TANG Hailong,WANG Min,JU Tangtong,DAI Yue,WANG Meiling,MA Yongqing,3,ZHENG Ganhong
(1.School of Materials Science and Engineering,Anhui University,Hefei 230039,China;2.School of Physics and Optoelectronic Engineering,Anhui University,Hefei 230039,China;3.Institute of Physical Science and Information Technology,Anhui University,Hefei 230039,China)
Abstract: TiO2/graphene composite was synthesized in the vapor environment of isopropanol.In order to improve the properties of composite,N-doped of TiO2/graphene with different N/Ti molar ratio was prepared in the vapor environment of deionized water and used urea as the source of nitrogen.The N-doped occupies in the interstitial sites of TiO2 lattice,substitutes for O element in TiO2 and for C element in graphene,and simultaneously changes the chemical states of Ti and O elements in TiO2.N-doped changes the morphology of TiO2 from nano-sheets to nanoparticles,accompanying with the decrease in specific surface area of the composites,first increases the particle size of TiO2 and then decreases,and alters the vibration modes of Ti-OTi.The composite with RN/Ti=2 exhibits the enhanced photocatalytic degradation performance to methylene blue,and the degradation rate increases from 7.7×10−2 min−1 for the undoped composite to 9.6×10−2 min−1.
Key words: TiO2/graphene;vapor-thermal;nitrogen-doped;photocatalytic performance
Anatase TiO2has been recognized as an important photocatalyst material due to its stable physical and chemical properties,excellent catalytic performance,non-toxic,low cost and rich abundance[1-6].Its conduction band is composed of the Ti 3d state,valence band maximum is mainly contributed by the O 2p states and band gap energy (Eg) is 3.2 eV.Therefore,the excitation of electron-hole pair requires the photon with energyhν≥ 3.2 eV or wavelengthλ≤ 387 nm,i e,the photo response of TiO2lies in the ultraviolet region,the energy in which is less than 5% of the total energy of solar spectrum[7].Hence,it is of great interest to increase the spectral response range of TiO2.However,the improvement of photocatalytic performance is determined by the synergistic effects of spectral response range,photon conversion efficiency,effective separation and rapid migration of photo-generated electron-hole pairs.The photo-generated electron-hole pairs will recombine and vanish away in 10 -100 ns,much shorter than the time for electrons to be captured and transferred (100 ns -1 ms)[8,9],so majority of carriers cannot migrate to the surface of the catalyst,where the photocatalytic reaction occurs,consequently reducing the photocatalytic reactivity.As a result,how to restrain the carrier recombination becomes the other key issue.So far,many efforts have been made to solve these problems,such as doped,building the heterojunction,modifying the surface with noble metals or quantum dots,controlling the exposed crystal surface,and combining with nano-carbonaceous materials[10-13].Among them,the strategy of combining TiO2with graphene has been paid the extensive attention.
Graphene is an atomic sheet of sp2-bonded carbon atoms that are arranged into a honeycomb structure,which possesses the high thermal conductivity (5000 W·m-1·K-1),rapid carrier mobility (200000 cm2·V-1·s-1),very large theoretical specific surface area (2630 m2·g-1),and the high transparency due to its one-atom thickness[14,15].The combination of graphene with TiO2is promising to improve photocatalytic performance in view of the following aspects.The adsorption to pollutant molecules can be enhanced.For an example,methylene blue molecules are prone to be adsorbed at the surface of grapheneviaπ-π conjugation.It can broaden the photo-response range,which is attributed to the Ti-O-C bond that causes the red shift of absorption edge.It can inhibit the recombination of photo-generated carriers,because the photo-generated electrons that transfer from TiO2to graphene can rapidly move away,reducing the probability of recombination between electrons and holes,and effectively promoting the separation of photo-generated electron-hole pairs in TiO2[16,17].
Recently,many efforts devoted to further improve the photocatalytic performance of TiO2/graphene composites,such as the noble metal modification and the doped with metal (including transition metal) or non-metallic element,etc[18-21].The transition metal doped in TiO2was reported to form a local d-state deep energy level,acting as the recombination center and hence reducing the photocatalytic activity[22].Many researches claimed that nitrogen (N)-doped can improve the visible-light driven photocatalytic activity of TiO2[23-25],and revealed that the chemical states of doped N element in TiO2are as follows: NOximpurities[26],Ti-N bond[27],Neutral NO radical and NO22−radical ion[28],N• center (single nitrogen atom center).In fact,there are still many problems worthy of further investigation: the mechanism of the visible-light response induced by N-doped has not been fully understood yet.Some researches attributed this to the narrowing of the band gap due to the hybridization of N 2p and O 2p orbitals.However,the theoretical calculation results showed that the N-doped does not narrow the band gap of TiO2,but introduces the local N 2p state near the valence band maximum.It is unclear whether the N-doped introduces Ti 3d defect state and oxygen vacancy.What differences arise between heavy doped and light doped?It was suggested that the band gap will narrow after the high concentration oxygen vacancies are replaced by N.However,other results showed that the band gap narrowing was not observed for the N heavily-doped TiO2.
So far,there have been many reports on N-doped in single-phase TiO2,but few reports on N-doped in TiO2/graphene nanocomposite.In addition,in previous reports,majority of TiO2/graphene nanocomposites were synthesized through the hydrolytic reaction of the Ti-containing source using water as the solvent,and the resulting TiO2/graphene composite consisted of granular TiO2randomly scattered on graphene.In this work,firstly,TiO2/graphene nanocomposites were synthesized using non-aqueous isopropanol as the solvent,and the reaction was performed in the vapor environment of isopropanol,expressed as the vapor thermal method hereafter.Compared with the traditional hydrolytic method,the vapor thermal method slowly provides the water (through the etherification reaction of isopropanol) needed for the hydrolysis of Ti source,and slows down the reaction speed,so that TiO2can nucleate and grow uniformly on graphene and realize the intimate contact between TiO2and graphene.The results of electron microscopy showed that TiO2exhibited the nano-sheet morphology.Then,using urea as the nitrogen source,N-doped of TiO2/graphene was performed in the vapor environment of deionized water.Imaginably,it is easier to achieve the uniform N-doped in such the TiO2nano-sheet,compared with the bulk TiO2.The doped N exists in forms of different chemical states;simultaneously,the chemical states of Ti and O elements in TiO2are changed,which depend on the degree of N doped.In addition,N doped also affects the morphology and specific surface area of the composites.
2.1 Synthesis of TiO2/ graphene
2 mg of graphene (r-GO,>98%,Aladdin) and 18 mL isopropyl alcohol (IPA,99.7%,SCR) were mixed under continuous ultrasonic vibration for 10 min to make graphene fully dispersed in IPA.Next,0.03 mL diethylenetriamine (DETA,99%,Aladdin) was added under continuous ultrasonic vibration for 10 min.Finally,2 mL titanium (IV) tetraisopropanolate (TIP,95%,Aladdin) was added.After ultrasonic vibration for 20 min,the mixture was transferred to a teflon internal kettle.The teflon outer kettle was filled with 30 mL deionized water (DI) or IPA,as shown in Scheme 1(a).The reaction solution kept at 200 ℃ for 24 h.The precipitation was washed repeatedly with absolute ethanol to remove the organic residual,and dried at 60 ℃.The sample was named as N0.
2.2 Nitrogen-doped
300 mg TiO2/graphene composites were mixed to 100 mL deionized water (DI) under continuous ultrasonic vibration for 10 min.Urea was added to the mixture according to N: Ti molar ratioRN/Ti=0.5,1,2,3,4,and 5,respectively.The mixture was placed in a quartz cup after ultrasonic treatment for 20 min.Pour 150 mL deionized water among the Hastelloy kettle,reaction temperature and reaction time were 180 ℃ and 12 hours,respectively.The sample need deionized water cleaning 3-4 times,then 60 ℃ drying.According to the molar ratio,the obtained samples were named N0.5,N1,N2,N3,N4,and N5,respectively.The reaction device is shown in Scheme 1(b).
Scheme 1 The schematic plot of the reaction vessel for the vaporthermal synthesis: (a) TiO2/graphene composite;(b) N doped
2.3 Characterization
The crystal structure of the samples was investigated by X-ray diffraction (XRD) using an X-ray diffractometer (XRD,Rigaku Industrial Corporation,Osaka,Japan) with Cu Kα radiation (λ=1.5406 Å)(operated at 40 kV and 100 mA).Scanning electron microscopy (SEM;S-4800,Hitachi,Tokyo,Japan)and transmission electron microscopy (TEM;JEM-2100,JEOL,Tokyo,Japan) were used to observe the morphology.The ultraviolet visible diffuse reflectancespectra (UV-Vis DRS) of samples were tested on a Shimadazu U-4100 spectrometer (U-4100,Shimadazu Corporation,Tokyo,Japan).X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ESCALAB 250Xi (Thermo Scientific Inc.,USA).Chemical bonds of the photocatalysts were tested by the Fourier transform infrared (FT-IR) spectra (Vertex 80/Hyperion2000,Bruker,Germany).The Brunauer-Emmett-Teller (BET) specific surface areas were calculated based on N2adsorption-desorption isotherms measured at 77 K using the gas adsorption apparatus(Autosorb-iQ,Quantachrome Instruments,USA).The pore size distribution was calculated by the BJH method.Electron spin resonance (ESR) signals of reactive species spin trapped by 5,5-dimethyl-L-pyrroline-N-oxide (DMPO) were determined on a Bruker EMX plus 10/12 (equipped with Oxford ESR910 Liquid Helium cryostat).2.5 mg of photocatalysts were dispersed in 1 μL DMPO/methyl alcohol solution and DMPO/H2O solution for detection of superoxide radicals (•O2-) and hydroxyl radicals (•OH),respectively.
The photocatalytic activity was realized by the degradation of methylene blue,which was 20 cm away from xenon lamp (300 W,16 A).The experimental process is as follows: 50 mg catalyst was added into 100 mL methylene blue solution with the concentration of 10 mg/L,and the samples were kept in the dark room for 30 min to achieve the adsorption-desorption equilibrium.After the light irradiation,the solution was extracted every 10 min.After the high-speed centrifugation,the concentration of methylene blue was analyzed by an UV-Vis spectrometer (UV-3200S,MAPADA,Shanghai,China) and calculated by a calibration curve.
3.1 Morphology and structure
Fig.1 shows the TEM images of N0 (a) and N2(c) and the SEM images of N0 (b) and N2 (d).For the TiO2/graphene composites (N0) synthesized in isopropanol vapor,the bare graphene and granular TiO2,most common morphology in previous reports[29-31],could hardly be observed from the TEM and SEM images.Instead,TiO2exhibits the nano-sheet morphology and TiO2nano-sheets uniformly grow at the surface of graphene,as shown in Figs.1 (a,b).
Fig.1 TEM images of N0 (a) and N2 (c) and SEM images of N0 (b)and N2 (d)
N-doped of TiO2/graphene was carried out through hydrolysis of urea in the water vapor,and the TEM and SEM of N2 sample are shown in Figs.1(c,d).It is observed that the morphology of TiO2was in the form of particles.This indicates that in the process of N-doped,the morphology of TiO2changes from nano-sheet to nano-particles.
The XRD result of pure graphene sample (N0) is shown in the inset of Fig.2.There is a broad diffraction peak at 2θ=25.2°,namely the (002) diffraction peak of reduced graphene[32-34].The N0 sample crystallizes to the single-phase anatase structure,and (101) diffraction peak at 2θ=25.2° was significantly broadened compared with that of the N-doped sample because,on the one hand,the reduced graphene has a broad peak at 2θ=25.2°;on the other hand,TiO2nano-sheet has a very small grain size.The grain size of N0,calculated by the Scherrer formula,is about 8.6 nm.All N-doped samples N0.5-N5 are anatase TiO2phase.Compared with the N0 sample,the diffraction peak intensity of (101)faces of N0.5-N5 was significantly increased,indicative of the better crystallization of TiO2in the process of N doped,and the crystallite size of TiO2changes between 9.4 and 12.3 nm,as shown in Fig.3.With the increase of N/Ti mole ratio,the size of TiO2increases first and then decreases.
Fig.2 XRD patterns of the purchased graphene and N0 (inset),and the N-doped samples N0.5,N1,N2,N3,N4,and N5 (main panel)
Fig.3 The relationship between crystallite size (nm) and the molar ratio of N/Ti
The FT-IR spectra of N0-N5 samples are shown in Fig.4.The vibration modes at 1623 and 3368 cm−1of all sample are attributable to the O-H vibration of adsorbed water and hydroxyls[35].Microscopically,the adsorption of TiO2to water results from the interaction between oxygen vacancy at the surface of TiO2particles and water molecules[36].Therefore,the ratio of oxygen vacancy may determine the strength of O-H vibration mode,which is consistent with the XPS results in Table 1.Some weak absorption peaks can be attributable to the C=O vibration (1791cm−1) of carboxyl COOH,tertiary C-OH stretching vibration (1395 cm−1),and alkoxy C-OH stretching vibration (1050 cm−1)[37].In addition,the vibration modes at 2915 and 2848 cm−1are attributed to CH2anti-symmetric stretching vibration,υas(CH2) and CH2symmetric stretching vibration,υs(CH2),respectively[38].
Fig.4 The FT-IR spectra of N0-N5
The strong absorption band in the range of 400-700 cm−1results from the Ti-O or Ti-O-Ti vibration.The N0 sample presents two peaks at 486.4 and 676.2 cm−1,respectively.For N-doped samples,the position and relative strength of Ti-O-Ti vibration peaks change with the increase ofRN/Ti,which indicates that N-doped cause the distortion of the lattice of TiO2[39,40].Furthermore,all N-doped samples present weak absorption peaks between 880 and 1050 cm−1,resulting from the tensile vibration mode of Ti-N bond due to N-doped[41,42],i e,N atom replaces O atom in the lattice of TiO2or N atom occupies the oxygen vacancy at the surface of TiO2particles.
3.2 Analysis on chemical state of element
In order to investigate the variation of the element chemical state (CS) with N-doped,the core energy level XPS spectra of C 1s,N 1s,O 1s,and Ti 2p were measured on N0,N2,and N5,as shown in Fig.5.The chemical state of each element and its corresponding binding energy (BE) and the ratio are listed in Table 1.
Fig.5 Core level XPS spectra of C1s (a,b,c),N1s (d,e,f),O1s (g,h,i) and Ti2p (j,k,l) for N0,N2 and N5.The red solid line is the experimental curve,the black dashed line is the fitted curve and the blue open circles are the sum of the fitted curves
Table 1 The chemical state (CS) of C,N,O and Ti elements and its correspoding binding energy (BE) and the ratio for N0,N2,and N5 samples
The XPS results of C 1s are shown in Figs.5 (a,b,c).The chemical states of C element are as follows:(1) C=C with the binding energy being 284.8 eV,corresponding to the C atom in graphene[23].(2) C-N with the binding energy being 286.1 -286.4 eV due to the N atom replacing the C atom in graphene or adsorbed on graphene[23,43,44].(3) C=O with the binding energy being 288.3 -288.6 eV,due to the combination of C atom in graphene and O atom in TiO2,indicating that TiO2contacts with graphene intimately[45].
The XPS results of N 1s are shown in Figs.5 (d,e,f).The chemical states of N element are as follows:
(1) substitutional nitrogen,i e,the N atom that replaces O atom of TiO2.Its binding energy is 397.8 eV.(2)interstitial nitrogen,i e,the N atom existed in the interstitial sites of TiO2lattice.Its binding energy is 399.6 eV.(3) graphitic nitrogen,i e,the N atom replacing C of graphene or adsorbed on graphene.Its binding energy is 401.3 eV.For undoped sample N0,there is interstitial nitrogen and graphitic nitrogen due to DETA added in the precursor solution during the synthesis of TiO2/graphene composite.The substitutional nitrogen was observed in N2 and N5.The ratio of substitutional nitrogen in the N5 sample (10.3%) is much lower than that in the N2 sample (22.7%),although the N/Ti molar ratio in the precursor solution was the highest during the synthesis of the N5 sample,because more N atoms in the N5 sample are combined with graphene[23,46-48].
The XPS results of O1s are shown in Figs.5 (g,h,i).The chemical states of O elements include Ti-O-Ti,C=O and oxygen vacancy (Ov),and the corresponding binding energies are 529.8-530.1,530.6-531.1,and 532.3-532.8 eV,respectively[49].As can be seen from Table 1,with the increase ofRN/Ti,the ratio of Ovdecreases from 12.0 % (N0) to 8.9 % (N5),possibly because N atoms occupy the oxygen vacancy at the surface of TiO2particles[50,51].This result is consistent with the appearance of Ti-N bond in the FT-IR spectrum.Furthermore,the decreases in the ratio ofOvwill reduce the capacity to adsorb water[36],weakening the O-H vibration,as shown in Fig.4.
Table 2 Textural parameters of all samples
The XPS results of Ti 2p are shown in Figs.5 (j,k,l).The chemical states of Ti include TiO2and TiOx(x<2),and the binding energy is 458.5 -458.8 eV and 460.2 eV,respectively.Generally,the more the oxygen vacancy (Ov),the more the TiOxratio,due to the valence equilibrium.However,with the increase ofRN/Ti,theOvratio decreases while the TiOxratio increases.The possible reasons are as follows: after the oxygen vacancy at the surface of TiO2particles is occupied by N atom,the N atom adsorbs O atom to form the NOximpurity[52].The electronegativity of oxygen atom (3.44)is greater than that of nitrogen atom (3.04),resulting in the increase of TiOxratio.
It was reported that the visible-light response for the N-doped TiO2can be attributed to the shift of valence band maximum towards the forbidden band.To test this,we measured the valence band XPS spectra of N0,N2,and N5,as shown in Fig.6.The valence band maximum of three samples hardly changes,locating around 2.0 eV.However,the intensity of valence band density of state (VBDOS) slightly increases after N-doped,maybe due to the decrease in theOvratio,because the valence band of TiO2is mainly contributed by the O 2p orbitals.
Fig.6 Valence band density of states for N0,N2,and N5
3.3 UV-Vis analysis
Fig.7 shows the UV-Vis DRS of N0 -N5.For all samples,the absorption edge near 380 nm originates from the electronic transition from the valence band to the conduction band of TiO2.And no obvious red shift of the absorption edge was observed,consistent with the result of VBDOS in Fig.6.Apart from the N2 and N5 samples,the absorption spectra of N0,N0.5,N1,N3,and N4 samples showed the “tail-like” absorption ranging from 387 nm to the visible-light region.The absorption in the visible-light region cannot be attributed to graphene,because the graphene mass is the same in all samples,instead,it may be the synergistic effects of impurities or defects in TiO2,such as O-H,Ti-N,Ov,TiOx,interstitial nitrogen and nitrogen adsorbed on the surface.
Fig.7 The UV-Vis DRS of N0-N5
3.4 BET analysis
The BET surface areas (SBET) of all samples were investigated by nitrogen adsorption-desorption isotherms.Fig.8 shows the results of N0 and N2,representatively.TheSBETand the average pore diameter(Dp) of all samples are listed in Table 2.TheDpof N0 is about 5.8 nm,while theDpof N0.5-N5 varies in 3.0-3.5 nm after N-doped.TheSBETvalue of N0 sample is the largest,reaching 287.8 m2·g−1.N-doped causes the decrease ofSBET.TheSBETof N0.5-N5 changes in the range of 121.6-149.7 m2·g−1,approximate to the previously reported value[42,47,52].The decrease ofSBETdue to N-doped maybe results from the variation of the morphology of TiO2from nano-sheet for the sample without N-doped to nano-particle after N-doped.
Table 3 The values of adsorption efficiency (Ae),degradation efficiency (De) and apparent reaction rate constant Kapp of all samples
Fig.8 Nitrogen adsorption-desorption isotherms for N0 and N2 samples.The insets show the pore size distribution calculated by using the BJH method
3.5 Photocatalytic degradation
In order to study the influence of N-doped on the photocatalytic degradation ability,photocatalytic degradation experiments were carried out under the radiation of the xenon lamp,using methylene blue (MB)as the target pollutant.The results are shown in Fig.9.Negative time denotes the adsorption process to MB in the dark room,and positive one denotes the photo-degradation process of MB under the light irradiation.The concentration of MB att=30 min is referred to as the initial concentrationC0.After 30 minutes adsorption,the concentration decreases toCeatt=0.The adsorption efficiency (Ae),calculated by (C0−Ce)/C0[53],is listed in Table 3.The degradation efficiency (De) is calculated by (Ce−C)/Ce,whereCis the concentration at any irradiation timet.TheDevalues att=60 min are also listed in Table 3.The degradation process can be fitted using the pseudo first-order kinetic model ln[Ce/C]=Kappt,whereKappis the apparent reaction rate constant.The absorption coefficientAeof N0 without N-doped is the highest,reaching 85.9 %,mainly due to its highest specific surface area.The adsorption performance of N-doped samples significantly decreases,and the adsorption coefficient (Ae) changes between 2.5% and 30.5%.This is because the nano flake of TiO2becomes particle after N-doped,and the specific surface area of the sample decreases.When N/Ti molar ratio is 2,the photocatalytic degradation performance of MB is enhanced,and the degradation efficiency (De) increases from 7.7×10−2(N0) to 9.6×10−2min−1(N2).
Fig.9 Variation of MB concentration (C/C0) with time in the presence of N0 -N5 under the xenon lamp
3.6 ESR analysis
There are no significant superoxide radical (•O2−)signal and hydroxyl radical •OH signals in the ESR spectra of the three samples before illumination.After the light irradiation,both the typical four-line spectra of •OH with relative intensities of 1:2:2:1 and sixline spectra of •O2−appear in the ESR spectrum.Fig.10 shows the height of the •OH signal peak at the magnetic field 3338 Oeand the height of the •O2−signal peak at the magnetic field 3337 Oe.The •OH signal of the N2 sample is the strongest.At the same time,the •OH signals of the three samples are all stronger than the•O2−signal,indicating that •OH plays a more important role in the photocatalytic degradation than •O2−does.
Fig.10 ESR spectra of radical adducts trapped by DMPO in N0 (a,b),N2 (c,d) and N5 (e,f) dispersions: DMPO-•OH formed in irradiated aqueous dispersions (a,c,e),DMPO-•O2- formed in irradiated methanol dispersion (b,d,f)
Therefore,the possible reason for the better photocatalytic degradation performance of the N2 sample is that photo-generated electrons are captured by defects,which reduces the probability of recombination between photo-generated holes and electrons,enabling more holes to reach the particle surface and to react with water to generate hydroxyl radicals •OH,i e,h++H2O→•OH+H+[53].The pollutant molecules can be degraded by •OH into non-toxic CO2and water.
In summary,N-doped strongly influences on the morphology,the crystallinity,the specific surface area,the optical absorption of TiO2/graphene and the chemical states of Ti and O,consequently affecting the photocatalytic degradation performance.The photocatalytic degradation performance is enhanced asRN/Ti=2 compared with the sample without N doped.The results of ESR analysis shows that photo-generated electrons are captured by defects and impurities,and photo-generated holes play a major role in the photocatalytic degradation.