I 2-Hydrosol-Seeded Growth of (I 2)n -C-Codoped Meso/Nanoporous TiO 2for Visible Light-Driven Photocatalysis
Pu Xu,†Jun Lu,‡Tao Xu,*,§Shanmin Gao,*,†,|Baibiao Huang,|and Ying Dai |
School of Chemistry and Materials Science,Ludong Uni V ersity,Yantai 264025,Shandong,P.R.China,Department of Physics and Department of Chemistry and Biochemistry,Northern Illinois Uni V ersity,DeKalb,Illinois 60115,and State Key Laboratories of Crystal Materials,Shandong Uni V ersity,Jinan,250100,P.R.China
Recei V ed:February 23,2010;Re V id Manuscript Recei V ed:April 18,2010
We report a template-free process to fabricate (I 2)n -C-codoped meso/nanoporous TiO 2nanocrystallites.I 2nanoparticles in I 2-hydrosol were ud as eds to initial the nucleation of a precursory TiO 2shell formed by the hydrolysis of organotitanium molecules.The hybridized jumbles were further calcinated at different temperatures.The resulting (I 2)n -C-codoped meso/nanoporous TiO 2structure was confirmed by transmission electron microscopy (TEM),X-ray diffraction (XRD),Brunauer -Emmett -Teller (BET)isotherm adsorption of N 2.UV -visible diffu reflectance spectroscopy (DRS)showed that (I 2)n -C-codoped porous TiO 2samples have a strong absorption b
etween 400-700nm.The formation mechanism of (I 2)n -C-codoped meso/nanoporous TiO 2spheres is discusd.Under visible light radiation,our samples exhibit superior ability in the photocatalytic degradation of methylene blue to that of the only C-doped TiO 2or to the commercial P25TiO 2nanoparticles.The origin of the enhanced visible light -light driven photocatalytic degradation is attributed to the continuous states in the band gap of TiO 2introduced by I 2doping.Factors that contribute to the enhanced photocatalytic reaction are also studied,including the enhanced thermostability and conductivity of anata TiO 2due to C doping,the location of I 2doping,as well as the morphology of the TiO 2structures.
1.Introduction
Semiconductor-catalyzed photo-oxidation using solar light as energy source is one of the efficient process for rapid and low-cost degradation of organic pollutants.Among various miconducting materials reported so far as photocatalysts,TiO 2has attracted much attention for environmental purification due to its low cost,high photostability and environmental benign properties.1-4Generally,in the TiO 2-catalyzed photo-oxidation process,the photoexcited electrons in the conduction band (CB)of TiO 2(-4.2eV vs vacuum)can be readily captured by O 2to form superoxide anions (O 2-)as well as other oxygen species,whereas the holes in the valence band (VB,-7.4eV vs v
acuum)reside below the highest occupied molecular orbitals (HOMO)of most organic pollutes,allowing aggressive oxidizing (ripping electrons off)the organic pollutes.1-4
However,a major drawback that impedes the practical applications of pristine TiO 2-bad photocatalytic oxidation is the fact that the large band gap of TiO 2(∼3.2eV)only partially overlaps with the solar spectrum in the UVA (400-320nm)and UVB (320-290nm)regions.As a conquence,only UV light (<6%of the total solar energy)can be utilized to generate electron -hole pairs and to initiate the photocatalytic oxidation process.5-11Besides,the photogenerated electron -hole pairs can either be trapped or recombine at the defect states inside the bulk of TiO 2structure before reaching the external surface.12The unwanted charge pathways further diminish the efficiency of the TiO 2-bad photocatalytic oxidation.13In the regards,
rearch efforts that can produce TiO 2structures posssing the following properties simultaneously are exceptionally warranted,including (i)lower band gap or introduction of color center that can result in visible light-driven electron -hole pairs;(ii)good electrical conductivity for rapid transport of photoinduced charge carriers to the external sites (iii)high surface area,thus more external sites for trapping charge carriers and adsorbing reactant species;and (iv)high quality of anata crystalline TiO 2to ensure a low density of charge trap states inside the bulk of TiO 2.
An effective way to tackle the challenges is to nsitize TiO 2with nonmetal elements such as N,14F,15S,16C,17,18and B,10,19which act as an electron donor or acceptor in the band gap of TiO 2,thus to shift or extend the band edge of TiO 2toward visible-light region.11,20,21Another notable dopant element is iodine,which also create an intermediate band in the forbidden zone of TiO 2band gap.This doped band can either accept photoelectrons from VB of TiO 2or electrons in this band can be photoexcited to the CB of TiO 2.22-24Nearly all doping methods require heat treatment of the TiO 2in the prence of dopant precursors.However,the heat treatment often exhibits an antithetic effect.On one hand,heat treatment helps improve the crystallinity of the TiO 2.On the other hand,heat treatment can cau pha transition of TiO 2from photoactive anata to less active rutile and excessive heat treatment can lead to unwanted sintering of the TiO 2nanocrystallines.25Carbon doping,as a remedy to mitigate this dilemma,can significantly stabilize the anata structure of TiO 2under high temperature,and thus inhibits the sintering of TiO 2nanocrystallines.26Amorphous carbon also acts as a strong adsorbent for organic pollute molecules.27In addition,the conductivity of TiO 2can significantly be enhanced through carbon doping and modifica-tion.28This attractive feature brought by carbon doping allows
*To whom correspondence should be addresd.E-mail:txu@niu.edu (T.X.);gaosm@ustc.edu (S.G.).†
Ludong University.‡
Department of Physics,Northern Illinois University.§
Department of Chemistry and Biochemistry,Northern Illinois University.|
Shandong University.
J.Phys.Chem.C 2010,114,9510–9517
951010.1021/jp101634s 2010American Chemical Society
Published on Web 05/05/2010
prompt transport of charge carriers to the external surface sites, where the desired photocatalytic reactions occur.
In terms of the morphology of the TiO2structures,mesopo-rous TiO2nanocrystals with a large specific surface area provide more active sites for adsorbing pollute molecules.29,30Mean-while,the light absorption efficiency can be enhanced in the porous structure due to multiple scattering.31
Bad on the above state-of-the-art design strategies for TiO2-bad photocatalysts,in this work,we aim to synthesize high surface area(I2)n-C-codoped anata TiO2.The I2doping is confined preferentially in the surface/near surface region of the TiO2structure in order to accommodate carriers in the surface region.The C doping is suppod to be throughout the TiO2 matrix in order to enhance the anata pha as well as its conductivity.It would be difficult to achieve all of the goals using conventional doping methods that are either through dissolving I2in organotitanium precursory solvents,which inevitably leads to ubiquitous doping of I2in the bulk of TiO2,22 or through sublimation of I2in TiO2powders that suffers from inhomogeneous doping on all TiO2particles due to the limited diffusion pathways of I2in the matrix of TiO2.22
Instead of adding I2or its precursors into the matrix of TiO2 as in the ca of conventional I2doping methods,we report herein a unique counter strategy,in which the TiO2precursor is added into the I2source to overcome the above problems.In this process,the I2nanoparticles in the I2-hydrosol rve as eds for nucleation of small carbon-containing TiO2particles formed from the hydrolysis of the organotitanium precursors.This process provides veral advantages over the conventional doping methods:(1)The homogeneity of I2doping due to the nanoscopic mixing of TiO2and I2nanoparticles prior to calcination;(2)more importantly,as the I2nanoparticles are surrounded by TiO
2nanoparticles,the diffusion of I2will have to start from the surface region of TiO2nanoparticles toward their bulk;hence,we can confine the I2doping preferential in the surface(or near surface)region of TiO2;(3)the creation of gaous species during calcination process in air synergistically facilitates the formation of porous TiO2structure.This template-free soft synthetic strategy allows the doping and pore formation to be accomplished simultaneously during calcination,while the amount and position of doping elements can be readily controlled under mild conditions.
Resulting from the deliberate synthetic designing strategies, the prepared anata meso/nanoporous TiO2exhibited extremely high temperature stability and significantly enhanced photo-catalytic degradation of methylene blue in comparison to commercial P25photocatalyst and to non-I doped TiO2under visible light irradiation.Meanwhile,we explore the insightful understanding in the factors that influence the photocatalytic activity of TiO2,including specific surface area(SSA),pore structure and size distribution,morphology of as-prepared nanoparticle,etc.This effort,in turn,will provide more basic science for designing better TiO2-bad photocatalysts.
2.Experimental Procedures
毛氏红烧肉
2.1.Sample Preparation.All chemicals ud in this study were reagent grade from Tianjin Ruijinte Chemical Reagent Co. without any further purification.A typical procedure for preparing the I2-C-codoped bimodal meso/nanoporous TiO2is described as following.First,0.500g of iodine crystals were dissolved in20mL absolute ethanol and slowly added drop-wi into120mL of distilled water at60°C with continuous stirring.The I2hydrosol was formed after stirring for0.5h and then cooled to room temperature.A total of10mL of tetrabutyl ortho-titanate(TBOT)was added at a rate of less than30drops/ min to the I2hydrosol,which leads to a brownish precipitate of floccules immediately.The color of the precipitate turns to bright yellow as the amount of TBOT incread.The resulting suspension was continuously stirred for an additional2h at room temperature and allowed to stand overnight.Finally,the precipitate wasfiltered out,washed by deionized water and absolute ethanol multiple times,and dried in an oven at80°C for6h.In order to obtain mesoporous(I2)n-C-codoped TiO2, the as-obtained precipitate needs to be further thermally treated at elevated temperatures.The samples after heat treatment were denoted as PIT-T,where the cond T refers to temperature ud for the heat treatment.For comparison,only C-doped mesoporous TiO2was also prepared in a similar way at400°C but without I2hydrosol(denoted as T-400).In addition, commercial P25(∼80%anata and∼20%rutile,Deguass)was ud as a reference.
2.2.Characterization.The phas of thefinal products were identified using X-ray diffractometer(XRD)(Rigaku D/max-2500VPC)with Ni-filtered Cu K R radiation at a scanning rate of0.02°s-1from20°to70°.
The morphology of the samples was obrved with a Hitachi model H800transmission electron microscope(TEM)using an accelerating voltage of150kV.
Ultraviolet-visible(UV-vis)diffu reflection spectra(DRS) were recorded on a Shimadzu UV-2550UV-vis spectropho-tometer in the range200-800nm at room temperature. Porous structure and BET surface area were characterized by an N2adsorption-desorption isotherm(ASAP-2020Mi-cromeritics Co.,USA).The samples were degasd at180°C prior to BET measurements.The pore volume and pore diameter distribution were derived from desorption branches of the iso-therms by the Barrett-Joyner-Halenda(BJH)model.The BET surface area was calculated from the linear part of the Brunauer-Emmett-Teller(BET)plot.
The X-ray photoelectron spectra(XPS)measurements were carried out on an X-ray photoelectron spectrometer(ESCALAB MK II)using Mg K R(1253.6eV)X-rays as the excitation source,with C1s(284.6eV)as the reference.Fourier transform infrared(FT-IR)spectra were recorded on a Nicolet Nexus670 spectrophotometer.
The conductivities of the PIT-T materials were measured on the pellets of the materials using four-probe method.Pellets(10 mm in diameter,0.5mm in thickness)of the powder materials were prepared using a press(pressure)30MP).A constant current of0.1mA was supplied through thefirst and fourth probes,while the voltage is measured between the cond and third probe.Current and voltage were supplied and measured by a HP3458A Multimeter.
2.3.Photocatalytic Activity Measurements.The photo-catalytic activity of the as-prepared(I2)n-C-codoped meso/ nanoporous TiO2was evaluated by measuring the decomposition rate of methylene blue at room temperature.For comparison, the same measurements were also performed on the C-doped meso/nanoporous TiO2(T-400)and commercial P25.The temperature of the photocatalytic reaction was maintained at 30°C.The visible light was obtained from a300W tungsten arc lamp(Zhejiang Electric Co.,Ltd.)with a glassfilter (transparent forλ>400nm).For a typical photocatalytic experiment,a total of200mg of the PIT-T sample was added to200mL of methylene blue aqueous solution(5.0×10-4mol/ L)in a custom-made quartz reactor.The methylene blue concentration was monitored by UV-vis spectroscopy during the entire experiment.Prior to irradiation,each suspension was
(I2)n-C-Codoped Meso/Nanoporous TiO2J.Phys.Chem.C,Vol.114,No.20,20109511
magnetically stirred in the dark for 60min to ensure an adsorption -desorption equilibrium between methylene blue and photocatalysts,which was then irradiated by visible light.After 5or 10min intervals during visible light illumination,about 3mL aliquots were taken out and centrifuged to remove the trace particles.The absorbance of the centrifuged solution was measured in the range 500-800nm using a UV -vis spectro-photometer (Shimadzu UV-2550).
小刺猬背西瓜3.Results and Discussion
3.1.Sample Structure.3.1.1.Particle Morphology Studied by Transmission Electron Microscopy (TEM).(I 2)n -C-codoped TiO 2was prepared via the hydrolysis of TBOT in I 2hydrosol without any addition of templates or surfactants,followed by a heat treatment in air at elevated temperatures for 3h.Figure 1shows the TEM images of I 2hydrosol and the (I 2)n -C-codoped TiO 2samples before and after heat treatment at different temperatures.The average particle size of I 2in the hydrosol is estimated to be 5.4nm with a wide particle size distribution (Figure 1a).After TBOT was hydrolyzed in I 2hydrosol,the newly formed particles showed an average size of 17nm,as prented in Figure 1b.No porous structure was obrved at this point.It is the subquent heat treatment that creates the mesoporous structures,as shown in Figure 1c -e,which clearly show that each individual spherical particle (condary particle)consists of a large number of much smaller nanoparticles (prim
ary particles).The four samples including PIT-200,PIT-400,PIT-600,and PIT-800were characterized by TEM as shown in Figure 1c -f,respectively.The apparent common feature of the four samples is that they all consist of pudo spherical TiO 2particles with diameter approximately 50nm.
Furthermore,each individual sphere in PIT-200,PIT-400,and PIT-600is compod of a large number of much smaller and looly packed TiO 2nanoparticles (a few nanometers in diameter),termed as primary nanoparticles.As a result,the interstitial voids among the primary nanoparticles constitute a short-range disordered nanoporous structure,which is termed as primary nanopores.In comparison,the voids among the large spheres (termed as condary mesoparticles)also create pores,termed as condary mesopores.The morphology of the primary pores gradually changes as the size of the primary nanoparticles increas with the increasing temperature of heat treatment.Eventually,the primary pores collap as the primary nanopar-ticles fud together at 800°C (sample PIT-800),whereas the condary pores remains as shown in Figure 2f.
3.1.2.Crystal Structure Studied by X-ray Diffraction (XRD).Powder X-ray diffraction (PXRD)is ud to investigate the changes of structure and crystallite sizes of the as-prepared (I 2)n -C-codoped TiO 2at different stages of the heat treatments.Figure 2a shows the wide-angle XRD patterns of the samples before and after heat treatment at various temperatures from 200to 800°C.The sample befo
re thermal treatment appears to be amorphous for TiO 2pha,which is probably due to the fact that hydrolysis of TBOT is relatively incomplete at room temperature,and large amounts of unhydrolyzed alkyls remain in the xerogel powders.As a result,the adsorbing of unhydro-lyzed alkyls onto the surface of the as-formed TiO 2particles could prevent further crystallization of TiO 2.32The three low diffraction peaks should be the iodine (JCPDS No.71-1370).Upon heat treatment,the wide-angle XRD results of all PIT-T samples showed an anata TiO 2pha,indicating that the crystallization of the samples is achieved.At low temperature,except for tho of the dominant anata TiO 2,another XRD peak (2θ)30.9°)with low intensity is detected for PIT-200sample,which can be abscribed to (121)diffraction plane of brookite TiO 2pha.It worth to note that no rutile pha is detected even for sample calcined at 600°C (PIT-600),indicating an excellent thermal stability of our samples against pha transformation,which can be ascribed to the carbon doping.28For sample PIT-800which is calcined at 800°C,the predominant pha is still anata TiO 2,which is accompanied by a small portion of rutile pha.Since the heat treatment is a necessary step for doping and for the formation of bimodal meso/nanoporous structure,the high thermal stability of our samples is a favorable feature for maintaining anata (photo-active pha)under high temperature treatment.27In addition,as the calcination temperature increas,the peaks assigned
to
Figure 1.TEM images of I 2hydrosol (a),the precursor of I 2-C-codoped TiO 2(b),and I 2-C-codoped TiO 2after thermal treatment at 200(c),400(d),600(e),and 800°C
(f).
Figure 2.Wide-angle (a)and small-angle (b)XRD patterns of I 2-C-codoped meso/nanoporous TiO 2after thermal treatment at 200,400,600,and 800°C (A:anata;R:rutile;B:brookite).
9512J.Phys.Chem.C,Vol.114,No.20,2010Xu et al.
anata become sharper and more inten due to the formation of larger grains as summarized in Table 1.The average crystallite sizes of(I2)n-C-codoped TiO2are4.3,6.1,7.8,and 15.2nm for PIT-200,PIT-400,PIT-600,and PIT-800,respec-tively,which are estimated from the full width at half-maximum of the diffraction peak using the Scherrer equation.This trend is also in consistence with our TEM obrvation.
Figure2b shows the small-angle XRD patterns of the same
samples.The strong diffraction peak at about1°for samples
PIT-200,PIT-400,and PIT-600indicates the prence of a
typical short-range disordered microstructure framework,which
is not obrved in sample PIT-800.The peaks in small-angle
养育的近义词range normally suggest the existence of a large lattice plane
distance related to the porous structure.This fact demonstrates
that our mesoporous TiO2posss relatively high stability
against thermal collap.Further elevation of the temperature
leads to the collap of the porous structure,which is evidenced
女职工保护法by the disappearance of diffraction peaks at small-angle range
for sample PIT-800.The collap of porous structure in sample
PIT-800is in agreement with the TEM study(Figure1f),
showing the diminished porosity inside the particles.
3.1.3.Specific Surface Area(SSA)and Pore Size Distribu-tion Measured by BET.The pore size and specific surface area of the samples are further characterized by nitrogen adsorption-
desorption isotherm measurements.Figure3a prents the
nitrogen adsorption-desorption isotherms.The results indicate
a hierarchically bimodal pore-size distribution for samples PIT-
200,PIT-400,and PIT-600.33Thefirst hysteresis loops in Figure
3a for samples PIT-200,PIT-400,and PIT-600located at relative
布尔运算low pressure are associated with the framework-confined
primary nanopores consisting of the interstitial voids among the
smaller nanoparticles.The shape and position of this loop are
slightly different,depending on the temperatures of heat
treatment.For samples PIT-200and PIT-600,the position of
thefirst loop is located at0.4<P/P0<0.6,while for sample PIT-400,thefirst loop is located at0.5<P/P0<0.8.The monolayer adsorption completes when the relative pressure
reaches0.6,0.8,and0.5for samples PIT-200,PIT-400,and
PIT-600,respectively,which indicates that sample PIT-400has
a greater primary pore size than that of samples PIT-200and
PIT-600.The cond hysteresis loop located at0.85<P/P0< 1.0is related to condary pores compod of the voids among large condary particles.It should be pointed out that for sample PIT-800,only one hysteresis loop at high relative pressure range (0.9-1.0)is obrved,which is associated with the adsorption-desorption in the condary pores consisting of the voids among large particles.34This is consistent with the small-angle XRD and TEM results.
The hierarchically bimodal pore-size distribution is further confirmed by its corresponding pore-size distribution,which is determined from the Barret-Joyner-Halenda(BJH)desorption isotherms as shown in Figure3b.The results suggested that samples PIT-200,PIT-400,and PIT-600showed a bimodal pore-size distribution consisting of smaller primary pores and larger condary pores.For the samples PIT-200,PIT-400,and PIT-600,the average pore diameters of primary pores are3.2,4.7, and3.3nm,and the average pore diameters for condary pores are42,46,and52nm,respectively.However,PIT-800sample only showed the prence of condary po
res with an average pore diameter of38nm,indicating the thermal collap of the primary pores,namely the fu of the primary nanoparticles at high temperature.Again,this result agrees with the small-angle XRD and TEM results.
Table1summarized the physical properties of the(I2)n-C-codoped meso/nanoporous TiO2samples obtained after heat treatment at various temperatures.The pore volume and specific surface area decrea with the increa of the heat treatment temperatures.The meso/nanoporous structures with high specific surface area are of particular interest,since they can provide more active sites to adsorb pollute molecules.Con-quently,this may enhance catalytic activity,as demonstrated later in the measurement of photocatalytic activity on degrada-tion of methylene blue.
3.1.
4.X-ray Photoelectron Spectra(XPS).XPS is a surface nsitive technique and is ud to confirm the prence and chemical states of I2,C,and Ti in our samples.Figure4a shows the XPS survey spectra of the samples calcined at400and600°C,indicating the prence of Ti,O,C,and I.The Ti2p high-
resolution XPS spectrum(Figure4b)shows two peaks at binding energies of458.1eV(Ti2p3/2)and463.9eV(Ti2p1/2).The Ti2p peak shows a slight deformation on the lower side
of the binding energy,corresponding to the different oxidation states of Ti,which can be wellfitted into two peaks of Ti4+and Ti3+.35 The existence of surface Ti3+can form a defect and act as a hole trap to promote charge transfer and thus suppress the recombination of electron-hole pairs.36
There are two peaks in the C1s high-resolution XPS spectrum (Figure4c)at the binding energies of285.7and289.5eV.The peak at285.5eV is an adventitious signal due to elemental
TABLE1:Physicochemical Properties of PIT Samples from N2Sorption Analysis and XRD Pattern a
sample
S BET
(m2/g)
pore volume
(cm3/g)
average pore size
(nm)
crystal size
(nm)
PIT-200226.80.388 5.486 4.3 PIT-400186.60.3438.586 6.1 PIT-600118.20.309 5.2497.8 PIT-80021.210.25436.26815.2
个人先进材料a BET surface areas were calculated by the multipoint BET method from the linear part of the BET plots.Single point absorption total pore volumes were obtained from the volume of N2 adsorbed at P/P0)0.995.Average pore diameters were estimated using the desorption branch of the isotherm and the BJH formula. Crystal size was determined
form the XRD pattern using the
Scherrer equation.
Figure3.N2adsorption-desorption isotherms(a)and the correspond-
ing BJH pore size distributions(b)of the as-prepared PIT samples. (I2)n-C-Codoped Meso/Nanoporous TiO2J.Phys.Chem.C,Vol.114,No.20,20109513
carbon from the carbon tape ud in the measurements,37whereas the peak at 289.5eV could be assigned to the formation of carbonate species which can induce the narrowing of the band gap of the doped titania.38It suggested that carbon may substitute for some of the lattice titanium atoms and form a Ti -O -C structure.39The carbon contained in a titanium alkoxide precur-sor could be incorporated into the lattice of TiO 2.
ponroThe XPS spectrum of the I 3d region (Figure 4d)shows doublet peaks at 620.5eV (I 3d3/2)and 630.5eV (I 3d5/2),which is equivalent to tho in molecular I 2.40,41In addition,with the increa of the temperature during heat treatment,the concentration of iodine molecules on the surface of the PIT samples decread due to sublimation of I 2at high temperature,which is also confirmed by the c
olor change of the samples during the heat treatment.The color of the samples changed from brown,to slight brown and fawn,and finally to white when the temperature incread from 200to 800°C,as shown is Figure 5.
3.1.5.Propod Formation Mechanism of I 2-C-Codoped Porous TiO 2.Different from the conventional doping method,in which the iodine (or its precursors)are added to the matrix
of TiO 2(or precursors of TiO 2),we adopt a counter strategy to add precursors of TiO 2into the matrix of I 2nanoparticles formed in the I 2hydrosol.A possible multistep mechanism,illustrated in Figure 6,is propod to interpret the formation of the (I 2)n -C-codoped meso/nanoporous TiO 2.First,when TBOT was added to I 2hydrosol at room temperature,the Ti(OBu)4molecules adsorbed onto the surface of I 2nanoparticles and slowly hydrolyzed to become a titania precursory layer on the I 2nanoparticles.The primary TiO 2particles formed as a shell on the surface of I 2particles.In other words,the I 2particles appears as nucleation sites for TiO 2spheres via the hydrolysis of TBOT.This is confirmed by the TEM images in Figure 1,panels a and b,showing that the particle size of I 2hydrosol increa upon the hydrolysis of Ti(OBu)4in I 2hydrosol due to the formation of precursory shell of (I 2)n -C-codoped TiO 2on the I 2particles.
At room temperature,the hydrolysis of Ti(OBu)4is relatively slow and incomplete.Therefore,a large amount of unhydrolyzed alkoxyls still remains in the xerogel powders.42The subquent heat treatment of the xerogel at elevated temperatures leads to a ries of reactions among I 2,alkyls,and amorphous TiO 2with the prence of O 2.We believe that the meso/nanoporous anata TiO 2doped with I 2and C is a product of the following synergetic process:(1)The I 2sublimes and a portion of the alkyls are oxidized into CO 2and H 2O.Conquently,the resulting gaous products pulverize the TiO 2shell,leading to the formation of the primary pores.(2)Part of the I 2along with some carbon pyrolyzed from the alkyls attaches to surface of the TiO 2primary particles.The further oxidation of I 2is more or less prevented by the shell of primary TiO 2nanoparticles and the remaining Ti-OR groups.(3)The amorphous TiO 2crystallizes into anata TiO 2(Figure 6)under heat treatment.(4)Finally,the aggregation among condary particles results in the condary pores consisting of the voids among the condary particles during the heat treatment.
3.2.Photophysical,Electrical,and Photocatalytic Oxida-tion Studies.3.2.1.UV -Vis Diffu Reflectance Spectra.Figure 7collects the UV -vis spectra of the prepared (I 2)n -C-codoped meso/nanoporous TiO 2samples (PIT-T),the undoped TiO 2sample (P25),as well as the only C-doped meso/nanoporous TiO 2(T-400).In comparison to the undoped P25and the only C-doped T-400sampl
es,the spectra of I 2-C-codoped TiO 2samples (PIT-200,PIT-400,PIT-600,and PIT-800)obtained at 200,400,and 600°C exhibit a strong
broad
Figure 4.XPS spectra of PIT calcined at 400and 600°C:survey spectrum (a)and high resolution XPS spectra for Ti 2p (b),C1s (c),and I 3d
(d).
Figure 5.Photos of (I 2)n -C-codoped meso/nanoporous TiO 2samples (PIT-T),the sample prior to heat treatment,and only C-doped meso/nanoporous TiO 2sample
家风作文600字
(T-400).
Figure 6.Schematic of the formation mechanisms for the bimodal (I 2)n -C-codoped meso/nanoporous TiO 2nanoparticles.
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