Preparation and Photocatalytic Properties of N-DopedGraphene/ TiO2 Composites

N-doped graphene (NG)/TiO2 composites were prepared by a two-step hydrothermal method using HF as the surface etchant and urea as the nitrogen source.*emorphology, structure, and bonding conditions of the NG/TiO2 composites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), and photoluminescence (PL) spectroscopy. *e effects of NG on the lifetime of photogenerated electron-hole pairs, adsorption capacity, and photocatalytic activity of the composite photocatalysts were also investigated. *e photocatalytic activity was evaluated under ultraviolet light and sunlight irradiation. *e recovery testing was completed under ultraviolet light irradiation.*e results show that TiO2 was uniformly loaded on the NG surface by chemical bonding. *e introduction of NG effectively inhibited the recombination of photogenerated electron-hole pairs and improved both the adsorption capacity and photocatalytic activity of the composites. *e 7 wt% NG/TiO2 showed the best adsorption capacity of methyl orange (MO). *e best photocatalytic activity occurred for 5 wt % NG/TiO2 composites, and after four recovery tests, the photocatalytic degradation of MO under 60min ultraviolet light irradiation exceeded 90%.


Introduction
In recent years, TiO 2 has become widely used in the field of photocatalysis because of its strong oxidizing ability, high chemical stability, low cost, nontoxicity, and environmental friendliness [1,2]. TiO 2 can undergo photocatalytic reactions under mild reaction conditions and will generate hydroxyl radicals (·OH) [3,4]. However, TiO 2 only responds to ultraviolet (UV) light, which is less than 5% of the solar spectrum, leading to a very inefficient use of solar energy. Also, the recombination rate of photoexcited charge carriers is high, so the quantum efficiency of TiO 2 is low. erefore, effectively suppressing the recombination of photoexcited carriers in the photocatalytic processes and improving the utilization of solar light are urgent problems to be solved.
Graphene is a two-dimensional carbon material formed by a single layer of C atoms with sp 2 hybridization. It has high conductivity, high stability, and a large specific surface area (theoretical value of 2630 m 2 /g). Since nitrogen atoms have an atomic radius similar to that of carbon atoms, the carbon structure can be doped with nitrogen atoms serving as electron donors, and the resulting N-doped graphene (NG) exhibits many new properties including a band gap, n-type conductivity, and increased free carrier density, thereby improving the conductivity and stability of graphene [5][6][7]. NG can provide a carrier for TiO 2 photogenerated electrons, reduce the recombination of photoexcited carriers, and improve the efficiency of photocatalytic reactions. erefore, NG/TiO 2 composites may be ideal new photocatalysts.
Compared with the methods of precipitation and sol-gel, the material prepared by the hydrothermal method has the advantages of high purity, adjustable particle size, adjustable morphology and crystal surface, and simple reaction conditions. Yan et al. [8] prepared RGO/TiO 2 composites with high photocatalytic performance by a two-step hydrothermal method. Based on this previous work, the raw materials were modified and new NG/TiO 2 composites were also synthesized by a two-step hydrothermal method. In this work, Ti(SO 4 ) 2 was used as the titanium source, urea as the nitrogen source, and HF as the surface etchant. e samples were characterized by various methods to determine the morphology and structure of NG/TiO 2 . e effects of introducing NG on photogenerated electron-hole pair lifetimes, adsorption performance, and photocatalytic performance were also studied. e photocatalytic properties of the composites for the degradation of methyl orange (MO) were tested under UV and sunlight conditions, and the catalysts were tested several times to investigate their cyclability. e preparation of anatase nano-TiO 2 was completed using the hydrothermal method. 30 mL HF (160 mmol/L), 50 mL H 2 O, and 192 mg Ti(SO 4 ) 2 were added to a plastic beaker. Because Ti(SO 4 ) 2 can be hydrolyzed, the HF was added to water first, and then the Ti(SO 4 ) 2 was added to the mixture. Next, the prepared solution was sonicated and then magnetically stirred for 15 min each, followed by the transfer of the reaction solution to a 100 mL Teflon-lined reactor, and subsequently incubated at 180°C for 12 hours. Finally, the obtained product was centrifuged, and the impurities were removed by washing with absolute ethanol and deionized water several times, and the sample was dried in a vacuum oven at 60°C for 12 hours. e obtained white powder was anatase-type nano-TiO 2 powder.

Preparation of NG.
A modified Hummers method was used for graphene oxide (GO) preparation. First, 30 mL of the GO dispersion was added to a beaker, and then urea was added in a mass ratio of 1 : 5 GO to urea. After magnetically stirring for 30 minutes, the solution was transferred to a Teflon-lined reactor, incubated at 160°C for 3 hours, naturally cooled, and then centrifuged several times, giving the final product NG, after lyophilization.

Preparation of NG/TiO 2 Composites
. 60 mg of TiO 2 powder, different masses of NG (1%, 3%, 5%, 7%, 9% by mass of TiO 2 ), and 50 mL of water were added to test tubes. After sonication for three hours, each mixed solution was poured into a 100 mL Teflon-lined reactor and was incubated at 180°C for 12 hours. e obtained product was centrifuged, and the impurities were removed by washing with absolute ethanol and deionized water several times, and the sample was dried in a vacuum oven at 60°C for 12 hours. e obtained powders were NG/TiO 2 composites.

2.3.
Characterization. X-ray diffraction (XRD, Bruker AXS D8 Advance) with a Cu target and Kα radiation (λ � 0.15406 nm) was used for phase identification of the samples in the range of 10°-80°. e morphology of the samples was examined by a field emission scanning electron microscope (FESEM, Hitachi SU8010). e chemical state of the sample was analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250 XI), and the structure of the sample was analyzed using Raman spectroscopy (RAMAN, Renishaw RM 2000). e absorption range and absorption properties of the sample were measured by ultraviolet-visible diffuse reflectance spectroscopy (UV-VIS DRS, PerkinElmer Lambda 650) and the recombination efficiency of the sample electron-hole pairs was characterized using photoluminescence spectroscopy (PL, HR 320). is mixture was stirred in the dark and samples were taken every 10 minutes. After centrifugation, the supernatant was taken, and the absorbance was measured at λ max � 464 nm using an UV-visible spectrophotometer to calculate the residual dye concentration.
e MO adsorption amount of the composites was calculated according to [9] where q is the adsorption capacity; Ca0 is the initial concentration of MO; Cat is the concentration of MO when adsorbed; V is the solution volume; and m is the mass of the adsorbent.

Photocatalytic Degradation
Test. 0.1 g of each composite having a different NG doping ratio was added to 100 mL of a 20 mg/L MO solution (pH � 3), and the mixture was stirred for one hour in the dark. In order to study the photocatalytic degradation ability of composites, catalytic reactions were carried out under ultraviolet light and sunlight, respectively. In order to study the effect of H 2 O 2 on the reaction rate of the composites, 2 mL H 2 O 2 was added to samples under ultraviolet light and sunlight, respectively. e ultraviolet light source was a 250 W high-pressure mercury lamp (365 nm ultraviolet light), which was 15 cm away from the mixture and was sampled every 10 min. After centrifugation, the supernatant was taken, and the absorbance was measured by a UV-visible spectrophotometer to calculate the photocatalytic efficiency. e solar photocatalytic test was conducted over a 5-hour period outdoors on a sunny day. e photocatalytic efficiency was calculated according to [9] where Cd0 is the initial concentration of MO and Cdt is the concentration of MO when the reaction time is t.

Recovery
Test. 0.1 g of 5 wt % NG/TiO 2 was added to 100 mL of a 20 mg/L MO solution (pH � 3) and stirred for 60 min in the dark to bring the MO into an adsorption equilibrium. e photocatalytic reaction was then carried out under a UV lamp and sampled at 60 min. After centrifugation, the supernatant was taken, and the absorbance was measured by a UV-visible spectrophotometer to calculate the photocatalytic efficiency, and the calculation formula was the same as formula (2). en, the precipitate and the photocatalyst were washed together and dried, and photocatalytic testing was repeated after the recovery. ese indicate that the prepared TiO 2 and NG/TiO 2 composites are mainly anatase crystal types. e diffraction peaks of the NG/TiO 2 composites are significantly weaker than those of TiO 2 , which is due to the interaction between NG and TiO 2 . In the NG/TiO 2 composites, the (002) characteristic diffraction peak of graphene did not appear near 2θ � 26°b ecause this diffraction peak overlaps with the (101) crystal plane diffraction peak of the anatase phase TiO 2 [10,11].

Results and Discussion
When calculated by the Scherrer equation, the TiO 2 nanoparticles were 43 nm. e Scherrer equation is as follows: D is the average thickness of the crystal grains perpendicular to the crystal plane direction, nm; K is the Scherrer constant, 0.89; c is the X-ray wavelength, which is 0.154056 nm; B is the half-height width of the diffraction peak of the measured sample; θ is the Bragg diffraction angle. Figure 2, it can be seen that TiO 2 and NG/TiO 2 composites have undergone agglomeration phenomena, because there are a few crystal surfaces on the surface of photocatalyst, due to HF etching. e etching mainly occurs on the high surface energy crystal face (001) and continues along the (001) direction [8], making it easy for particles to agglomerate in order to achieve a stable state. Figure 2(b) is the SEM image of the NG/TiO 2 composites. e morphology of TiO 2 did not change and the loading was good through the secondary hydrothermal treatment. Some of the N-doped graphene thin layers are stacked together to form a multilayer structure.

FESEM. From
is edge curl and fold appearance was attributed to the defect structure caused by the nitrogen-doped graphene crystal [12].

XPS.
e chemical composition of NG/TiO 2 nanocomposites was investigated by XPS. e characteristic peaks of C, F, N, O, and Ti can be observed in Figure 3(a). C 1s (Figure 3(b)) has a binding energy of 284.3 eV, indicating the presence of a C-C bond, which is a characteristic peak for graphene. e peaks at binding energies of 285.5 and 288.8 eV correspond to C-O and C�O, respectively, and were caused by possible residual hydroxyl functional groups of graphene [13]. Figure 3(c) is the deconvoluted peak of O 1s, where the peak at 531.3 eV corresponds to Ti-O-C, the peak at 530.6 eV corresponds to C�O and O�C-OH, and the peak at 533.4 eV corresponds to C-O [14]. In Figure 3(d), the characteristic peaks of Ti 2p3/2 and Ti 2p1/2 appeared at binding energies of 458.7 and 464.5 eV, respectively, which confirmed the existence of TiO 2 [15]. e peak at 459.2 eV corresponds to C-Ti, indicating that there is an interaction between TiO 2 and NG. e N 1s (Figure 3(e)) peak has binding energy at 399.5 eV, indicating that there may be C-N present, proving that the N atom has been doped into the structure of graphene. e F 1s (Figure 3(f)) spectra can be deconvoluted into two peaks. e peak located at a binding energy of 684.7 eV was caused by the exchange of ligands between F and hydroxyl groups on the surface of TiO 2 to form ≡Ti-F, and Fchemisorbed on the surface of the nano-TiO 2 [16]. e peak near 690.2 eV indicated that F is doped into the TiO 2 lattice [17].

Raman Spectrum.
It can be seen from the Raman spectrum (Figure 4(a)) that characteristic Raman scattering peaks of anatase phase TiO 2 were observed at 144.4 cm −1 (E g ), 395.9 cm −1 (B 1g ), 515.4 cm −1 (A 1g ) and 636.3 cm −1 (E g ). Figure 4(b) shows that the characteristic Raman scattering peaks for the D-band and G-band of graphene were observed near 1364 cm −1 and 1578 cm −1 , respectively. After the formation of the NG/TiO 2 composites, the characteristic Raman peaks of the anatase TiO 2 still existed, and the Raman peaks of the composites moved toward higher wavenumbers, indicating that a chemical interaction between NG and TiO 2 molecules took place [18,19]. However, due to the addition of NG, the impurity level or defect level will appear in the spectra of the composites, and the appearance of the impurity level or defect level will also cause the Raman peaks to move toward higher wavenumbers [18][19][20].

UV-VIS DRS.
e UV-visible diffuse reflectance spectrum ( Figure 5(a)) shows that pure TiO 2 has a light absorption range below 400 nm and can only absorb ultraviolet light. is is due to the electron transfer from the valence band (O 2p) to the conduction band (Ti 3d). Compared with pure TiO 2 , the light absorption boundary of the NG/TiO 2 composites was red-shifted, and the absorption range of the photocatalyst was extended to the visible light region. e absorption intensity was also improved due to the introduction of nitrogen atoms into the carbon grid of graphene. TiO 2 grew on the N-doped graphene layer in situ through Ti-O-C bonding and hybridized with the C 2p and O 2p atomic orbitals under high Journal of Chemistry temperature and pressure [21]. is resulted in the increase of the valence band edge, thereby reducing the width of the forbidden band [22], and showed strong light absorption with visible light response ability. e band gap energies of TiO 2 , 3 wt % NG/TiO 2 , 5 wt % NG/TiO 2 , and 7 wt % NG/TiO 2 were calculated by formula E g � 1240/λ and were approximately 3.18 eV, 2.98 eV, 2.92 eV, and 2.96 eV, respectively ( Figure 5(b)).

Photoluminescence.
e photoluminescence spectrum is a way to characterize the electron-hole pair recombination efficiency: low emission intensity indicates that the recombination rate of photogenerated electron-hole pairs is slow, and the separation of electrons and holes is effective [23], which is beneficial to photocatalytic reaction. e spectrum in Figure 6 shows that the photoluminescence of the NG/TiO 2 composites with different amounts introduced, including 0 wt %, 1 wt %, 3 wt %, 5 wt %, 7 wt %, and 9 wt %. e emission intensity of pure titanium dioxide is the highest, indicating that the electron-hole pair has the fastest recombination rate and the lowest photocatalytic efficiency. e presence of NG can be used as a carrier of photogenerated electrons, which has charge transfer with the semiconductor and can effectively inhibit the recombination of photogenerated electron-hole pairs. 5 wt % NG/TiO 2 has the lowest emission intensity, indicating that the introduction of 5 wt % NG can transfer electrons well and can effectively separate electrons and holes, and therefore the photocatalytic effect will be the most significant. Introducing insufficient amounts of NG is the reason that NG, as a carrier, cannot transport the photogenerated electrons completely and effectively, and some of the excess photogenerated electrons will recombine with holes, affecting the photocatalytic reaction efficiency. Excessive NG can reduce the effective components of photoelectron-hole pairs, and at the same time, the photoelectron hole can use NG as the recombination center to recombine within it [24], affecting the photocatalytic reaction efficiency.
In general, different characterization techniques verify and compensate for each other and comprehensively evaluate the composites from different angles.

Suction and Desorption Balance Tests.
e adsorption of dye by a photocatalyst plays an important role in dye degradation [25,26]. MO is a monosulfonate azo compound. Under acidic conditions, the -SO 3− in MO interacted with the positively charged center on the surface of TiO 2 and adsorbed on the surface in a vertical manner. Such an adsorption mode left a small amount of MO on the surface of    TiO 2 [27]. Figure 7 shows the adsorption curves of MO for samples with different NG doping. e amount of adsorption of MO on the sample increased with time, and the adsorption was effectively saturated after 60 min. e adsorption performance of the composites with different NG loadings was better than that of TiO 2 , and the adsorption amount increased at first and then decreased with the increase of NG loading. 7 wt % NG/TiO 2 had the best adsorption performance, and the adsorption capacity after 60 min was 9.27 mg/g. is amount of NG effectively improved the adsorption performance of TiO 2 , but the amount of NG had a limit. is was because NG itself had a very large specific surface area and provided more sites for the adsorption of MO. However, as the amount of NG became too large, it reduced the specific surface area due to overlapping with itself, which then affected the adsorption of MO by the catalyst.      Table 2, it can be seen that the degradation efficiency of 5 wt % NG/TiO 2 for MO reached 46.97% in 120 min under sunlight condition. e degradation efficiency of TiO2 for MO reached 26.29% in 120 min under sunlight condition. e degradation efficiencies of other NG-added photocatalysts for MO reached 42.97% (7 wt % NG/TiO 2 ), 39.08% (9 wt % NG/TiO 2 ), 35.76% (3 wt % NG/TiO 2 ), and 33.27% (1 wt % NG/TiO 2 ) in 120 min under sunlight condition. e experimental results showed that the degradation rate under sunlight conditions was lower than that under ultraviolet light. is was because UV was a single wavelength high-pressure mercury lamp at 365 nm, whereas the sunlight was a full-spectrum light source containing ultraviolet and visible light in each wavelength band. Whether it was pure TiO 2 or the NG/TiO 2 composites, the degradation efficiency under UV conditions was higher than sunlight, and the addition of H 2 O 2 in the photocatalytic system promoted the photocatalytic degradation performance of the photocatalyst [28]. Compared with pure TiO 2 , the photocatalytic efficiency of the composites increased with the addition of NG. As the NG content increased, the catalytic performance of different photocatalysts increased at first and then decreased. Comparing NG/TiO 2 with two other photocatalysts (RGO/TiO 2 [9] and N-TiO 2 /RGO [29]) under the same conditions, the results showed that NG/TiO 2 exhibited the highest degradation efficiency for MO. e main reasons are as follows: (1) NG had a large specific surface area and an extremely high electron mobility. e combination of NG and TiO 2 significantly increased the specific surface area and electron mobility of the catalyst. As the photon contact probability increased, the absorption intensity of the dye molecules and photons increased. e higher electron mobility increased the photogenerated electron transfer rate of TiO 2 , inhibited the photogenerated electron-hole pair recombination, and thus had better photocatalytic performance. (2) N-doping narrowed the forbidden bandwidth of the photocatalyst, improved the responsivity of the photocatalyst in the visible light region, and thereby improved the utilization of sunlight. (3) With the increase of NG content, the effective component (TiO 2 ) of the photocatalyst decreased, and the photocatalytic efficiency also decreased. e reason why the 7 wt % NG/TiO 2 with the largest adsorption capacity did not exhibit the best photocatalytic performance may be due to the decrease of TiO 2 in the photocatalyst, which led to the photocatalyst not being able to completely oxidize the MO adsorbed on the surface.

Recovery Test.
In order to evaluate the chemical stability of the catalyst and the prospective industrial applications, a 5 wt % NG/TiO 2 sample with excellent photocatalytic performance was subjected to multiple recovery cycle tests under ultraviolet conditions. Figure 9 shows the degradation

Conclusions
NG/TiO 2 composites were successfully synthesized by a twostep hydrothermal method with HF as the surface etchant and urea as the nitrogen source of NG. e morphology was optimal, and the chemical bond between NG and TiO 2 resulted in a firm loading. By adding different amounts of NG, the recombination of photogenerated electron-hole pairs was effectively suppressed, the lifetime of carriers was prolonged, and the ability to respond to visible light was improved. e photocatalytic performance test showed that NG/TiO 2 composites had better adsorption performance and photocatalytic performance than pure TiO 2 . Specifically, 7 wt % NG/TiO 2 showed the best adsorption effect on methyl orange, and the adsorption capacity after 60 min of reaction time was 9.27 mg/g. However, 5 wt % NG/TiO 2 showed the best photocatalytic activity. Under the conditions of UV light with the addition of 2 mL H 2 O 2 , the degradation efficiency of methyl orange reached 100% after 30 min, and it was completely degraded after 60 min. Under the conditions of sunlight and 2 mL of H 2 O 2 , the degradation efficiency of methyl orange reached 98.9% after 120 min, and the degradation efficiency reached 99.85% after 180 min. e degradation efficiency of methyl orange reached 94.67% after 240 min in pure solar illumination. After 300 min of degradation time, the efficiency reached 99.48%. After five reaction cycles, the degradation rate of methyl orange was still more than 90% at 60 min, showing a stable photocatalytic activity. erefore, NG/TiO 2 composites are a new type of photocatalytic material that is ideal for industrial applications.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.