Materials Letters 89 (2012) 77–80

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Materials Letters

0167-57

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n Corr

E-m

journal homepage: www.elsevier.com/locate/matlet

Hindered sintering behaviors of titania nanoparticle-based materials

Kathy Lu n, Yongxuan Liang, Wenle Li

Materials Science and Engineering Department, Virginia Tech, Blacksburg, VA 24061, USA

a r t i c l e i n f o

Article history:

Received 21 July 2012

Accepted 25 August 2012Available online 1 September 2012

Keywords:

Nanoparticles

Sintering

Organic

Microstructure

Porous materials

Surfaces

7X/$ - see front matter & 2012 Elsevier B.V.

x.doi.org/10.1016/j.matlet.2012.08.109

esponding author. Tel.:þ1 540 231 3225; fax

ail address: klu@vt.edu (K. Lu).

a b s a t r a c t

Porous material sintering is gaining more attraction because of the need for catalyst supports, sensors,

solar cells, or other desired interactions with the enviroment. In this study, anatase TiO2 nanoparticle

suspensions are cast and sintered into porous materials. Surprisingly, suspension preparation condition

has a drastic effect on sintered grain size, relative density, and shrinkage. When the suspension is

prepared at basic condition with polyacrylic acid (PAA) as the dispersant, the sample shows faster

shrinkage with much larger grain sizes. When the suspension is prepared at acidic condition with

polyethylene glycol (PEG) as the dispersant, the sample has much slower shrinkage with minimal grain

growth. The fundamental cause is the special organic residuals adsorbed on particle surfaces. This

provides a simple yet extremely effective approach to produce nano grain-sized and highly porous

materials.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Porous nanoparticle-based materials are of scientific andtechnological importance because of their vast ability to adsorband interact with atoms, ions, and molecules on their largesurfaces (interior and exterior) and in the nanometer- tomicron-sized pore space. This family of materials has demon-strated important uses in various fields such as ion exchange,separation, catalysis, filtration, bone implant, sensing, and biolo-gical molecular isolation and purification [1–3]. They also offernew opportunities in guest–host synthesis and molecular manip-ulation and reaction in the nanoscale for making nanoparticles,nanowires, and other quantum nanostructures.

TiO2 has attracted much interest in recent years due to itsexcellent photocatalytic activity [4] and has been applied tophotocatalysis [5,6], water photoelectrolysis [7,8], light inducedsuper-hydrophilic/super-hydrophobic surfaces [9,10], dye-sensitized solar cells [11–15], and sensors [16,17]. There havebeen great efforts to maintain the porous and nanoscale micro-structures of TiO2 while keeping the more open anatase crystal-line phase.

In this paper, TiO2 green samples are made from differentsuspension conditions and sintered to different porous states. Theeffects of suspension preparation conditions on sintered density,shrinkage, and grain size are evaluated by scanning electronmicroscopy and FT-IR. The fundamental mechanism for thesintering behavior difference is proposed and a simple, effective

All rights reserved.

: þ1 540 231 8919.

approach is offered for sintering porous, highly open nanostruc-tured materials.

2. Material and methods

TiO2 nanoparticles with a specific surface area of 50 m2 g�1

and average particle size around 34 nm were used (P25, EvonikDegussa Corporation, Parsippany, NJ). For the basic TiO2 nano-particle suspension preparation, polyacrylic acid (PAA, [CH2–CH(COOH)]n, Mn¼1800, Sigma-Aldrich, St. Louis, MO) was usedas the dispersant. PAA is an ionic dispersant with a configurationsensitive to pH. The optimal approach for PAA to adsorb on theTiO2 particle surfaces is first go to acidic pH of 1–2, beforeadjusting to a pH of 8–9. The optimum PAA amount is �3 wt%based on TiO2 nanoparticles, as confirmed by potentiometrictitration. During the suspension preparation, TiO2 nanoparticleswere added in NH4OH solution along with 3 wt% PAA (based onTiO2 weight). Ball milling was used to mix the TiO2 suspensionand disperse the TiO2 nanoparticles. Suspensions of approxi-mately 30 vol% TiO2 solids loading were obtained at a final pHof �9.0.

For the acid-based TiO2 nanoparticle suspension preparation,polyethylene glycol (PEG, H–(O–CH2–CH2–)n–OH, Mn¼400,Sigma-Aldrich, St. Louis, MO) was dissolved in distilled waterand balled milled for mixing. TiO2 nanoparticles were added in0.15 M HNO3 solution along with 0.1 wt% PEG (based on TiO2

weight). PEG is a nonionic dispersant and does not dissociate insuspension, so there is no need to adjust pH in a complicatedmanner. In order to make high solids loading TiO2 nanoparticlesuspension, only 0.1 wt% PEG (based on TiO2 nanoparticles) was

K. Lu et al. / Materials Letters 89 (2012) 77–8078

used. Suspensions of approximately 30 vol% TiO2 solids loadingwere obtained. The final pH of the TiO2 nanoparticle suspensionswas fixed at �1.7.

Sintering of the green TiO2 samples after drying was per-formed in a furnace (E3504 Front Load Furnace, Deltech, Inc, IL,60007) in air. The sintering temperature was 450 1C and 650 1Cwith different dwelling time (1 h, 24 h, and 72 h) and the heatingrate was 10 1C/min. Such sintering temperature selection wasbased on the intent of maintaining anatase TiO2 phase andverified by our XRD work. The green and sintered TiO2 sampleswere polished into regular shapes, and then the dimensions of thesamples were measured by a Vernier caliper. The absolute densitywas calculated based on the sample volume and weight. Therelative density was obtained using the theoretical density ofanatase TiO2 at 3.84 g/cm3 and rutile at 4.26 g/cm3 (20% rutile and80% anatase phase for P25).

The functional group analysis for the TiO2 samples was carriedout using Fourier transform infrared (FT-IR) spectroscopy. FT-IRphotoacoustic spectra were collected using a Nicolet ImpactModel 400 instrument (GMI, Inc., Ramsey, Minnesota). Thespectra were obtained within a nitrogen-purged environment.The microstructure of the TiO2 samples was analyzed using a fieldemission scanning electron microscope (SEM, LEO550, Carl ZeissMicroImaging, Inc, Thornwood, NY). The samples were sputtercoated with a Au layer to reduce charging of the samples.

3. Results and discussion

Table 1 shows the relative density and linear shrinkage((length of green sample� length of sintered sample)/length ofgreen sample) of the PAA-based (basic condition) and PEG-based(acidic condition) samples at green state and after sintering atdifferent conditions. Even though both kinds of samples start withvery similar densities (�49%), the TiO2 sample prepared with PAAshows faster densification and larger shrinkage. This difference issmall after 450 1C, 1 h sintering and may even be negligible.However, when the samples are sintered at 450 1C for 24 h, thereis almost two times of shrinkage difference with the absolutedensity difference at about 5%. At higher temperatures (such as650 1C, 1 h), the PAA-based sample has about 12% higher density(65.44% vs. 53.74%) and almost twice of the shrinkage (6.82% vs.3.61%). With the holding time increase to 24 h and 72 h at 650 1C,the density difference becomes smaller, at about 1–2%. The linearshrinkage difference persists even though the gap is diminishing.This indicates that the PAA-based and PEG-based samples havevery different sintering kinetics before at least 75% dense, a veryimportant density range for many porous material applicationsmentioned before.

Fig. 1 shows that the TiO2 samples prepared with PEG have muchslower grain growth. The green samples are made from the samenanoparticles with the same homogenous microstructure and density

Table 1Relative density and linear shrinkage of PAA-based and PEG-based TiO2 samples at dif

PAA, basic condition

Relative density Linear shrin

Green sample 48.24%70.38%

450 1C, 1 h 50.31%70.78% 1.6%70.1

450 1C, 24 h 59.59%71.87% 2.74%70.2

650 1C, 1 h 65.44%72.36% 6.82%70.1

650 1C, 24 h 67.76%72.87% 13.53%70.4

650 1C, 72 h 72.49%72.28% 15.0%70.5

and are thus omitted. After 450 1C 1 h sintering, the microstructuresare still very similar and grain size difference is not visible. However,as the temperature increases to 650 1C, the grain size differencebecomes apparent and increases with the sintering time. For the650 1C, 1 h sintered samples (Fig. 1b), the grain sizes are39.54718.52 nm for the PAA-based sample and 30.42715.99 nmfor the PEG-based sample. For the 650 1C, 24 h sintered samples(Fig. 1c), the grain sizes are 102.25732.04 nm for the PAA-basedsample and 39.2678.55 nm for the PEG-based sample. However,with prolonged sintering at 650 1C (after 72 h, Fig. 1d), the grain sizedifference becomes small again, 123.08731.78 nm for the PAA-basedsample and 81.47729.77 nm for the PEG-based sample, even thoughthe PEG-based sample has less grain facets. It should be mentionedthat each of these measurement is based on 100 grains. This grainsize trend is fairly consistent with the density and shrinkage data inTable 1. This finding surprisingly and desirably provides a veryeffective approach for controlling the grain size at o100 nm withhigh porosity (r70% dense). In addition, the anatase phase ismaintained for the desired applications mentioned earlier by keepingthe sintering temperature at o700 1C.

Based on sintering fundamentals, sintering density increase,shrinkage, and grain growth are a result of atomic diffusion:surface diffusion dominant at lower temperatures (densities)and grain boundary diffusion dominant at higher temperatures(densities). At the same time, surface diffusion contributes tograin growth (coarsening) and grain boundary diffusion contri-butes to shrinkage. Other sintering mechanisms, even if present,are not believed to contribute significantly to the sinteringprocess. These two dominant sintering mechanisms are activesimultaneously with the relative contribution changing based onthe sintering temperature. In this work, the sintering temperatureis fairly low (450–650 1C) in order to maintain the anatase phasefor TiO2. While grain boundary diffusion contributes to shrinkage,it is most likely that surface diffusion triggers the sinteringbehavior difference.

In order to understand the factors that contribute to the slowersintering for the PEG-based samples, both energy dispersivespectroscopy (EDS) and Fourier Transform Infrared Spectroscopy(FTIR) analyses are conducted. EDS results do not show anyconsistent TiO2 stoichiometry difference. The FT-IR results fordifferent samples are shown in Fig. 2. At wavenumber greaterthan 2000 cm�1 (Fig. 2a), the PEG-based sample shows more –CH2 functional group content at low sintering temperatures butthis trend reverses at high sintering temperatures, which cannotexplain the shrinkage and grain growth differences. At wavenum-ber lower than 2000 cm�1 (Fig. 2b), while the peaks for theorganic functional groups (such as –CH3) at 1400–1800 cm�1

decrease with sintering, the peak at 1090 cm�1 persists. The1090 cm�1 wavenumber peak corresponds to the C–OH andC–O–C functional groups. Since PAA has no C–OH or C–O–Cgroups in the molecular structure, this means that the PEG-based sample has residual C–OH and C–O–C groups on the TiO2

ferent sintering conditions. Each data point is based on at least 3 samples.

PEG, acidic condition

kage Relative density Linear shrinkage

49.16%70.46%

1% 49.90%70.56% 1.28%70.09%

6% 54.25%72.24% 1.5%70.19%

5% 53.74%71.37% 3.61%70.46%

2% 66.78%72.02% 7.43%70.28%

0% 70.61%71.83% 10.78%70.48%

Fig. 1. Microstructures of sintered TiO2 samples at different conditions (For each

image, the left is for the PAA-based sample and the right is for the PEG-based

sample): (a) 450 1C 1 h, (b) 650 1C 1 h, (c) 650 1C 24 h, and (d) 650 1C 72 h.

Fig. 2. FT-IR results for different TiO2 samples, showing clear difference at

1090 cm�1 wave number.

K. Lu et al. / Materials Letters 89 (2012) 77–80 79

grain surfaces, which hinders TiO2 species diffusion and thussintering. It was reported that hydroxyl-terminated ether mole-cules react with nanoparticle surfaces and chemically adsorb

there and thus can hinder sintering [18]. However, agglomerationwas considered as the major factor in the previous study,shrinkage and grain size differences are not detected and diffu-sion mechanism was not mentioned. In this study, the hinderingeffect is embodied for both the surface diffusion and grainboundary diffusion mechanisms since the dispersant adsorptionis prevalent on initial TiO2 surfaces. After 72 h of sintering at6501C, the C–OH and C–O–C group intensity diminishes. This isaccompanied by the smaller density difference shown in Table 1and smaller grain size difference seen in Fig. 1d. In this study,agglomerates are not the contributing factor as both kinds ofsamples have homogeneous microstructures.

Thermogravimetric analysis (TGA) was also carried out. Itshows that the PAA, basic condition sample loses weight moredrastically before 400 1C (slope change). The PEG, acidic sample,however, does not show the drastic change of the weight loss.This is consistent with the more persistent presence of thefunctional groups on the TiO2 samples with PEG as the dispersant.

Although un-tested, the above results suggest that any dis-persant providing residual C–OH and C–O–C groups on the TiO2

grain surfaces at the studying temperatures should be able tohinder the diffusion and sintering of TiO2 nanoparticles. Polymerscontaining multiple C–OH and C–O–C groups in every repeating

K. Lu et al. / Materials Letters 89 (2012) 77–8080

unit are promising candidates (e.g. polyesters, polyols, and poly-alcohols), but each dispersant should be separately evaluated todetermine the efficiency and effective temperature range forhindering TiO2 nanoparticle sintering. It should also be mentionedthat the pH difference does not play a role in modifying thesintering behavior of TiO2 nanoparticles, although different pHvalues were used for dispersing purpose. All the samples weredried before sintering, so neither pH nor electrostatic forcesshould affect sintering.

4. Conclusions

In this study, TiO2 nanoparticle suspensions are prepared withdifferent dispersants at different pH conditions. Even though thestarting TiO2 nanoparticles and the green density/microstructureare the same, the dispersant has a surprising effect on sinteringshrinkage and grain growth. The PAA-based sample has fasterdensity increase, shrinkage, and grain growth. The hindranceeffect for the PEG-based sample sintering is attributed to theresidual functional groups on the TiO2 surface from the PEGdispersant used. This study offers a new approach to control thegrain growth and shrinkage for porous, low density samplesintering and has significance for catalyst supports, sensors,filters, solar cells, and other applications requiring highly porous,large surface area microstructures.

Acknowledgment

The authors acknowledge the financial support from NationalScience Foundation under Grant no. CMMI-0969888.

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