Coating mechanism of nano-TiO2 films on the surface of ultra-fine calcined coal kaolin particles

International Journal of Minerals, Metallurgy and MaterialsVolume 18, Number 4, August 2011, Page 487Do:10.1007/8126130110467-0Coating mechanism of nano-TiO2 films on the surface of ultra-finecalcined coal kaolin particlesHai Lin and ying-bo DongSchool of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China(Received: 19 July 2010; revised: 5 August 2010; accepted: 23 August 2010)Abstract: a new type of mineral composite was made by calcined coal kaolin. The interaction mechanism of an inorganic modification reagent TiOSO with the surface of ultra-fine calcined coal kaolin particles(substrate)was studied by X-ray photoelectron spectroscopy. Theresults show that chemisorption exists in the phase boundary between the modification agent and the substrate surface while physical ad-sorption occurs on the modification layers of hydrate titanium dioxide. The interaction force was calculated and analyzed according toDLvO theory between ultra-fine calcined coal kaolin particles and hydrate titanium dioxide nano-particles in the modification system. It isshown that the both electrostatic force and van der Waals force are attractive and the coacervation between ultra-fine calcined coal kaolinparticles and hydrate titanium dioxide nano-particles leads to the coating of hydrate titanium dioxide on the surface of ultra-fine calcined coalkaolin particles.Keywords: calcined coal kaolin; titanium dioxide; coatings; modification; mechanist1 IntroductionOn the basis of the above results, this paper has studiedthe interaction mechanism between ultra-fine calcined coalIndustrial mineral resources are abundant in China, the kaolin and titanium dioxide films by means of moden testvalue of which will be greatly increased by the processes of methods and theoretical analysis. The research providesultra-fine grinding and surface modification. a lot of func- measures and theoretical basis to reveal the mechanism oftional mineral materials and fillers have appeared; however inorganic coatings on the surface of ultra-fine industrialmost of the studies are still focused on the development and mineral particles, which is very important to the preparationapplication of industrial minerals, ignoring the study on theof new types of mineral composites with industrial mineralmechanism[1]. On the contrary, there are a large number of particles and the guidance for modification methodsreports about organic modification on the surface of mineral2. Experimental procedureparticles, such as calcium carbonate, kaolin, and quartz,while there are few reports on the inorganic modification 2. 1. Materials and apparatusmechanism [2-5]The sample was coal kaolin made in the laboratory [7], ofCoal kaolin is a kind of important industrial mineral re which the main component is 2SiO2 Al2O3 2H2O. Thesource. A new type of mineral composite, substituting for whiteness of the sample is 88.50%(blue-ray brightness), thetitanium dioxide, has been made successfully with ultra-fine particle size of-2 um is 91.88%, dso is 0.95 um, the specificcalcined coal kaolin as a substrate and TiOSO4 as a modifi- surface area is 3. 35 m/g, the oil absorption value is 35. 20 gcation reagent. The results of transmission electron micros- per 100 g, and the hiding power is 146. g/m. The appa-copy show that an even and compact nano-TiO2 film has ratuses of surface analysis were TEM and an XSAM800been coated on the surface of kaolin particles [6]multifunctional surface analysis system produced by中国煤化工Correspondingauthor:HaiLinE-mail:linhai@cesustb.odu.cnSpringerc Univerity of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2011CNMHGInt J. Miner. Metall. Mater., VoL 18, No 4, Aug 2011KRATOS Co in England; the test condition is Al Ka(12 kv15mA)2. 2. MethodsThe test was operated in a self-modified reactor withonstant temperature control. A certain amount of coal kao-lin(substrate)and water were added in the reactor; the substrate was made fully dispersed in water. After heating thereaction mixture to the required modified temperature in arater-bath. some titanium salt solution was added as amodification reagent. The final modified products were obtained by centrifugation, filtration, and drying [8Fig 1. TEM image of the modified product.The technology of coating a TiO2 film on the surface ofultra-fine calcined coal kaolin particles was studied. It wasshown that temperature, time, amount of surface modifica-tion agent, pulp density, and stirring velocity were importantfactors influencing the effect of the coating. The optimumcondition determined by experiments were the temperatureof 85%C. the time of 3 h. the amount of surface modificationagent of 0. 75 g/g, the pulp density of 5 g/L, the modificationH value of 8.0, and the stirring velocity of 375 r/min. Thefinal coating product, which was acquired using the opti-mum condition, was tested and analyzed by SEM and TEMThe coating mechanism of the modification reagent and Fig. 2. Enlarge drawing of the substrate and film at the in-substrate was studied by X-ray photoelectron spectroscopy terface.(XPS). By XPS, not only surface information but also deepdistribution data can be obtained. therefore it is a useful energy produced by an ion gun to bombard the samplesmethod in sorption study. Accurate detection information surface, then surface atoms absorbing enough energy canalso can be obtained on the electron binding energy of escape from the samples surface because of energy transferatomic orbitals by XPS, so that whether the change of After that, the escaped ions are examined by mass spectro-chemical environment occurs can be judged by the shift of graphy, and then the secondary ion mass spectrum will bebinding energy. Therefore, we can get the information about obtained. If the spattered surface is examined by XPs,themodification particles from both the surface and interiorep anatoly result will be observed. The main function of3. Results and discussionthe nondestructive deep anatomy technology is to test theparameters affected by the thickness of samples [9]. The in-TEM images in Figs. I and 2 show that an even and tensity of signals in XPS is determined by the photoeleccompact film of nano-TiO2 has been coated on the surface trons energy and escape velocity, while the mean free pathof the substrate after the interaction between the modifica- is a correlative parameter, which can determine the escapetion reagent and substratedepth of ions. When the energy of electrons is fixed, withThe deep anatomy test technology, XPS, has been intro-the altering of shooting angle, the actual escape depth ofduced on the study of the coating mechanism of the modifielectrons will change, and the information about the sam-cation reagent and substrate. There are two kinds of deepe's surface and depth will be obtainedanatomy test technology: nondestructive technology and de-The XPs test results of the substrate and modificationstructive technology, which can both analyze the elementaryagent shown in Figs 3-6 indicates that before the reactioncomposition and chemical constitution of the filmy with the reagent, the electron binding energies of Si2p, Al2pcross-section deeplyand ols for中国煤化工are10270,74.35,lon spatter surface analysis is a kind of ideal destructive and 532.15 eCNMHGreaction with thedeep anatomy technology that usually uses ions with enough substrate the electron binding energies of Ti2p and olsH. Lin et aL, Coating mechanism of nano-TIO2 flms on the surface of ultra-fine calcined coal kaolin particles600006000050000≌5000040000A30000O(A)a(A)Si2p3000020000Ti(10000Al2p400600800100012001400Binding energy!eⅤ40060080000012001400Binding energy /evFig 3. XPS wide scanning image of the substrate.Flg. 5. XPS wide scanning image of the modification re-3500Si2p 102.70eV30004000Ti2p 459. cV250020003000l5002500100~150011005Binding energy /ev1400}(b)Al2p 74.35 evBinding energy /evols 530.50eV12002000800Binding energy /eV5385365345325305285262000ols 532. 15eVBinding energyFig. 6. XPS seanning image of the modification reagent:(a)TLp; (b)o1s.for the modification reagent are 459.05, and 530.50 ev, re-800The XPS test results(Fig. 7-9) of coating products afterhe reaction between the substrate and modification reagentshow that no data of the substrate particles have been de-536534532530528tected at the angle of 30 or 90o with nondestructive deepBinding energy/eⅤanatomy te中国煤化工 angle of electrons)Fig. 4. XPS seanning Images of the substrate before the reac- but the dataCNMH GIde on the substratedion:(a)S(2p:( b)Al2p;(c)oI.surfaceInt J. Miner. Metall Mater VoL 18, No,4, Aug 2011400060000Ti2p 45905 eV3500500003000840000≥25000(A)TI(A)2000200001500S2100001000470Binding evergy/eⅤ10001400ols 530.55eVFig 8. XPS wide scanning image of the modification product20000at the angle of 90.above are 530.55 and 530.45 eV respectively; the electronbinding energies of ols are 530.55 and 530.45 eV respec-12000tively. Compared with the electron binding energies of Ti2pand Ols of the reagent before the reaction, the energy of8000Tip does not change, moreover the shift of Ols is in theerror limit(+0. 1 ev) of the apparatus used. Therefore, it in-4000538536534532530528526dicates that the chemical conditions of hydration titaniuming evergy/eⅤdioxide on the surface of the particles(except the interphasef hydration titanium dioxide and the substrate)are notFIg. 7. XPS scanning images of the modification product atchanged; after the reaction between the modification reagentthe angle of 30: a Ti2p;(b)ols.he adsorption between hyThe electron binding energies of Tip in the film on the sur- dioxide particles on the surface of modification products isface of the modification particles at the angles mentioned physical adsorption.4000(a)Ti2p 459.05 eVOIs 530.45eV2000035003000160025001500536534532530528526Binding evergy /evBinding evergy /evFlg, 9. XPS scanning image of modification product at the angle of 90: (a Tip;(b)olsBased on the test results above, the argon sculpture in the binding energy of A2p and Si2p are 74.95 and 102.1XPS has been applied for 2 min to find out the essence of ev, respectively. Compared with the binding energy of theabsorption between the substrate particles and hydration ti- substrate befd中国煤化 If electron bindingtanium dioxide. The data about both the substrate and modi- energy of a255eV. which arefication reagent have been found after argon sculpture, and beyondCNMHGherefore it showsH. Lin et al, Coating mechanism of nano-TiO2 fillms on the surface of ultra-fine calcined coal kaolin particlesthat the chemical condition of the substrate eigenelements 4. 2. Van der Waals forceAl and Si has changed after the modification, so that the ad-Generally, van der Waals force is produced by the actionssorption action between hydration titanium dioxide and the of many atoms(molecule), so its expression is significantlysubstrate is a kind of chemiadsorption. Moreover, thedifferent from the actions of a single atom. Van der Waalschemical condition of the reagents eigenelement Ti has alsoforce was discovered by Hamarker with the principle of ad-been changed after modification( the shift of binding energy ditive property. In this research, the substrate particle is ais +0.35 ev)sheet, while the titanium dioxide particle is a sphere, so theirThe process of the coating on the surface of ultra-fine cal- reaction seems to be a process between the plane and sphere,cined coal kaolin has been completed by chemiadsorption and the expression is as the followingbetween the reagent and substrate and by physical actionsbetween hydration titanium dioxide particles. AccordinglyU, =-AR/(6)the modification layer on the surface of the substrate is even, Fw=AR/(6H)where Uw is the van der Waals energy per area, Jm A the4. Calculation and analysisHamarker constant in vacuum, J; R the radii of sphericalSubstrate particles in the modification system are very Waals force per area, N i of particles, m; Fw the van derfine and the hydration titanium dioxide is a kind of ultra-fineFormula of Hamarker constant is as the followinggrain, so their process specific surface areas and specificsurface energy are large. This system belongs to astaticism 4=7 C02=r202 3aovcrease of the surface area, and then the specific surfaceergy will decrease in the process of coacervation amongwhere p is the mass density, kg m;C the coefficient ofticles, which is an irreversible process [10].dispersion interaction energy; a the atomic polarizability,c'm5; ho the planck constant, 6.626 10*J-s;v the elec-4. 1. Interaction between particlestronic rotating frequency, s"; En the dielectric constant,There are two kinds of interactions between particles in8854×102c2rlm1.cluding short-range force and long-range force, the formerThe key step to calculate the van der Waals energy ofonly occurs between particles within the confines of 2 nm. objects with Eqs.(1)and(2)is to work out the Hamarkera kind of short-range repulsion, Bohr-repulsion, will occur constant. The constant indicates the characteristic of molein the electron cloud of particles on the surface when the cule interaction in vacuum. However, some interactions ofdistance between two particles reaches 1 nm. On the con- particles always occur in some medium, for instance the actrary, a kind of strong short-range attraction will occur for tions of mineral particles in aqueous medium. Hamarkerthe contact and chemical reaction between two particles constant A in this condition is an effective constant, whichthrough the covalent bond, the coordinate bond, the electro- involves the effect of medium moleculesvalent bond, or even the hydrogen bond.Assuming Al is the Hamarker constant of particle 1 inLong-range force occurs between particles from 5 to100 vacuum, A33 is the constant of medium 3 in the same condinm. Main long-range forces between particles in aqueous tion, and then the hamaker constant A13 between them ismedium include van der waals force, electrostatic forceproduced by the overlap of the electrical doub,dt=+h-243~(√-√43)2by the absorbehydration shell effeThe Hamarker constant A132 between particles 1 and 2 inand hydrophobic effect on the surface of particlesthe medium 3 isResults of this research on the modification system of A2=(VAI-V433)x(42sitonion show that both short-range force andIt can be seen from the equations above thatlong-range force including van der Waals force and electro- Hamarker co中国煤化工 cles in the mediumeenparticles anare affected Ititanium dioxideCNMHGthe constant of themediumuun.MOreover, wnen di<0, the van derInt J. Miner. Metall Mater VoL18, No, 4, Aug 2011Waals force will be repelled.charge be attracted easily on the substrate surface with43. Electrostatic forcenegative electric charge, which accordingly weaken thecoacervation among hydration titanium dioxide particles; onElectrostatic effect among particles will occur when the the contrary, the positive electric charge of coacervationparticles are close enough for the electrical double layers torvation of a partoverlap each other The origin of the electrostatic effect isof hydration titanium dioxide particles. According to thethe attraction or repulsion of the electriferous ions in dif-electrostatic theory, Coulomb force is relative to the distancefused layers close to each other, so this electrostatic force and electric charge of particles, is proportional to electricwill always be repulsion among same particles and attrac- charge and inversely proportional to the square of distance.tion among different onesThe increase of the hydration titanium dioxide particleOn the condition of pH 8.0 in this research, the electrical electric charge will enhance the electrostatic repulsionproperty of the substrate is negative; however, the hydration among them, which can bafile their further coacervationtitanium dioxide particle hydrolyzed by titanyl sulphate is Moreover, these increased electric charges of hydration tita-positive. Therefore, the electrostatic force between the sub- nium dioxide strengthen the Coulomb force, leading to astrate and hydration titanium dioxide should be an attraction strong electrostatic attraction.forcBased on the discussion above, according to dlvo the444. Total energy (Ur)ory, both the van der Waals force and electrostatic forcebetween the substrate particles and hydration titanium dioxAccording to DLvO theory, the total energy (Ur)in-ide are attraction forces. According to the mutual-agglo-volves van der Waals energy and electrostatic energy; themeration theory, in the modification system, the coacerva-expression is given as followstion process occurs between the substrate and hydration tita-Uw+tW(6) nium dioxide particles hydrolyzed by titanyl sulphate, whickpromotes the coating of hydration titanium dioxide on theIn the modification system studied by the present study, substrate surface, and then a film of hydration titanium di.the interaction between the substrate and ultra-fine hydrationtitanium dioxide particles is a kind of interaction betweenoxide will be coated on the surface of substrate particles.the heterogeneous5 ConclusionsAccording to Refs. [11-15], the Hamarker constants can1 )An effectual research measure and an analyticalbe gotten: mica, 10.0x10 J; hydration titanium dioxideIng197x10 J; water in vacuum, 3.7x10-20 J. Assume the coating film on ultra-fine industrial mineral particlesabstrate is particle 1, hydration titanium dioxide is particle2, the medium is 3; the component and structure of the sub-(2)The angle changing technology and etching technolstrate are similar to mica's, so it can be obtained approxi- ogy of X-ray photoelectron spectroscopy have been appliedmately. that A1=10.0×1020J,Ax=197×100Jin this study. The results show that chemiadsorption occursA3337x10 J then the effective Hamarker constants of on the interphaseetween ultra-fine calcined coal kaolinthe substraate and hydration titanium dioxide in aqueous me- particles and hydration titanium particles, but the sorptiondium can be obtained.4x=(1-V)×(<-√)=layer is physical adsorption.=3115×100J(7()The theoretical calculation indicates that both the vanA1320, indicates that the van der Waals force between theder waals force and electrostatic force between ultra-finesubstrate and hydration titanium dioxide particles is an at- calcined coal kaolin and hydration titanium particles are attraction forcetraction forces. Based on the mutual-agglomeration theoryAt the early stage of the hydrolyzation process, theultra-fine calcined coal kaolin and hydration titanium parti-cific surface area of hydration titanium dioxide is very large cles can be aggregated in the modification system, and thebecause of its fine particles. 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