949 min −1 ) of the MO solution (Figure 8d). This greatly enhanced photocatalytic efficiency towards the MO in the presence of the MB is consistent with previous literature [67,68] and also indicates a possible competition between different dyes during the catalytic procedure and the mutual influence (promotion or suppression) dyes have on each other's degradation rate depending on the specific dye. Meanwhile, 1:1 C, N-TiO 2 @ACG @ACG also had a good removal effect on the MB from mixed dye solutions, with 92% MB removal in 45 min and a photocatalytic rate of k = 0.03595 min −1 . After 45 min of photocatalytic degradation by 1:1 C, N-TiO 2 @ACG, the mixed solution became obviously clear (as shown in the inset in Figure 8b), indicating that 1:1 C, N-TiO 2 @ACG demonstrated excellent photocatalytic capability for both single and mixed dye solutions.
The photocatalytic degradation of 1:1 C, N-TiO 2 @ACG towards the MB was operated under different pH values as shown in Figure 9a with the HCl and NaOH solution to adjust the pH value. The MB can remain stable at different pH ranges, especially in acidic conditions, possibly due to its cationic characteristics. At the pH value of less than 6.4, the degradation rate of the MB decreased significantly, being 60% at a pH of 2.5. The degradation rate became higher when the pH increased, and the MB was completely degraded within 30 min at pH = 10, and within 20 min at pH = 11.7. This trend is consistent with that of previous literature [69] and is possibly due to more hydroxyl (•OH) active radicals being generated and a strong attraction between the cationic MB and the photocatalyst under strong alkaline conditions, resulting in the enhanced degradation effect. The stability of 1:1 C, N-TiO 2 @ACG was examined through three successive cycling experiments of MB degradation. As presented in Figure 9b, 1:1 C, N-TiO 2 @ACG still maintained an over 80% removal rate for MB degradation after four recycles, demonstrating its excellent reusability and practicability.  The photocatalytic degradation of 1:1 C, N-TiO2@ACG towards the MB was operated under different pH values as shown in Figure 9a with the HCl and NaOH solution to adjust the pH value. The MB can remain stable at different pH ranges, especially in acidic conditions, possibly due to its cationic characteristics. At the pH value of less than 6.4, the degradation rate of the MB decreased significantly, being 60% at a pH of 2.5. The degradation rate became higher when the pH increased, and the MB was completely degraded within 30 min at pH = 10, and within 20 min at pH = 11.7. This trend is consistent with that of previous literature [69] and is possibly due to more hydroxyl (•OH) active radicals being generated and a strong attraction between the cationic MB and the photocatalyst under strong alkaline conditions, resulting in the enhanced degradation effect. The stability of 1:1 C, N-TiO2@ACG was examined through three successive cycling experiments of MB degradation. As presented in Figure 9b, 1:1 C, N-TiO2@ACG still maintained an over 80% removal rate for MB degradation after four recycles, demonstrating its excellent reusability and practicability. To better understand the possible photocatalytic degradation mechanisms, free radical trapping experiments were performed. Herein, silver nitrate (AgNO3), isopropanol (IPA), ethylene diamine tetraacetic acid (EDTA) and benzoquinone (BQ) were added as specific trappers of e − , •OH, h + and •O 2− , respectively and the corresponding results are illustrated in Figure 10. As can be seen, the presence of all trappers reduced the degradation efficiency, and the addition of BQ and EDTA greatly inhibited the photocatalytic activity, indicating that •O 2− and h + radicals play a major role in the photocatalytic degradation process. To better understand the possible photocatalytic degradation mechanisms, free radical trapping experiments were performed. Herein, silver nitrate (AgNO 3 ), isopropanol (IPA), ethylene diamine tetraacetic acid (EDTA) and benzoquinone (BQ) were added as specific trappers of e − , •OH, h + and •O 2− , respectively and the corresponding results are illustrated in Figure 10. As can be seen, the presence of all trappers reduced the degradation efficiency, and the addition of BQ and EDTA greatly inhibited the photocatalytic activity, indicating that •O 2− and h + radicals play a major role in the photocatalytic degradation process.
ical trapping experiments were performed. Herein, silver nitrate (AgNO3), isopropanol (IPA), ethylene diamine tetraacetic acid (EDTA) and benzoquinone (BQ) were added as specific trappers of e − , •OH, h + and •O 2− , respectively and the corresponding results are illustrated in Figure 10. As can be seen, the presence of all trappers reduced the degradation efficiency, and the addition of BQ and EDTA greatly inhibited the photocatalytic activity, indicating that •O 2− and h + radicals play a major role in the photocatalytic degradation process. Based on the aforementioned analyses, a possible degradation mechanism of 1:1 C, N-TiO2@ACG towards organic dyes was proposed, as shown in Figure 11. ACG possesses a porous structure and increases the specific surface area, which facilitates light absorbance and contact with pollution. Furthermore, 1:1 C, N-TiO2@ACG has a narrowed band gap. After irradiation, the electrons in the valence band (VB) of TiO2 will absorb photon energy from solar radiation and electron (e − ) hole (h + ) pairs in the CB and VB are excited, respectively. Normally, e − and h + will readily recombine, leading to low activity. Here, ACG can transfer the photogenerated electrons and suppress the recombination rate. e − can easily reduce the dissolved oxygen in water to form •O2 − , and accordingly, holes can also oxidize OH − and H2O molecules adsorbed on the surface of TiO2 to produce •OH. Based on the aforementioned analyses, a possible degradation mechanism of 1:1 C, N-TiO 2 @ACG towards organic dyes was proposed, as shown in Figure 11. ACG possesses a porous structure and increases the specific surface area, which facilitates light absorbance and contact with pollution. Furthermore, 1:1 C, N-TiO 2 @ACG has a narrowed band gap. After irradiation, the electrons in the valence band (VB) of TiO 2 will absorb photon energy from solar radiation and electron (e − ) hole (h + ) pairs in the CB and VB are excited, respectively. Normally, e − and h + will readily recombine, leading to low activity. Here, ACG can transfer the photogenerated electrons and suppress the recombination rate. e − can easily reduce the dissolved oxygen in water to form •O 2 − , and accordingly, holes can also oxidize OH − and H 2 O molecules adsorbed on the surface of TiO 2 to produce •OH. Therefore, under the co-function of h + , •O2 − and •OH, organic dyes such as MB and MO molecules were degraded to CO2 and H2O.

Conclusions
In summary, spent coffee grounds-derived carbon that supported C, N doped TiO2 (C, N-TiO2@ACG) composite photocatalysts were prepared. Herein, spent coffee grounds-derived carbon can provide attachment sites for TiO2, increase contact with pollutants, and suppress the recombination of electron-hole pairs. Furthermore, C, N co-doping extends the light absorption region of TiO2 to the visible region, which greatly improves the photocatalytic activity. Correspondingly, the band gap energy of C, N- Figure 11. Schematic illustration of the proposed photocatalytic mechanism of 1:1 C, N-TiO 2 @ACG towards MB and MO.

Conclusions
In summary, spent coffee grounds-derived carbon that supported C, N doped TiO 2 (C, N-TiO 2 @ACG) composite photocatalysts were prepared. Herein, spent coffee groundsderived carbon can provide attachment sites for TiO 2 , increase contact with pollutants, and suppress the recombination of electron-hole pairs. Furthermore, C, N co-doping extends the light absorption region of TiO 2 to the visible region, which greatly improves the photocatalytic activity. Correspondingly, the band gap energy of C, N-TiO 2 @ACG decreases from 3.31 eV of TiO 2 to 2.34 eV. Consequently, the optimal 1:1 C, N-TiO 2 @ACG delivers remarkable photocatalytic activity with the degradation efficiency of MB reaching 96.9% within 45 min, as well as a high reaction rate of 0.06348 min −1 , approximately 4.66 times that of TiO 2 (0.01361 min −1 ). Furthermore, it also demonstrated greatly enhanced photocatalytic efficiency towards the MO in the presence of the MB compared with the single MO solution. In addition, it demonstrated superior degradation performance towards the MB in alkaline conditions. This work provides a reference and extension method for the further development of TiO 2 -based photocatalysts and the high value-added utilization of coffee grounds.
Author Contributions: Conceptualization, Z.C. and F.R.; methodology, X.G. and J.W.; formal analysis, X.G. and J.W; investigation, X.G., J.W. and Z.S.; writing-original draft preparation, Y.J. and J.W.; writing-review and editing, Y.J. and P.R.; supervision, Y.J. and P.R. All authors have read and agreed to the published version of the manuscript. Data Availability Statement: The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.

Conflicts of Interest:
The authors declare no conflict of interest.