Photodegradation performance and recyclability of a porous nitrogen and carbon co-doped TiO₂/activated carbon composite prepared by an extremely fast one-step microwave method

Abstract

In recent years, microwave sintering has become a common tool in materials science because of its rapid heating rate and high energy utilization efficiency. In this study, a fast one-step microwave method (3 min) was employed to prepare a porous nitrogen and carbon co-doped TiO₂/activated carbon ((C,N)-TiO₂/AC) composite bulk material. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), and electrical conductivity measurements. Photodegradation and recyclability experiments were also performed. For comparison, a sample was made by physically mixing (C,N)-TiO₂ and AC, and was characterized and examined by the same technique. The photodegradation of a methyl orange solution by the porous (C,N)-TiO₂/AC bulk revealed its good photocatalytic properties, with 94% photodegradation occurring within 100 min of irradiation following the initial 4 h of adsorption in the dark. The porous (C,N)-TiO₂/AC bulk also demonstrated very good recyclability, achieving 94% photodegradation in the 6th recycle. The results show that the composite material can be used in commercial photodegradation processes because of its low cost, simple method of preparation, and good recyclability.

---

1. Introduction

In recent years, titanium dioxide (TiO₂) has attracted increasing attention as a photocatalyst because of its many advantages, including its low toxicity, safety, excellent physicochemical stability, relatively low cost, and excellent photocatalytic properties under solar irradiation. However, its application in photodegradation processes has been hindered due to several shortcomings. First, the energy bandgap of anatase TiO₂ is 3.2 eV,¹ and the excitation wavelength that can be used is less than or equal to 387.5 nm, i.e., ultraviolet light, which accounts for only a small portion (3–5%) of the solar spectrum. Second, TiO₂ suffers from low quantum efficiency because of the rapid recombination rate of the electron–hole pairs generated during the photocatalytic reaction.^2,3 Finally, the powder TiO₂ can cause secondary pollution, as it remains a challenge to isolate and recycle TiO₂ unless special, costly equipment is available.

Much effort has been made toward increasing the use ratio of solar light during photodegradation, such as doping with metallic elements (commonly, Cu, Fe, Co, Ag, and Au)^4–8 and nonmetallic elements (e.g., N, C, S, and F).^9–16 As co-dopants, C and N have been demonstrated to greatly decrease the energy bandgap of TiO₂ and enhance the absorption of visible light, thereby improving the use ratio of solar light.^17,18

The quantum efficiency of TiO₂ can be upgraded by designing appropriate TiO₂-based composites. In a suitable composite material, the photogenerated electron–hole pairs would have longer lifetimes because a mechanism to separate the photogenerated electron–holes pairs would be operative,¹⁷ and pathways would be available that would enable egress of the carriers from the photocatalyst particles to effect photodegradation.¹⁷

To produce recyclable photocatalysts, immobilization of the particles has been well investigated, and supports such as concrete,^19,20 silica,^21,22 silk mask paper,²³ glass,^24–27 ceramic tile,²⁸ cement,^29,30 activated carbon (AC),^31–35 carbon felt,^36,37 and ore^38–40 have been used. Among these, AC has a high surface area and good adsorption ability because of its porous structure. Consequently, many researchers have attempted to immobilize TiO₂ on the surface of AC, using methods such as sol–gel,^41–43 deposition,^1,32,44,45 dip coating,^32,33,46,47 wet chemical,³⁴ hydrolysis,³² and microwave-assisted synthesis.^48–53 However, these methods are limited by their complexity, long processing times, and the requirement of costly equipment. Further, TiO₂ is easily dislodged from the AC surface, affording poor prospects for repeated utilization. Therefore, a new method must be developed that is simple and inexpensive and produces a more durable photocatalyst.

In this paper, we introduce a one-step microwave method for the preparation of a bulk composite of C- and N-co-doped TiO₂ ((C,N)-TiO₂) and AC (denoted as (C,N)-TiO₂/AC), which is both porous and recyclable. Only a short one-step microwave heating process was necessary after the starting materials (anatase TiO₂ (ATiO₂)), guanidine hydrochloride (CH₅N₃·HCl), and AC were mixed. This microwave method has the advantages of being fast, low cost, and simple, as well as the ability to fabricate porous structures with good photocatalytic properties and satisfactory recyclability. The porous (C,N)-TiO₂/AC bulk demonstrated great potential applicability in the photodegradation of a model organic pollutant.

2. Experimental

2.1 Starting materials

ATiO₂ (99.8%, AR) and CH₅N₃·HCl (99.0%, AR) were purchased from Aladdin Industrial Corporation. The AC (99.0%, AR) was purchased from Tianjin Kermel Chemical Reagent Company.

2.2 Preparation of porous (C,N)-TiO₂/AC bulk

In an agate mortar, ATiO₂, CH₅N₃·HCl, and AC were uniformly mixed in a 3 : 2 : 15 mol ratio. After transferring the mixture into a ceramic crucible (50 mL, Materials Business of Dacheng Kiln, Jingdezhen City, China) with a lid, the ensemble was irradiated for 3 min in a microwave oven (EG823LC7-NR3 23L, Midea, China) at 400 W power under 1 atmospheric pressure, i.e., 1.013 × 10⁵ Pa. The temperature of the microwave oven (302.5 °C) was measured immediately after microwave irradiating. The temperature and pressure of the microwave oven when the photocatalyst was prepared are shown in Table 1. For comparative purposes, another sample was prepared by physically mixing ATiO₂, CH₅N₃·HCl, and AC, followed by hot-pressing (DCHP-2000A-03, HLSTAR, USA) at 300 °C under 6.5 × 10⁷ Pa. This sample, denoted as P(C,N)-TiO₂/AC, was subjected to the same experiments and characterization steps (except for XPS) as the microwaved material.

Table 1 Temperature and pressure of the prepared (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC

Sample	(C,N)-TiO2/AC	P(C,N)-TiO2/AC
Temperature (°C)	302.5	300
Pressure (Pa)	1.013 × 10^5	6.5 × 10^7

---

2.3 Characterization

The lattice structures of the prepared samples were characterized by X-ray diffraction (XRD; X'Pert PRO MPD, Panalytical, the Netherlands) at a scan rate of 8° min⁻¹. We used scanning electron microscopy (SEM; Zeiss Ultra 55, Carl Zeiss, Germany) to obtain morphological and structural information. X-ray photoelectron spectroscopy (XPS; ESCALAB 250, Thermo Fisher Scientific) was performed to investigate the chemical states of C, N, O, and Ti. Photoluminescence (PL) spectra (F4500 Fluorescence Spectrophotometer, Hitachi, Japan) were measured to analyze the recombination information of the photogenerated electron–hole pairs. A ZEM-3 system (Ulvac-Riko, Japan) was utilized to measure the electrical conductivity of the samples. A UV-2550 instrument (Shimadzu, Japan) was used to obtain the UV-Vis diffuse reflectance spectra, from which energy bandgap (E_g) values were calculated according to eqn (1):²⁴

αhν = A(hν − E_g)² (1)

here, A is the absorption constant for indirect transitions, α is the absorption coefficient, and hν is the incident photon energy. By taking hν as the x-axis and (αhν)^1/2 as the y-axis, E_g can be determined as the crossing point between the line extrapolated from the linear part of the curve and the x-axis of the plot.

2.4 Dark adsorption behaviour and photocatalytic properties

Dark adsorption experiments and photocatalytic testing were carried out using a photocatalysis apparatus (W350, Shanghai Lansheng Electronics Co.). The photocatalyst sample (3.5 g) was put into a methyl orange (MO) solution (100 mL, 10 mg L⁻¹). During dark adsorption, the reactor was wrapped with a tin foil to exclude light. Each hour, an aliquot of the MO solution (3 mL) was removed and subjected to UV-Vis absorbance analysis. After 4 h, the tin foil was removed, and the reactor was exposed to irradiation from a xenon lamp (XQ350, Shanghai Lansheng Electronics Co.). The wavelength range of the xenon lamp was 380–1100 nm, and the selected current intensity was 9 A.

The dark adsorption ability and photocatalytic properties of the TiO₂ sample at time t were obtained by comparing the residual concentration of MO at t to its initial concentration. The concentration of the MO solution was evaluated according to the method described by Zhong Wei.⁵⁴ The dark absorption and degradation rate of the MO solution were determined by the following formula:⁵⁴

E_d = (A₀ − A_t)/A₀ = (C₀ − C_t)/C₀ (2)

where C₀ (mg L⁻¹) is the initial concentration of the MO solution during the dark adsorption or photodegradation period, A₀ is the initial absorbance intensity of the MO solution, C_t (mg L⁻¹) is the concentration at time t (min), and A_t is the corresponding absorbance intensity of the MO solution at time t (min).

3. Results and discussion

3.1 XRD analysis

Fig. 1(a) shows the XRD patterns of the composite samples and the starting materials. Fig. 1(b) and (c) present the enlarged diffraction patterns between 26° and 30° of the (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC composites. The diffraction peaks observed at 25.3°, 37.8°, 48.0°, 53.9°, 55.0°, 62.7°, 68.8°, 70.2°, and 75.0° correspond to the (101), (004), (200), (105), (211), (213), (116), (220), and (215) crystal planes of ATiO₂ (PDF No. 21-1272), respectively, while the diffraction peak located at 26.6° can be indexed to the (003) crystal plane of AC (PDF No. 26-1079). The characteristic peak positions of ATiO₂ do not shift in both (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC. However, the intensities of the characteristic peaks decrease after being processed by microwave, because C and N atoms from CH₅N₃·HCl are partially doped into the ATiO₂ lattice structure and they deteriorate the crystal structure of ATiO₂.⁵⁴ Finally, no diffraction peaks except those owing to ATiO₂ and AC are observed in the XRD patterns of either composite, indicating the physical mixing of the components. The average crystallite size of TiO₂, (C,N)-TiO₂/AC, and P(C,N)-TiO₂/AC was estimated by using Scherrer's equation¹⁵ (as listed in Table 2). The average crystallite sizes of (C,N)-TiO₂/AC (14.8 ± 0.3 nm) and P(C,N)-TiO₂/AC (15.0 ± 0.3 nm) are slightly smaller than TiO₂ (15.6 ± 0.4 nm), which may be the dopants inhibit the grain growth;^13,53 this result is in good agreement with those reported earlier.^13,53 As a result, the surface area of the photocatalyst may increase and lead to high photocatalytic activity.^36,51

Fig. 1 (a) XRD patterns of the starting materials (ATiO₂ and AC), and the composite samples. Enlarged diffraction patterns between 26° and 30° are shown for (b) P(C,N)-TiO₂/AC and (c) (C,N)-TiO₂/AC.

Table 2 Average crystallite size of TiO₂, (C,N)-TiO₂/AC, and P(C,N)-TiO₂/AC

Samples	TiO2	(C,N)-TiO2/AC	P(C,N)-TiO2/AC
Crystallite size (nm)	15.6 ± 0.4	14.8 ± 0.3	15.0 ± 0.3

---

3.2 SEM analysis

Fig. 2(a) and (c) present photographic images of the (C,N)–TiO₂/AC and P(C,N)-TiO₂/AC samples, and Fig. 2(b) shows the opposite side of the sample depicted in Fig. 2(a). The (C,N)-TiO₂/AC bulk has many pores ranging widely in size, from the largest being at ∼0.5 cm to the smallest at nanosize dimensions (Fig. 2(a) and (b)). We suspect that the microwaved photocatalyst has even smaller pores than those determined by SEM. The AC particles appear to be homogeneously distributed in the P(C,N)-TiO₂/AC bulk because of the even, gray colour of the composite (Fig. 2(c)). However, no holes are observed in P(C,N)-TiO₂/AC. SEM images of the (C,N)-TiO₂/AC sample at magnifications of 74× and 10 000× are shown in Fig. 2(d) and (e), respectively. Even under high magnification, the holes in (C,N)-TiO₂/AC are present in a wide size range. This wide pore size distribution reflects the inhomogeneity of the forces caused by the expansion of gases under microwave irradiation. The (C,N)-TiO₂/AC composite has spherical, uniform, loosely accumulated particles with diameters of about 40 nm (Fig. 2(f) (200 000×)).

Fig. 2 Photographic images of (a and b) opposite sides of the (C,N)-TiO₂/AC sample, and (c) the P(C,N)-TiO₂/AC sample. (d–f) SEM images of (C,N)-TiO₂/AC at different magnifications.

The (C,N)-TiO₂/AC composite has a large surface area because of the large number of pores and loosely accumulated particles.^32,41,50 The large surface area should improve the photocatalyst's ability to adsorb MO and absorb sunlight over time, and thereby, increase the MO degradation rate.

3.3 XPS analysis

Fig. 3 presents the XPS-determined chemical states of each element in TiO₂ and (C,N)-TiO₂/AC. Fig. 3(a) shows the N 1s XPS spectrum for (C,N)-TiO₂/AC, in which the N 1s core level peaks were deconvoluted into two component peaks at 399.6 and 400.4 eV. The peak at 399.6 eV is generally ascribed to the Ti–N bond,^54,55 likely from the replacement of oxygen atoms in the TiO₂ lattice by nitrogen atoms from CH₅N₃·HCl during the preparation. The peak at 400.4 eV is variously assigned to bonds such as N–C, N–O, and N–N.⁵⁴ Fig. 3(b) shows the C 1s XPS spectrum of (C,N)-TiO₂/AC. After fitting the XPS envelopes, three peaks (284.8, 285.9, and 289.1 eV) are observed. The peak at 284.8 eV corresponds to C–C bonded species which came from the adventitious carbon and the AC.⁵⁶ The peaks located at 285.9 and 288.8 eV can be assigned to C–O and C=O species, respectively, which are derived from the substitution of the lattice titanium atoms with the formation of Ti–O–C structures.^13,14,57

Fig. 3 XPS spectra. (a) N 1s and (b) C 1s obtained from (C,N)-TiO₂/AC; Ti 2p obtained from (c) TiO₂ and (d) (C,N)-TiO₂/AC; and O 1s obtained from (e) TiO₂ and (f) (C,N)-TiO₂/AC.

The representative Ti core level XPS spectrum of (C,N)-TiO₂/AC depicted in Fig. 3(d) shows two peaks centered at 459.5 and 465.2 eV, which may be indexed to the Ti 2p_3/2 and Ti 2p_1/2 photoelectrons in the Ti⁴⁺ chemical state, respectively. The spin–orbit splitting between these two bands is 5.7 eV, thus confirming that Ti exists mainly in the Ti⁴⁺ state in the composite.⁵⁸ It can be seen from Fig. 3(c) and (d) that the binding energies of the Ti 2p_3/2 and Ti 2p_1/2 photoelectrons of (C,N)-TiO₂/AC are 459.5 and 465.2 eV, respectively, slightly higher than those in TiO₂ (458.8 and 464.5 eV),^17,59 indicating that C and N atoms have been doped in the TiO₂ lattice and resulted in oxygen vacancy defects. The structural integrity of TiO₂ is thus modified by the C and N dopants. These results are in accord with the previously published data.^60–62

The O 1s XPS spectra of TiO₂ and (C,N)-TiO₂/AC are shown in Fig. 3(e) and (f). The O 1s region of TiO₂ can be fitted to two peaks (530.0 and 531.0 eV), whereas that of (C,N)-TiO₂/AC can be fitted to three peaks (530.7, 531.0, and 532.7 eV). The peaks at 530.0 and 530.7 eV correspond to lattice oxygen atoms in Ti–O linkages.^49,59 The increase in the Ti–O binding energy in (C,N)-TiO₂/AC may be attributed to the C and N dopants, which affect the electron binding energy of the O atoms.⁴⁹ The peak with a binding energy of 531.0 eV is attributed to surface-adsorbed components such as hydroxyl groups.⁴⁹ Table 3 shows that the ratio of oxygen to titanium atoms in the TiO₂ sample is slightly larger than 2, indicating an excess of oxygen in the XPS-measured TiO₂. This ratio further suggests the presence of hydroxyl groups in TiO₂, probably owing to the decomposition of water vapor during sample preparation by microwave irradiation, particularly in the presence of AC.^31,34,35 Thus, the content of surface hydroxyl groups adsorbed on the TiO₂ sample was nearly 4.52%, because the ratio of oxygen to titanium atoms in the pure TiO₂ should be 2. However, the ratio of oxygen to titanium atoms in (C,N)-TiO₂/AC is much larger than 2 (Table 3). This is also because some carbon atoms have replaced the lattice titanium atoms. The characteristic peak at 532.7 eV may be attributed to C–OH groups,⁶³ which are derived from hydroxyl group adsorption on the AC surface.

Table 3 Ti and O contents of the composite samples determined by XPS

Samples	TiO2	(C,N)-TiO2/AC
Ti (at%)	26.2	11.06
O (at%)	56.92	33.73

---

3.4 DRS analysis

Fig. 4(a) presents the UV-Vis DRS of TiO₂, (C,N)-TiO₂/AC, P(C,N)-TiO₂/AC, and AC. Fig. 4(b) shows the (αhν)^1/2–hν curves of TiO₂ and the two composite samples. Compared with TiO₂, the absorption boundaries of the (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC composites are obviously red-shifted into the visible region. Furthermore, both composites show significantly enhanced optical absorption in the 450–800 nm wavelength region. Red-shifting indicates a reduction in the energy bandgap (E_g). This value can be estimated from the plot of (αhν)^1/2 versus (hν). The crossing point of the tangent of the curve and the horizontal axis is approximately equal to the indirect E_g of TiO₂. The E_g values obtained for TiO₂, (C,N)-TiO₂/AC, and P(C,N)-TiO₂/AC by this method are 3.14, 2.53, and 2.55 eV, respectively. The E_g values of (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC are nearly identical, probably because they are basically physical mixtures of (C,N)-TiO₂ and AC. A trace chemical bonding between (C,N)-TiO₂ and AC may have occurred in (C,N)-TiO₂/AC, which would account for its slightly lower E_g value than that for P(C,N)-TiO₂/AC, because C can easily bond with Ti as the C was doped in (C,N)-TiO₂ under the high temperature. Formation of chemical bonding between TiO₂ and AC under microwave irradiation has also been reported earlier.^42,64

Fig. 4 (a) UV-Vis absorption spectra of starting materials and composite samples. (b) Plots of (αhν)^1/2 versus hν for TiO₂ and the composite samples. The value of E_g can be obtained by extrapolating the linear part of the plot to the x-axis, where the crossing point represents E_g.

There are two reasons why the E_g of (C,N)-TiO₂/AC is reduced in the visible region. The first is related to the dopant C and N atoms, which generate impurity energy levels in TiO₂.¹⁸ The second is E_g narrowing; because the level of (C,N) co-doping in TiO₂ in this work is high, E_g is reduced and the absorption boundary is red-shifted.

The optical absorption by (C,N)-TiO₂/AC in the visible region also increases because of two reasons. Besides the above-mentioned reduction of E_g, the addition of AC contributes to the optical absorption by (C,N)-TiO₂/AC, as it can greatly increase the optical absorption rate in the visible region. Indeed, we believe that the enhanced absorption ability of (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC in the visible light region is contributed mainly by the AC.

3.5 PL analysis

PL in TiO₂ originates from the recombination of photogenerated charges. Thus, PL spectra are usually employed to investigate the overall efficiency of charge carrier trapping, immigration, and transfer. The lifetimes of the photogenerated electrons and holes in a semiconductor can be qualitatively determined based on its PL spectrum. A lower PL intensity indicates a lower recombination rate. The PL spectra of TiO₂, (C,N)-TiO₂/AC, P(C,N)-TiO₂/AC, and AC after excitation at 296 nm in the 350–550 nm wavelength region are shown in Fig. 5. Five major emission peaks of TiO₂ were observed, located at 398, 451, 468, 483, and 493 nm. The peak at 398 nm is attributed to a direct transition from the conduction band to the valence band, while the remaining peaks are ascribed to exciton effects derived from surface states and lattice defects.^13,59 The PL peaks at 451 and 468 nm are due to band-to-band transitions, while those at 483 and 493 nm are due to intra-band transitions within the energy level traps or surface states. From Fig. 5, we can also see that the emission peak intensities at 398 nm for (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC are much lower than that for TiO₂. These results suggest that the lifetimes of the photogenerated carriers in the (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC samples are longer than those in TiO₂.

Fig. 5 PL spectra of the starting materials and composite samples.

The prolongation of the photogenerated carrier lifetimes in the (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC samples is considered to arise from the inhomogeneous structures of (C,N)-TiO₂ and AC. The addition of AC is suggested to provide AC-containing TiO₂-based composites with good electron-transfer ability and sufficient interfacial contact with TiO₂. To verify this assertion, we performed electrical conductivity measurements for the as-prepared samples. Table 4 shows the electrical conductivity values for (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC as of 2.5 and 3.1 s m⁻¹, respectively (we also attempted to assess the electrical conductivity of (C,N)-TiO₂, but the value was too low to detect). These data confirm our suggestion, as do reports in the literature,⁶⁴ that the added AC in an AC-containing TiO₂-based composite can impart good electron-transfer ability and provide sufficient interfacial contact with TiO₂, since electrons are possibly transferred from the conduction band of TiO₂ to AC. Therefore, the addition of AC can reduce the recombination of photogenerated electron–hole pairs.

Table 4 Electrical conductivity determinations for (C,N)-TiO₂, (C,N)-TiO₂/AC, and P(C,N)-TiO₂/AC^b

a Value below the limit of detection of the ZEM-3 instrument.b EC represent Electrical Conductivity.
Sample	(C,N)-TiO2	(C,N)-TiO2/AC	P(C,N)-TiO2/AC
EC (s m^−1)	^a	2.5	3.1

---

The lower recombination rate of the photogenerated electron–hole pairs can improve the photocatalytic quantum efficiency of the porous (C,N)-TiO₂/AC bulk, which can greatly enhance its degradation performance toward organic pollutants.

3.6 Dark adsorption and photocatalytic degradation of MO

We next examined the dark adsorption and photodegradation rate of MO using (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC as photocatalysts. The initial volume of the MO solution was 100 mL, the initial MO concentration was 10 mg L⁻¹, and the loading photocatalyst was 3.5 g. The desorption–adsorption equilibrium of both (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC can be achieved after 4 h (Fig. 6(a)). (C,N)-TiO₂/AC has more dark adsorption than P(C, N)-TiO₂. More MO on the surface of photocatalysts is advantageous for the photochemical reaction between MO and photocatalysts.^18,42 Therefore, the solutions were firstly adsorbed with the photocatalyst in the dark for 4 h (dark adsorption). Then, they were exposed to irradiation from the xenon lamp for the photocatalytic experiment. The experimental results are shown in Fig. 6(b). The dark adsorption properties and, in particular, photocatalytic ability of (C,N)-TiO₂/AC are better than those of P(C,N)-TiO₂/AC. After dark adsorption, the rate of MO degradation for the (C,N)-TiO₂/AC bulk reaches 94% after 100 min of illumination, whereas that for P(C,N)-TiO₂/AC only reaches 88% even after 12 h of illumination. The better photocatalytic ability of (C,N)-TiO₂/AC originates predominantly from its pore structure, since the E_g values of both materials are nearly identical. The pores increase the surface area of the (C,N)-TiO₂/AC bulk, and therefore, increase the rates of dark adsorption and photodegradation. Another reason may be the better interfacial contact between (C,N)-TiO₂ and AC in (C,N)-TiO₂/AC than in P(C,N)-TiO₂/AC. This result, together with the lower E_g value for (C,N)-TiO₂/AC than that for P(C,N)-TiO₂/AC, is observed because of the presence of trace of chemical bonding between (C,N)-TiO₂ and AC in (C,N)-TiO₂/AC.

Fig. 6 (a) Dark adsorption and (b) photocatalytic degradation of MO by (C,N)-TiO₂/AC and P(C,N)-TiO₂/AC.

To test the recyclability of the (C,N)-TiO₂/AC bulk, repetitive degradation tests were performed. The results are shown in Fig. 7. In the sixth recycle, after 4 h of dark adsorption and 2.67 h of photocatalysis, the degradation of MO can become as high as 94%. This indicates that the (C,N)-TiO₂/AC bulk have a very good recyclability.

Fig. 7 Recyclability of (C,N)-TiO₂/AC.

4. Conclusion

We herein show that the preparation of a porous (C,N)-TiO₂/AC bulk composite by a microwave method required only uniform mixing of CH₅N₃·HCl, TiO₂, and AC, followed by microwave irradiation for 3 min. The prepared sample exhibited good dark adsorption and photodegradation abilities, as well as satisfactory recyclability. The dark adsorption properties arose from the pore structure of the composite, and the satisfactory photodegradation behavior is attributed to the enhanced UV-Vis absorption, good dark adsorption of MO, and narrowed energy bandgap owing to the presence of C and N dopants, as well as good electron transfer ability because of the composite structure of (C,N)-TiO₂ and AC. We believe that this porous (C,N)-TiO₂/AC composite material can be used in photodegradation applications owing to its low cost, cheap preparation equipment (a household microwave oven is suffice), simple and rapid preparation procedure, and low energy consumption. Besides, the sample is recyclable and the bulk is easily separated from the solution, without requiring any expensive equipment.

Acknowledgements

The authors gratefully acknowledge financial support for this work by the National Natural Science Foundation of China (51672090, 51372092).

References

1. G. Li Puma, A. Bono, D. Krishnaiah and J. G. Collin, J. Hazard. Mater., 2008, 157, 209–219.
2. S. Singh, H. Mahalingam and P. K. Singh, Appl. Catal., A, 2013, 462–463, 178–195.
3. M. Pelaez, N. T. Nolan, S. C. Pillai, M. K. Seery, P. Falaras, A. G. Kontos, P. S. M. Dunlop, J. W. J. Hamilton, J. A. Byrne, K. O'Shea, M. H. Entezari and D. D. Dionysiou, Appl. Catal., B, 2012, 125, 331–349.
4. D. Zhang, Transition Met. Chem., 2010, 35, 933–938.
5. H. M. Ibrahim, World J. Microbiol. Biotechnol., 2015, 31, 1049–1060.
6. O. P. Linnik, M. A. Zhukovskiy, G. N. Starukh, N. P. Smirnova, N. V. Gaponenko, A. M. Asharif, L. S. Khoroshko and V. E. Borisenko, J. Appl. Spectrosc., 2015, 81, 990–995.
7. J. Ma, H. He and F. Liu, Appl. Catal., B, 2015, 179, 21–28.
8. H. Liu and R. Xiong, J. Wuhan Univ. Technol., Mater. Sci. Ed., 2013, 28, 44–47.
9. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271.
10. G. Zhu and K. Zhou, Sci. China, Ser. B: Chem., 2007, 50, 212–216.
11. E. A. Konstantinova, N. T. Le, P. K. Kashkarov, A. A. Zaytseva and V. G. Kytin, Moscow Univ. Phys. Bull., 2014, 69, 180–184.
12. S. Sakthivel and H. Kisch, Angew. Chem., 2003, 42, 4908–4911.
13. X. F. Lei, X. X. Xue, H. Yang, C. Chen, X. Li, M. C. Niu, X. Y. Gao and Y. T. Yang, Appl. Surf. Sci., 2015, 332, 172–180.
14. Q. Deng, Y. Liu, K. Mu, Y. Zeng, G. Yang, F. Shen, S. Deng, X. Zhang and Y. Zhang, Phys. Status Solidi A, 2015, 212, 691–697.
15. A. Khalilzadeh and S. Fatemi, Clean Technol. Environ. Policy, 2013, 16, 629–636.
16. E. P. Lokshin, T. A. Sedneva, A. T. Belyaevskii and V. T. Kalinnikov, Theor. Found. Chem. Eng., 2008, 42, 590–594.
17. H. Chen, K.-F. Chen, S.-W. Lai, Z. Dang and Y.-P. Peng, Sep. Purif. Technol., 2015, 146, 143–153.
18. X. Yang, C. Cao, L. Erickson, K. Hohn, R. Maghirang and K. Klabunde, J. Catal., 2008, 260, 128–133.
19. M. M. Ballari, M. Hunger and G. Husken, Journal, 2009, 409–414.
20. Y. Wang, Y. Li and W. Zhang, Environ. Sci. Pollut. Res., 2015, 22, 3508–3517.
21. J. Oguma, Y. Kakuma, M. Nishikawa and Y. Nosaka, Catal. Lett., 2012, 142, 1474–1481.
22. Q. Dai, N. He, K. Weng, B. Lin, Z. Lu and C. Yuan, J. Inclusion Phenom. Macrocyclic Chem., 1999, 35, 11–21.
23. L.-Z. Sha, H.-F. Zhao and G.-N. Xiao, Fibers Polym., 2013, 14, 976–981.
24. R. Ananthakumar, B. Subramanian, S. Yugeswaran and M. Jayachandran, J. Mater. Sci.: Mater. Electron., 2012, 23, 1898–1904.
25. V. I. Shapovalov, O. A. Shilova, I. V. Smirnova, A. V. Zav'yalov, A. E. Lapshin, O. V. Magdysyuk, M. F. Panov, V. V. Plotnikov and N. S. Shutova, Glass Phys. Chem., 2011, 37, 150–156.
26. O. V. Savvova and L. L. Bragina, Glass Ceram., 2010, 67, 184–186.
27. J. Kumar and A. Bansal, Water, Air, Soil Pollut., 2013, 224, 1–11.
28. V. Petrovič, V. Ducman and S. D. Škapin, Ceram. Int., 2012, 38, 1611–1616.
29. A. R. Jayapalan, B. Y. Lee and K. E. Kurtis, Nanotechnology in Construction 3, Springer, Berlin, Heidelberg, 2009, pp. 267–273.
30. M. Janus, K. Bubacz, J. Zatorska, E. K. Nejman, A. Czyzewski, J. Przepiórski and A. W. Morawski, React. Kinet., Mech. Catal., 2014, 113, 615–628.
31. Z. Zhang, F. Yu, L. Huang, J. Jiatieli, Y. Li, L. Song, N. Yu and D. D. Dionysiou, J. Hazard. Mater., 2014, 278, 152–157.
32. A. H. El-Sheikh, A. P. Newman, H. Al-Daffaee, S. Phull, N. Cresswell and S. York, Surf. Coat. Technol., 2004, 187, 284–292.
33. Y. Tao, C.-Y. Wu and D. W. Mazyck, Ind. Eng. Chem. Res., 2006, 45, 5110–5116.
34. R. C. Asha and M. Kumar, Desalin. Water Treat., 2014, 54, 3279–3290.
35. Z. Zhang, Y. Xu, X. Ma, F. Li, D. Liu, Z. Chen, F. Zhang and D. D. Dionysiou, J. Hazard. Mater., 2012, 209–210, 271–277.
36. Y. J. Li, W. Chen, L. Y. Li and M. Y. Ma, Adv. Mater. Res., 2010, 146–147, 1754–1760.
37. Y. Li, W. Chen, L. Li and M. Ma, Sci. China: Chem., 2014, 54, 497–505.
38. P. Ševčík, G. Čík, T. Vlna and T. Mackul'ak, Chem. Pap., 2009, 63, 249–254.
39. L. Shaomin, G. Gaoli, X. Bihua, G. Wenqi and M. Guanjun, J. Wuhan Univ. Technol., Mater. Sci. Ed., 2009, 24, 557–561.
40. C. Wang, H. Shi and Y. Li, J. Wuhan Univ. Technol., Mater. Sci. Ed., 2012, 27, 603–607.
41. C. Liu, Y. Li, P. Xu, M. Li and M. Zeng, Mater. Chem. Phys., 2015, 149–150, 69–76.
42. F. Tian, Z. Wu, Q. Chen, Y. Yan, G. Cravotto and Z. Wu, Appl. Surf. Sci., 2015, 351, 104–112.
43. F. Tian, Z. Wu, Y. Yan, X. Ge and Y. Tong, Korean J. Chem. Eng., 2015, 32, 1333–1339.
44. B. Tryba, A. W. Morawski and M. Inagaki, Appl. Catal., B, 2003, 41, 427–433.
45. A. H. El-Sheikh, Y. S. Al-Degs, A. P. Newman and D. E. Lynch, Sep. Purif. Technol., 2007, 54, 117–123.
46. Y. Liu, S. Yang, J. Hong and C. Sun, J. Hazard. Mater., 2007, 142, 208–215.
47. Z. He, S. Yang, Y. Ju and C. Sun, J. Environ. Sci., 2009, 21, 268–272.
48. P. I. Liu, L. C. Chung, C. H. Ho, H. Shao, T. M. Liang, R. Y. Horng, M. C. Chang and C. C. Ma, J. Colloid Interface Sci., 2015, 446, 352–358.
49. P.-I. Liu, L.-C. Chung, H. Shao, T.-M. Liang, R.-Y. Horng, C.-C. M. Ma and M.-C. Chang, Electrochim. Acta, 2013, 96, 173–179.
50. X. Ge, F. Tian, Z. Wu, Y. Yan, G. Cravotto and Z. Wu, Chem. Eng. Process., 2015, 91, 67–77.
51. D. Liu, Z. Wu, F. Tian, B.-C. Ye and Y. Tong, J. Alloys Compd., 2016, 676, 489–498.
52. X. Ge, Z. Wu, Z. Wu, Y. Yan, G. Cravotto and B.-C. Ye, J. Taiwan Inst. Chem. Eng., 2016, 64, 235–243.
53. F. Tian, Z. Wu, Y. Tong, Z. Wu and G. Cravotto, Nanoscale Res. Lett., 2015, 10, 360.
54. W. Zhong, Y. Yu, C. Du, W. Li, Y. Wang, G. He, Y. Xie and Q. He, RSC Adv., 2014, 4, 40019–40028.
55. L. Yu, X. Yang, J. He, Y. He and D. Wang, J. Mol. Catal. A: Chem., 2015, 399, 42–47.
56. X. Li, P. Liu, Y. Mao, M. Xing and J. Zhang, Appl. Catal., B, 2015, 164, 352–359.
57. S. K. Parayil, A. Razzaq, S.-M. Park, H. R. Kim, C. A. Grimes and S.-I. In, Appl. Catal., A, 2015, 498, 205–213.
58. L.-L. Tan, W.-J. Ong, S.-P. Chai, B. T. Goh and A. R. Mohamed, Appl. Catal., B, 2015, 179, 160–170.
59. R. Jaiswal, J. Bharambe, N. Patel, A. Dashora, D. C. Kothari and A. Miotello, Appl. Catal., A, 2015, 168–169, 333–341.
60. Y.-H. Qin, Y. Li, T. Lam and Y. Xing, J. Power Sources, 2015, 284, 186–193.
61. K. Huang, K. Sasaki, R. R. Adzic and Y. Xing, J. Mater. Chem., 2012, 22, 16824–16832.
62. Y. Fan, C. Ma, B. Liu, H. Chen, L. Dong and Y. Yin, Mater. Sci. Semicond. Process., 2014, 27, 47–50.
63. D. Wang, Z. Geng, B. Li and C. Zhang, Electrochim. Acta, 2015, 173, 377–384.
64. E.-C. Su, B.-S. Huang, C.-C. Liu and M.-Y. Wey, Renewable Energy, 2015, 75, 266–271.

---