C-doped anatase TiO2: Adsorption kinetics and photocatalytic degradation of methylene blue and phenol, and correlations with DFT estimations

Journal of Colloid and Interface Science 547 (2019) 14–29

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Regular Article

C-doped anatase TiO2: Adsorption kinetics and photocatalytic degradation of methylene blue and phenol, and correlations with DFT estimations Juan Matos a,b,⇑, José Ocares-Riquelme a,c, Po S. Poon a, Ricmary Montaña a, Ximena García c, Kilver Campos d, Juan C. Hernández-Garrido e, Maria M. Titirici f a

Hyb&Car Group, Biorefinery Department, Technological Development Unit, University of Concepcion, Av. Cordillera, 2634, Parque Industrial Coronel, Coronel, Chile Millennium Nuclei on Catalytic Processes Towards Sustainable Chemistry (CSC), Chile c Department of Chemical Engineering, University of Concepcion, Barrio Universitario s/n, Edmundo Larenas, Concepcion, Chile d Centre of Physics, Venezuelan Institute for Scientific Research, Km. 11, Pan-American Road, Caracas, Venezuela e Department of Materials Science and Metallurgy Engineering and Inorganic Chemistry, Faculty of Sciences, University of Cadiz, Puerto Real, Cadiz, Spain f Queen Mary University of London, School of Engineering and Materials Science, Mile End Road, E14NS London, United Kingdom b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 15 January 2019 Revised 9 March 2019 Accepted 23 March 2019 Available online 25 March 2019 Keywords: C-doped TiO2 Adsorption Photocatalysis Methylene blue Phenol DFT estimations

a b s t r a c t This work shows an easy and eco-friendly methodology to obtain almost pristine anatase phase of TiO2 by using furfural, a biomass-derived molecule, as a bio-template. The photocatalytic activity was studied following the degradation of methylene blue and phenol under artificial solar irradiation. Results were compared against those obtained on a commercial pristine anatase TiO2. The pseudo first-order, the secondorder and the intraparticle diffusion kinetic models were verified. The textural and surface chemistry properties of the materials were correlated with the surface density of molecules adsorbed in equilibrium. The reaction-rate showed an almost perfect quadratic regression as a function of the surface density. Theoretical estimations of the density of states by DFT + U were performed showing that the total electron charge in the oxygen bonded to anatase TiO2 increased due to carbon doping in agreement with the prediction of appearance of atomic orbitals 2p from carbon atom in the hybrid material. C-doping is responsible of the red-shift from 3.14 to 2.94 eV observed for a Ti15O32C super-cell than pristine anatase Ti16O32. The increase in the activity of the C-doped TiO2 photocatalyst was due to the decrease in the energy band-gap promoting a higher absorption of photons from the visible light. Ó 2019 Elsevier Inc. All rights reserved.

⇑ Corresponding author at: Hyb&Car Group, Bioenergy Department, Technological Development Unit, University of Concepcion, Av. Cordillera, 2634, Parque Industrial Coronel, Coronel, Chile. E-mail address: [email protected] (J. Matos). https://doi.org/10.1016/j.jcis.2019.03.074 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

J. Matos et al. / Journal of Colloid and Interface Science 547 (2019) 14–29

1. Introduction The delivery of more sustainable forms of energy is a challenge for the science of the XXI century [1–4]. Investments in semiconductor technologies associated with solar energy production [1] and solar environmental remediation [2] have remarkably increased in the last years [4]. Besides the economic aspects, the low semiconductor activity under visible light, the high recombination rate of photogenerated electron–hole pairs, and the low stability of photocatalysts in terms of their recovery and reutilization, limit the largescale implementation of photocatalytic processes in solar environmental remediation [2]. In this sense, the C-based and C-doped semiconductors have played a special role in heterogeneous photocatalysis due to their potential to operate in the visible range [4–17]. The enhanced photocatalytic performance of C-TiO2 composites have been attributed to both porosity of carbon supports and to a strong interfacial electronic effect between the two components [5–8,18–21]. The photochemical reactivity of carbon-based materials has been studied in several applications such as treatment of polluted water with dyes [7,18], phenol [19], chlorophenols [20], cyanobacteria [21], hydrogen photoproduction [6,16], and solar cells [8,13,14]. Novel carbon-containing TiO2-based photocatalysts have been recently reported for multiple energy storage [22], as high-performance supercapacitors [23], as efficient photocatalysts for the degradation of pollutants [24,25] and for hydrogen evolution reaction [26–29]. Graphitic carbon nitrides have been also reported for the water splitting reaction [26,28] while C-doped TiO2 have shown an excellent performance in the degradation of organic pollutants and disinfection [24,25,30]. Theoretical studies of the influence of C-doping upon the optoelectronic properties of TiO2 have been widely reported [31–35]. Gao and co-workers [31] and Xu and co-workers [32] using firstprinciple calculations based on the density functional theory (DFT) have reported that the electronic structure of C-doped anatase TiO2 with different oxidation states influences the crystal structure, the energy gap, the charge density, and the optical properties of TiO2. However, most of these theoretical studies [31–35] provide different explanations for the enhanced photoactivity of TiO2. By contrast, two highly-detailed studies using DFT reported by Tsetseris [36,37] have shown the role of the C-doped upon the anatase [36] and rutile [37] crystalline phases of TiO2. These works showed the evolution of carbon dopants as point defects, which are intimately related to the electronic levels in the TiO2 band gap [36] and responsible of the photocatalytic activity and photovoltaic applications [37]. The present work is aimed to prepare C-doped TiO2-based material with very low carbon content (1 wt%) and to verify its influence upon the kinetics of adsorption and the photocatalytic activity in the degradation of methylene blue and phenol by using artificial solar irradiation. These molecules were selected as target pollutants because they are clear different in molecular size and chemical behaviour, besides they are representative in polluted waters studies. Experimental results were interpreted in terms of the electron charges and energy band gap theoretically estimated by using the density of states by DFT + U of pristine anatase TiO2 and C-doped TiO2. We believe that the understanding of carbon doping on anatase phase of TiO2 is still an important issue because most of theoretical estimations in this area are compared against commercial TiO2-P25, which, in our judgment, is not correct because TiO2-P25 is a mixture of anatase and rutile phases. 2. Experimental 2.1. Materials and synthesis of hybrid C-TiO2 materials Analytical grade furfural (
98% purity, Merck), titanium (IV) isopropoxide (97% purity, Aldrich), methylene blue (>82% purity,

15

Merck) and phenol (
98% purity Sigma-Aldrich) were used. Absolute ethanol was purchased from Merck (
99.5% high purity). Fig. S1 (supplementary material) shows a scheme of a typical solvothermal synthesis. In short, 0.5 g of furfural (Fu) and 0.5 g of titanium (IV) isopropoxide were dissolved in 9 mL of ethanol. The resulting solution was sealed into a glass vial inside a Teflonlined autoclave, followed by solvothermal treatment at 180 °C for 16 h. The obtained brown solid was filtered and washed several times with absolute ethanol. The material was dried under staticair at 100 °C for 2 h. The resulting hybrid material was denoted as Fu-TiO2-C. Different procedures were conducted to achieve the best-controlled calcination of the Fu-TiO2-C sample to achieve 1 wt% in carbon content. The most reproducible calcination was performed in an oven under static air, heating from ambient to 550 °C by using consecutive heating stages and then, by 5 h. The calcined sample was denoted C(1%)-TiO2. This value for carbon content was selected for the sake of simplicity in the construction of the theoretical TiO2-based super-cell described below. The so prepared C-doped TiO2 is mainly composed by anatase phase, as discussed below. Due to this, TiO2 from Merck (Reag. Ph Eur purity) was employed for comparative reasons [38] instead of TiO2-P25, because TiO2 from Merck consists of polyhedral particles of pure anatase phase with a BET surface area of 25 m2 g1 and surface pH ca. 7.3. This commercial sample is denoted as TiO2-100%.

2.2. Characterization of materials Elemental analysis were conducted in a Vario EI elemental analyzer to determine the mass fractions of carbon, hydrogen, nitrogen (CHN) of the samples. The CHN analysis was obtained by the combustion of the sample in excess of oxygen and using various traps to collect the combustion products: carbon dioxide, water, and nitric oxide, among others. The masses of these combustion products were used to calculate the composition of the sample by comparison with the internal calibrations patterns for C, N, and H. Oxygen content was determined by the difference between 100% and C, H, and N fractions (%) using the remaining fraction of Ti and O obtained from thermogravimetric analysis of the samples. Thermogravimetric analysis (TGA) under O2 flow was performed to obtain the residual mass of samples composed by titanium and oxygen. Textural characterization was performed by N2 adsorption–desorption isotherms at 77 K. The full isotherms in the range of 4  103 to 84 kPa were measured in a Gemini VII 2390 t equipment from Micromeritics. Surface area, micropore volume, and pore mean diameter were obtained by Brunauer–Emme t–Teller (BET), Dubinin-Radushkevich (DR) and Horvath–Kawazoe (HK) methods, respectively. Surface pH (pHPZC) of materials were obtained from the change of pH in solution as a function of the solid concentration [38] following the pH in aqueous suspension with respect to time until a constant pH is observed. Powder Xray diffraction (XRD) patterns were recorded in the range of 2h = 3–90° with a Bruker D4 Endeavor using CuKa (0.15418 nm) radiation. Diffuse reflectance ultraviolet/visible spectra (DR/UV– vis) were performed at 300 K in the range 200–800 nm using a UV–VIS–NIR spectrophotometer (Varian Cary-5000). Scanning electron microscopy (SEM) images were obtained in a Jeol JSM6380LV equipment. Histograms of spheres were performed measuring 70 different spheres and the data was processed by ImageJ program. Transmission electron microscopy (TEM) studies were performed in a JEOL 2010F 200 kV microscope, with 0.19 nm spatial resolution equipped with a JEOL High Angle Annular Dark Field (HAADF) detector. It permits the acquisition of both HAADF-STEM images and compositional analysis data (spot mode or elemental mapping) using a 0.5 nm electron probe. High-resolution HRTEM images and the corresponding digital diffraction diagrams (DDPs)

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was carried out to confirm the nature of the crystals that make up the agglomerates. 2.3. Adsorption in the dark and photocatalytic tests. 2.3.1. Experimental conditions The kinetics of adsorption in the dark and the photodegradations of methylene blue (MB) were performed at 25 °C under stirring 62.5 mg of the selected photocatalyst in 125 mL MB solution for a catalyst weight of 0.5 g L1. The initial concentration of MB was 12.5 ppm, corresponding to 39.1 lmol L1 (4.89 lmol, ca. 2.95  1018 molecules). On the other hand, the kinetics of adsorption in the dark and the kinetics of phenol (PhOH) photodegradation were performed at 25 °C under stirring 10 mg photocatalysts in 20 mL PhOH solution for a catalyst weight of 0.5 g L1. The initial concentration of PhOH was 25 ppm, ca. 266.0 lmol L1 (5.32 lmol, ca. 3.20  1018 molecules). Although the initial concentration of PhOH is two times higher than that of MB, the number of molecules are similar (ca. 3  1018 molecules). These conditions were chosen to compare the experimental results with the theoretical values because these estimations are based in terms of a fix number of atoms, and therefore, a fixed number of adsorption and active sites are accessible for the photodegradation reactions. 2.3.2. Irradiation source and analysis Open-to-air air batch photoreactors were employed [39]. They consist of 200 mL or 50 mL cylindrical Pyrex flasks provided with a bottom optical window of 6 cm or 4 cm in diameter, for MB and PhOH photodegradations, respectively. Irradiation was performed by using a solar simulator (solar-box) with a Xe-lamp (450 W.m2) emitting the solar spectrum. Total radiation was verified with a pyranometer and the total photon flux was estimated ca. 4  1017 photonscm2 s1. The samples were maintained in the dark for 60 min to achieve adsorption equilibrium in the dark. This time was selected because of the preliminary studies of the MB and PhOH kinetics of adsorption in the dark, explained in Section 3.2. After this time in the dark, the kinetics of MB and PhOH photodegradations were followed. Aliquots of MB were taken off from solution, centrifuged and analysed by using an UVspectrophotometer Perkin Elmer, Lambda 35 at 664 nm. For PhOH studies, several aliquots were taken off and filtered, and the concentration of PhOH and the main intermediate products formed along reaction were analysed by HPLC using similar experimental conditions previously reported [40]. These products were hydroquinone (HQ) and benzoquinone (BQ) No catechol or resorcinol were detected along reaction. The kinetics of adsorption and photodegradation were performed at least by duplicate and the experimental error was below 2% in all the cases. However, since some plotted points are too close one to each other, the error bars were not included in Figs. 7, 9 and 10, to facilitate the visualization of the kinetic results. The first-order apparent rate-constant (kapp) of MB and PhOH photodegradation was obtained from the linear regression of the kinetic data. This kinetic parameter was selected to compare photocatalytic activity obtained on Fu-TiO2-C and C (1%)-TiO2 samples against that obtained on the commercial and pure crystalline anatase TiO2 (TiO2-100%) from Merck.

O atoms. Plane-waves cut-offs for the wave function and enhanced density were 40 for Ti and 400 Ry for C and O atoms, respectively, which was determined performing a series of total energy calculations varying de cut-off energy. For the structural relaxation, the Methfessel-Paxton method [47] was used with a parameter of 0.02 Ry, and for the density of states (DOS) calculations the tetrahedron method was used [48]. For the k-point integration, we use a 4  4  2 mesh for the primitive cell of pure anatase (TiO2), and a 2  2  2 for the super-cells with carbon impurities. In the structural optimizations, all the atoms were relaxed until the residual forces are below 0.001 Ry/Å. Additionally, the DFT + U approach were included in the calculations and for various calculations the parameter used was 3.5. 3. Results and discussion 3.1. Characterization of materials 3.1.1. Elemental analysis Table 1 shows a summary of the elemental analysis (C, H) and residual mass (Ti, O) of samples after calcination. Fu-TiO2-C sample contains 19.2 wt% and 2.3 wt% for C and H, respectively. These results agree with the residual mass of ca. 78.0 wt%. By contrast, the elemental composition of C(1%)-TiO2 sample was only about 0.88 wt% and 0.02 wt%, for C and H, respectively, indicating that after calcination the sample is mainly composed by Ti and O (ca. 99 wt%) with a C content ca. 1 wt. This is an important issue because the C-doped TiO2-based super-cell (Ti15O32C) constructed for the theoretical estimations (Section 3.3) is based on the approximation of the intercalation of one C atom by one Ti atom, in a Ti15O32C super-cell. This super-cell contains a theoretical carbon proportion ca. 1 wt% with respect to its molecular weight. Table 1 shows values below 0.05 wt% and below 0.01 wt% for C and H, respectively in the commercial TiO2-100 wt%, and the residual mass obtained indicating that Ti + O composition is higher than 99.9 wt%. 3.1.2. SEM, XRD and HRTEM analysis. Fig. 1a–d shows the SEM images for Fu-TiO2-C and C(1%)-TiO2 samples, respectively, while Fig. 1e shows the histograms representing the sphere´s size distribution for these two samples. Fig. 1a shows that Fu-TiO2-C sample is composed by microspheres with a mean size ca. 4.0 ± 1.6 lm. Some spheres are still connected to each other showing the interface zone between them (indicated by white arrows), suggesting that the coalescence mechanism was the driving force for the formation of this material [49]. The calcined C(1%)-TiO2 sample is also composed by microspheres (Fig. 1c), but the interface zone among spheres was not detected. It is interesting to highlight that not only the mean sphere size was clearly lower in the calcined sample, ca. 1.7 ± 0.9 lm, but also the spheres size distribution shifted to spheres with lower size as consequence of the carbon calcination (Fig. 1e). This result suggests that TiO2-C is a nanostructured material and agrees with the elemental analysis (Table 1) where Ti + O composition was ca. 99 wt% in the C(1%)-TiO2 sample. Another important aspect is the roughness of the surface of samples. It can be seen from the

2.4. Theoretical methods The total energy of pure anatase TiO2 and C-doped TiO2 was calculated within the generalized gradient approximation (GGA) of the Density Functional Theory (DFT) [41,42]. The Perdew-BurkeErnzerhof (PBE) functional [43,44] with the Vanderbilt ultra-soft pseudopotential [45] as implementation of quantum ESPRESSO [46] were used. Valence electrons included in the calculations were 3s, 3p, 3d, and 4s for Ti atom, while 2s, and 2p were used for C and

Table 1 Summary of elemental analysis and residual mass of catalysts.

a

Sample

C (%)

H (%)

Ti + O (%)a

Fu-TiO2-C C(1%)-TiO2 TiO2-100%

19.2 0.88 <0.05

2.3 0.02 <0.01

78.0 99.0 >99.9

Values obtained from residual mass in TGA.

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Ocurrence (%)

(e)

Fu-TiO2-C

C(1%)-TiO2

30 20 10 0 1

2

3

4

5

6

7

8

9

10

11

Sphere´s diameter (µm) Fig. 1. SEM images of C-doped TiO2 photocatalysts. (a and b): Fu-TiO2-C. (c and d): C(1%)-TiO2. (e) Histograms of the sphere´s size distributions for the samples.

magnified SEM images that not only the Fu-TiO2-C sample (Fig. 1b) but also C(1%)-TiO2 sample (Fig. 1d) showed a rough surface. This is an important parameter to consider because an excessive roughness may influence in the scattering of incidents photons during reaction affecting the photoactivity. This fact will be discussed below with the HRTEM analysis (Figs. 3 and 4). On the other hand, Fig. 2a and b shows the XRD patters for FuTiO2-C and C(1%)-TiO2 samples, respectively. The diffraction peaks for anatase and rutile phases were identified according to the PDF card 84-1286 and PDF card 88-1175, respectively. The strong diffraction peaks at 2h = 25.3°, 37.8°, 48.0°, 55.1°, 62.7° and the soft diffractions peaks at 2h = 36.9°, 38.6°, 70.2°, and 74.0°, can be indexed to the (1 0 1), (0 0 4), (2 0 0), (2 1 1), (2 0 4), (1 0 3), (1 1 2), (2 2 0), (1 0 7), respectively, to the crystal planes of anatase TiO2 (PDF card 84-1286). Additional diffraction peaks were observed at 2h = 54.2°, 69.0°, 74.4°and 82.3° that can be indexed to the (2 1 1), (3 0 1), (3 2 0), (3 2 1), respectively, to the crystal planes of rutile TiO2 (PDF card 88-1175). In conclusion, anatase is the most important crystalline phase with ca. 95% and 85% for Fu-TiO2-C and C(1%)-TiO2, respectively. In a previous study

of electrooxidation of formic acid on Pd-based catalysts supported on TiO2-C samples [50], our group reported that anatase phase is the most important phase obtained by solvothermal synthesis (ca. 90%). This is expected because the temperature of synthesis (180 °C) is much lower than the thermodynamic temperature required to promote the transition from anatase to rutile phase, which is ca. 800 °C [51]. The calcination did not introduce an important transition to rutile phase (only 15%) due to the low calcination temperature (550 °C). Thus, despite of the high carbon content (ca. 19 wt% C, Table 1) for Fu-TiO2-C sample, no shifting was observed for the main important diffraction peaks of anatase or rutile phase. A similar crystallographic stability was also observed for the C (1%)-TiO2 sample obtained after calcination (ca. 1 wt% C, Table 1). On the contrary, it is clear from Fig. 2a, that in presence of high carbon concentration, the peaks are wider and with lower intensity that the peaks observed for the sample calcined with low carbon content (Fig. 2b) which are sharper and with the double of intensity, suggesting an important sintering of TiO2 crystallites due to the calcination of the Fu-TiO2-C sample. The mean size of

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J. Matos et al. / Journal of Colloid and Interface Science 547 (2019) 14–29

Fig. 2. XRD patterns. (a): Fu-TiO2-C. (b): C(1%)-TiO2. [A: anatase; R: rutile phases].

nanocrystallites, obtained by using the Scherrer’s formula [49,50], were ca. 9.3 nm and 25.1 nm for Fu-TiO2-C and C(1%)-TiO2 samples, respectively. Thus, although the mean size of sphere decreased after calcination as showed the histograms from Fig. 1c, sintering during the calcination led to an increase in the mean size of TiO2 crystallites. Fig. S2 (supplementary) shows schematically, how the decrease of the mean sphere size and the increase of TiO2 nanocrystallites by sintering during the calcination, can occur simultaneously. For a better understanding of the C-doping effect upon the lattice variation of TiO2, digital electron diffraction patterns (DDPs) were obtained from HRTEM analysis. Fig. 3 shows the main crystalline faces indexed by electron diffraction obtained from the HRTEM analysis performed to Fu-TiO2-C and C(1%)-TiO2 samples. It can be seen from the HRTEM images and the DDPs from Fig. 3 that the structure of the two samples is made up of agglomerates of crystals with nanometric-size, confirming that the present hybrid materials are nanostructured. Fig. 3 shows the presence of TiO2 nanocrystals oriented along the zone axis [0 1 0] and [1 1 1] whose crystallographic relationships can be indexed to the anatase phase being 101 and 011, the main crystalline faces observed in both samples. Rutile phases were not detected in this analysis, indicating that the structural framework of the present materials is mainly constituted by anatase phase in agreement with the XRD patterns from Fig. 2. In addition, the influence of carbon content upon the roughness of the crystallites of TiO2 can be seen from HRTEM images of the nanocrystals of both samples (Fig. 4). Most of these crystallites present well-defined morphologies where the presence of crystalline faces is quite evident. Deepening the analysis of the exposed surface of these facets, it has been observed that the TiO2 crystals of the Fu-TiO2-C sample exhibit a slightly rougher surface than those of the C(1%)-TiO2 sample crystals. This HRTEM analysis agree with the morphological observations from Fig. 1. Thus, a lower surface roughness showed by the calcined sample suggest a lower scattering of incident photons enhancing the light harvesting efficiency factor as reported elsewhere by our group [8]. In addition, Lee and co-workers have shown that an increase in the sidewall roughness of crystalline phases in photoactive semiconductors is responsible of a loss of transmission of light and an enhancement in the scattering loss in optoelectronic devices [52].

3.1.3. Surface pH and textural properties. Fig. S3 (supplementary) showed the plot of pH of samples as a function of weight of material. These values were estimated from the results obtained at steady-state conditions after ca. 90 min starting from neutral pH. Therefore, the values so obtained, can be considered as the pH at the zero-point charge (pHPZC). These values are included in Table 2. The surface pH of Fu-TiO2-C is clearly acid (ca. 3.9) in agreement with the presence of carboxylic acid groups in the solid remaining from the solvothermal process [7]. After calcination, the surface pH increases up to a less acid pH (ca. 5.3), consequence of the evolution of carboxylic groups during the calcination. It must be point out that the lightly basic pH of the commercial TiO2 (ca. 7.3) agrees with the lack of defects [53] in the crystalline framework of pristine anatase. On the other hand, Fig. 5 shows the adsorption-desorption N2 isotherms and a summary of textural and porosimetry results is compiled in Table 2. Both adsorption isotherms are type IV(a) with a hysteresis loop indicating that the present materials are composed mainly by a mesoporous framework with a contribution of micropores. Thommes and co-workers [54] have reported that the adsorption in mesopores is determined by the adsorbentadsorptive interactions and by the interactions between the molecules in the condensed state. Thus, these phenomena are discussed below for the adsorption of MB and PhOH considering a pseudo first- and second-order kinetics for the adsorption in the dark as well as the intraparticle diffusion model. The hysteresis loop from Fig. 5a for Fu-TiO2-C is different from that observed in Fig. 5b for the calcined sample. Fig. 5a showed a H4 hysteresis loop with a closing desorption branch at ca. 0.4 relative pressure. This is associated with materials containing both micropores and small mesopores [54] as confirmed by the pore volume values (Table 2), where the contribution of micropores is only ca. 18% of the total pore volume and with an average pore diameter of 6.2 nm, which corresponds to small mesopores. By contrast, the very steep desorption branch observed for the C (1%)-TiO2 sample (Fig. 5b) is a characteristic feature of H2(a) hysteresis loops [54] which is associated with a more complex pore structure and with the pore blocking of the material as indicated by the remarkable decrease in BET surface area, ca. 13 m2 g1 (Table 2) against 120 m2 g1 for the non-calcined sample. It has been reported [54] that calcination processes had widened the entrances to spherical pores. In agreement, the calcined sample showed only ca. 9% for the micropores contribution and the average pore diameter increased up to ca. 14.5 nm, which is more than two times wider than that for the non-calcined sample. Table 2 shows that the commercial TiO2 sample is composed by mesopores and this sample is characterized by a low surface area ca. 25 m2 g1 and with an average pore diameter ca. 13.4 nm [38]. 3.1.4. DR/UV–Vis Fig. 6 shows the diffuse reflectance UV–vis spectra, (DR/UV–vis) plotted as the Kubelka-Munk function of the reflectance F(R), for Fu-TiO2-C and the calcined C(1%)-TiO2 samples. Pristine anatase commercial semiconductor (TiO2-100%) is also included for comparative purposes. DR/UV–vis spectra of the non-calcined sample showed a much higher intensity and full absorption spectra along the UV and visible ranges. This is expected because Fu-TiO2-C showed ca. 12 wt % in carbon content while the calcined and commercial samples contain only ca. 0.9 wt% and <0.05 wt% for C(1%)-TiO2 and TiO2100% samples, respectively. Titirici and coworkers [55] have shown that photosensitive functional groups on the surface of carbon promotes the absorption of photons in the visible range. This is due to the low energy gap for the p-to-p* excitation of functional oxygen groups such as carbonyl, ketones and pyrones [55]. This excitation is particularly truth for the high carbon

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Fig. 3. HRTEM images and DDPs patterns. (a and b): Fu-TiO2-C; (c and d): C(1%)-TiO2.

content samples such as the present Fu-TiO2-C as can be seen from Fig. 6. It can be seen that Fu-TiO2-C absorbs much more photons than C(1%)-TiO2 and TiO2-100% both at UV as in the visible region indicating that the sphere framework is responsible for an enhancement in the optoelectronic response of TiO2 by an increase in the light scattering [56] within the sphere. The band gap energies (Ebg) of the samples were estimated from the tangent lines in the plots of F(R) as a function of the energy. These values were 3.15 eV, 3.03 eV, and 3.24 eV, for Fu-TiO2-C, C (1%)-TiO2 and TiO2-100%, respectively, (Table 2). These values indicate that the calcined sample with ca. 1 wt% C doping showed an important red-shift with a decrease in the energy band gap down to 3.03 eV (absorption edge ca. 410 nm) while the commercial sample showed 3.24 eV which is expected for pristine anatase [57]. Due to the very broad absorption spectra of the noncalcined sample, the energy band gap cannot be accuracy estimated by this way, but as a first approach, considering only the UV-region, an energy band gap about 3.15 eV was estimated, indicating that carbon contribution is responsible of the decrease in the energy band gap of the semiconductor. 3.2. Methylene blue removal and conversion 3.2.1. Adsorption in the dark Fig. 7a shows the kinetics of MB adsorption in the dark on the present samples. It can be seen a slightly change of only 2–4% in MB adsorption between 60 and 90 min. This range is almost similar for the three samples and it is a clear indication that equilibrium of MB adsorption has been reached after 60 min. So, this time was considered as the minima time required to achieve the equilibrium of adsorption before to start the irradiation in the photocatalytic test.

It is clear from Fig. 7a that Fu-TiO2-C adsorbs much more MB than C(1%)-TiO2 and TiO2-100%. This result can be explained in terms of the surface pH and the BET surface area listed in Table 1, and Table 2, respectively. Fu-TiO2-C sample showed the highest surface area and the more acid surface pH, and therefore, an important trend to adsorb basic amines such as MB with a high dissociation constant (pKb) in water and a high half-neutralization potential [5] was expected. However, despite of TiO2-100% sample showed two times higher BET surface area that C(1%)-TiO2 sample, the MB adsorption on the former was lower indicating that other variables influence the MB adsorption on these materials. Thus, for a better understanding of the adsorption process of MB, considering the values from the kinetics of MB adsorption (Fig. 7a) the fits of the kinetic data with the pseudo first-order, the pseudo second-order, and the intraparticle diffusion model (IPD) were verified. The pseudo first-order model is given by the Eq. (1) [58,59]:

dnadst =dt ¼ k1 ðneq  nadst Þ

ð1Þ

where k1 (min1) is the pseudo first-order rate constant for the adsorption, nads-t is the amount of MB adsorbed (lmol) at time t (min) and neq is the amount adsorbed at equilibrium (lmol). The integration of Eq. (1) at the initial conditions (nads = 0 at t = 0) gives the equation (2).

 logðneq  nads Þ ¼ logðneq Þ 

 k1 t 2:303

ð2Þ

The pseudo second-order equation [60,61] may be expressed by Eq. (3):

dnadst =dt ¼ k2 neq  nadst


2

ð3Þ

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Fig. 4. Composition of HRTEM images comparing the roughness of the crystalline surfaces. (a–c): Fu-TiO2-C; (d–f): C(1%)-TiO2.

Table 2 Textural properties, surface pH (pHPZC) and energy band gap (Ebg) of samples.

d e f

Vlporeb (cm3 g1)

Vmesoc (cm3 g1)

Vtotd (cm3 g1)

Dporee (nm)

pHPZC

Ebg (eV)

Fu-TiO2-C C(1%)-TiO2 TiO2-100%

120 13 25f

0.033 0.004 –

0.155 0.043 0.108f

0.186 0.047 0.110f

6.2 14.5 13.4f

3.9 5.3 7.3f

3.15 3.03 3.24

SBET, BET Surface area. Vmeso, volume of mesopores. Vlpore, the micropore volume. VTot, total volume of pores. Dpore, average pore diameter. Taken from Ref. [38].

140

35

(a)

(b)

30

120 Vol. ads. (cm3/g, STP)

c

SBETa (m2 g1)

Vol. ads. (cm3/g, STP)

a b

Support

100 80 60 40 20

25 20 15 10 5 0

0 0

0.2

0.4

0.6

0.8

Relave Pressure (P/Po)

1

0

0.2

0.4

0.6

0.8

Relave Pressure (P/Po)

Fig. 5. Adsorption/desorption N2 isotherms at 77 K. (a): Fu-TiO2-C. (b): C(1%)-TiO2.

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J. Matos et al. / Journal of Colloid and Interface Science 547 (2019) 14–29

25

F (R)

regression of the curve nads-t = f(t0.5), expressed by the Eq. (5) where kp is the IPD rate constant (lmol min0.5), and C is a constant (lmol) attributed to the extension of the boundary layer thickness [64].

Fu-TiO2-C C(1%)-TiO2 TiO2-100%

20

21

15

nadst ¼ C þ kp  t0:5

10

The plots for the MB adsorption in the dark on Fu-TiO2-C, C(1%)TiO2 and TiO2-100%, considering the pseudo first-order, the pseudo second-order, and the intraparticle diffusion model (IPD) are shown in Figs. S4–S6 (supplementary), respectively. Table 3 listed the values of k1, k2 and kp and the regression factors obtained from the kinetic treatments for MB adsorption. The values obtained for k1, k2 and mainly in kp on the Fu-TiO2-C sample are clearly higher than values obtained on the other samples. This can be ascribed to the higher surface area and a to the lower acidic surface pH in this sample suggesting these properties induce a faster kinetics of MB adsorption. The regression factors in Table 3 showed that Fu-TiO2-C fitted much better on a pseudo firstorder kinetic model while C(1%)-TiO2 and TiO2-100% samples fitted better on a pseudo second-order kinetic model. These results agree with the high carbon content observed for the Fu-TiO2-C sample (ca. 19 wt%, Table 1) in comparison of the low and negligible carbon contents, ca. 0.9 wt% and <0.05 wt% for C(1%)-TiO2 and TiO2100%, respectively. A higher carbon content would promote a higher adsorption capacity due to a more developed pore framework, suggesting a physisorption mechanism [58,59] for MB on Fu-TiO2-C, despite of this sample is characterized by a very acidic surface pH (Table 2). On the contrary, the high regression factors observed for the pseudo second-order model on C(1%)-TiO2 and TiO2-100% samples, suggest a chemisorption mechanism [60,61] for the MB adsorption on these solids, even despite of these samples are lightly acid or neutral (Table 2). It should be highlighted that the three samples fitted very well (R2kp > 0.94 in most of cases) with the intraparticle diffusion model suggesting that the adsorption of MB is controlled by this model. This result is consistent with the fact that the present samples are complex materials, composed of micro and mesopores as discussed above (Table 2). The higher the Vlpore/Vtot ratio and the more acid the surface pH (Table 2) of the solids, the higher the constant C (Table 3) for the MB adsorption. In conclusion, both the Vlpore/Vtot ratio and pHPZC are the driven-force for the adsorption of MB according to IPD model. This can be inferred from the fact that despite of the surface area for TiO2-100% is two-times higher than that of C(1%)-TiO2, the C constant from IPD model was remarkably lower in TiO2-100%. This is due to the negligible micropore volume on this sample (Table 2) suggesting that the presence of micropores is mandatory to facilitate the diffusion of MB from mesopores and thus, promote a higher adsorption capacity from the bulk of solution.

5 0 200

300

400

500

600

700

800

Wavelength (nm) Fig. 6. Diffuse reflectance UV–visible spectra (DR/UV–Vis).

Fig. 7. (a): Kinetics of MB adsorption in the dark. (b): Kinetics of MB photodegradation under artificial solar irradiation. (c): Linear regression of the kinetic data from Fig. 7b.

where k2 is the pseudo second-order rate constant (lmol1min1) for the adsorption. Applying the initial conditions, Eq. (3) can be integrated to obtain Eq. (4).

ð1=neq  nadst Þ ¼ ð1=neq Þ þ k2  t

ð5Þ

ð4Þ

The fractional approach to equilibrium change is done according to a function of (Dt/r2)0.5, where r is the radius of adsorbent particle and D is the effective diffusivity of solute within the particle [62,63]. The initial rate of IPD [63] is obtained from the liner

3.2.2. Photodegradation of methylene blue Some preliminary aspects must be point out before discussing the photocatalytic degradation of MB. Interesting contradictorily points of views [65–69] have been established regarding the use of dyes and azo-dyes in photocatalytic degradation tests. Vinodgopal and coworkers reported [65] some dyes can absorb visible light and photoassist to semiconductor during photodegradation. Ohtani and coworkers [66] established that MB absorbs photons from visible light and therefore it would not be an appropriate substrate for the evaluation of the photocatalytic activity under visible light irradiation of semiconductors. Herrmann and coworkers have reported [67] that the color fading of dyes and azo-dyes under visible light irradiation depends on the experimental conditions, mainly the source of the irradiation. Our group have reported in an earlier work of MB photodegradation by using N-doped porous carbons [5] that at high photon flux, the MB photodegradation

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J. Matos et al. / Journal of Colloid and Interface Science 547 (2019) 14–29

Table 3 Summary of the kinetics parameters for the MB and PhOH adsorption considering the pseudo first-order, the pseudo second-order, and the intraparticle diffusion model (IPD).

a b c d e f g

Target molecule

Sample

k1a (min1)

R2k1b

k2c (lmol1 min1)

R2k2d

kpe (lmol min0.5)

R2kpf

Cg (lmol)

MB

Fu-TiO2-C C(1%)-TiO2 TiO2-100%

0.054 0.039 0.037

0.929 0.640 0.845

0.360 0.318 0.221

0.826 0.914 0.985

0.091 0.016 0.038

0.954 0.944 0.994

0.880 0.830 0.537

PhOH

Fu-TiO2-C C(1%)-TiO2 TiO2-100%

0.066 0.044 0.051

0.982 0.981 0.968

1.35 0.208 0.328

0.821 0.865 0.851

0.071 0.113 0.093

0.909 0.978 0.973

0.010 0.100 0.477

Pseudo first-order rate-constant. Linear regression factor of the first-order rate-constant. Pseudo second-order rate-constant. Linear regression factor of the second-order rate constant. Intraparticle diffusion model rate-constant. Linear regression factor for the intraparticle diffusion model rate-constant. Extension of the adsorbate on the boundary layer thickness according to IPD model.

takes place at steady-state conditions in terms of the energy supplied. Similar approaches have been reported for the photocatalytic degradation of MB by Inagaki and coworkers [68] on carbon coated TiO2. Preliminary experiment of direct photolysis of MB (in absence or photocatalysts) is included in the Fig. 7b. It can be seen direct photolysis of MB is only ca. 5% after 3 h irradiation. It means that photolysis of MB is negligible using the photon flux of the artificial solar light employed in this work. Accordingly, the linear regression for the photolysis (Fig. 7c) yields a very low first-order apparent rate-constant of ca. 3  105 min1 (Table 3) which is 2-order magnitude lower than values obtained in presence of photocatalysts. Therefore, a photoactive semiconductor is required to induce the photooxidation of the dye [7]. We can infer that color fading problem of MB reported by Ohtani and coworkers [66] during visible irradiation is negligible, because in the present study, color back to the photoreactor was not detected after turn-off the irradiation. Thus, in agreement with Herrmann and coworkers [67] whom work in similar experimental condition of high photon flux of ca. 1017 photons cm2 s1 as in the present work, the MB disappearance showed in the Fig. 7b is due to a photocatalytic effect. Thus, the Fig. 7b shows the kinetics of MB photodegradation while Fig. 7c shows the linear regression of the kinetic data from Fig. 7b used to obtain the first-order apparent rate-constants (kapp) by Eq. (6).

LnðCo =Ct Þ ¼ kapp  t

ð6Þ

The kinetics of MB photodegradation in Fig. 7b shows that C (1%)-TiO2 sample is more photoactive than pristine anatase (TiO2-100%) and much more than Fu-TiO2-C. Table 4 listed a summary of the kinetic parameters obtained from the MB degradation. Fu-TiO2-C developed the lowest photoactivity in terms of kapp (2.72  103 min1) even despite of this sample adsorbed the highest MB quantity. The kapp obtained on the C(1%)-TiO2 sample was 7.75  103 min1 which is up to 1.6 and 2.8 times higher than TiO2-100% and Fu-TiO2-C, respectively. The photocatalytic activity in the degradation of MB on C(1%)-TiO2 sample is comparable to Cdoped TiO2 composites reported by Ma and coworkers [25]. However, no correlations were found between the photocatalytic activity (in terms of kapp) of the present samples with the texture or the surface pH, despite of the later seems to play the most important role in controlling photoactivity. Thus, for a better understanding of the influence of the textural properties upon the MB adsorption and photodegradation processes, the surface concentration also called surface density (dsur) [69] of MB on the present samples were estimated by Eq. (7), where neq is the amount of adsorbed

MB in the dark after 60 min (Table 4), SBET is the BET surface area (Table 2), and m is the mass of solid.

dsur ¼ neq =SBET  m

ð7Þ

The dsur values are listed in Table 4. The highest surface density for MB adsorption was obtained on the C(1%)-TiO2 sample. It can be attributed to a surface pH almost neutral (ca. 5.3, Table 2) and to a high diffusion rate (Table 4) due to the micropore contribution (ca. 9%, Table 2) which agrees with a high C constant according to IPD model discussed above. However, despite of Fu-TiO2-C showed two-times higher micropore volume contribution to the total volume of pores (Table 2) than C(1%)-TiO2, the surface density of the later is ca. 5.5 times higher suggesting that the present Cdoped TiO2 nanostructured material is more complex than expecting, and further aspects besides texture and surface pH must be considered to explain the photoactivity. On the other hand, it has been widely reported the photodegradation of MB is a unimolecular photocatalytic surface reaction [25,40,66–69]. It means that MB adsorption is followed by the photocatalytic degradation under UV–vis irradiation described by Eq. (8), where R is the reactant (MB or PhOH) and S an adsorption site on the surface of photocatalysts.

R + S MBAds ! Intermediates ! Products

ð8Þ

The global reaction-rate (vreac) [69] can be estimated by the equation (9) where vreac is expressed in lmol m2 min1 and dsur is obtained from Eq. (7).

vreac ¼ kapp  dsur

ð9Þ

The vreac values are listed in Table 4 showing that the higher the surface density the higher the reaction-rate. An almost perfect quadratic relationship between the global reaction-rate and the surface density is shown in the Fig. 8a suggesting that MB photodegradation is highly dependent of the surface density and this parameter is highly dependent not only on the textural and porosimetry properties but also on the surface chemistry of the material as suggest the kinetics parameters for the MB adsorption discussed above. For example, despite of Fu-TiO2-C showed a lower photocatalytic activity than pristine anatase TiO2 (TiO2-100%), after calcination, the photocatalytic activity of C(1%)-TiO2 increased up to ca. 3.7 times higher than TiO2-100% and ca. 15.5 times higher than Fu-TiO2-C. However, the surface density of the C(1%)-TiO2 is only ca. 2.3 and 5.5. times higher than that on TiO2-100% and Fu-TiO2C. Therefore, it seems to be logical to associate the increase in photocatalytic activity for MB photodegradation to other variables such as the changes in the optoelectronic properties and the surface chemistry caused by the carbon doping. The experimental

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J. Matos et al. / Journal of Colloid and Interface Science 547 (2019) 14–29

Table 4 Summary of the kinetic results. Adsorption in equilibrium (nads), surface density (dsur), first-order apparent rate-constants (kapp), linear regression factor (R2), global reaction-rate (vreac), and photocatalytic activity relative to TiO2-100% (/rel). dsurb (lmol m2)

kappc 103 (min1)

R2d

vreace 103 (lmol m2 min1)

/relf

g

g

0.22 1.2 0.53

0.03 2.72 7.75 4.75

0.707 0.942 0.985 0.987

g

1.66 0.95 0.83

0 0.24 3.7 1.0

0.02 1.84 5.75 3.95

0.746 0.971 0.986 0.987

g

nadsa (lmol)

Target molecule

Sample

MB

Photolysis Fu-TiO2-C C(1%)-TiO2 TiO2-100% Photolysis Fu-TiO2-C C(1%)-TiO2 TiO2-100%

g

g

0.60 1.04 1.26

0.50 8.0 5.0

PhOH

0.60 9.3 2.5 0.92 46.0 19.8

0 0.05 2.3 1.0

a

Obtained after 60 min adsorption in the dark. Surface density estimated from Eq. (7) using 62.5 mg and 10 mg as photocatalysts weights for MB and PhOH, respectively. c kapp obtained from Eq. (6). d Linear regression factor. e Global reaction-rate obtained from Eq. (9). f /rel obtained from (vreac-i/vreac-TiO2-100%). g Direct photolysis experiments are conducted in absence of photocatalysts; thus, no adsorption of pollutants occurred in this process and estimations of surface density or global reaction-rate are not possible to do. b

Fig. 8. Relationship between the global reaction-rate and the surface density for the photocatalytic degradations. (a): Methylene blue; (b): Phenol.

Ebg-exp values were 3.24 eV, 3.15 eV, and 3.03 eV for TiO2-100%, FuTiO2-C, and C(1%)-TiO2, respectively. Yu and coworkers [33] and Li and coworkers [35] have found similar results. Thus, it can be concluded that the red-shift caused by the C doping is responsible for the increase in the photocatalytic activity for MB photodegradation. 3.3. Phenol removal and conversion 3.3.1. Adsorption in the dark A carefully study of adsorption and photodegradation of phenol was also performed. This molecule was selected because it is a Brönsted acid and therefore, it presents an opposite chemical nature than MB. Fig. 9a shows the kinetics of MB adsorption in the dark on the present samples. Similar than MB adsorption, a slightly change of only 2–6% in PhOH adsorption was observed between 60 and 90 min, the three suggesting the equilibrium of adsorption has been almost totally reached after 60 min for the three samples. So, this time was considered before to start the photocatalytic test. Table 4 shows that Fu-TiO2-C sample adsorbs less PhOH than C (1%)-TiO2 and TiO2-100% samples. This result was expected because the acid nature of phenol and the acid surface pH in FuTiO2-C sample (Table 1) which indicates an opposite trend in the PhOH adsorption than that observed for MB (Fig. 7a). Nevertheless, for a better understanding of the influence of the textural and chemical properties of the present materials upon

the PhOH uptake, the plots for the PhOH adsorption in the dark on Fu-TiO2-C, C(1%)-TiO2 and TiO2-100%, considering the pseudo first-order, the pseudo second-order, and the intraparticle diffusion model (IPD) were performed. These results are shown in Figs. S7–S9 (supplementary), respectively. Table 3 listed the values of k1, k2 and kp and the regression factors obtained from the kinetic treatments. The C constants according to IPD model (Table 3) followed an opposite trend for PhOH adsorption than that observed for MB. The more basic the sample the higher the C constant suggesting that pHPZC is the driving-force for the adsorption with TiO2-100% showing the highest adsorption, ca. 1.26 lmol (Table 4). Table 3 shows all samples fitted much better the pseudo firstorder kinetics for PhOH adsorption suggesting a physisorption mechanism influenced by the IPD model but with C constants clearly lower than in the case of MB. This result was expected because the kinetic diameter of PhOH [70] is clearly lower (0.67 nm) than the longitudinal length [71] of MB (1.42 nm), and therefore, the ratio between the average pore diameter of FuTiO2-C (Table 2) and the kinetic diameter of PhOH and the longitudinal length [71] of MB, results in values ca. 9.3 and ca. 4.4, for PhOH and MB, respectively. Same trends were found for the other samples, and thus, it can be concluded the diffusion of PhOH from the bulk of solution to the pores of materials is not limited and therefore, lower C constants are expected for PhOH than for MB according to IPD model (Table 4).

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J. Matos et al. / Journal of Colloid and Interface Science 547 (2019) 14–29

the present samples is clearly much higher than that for the MB, the kapp obtained for the PhOH photodegradation were clearly lower than the values obtained for the MB photodegradation (Table 4). This can be attributed to the fact that PhOH is a more refractory molecule than MB [40]. In addition, despite of the C constant according IPD model (Table 3) and the PhOH uptake in equilibrium (nads, Table 4) suggest that TiO2-100% has a higher affinity to adsorb phenol than C(1%)-TiO2, the C-doping plays the most important role for the photoactivity of TiO2, due to a higher absorption of photons from visible light range as discussed above [4,7,9,15]. This inference was verified in the same way than in case of MB by plotting the global reaction-rate as a function of the surface density. Fig. 8b shows a perfect quadratic relationship between these parameters suggesting that not only C-doping but also the surface density of PhOH molecules play the most important factors affecting the photocatalytic activity of C-doped TiO2based nanostructured materials. As discussed above, the surface density is affected by the pore size distribution and by the pHPZC of the solids. Despite Fu-TiO2-C showed two times more micropore volume contribution to the total pore volume than C(1%)-TiO2, the later showed 16 times more surface density because of the less acid surface pH (Table 2). In terms of the reaction-rate (vreac, Table 4), the enhancement in the surface density of PhOH molecules on C(1%)-TiO2 is responsible for the increase in the photoactivity up to 50 and 2.4 times higher than Fu-TiO2-C and TiO2-100%, respectively.

3.3.2. Phenol photodegradation Fig. 9b and c shows the kinetics of PhOH photodegradation and the linear regression of the kinetic data, respectively. The firstorder apparent rate constants (kapp) obtained from the linear regressions are listed in Table 4. Similar than MB, a preliminary experiment of direct photolysis for PhOH (in absence or photocatalysts) was performed, Fig. 9a shows that photolysis of PhOH is only ca. 3% after 3 h irradiation concluding PhOH degradation by photolysis is negligible using the present experimental conditions. Accordingly, the linear regression for the photolysis (Fig. 9c) yields a very low first-order apparent rate-constant of ca. 2  105 min1 (Table 4) which is 2-order magnitude lower than values obtained by using photocatalysts. The photocatalytic activity observed for the photodegradation of PhOH on C(1%)-TiO2 is comparable to that reported by Tryba and coworkers [9] with 80–90% PhOH conversions after 4 h irradiation. Contrary to the adsorption trends, the kinetic of PhOH photodegradation shows the similar behaviour than that observed for MB photodegradation. It means that C(1%)-TiO2 sample is more photoactive than TiO2-100% and much more than Fu-TiO2-C. Table 4 shows that the kapp values obtained on C(1%)-TiO2 sample is ca. 1.5 and 3.1 times higher than that on TiO2-100% and Fu-TiO2C, respectively. Despite of the surface density of PhOH adsorbed on

0.30 TiO2(100%)

HQ + BQ (μmol)

Fig. 9. (a): Kinetics of PhOH adsorption in the dark. (b): Kinetics of PhOH photodegradation under artificial solar irradiation. (c): Linear regression of the kinetic data from Fig. 9b.

3.3.3. Intermediate products from PhOH degradation The main intermediate products detected during the PhOH photodegradation were hydroquinone (HQ) and benzoquinone (BQ), which in the present experimental conditions (ambient temperature and almost neutral pH of solution), both are commonly found in thermodynamic equilibrium [9–11,19,40]. The kinetic trends for the appearance and disappearance of HQ + BQ are plotted together in Fig. 10. These kinetic trends are very similar suggesting that the mechanism of reaction is the same on the present samples and therefore, it is independent of the changes in the texture or chemical properties of the present materials. The maxima values detected for the HQ + BQ concentration were ca. 0.11 lmol (at 30 min reaction), ca. 0.23 lmol (at 45 min reaction) and ca. 0.24 lmol (at 45 min reaction) for C(1%)-TiO2, Fu-TiO2-C and TiO2-100%, respectively. A clear decrease in the HQ + BQ concentration (50% lower) was detected for the C(1%)-TiO2 sample, which is in good agreement with a higher photocatalytic activity discussed above. In other words, the higher the photoactivity the lower the concentration of the intermediate products detected. From a molecular point of

FU-TiO2-C

C(1%)-TiO2

0.20

0.10

0.00 0

60

120

180

240

300

360

Time (min) Fig. 10. Kinetics of appearance and disappearance of the main intermediate products detected along the photodegradation of phenol under artificial solar irradiation.

J. Matos et al. / Journal of Colloid and Interface Science 547 (2019) 14–29

view, these values are in good agreement with the initial concentration of PhOH (5.32 lmol). It means that between 2 and 4% of phenol molecules have been converted into HQ + BQ molecules. These values agree with the PhOH photocatalytic degradation (Fig. 9b) showing between 10 and 15% conversion between 30 and 45 min reaction. It means, that besides CO2 (final product of the mineralization) some others molecular intermediates have been formed, probably catechol and in less proportion resorcinol [40], but the concentrations of these would be below of the limit of detection of the HPLC equipment because they were not detected in the present work. 3.4. Theoretical estimations The theoretical estimations for TiO2 and C-doped TiO2 were performed considering a pristine anatase crystalline structure of TiO2 showed in the Fig. 11a, which has a tetragonal body-centred lattice (space group I4I/amd, Z = 4), with the values of crystal lattice parameters as follows: a = b = 3.7845 Å, c = 9.5143 Å, u = 0.20806 [72]. The tetragonal unit cell of anatase contains four Ti atoms and eight O atoms. The nearest coordination sphere of a Ti atom is a distorted octahedron of eight O atoms, whereas the nearest coordination sphere of an O atom is a distorted triangle of Ti atoms. For C-doped anatase TiO2, a (2  2  2) Ti16O32 super-cell was used for the calculations by means of 2-fold translations of the primitive unit cell Ti4O8 along the ax and ay vectors of translations. There are many discrepancies about what the most stable configuration for the C-doped TiO2-based materials is [36,73,74]. This is due to most of the theoretical studies use different dopant concentrations [75] and as expected, the dopant content influences the intercalation or substitutional mechanisms. In addition, carbon doping can be associated to substitutional oxygen by carbon (anion doping) or by the intercalation of one C atom at a Ti site (cation doping). The calculated structures suggest that cation-doped C atoms form a carbonate-type structure or even oxycarbide as reported earlier by our group [50], whereas anion-doped C atoms do not invoke any significant structural change [73]. Kamisaka and coworkers [73] showed that changes in the energy absorption in Cdoped TiO2 can be ascribed to the transitions from the valence band to the impurity states. These transitions should be able to

25

promote photocatalytic reactions, because electron holes in the valence band are crucial for this process. Keeping this in mind, the present structural model is constructed under the basis that the minimal quantity of carbon required to dope a stable TiO2-based structure would be that one obtained after the substitution of one C atom by one Ti atom (cation doping). Therefore, the modelling of C atom defect was performed by replacing one Ti atom in the Ti16O32 super-cell by one C atom. This replacement yields the model Ti15O32C in the Fig. 11b. As pointed out in the experimental section, this approximation was done because this C-doped super-cell contains theoretically ca. 1 wt% C relative t its molecular weight. Thus, this C content is like that obtained experimentally (ca. 0.9, Table 1). In addition, because of the similitude in the oxidation states between Ti and C (both as Ti+4 and C+4), this substitution would have a minimum effect on the magnetic moment in the anatase structure and the estimation of the band structure for the substitutional defect is the most close to the real situation [76–78]. Fig. 12 shows the electronic density of states (DOS) of both the non-doped pristine anatase super-cell Ti16O32 (Fig. 12a) and the Cdoped super-cell Ti15O32C (Fig. 12b). Table 5 listed a summary of the theoretical estimations including the total electron charge (ETotal-charge) and partial electron charge (EPartial-charge) for s, p, and d, orbitals. Table 5 also contains a comparison between the theoretically estimated and the experimentally obtained values for the energy band-gap, Ebg-theo and Ebg-exp, respectively. The Ebg-theo was calculated from the difference between the conduction and valence band showed in Fig. 12. These values were ca. 3.14 eV and ca. 2.94 eV for the pristine Ti16O32 and C-doped Ti15O32C super cells, respectively. These values are in good agreement with the experimental data obtained in the present work, ca. 3.24 eV and ca. 3.03 eV for pure anatase and C-doped TiO2, respectively and agree with values reported by different groups [7–8,31–35]. By contrast, the present theoretical estimations are remarkably higher than those obtained by Tsetseris [36] and Asahi and co-workers [79] who reported values ca. 1.9 eV and 2.0 eV, respectively. This may be due to the present estimations were performed introducing C atoms as substitutional impurity.

Fig. 11. (a): Structure of pristine anatase TiO2 cell. (b): C-doped TiO2 super-cell. [Ti: red spheres, O: light gray, C: light yellow]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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J. Matos et al. / Journal of Colloid and Interface Science 547 (2019) 14–29

MB and PhOH is most likely promoted by reactive oxygen species (ROS) formed upon irradiation of the materials, mainly hydroxyl radical (OH) and superoxo anion radical (O 2 ). This has been widely reported in the literature for different carbon-containing photocatalysts such as C-doped mesoporous anatase [24], graphene-TiO2 nanosheets [25], hybrid TiO2@g-C3N4 core-shells hollow nanospheres [26], TiO2-based microspheres [30], activated carbon/TiO2 composites [84,85], and biogenic TiO2-SiO2 composites prepared from rice husks [86]. As discussed above for the PhOH photodegradation, the same intermediate products (HQ and BQ) were detected using any of the three samples, independently of the C-content. Therefore, it is expected that mainly hydroxyl radical (OH) besides superoxo anion radical (O 2 ) would be the most important ROS formed under irradiation and therefore the MB and PhOH degradations occur by consecutive first-order isokinetic reactions [40]. According to previous works [9–11,19,40], the hydroxy radical (OH) is formed by the interaction between hydroxyl groups from adsorbed water molecules with the holes (h+) formed on the surface of the irradiated TiO2 by means of reactions (10)–(12). UV photons create electron hole pairs in TiO2 according to Eq. (10) which separate because of electron transfer reactions by Eq. (11) and (12). Fig. 12. Electronic density of states (DOS) of materials. (a): Pristine anatase TiO2 cell. (b): C-doped TiO2 super-cell.

TiO2 + hm ! e + pþ

ð10Þ

DFT estimations permit to understand in a better manner this contradictorily results because when a C atom is introduced as substitutional, it creates defect levels in the anatase band gap [73]. In consequence, a decrease in the energy band-gap is expected because of the presence of C-related gap states [36] due to electronical charges in 2p orbitals, indicated by the sharp peak around 0 eV in the Fig. 12b for the C(1%)-TiO2 sample. It is clear from data in Table 5 that the chemical environment influences the partial charge of the orbital 2p in O atoms. For example, the value for Epartial-charge in O atoms with respect to C atoms is lower than that obtained with respect to Ti atoms (4.8404 against 5.0531). The presence of C-related gap states agrees with an enhancement of the visible light absorption as suggested by Asahi and co-workers [79], Sakthivel and Kisch [80], Khan and coworkers [81]. Yang and co-workers [82] have also reported for Ndoped TiO2, that 2p states for N atoms lie within the band gap in the substitutional N to O structure and interstitial N-doped TiO2 supercell, which results in the reduction of the photon-transition energy and absorption of visible light as in the present case for C-doped TiO2. Sun and co-workers reported [83] from DFT calculations, that C-doping influence the electron total charge of Ti as in the present study where small changes in the partial and total electron charge were obtained (Table 5). The present results suggest a fast electron injection from the excited functional oxygen groups on the carbon surface to the conduction band of TiO2 in a similar way than dye-sensitized solar cells.

O2 + e ! O2  (ads)

ð11Þ

OH + pþ ! OH .

ð12Þ

3.5. General discussion It should be mention that given the low degradation yield for the non-catalyzed reaction (direct photolysis), the oxidation of

Then, OH° radicals created by Eq. (12) react with phenolic compounds to produce hydroxylated aromatic compounds, mainly hydroquinone in equilibrium with benzoquinone (Fig. 10), and then aliphatic fragments resulting from the opening of these intermediates producing CO2. Thus, it is expected that MB photodegradation follows a similar reaction mechanism (oxidation by hydroxylation) as reported elsewhere [7,67] because no hypsochromic shifts were observed in the wavelength of the UV–visible spectra along reaction [86]. However, despite of the photo-assistance from the dye to the semiconductor has been discarded in the present experimental conditions [5,7], C-doped TiO2-based hollow microspheres develops interesting optoelectronic properties due to the presence of quantum carbon dots, recently reported by our group [87]. The presence of carbon dots represented by the C-doped supercell in Fig. 11b is responsible of the red-shift in the semiconductor. As discussed above (Fig. 6), an absorption shift of 410 nm was obtained in the DR/UV–Vis spectra for the C(1%)-TiO2 material. This shift corresponds to an energy band gap of about 3.03 eV which is lower than the value for the commercial TiO2 employed in this study (3.24 eV) for pristine anatase TiO2. This energy band gap indicates that C(1%)-TiO2 is not only photoactive under UV-irradiation but also, it is photoactive under visible irradiation in agreeing with the theoretical estimations performed by DFT. The electronic density of states from Fig. 12 showed that the changes in the optoelectronic properties of TiO2 can be attributed to a strong interaction between a Ti atom of

Table 5 Theoretical estimations. Total electron charge (ETot-charge), partial electron charge (EPartial-charge) for s, p and d, orbitals, and estimated energy gap band (Ebg-theo). Sample

Atom

ETot-charge

EPartial-charge-s

EPartial-charge-p

EPartial-charge-d

Ebg-theo (eV)

Ebg-exp (eV)

Ti16O32

Ti O

10.1960 6.8557

2.4131 1.7990

5.9918 5.0567

1.7911 0.0000

3.14

3.24

Ti15O32C

Ti C O (C) O (Ti)

10.1887 3.4484 6.6621 6.8549

2.4150 1.3949 1.8217 1.8018

5.9917 2.0535 4.8404 5.0531

1.7820 0.0000 0.0000 0.0000

2.94

3.03

J. Matos et al. / Journal of Colloid and Interface Science 547 (2019) 14–29

TiO2 with one C atom. This interaction would induce a negative charge on TiO2 surface [85] enhancing the charge transfer process +4 from O which is responsible for the band-gap [7,8]. Thus, it 2 to Ti seems to be logical that under UV–visible irradiation, the C atom playing the role of dopant would photo-assist to TiO2 by transferring electron density. This inference agrees with the presence of Crelated gap states (Fig. 12b) also reported by other group [78-81] whom propose the photo-assisting role of surface carbon species as early reported by our group [5]. Finally, following the theoretical approach early reported by our group [7], the values of the valence and conduction bands, EVB and ECB, respectively, can be estimated for C(1%)-TiO2. These values were ca. 3.60 V and ca. 0.64 V for EVB and ECB, respectively, which are clearly different from those reported [88] for TiO2, ca. 2.9 V and ca. 0.3 V. The present estimated values suggest that both hydroxyl radicals (°OH) and superoxo anion radical (O2°) can be generated not only by UV but also by visible light irradiation of Cdoped TiO2-based photocatalysts. Thus, the present nanostructured and mesoporous TiO2-based materials material showed different thermodynamic characteristics than those of TiO2 prepared by conventional sol-gel or chemical vapor deposition methodologies.

4. Conclusions A two-pot procedure of solvothermal synthesis followed by calcination was performed to obtain a nanostructured material mainly composed by anatase TiO2 with ca. 1 wt% in C as a structural dopant. The experimental results of the photocatalytic activity of TiO2 were correlated with theoretical estimations by using the density of states by DFT + U approach. The kinetics of methylene blue and phenol photodegradation were followed under simulated solar irradiation and results of photoactivity were compared against those obtained on a pristine anatase commercial TiO2. The pseudo first-order, the second-order and the intraparticle diffusion kinetics models were verified. The textural and surface chemistry properties of the materials were correlated with the surface density of molecules adsorbed in equilibrium. The global reactionrate showed a perfect quadratic regression as a function of the surface density of molecules adsorbed. The presence of high carbon content promotes the physisorption of MB while the almost pristine TiO2 (only 1% C-doping) and commercial TiO2 were characterized by the chemisorption mechanism of MB. Phenol adsorption is characterized by the physisorption mechanism on all the samples. The MB adsorption is highly dependent of the intraparticle diffusion model showing a less trends to diffuse from the bulk of solution to the pore framework in comparison of phenol that did not presented diffusional limitations. The textural and surface chemistry properties of the materials were correlated with the surface density of molecules adsorbed in equilibrium. In summary, the present results for the MB and PhOH photodegradations are consistent with previous works [4,20,33]. We reported an important increase in the visible-light photocatalytic activity of C-doped TiO2 in the photodegradation of MB in comparison to other C-doped TiO2 photocatalyst [33], and an enhancement in the photocatalytic degradation of an aromatic molecule such as phenol with anatase-TiO2 doped with very low quantities of carbon [20]. It must be highlighted that the present work demonstrated C-doping did not affect the mechanism of reaction as shown in the case of phenol where the product distributions followed the same trend than that on the pristine anatase TiO2. In addition, the stablished hypothesis was demonstrated because the present DFT estimations are consistent with the experimental results, not only for the increase in the photocatalytic activity of C-doped TiO2 discussed above for the MB and PhOH photodegrada-

27

tions, but also when C-doped concentration is very low, 1 wt%, as in the present C(1%)-TiO2 sample, the harvesting of solar energy is favored [4] due to a low surface roughness. By contrast, when carbon content is high, Fu-TiO2-C sample, the photocatalytic activity is even lower than that for the reference pristine anatase probably due to an increase in the spin-polarized state of anatase structure [76,78] and to a very acid surface pH which concomitantly led to a faster recombination of hydroxyl radicals. The so-called global reaction-rate here proposed, is an innovative kinetic approach showing a perfect quadratic regression of the photocatalytic activity as a function of the surface density. Thus, it can be also called surface reaction-rate. However, the enhancement in the photocatalytic activity was not only attributed to the enhancement in the surface density but also to the carbon doping with the crystalline framework of TiO2. Theoretical values for the energy band gap of pristine anatase TiO2 and C-doped TiO2 were estimated and a red-shift from 3.14 eV to 2.94 eV was found for a Ti15O32C super-cell in comparison than pristine anatase TiO2. These results were consistent with the experimental energy values obtained and with the increase in the photocatalytic activity of C-doped TiO2. It can be concluded that the increase in the photocatalytic activity of the C-doped TiO2-based material was due to the decrease in the energy bandgap promoting a higher absorption of photons from the visible light in agreement with the theoretical prediction of C 2p orbitals appearance in the C-doped material. It can be concluded that the increase in the photocatalytic activity of the C-doped TiO2 photocatalyst was due to the decrease in the energy band-gap promoting a higher absorption of photons from the visible light in agreement with the theoretical prediction of appearance of atomic orbitals 2p from carbon atoms in the hybrid material. Similar approaches are being conducted by our group for the understanding of N-, S-doped and P-doped TiO2 since it has been widely reported that these photocatalysts have shown an increase in the photocatalytic degradation of dyes [4,5,27] and other organic molecules [17,20,21] in presence of the these elements. Acknowledgements J. Matos acknowledges the Chilean projects: FONDECYT 1161068, CONICYT PIA/APOYO CCTE AFB170007, and the Millennium Science Initiative of the Ministry of Economy, Development and Tourism, Chile, grant Nuclei on Catalytic Processes towards Sustainable Chemistry (CSC). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.03.074. References [1] V. Balzani, A. Credi, M. Venture, Photochemical conversion of solar energy, ChemSusChem 18 (2008) 26–58. [2] S. Malato, P. Fernández-Ibáñez, M.I. Maldonado, J. Blanco, W. Gernjak, Decontamination and disinfection of water by solar photocatalysis: recent overview and trends, Catal. Today 147 (2009) 1–59. [3] C. Pelekani, V.L. Snoeyink, Competitive adsorption between atrazine and methylene blue on activated carbon: the importance of pore size distribution, Carbon 38 (2000) 1423–1436. [4] T.J. Bandosz, J. Matos, M. Seredych, M.S.Z. Islam, R. Alfano, Photoactivity of Sdoped nanoporous activated carbons: a new perspective for harvesting solar energy on carbon-based semiconductors, Appl. Catal. A: Gen. 445–446 (2012) 159–165. [5] J. Matos, M. Hofman, R. Pietrzak, Synergy effect in the photocatalytic degradation of methylene blue on a suspended mixture of TiO2 and Ncontaining carbons, Carbon 54 (2013) 460–471. [6] J. Matos, T. Marino, R. Molinari, H. García, Hydrogen photoproduction under visible irradiation of Au-TiO2/activated carbon, Appl. Catal. A: Gen. 417–418 (2012) 263–272.

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