Optimization of thermal exfoliation of graphitic carbon nitride for methylparaben photocatalytic degradation under simulated solar radiation

rsc.li/materials-a Journal of Materials Chemistry A Materials for energy and sustainability rsc.li/materials-a ISSN 2050-7488 COMMUNICATION Zhenhai Wen et al. An electrochemically neutralized energy-assisted low-cost acid-alkaline electrolyzer for energy-saving electrolysis hydrogen generation Volume 6 Number 12 28 March 2018 Pages 4883-5230 Journal of Materials Chemistry A Materials for energy and sustainability This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. Accepted Manuscript View Article Online View Journal This article can be cited before page numbers have been issued, to do this please use: J. Plaza Morales, A. Arencibia and M. López-Muñoz, J. Mater. Chem. A, 2023, DOI: 10.1039/D3TA01109G. ARTICLE Please do not adjust margins Please do not adjust margins Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x Optimization of thermal exfoliation of graphitic carbon nitride for methylparaben photocatalytic degradation under simulated solar radiation Jorge Plaza,a Amaya Arencibiab and María José López-Muñoz*a Previous studies have shown that the exfoliation of graphitic carbon nitride (g-C3N4) is essential to obtain materials with good photocatalytic properties. However, in most works the influence of the exfoliation variables were investigated in an unsystematic way by changing the levels of one factor at a time. In this work, a full factorial design 32 was employed to evaluate the influence of temperature and time used in the thermal exfoliation of bulk g-C3N4 obtained from urea (bulk-U), on the photocatalytic performance for 5 mg L−1 methylparaben degradation under simulated solar radiation. Based on the thermal stability of the starting bulk-U, the levels of the design were set in the range 400 to 450 oC for temperature and 2 to 6 h for time. The results showed an enhancement of the photocatalytic activity with the increase of both factors, obtaining the maximum response at T = 450 °C and t = 6 h. An economic evaluation at laboratory scale including the main costs derived from the synthesis stage and the photocatalytic degradation procedure was also performed, comparing all the exfoliated materials. The total expenses could be minimized without compromising a good photocatalytic activity with the material obtained by exfoliation of bulk-U at 450 oC for 2 h. A detailed characterization of the materials was carried out by XRD, FTIR, adsorption-desorption of N2, TEM, SEM, UV-Vis DR, and PL spectroscopy. It was inferred that the enhancement of the photocatalytic performance induced by the exfoliation was mainly related to the consequent increase in the surface area and the improvement in the separation of photogenerated charge pairs derived from the unpacking of the stacked layers. 1. Introduction In recent years polymeric graphitic carbon nitride (g-C3N4) has attracted much attention as a metal-free semiconductor for photocatalytic applications since it shows a band gap ca. 2.7 eV which makes it active under visible-spectrum light, is non-toxic and exhibits a relatively high stability to photo-corrosion and acid or base solutions. Moreover, g-C3N4 can be easily synthesized by the polycondensation of a variety of organic precursors containing both carbon and nitrogen such as urea, cyanamide, dicyandiamide or melamine.1,2 However, the bulk gC3N4 also has a number of shortcomings that limit its photocatalytic efficiency, among which: (i) a high rate of electron-hole pairs recombination, (ii) a low surface area and (iii) a moderate water oxidation ability due to the energy position of the upper limit of the valence band (VB) at about 1.4 V vs. NHE.3 The latter means that the photo-generated holes of g-C3N4 are able to attain only oxygen evolution from water oxidation, instead of the formation of oxidant hydroxyl radicals (HO•) hence limiting the applications in photocatalytic depuration processes.4 To overcome the above-mentioned disadvantages and enhance the photocatalytic performance of g-C3N4 different strategies have been proposed, such as the exfoliation of the 2D-stacked layers that build the graphite-like structure of bulk g-C3N4. The exfoliation into single-layered nanosheets, feasible due to the weak Van der Waals forces between the layers, leads to both the increase of the specific surface area hence providing a higher number of exposed catalytic centres, and the improvement of the electron transport ability along the inplane direction.5 As a result, the exfoliated materials show a higher photocatalytic activity compared to their bulk counterparts. Different approaches have been previously reported to attain the exfoliation of bulk g-C3N4 including mechanical, supercritical, hydrothermal, liquid-phase and thermal exfoliation methods.4,6 Among them, thermal exfoliation has attracted great attention as it is a simple and effective procedure. Previous works have reported that the experimental variables of the etching process, such as time or temperature, can have a significant influence on the optical and electronic properties of the materials obtained.7 However, to the best of our knowledge, the experiments that showed such effect were performed in an unsystematic way by changing the levels of one factor at a time. On this basis, in the present work a systematic approach using factorial experimental design was carried out to screen the influence of the temperature and time of thermal exfoliation of bulk g-C3N4 on the photocatalytic properties of the obtained a. Departamento de Tecnología Química y Ambiental; ESCET, Universidad Rey Juan Carlos, 28933, Móstoles, Madrid, Spain. Email: [email protected]. b. Departamento de Tecnología Química, Energética y Mecánica. ESCET, Universidad Rey Juan Carlos, 28933, Móstoles, Madrid, Spain. Page 1 of 9 Journal of Materials Chemistry A Journal of Materials Chemistry A Accepted Manuscript Open Access Article. Published on 20 April 2023. Downloaded on 4/21/2023 11:15:16 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3TA01109G ARTICLE Journal Name 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins materials. Methylparaben (MeP, C8H8O3) was evaluated as target molecule in the simulated solar light-mediated photocatalytic reactions. MeP is the methyl ester of phydroxybenzoic acid, widely present as a preservative in formulations of cosmetics and personal care products due to its broad anti-microbial and anti-fungal properties. Despite it is substantially removed in wastewater treatment plants (WWTPs), it has been reported the presence of MeP in aquatic systems including freshwater, brackish and marine water, sediments, and biota.8 This is a matter of concern since previous studies have warned of a potential estrogenicity and endocrinedisrupting properties of parabens,9 what it makes important to investigate their removal from the aqueous medium. 2. Experimental 2.1. Synthesis of g-C3N4 materials The bulk g-C3N4 sample (bulk-U) was synthesized by thermal polymerization using urea as precursor. Briefly, 15 g urea were dissolved in 20 mL of Milli-Q® water and the homogeneous solution was placed in three covered crucibles, which were heated in a muffle furnace two consecutive steps at 400 °C for 2 h, and 450 °C for 2 h at 5 °C min−1 heating rate. After cooling to room temperature, the solid obtained was grounded and sieved to a particle size < 200 μm. The exfoliation of bulk-U was carried out by thermal etching at different temperatures in the range 400 to 450 °C for 2, 4, or 6 h in air atmosphere. The solids obtained were labelled as Uxxx-y, where “xxx” represents the temperature applied and “y” the duration of treatment. As an example, the sample U425-4 was obtained by exfoliation of bulk-U at 425 °C for 4 h. 2.2. Characterization X-Ray Diffraction (XRD) patterns were acquired using Philips XPERT MPD equipment with Cu Kα radiation (λ= 1.5414 Å) as Xray source. Data were recorded in the range 10 to 80° at a 0.01° step size. The adsorption−desorption isotherms were determined in a Micromeritics Tristar 3000, using N2 at -196 °C. Previously to the analysis, the samples were subjected to two degasification steps under nitrogen at 90 °C and 200 °C for 5 h. BET surface area (SBET) was determined using adsorption data in the relative pressure (P/P0) between 0.05-0.2. The morphology and structure of the synthesised materials was determined by scanning electron microscopy (SEM) with a JEOL JSM-7900F at 1−30 kV equipment and transmission electron microscopy (TEM), performed on a STEM JEOL F200 microscope operating at 200 kV. The total content of C, H, N and O was evaluated using a CHNS-O Analyzer FLASH 2000 Thermo Scientific. The optical properties of the samples were investigated by UV–vis diffuse reflectance (DR) spectroscopy using a Varian Cary 500 Scan UV-Vis-NIR spectrophotometer equipped with an integrating sphere. The wavelength range analysed was 200 to 800 nm with a spectral scan rate of 100 nm min–1. The energy band gap values were obtained by extrapolating a linear fitting in the Tauc plot. Photoluminescence (PL) spectra were obtained with an Agilent Cary Eclipse spectrophotometer using an excitation wavelength of 350 nm. Time-resolved fluorescence emission spectra were recorded at room temperature with a Mini-tau system from Edinburgh Instruments, using an EPL-375 picosecond pulsed diode laser. As excitation source, it was used an EPL-375 picosecond pulsed diode laser with emission at 372 nm and the emission was recorder with a band pass filter at 420 nm. Infrared spectra of the samples were recorded with 4 cm−1 resolution and 64 scans in the wavenumber range of 4000–500 cm−1 using a Perkin Elmer Frontier IR spectrophotometer equipped with an ATR module. Thermograms of the bulk material were analysed by a thermogravimetric analyser model TGA/DSC 1 with a 5 °C min−1 heating ramp and a constant air gas flow of 100 mL min−1. 2.3. Photocatalytic reactions The photocatalytic reactions for MeP degradation were conducted in a reactor of 1 L capacity placed in a solar-radiation simulator (Solarbox 1500). The solar chamber was provided with a 1,500 W Xenon lamp and a  < 300 nm cut-off filter, obtaining an average light intensity of 329 W m−2 in the 300-800 nm range (StellarNet Spectrometer). The initial concentration of MeP was set at 5 mg L−1 with a loading of g-C3N4 catalyst of 0.25 g L−1. The suspension was magnetically stirred and continuously saturated with an air flow. Aliquots taken over reaction time were filtered through a 0.22 μm syringe-driven PTFE filter for analysis in a HPLC equipment (Agilent 1260 Infinity II) provided with a reversed phase column (Agilent Poroshell 120 EC-C18). Methanol–H2O (30:70, v/v) was used as mobile phase with a flow rate of 0.8 mL min−1. 2.4. Factorial experimental design Multivariate experimental design was carried out following the methodology of response surface. Analysis of the experimental data was carried out by the statistical graphics software system STATGRAPHICS Centurion Version XVI. 3. Results and discussion 3.1. Experimental design Preliminary experiments were carried out using bulk-U under simulated solar radiation. This photocatalyst could effectively degrade MeP (5 mg L-1) up to > 99% within 220 min irradiation. The first-order kinetic model was used to fit the experimental profile of MeP concentration as a function of irradiation time, obtaining a kinetic constant, k = 0.0087 min-1 (R2= 0.985). The factorial design methodology was then used employed to evaluate the influence of two factors of the thermal exfoliation of bulk-U, namely temperature (X1) and duration time (X2), on the resulting photocatalytic properties. To establish the levels of both factors in the experimental design, a thermogravimetric analysis (TGA) of bulk-U was initially performed to assess its thermal stability in air (Fig. 1). Fig. 1a displays the TGA curve obtained as a function of temperature. It is observed an initial decrease in weight of ca. 5 % within the temperature range 20 to 100 °C, which is associated with the removal of physisorbed water from the catalyst surface. Next, bulk-U exhibited a high Page 2 of 9Journal of Materials Chemistry A Journal of Materials Chemistry A Accepted Manuscript Open Access Article. Published on 20 April 2023. Downloaded on 4/21/2023 11:15:16 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3TA01109G Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3 Please do not adjust margins Please do not adjust margins thermal stability up to 360 °C above which, it diminished slightly up to ca. 450 °C. Beyond this temperature, the weight percentage decreased drastically until the decomposition of the material over 600 °C.10 Also, the influence of the thermal treatment duration on the mass yield of the exfoliated materials was evaluated by TG analysis in the range 400 to 500 °C (Fig. 1b). As shown, a drastic loss of mass (> 95 %) was observed within 1 and 2 h at 500 and 475 °C, respectively. By contrast, a more gradual decline of the percentage weight over time, indicative of a higher thermal stability, could be noted at lower temperatures. Even though, a mass loss around 85% was observed at 450 °C after 6 h, which led to establish the latter experimental conditions as the upper values for temperature (X1) and time (X2) factors, respectively, to avoid a very scarce mass yield. 0 100 200 300 400 500 600 0 20 40 60 80 100 % Weight bulk g-C3N4 Temperature (ºC) 360 H2O loss 20 Decomposition (a) 012345678 0 20 40 60 80 100 (b) % Weight bulk g-C3N4 Time (h) 450 ºC 475 ºC 400 ºC 425 ºC 500 ºC Fig. 1 Thermogravimetric analysis of bulk-U in air as a function of (a) temperature and (b) time. The arrows included in (a) indicate the starting point of the analysis in (b). Table 1 Design matrix and experimental results of the photocatalytic response (% MeP degradation at reaction times of 30, 45 and 60 min) and the calculated cost response (relative costs involved at lab scale). Code Factors Exfoliated sample Experimental result (% MeP deg.) Kinetic constant Calculated Exp X1 X2 T (ºC) t (h) (ID) t = 30 min t = 45 min t = 60 min k (min−1) relative cost (%) 1 0 0 425 4 U425-4 55.7 70.4 85.3 0.033 52.4 2 0 0 425 4 U425-4 57.8 70.4 85.6 0.036 53.5 3 0 0 425 4 U425-4 60.9 74.3 84.4 0.035 51.7 4 −1 −1 400 2 U400-2 30.3 40.2 48.0 0.011 88.8 5 −1 0 400 4 U400-4 38.7 51.3 63.0 0.025 58.2 6 −1 +1 400 6 U400-6 65.3 80.7 91.1 0.039 52.5 7 0 −1 425 2 U425-2 48.4 62.4 74.2 0.027 50.2 8 0 +1 425 6 U425-6 87.5 95.7 98.8 0.065 51.7 9 +1 −1 450 2 U450-2 68.5 82.4 93.8 0.044 40.3 10 +1 0 450 4 U450-4 92.0 98.3 99.6 0.091 57.9 11 +1 +1 450 6 U450-6 84.6 94.6 98.6 0.068 100 According to the above considerations a full factorial design 32, two factors at three levels, was proposed setting the upper and lower limits for temperature and time factors at 400 and 450 °C, and 2 and 6 h, respectively. The design consisted of 11 experiments, comprising 4 factorial points, 4 axial points and 3 replications of the central point, the latter used to determine the experimental error. Table 1 displays the actual values and code levels of temperature and time factors used in each experiment and the label assigned to the exfoliated samples. For each run, the experimental percentages of MeP photocatalytic elimination with simulated solar radiation after 30, 45 and 60 min were taken as design response variables (Table 1). The photocatalytic results were well fitted to a firstorder kinetic model with all materials (calculated kinetic constants shown in Table 1). It is worth noting that compared with the results obtained with bulk-U indicated above (> 99 % MeP degradation within 220 min irradiation) the exfoliation of the material brought out a very significant enhancement of the photocatalytic performance, attaining in some cases  99 % MeP degradation within 60 min irradiation. The experimental reaction results obtained were fitted to a second-order polynomial equation (response function): (1) 𝑌 = 𝑏 0 + 𝑏 1 𝑋 1 + 𝑏 2 𝑋 2 + 𝑏 12 𝑋 1 𝑋 2 + 𝑏 11 𝑋 2 1 + 𝑏 22 𝑋 2 2 where Y represents the % MeP elimination at a fixed time; X1 and X2 are the factors evaluated (temperature and time); b0 is the independent coefficient; b1 and b2 are the linear coefficients for temperature and time; b12 represents the binary interactions; and b11 and b22, the quadratic interactions. Validation of the mathematical model was performed by the Ftest for analysis of variance (ANOVA). For the significance of the effects and interactions of the factors on the response, a confidence level of 95 % was used. The statistical significance was considered for P-value < 0.05 (5%). Table 2 shows the statistical equations obtained on the dimensionless scale (−1 0 1) for the photocatalytic responses Page 3 of 9 Journal of Materials Chemistry A Journal of Materials Chemistry A Accepted Manuscript Open Access Article. Published on 20 April 2023. Downloaded on 4/21/2023 11:15:16 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3TA01109G ARTICLE Journal Name 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Y30, Y45 and Y60. As can be seen, the correlation coefficient values (R2) indicate a good model adequacy, corroborated by the appreciable degree of correlation between the predictive values of the model and the experimental values observed for the responses analysed (Fig. 2a). Table 2 Statistical models obtained for the responses analysed. Calculated response function R2 Photocatalytic responses Y30 = 60.86 + 18.47 X1 + 15.03 X2 + 0.41 X12 – 4.75 X1 X2 + 2.97 X22 R2=0.977 Y45 = 73.30 + 17.21 X1 + 14.33 X2 – 0.95 X12 – 7.07 X1 X2 + 3.33 X22 R2=0.946 Y60 = 85.04 + 14.99 X1 + 12.06 X2 – 3.67 X12 – 9.57 X1 X2 + 1.48 X22 R2=0.987 Cost response Y’cost = 49.61 – 0.22 X1 + 4.15 X2 + 12.85 X12 + 24.00 X1 X2 + 5.75 X22 R2=0.950 The determination of the significant factors generated among the linear, binary, and quadratic interactions were obtained from the standardized Pareto diagram (Fig. 2b). The Pareto chart shows that the first-order effects for temperature (X1) and time (X2) are relevant. Hence it could be inferred that an increase in these variables might enhance the efficiency of the exfoliated material for the photocatalytic degradation of MeP. For a clearer analysis, Fig. 3 displays the response surfaces and the contour representations of the calculated polynomial functions for MeP removal at 30, 45 and 60 min of irradiation included in Table 2. As can be observed, there is an increasing trend of the response variable with the increase of both temperature and time of exfoliation of bulk-U, obtaining the maximum response at T = 450 °C and t = 6 h (U450-6). Accordingly, the factorial design points to a further increase of the upper levels of both factors to improve the photocatalytic performance of the exfoliated materials. However, the thermogravimetric experiments (Fig. 1) demonstrated that such increase of temperature or time would lead to a significant decrease of the mass yield of the catalyst thus highly increasing the costs associated to the elimination of the target pollutant. 0 20 40 60 80 100 0 20 40 60 80 100 (a)(a) % Degradation of MeP at 5 min % Degradation of MeP at 30 min % Degradation of MeP at 60 min Predicted values Experimental values 0 2 4 6 8 (b) X22 X12 X1X2 X2 Standaridized effect X1 Positive effect Negative effect Fig 2 (a) Correlation between predicted and experimental values for the photocatalytic degradation of MeP at different irradiation times. (b) Pareto diagram of standardized effects of the factorial design proposed for the response at 45 min of photocatalytic reaction (Y45). Fig. 3 Representative response and contour surfaces of the effect of temperature and exfoliation time of bulk g-C3N4 on the photocatalytic performance for % MeP removal (scale from 0 to 100 %) at different solar irradiation times: (a) 30, (b) 45, and (c) 60 min. 3.2. Economic evaluation The actual viability of a synthesized material for its use in photocatalytic applications should be linked not only to its photoactivity, but also to economic aspects ranging from the synthesis of the catalyst to the decontamination process for its use. To contemplate the latter consideration, a first approximation of global costs at lab scale was carried out including all the materials evaluated in the factorial design shown above. For the economic study both factors (the exfoliation temperature and time of bulk-U) were kept as in the previous section, setting the costs to attain a 99.5% degradation of MeP as the variable response (Table 1). The costs calculated were fitted to the second-order polynomial (equation 1) to obtain the response function, which was labelled as Y’cost (Table 2). The economic assessment comprised, on the one hand, the costs associated with the whole synthesis process including the expenses of the precursor reagent and the electrical costs for both the thermal polycondensation of urea and the postPage 4 of 9Journal of Materials Chemistry A Journal of Materials Chemistry A Accepted Manuscript Open Access Article. Published on 20 April 2023. Downloaded on 4/21/2023 11:15:16 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3TA01109G Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5 Please do not adjust margins Please do not adjust margins thermal treatment of bulk-U to obtain the exfoliated material. The latter costs are mainly governed by the temperature and duration of the treatments which, in turn, determine the electricity needed during the synthesis stage. On the other hand, the analysis of total expenses also considered those associated with the photocatalytic reaction as a function of its duration to attain the desire concentration of the target pollutant. The reaction time depends on the activity exhibited by the photocatalyst and includes the expenses for aeration, and the electrical costs due to the irradiation within the solar simulation box and the magnetic stirring of the suspension. Due to the variability of the costs evaluated, mainly those related to the electricity use, they have been expressed in relative terms taking as reference the total expenses of the material that involved the highest costs, U450-6, for which a value of 100 % was assigned (Table 1). Therefore, a timeless reading of the results obtained in the present work can be made. As it can be observed in Table 2 the fitting of the results to the equation Y’costs yielded a good regression coefficient for the estimation. Fig. 4 displays the response surface and the contour graph expressed as relative costs. As can be seen (Fig. 4a and b), the minimum costs were estimated for an exfoliation time over 2−2.4 h and a temperature in the range 435−450 C, experimental conditions leading to a reduction ca. 60 % of costs compared to the reference material, U450-6. These results and, in general, the shape of the response surface clearly shows the balance between the costs associated to the synthesis stage and those derived from the photocatalytic process. To minimize the expenses of the photocatalytic stage it is necessary to get the materials with the higher activity. As seen in Fig. 3, the latter is associated to the increase of the exfoliation temperature and time, which leads to the increase of electricity costs in the synthesis stage. As a result, the overall cost grows up to its maximum value (U450-6). Likewise, but in the opposite direction, it occurs for the photocatalysts synthesized at the lower temperatures and times. In this case the costs of the synthesis stage are greatly decreased, but since the materials obtained show a lower photoactivity a longer irradiation time is required for attaining the degradation of MeP, hence increasing the overall costs to a value close to the reference value (92 % for U400-2). To a better understanding Fig. 4c and d show a graphical comparison between the distribution of costs calculated for the most economic material, U450-2, and the two high-cost materials, U400-2 and U450-6. For U400-2, the photocatalytic reaction accounts for 78.9 % of the total cost whereas only 21.1 % stands for the synthesis step. Quite the opposite, in the U4506 material only 11.0 % of costs are derived from the MeP degradation process while 89.0 % are ascribed to the synthesis of the catalyst. In between of both, the optimal cost value found for U450-2 derives from a distribution of costs of 68.5 % and 31.5 % for the catalyst synthesis and the photocatalytic reaction, respectively. It should be noted that the above estimation of costs is a first approach to the expenses related to a laboratory scale experimentation and hence, for practical applications it would be necessary to evaluate additional aspects such as the scaleup of the synthesis of the material, the use of direct solar radiation or the reuse of the photocatalyst. Nevertheless, the economical estimation done shows up that not only the efficiency within a certain time but the expenses for synthesis and reactions should be considered in the development of photocatalysts. Page 5 of 9 Journal of Materials Chemistry A Journal of Materials Chemistry A Accepted Manuscript Open Access Article. Published on 20 April 2023. Downloaded on 4/21/2023 11:15:16 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3TA01109G ARTICLE Journal Name 6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Fig. 4 (a−b) Representative response and contour surfaces of the relative costs associated with the degradation of 99.5% of MeP as a function of the effect of temperature and exfoliation time. (c−d) Breakdown of the percentages of costs associated with the material with the lowest relative cost (U450-2; inner circle) and higher costs (U400-2 and U450-6; outer circle for c and d, respectively). 3.3. Characterization of the photocatalysts To understand the influence of the exfoliation variables analysed in the factorial design on the photocatalytic performance of the materials, they were characterized by XRD, N2 adsorption-desorption isotherms, FTIR, elemental analysis, TEM, SEM, UV-Vis DR, and PL spectroscopy. Fig. 5a displays the XRD patterns of selected samples. The weak diffraction signal at 2θ = 13.2° assigned to the (1 0 0) plane is associated with the ordering of heptazine units linked by hydrogen bonds within the 2D layer of the materials. The diffraction signal at 2θ = 27.4° is ascribed to the (0 0 2) plane of graphitic carbon nitrides and related to the Van der Waals forces that generate the periodic stacking between layers along the c axis.11–13 In comparison to the bulk-U sample the exfoliated samples showed a decrease of the intensity of both peaks, more significant for the diffraction at 27.4°. The enlarged view of the (0 0 2) peak (Fig. 5b) demonstrates that the gradual decrease of the signal, which denotes an increasing exfoliation degree, is linked to the increase of the temperature and duration of the thermal treatment (lower intensity in U450-4 and U450-6). The reduction of the intensity of the (1 0 0) signal is indicative of the breaking of intralayer H-bonds and the interlayer stacking hence proving the exfoliation achieved.12,14,15 In turn, this is in accordance with the better photocatalytic activity shown by the latter samples in comparison to the materials exfoliated at lower temperatures, as observed in Fig 3. The N2 adsorption−desorption isotherms of the exfoliated materials (Fig. 5c) showed a gradual change in the shape by increasing temperature and time of exfoliation. According to the classification collected by the IUPAC, the isotherms of bulkU and samples exfoliated at 400 and 425 C can be classified as type III isotherms. For the materials obtained at a higher temperature and time, a change to type IV isotherms was observed with an increase in adsorption at moderate-high pressures along with the presence of H3-type hysteresis loop, which indicates the development of a certain mesoporosity.16,17 Table 3 summarizes the BET surface area and total pore volume of representative samples. As can be observed both parameters increase with temperature and time of exfoliation. The BET surface area increases with the temperature of exfoliation from 23 cm2 g−1 (bulk-U) to 49 cm2 g−1 (U400-2) and 124 cm2 g−1 (U450-2), being further enhanced to 279 cm2 g−1 (U450-6) by increasing at 450 C the exfoliation time from 2 to 6 h. Following a similar trend, the total pore volume increases from 0.027 (bulk-U) to 0.679 cm3 g−1 (U450-6). These results confirm the success of the thermal treatment for unpacking the layers of the graphitic structure into 2D nanosheets, in agreement with previous results17,18, and the significant influence of the exfoliation variables (temperature and time) on the textural properties of the resulting materials. Also, the increase of the surface area provides a higher number of active sites that can account for the improvement in the photocatalytic activity shown by the materials (Table 1). The characteristic functional groups of the catalyst structure were probed by FTIR spectroscopy. As shown in Fig. 5d, the FTIR spectra of the bulk and exfoliated materials are quite similar despite the respective thermal treatments. In all samples a broad band centred at ~ 3200 cm−1 is observed, which is indicative of the presence of hydrogen bonding. Also in this region, in the 3100−3600 cm−1 range, the stretching vibration modes of N-H and N-H2 bonds are distinguishable, indicating the presence of primary and/or secondary amino functional groups. The band displayed at 1623 cm−1 corresponds to the NH2 bending vibration, and the band at 1540 cm−1 is assigned to the C=N stretching vibrations of the aromatic ring. Besides, the bands centred at 1397, 1314 and 1227 cm−1 are related to the stretching and rotation vibrations of the C-N of the heptazine rings. Finally, at lower frequencies, the narrow band at around 810 cm−1 is associated with the out-of-plane vibration of the heptazine ring group.19,20 10 20 30 40 50 60 70 80 (a) 13.2 º 27.4 º U450-6 U450-4 U450-2 U400-2 Intensity (a.u.) 2 (deg.) U425-2 20 22 24 26 28 30 32 34 24 28 32 36 (b) Intensity (a.u.) 2 (deg.) U400-2 U425-2 U450-2 U450-4 U450-6 bulk-U 2 (deg.) 0.0 0.2 0.4 0.6 0.8 1.0 0 100 200 300 400 500 (c) Adsorbed quantity (cm3/g STP) Relative pressure (P/P0) U450-6 U450-4 U450-2 U425-2 U400-2 bulk-U 4000 3500 3000 2500 2000 1500 1000 500 (d) N N NN N NN NN N NH2 NH Absorbance (a.u) Wavenumber (cm-1) bulk-U U450-2 U400-2 U450-4 U425-2 U450-6 Fig. 5 (a) XRD patterns (b) enlarged view of (0 0 2) peak, (c) N2 adsorption−desorption isotherms and (d) FTIR spectra of bulkU and exfoliated materials. Table 3 shows the C:N molar ratio of the materials, obtained by elemental analysis. The complete polymerization of the urea precursor to g-C3N4, would lead to a C:N ratio of 3:4. However, all the materials show a lower ratio, which is related to incomplete polymerization and oxidation of the carbon nitride structure. In addition, no significant differences in the C:N ratio could be observed despite the variation of the exfoliation conditions. This result indicates that the thermal treatment does not substantially affect the ratio between carbon and nitrogen content of the urea-derived materials, in agreement with previous reports.21 Page 6 of 9Journal of Materials Chemistry A Journal of Materials Chemistry A Accepted Manuscript Open Access Article. Published on 20 April 2023. Downloaded on 4/21/2023 11:15:16 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3TA01109G Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7 Please do not adjust margins Please do not adjust margins The morphology and structure of C3N4 materials were investigated by scanning (SEM) and transmission electron microscopy (TEM). The SEM image of the bulk sample (Fig. 6a) displays disordered solid agglomerates, giving rise to a morphology formed by criss-cross smooth blocks. After the thermal treatment at 450 C (U450-6) (Fig. 6b), the surface of the aggregates is less smooth evidencing an irregular delamination of the structure compared to the reference material. In addition, the high-magnification of this image (Fig. 6c) reveals that the 2D nanosheets tend to bend, making curled in-plane edges to minimize the surface free energy.7,18 In the TEM micrographs, the parent bulk-U (Fig. 7a) displays a marked agglomeration of disordered layers with numerous in-plane voids, which some authors have related to the release of ammonia gas during the synthesis process22. After thermal etching at 400 °C for 2 h (U400-2), the nanosheets appeared as large curved layers, as shown in Fig. 7b. In agreement with the SEM images and previous reports, the edges of the layers tend to bend to reduce surface energy.19 The further increase in time and temperature (Fig. 7c). causes a much lower degree of agglomeration and a smaller thickness of the sets of stacked layers (~10 nm), which is indicative of a fewer number of piled-up layers. This is consistent with the increase of the surface area of the material (Table 3) and the decrease in the intensity of the X-Ray diffraction signal at 2θ = 27.4°, compared to bulk-U (Fig. 5a and 5b). The UV-VIS DR spectra of bulk-U and the exfoliated materials showed their absorption in the blue region of the visible spectra with a maximum in the UVA region at 378 nm (Fig. 8a). Table 3 lists the band gap values (Eg) calculated from the Tauc plot displayed in the Fig. 8b. As can be observed, there is not a significant change in the Eg values with varying the exfoliation conditions. This result is in agreement with those reported by other authors that also used urea as g-C3N4 precursor and found that the band gap of the bulk material, ca. 2.80 eV, was just slightly increased to 2.88 eV by the exfoliation treatment.22,23 In contrast, the band gap variations using other precursors (e.g., melamine or thiourea) tend to be more pronounced when the material is exfoliated, increasing by more than 0.4 eV.24,25 Table 3 Summary of the textural and optical properties of bulkU and exfoliated samples. Samples SBET (m2g-1) Vpore (cm3g-1) Absorption edge (nm) Eg (eV) Emission peak (nm) C:N ratio bulk-U 23 0.027 419 2.856 438 3:4.7 U400-2 49 0.081 413 2.900 438 3:4.7 U425-2 67 0.123 415 2.881 438 3:4.6 U450-2 124 0.271 414 2.895 440 3:4.6 U450-4 209 0.400 414 2.891 440 3:4.6 U450-6 279 0.679 416 2.869 444 3:4.6 PL spectra were used as an approach to qualitatively evaluate the extent of recombination of photogenerated electron–hole pairs. In general, a high PL emission denotes a significant recombination of the charge carriers. As shown in Fig. 9a, the PL spectra display a broad emission in the range 400 to 600 nm. Gaussian fitting to the photoluminescence profile was carried out to identify the constituents of this emission (Fig. 9b), obtaining three main bands centred at around 429, 450, and 480 nm, which is in accordance with previous works that assigned them to transitions between the sp3 C−N σ band, the sp2 C−N π band, and the lone pair (LP) of the bridge nitride atom states, specifically, to σ* σ*−LP, π*−LP, and π*−π, transitions, respectively.26 Fig. 6 SEM images of (a) bulk-U, (b) U450-6 and (c) high-magnification of the area included in the red box. Page 7 of 9 Journal of Materials Chemistry A Journal of Materials Chemistry A Accepted Manuscript Open Access Article. Published on 20 April 2023. Downloaded on 4/21/2023 11:15:16 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3TA01109G ARTICLE Journal Name 8 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Fig. 7 TEM images of (a) bulk-U and the exfoliated materials (b) U400-2 and (c) U450-6. 250 300 350 400 450 500 550 600 0.0 0.5 1.0 1.5 2.0 2.5 (a) F(R)  (nm) bulk-U U400-2 U425-2 U450-2 U450-4 U450-6 2.6 2.8 3.0 3.2 3.4 0.0 0.5 1.0 1.5 2.0 2.5 3.0 (b) (h)1/2 Photon energy (eV) bulk-U U400-2 U425-2 U450-2 U450-4 U450-6 Fig 8 (a) UV-Vis DR spectra, (b) Tauc plot to calculate the band gap energy of the synthesised materials. Fig. 9a also shows that the emission intensity gradually decreased from bulk-U to the materials exfoliated at increasing temperature and time. The decrease of the PL emission peak is indicative of a progressive suppression of the radiative electron–hole recombination in the exfoliated materials, hence favouring a higher lifetime of the charge carriers as the extent of the exfoliation increases (U450-4 and U450-6).14 To further elucidate, the time-resolved fluorescence decay profiles were measured and fitted to a bi-exponential decay function, and radiative lifetimes (1 and 2) and the relative percentages of photoexcited charge carriers were determined (Fig. 9c). The shortest lifetime (1) increased from 3.8 ns (bulk-U) to 4.8 ns and 5.9 ns when the material was exfoliated by thermal etching 2 h at 400 C (U400-2), and 6 h at 450 C (U450-6), respectively. The same trend was observed for 2, which increased from 26.2 ns (bulk-U) to 31.5 ns (U400-2), and 40.0 ns (U450-6), respectively. The observed increase in the lifetime of the charge carriers as the degree of exfoliation increases, also reported in previous studies, can be related to the improved electron transport.7,28 The development of intralayer mesoporosity and the decrease of the layer thickness promoted by the exfoliation (Fig. 5, 6 and 7) might facilitate the transport of charge carriers to the surface thus avoiding their recombination.24,27 Therefore, the more successful the g-C3N4 exfoliation, the higher photocatalytic activity might be expected, as the experimental results show (Table 1). 400 450 500 550 600 (a) bulk-U U400-2 U425-2 U450-2 U450-4 U450-6 Intensity (a.u.) Wavelength (nm) 400 450 500 550 600 (b) P3 P2 Experimental data Fitted Intensity (a.u.) Wavelength (nm) P1 0 20 40 60 80 100 (c) Intensity (a.u.) Time (ns) bulk-U U400-2 U450-6 Fitting Samples τ1 [ns]–Rel% τ2 [ns]–Rel% 3.8−58 26.2−42 4.8−60 31.5−40 5.9−56 40.0−44 Fig 9 (a) PL spectra of bulk-U reference and exfoliated materials, (b) Profile analysis by gaussian fitting for bulk-U and (c) Timeresolved fluorescence decay spectra monitored at 430 nm under 350 nm excitation for bulk-U, U400-2 and U450-6. 4. Conclusion In the present work, a systematic approach using a 32 factorial experimental design was carried out to investigate the influence of the exfoliation temperature and time of bulk g-C3N4 obtained from urea (bulk-U) on the photocatalytic performance for methylparaben degradation. The results showed a significant influence of the exfoliation variables: the higher the temperature and time of the bulk thermal treatment, the better the resulting photocatalytic performance. The upper limit of both variables was, however, determined by the thermal stability of the bulk-U, and it was established at 450 C and 6 h to avoid a low percent mass yield in the synthesis of the photocatalysts. The estimation at laboratory scale of the costs related to the synthesis stage and the photocatalytic procedure pointed out to the exfoliated material U450-2 as the one that minimises the energetic cost of the synthesis process, without Page 8 of 9Journal of Materials Chemistry A Journal of Materials Chemistry A Accepted Manuscript Open Access Article. Published on 20 April 2023. Downloaded on 4/21/2023 11:15:16 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D3TA01109G