Kinetic mechanism of TiO2 nanocarving via reaction with hydrogen gas

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Kinetic mechanism of TiO2 nanocarving via reaction withhydrogen gas

Sehoon Yoo,a) Suliman A. Dregia, and Sheikh A. Akbarb)

Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210

Helene Rick and Kenneth H. SandhageSchool of Materials Science and Engineering, Georgia Institute of Technology,Atlanta, Georgia 30332

(Received 21 January 2006; accepted 18 April 2006)

Dense polycrystalline titania (TiO2, rutile) was converted into oriented arrays ofsingle-crystal titania nanofibers by reaction with a noncombustible, hydrogen-bearinggas mixture at only 680–780 °C. Such nanofiber formation resulted from anisotropicetching (“nanocarving”) of the titania grains. The fibers possessed diameters of20–50 nm and lengths of up to several microns, with the long fiber axes orientedparallel to the [001] crystallographic direction of rutile. Mass spectroscopy andinductively coupled plasma spectroscopy indicated that oxygen, but not titanium, wasremoved from the specimen during the reaction with hydrogen. The removal ofsubstantial oxygen and solid volume from the reacting surfaces, without an appreciablechange in the Ti:O ratio at such surfaces, was consistent with the solid-state diffusionof titanium cations from the surface into the bulk of the specimen. Thereaction-induced weight loss followed a parabolic rate law, which was also consistentwith a solid-state diffusion-controlled process.

I. INTRODUCTION

A significant level of research is currently focused onthe creation of nanostructured materials for a variety ofapplications, including for photonic, electronic, andchemical sensing devices.1,2 However, the widespreadutilization of nanostructured materials has been compli-cated by the often-conflicting demands for precise con-trol of fine features (down to the nanometer scale) and forlarge-scale mass production. Recently, Yoo et al. devel-oped a novel technique, referred to as “nanocarving,” forconverting bulk polycrystalline titania (TiO2) surfacesinto arrays of titania nanofibers.3 The nanocarving proc-ess is a relatively simple and scalable approach that in-volves the reaction of bulk polycrystalline titania with anoncombustible hydrogen-bearing gas mixture at a mod-est temperature (e.g., reaction with a 5% H2/95% N2

mixture at 700 °C).3 This process yields oriented arraysof single-crystal nanofibers over the exposed TiO2 sur-faces. Unlike other approaches that have relied upondeposition and growth (i.e., additive processes) to syn-thesize one-dimensional nanostructures,4–6 the titania

nanofibers generated by the nanocarving technique resultfrom the anisotropic etching of titania grains (i.e., a sub-tractive process). The objective of the present work wasto evaluate the mechanism of nanocarving through theuse of chemical analyses (mass spectrometry and induc-tively coupled mass spectroscopy), thermogravimetricanalyses, and microstructural analyses.

II. EXPERIMENTAL

The starting material was commercial anatase TiO2

powder (99.9% purity, Alfa Aesar, Ward Hill, MA) thatpossessed an average particle size of 32 nm. Batches of0.3 g TiO2 powder were formed into disks by single-endcompaction in a stainless steel die under a uniaxial pres-sure of 17.5 MPa. The TiO2 disks were placed in a hori-zontal tube furnace (Lindberg/BlueM, Asheville, NC)and sintered in air by heating at 5 °C /min to a set tem-perature of 1100–1400 °C and then holding for up to48 h. The sintered titania disks were 0.8 mm thick and10 mm in diameter. X-ray diffraction analyses indicatedthat the sintered titania disks were composed of rutile.3

Weight and dimension measurements indicated thatthe sintered rutile disks possessed densities of 4035 ±85 kg/m3, which corresponded to 94.9 ± 2.0% theoreticalvalue for rutile (4250 kg/m3).7 The average grain sizeafter sintering was determined by linear intercept analy-ses (ASTM E 112-96).8

a)Present address: School of Materials Science and Engineering,Georgia Institute of Technology, Atlanta, Georgia 30332.

b)Address all correspondence to this author.e-mail: [email protected]

DOI: 10.1557/JMR.2006.0225

J. Mater. Res., Vol. 21, No. 7, Jul 2006 © 2006 Materials Research Society1822

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The sintered disks were placed in a fused silica reac-tion tube within a horizontal tube furnace (Lindberg/BlueM). After attaching O-ring-sealed end caps to theends of the reaction tube, the tube was purged with a 5%H2/95% N2 gas mixture, then heated at 5 °C/min to apeak temperature of 680–780 °C, and held at this tem-perature for 8 h with a flow rate of 500 ml/min. Theoxygen and water vapor partial pressures within theheated furnace were evaluated with an oxygen sensor(CG 1000, Ametek, Paoli, PA) and a humidity sensor(HMP234, Vaisala, Woburn, MA). The measured oxy-gen partial pressures ranged from 3.0 × 10−17 Pa, and themeasured water vapor partial pressures ranged from 2.8 ×101 Pa. In some cases, the nanocarved (nanofiber-bearing) disks were re-oxidized by exposure to ambientair at 700 °C for 8 h.

The exhaust gas products of the nanocarving reactionwere evaluated through mass spectroscopy and induc-tively coupled plasma spectroscopy. Two titania diskspecimens were placed inside a fused silica tube in ahorizontal tube furnace. After attaching O-ring sealedend caps to each end of the fused silica tube, the 5% H2/95% N2 gas mixture was passed over the titania samplesat 1000 ml/min as the furnace was heated at 5 °C/min to700 °C. Upon exiting the furnace, the exhaust gas waspassed through a heated (120 °C) stainless steel gas line.The heated gas line was split using a tee into two streams.One gas stream was passed into a gas analysis system(QIC-20 Gas Analysis System, HIDEN Analytical, War-rington, UK) operating in mass mode with a secondaryelectron multiplier, whereas the other stream was passedthrough an oil bubbler and then exhausted to a fumehood. Upon cooling to room temperature, the exhaustend of the silica reaction tube was swabbed with cotton-tipped applicators to pick up potential condensationproducts from the exhausted gas. The compositions ofthe swabbed applicators and virgin (control) applicatorswere evaluated with inductively coupled plasma massspectroscopy (ICP-MS; Perkin-Elmer Elan 6000 ICP-MS, Wellesley, MA).

Thermogravimetric analysis (TGA; Pyris TGA, Per-kin-Elmer, Boston MA) was used for dynamic evaluationof the weight change of the titania specimens duringreaction with the H2/N2 gas mixture. The titania diskspecimens were suspended vertically in a platinum bas-ket within the controlled atmosphere TGA furnace. A 5%H2/95% N2 gas mixture was passed through the furnaceat a flow rate of 500 ml/min as the furnace was heated ata rate of 5 °C/min up to a peak temperature in the rangeof 680–780 °C.

A field emission gun scanning electron microscope(XL-30, FEI/Philips, Peabody, MA) was used to char-acterize the surface morphologies of the titania diskspecimens before and after exposure to the H2/N2 gastreatment. A gold-palladium coating was applied to the

specimens to prevent electron charging within the elec-tron microscope.

III. RESULTS AND DISCUSSION

As shown in the secondary electron image of Fig. 1(a),exposure of the TiO2 disk specimens to the 5% H2/95%N2 gas mixture at 700 °C for 8 h resulted in the formation

FIG. 1. Secondary electron images of TiO2 disk surfaces after expo-sure for 8 h at 700 °C to: (a) a 5% H2/95% N2 gas mixture, (b) 100%N2, and (c) a 5% H2/95% Ar gas mixture.

S. Yoo et al.: Kinetic mechanism of TiO2 nanocarving via reaction with hydrogen gas

J. Mater. Res., Vol. 21, No. 7, Jul 2006 1823



of oriented arrays of nanofibers on the titania specimensurface. To confirm that hydrogen was the active reactantgas species, titania specimens were exposed to pure flow-ing N2 under otherwise similar conditions as for the 5%H2/95% N2 nanocarving process (700 °C, 8 h). The sur-face morphology of the specimens after the nitrogentreatment is shown in the secondary electron image inFig. 1(b). No evidence of nanofiber formation was de-tected with this specimen; that is, the surface morphologyin this case (faceted titania grains) was identical to thatfor the starting as-sintered titania specimens. Furtherconfirmation of the active nature of hydrogen in thenanocarving process was obtained by exposing the titaniadisk specimens to a flowing 5% H2/95% Ar gas mixtureunder similar conditions as for the 5% H2/95% N2 treat-ment (700 °C, 8 h). The secondary electron image inFig. 1(c) reveals that this 5% H2/95% Ar treatmentyielded similar nanofiber arrays, as shown in Fig. 1(a) forthe 5% H2/95% N2 treatment.

Potential chemical reactions between TiO2(s) andH2(g) include the following:

TiO2�s� + 3H2�g� → TiH2�s� + 2H2O�g� , (1)

TiO2�s� + H2�g� → TiO�g� + H2O�g� , (2)

TiO2�s� + xH2�g� → TiO2−x�s� + xH2O�g� . (3)

The first reaction was thermodynamically unfavored;that is, at 700 °C for H2(g) and H2O(g) partial pressuresof 0.05 atm and (2.8–6.8) × 10−4 atm [(2.8–6.9) × 101 Pa,as measured by the water vapor sensor], respectively, theGibbs free-energy change for reaction (1) was calculatedto be +308.4 to 323.2 kJ/mol.9 Previous x-ray diffraction(XRD) analyses, x-ray photoelectron spectroscopy(XPS), and electron diffraction (ED) analyses also failedto detect the presence of titanium hydride in the nanofi-bers on the reacted titania surfaces.3,7 The vapor pressureof TiO(g) formed by reaction (2) should be extremelylow at 700 °C. The equilibrium vapor pressure of TiO(g)generated over a TiO2(s) surface for H2(g) and H2O(g)partial pressures of 0.05 atm and (2.7–6.8) × 10−4 atm[(2.8–6.9) × 101 Pa, as measured by the water vaporsensor], respectively, at 700 °C is only (1.6–4.0) ×10−27 atm.9

The formation of TiO(g), or other potential volatileTi-bearing gas species, was evaluated by mass spectros-copy. A representative analysis of the gas exhaust ob-tained after 1 h exposure of titania disk specimens to aflowing 5% H2/95% N2 gas mixture at 700 °C is shownin Fig. 2 (similar analyses were generated from gassamples obtained through 7 h reaction). The peaks lo-cated at 28 and 44 amu corresponded to N2 and CO2,respectively. The appearance of a CO2 peak was presum-ably due to the dissociation of volatile organics from the

pump oil upon exposure of the organics to the ion beamused in mass spectroscopy. The absence of peaks beyond44 amu indicated that titanium-bearing gas species werenot detected in the exhaust gas stream. However, Ti-bearing gas species may have condensed at the coolerend of the reaction tube, upstream of the mass spectrom-eter. To evaluate this possibility, cotton-tipped applica-tors were used to swab the interior of the exhaust end ofthe reaction tube after 8 h of reaction. ICP-MS analysesof such cotton swabs are listed in Table I. The titaniumconcentrations obtained from cotton applicators afterswabbing (46.7, 59.2 ppm) were similar to the valuesobtained from the virgin applicators without swabbing(54.4, 59.3 ppm); that is, a titanium condensate was notdetected at the cooler end of the reaction tube. Hence, themass spectroscopic and ICP analyses failed to detect thepresence of volatile Ti-bearing species during the nano-carving process.

With reaction (3), oxygen is removed from the titaniasurface to form water vapor during nanocarving, so as toleave behind a reduced form of titania, TiO2−x. The colorof the titania specimens changed to blue-grey after the5% H2/95% N2 heat treatment, which has also been re-ported by other authors upon partial reduction of titaniato form TiO2−x.

10 Thermogravimetric analysis (Fig. 3)indicated that the titania specimens underwent a weightloss of 0.15 mg/cm2 after exposure to the 5% H2/95% N2

FIG. 2. Mass spectroscopic analyses of exhaust gases during the ex-posure of TiO2 specimens to a flowing 5%H2/95% N2 gas mixture at700 °C. The peak at 28 amu corresponds to N2, followed by anothersmaller peak at 29 amu, which corresponds to a N2 isotope. A CO2

peak appears at 44 amu. No molecules containing titanium were de-tected.

TABLE I. Titanium concentrations, measured by ICP-MS analyses, ofcotton-tipped applicators used to swab the reaction tube after 8 h of theH2/N2 treatment and from unused (virgin) cotton-tipped applicators.

Sample Ti concentration (ppm)

Used applicator tip a 59.2Used applicator tip b 46.7Unused applicator tip a 54.4Unused applicator tip b 59.3


J. Mater. Res., Vol. 21, No. 7, Jul 20061824



gas mixture for 8 h at 718 °C. Scanning electron micro-scope analyses of specimen cross-sections indicated thatthe depth of the nanocarved region after this treatmentranged from 0.7 to 1 �m. From these data, along with themeasured values of the starting specimen surface area(0.870 ± 0.004 cm2) and density, the volume percentageof oxygen removed from the nanocarved region was cal-culated to be 17.0%. Despite such a substantial loss ofoxygen, XRD, XPS, and ED analyses have indicated thatthe nanofibers formed on the surfaces of the reacted ti-tania specimens were composed of the rutile polymorphof titania3,11; that is, such oxygen loss did not result inthe formation of other distinguishably different titaniumoxide phases (e.g., Ti3O5, Ti2O3, TiO, etc.) on the speci-men surfaces.

Retention of the Ti:O stoichiometry of rutile with sucha large loss of surface oxygen, in the absence of forma-tion of a volatile Ti-bearing species, may have been theresult of a solid-state diffusion process. Indeed, priorwork by several authors has shown that ion-sputteredtitania surfaces that are depleted of oxygen with respectto TiO2 (due to preferential loss of oxygen during sput-tering) may be restored to the TiO2 stoichiometry byannealing in ultrahigh vacuum at modest temperatures(200–600 °C).12–14 Such chemical restoration has beenattributed to solid-state diffusion.15–17 Two solid-statediffusion processes may be considered during nanocarv-ing: (i) excess titanium may diffuse away from the re-acting surface towards the bulk of the polycrystallinetitania specimen, or (ii) additional oxygen may diffusefrom the bulk of the specimen to the reacting surface(Fig. 4). However, the latter process would not have beenconsistent with the observed nanocarved structure, as thisprocess would have resulted in replacement of the lostoxygen and refilling of the void space. Henderson18

showed, through the use of static secondary ion massspectrometric (SSIMS) analyses of 18O and 46Ti, that the

restoration of the Ti:O stoichiometry of ion-sputtered(oxygen-depleted) TiO2 surfaces over 700 K was causedby rapid titanium cation diffusion from the surface intothe bulk. Several authors19 have reported that trivalenttitanium interstitials, Tii

��, are the dominant ionic defectsin rutile at temperatures below 1100 °C for oxygen par-tial pressures below 10−8 atm (i.e., for conditions includ-ing those used during nanocarving: T � 780 °C, pO2 �9.1 × 10−17 Pa). The rate of diffusion of titanium inter-stitials in rutile has also been found to be much faster (byseveral orders of magnitude) than that of oxygen diffu-sion.20–26 Hence, such prior work strongly suggests thatthe solid-state diffusion of trivalent titanium interstitialswas responsible for the retained rutile stoichiometry atthe specimen surfaces during nanocarving at �780 °C inthe 5%H2/95% N2 atmosphere.

Experimental support for such a solid-state diffusionprocess was provided by TGA. The square of the weightloss per area, (�m/A)2, is plotted versus reaction time inFig. 3 for temperatures of 682, 718, and 772 °C. For allthree temperatures, the fit of the data to straight linesindicated that the weight loss followed a parabolic ratelaw. The parabolic thermogravimetric rate constant kg

may be obtained from the slope of each line in Fig. 3,according to the rate equation:

��m�A�2 = kgt . (4)

Although the temperature range examined (682–772 °C)was relatively narrow, a plot of ln(kg) versus 1/T (Fig. 5)yielded a straight line, which was consistent with Arrhe-nius behavior. From the slope of this line, an activationenergy of 57.3 ± 9.0 kJ/mol was obtained, which wasnot far from the activation energy value (52.3 kJ/mol)obtained by Rekoske et al.27 who examined the

FIG. 3. Square of weight loss per area, (�m/A)2, of dense TiO2 speci-mens versus time t for 3 different H2/N2 treatment temperatures (682,718, and 772 °C). The R2 values associated with the fit of the 682, 718,and 772 °C data to the straight lines were 0.9925, 0.9911, and 0.9910,respectively.

FIG. 4. Schematic illustration of TiO2 nanocarving. Two solid-statediffusion processes may be considered during nanocarving: (1) tita-nium cations may diffuse away from the reacting surface towards thebulk of the polycrystalline titania specimen, or (2) oxygen anions maydiffuse from the bulk of the specimen to the reacting surface.





thermally activated rate of TiO2 weight loss during re-action with H2 at 350–500 °C. The effect of the averagegrain size of the titania specimens on the rate of reactionwas also examined by TGA. The values of the averagegrain size, which were varied by annealing the titaniaspecimens at different temperatures (1100–1400 °C, allfor 6 h), are shown in Table II. While the reaction ofthese specimens with 5% H2/95% N2 at 712 °C resultedin parabolic kinetics [Fig. 6(a)], the parabolic thermo-gravimetric rate constant decreased as the average grainsize of the titania specimens increased (Table II). Thisobservation suggested that grain-boundary diffusion wasan important transport mechanism. The effective diffu-sion coefficient Deff for the migration of trivalent tita-nium cations as interstitial ions through the titania latticeand along grain boundaries may be expressed by

Deff = Dl +�

G�Dgb − Dl� , (5)

where Dl and Dgb are the lattice and grain boundarydiffusivities, respectively, G is the grain size, and � is thegrain boundary width.28 If the grain boundary width � isassumed not to vary with the grain size, then the effectivediffusion coefficient should be linearly dependent on theinverse grain size, as shown below.

Deff �1

G. (6)

Because the value of a solid-state diffusion controlledparabolic rate constant kg should be directly proportionalto the effective diffusion coefficient, a plot of kg versus1/G should then yield a straight line. This was found tobe the case, as shown in Fig. 6(b). Hence, the dependenceof the nanocarving reaction rate on reaction time andon grain size were both consistent with a solid-statediffusion-controlled process.

Experiments involving porous and dense titaniasamples are also consistent with a solid-state titaniumdiffusion mechanism. Porous and dense TiO2 powdercompacts were prepared by varying the powder pressingforce (up to 17.5 MPa) during compaction. Both speci-mens were then placed side by side in the tube furnaceand exposed to the flowing 5% H2/95% N2 gas mix-ture for 8 h at 700 °C. Secondary electron images ofthe surfaces of both specimens are shown in Fig. 7.As expected from the other experiments described ear-lier, extensive nanofiber formation occurred for thedense TiO2 sample. However, such extensive nanofiber

FIG. 5. Plot of ln[kg] versus 1/T obtained from thermogravimetricanalyses of dense titania specimens during exposure to a flowing 5%H2/95% N2 gas atmosphere.

TABLE II. Values of the average grain sizes of TiO2 specimens afterannealing for various temperatures for 6 h, as measured by the linearintercept method.

Sinteringtemperature,

T (°C)

Averagegrain size,

G (�m)

Parabolic rateconstant, kg

(mg2/cm4s)

1100 1.57 1.18 × 10−6

1200 2.74 8.85 × 10−7

1300 4.59 6.70 × 10−7

1400 7.70 4.66 × 10−7

FIG. 6. (a) Square of weight loss per area, (�m/A)2, of dense TiO2

specimens (prepared by annealing at different temperatures) versustime t upon exposure to a 5%H2/95% N2 gas mixture at 712 °C. Thesamples were annealed at (1) 1100 °C, (2) 1200 °C, (3) 1300 °C, and(4) 1400 °C. (b) Plot of rate constant kg versus 1/(grain size). Thevalues of the rate constants were obtained from the slopes of the linesin (a).





formation was not detected for the porous TiO2 speci-men, although evidence of etching of some of the titaniagrains was detected. The lack of nanofiber formation inporous specimens indicated that the nanocarving was notcaused by volatile Ti-bearing species since such a proc-ess would even be more enhanced in the porous speci-mens.

Secondary electron images of a cross-section of theporous specimen after exposure to the 5% H2/95% N2

gas mixture are shown in Fig. 8. A higher magnificationimage of the circled area in Fig. 8(a) is presented in Fig.8(b). Grains located several tens of microns from theexternal surface also exhibited some etching, which in-dicated that H2 gas could access the bulk region throughthe pores and create excess titanium interstitials by thefollowing reaction:

TiTi + 2Oo = Tii�� + 3e� + O2�g� . (7)

Contrary to the dense titania specimens, small volumesof titania surrounded by pores within the porous speci-men [as shown in Fig. 9(b)] could quickly become satu-rated with excess titanium interstitial cations generatedby the reaction of titania with hydrogen. Such local satu-ration may have inhibited the generation of additionaltitanium interstitial cations; that is, the lack of a sink forthe additional titanium cations may have locally impededthe nanocarving process.

Recent scanning tunneling microscopy studies haveshown that the re-oxidation of reduced TiO2 resulted inthe growth of new TiO2 layers by the migration of tita-nium interstitials to the titania surface and then reactionwith gaseous oxygen.29–34 In the present case, re-oxidation of the nanocarved titania surfaces by exposureto ambient air at 700 °C for 8 h also resulted in the for-mation of new titania (Fig. 10) in the form of nanorods(i.e., of lower aspect ratio than the nanofibers formed

FIG. 7. Secondary electron images of the surfaces of (a) dense and(b) porous TiO2 specimens after exposure to a flowing 5%H2/95% N2

gas mixture at 700 °C for 8 h.

FIG. 8. Secondary electron images of a cross-section of the porousTiO2 specimen shown in Fig. 7(b) after exposure to a flowing 5%H2/95% N2 gas mixture at 700 °C for 8 h: (a) low magnification and(b) high magnification image of the area enclosed by the circle in (a).





during the nanocarving process). Therefore, the forma-tion of nanofibers during nanocarving, and of nanorodsduring reoxidation, appears to occur as follows. Whendense, polycrystalline TiO2 with a grain size in the rangeof 1.6–7.8 �m reacts with a flowing 5% H2/95% N2 gasmixture at 680–780 °C, oxygen is lost to the gas phase aswater vapor and excess titanium interstitials are gener-ated on the surface of the titania specimen. The excesstitanium interstitials migrate back into the bulk of thespecimen via diffusion through the lattice and alonggrain boundaries. Upon re-oxidation of the nanocarvedspecimen, the titanium interstitials migrate back to thespecimen surface to react with oxygen and reform titaniananorods.

IV. CONCLUSIONS

The formation of nanofibers (“nanocarving”) on thesurface of dense, polycrystalline titania upon reactionwith a hydrogen-bearing (5%H2/95% N2) gas mixture at680–780 °C was evaluated with mass spectroscopy(MS), inductively coupled plasma (ICP) analyses, TGA,and electron microscopy. MS and ICP analyses failed todetect the formation of volatile titanium-bearing gas spe-cies, which indicated that only oxygen was removedfrom the specimen. Although appreciable oxygen andsolid volume were lost from the specimen surface duringnanocarving, the rutile phase was retained at the externalsurface (i.e., the Ti:O ratio was maintained near 1:2).These observations were consistent with the formation ofexcess titanium interstitials at the reacting surface andthen solid-state diffusion of such interstitials into thebulk of the titania specimen. TGA data indicated that thereaction followed a parabolic rate law, which was con-sistent with such a solid-state diffusion mechanism. Theparabolic thermogravimetric rate constant, kg, was in-versely proportional to the average titania grain size,which indicated that grain-boundary diffusion was im-portant during this modest-temperature process.

ACKNOWLEDGMENTS

Mr. Gene Weeks (Laboratory for EnvironmentalAnalysis, University of Georgia, Athens, GA) is ac-knowledged for conducting ICP-MS analyses. Financialsupport for this work was provided by the National Sci-ence Foundation (DMR-0309558).

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FIG. 10. Secondary electron images showing nanorod formation uponreoxidation (700 °C, 8 h, ambient air) of a nanofiber-formed TiO2

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