Synergized mechanistic and solar photocatalysis features of N-TiO 2 functionalised activated carbon

A TiO2 photocatalysts was successfully functionalised by employing nitrogen (N) as a dopant on activated carbon (AC) support as synergist. Two different types of activated carbon adopting namely Garcinia mangostana and palm shell as precursor were chosen as an activated carbon support. Thus the synthesized samples were examined for its physical and chemistry properties through advanced microscopic and spectroscopic techniques. The results revealed the contribution of adsorbent support through the rich surface area while doping of nitrogen contributed for effectively utilizing the incident photons by narrowing the band gap energy. The synergetic adsorption-photocatalytic activity was investigated by adopting batik dye, Remazol Brilliant Blue Dye (RBB) as model pollutant. Thus the N-TiO2 functionalised activated carbon demonstrated excellent adsorption-photocatalytic activity with 80% removal efficiency in 6 h. The synergism of adsorption-photocatalysis portrayed the alternative for treating recalcitrant RBB a predominant dye found in batik textile industry wastewater.


Introduction
Remazol Brilliant Blue dye (RBB) is a harmful anthraquinone dye used mainly in the textile industries.The RBB is one of the most common dyes and often used as a primary material in the production of polymeric colorants [1] and classified as recalcitrant and toxic organic pollutants [2].This injurious dye is capable of damaging the flora and fauna in the aquatic and terrestrial ecosystem [3].Therefore, an effective treatment is mandatory to eliminate this complex organic pollutant for the benefit of public health and ecological safety.Moreover, their chemical structures make more resistivity towards the existing conventional physicochemical techniques [4][5][6][7].The practiced or available conventional treatment method is only able to transfer the RBB from one phase to another and not to fully transform it into harmless compounds.Thus some alternative technologies are in demand to effectively eliminate RBB from the aqueous solution.
The well-known Advanced Oxidation Processes (AOPs) shows great ability in addressing this issue.Off these the heterogeneous photocatalysis employing TiO 2 as photocatalyst had proven it ecofriendly successfulness in addressing various organic pollutants.This was mainly attributed to its excellent versatile characteristic of possessing high chemical stability, non-toxicity, cost effective, etc. [8][9][10][11].However, the efficiency and practicality of TiO 2 is restricted by its well-known obstacles [12,13,14].Attempts have been made to overcome its obstacles through doping with nonmetal, noble metal, incorporation with organic semiconductor, etc. [15,16,17].Among the adopted strategy doping with non-metals like C, N, S and F easily incorporates into the lattice of TiO 2 and shifts the band gap energy towards the absorption of visible light [18,19,20].On the other hand, such doping is one of the ease and economical methods for promoting sustainable approach in the current situations.
Furthermore, functionalising TiO 2 on porous activated carbon (AC) will lead to synergisation and would exhibit greater ability for treating organic compounds.This superior contribution is sustained by its high specific surface area, high degree of surface reactivity and relatively low cost [21][22][23][24][25].Therefore, activated carbon will be one of the most cost effective supports with proven adsorption capacity for synergizing the adsorption-photocatalytic oxidations process in upsurge the photacatalytic reaction with organic pollutants especially textile dye [26,27,28].
The present work embarks on intensification of N-TiO 2 with AC for synergized performance for treating the aquatic pollutant.The nitrogen doped TiO 2 was successfully synthesized and was functionalised on two different AC derived from Garcinia Mangostana shell (GAC) and palm shell (CAC).Off these GAC was prepared in our laboratory and CAC was commercial obtained from the market.Thus fabricated N-TiO 2 /AC was characterized and functionalized for the removal of RBB dye under direct sunlight.This present work will be a promising alternative solution for destructing such recalcitrant pollutant through a sustainable approach.

Functionalisation of N-TiO 2 on AC
The details on preparation of AC from Garcinia Mangostana shell and N doping are detailed in our earlier reports [29].The functionalisation of N-TiO 2 onto AC was carried out by adding 1 g of N-TiO 2 and AC each into 1 L of Milli-Q water.The mixture was homogenized through prolonged stirring at room temperature (27 ± 2 °C).Then the product was centrifuged and dried over night at 100 ± 2 °C.The dried product was then carbonized in the furnace at 400 °C for an hour.

Characterization
Crystallography identifications (XRD) were performed using a Bruker diffractometer (D8) with Cu K α emission of wavelength (λ) 1.5406 Å.The angular 2θ diffraction was varied between 10 and 80° with the data collection completed at the step size of 0.02° with the step time of 1 s.The crystallite sizes were calculated by using the Debye-Scherrer equation.Field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray (EDX) silicon drift detector (SDD) (Hitachi SU-8000) was used to investigate the morphology and the inorganic compositions of prepared photocatalysts.Brunauer-Emmett-Teller (BET) surface area, pore volume, and Barret-Joyner-Halenda (BJH) pore size distribution (PSD) based on N adsorption-desorption isotherms were analyzed by QuantachromeAutosorb Automated Gas Sorption System.The degassing of the sample was carried out at 150 °C for 5 h with nitrogen gas.Thermo Nicolet iS10 Fourier Transform Infrared (FTIR) Spectroscope was used to analyze the functional group transmittance between 500 and 4000 cm −1 .X-ray photoelectron analysis (XPS) was carried out using Axis Ultra DLD instrument of KRATOS using monochromatic Al-K α radiation (225 W, 15 mA, 15 kV).The binding energy (BE) of adventitious C ls = 284.9eV was used as reference.The observed spectrums were indicated by a curve-fitting process, assuming the Gaussian-Lorentzian function.The optical absorption study was evaluated using UV-Vis Spectrophotometer (Shimadzu UV-2600) equipped with BaSO 4 diffuse reflectance integrating sphere (DRS) at ambient temperature in the wavelength range between 200 and 800 nm.

Synergetic Adsorption-photoactivity Evaluation
Synergetic adsorption-photocatalytic study was carried out in a batch reactor of 1 L capacity with a working volume of 250 mL.The reactor adopted a simple glass beaker, under stirred condition using a magnetic stirrer.A dedicated adsorption process was carried out in a dark condition until adsorption equilibrium (1 h) was achieved.This was followed by photocatalysis under direct solar light.All the photocatalytic experiments were carried in a good sunny day under identical conditions.The intensity of sunlight which varied between 80 and 210 W•m −2 was measured using LT Lutron LX-101 light meter.The intensified adsorption-photocatalytic experiments were performed with a synergized catalyst of 1 g with 50 mg/L of initial dye concentration.Prior to the experiments the dye solutions was vigorously stirred for uniform dye concentration.The samples were collected at regular time interval, filtered and the supernatant was subsequently analyzed for the residual dye concentration.The concentrations of RBB solution were determined by measuring the absorbance at 596 nm with the UV-Vis spectrophotometer.

Structural and Morphology Properties
Figure 1 shows the XRD pattern for N-TiO 2 , N-TiO 2 /CAC and N-TiO 2 /GAC.All the diffraction peaks in Figure 1 showed the complete formation of body-centered tetragonalanatase phase of TiO 2 .This was indexed according to JCPDS card No. 21-1272 and the lattice plane was confirmed by diffraction peaks (101), ( 004), ( 200), ( 211), (204), and (301) at 2θ = 25.3, 37.8, 48.0, 55.1, 62.7, 76.0°.No diffraction peaks, either related to rutile or brookite phases were observed in the prepared N-TiO 2 samples.However, a minor primitive rutile phase peak was detected at 27.5° (110) in the composite samples.This was mainly attributed to the heat treatment adopted in the functionalisation step [5].Further, the functionalisation leads to instability of anatase phase.The presence of amorphous carbon in both the synergized samples proved the formation of the adsorbent as a synergist to the photocatalyst.The average particle sizes of TiO 2 calculated from Debye-Scherrer equation were found to be 33.49nm for N-TiO 2 and 61.91 nm for both N-TiO 2 /CAC and N-TiO 2 /GAC.The morphology and texture of the prepared samples were depicted in Figure 2. The spherical morphology with relatively uniform pores was inferred for the N-TiO 2 as shown in Figure 2a and 2b.Meanwhile, the functionalised illustrated a fairly uniform morphology with a minimum number of pores as portrait in Figure 2c and 2d.However, the pore size of the N-TiO 2 /CAC is larger and even as compared to N-TiO 2 /GAC.The images in Figure 2e and 2f clarify the distributions of the pores derived after the functionalisation of N-TiO 2 .The EDX spectra of the prepared catalysts are illustrated in Figure 3 and the elemental mapping shows that N-TiO 2 /GAC contain 48.58 wt% of C, 29.48 wt% of O, 14.89 wt% of Ti, 6.44 wt% of N and 0.61 wt% of P. The trivial occurrence of phosphate was due to the usage of H 3 PO 4 acid during the preparation of GAC.Meanwhile, N-TiO 2 /CAC comprise of 55.84 wt% of C, 19.43 wt% of O, 19.56 wt% of Ti and 5.17 wt% of N.Both the photocatalyst functionalised samples recorded a highest percentage of carbon that indicates good attachment of N-TiO 2 with AC [30,31].

Surface Area Analysis
Figure 4 portraits the adsorption-desorption hysteresis and pore size distribution (PSD) of the prepared intensified photocatalysts.Table 1 lists the BET surface area; pore volume and average pore diameter of the samples.From the table, it is observed that the surface area and pore volume of pure GAC is higher than that of pure CAC due to the appropriate synthesis method of GAC.N-TiO 2 exhibited type II hysteresis of BET isotherm with cylindrical pores.The GAC functionalised with photocatalyst showed a type IV classification of BET isotherm, a common type for AC, indicates that dominance of the AC in the surface pores.The functionalised nano sized N-TiO 2 has percolate into the macro and mesopores blocking the micropores resulted in decline of surface area of GAC.This is further clarified through decrease in pore volume of N-TiO 2 /GAC [5,32,33,34].Similarly, the N-TiO 2 /CAC showed type IV isotherm, however with lesser N-TiO 2 blocking the micropores as justified in our FESEM images give rise to the higher surface area.This was further substantiated through marginal decrease in the pore volume (0.4357 cm 3 /g).Further, the percolation effect and the surface presence of the N-TiO 2 on the AC pores is well observed from the morphological images which contribute to the improved photocatalysis.

FTIR Analysis
Figure 5 explained the functional group transmittance of the prepared photocatalysts.In Figure 5(c), a strong broad distorted peak was inferred in the spectrum between 3300 and 3500 cm −1 indicated the presence of hydroxyl groups (O-H) from the acid base character modification of prepare photocatalyst [31].It also observed in the spectrum that there is a weak intensity of primary amines (N-H) between 3200 and 3500 cm −1 .Such amines are contributed by the incomplete removal of HNO 3 [29].However a stretching vibration of H-O-H bonding of water was recorded for spectrum between 1500 and 1800 cm −1 .Figure 5a and 5b showed the ν OH vibration shifted from 3300-3500 cm −1 to 3600-4000 cm −1 .These vibration shifts was attributed to the incremental positive charge derived from O-H groups, absorbed on the surface of the N-TiO 2 [31].Furthermore, between 1400-1600 cm −1 region shows another strong peak which can be attributed to C=C and C=O skeleton vibration of the aromatic ring.The presence of signals from 2250-2500 cm −1 is mainly due to the impurities of N doping.However, the impurity does not tamper either the chemical structure of the composite nor the performance of it [35,36].

XPS Analysis
The obtained chemical state spectrums of the prepared photocatalyst are displayed in Figure 6.X-ray spectroscopy was applied to the spectral regions of Ti 2p between binding energy 459.1 and 464.8 eV was indexed to the Ti 2p 3/2 and Ti 2p 1/2 photoelectrons in the Ti 4+ chemical state respectively [25].The presence of Ti is considered as an active element for the photocatalytic activity.Meanwhile, two distinguish peaks were detected under C 1s spectra.The peaks at 284.5 eV matches to the C-C bonded species which mainly attributed from the activated carbon.The peak located at 288.8 eV is assigned to the formation of C=O species that are resultant from the substitution of lattice titanium atoms with the formation of Ti-O-C bond structures.The fitted O 1s spectra at 531.6 eV indicated the presence of Ti-OH species.A peak centered at 532.7 eV is assigned to the formation of Ti-O-C bond.The N 1s spectra showed the presence of N-H at 399.2 eV contributed to the anionic doping that supported the visible light enhancement [37].

UV-Vis Absorption Spectra
The UV-visible absorption spectra obtained for the samples were presented in Figure 7.The obtained spectrum illustrates the light response of N-TiO 2 and N-TiO 2 functionalised AC.All the prepared samples exhibited an enhanced absorption in the visible light region along with a notable shift towards longer wavelength.Among them N-TiO 2 /AC emerged with a greater adsorption under visible light region and was primarily attributed to the contribution of the carbon support because of synergization due to the black characteristic of AC [30,38].Moreover, the carbon further contributed to an increase of sacrificial electric charge of oxides in N-TiO 2 [31,39].

Synergetic Adsorption-photocatalytic Activity
The synergetic adsorption-photocatalytic degradation of RBB Dye with the prepared samples were shown in Figure 8.The N-TiO 2 /GAC resulted with a maximum removal efficiency of 80%; while N-TiO 2 /CAC resulted with 45% degradation within 6 h of solar light illumination.The nonfunctionalised showed a poor performance with 30% removal efficiency.A blank experiment was carried out to ensure the photolysis effect.The improved photodegradation performance of N TiO 2 funcationalised samples revealed the homogenous deposition of TiO 2 on to the surface of AC.This facilitated the active electron mobility resulted in generation of active radicals and is clear from the topography obtained from electron microscope.
Under the dark condition, both the N-TiO 2 supported with AC show great adsorption efficiency due to the presence of RBB as RBB + form [40]. N-TiO 2 /CAC with higher surface area shows better adsorption ability comparing with N-TiO 2 /GAC.This rich surface area offering more sites to adsorb RBB resulted in further improving the photocatalytic performance.Additionally, when the sample is exposed to solar irradiation, higher concentration of RBB attached on the photocatalysts surface plays a key role for the acceleration of photochemical reactions between RBB and photocatalysts.It is well known that N doping enables visible light absorption ability of TiO 2 [37].In addition, the dopant has tendency to exist above the valence band for the exchange of nitrogen (N 2p) [25].Consequently, the N-TiO 2 illuminated by solar light triggers the electron of valence band to conduction band leaving behind a hole.The excited electrons expected to shift from the N 2p states above the valence band of TiO 2 to the conduction band and get trapped by the surface absorbed O 2 .Thus excited electron-hole pairs activate the redox reactions on the surface of the titania, generates active species [25].These generated species strongly oxidize RBB continuously leading to the formation of harmless CO 2 and H 2 O mostly with minor portion of harmless intermediates.The obtained promising result was attributed to functionalized N-TiO 2 anatase stability and its robust utilization of photons.Moreover, the AC has proven its ability in contributing for enhancement of photocatalysis reaction through synergistic effect obtained by forming a strong interface between both the AC and N-TiO 2 phase [41].The AC support also contributed for the enhanced photocatalysis by providing rich surface area that allowed the concentration of RBB dye around the funcationalised titania [42].This phenomenon well supported the photo-degradation process resulting from the close interface [22].Furthermore, when the dye was exposed to solar light it undergoes photosensitized oxidation where an electron is excited from the dye and move into the conduction band of TiO 2 .As a result, the RBB dyes transformed into cationic dye radicals and undergo further degradation.The interaction of pollutant with photocatalyst driven under the solar irradiation is schematically illustrated in Figure 9.

Conclusions
Synergetic N-TiO 2 functionalised AC was successfully synthesized and demonstrated its synergized effect through a photocatalysis reaction driven under solar energy.The attachment of N-TiO 2 had improved absorbance towards the visible light region ensuring the maximum and active utilization of solar spectrum.The carbon derived from the AC also acted as a key contributor for enhancement of photocatalytic activity by shrinking the gap energy.The inherent synergisation well contributed for the completed removal RBB dye by utilizing solar energy.The synergetic effect also contributed for adsorption that further enhanced the removal of dye.Thus the present study established the vibrant applicability of the developed functionalised AC in promoting a sustainable environmental remediation.

Figure 9 .
Figure 9. Mechanistic interaction of RBB with N-TiO 2 excited under solar energy.

Table 1 .
BET surface area, pore volume and average pore diameter of the AC and prepared photocatalysts.