Effect of biochar on the photocatalytic activity of nitrogen-doped titanium dioxide nanocomposite in the removal of aqueous organic pollutants under visible light illumination

Document Type : Research Paper

Authors

1 Department of Chemistry, Amirkabir University of Technology, Tehran, Iran.

2 Department of Chemistry, University of Zanjan 45195-313, Zanjan, Iran

3 Department of Chemistry, University of Zanjan 45195-313, Zanjan, Iran.

4 Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University, Šlechtitelů 27, 783 71 Olomouc, Czech Republic

Abstract

Biochar, as a low-cost carbon obtained from barley straw, was used for the simple sol–gel synthesis of visible light photocatalysts comprising N-doped TiO2/biochar nanocomposite (N-TiO2/C) and thermally treated N-TiO2/C. The nanocomposites were characterized by SEM, EDX, TEM, XRD, BET, FTIR, DRS UV-vis, and PL measurements. The doped TiO2 catalyzed the photodegradation of rhodamine B (RhB) in aqueous dispersion under visible light illumination where the N-TiO2/C nanocomposite with a band-gap of 2.96 eV and large surface area (206 m2 g-1) showed the highest photocatalytic activity and degrading 99% of RhB under visible light of a 40-Watt white LED lamp within 105 min. Photoluminescence (PL) spectroscopy experiments revealed the effective separation of charge carriers by the N-doped TiO2 materials. The presence of carbon enhances the photocatalytic activity of N-TiO2 material by decreasing the band gap, enhancing the visible light absorption, reducing the reflection of light, enhancing the adsorption of RhB and intermediates on the N-TiO2 surface, thus prolonging the separation electrons (e-) and valence band hole (h+).

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INTRODUCTION

Dyes are of the principal pollutants of water because of their high chemical oxygen demand (COD) and extensive absorption of visible light radiation [1]. The resulting water pollution leads to a global decrease in the availability of clean and safe water from available resources.The photocatalytic strategy is an effective and cleaner option for treating the organically polluted water bodies [2]. The degradation of organic dyes using light and semiconductors occurs through a photocatalytic or photo-assisted (photosensitized) oxidation mechanism depending on the process of charge carrier generation [3]. The photocatalytic purification of dye wastewater by irradiated TiO2 has proven its effectiveness due to the high efficiency, stability, and relative non-toxicity of TiO2 [4]. However, the practical application of TiO2 alone, while using solar energy, is hindered by its low adsorption ability and the essential requirement of exclusive UV radiation for its surface photoactivation primarily due to a large optical band gap (Eg = 3.2 eV). Additional drawbacks of TiO2 materials are limited quantum yields because of high recombination rates of photon-induced electron-hole pairs that lead to limited applications under solar light [5] and weak separation efficiency of photocarriers, which results in low photocatalytic activity.

Doping is an effective method to extend the light absorption and photoactivation of TiO2 to the visible light region and enhancing the utilization efficiency of solar energy [6]. Nitrogen doping is by far the most studied system and an ideal option for the preparation of visible-light responsive TiO2 [6-8]. Zhu et al. prepared N-doped TiO2 nanofibers with a shell of TiO2 anatase [9]; higher photocatalytic activity under visible-light irradiation was related to the effective adsorption of organic molecule and the activation of O2 by the surface layer. Sakthivel et al. synthesized the N-TiO2 photocatalysts and used them for the mineralization of 4-chlorophenol under visible light [10].

Kamat et al. reported on the photodegradation of dyes pre-adsorbed on the surface of TiO2 particles under visible light [11]; only the molecules that were in direct contact with the TiO2 surface underwent photodegradation. Zhang et al. studied the decomposition of dyes in an aqueous TiO2 dispersion under irradiation by visible light [12] where the dyes adsorbed on the surface of TiO2 particles act as sensitizer and inject electrons into the conduction band of TiO2 from their respective excited states and promote self-photosensitized decomposition [13]. Chung et al. synthesized N- and S-codoped TiO2 (NS-TiO2) and studied their visible light photocatalytic activity [14]; post thermal treatment of NS-TiO2 decreased the photocatalytic activity. In addition, carbon modified TiO2 can indeed improve the visible light photocatalytic activity [15] where carbon species serve a surface sensitizer and improve the visible light activity of carbon-modified TiO2 [16]. Furthermore, Wang et al. showed that nanosized carbon on the surface of TiO2/carbon composite improves the separation of photogenerated electrons and holes by capturing the electrons, which sensitizes TiO2 for the absorption of more light on the catalyst surface [17].

Biochar is a carbon material prepared via the pyrolysis of plant and animal-based biomass under an inert atmosphere [18] and comprises abundant oxygen functional groups, an aromatic surface, a large surface area endowed with high porosity [19]. Owing to its high specific surface area and rich functional groups [20], biochar is an excellent sorbent to remove organic pollutants from aqueous solutions. Cai et al. developed composites made up of supported TiO2 on biochar and used them as adsorptive photocatalyst for degradation of organic pollutants [21]. Acid pre-treated biochar (pBC) has been used as a support for TiO2 to fabricate photocatalyst, TiO2/pBC [22]. Kim et al. studied photocatalyst TiO2 supported on biochar for the degradation of sulfamethoxazole [23]. TiO2-coated biochar composites were also deployed for the removal of safranine T [21]. The results have demonstrated that loading of TiO2 enhanced the adsorption ability of biochar and the high specific surface area of the biochar synergistically promoted the photocatalytic activity of TiO2.

In this study, nitrogen-doped TiO2/carbon nanocomposite is developed to obtain a highly active and visible-light photocatalyst that benefits from the N-doping effect, cooperative adsorption, and sensitization performance of biochar.

MATERIALS AND METHODS

Materials

Titanium (IV) isopropoxide (Ti(OCH(CH3)2)4, TTIP) as the TiO2 precursor, rhodamine B (tetraethylrhodamine, RhB), acetylacetone, urea, absolute ethanol and other chemicals were of analytical reagent grade purchased from Merck and were used without further purification. Deionized (DI) water was used for all the experiments. The TiO2 (P25) sample by Degussa was used as reference, a nonporous powder material comprising a mixture of anatase and rutile (80:20) with a BET surface area of about 50 m2 g-1 and an average particle size of about 30 nm [24].

UV–vis spectra of the solutions were run on an Analytik Jena Specord 210 Plus Spectrophotometer. Photoluminescence emissions were recorded on a LS 55 Perkin Elmer Fluorescence Spectrometer. Fourier transform infrared (FTIR) spectra were taken using a Thermo Scientific Nicolet iS10 FTIR Spectrometer. Powder X-ray diffraction patterns were collected at a X’Pert Pro Company, wavelength 1.5406 Å (Cu Kα), voltage 40 kV, current 40 mA. The average crystallite size (DScherrer) was calculated by the Scherrer equation Eq. (1) [25]:

DScherrer = kλ / (β . cos Ɵ) (1)

where k is the shape factor; λ represents the x-ray wavelength used for the measurement; β is the line width (FWHM) in radians, and Ɵ is the Bragg angle. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) were taken using a FEI QUANTA200 ESEM. Transmission electron microscopic (TEM) images were generated by an EM 208S electron microscope. N2 adsorption (Belsorp-mini, Japan) was used for determining the surface area (Brunauer-Emmett-Teller (BET) method), and N2 desorption was used for the estimation of pore volume and pore size distribution (Barrett-Joyner-Halenda (BJH) method). Light absorptions in UV and visible ranges were studied by UV-visible diffuse reflectance spectroscopy (UV-vis DRS; AvaSpec-ULS2048LTEC). Band gap energies of the photocatalysts were estimated from the plots of light absorptions using Tauc’s equation(Equation 2) [26]:

 (2)

where B is a constant; α is molar extinction coefficient; h is photon energy; Eg is average energy gap; and n is ½ for allowed direct transition (plotted as a(h)2 versus E).

Synthesis and treatment of biochar

The barley straw was used as the biomass obtained from farms in the city of Zanjan, Zanjan Province, Iran. The barley straw was dried at room temperature and then milled to pass a 0.50 mm size screen. The milled barley straw was first dewaxed in a Soxhlet apparatus using ethanol 96% and dried at 55 °C for 24 h. Then, it was pyrolyzed at 500 °C under a nitrogen flow at 60 sccm in a tubular furnace for 2 h with the heating rate of 5 °C min-1. The obtained biochar was cooled and washed at least three times with DI water. This was followed by drying at 80 °C for 24 h.

For acid treatment, 0.10 g of the obtained dried biochar was dispersed in 15 mL of 65% HNO3 and refluxed at 80 °C for 1 h. The ensuing treated biochar was separated by centrifugation and washed with DI water until the pH of the filtrate reached 7. Finally, it was oven-dried at 105 °C for 12 h.

Preparation of nitrogen doped TiO2 (N-TiO2)

Doped TiO2 hybrid material was prepared using the sol-gel method. As a dopant source, 3.3 mmol (0.2 g, 60.06 g/mol) of urea was dissolved in 12 mL of absolute ethanol. It was added to 3.4 mmol of titanium (IV) isopropoxide (TTIP) and 7.8 mmol of acetylacetone dissolved in 5 mL of absolute ethanol under vigorous stirring, followed by adding deionized water (0.4 mL). After 10 minutes, the solution was transferred to a 25 mL Teflon lined stainless-steel autoclave, and the solvothermal treatment was performed at 115 °C for 12 h. The resulting cream-color powder was washed several times with deionized water and methanol by centrifugation. The washed sample was dried at 60 °C for 24 h.

Synthesis of nitrogen doped TiO2/carbon nanocom-posite (N-TiO2/C)

A mixture of 0.02 g of acid treated biochar and 3.3 mmol of urea in 12 mL of absolute ethanol was prepared and sonicated using an ultrasound bath for 30 min. It was added to 3.4 mmol of TTIP and 7.8 mmol of acetylacetone in 5 mL of absolute ethanol under vigorous stirring. It was followed by the addition of 0.40 mL of deionized water and the stirring continued for additional 10 minutes. The mixture was transferred to a 25 mL Teflon lined stainless-steel autoclave and aged at 115 °C for 12 h. The resulting brown solid was dried at 70 °C for 12 h. Then it was washed several times with deionized water and methanol and dried at 60 °C for 24 h to get the N-TiO2/C sample.

The dried N-TiO2/C gel was subsequently ground into a fine powder and calcined at 500 °C with the rate of 5 °C min-1 under N2 for 2 h to yield the gray N-TiO2/C(500).

Photoreactor and light source

A 40-Watt white LED lamp was positioned at 5 cm above a thermostated Pyrex cylindrical double-walled reactor which was placed in a black wooden box. The temperature of the photocatalytic reaction was maintained at 25 °C by water circulation.

Photocatalytic properties of the samples

For photocatalytic activity evaluations, the photodegradation of RhB was studied as a model compound because it is a common hazardous contaminant present in industrial wastewater [27]. An aqueous TiO2 dispersion was prepared by adding 40 mg of photocatalyst (TiO2, N-TiO2 or N-TiO2/C) powder to a 40 mL solution containing the RhB at 10 ppm concentration. Prior to irradiation, the dispersions were magnetically stirred in the dark for 30 min to secure the establishment of an adsorption/desorption equilibrium of RhB on the photocatalyst surface. At given irradiation time intervals, the dispersion was sampled (1 mL), diluted to 3 mL by DI water, centrifuged to separate the photocatalyst particles and UV-vis spectra of the supernatant (measured over the range of 200-800 nm) were recorded using an Analytik Jena Specord 210 Plus Spectrophotometer. All the photodegradation performances were assessed under visible light.

RESULTS AND DISCUSSION

Nitrogen-doped TiO2 catalysts were successfully synthesized by an easy and low-cost one-step solvothermal method. In the preparation of N-TiO2/C nanocomposite, biochar is probably grafted onto the surface of TiO2 via C–O–Ti bonds under mild solvothermal synthesis conditions (115 °C for 12 h). This structure is suitable for the charge transfer upon light excitation [28]. A very high temperature (> 800 °C) is required for the synthesis of carbon-doped TiO2, wherein carbon substitutes for some of the lattice oxygen atoms [29]. N-TiO2/C nanocomposite prepared via this direct solvothermal synthesis is significantly brown, showing an electronic interaction and different bonding in this highly interpenetrated material than that of a gray colored physical mixture of carbon and TiO2. Toward a better understanding of the influence of carbon, N-TiO2 without carbon was also prepared. Notably, the results obtained by N-TiO2 and N-TiO2/C hybrid compounds demonstrated a significant effect of carbon on textural properties. In order to study the effect of heat treatment on the photocatalytic activity of N-TiO2/C, it was calcinated at 500 °C.

Surface properties

The N2 sorption isotherms and the corresponding BJH pore size distribution curves of the prepared nitrogen-doped TiO2 samples are depicted in Fig. 1 (A) and 1 (B), respectively. As displayed in Fig. 1, the nitrogen isotherms of N-TiO2, N-TiO2/C and N-TiO2/C(500) indicate the presence of mesoporous materials [30]. According to IUPAC classification [31], these doped materials exhibit isotherms type IV and their hysteresis are of triangular shape that is classified as H2 loops, indicating that the prepared composites have mesoporous structure with ink-bottle like pores [31].

The surface properties of the synthesized materials are summarized in Table 1. The most significant increase in the BET area is for N-TiO2 compared to TiO2. The surface area and pore volume of N–TiO2 were determined to be 220 m2 g-1 and 0.16 cm3 g-1, respectively. The BET specific surface area of titania increased with the N-doping due to the suppression of the TiO2 crystal growth by urea. Although the surface area of N–TiO2 was decreased to some extent by the addition of treated biochar, it is approximately four times higher than that of TiO2. The specific surface area (from 206 m2 g-1 to 80 m2 g-1) and pore volume (from 0.16 cm3g-1 to 0.09 cm3g-1) decreased with the calcination of N-TiO2/C due to the sintering and crystal growth of TiO2 particles. Correspondingly, the pore diameter of N-TiO2/C(500) material increased from 1.21 nm to 2.38 nm (Table 1). The pore volume (0.16 cm3 g-1) and pore diameter (1.21 nm) of N-TiO2 and N-TiO2/C are the same. These findings are consistent with our previous statements that TiO2 is not doped with carbon in N-TiO2/C material.

For the synthesized materials the mesopore surface area (Smeso (m2 g-1)) was equal to the difference between the total surface area (SBET) and the slop of t-plot that shows mesoporous plus external surface area (Table 1) [32]; N-TiO2/C nanocomposite exhibited the highest mesoporous surface area (98 m2g-1) that would provide more opportunities for the interaction of reactants with active sites of the photocatalyst, thus leading to greater activities. As expected, the comparison of N-TiO2 with N-TiO2/C shows that the mesopore surface area was increased by the presence of biochar from 80 m2 g-1 to 98 m2 g-1. The most noticeable point is the decrease of mesoporous surface area from 98 m2 g-1 for N-TiO2/C to 23 m2 g-1 in N-TiO2/C(500) upon calcination due to the carbon degradation during calcination at 500 °C (Table 1). The high surface area and porosity of N-TiO2/C material is indicative of a larger number of photocatalytic surface-active centers and the adsorption sites for compounds, with enhanced ease of reactants’ transport through the mesopores.

FTIR spectroscopy

Fig. 2 shows the spectra of nitrogen doped TiO2 samples in the range of 400–4000 cm−1. The strong and very broad peaks in the region 500-1080 cm−1 can be considered as the stretching vibration of Ti-O and Ti–O-Ti bonds [33] combined with the stretching vibration of Ti–O–C bonds [34]. N-TiO2 and N-TiO2/C show a very broad peak at about 3448 cm-1 that is assigned to the stretching vibration of adsorbed water and hydroxyls on the surface of TiO2, and the peak appearing at 1630 cm-1 is attributed to the bending vibration of the O–H bond in hydroxyls and adsorbed water (Fig. 2(a) and (b)) [35]. In comparison to N-TiO2, the band for N-TiO2/C nanocomposite span remarkably over a wider range from 3675 to 2672 cm-1, suggesting that the N-TiO2/C surface is richer than N-TiO2 in hydroxyl groups, which can be a piece of evidence for the existence of higher Ti–OH [36]. For this reason, H2O is more easily adsorbed on the surface of this biochar modified nanocomposite. The hydroxyl groups present on the surface directly generate OH radical when reacted with the photogenerated hole during the photocatalysis. As adsorbed water and hydroxyls play an important role in the photocatalytic activity [37], the strong intensity of the peak at 1630 cm-1 indicates a high content of adsorbed water and hydroxyls in the sample, which is helpful for enhancing the photocatalytic activity of N–TiO2/C. It has been shown that hydrogen-bonding interactions between adsorbed water and TiO2 stabilize photogenerated charge carriers (e- and h+) in nanocrystalline TiO2 and suppress their recombination [35]. The peaks at 1527 cm-1 and 1421 cm-1 are from the impurities, probably acetyl acetonate, that has not been completely removed in the wash. It can be observed that the intensities of absorption bands of adsorbed water and oxygen-containing functional groups such as C-O and Ti-OH (3448, 1630, 1527 and 1421 cm-1) were dramatically reduced during the calcination at 500 °C in N-TiO2/C(500), Fig. 2(c). The presence of a small peak at 1034 cm-1 in the spectrum of N-TiO2/C was attributed to the C-H stretching vibration from the unreacted precursor TTIP reflects the incomplete substitution with OH groups of H2O. The bands observed at 2960, 2924, and 2849 cm-1 are also assigned to the C-H stretching vibration of organic species remaining on the sample surface.

For comparison, the FTIR spectra of biochar and acid treated biochar are shown in Fig. 3.

SEM and TEM analyses

The morphology and microstructure of the prepared N-TiO2/C were probed with SEM (see Fig. 4). It could be observed that the N-TiO2/C particles are spherical with an uneven distribution of agglomerated particles. EDX analysis showed that N-TiO2/C consists of 0.21 wt% doped nitrogen and 3.05 wt% carbon (Fig. 4). The Ti and O content in N-TiO2/C were 60.97 wt% and 35.76 wt%, respectively, the molar Ti:O ratio being 1:1.76 (33.69:59.17). It is evident that the nanocomposite is, in fact, a non-stoichiometric compound with surface oxygen vacancies and the formula of N-TiO2-x/C. It is usual that N-doping and the formation of oxygen vacancies and electrons occur together to support the neutrality of the lattice. It has been proven [38] that two adjacent Ti atoms of the removed oxygen atom are reduced to Ti3+. As a result, donor states about 0.75–1.18 eV below the conduction band edge ensued [39]. The coupling of N-doping with the formation of oxygen vacancies prevent the recombination of the photogenerated e-/h+ recombination as well as enhance the visible light absorbance [40]. Additionally, the obtained and coordinatively unsaturated Ti species may act as catalytic sites [38]. Interestingly, the presence of surface Ti3+ defects can also enhance the photocatalytic performance by increasing the oxygen adsorption in conjunction with trapping the photogenerated electrons, which prevents the electron-hole recombination [41]..

Studies of the N-TiO2/C nanocomposite by TEM showed that it is composed of irregular sphere-like nanoparticles with uniform distribution and size 3-10 nm, Fig. 5. The small size and uniform distribution of particles may be favorable for the adsorption and photoreactions.

XRD characterization

XRD patterns displayed in Fig. 6 pointed out the clear presence of a distinctive anatase phase in N-TiO2/C and N-TiO2/C(500) samples. In terms of the rates of recombination e-/h+ and adsorptive affinity for organic compounds, TiO2 anatase phase goes beyond TiO2 rutile phase [42]. There are no peaks for the dopant N and modifier C due to their low weight percent in the doped TiO2. Without changing the anatase crystal phase, both materials exhibited almost identical XRD patterns differing only in terms of the intensity of the anatase diffraction lines. The diffraction peaks of N-TiO2/C became sharper and their intensity increased via calcination in N-TiO2/C(500). Meanwhile, the average crystallite size, calculated by the Scherrer method[25], increased with the calcination temperature from 10.3 nm for N-TiO2/C to 33.1 nm in N-TiO2/C(500).

Optical properties

Fig. 7 depicts the UV–vis absorption spectra for the prepared yellow-cream N-TiO2, brown N-TiO2/C, gray N-TiO2/C(500) and white TiO2. Following the introduction of nitrogen, both the N-TiO2 and N-TiO2/C demonstrate a noticeable red shift in the absorption band and a wide background absorption in the visible light region compared to TiO2, which is a characteristic pattern of visible-light responsiveness. However, the TiO2 sample responded only to the light of wavelengths shorter than 400 nm (Fig. 7(a)). Although the band gap energies of N-TiO2 and N-TiO2/C are not so low, their absorbances are not decreased rapidly as for TiO2; they show a continuous curve until their absorbance reached zero at about 650 nm (Fig. 7(b) and (c)). The continuous absorbance spectra for N-TiO2 and N-TiO2/C suggest a broad size distribution of the anatase nanocrystals. Various band gap is the result of a broad crystal size distribution, since the band gap energy of nanosized TiO2 particles is strongly dependent on the crystal size [43]. Therefore, the long-ranged continuous visible-light absorbance spectrum of the N-TiO2 and N-TiO2/C results from the overlap between various absorbance spectra of nanocrystals. Accordingly, the wide background visible-light absorbances of N-TiO2 and N-TiO2/C show that these are efficient visible light-active photocatalysts. The band in the region below 400 nm is ascribed to the charge transfer process from O2- (valence band) to Ti4+ (conduction band) under UV illumination [44]. The N-doping results in visible-light responsive photocatalyst by narrowing the band gap [45]. Additionally, it can be observed that the visible light background absorption of N-TiO2/C sample is higher than that of N-TiO2 due to the biochar effect as it can absorb visible light and prevent the reflection of light [17, 46]. Fig. 7(d) shows that after calcination at 550 °C, the composite N-TiO2/C(500) displayed almost the same absorption band but the background visible-light absorption decreased. All these findings confirm that carbon species are modified on the surface of the N-TiO2/C photocatalyst, forming the nanocomposite. Bonds such as Ti–O–C and Ti–OCO can be formed on the surface of TiO2 by the biochar layer [17]. The close contact between the biochar carbon and N–TiO2 facilitates the charge separation by trapping the photo-generated electrons [47]. The biochar species on the surface of the N-TiO2/C sample serve as a surface sensitizer and enhances its visible-light photocatalytic activity by harvesting more visible light and the separation of photogenerated e-/h+ [48].

The Kubelka-Munk function was used to calculate the band gap by extrapolating the linear portion of the Tauc plot (Fig. 8). The band gap for N-TiO2, N-TiO2/C and N-TiO2/C(500) were found to be 3.08, 2.98 and 3.07 eV, respectively, which are lower than that of commercial TiO2, ~ 3.20 eV [26]. The N-TiO2/C showed the lowest band gap energy and the highest visible light absorbance among the nitrogen-doped TiO2 materials. In the case of N-TiO2/C(500), although N dopant moved out from the TiO2 at the high temperature of 500 °C, N-TiO2/C(500) showed lower band gap energy compared to P25 because oxygen atom vacancies and probably some N dopant were still located on the surface of N-TiO2/C(500). The decrease in the band gap of N-TiO2 suggests the localized nature of nitrogen species in the TiO2 lattice [49] which occupy some of the oxygen positions in the lattice. The nitrogen doping creates an N-induced mid-gap level by hybridization of nitrogen 2p states and oxygen 2p states at the top of the valence band [50], which are responsible for the visible-light absorption. The creation of oxygen vacancies and color centers such as Ti3+ also enhances the absorption of visible light [51]. Therefore, the doped nitrogen atoms and oxygen vacancies are involved in the decreasing band gap energy [52].

Photoluminescence

Photoluminescence (PL) test was conducted to reveal the trapping and transfer of charge carriers in the materials. The measured PL emission spectra of the prepared nitrogen doped TiO2 samples are presented in Fig. 9. The emission intensity of N-TiO2(500) was lower than TiO2. In addition, the emission spectra of N-TiO2 and N-TiO2/C were the same and their intensities were considerably lower than that of N-TiO2(500). The lower PL intensity explains that N-TiO2 samples efficiently hold the separation of the charge carriers and retard their recombination compared to TiO2. The lower recombination rate of photogenerated e--h+ pairs results in the lower PL emission spectra [53]. The influence of the enhanced charge separation is displayed in their photocatalytic reaction.

Photocatalytic activity

The process of RhB removal on the prepared composites N-TiO2, N-TiO2/C and N-TiO2/C(500) was evaluated under visible light irradiation and the results are shown in Fig. 10. During the reaction, the absorption bands of RhB dye gradually decreased, then its color changed from magenta to green, then yellow, and finally disappeared (Fig. 11); N-doped TiO2 sample removed 94% of the RhB after 150 min. When modified with 3 wt% biochar, the N-TiO2/C photocatalyst removed with noticeably higher photocatalytic rate, 97% of the RhB in 105 min, suggesting its relatively high adsorption ability towards RhB due to its high surface area, porosity and high surface hydroxyl groups as mentioned before. It is clearly seen that the biochar in N-TiO2/C provides active sites for enhancing the adsorption of RhB. When the calcinated N-TiO2/C was used, the ratio of the removed RhB was only 22% after 180 min, showing the effect of lower specific surface (Table 1). There are also other studies which demonstrate the significant effect of crystallinity and specific surface area on the photocatalytic activity of TiO2 composites [54]. Considering the appropriate band gap, large specific surface area and good adsorption ability, the N-TiO2/C photocatalyst manifested the best photocatalytic performance for the degradation of RhB among all other samples.

As seen in Fig. 10 and Fig. 12, under visible light and in the presence of N-TiO2 and N-TiO2/C photocatalysts, the maximum absorption peak of RhB shifted gradually from 554 nm to 497 nm, implying that there were different intermediate products. The most probable intermediates for RhB are N-de-ethylated species [55]: N,N,N′- triethyl rhodamine (maximum peak 537 nm), N,N′-diethyl rhodamine (521 nm), N-ethyl rhodamine (505 nm), and thoroughly de-ethylated rhodamine (495 nm). Fig. 13 shows the changes of absorbance and wavelength shift for RhB solution maximum peak with time during the photocatalytic process.

The complete de-ethylation and more than 50% degradation of RhB occurred in the first 45 min and 120 min with the photocatalyst N-TiO2/C and N-TiO2, respectively (Fig. 13). The photodegradation of a dye assisted by TiO2 particles via N-de-ethylation occurs only when it is on the semiconductor surface, and not in bulk solution [56]. Since the band gap energy of the prepared N-doped TiO2 materials (about 3 eV) is not low enough for efficient visible light absorption, it is concluded that the adsorbed RhB dye serves as a sensitizer. Under the irradiation by visible light, the adsorbed RhB dye undergoes electronic excitation by absorbing photons at λ > 470 nm (about 2.6 eV). An excited RhB molecule then injects electrons into the conduction band of the TiO2 particles and initiates the chemical degradation [11, 12]. Therefore, RhB can undergoe photodegradation through a solution bulk reaction or a surface reaction [57]. The hydroxyl radicals (•OH), formed via the excitation of the electrons from the valence band to the conduction band of TiO2 under the light irradiation, promote the solution bulk reaction and lead to the photodecomposition of RhB in the aqueous solution. Moreover, the surface reaction is induced when the excited RhB injects electrons to the conduction band of the TiO2 and generates hydroxyl radicals. The hydroxyl radicals formed via this pathway cause the spontaneous de-ethylation of adsorbed RhB molecules [55], which shift the maximum peak from 554 to 497 nm in the UV-Vis spectrum. However, in the decomposition of RhB molecules via the solution bulk reaction the absorbance of RhB is simply decreased without a peak shift. As shown in Fig. 10 and Fig. 13, the absorbance spectra of the N-TiO2 and N-TiO2/C solutions demonstrate a significant peak shift from λ = 554 nm to λ = 496 nm, while no peak shift in the case of the N-TiO2/C(500) solution was observed. These results indicate that the surface reaction rates for N-TiO2 and N-TiO2/C were much larger than that obtained for N-TiO2/C(500). In fact, no surface reaction occurs on N-TiO2/C(500) photocatalyst (Fig. 10); the surface reaction rate on N-TiO2/C is higher than that of N-TiO2.

The high activity and desirable properties of our synthesized N-TiO2/C nanocomposite in terms of low amount of catalyst used and the power light, is clearly seen in comparison with the earlier reported doped TiO2 (Table 2).

In summary, during the process of RhB removal on the N-TiO2/C surface, the biochar enhances the surface adsorption of RhB on the photocatalyst, which causes an increase in surface reaction rate and a concentration effect for photodegrading RhB. The higher photocatalytic activity of N-TiO2/C with respect to N-TiO2 is mainly attributed to its higher surface adsorption of RhB, 40 wt% and 10 wt% in 30 min by N-TiO2/C and N-TiO2, respectively (Fig. 13). Additionally, the visible light absorbance of N-TiO2/C is higher than that of N-TiO2 due to its lower band gap energy and the presence of the modified biochar. UV-Vis spectra of N-TiO2/C solution reveal that the absorbance at 497 nm decreased dramatically within 60 min after the 45 min initial rapid surface reaction. These findings show that N-TiO2/C efficiently decomposed RhB under visible light irradiation via both the fast surface reaction and the solution bulk reaction. It appeared that the biochar is an efficient promoter that facilitates N-TiO2/C nanocomposite in driving effectively the photochemical degradation reactions by adjusting three main limitations of pure TiO2: inducing the absorbance of a higher amount of photoenergy in the visible region, enhancing the RhB dye adsorption, and assisting the effective separation of the photocatalytically produced electron-hole. This study offers the use of abundant and sustainable resources via an economical method to harness the usage of solar energy for wastewater treatment in the future.

CONCLUSION

Three materials of nitrogen-doped TiO2 including N-TiO2/C (modified with biochar), calcinated N-TiO2/C at 500 °C, and N-TiO2 were successfully synthesized by a simple one-step sol-gel method. As prepared N-doped TiO2 materials uncovered improved interesting properties compared to pure TiO2. The highest photocatalytic activity for the degradation of RhB dye in the visible region was obtained for N-TiO2/C nanocomposite (100% removal in 105 min), due to its favorable properties such as the highest mesoporous surface area and porosity, surface hydroxyls, and the lowest band gap energy. N-TiO2/C nanocomposite could decompose RhB through both dye-sensitization and photocatalytic pathways. It appeared that the widely available biochar is an efficient promoter that helps N-TiO2/C nanocomposite in driving effectively the photochemical degradation reactions by adjusting three primary limitations of pure TiO2: inducing the absorbance of a higher amount of photoenergy in the visible domain, enhancing the RhB dye adsorption, and assisting the effective separation of the photocatalytically produced electron-hole.

ACKNOWLEDGEMENTS

The authors are grateful for the financial support of this study by the University of Zanjan, Amirkabir University of Technology and the Iran National Science Foundation under Grant No. INSF 97009020.

DECLARATION OF COMPETING INTERESTS

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

 

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Nanochemistry Research
Volume 6, Issue 1
June 2021
Pages 79-93

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