Chemical Physics Letters 482 (2009) 134–138

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Chemical Physics Letters

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Active anatase (0 0 1)-like surface of hydrothermally synthesized titania nanotubes

Qiang Chen a, Gregory Mogilevsky a, George W. Wagner b, Jacob Forstater a, Alfred Kleinhammes a, Yue Wu a,*

a Department of Physics and Astronomy and Curriculum in Applied and Materials Sciences, University of North Carolina, Chapel Hill, NC 27599-3255, USAb US Army Edgewood Chemical Biological Center, Aberdeen Proving Ground, MD 21010-5424, USA

a r t i c l e i n f o

Article history:Received 5 August 2009In final form 1 October 2009Available online 3 October 2009

0009-2614/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cplett.2009.10.003

* Corresponding author. Fax: +1 919 962 0480.E-mail address: yuewu@physics.unc.edu (Y. Wu).

a b s t r a c t

Using 31P and 13C NMR with DFT calculations we demonstrate the exposed surface of titania nanotubes(TiNTs) is a stable, unterminated anatase (0 0 1)-like surface and is catalytically active under ambientconditions. We find that methanol dissociatively adsorbs on the surface of TiNTs agreeing with the pre-dicted activity of surface dissociation of organic molecules on the crystalline (0 0 1)-anatase surface.We further examined the catalytic activity of anatase power, TiNT, and nanosheets in catalytichydrolysis of S-[2-(diisopropylamino)ethyl]-O-ethyl methylphosphonothioate (VX) via 31P NMR anddemonstrate that titanate-like nanosheets are inactive in the reaction owing to their hydroxylated(0 0 1) surface.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Titanium dioxide (TiO2) is a promising crystalline material foruse in catalysis, solar energy harvesting and conversion, and envi-ronmental applications [1,2]. In anatase TiO2, the surface is primar-ily composed of thermodynamically stable (1 0 1) facets. However,for many chemical applications it is desirable to utilize the minor-ity (0 0 1) surface exploiting its high chemical activity [3]. Formany years, researchers have been seeking to prepare TiO2 materi-als with the dominant active anatase (0 0 1) surface [1]. Recently,anatase single crystals with a large percentage of active (0 0 1) fac-ets were synthesized by terminating the surfaces with fluorine [4].These synthesized powder samples were then treated at differenttemperatures to obtain anatase single crystals with a large per-centage of the (0 0 1) surface exposed. However, the chemicalactivity of this resulting material was not examined. Theoretical[5–7] and experimental [7] results have confirmed that water mol-ecules are dissociatively or chemically adsorbed on the anatase(0 0 1) surface. Therefore, the supposed active (0 0 1) surface onanatase nanoparticles is often hydroxylated or possibly deactivatedby the adsorption of water under ambient conditions.

We have shown [8] that hydrothermally synthesized nanotubu-lar TiO2 (TiNT) is actually titanium dioxide in which the anatase(0 0 1) surfaces curve along the anatase [0 1 0] axis to form multi-walled concentric tubes or Archimedean scrolls. Quantitative 1HNMR and thermogravimetric analysis (TGA) have suggested thatwater molecules are molecularly or physically adsorbed on the

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curved TiNT surface under ambient conditions [9]. Additionally,our previous experiments have demonstrated the chemical activityof TiNTs in dissociative adsorption of organic molecules [10,11],although at that time no detailed information about the surfacewas known. The TiNTs are normally 100 nm–1 lm long with 10–15 nm external diameter and 5–7 nm internal diameter [12–15].Moreover, the TiNTs possess large surface areas leading to a largequantity of active anatase (0 0 1)-like surface at ambient condi-tions [12–14].

Recently, we have confirmed the existence of TiO2 nanosheetswith the anatase (0 0 1)-like surface [9]. High resolution 1H nucle-ar magnetic resonance (NMR) investigations suggested that thesurface was covered by hydroxyl groups [9]. In addition, recenttheoretical work [16,17] has predicted that anatase nanosheetswith the anatase (0 0 1) surface will undergo surface reconstruc-tion for both clean and hydrated surfaces and be converted tomore stable lepidocrocite-like nanosheets [16,18]. This recon-structed structure is not expected to be active, as all of the Tiatoms in this structure are fully coordinated. However, thenon-hydrated nanotubes are expected to remain chemically activein ambient conditions just liked predicted for the anatase (0 0 1)surface.

Here we report that the curved anatase (0 0 1) surface of TiNTsis active in the dissociation of methanol under ambient conditionswith both experimental and theoretical 13C NMR results. Further-more, to directly demonstrate the relative chemical activities ofhydroxylated to bare (0 0 1)-anatase surfaces, we compare theactivities of nanosheets to TiNTs and anatase nanoparticles in cat-alytic hydrolysis of warfare chemical agent S-[2-(diisopropyl-amino)ethyl]-O-ethyl methylphosphonothioate (VX) with the useof 31P NMR.

250 200 150 100 50 0 -50

(b)

13C Chemical Shift (ppm)

48.0

66.0

64.0

49.9(a)

Fig. 1. 13C spectra of methanol adsorbed in TiNTs (a, red) before and (b, black) afterdesiccator drying. Both spectra were obtained via 1H to 13C CP and the shown signalis the sum of 51 200 scans. The echo applied in (b) filtered out the broadbackground at 100–200 ppm observed in (a).

Q. Chen et al. / Chemical Physics Letters 482 (2009) 134–138 135

2. Experimental and simulation details

2.1. Nanotube and nanosheet synthesis

TiNTs were synthesized by hydrothermal treatment of 32 nmanatase particles (Sigma–Aldrich) in 10 M NaOH at 140 �C for72 h. The resulting slurry was washed repeatedly with pure H2Oand 0.1 M HCl until the resultant pH was neutral. The slurry wasthen dried to obtain a powder consisting of nanotubes. Nanosheetswere synthesized as described in Ref. [9]. Nanotubes were sus-pended in pure H2O and ground in a Silica/ZrO grinding media ina BeadBeater grinder (BioSpec) for 45 min. After grinding, theresulting suspension was centrifuged and the supernatant solution,which contained suspended nanosheets, was decanted. The dec-anted solution was dried off on a heated glass slide.

2.2. Solid state NMR

2.2.1. 13C NMR study of methanol adsorptionAll 13C experiments were performed on a 400 MHz Oxford

superconducting magnet with a Chemagnetics console. All thespectra were acquired with magic-angle spinning (MAS) at a speedof 10 kHz. Samples were contained within a 4 mm MAS rotor.Ramped cross polarization (RAMP-CP) [19] and continuous wavedipolar decoupling were employed to enhance the weak 13C signalsat their natural abundance. For acquisition, a recycle delay of 3 s, a1H 90� pulse of 3.0 ls, and a contact time of 3 ms were employed.The spectrum obtained after drying the sample was acquired with200 ls echo to filter out a broad peak around 100–200 ppm, whichwas determined to be a background signal from the carbon-con-taining materials in the probe and MAS rotor used. The carbonylcarbon of glycine was used as a secondary reference for the 13Cchemical shifts (176.4 ppm relative to TMS). Synthesized TiNTswere stored in saturated methanol vapor at room temperaturefor a week before solid state NMR experiments were undertaken[8,10,11]. Following this, the MAS rotors were uncapped and driedin a desiccator for about two weeks. The spectrum was then retak-en for comparison.

2.2.2. 31P NMR study of VX decontaminationThe 31P NMR hydrolysis study on VX was performed under the

same experimental conditions as outlined in Refs. [20,21]. Thehydrolysis of VX on anatase powder and TiNTs were monitoredusing a Varian Unityplus 300 (B0 = 7 Tesla) NMR spectrometer inthe Doty Scientific 7 mm high-speed VT-MAS probe, while thaton nanosheets were monitored using a Varian INOVA 600(B0 = 14 Tesla) NMR spectrometer in the Doty Scientific 5 mmXC5 VT-MAS probe. All the samples are used as received withoutadding any water or drying. Spectra were acquired at a spinningspeed of 3 kHz. Direct polarization (DP) was used to obtain thespectra. H3PO4 (0 ppm, 31P) was used as the external shift refer-ence. VX (5 lL) was injected via a syringe into a column containinga powder of anatase, TiNTs, or nanosheets within the 7 mm NMRrotor. The rotor was sealed by its cap with an O-ring to reducethe escape of VX during NMR experiments. The spectra were takenafter different times to follow the chemical reaction. All spectrawere recorded as the sum of 128 scans with a recycle delay of 4 s.

2.3. DFT calculation

All density functional theory (DFT) calculations were performedwith CASTEP [22] using the Accelrys Materials Studio 4.3. The cal-culations were carried on a Beowulf Linux cluster at the ResearchComputation Center of the University of North Carolina at ChapelHill.

2.3.1. Structure optimizationThe calculations were performed at the DFT level with the

Perdew–Burke–Ernzerhof (PBE) [23] gradient-corrected ex-change-correlation functional. Ultrasoft pseudopotentials [24]were generated on-the-fly by CASTEP [22] and represented withplane wave basis sets. Valence states include 1s shell for hydrogen,2s and 2p shells for C, 2s and 2p shells for oxygen, and 3s, 3p, 3d,and 4s shells for Ti. The systems were modeled with methanolmolecules on TiO2 anatase (0 0 1) double-layer sheets. The slabswere repeating in the normal direction with a vacuum spacing ofat least 10 Å. The structures were optimized with delocalized inter-nal coordinate methods without symmetry constraints until theenergy, force, and displacement reached their convergence criteriaof 5 � 10�6 eV/atom, 0.01 eV/Å, and 5 � 10�4 Å, respectively. Thecut-off energy for the pseudopotentials was set to 610 eV and thefine FFT grid was set to 16 1/bohr. A 3 � 3 � 2 Monkhorst–Packgrid was used to sample the Brillouin zone of the single sheet sys-tem, which is represented as a (2 � 2) anatase (0 0 1) surface. A7 � 7 � 1 grid was used to sample double sheets system, whichis represented as a (1 � 1) anatase (0 0 1) surface. The calculationon methanol was performed on the gamma point of a10 Å � 10 Å � 10 Å supercell. All these parameters were justifiedon the calculation on anatase crystal. The computed values,a = 3.798 Å, c = 9.717 Å, were in excellent agreement with experi-mental values, a = 3.782 Å, c = 9.502 Å [25].

2.3.2. Ab initio chemical shifts calculationThe chemical shielding parameters were calculated using NMR-

CASTEP [24,25] in Materials Studio 4.3 [26]. The calculations wereperformed with the same parameters used for structure optimiza-tion. The calculated 13C absolute shielding values, riso, were con-verted to chemical shifts with respect to TMS, diso, usingdiso(ppm) = 175.0292 – 1.0469 � riso(ppm). The conversion was de-rived through a linear fit of the experimentally obtained chemicalshift values for the carbonyl and methylene carbons of crystallineglycine and the methane and methylene carbon of adamantaneto the absolute shielding values riso for these compounds calcu-lated at the same level.

Fig. 3. Calculated 13C chemical shift (ppm) of methanol (a) free, adsorbed (b) on thesurface, and (c) between layers of TiNTs.

136 Q. Chen et al. / Chemical Physics Letters 482 (2009) 134–138

3. Results and discussion

3.1. Methanol adsorption

3.1.1. 13C NMR spectra of methanol adsorptionFig. 1 compares the 13C spectra of methanol in TiNTs before and

after drying. Both spectra have two peaks: a narrow peak at 48–50 ppm and a broad peak at 64–66 ppm. After drying, the relativeintensity of these two peaks changes from 1.0:1.3 to 1.0:0.3. Also,the narrow 48.0-ppm peak broadens slightly and shifts to49.9 ppm. The significant intensity reduction and the peak broad-ening indicate that some loosely bound or mobile methanolmolecules are released into the environment upon drying. Theseare molecularly or physically adsorbed methanol molecules whichweakly interact with the TiNTs surface. The 66-ppm peak shifts to64-ppm after drying, however, its intensity does not differ signifi-cantly. This suggests that these methanol molecules have a stronginteraction with the surface of TiNTs, and are likely dissociativelyor chemically adsorbed on the surfaces of TiNTs. The broadnessof the peaks is due to the molecules’ restricted motion and/or dis-tribution on TiNTs surface.

In contrast, there is only one peak around 50 ppm for methanoladsorbed on anatase nanocrystals as shown in Fig. 2. In addition,the 13C spectra of methanol adsorbed on anatase nanocrystals(32 nm in size) acquired with cross-polarization (CP) techniquewere weak due to the high mobility of these molecules, whichprevents the efficient magnetization transfer with CP. Direct polar-ization (DP) successfully records the 13C spectrum of these highlymobile molecules, see Fig. 2a. The peak around 60-ppm observedin TiNTs and indicating adsorbed methanol is not significantcompared to the baseline.

3.1.2. Ab initio 13C chemicals shifts calculations of methanolIn order to understand how methanol molecules adsorb on

TiNTs, we performed ab initio chemical shifts calculations on themethanol–TiO2 system. Considering the short Ti–Ti distance(0.38 nm) [1], the long circumference of TiNTs (20–50 nm), andour computation capability, we modeled the system as methanolmolecules on (2 � 2) double layered anatase (0 0 1) sheets.

As seen in Fig. 3, the calculated 13C chemical shifts fit the exper-imental results well. The observed 48–50 ppm resonances are closeto the chemical shift of free methanol as seen in Fig. 3a. The methylcarbon of dissociatively adsorbed methanol molecule has thechemical shift of 67.6 ppm (Fig. 3b), which is close to the observed

250 200 150 100 50 0 -50

(b)

13C Chemical Shift (ppm)

(a)

Fig. 2. 13C NMR spectra of methanol on anatase nanocrystals: (a) direct polarizationwith 200 ls filtering at short recycle delay of 1.5 s before desiccator drying. (b) Afterdesiccator drying, RAMP-CP with 200 ls filter.

peaks at 64.0–66.0 ppm. Calculation on methanol absorbed be-tween the layers of TiNTs, as demonstrated in Fig. 3c, shows thatthe methyl 13C chemical shift is close to that of free methanol.Therefore, it can be concluded that the observed large intensityreduction of this peak (48–50 ppm) upon drying is due to the lossof free or weakly bound methanol molecules while the slightlyshifted resonance is from methanol molecules adsorbed betweenthe layers of TiNTs.

Both experimental and theoretical NMR results conclude thatmethanol molecules are adsorbed in different ways by TiNTs.The surface adsorbed methanol molecules are dissociativelybound to the undercoordinated Ti5c of TiNTs (Fig. 3b). In thischemical adsorption, the O–H bond of methanol is broken andthe proton is transferred to the O2c of TiNTs. There are alsoweakly bound or free methanol molecules, which easily evaporateat room temperature. The free or loosely bound methanol mole-cules are responsible for the narrow 48.0-ppm chemical shift peakand the intensity reduction upon drying. In addition, methanolmolecules could be absorbed between the layers of TiNTs. Theabsorption of these molecules is close to physical adsorption asthe 13C chemical shift is close to that of free methanol molecule.The existence of molecular adsorption between layers is possibledue to the weak hydrogen-bond-like interaction between the C–Hand O2c of the TiNTs. After evaporation, the physically absorbedmethanol molecules between layers are responsible for the49.9-ppm resonance observed in the dried TiNTs sample. Thepeak is slightly broader due to the restricted motion of methanolbetween layers. Their slight shift also suggests stronger interac-tion with TiNTs compared to those free methanol molecules onTiNTs surface.

The NMR experiments and ab initio calculations verify that thesurface of TiNTs is active in the dissociation of methanol underambient conditions, even though there are water molecules phys-ically adsorbed on the surface [9]. Many theoretical results pre-dicted that the flat anatase (0 0 1) surface is active and moleculessuch as water and methanol are only dissociatively adsorbed[26]. However, TiNTs retain stable and active (0 0 1)-like anatasesurface sites under ambient conditions.

3.2. Catalytic hydrolysis of VX

To demonstrate the difference in surface chemistry of cleanversus hydrated (0 0 1) anatase-like surfaces we performed the

Fig. 4. 31P NMR monitoring of VX hydrolysis on anatase nanoparticles at 11.5 min, 3.6 h, 15.8 h (left column); nanotubes at 11.5 min, 28.5 min, 1.9 h (center column); andnanosheets at 12.5 min, 12.8 h, 24 h (right column). Spinning sidebands observed for VX in the right column spectra of the nanosheets are a consequence of working at amagnetic field of 14 T while the other spectra were obtained at 7 T.

Q. Chen et al. / Chemical Physics Letters 482 (2009) 134–138 137

same decontamination experiment on nanosheets that wascarried out using multiwalled nanotubes reported in Ref. [20].There, multiwalled titania nanotubes proved to be far superiorto anatase nanocrystals and alumina in catalytically decomposingVX nerve agent through hydrolysis [20]. Here, we confirmed thatunlike the multiwalled titania nanotubes, nanosheets are not ac-tive in the decontamination of VX [20]. In situ 31P NMR is usedto monitor the hydrolysis of VX by recording the conversion ofVX represented by the NMR line with chemical shift of 52 ppmto the hydrolysis byproduct EMPA (ethyl methylphosphic acid)associated with the line near 22 ppm. Fig. 4 shows how the inten-sities of these lines evolve with time after the various sampleswere exposed to VX. Line intensities as function of time for VXon anatase nanoparticles, nanotubes (TiNTs) and nanosheets aredisplayed in Fig 4 left, middle and right column, respectively.Clearly, the half-life for VX hydrolysis is considerably longer onnanosheets (ca. 71 h) compared to multiwalled TiO2 nanotubes(TiNTs) (<1 h) and anatase nanocrystals (ca. 4 h) [20]. Unlike theanatase nanoparticles and the nanotubes where the reaction iscomplete in 15.8 and 1.9 h, respectively, the intensity of the52 ppm peak, a signature of VX, does not decrease even after24 h reaction time with nanosheets. Furthermore, the �22 ppmpeak corresponding to EMPA does not appear in the nanosheetscompared to the other materials. The hydroxyl groups on thenanosheets’ surface is shown to be quite stable and integratedinto the structure [9]. Owing to the necessity of the hydroxylgroups for structural stabilization, the nanosheets can be charac-terized as more titanate-like with an empirical formula of TiO2�x-H2O with x = 0.63 rather than TiO2 under ambient conditions [9].This study shows that titanate-like nanosheets are not active indissociative adsorption of molecules owing to their surface beingstably terminated with hydroxyl groups. However, anatase, espe-cially anatase-like nanotubes are active.

4. Conclusion

Using solid state NMR and ab initio calculations to investigatemethanol–TiNT interaction, we confirm that these hydrothermallysynthesized TiNTs are an air-stable material with a large number ofactive anatase (0 0 1)-like surface sites. Furthermore, we demon-strated that the hydrated (0 0 1) anatase-like surface, as it is foundin TiO2 nanosheets, is unable to decompose VX nerve agentthrough hydrolysis. Thus, the unterminated anatase (0 0 1)-likesurface, as found in TiNTs, is chemically active while the hydratedsurface is not active in dissociative adsorption of organic mole-cules. This finding provides a new direction in the search for activemetal oxide surfaces, whose surface chemistry is promising formany potential applications.

Acknowledgements

Sketch of methanol adsorption on TiNTs was generated withMaterials Studio 4.3. This work was supported by NSF, Grant No.DMR 0906547 and ARO, Grant No. DAAD19-03-1-0326.

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