Phase Transition of TiO2 Nanotubes: An X-ray Study as a Function of

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Phase Transition of TiO Nanotubes: a Xray Study as a Function of Temperature Francesca Anna Scaramuzzo, Alessandro Dell'Era, Gabriele Tarquini, Ruggero Caminiti, Paolo Ballirano, and Mauro Pasquali J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08297 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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The Journal of Physical Chemistry

Phase Transition of TiO2 Nanotubes: a X-ray Study as a Function of Temperature. Francesca A. Scaramuzzo,a * Alessandro Dell’Era,a Gabriele Tarquini,a Ruggero Caminiti,b,c Paolo Ballirano,d * Mauro Pasqualia * a

Dipartimento S.B.A.I, Sapienza Università di Roma - Via del Castro Laurenziano, 7 - 00161

Roma (Italy) b

Dipartimento di Chimica, Sapienza Università di Roma - Piazzale A. Moro, 5 - 00185 Roma

(Italy) c

CNIS, Centro di Ricerca per le Nanotecnologie Applicate all’Ingegneria, Sapienza Università

di Roma - Piazzale A. Moro, 5 - 00185 Roma (Italy) d

Dipartimento di Scienze della Terra, Sapienza Università di Roma - Piazzale A. Moro, 5 -

00185 Roma (Italy)

ABSTRACT. Here, a study aimed to determine the phase transition temperature of titanium dioxide nanotubes, both in presence and in absence of a Ti layer underneath, is reported. Titania nanotube arrays with different diameter and wall thickness were synthesized via anodic growth

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at different potential values on a titanium foil. Ex- and in-situ high temperature X-ray diffraction measurements were performed respectively on the arrays as such and on the powder obtained by detaching the nanotubes from the metallic substrate. Our results demonstrate that the presence of Ti dramatically influences the crystallization process decreasing the temperature formation of rutile, which tends to appear initially at the Ti/TiO2 interface.

INTRODUCTION. Physico-chemical properties, chemical stability, environmental compatibility and natural abundance make titanium dioxide an extremely exploited material for a variety of applications, ranging from solar energy harvesting to biomedical applications. 1-3 Titanium dioxide occurs in four different crystal structures, namely anatase, rutile, brookite and TiO2(B), the first two being the most widespread and used.4 Anatase and rutile are both tetragonal, but they differ in crystal habits: their TiO6 octahedra are highly distorted in anatase, while they have only a slight orthorhombic distortion in rutile. In other words, the two forms differ for Ti-Ti and Ti-O distances, which results in the fact that the anatase crystal structure can be described as consisting of octahedra connected by vertices, while that of rutile as consisting of octahedra connected by edges.5 For many applications, such as photoinduced redox reactions, crystalline TiO2 shows better performances than the amorphous one.6-7 However, it is easy to imagine that anatase and rutile, having different unit cells, show different electronic and ionic properties both as bulk and as nanostructured material,8 which results in different performances. Usually, rutile is exploited for surface chemistry studies and has been successfully used for photocatalytic degradation of methyl orange,9-11 while anatase is the elective phase in photoelectrochemistry, for high capacitance electrodes and for CO2 photocatalytic reduction.12-15 In principle, anatase is


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preferred to rutile in photoelectrochemistry because, despite a slight higher band gap (3.2 vs 3.0 eV), it shows oxygen vacancies well overlapped to the conduction band, thus increasing the probability of indirect interband transitions. On the other hand, rutile shows oxygen vacancy states at 0.75 and 1.18 eV below the conduction band minimum, which is lower than the H+/H2 redox potential and limits the electrons mobility. 16 However, some authors report that the use of mixed-phase TiO2 results in improved photocatalytic properties not shown by the two phases separately.17-19 Normally, rutile is more stable than anatase, and the phase transition, which is temperature-induced, is irreversible. The temperature at which the transition occurs is influenced by several parameters, such as impurities, morphology and heat flow conditions.20 Nowadays, with the growing and widespreading of nanotechnology, it is necessary to successfully perform the phase transition process determining both the transition temperature and the influencing parameters for the nanostructures as well as for the bulk. Titania nanomaterials can be obtained either in amorphous or in specific crystalline phase by tuning the synthetic methodology. For example, wet-chemistry approaches with heating of the reaction mixtures at well-defined temperatures can result in the synthesis of anatase nanowires,21 either anatase or rutile nanoparticles22-23 or, as an alternative, anatase and rutile mixture nanofoams.24 On the other hand, a similar synthetic strategy leads to the formation of amorphous titania nanotubes, which can be then crystallized with a post-synthetic heat treatment or close to ambient temperature.25-26 A totally different synthetic approach is the conventional electrochemical anodization, which allows the synthesis of titania nanotubes in well-ordered array form. This kind of nanostructure is widely exploited in many different fields, since it combines the functional features of titania with a controllable nanoscale geometry.27 The nanotubes synthesized in this way are amorphous, but they are easily crystallized by heating.28-31 For titania nanotube arrays with a well-defined


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morphology, it has been reported that amorphous/anatase transition happens above 550 K, while at less than 873 K rutile appears, becoming prevalent at 973 K. The relative intensity of the rutile peaks respect to anatase peaks increases with temperature, thus indicating the gradualness of the transition.32-33 However, since it has been demonstrated that the rutile nucleation begins at the nanotube–support interface and that the internal order of the array induces a preferential orientation during the anatase-to-rutile transition, it clearly results that the presence of residual Ti influences the whole process.34 Actually, it has been reported that for TiO2 nanotubes with diameter 700 K) the cell parameters of bulk and nanostructures tend to converge. It is worth noting that NTs cooled back to room temperature (RT) possesses cell parameters consistent with those of bulk anatase.


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Figure 6. Comparison between the cell parameters of anatase nanotubes and bulk as a function of T. Error bars are not reported since the uncertainty is within the size of the symbols.

The volume of NTs increases irregularly as temperature is raised (Figure 7).

Figure 7. Comparison between the volume and the c/a cell parameters ratio of anatase nanotubes and bulk as a function of T. For the sake of clarity only data of NTs synthesized at 60 V are reported. Error bars are not reported since the uncertainty is within the size of the symbols.


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This lack of regularity is apparently counterintuitive. However, this behavior is related to the combined effect of the expansion of the cell caused by heating and the release of strain. It is worth noting that the c/a cell parameters ratio clearly indicates that at 500 K the unit cell of NTs is significantly more compressed along the c-axis than bulk anatase. As already reported,45 in bulk anatase the z fractional coordinate of oxygen slowly changes with temperature (Figure 8), determining changes in different interionic distances and angles.

Figure 8. Comparison between the z fractional coordinate of oxygen of anatase nanotubes and bulk as a function of T. For the sake of clarity only data of NTs synthesized at 60 V are reported.

The z fractional coordinate of NTs passes from ca. 0.212 at 500 K to ca. 0.208 at 773 K. Similarly to cell parameters, the relatively large difference with the z fractional coordinate of bulk anatase at 500 K progressively reduces as temperature increases. Upon cooling back to room temperature the coordinate becomes very close to that of bulk anatase. At 500 K the Ti-O bond distances of NTs are 2 x 2.0114(18) Å and 4 x 1.9373(3) Å, indicating a strongly elongated octahedron that tends to regularize as temperature is increased (Figure 9). However, it is worth noting that the evolution of the mean Ti-O bond distance, i.e. , with temperature of NTs is perfectly consistent with that reported for bulk anatase.45


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Figure 9. Evolution of individual and mean Ti-O bond distances of NTs and bulk as a function of T. For the sake of clarity only data of NTs synthesized at 60 V are reported. Error bars are not reported since the uncertainty is within the size of the symbols.

CONCLUSIONS. Ex- and in-situ high temperature X-ray diffraction measurements were performed on titania nanotubes with different morphological features in order to determine their phase transition temperature. The results obtained highlight that the presence of Ti is not innocent in the crystallization process, since the rutile tends to appear initially at the Ti/TiO2 interface. Moreover, the presence of Ti decreases the rutile temperature formation: heating up to 773 K, in fact, rutile appears in the case of nanotubes anchored to the metallic substrate independently of their morphology. On the other hand, for pure nanostructured TiO 2 at the same temperature anatase is the only crystalline phase.

AUTHOR INFORMATION Corresponding Authors


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* To whom correspondence should be addressed: [email protected], [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Centro di Ricerca per le Nanotecnologie Applicate all’Ingegneria, Sapienza Università di Roma (CNIS) is kindly acknowledged. SUPPORTING INFORMATION SEM images, ex-situ XRPD of TiO2 nanotube arrays on Ti upon heat treatment in air at increasing time and under N2 atmosphere (PDF file). REFERENCES (1) Arango, A.C.; Johnson, L.R.; Bliznyuk, V.N.; Schlesinger, Z.; Carter, S.A.; Hörhold, H.H. Efficient Titanium Oxide/Conjugated Polymer Photovoltaics for Solar Energy Conversion. Adv. Mater. 2000, 12, 1689-1692. (2) Parkin, I.P.; Palgrave, R.G. Self-cleaning Coatings. J. Mater. Chem. 2005, 15, 1689-1695. (3) Larsson, C.; Thomsen, P.; Lausmaa, J.; Rodahl, M.; Kasemo, B.; Ericson, L.E. Bone Response to Surface Modified Titanium Implants: Studies on Electropolished Implants with Different Oxide Thicknesses and Morphology. Biomaterials 1994, 15, 1062-1074.


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Figure 1. Multi-holder, two-electrodes homemade electrochemical cell equipped with a Ni net (a) and a typical Ti/TiO2 NTs array sample obtained after anodization (b). 54x37mm (300 x 300 DPI)


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Figure 2. Ex-situ XRPD of TiO2 nanotube arrays on Ti synthesized at 10 V upon heat treatment at increasing temperature. Diagnostic peaks for titanium (stars), anatase (empty dots) and rutile (black dots) as reported in literature are also shown. 86x93mm (600 x 600 DPI)



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Figure 3. Ex-situ XRPD of TiO2 nanotube arrays on Ti synthesized at 60 V upon heat treatment at increasing temperature. Diagnostic peaks for titanium (stars), anatase (empty dots) and rutile (black dots) as reported in literature are also shown. 90x98mm (600 x 600 DPI)


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Figure 4. EDXD data of TiO2 nanotube arrays detached from the metallic support: NTs synthesized at 60 V (purple), NTs synthesized at 10 V (red), NTs synthesized at 10 V after suspension in chloroform (black). 65x53mm (300 x 300 DPI)



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Figure 5. Magnified view (20-85°2θ) of the in-situ HT-HRPD data set of NTs synthesized at 60 V. The diffraction pattern indicated as “303 back” was measured after cooling back the sample from 773 K. 65x53mm (600 x 600 DPI)


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Figure 6. Comparison between the cell parameters of anatase nanotubes and bulk as a function of T. Error bars are not reported since the uncertainty is within the size of the symbols. 82x82mm (300 x 300 DPI)



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Figure 7. Comparison between the volume and the c/a cell parameters ratio of anatase nanotubes and bulk as a function of T. For the sake of clarity only data of NTs synthesized at 60 V are reported. Error bars are not reported since the uncertainty is within the size of the symbols. 82x82mm (300 x 300 DPI)


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Figure 8. Comparison between the z fractional coordinate of oxygen of anatase nanotubes and bulk as a function of T. For the sake of clarity only data of NTs synthesized at 60 V are reported. 56x38mm (300 x 300 DPI)



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Figure 9. Evolution of individual and mean Ti-O bond distances of NTs and bulk as a function of T. For the sake of clarity only data of NTs synthesized at 60 V are reported. Error bars are not reported since the uncertainty is within the size of the symbols. 67x55mm (300 x 300 DPI)


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Phase Transition of TiO2 Nanotubes: An X-ray Study as a Function of

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