fabrication of nanotubular oxide layer on ti–24nb–4zr–8sn alloy … text.pdf · 2018. 1....

2 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

Fabrication of nanotubular oxide layer on Ti–24Nb–4Zr–8Sn alloy by electrochemical anodization

Anna Matras1*, Agata Roguska2, Małgorzata Lewandowska1

1Faculty of Materials Science and Engineering, Warsaw University of Technology (WUT), Warsaw, Poland, 2Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland, *[email protected]

Nanotubular oxide layer of TiO2 was fabricated by electrochemical anodization of Ti–24Nb–4Zr–8Sn alloy in electrolyte containing water, glycerin and ammonium fluoride. Physicochemical characterization was performed in order to evaluate the structural and chemical properties of obtained layer. Variable parameters such as voltage (10 V, 20 V and 30 V) and anodization time (10 min and 40 min) were applied to determine the influence of those factors on the morphology and chemistry of fabricated titania nanotubes. Scanning electron microscopy was used to assay the architecture of obtained nanotubular layer and the impact of anodization parameters on the produced structure. Chemical and structural analysis were conducted by energy dispersive X-ray spectros-copy coupled with scanning electron microscope and X-rays diffraction technique. Obtained results allowed to declare that morphology of the nanotubes de-pends on voltage applied and duration of the anodization. For higher voltage applied, the nanotubes with bigger diameter are obtained. For layers fabricated at 10 and 20 V, increase in anodization time results in obtaining more homogeneous oxide structure (the diameter distribution is narrow) as well as decrease of characteristic dimensions values (wall thickness and diameter). For structures anodized with 10 V, the average nanotubes diameter was 38 nm while for layer fabricated with 30 V, the average diameter was 101 nm. The most uniform nanotubular structure was fabricated by anodization at 20 V for 40 min. Chemical analysis revealed presence of such elements as titanium, oxygen, niobium and tin. However, thickness of the nanotubular oxide layer is about hundreds of nm, therefore additional examination need to be done to determine whether the nanotubes contains alloying elements (Nb or Sn) or the signal comes from the substrate. In general, the chemical composition of the anodized nanotubes corresponds to the composition of the substrate Ti2448 alloy.

Key words: titania nanotubes, electrochemical anodization, titanium alloy, surface analysis.

Inżynieria Materiałowa 1 (215) (2017) 2÷7DOI 10.15199/28.2017.1.1© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

Electrochemical anodization is a simple and versatile technique to produce ordered porous oxide layers on the surface of various me-tallic materials. Self-organized porous structures have been so far successfully obtained via electrochemical method, i.e. on aluminum [1, 2], zirconium [3, 4], titanium [5, 6] and niobium [7, 8]. The anodization process consists of two simultaneously occurring steps: formation of compact oxide layers on the surface of the substrate and its selective chemical dissolution. The morphology and proper-ties of obtained nanotubes depend on numerous process parameters. The key factor controlling the tube diameter is the anodization volt-age; generally, for higher voltage applied the nanotubes with big-ger diameters are obtained [9, 10]. Water content in the electrolyte influences the growth rate and the dissolution speed of the layer. Nanotubes with smooth walls are obtained in electrolytes with low water concentration, while ripples on the walls are formed for higher water content [11, 12]. As far as height of nanotubes is con-cerned, for increasing the anodization time thickness of the oxide layers increased.

Not only pure metals can be anodized, but the alloys as well. For example in the same way, titanium alloy can be modified to produce nanotubular layers and to improve the properties of the surface. Single-phase alloys are preferred over two-phases ones for obtaining nanotubes because multiple phase materials often suffer from selective dissolution [13, 14]. Over the last couples of years, beta titanium alloys have gained increasing popularity as a very important class of materials. They possess a good combination of high strength, low density, and good corrosion resistance as well as exhibit great processability. Beta alloys maintain the beta structure upon quenching from the beta phase field and contain enough beta stabilizing elements to avoid formation of the martensite, due to avoiding cooling through the martensite start line. The two types of beta stabilizers are known as eutectoid and isomorphous. The isomorphous stabilizers include Mo, V, Ta and Nb and the eutectoid

stabilizers include Cr, Mn, Fe, Co, Ni and Cu. Depending upon the amount of stabilizers present in an alloy, the alloy will be a meta-stable or stable beta Ti alloy [14÷16]. It was found that Ti–Nb alloy allows to obtain nanotubes over wider range of tube dimensions in comparison to pure Ti. Nanotubes anodized on Ti–Nb alloy can be fabricated with length in the range from 0.5 to 8 µm and diameter from 3 to 120 nm. It has been reported that great impact on the length of the nanotubes fabricated on Ti alloys has zirconium. The length is directly proportional to the content of Zr and it can be ad-ditionally controlled by applied voltage, as well as diameter of the nanotubes. The chemical composition of the anodized nanotubular oxide structure fabricated on an alloy corresponds to the composi-tion of the substrate alloy [10, 14].

In this paper, the nanotubular oxide layer was fabricated by elec-trochemical anodization of Ti–24Nb–4Zr–8Sn alloy in glycerin-based electrolyte containing fluoride ions. Physicochemical char-acterization was performed in order to evaluate the structural and chemical properties of obtained layer. Variable parameters such as voltage and anodization time was applied to determine the influ-ence of those factors on the morphology and chemistry of fabricated titania nanotubes.

2. MATERIAL AND METHODS

For the investigation, the Ti2448 alloy (Ti–24Nb–4Zr–8Sn, wt %) with a single β phase was used in the form of plates with thickness of 0.4 mm. All samples were ultrasonically cleaned with acetone, ethanol and deionized (DI) water, respectively, and then dried in air. Nanotubular oxide layers were fabricated by electrochemical anodi-zation using polar organic electrolyte based on glycerol (52 wt %), DI water (47.14 wt %) and ammonium fluoride (0.86 wt %). A di-rect current (DC) power supply was used to keep the voltage at a constant value (10, 20 and 30 V) for a set amount of time (10 and 40 min). After anodization process, the samples were rinsed with DI water and dried in air. Thermal annealing was performed at 450°C

NR 1/2017 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 3

for 3 h to obtain mechanically stable nanotubes well integrated with the substrate.

The morphology of the obtained nanotubular oxide layers were examined by a scanning electron microscope (SU-8000, Hitachi). The specimens were attached to the table using a conductive carbon tape to increase the amount of electric charges flowing. Observation was done at acceleration voltage of 5 kV and a current of 10 µA. The chemical compositions of the fabricated nanotubes were ana-lyzed by energy dispersive X-ray spectroscopy (EDS) coupled with scanning electron microscope. X-rays diffraction (XRD) using Cu Kα radiation (λ = 1.54 Å) was performed to evaluate the structural properties of the nanotubes. Diffraction patterns were recorded within 2θ angle between 10° and 100°. ImageJ software was used in order to estimate characteristic dimensions of the nanotubes, i.e. diameter, wall thickness and length. Statistics was performed based on images selected from different areas of the layer; 50 measure-ments were made for each sample.

3. RESULTS AND DISSCUSSION

In order to reveal the morphology and estimate characteristic di-mensions of nanotubular structures fabricated by electrochemical anodization SEM observation was performed. Figure 1 shows SEM micrograph of nanotubular oxide layers obtained at constant volt-age of 10 V. Nanotubes (NT) possess regular shape and ordered

structure, there are areas of fully formed tubes as well as places with initial stage of their formation. It has been shown that ripples on the walls of the NT are formed. The average values of character-istic dimensions of the nanotubes were calculated and the results are presented in Table 1. For the NT fabricated at 10 V for 10 min the average diameter was 43 nm and the wall thickness was 11.2 nm, while for the structures manufactured at 10 V for 40 min the char-acteristic dimensions decreased — the average diameter was 38 nm and the wall thickness was 10.3 nm. In Figure 2 distribution of the values of tubes diameter obtain at constant voltage of 10 V for 10 and 40 min are shown. Longer time of anodization results in obtain-ing more homogeneous nanotubular structure as the range of values distribution is narrow.

Fig. 1. SEM micrographs of nanotubes obtained at 10 V for 10 min (a) and 40 min (b)Rys. 1. Zdjęcie SEM nanorurek wytworzonych przy napięciu 10 V przez 10 minut (a) i 40 minut (b)

Fig. 2. Distribution of the nanotubes diameter obtained at 10 VRys. 2. Rozkład średnicy nanorurek wytworzonych przy napięciu 10 V

Table 1. Parameters of obtained nanotubesTabela 1. Parametry otrzymanych nanorurek

Voltage V

Diameter of nanotubes, nm Wall thickness of nanotubes, nm

10 min 40 min 10 min 40 min

10 43 38 11.2 10.3

20 66 54 11.7 9.5

30 61 101 — 10.7

a) b)


In Figure 3 the SEM micrograph of the oxide nanotubes obtained at voltage of 20 V is shown. The nanotubular structures are ordered and have regular spherical shape, fully formed nanotubes occupied the entire surface of the sample. There are ribs on the wall of the na-notubes (Fig. 3c). For the layers fabricated at 20 V for 10 min the av-erage tube diameter was 66 nm and the wall thickness was 11.7 nm, while for the structures obtained at the same voltage for 40 min the characteristic dimensions decreased and the average diameter was 54 nm and the wall thickness was 9.5 nm. Figure 4 presents the distribution of the diameter of the nanotubes fabricated at constant voltage of 20 V for 10 and 40 min. Similar as in the case of layers formed at 10 V, increase in anodization time results in narrow diam-eter distribution and obtaining more homogeneous oxide structure.

Figure 5 presents micrograph of nanotubular oxide structures manufactured at constant voltage of 30 V. The obtained nanotubes still possess regular spherical shape; however, they lose their or-dered arrangement. For the NT fabricated at 30 V for 10 min the estimated average diameter was 61 nm, whereas for the structures fabricated at 30 V for 40 min the values of measured dimensions increased and the average diameter was 101 nm and the wall thick-ness was 10.7 nm. The structures obtained at 30 V for 10 min did not possess fully formed nanotubular structure, the layer was po-rous therefore it was impossible to estimate the wall thickness. In Figure 6 distributions of the values of tubes diameter obtain at con-stant voltage of 30 V for 10 and 40 min are shown. Interestingly, longer time of anodization results in this case in obtaining broader

distribution of the pore diameter. However, it should be emphasized that the structures anodized for 10 min did not form fully nanotubu-lar structure but the porous ones. Figure 7 presents structures fabri-cated at 20 V and 30 V for 10 min. Such short time of anodization is not enough to obtain fully ordered nanotubes; in the micrographs the areas of initial stage of NT formation are clearly seen.

The most uniform nanotubular oxide structure was obtained by anodization at constant voltage of 20 V for 40 min. The decrease of characteristic dimensions values of the nanotubes with increas-ing fabrication time can be explained by the NT formation mecha-nism. In general, the process of formation of the nanotubes by elec-trochemical anodization can be divided into four stages. At first, a dense barrier oxide layer is formed. The following second step is selective chemical dissolution of oxide leading to formation of cy-lindrical nanopores. The next stage is creation of a bilayered struc-ture with nanotubes covered by the nanoporous layer. The last step is dissolution of the nanoporous layer which results in obtaining fully ordered nanotubular structure [17, 18]. In the micrographs pre-senting sample fabricated at 20 V for 10 min a typical structure with nanoporous layer (Fig. 7, area 2) covering the nanotubes (Fig. 7, area 1) can be observed. For the layer manufactured at 30 V, after 10 min of anodization structure with cylindrical nanopores is clear-ly visible (Fig. 7). In the initial stages of nanotubes formation, the values of average wall thickness and diameter of the oxide struc-tures are higher as they are influenced by nanoporous layer which is in the next steps dissolved to form thin wall of ordered nanotubes.



a) b)c)




Fig. 7. SEM micrograph of initial stage of nanotubes formation: a) 20 V, b) 30 VRys. 7. Zdjęcia SEM początkowych etapów powstawania nanorurek: a) 20 V, b) 30 V

To evaluate the chemical composition of the anodized oxide layer energy dispersive X-ray spectroscopy was conducted. Sam-ple with the most homogenous nanotubular structure fabricated at 20 V for 40 min was selected to perform the EDS analysis. The

percentage content of the various elements detected in the oxide layer is presented in Table 2. In comparison to the chemical com-position of the alloy before electrochemical treatment, the anodized layer has a higher content of oxygen and lower content of titanium.

a)

a)

b)

b)


The differences in content of alloying elements are not significant given that the depth of EDS analysis exceeds the layer thickness therefore the signal is disturbed by the substrate. Due to limitation of this method further investigation of the fabricated nanotubular layer need to be done to confirm its exact chemical composition. However, the obtained EDS results allow to indicate that chemical composition of the anodized nanotubes corresponds to the composi-tion of the substrate (Ti2448 alloy).

In order to examine the phase composition of the anodized layer, X-ray diffraction analysis was carried out. Examination was per-formed for the sample with the most uniform structure fabricated at 20 V for 40 min. The diffraction pattern obtained for annealed structure is presented in Figure 8. As-anodized layer possess an amorphous structure which changed into crystalline form after an-nealing in 450°C. In the diffraction pattern peaks from titanium β phase coming from the substrate can be clearly distinguished as well as peaks from titania which derived from the oxide layer. The ob-tained crystallographic form of the titanium oxide was anatase. This results corresponds to the conclusion found in the literature demon-strated that as-formed titania nanotubes usually possess an amor-phous structure which can be converted to anatase at temperature higher than approximately 280°C or a mixture of anatase and rutile at temperature higher than 450°C [19]. Ghicov et al. reported that elements such as niobium significantly increases the temperature of the crystalline transformation from anatase to rutile and shifts the temperature of structural collapse to a higher value. Therefore, the anatase–rutile transition in the Ti2448 alloy can take place at a higher temperature that in the in pure TiO2 nanotubes [20].

4. CONCLUSIONS

Anodic oxidation of the β-phase titanium Ti2448 alloy in a glycerol based electrolyte containing a suitable amount of fluoride ions leads to formation of a porous oxide layer in the form of nanotubes. The as-obtained nanotubular layer possess ordered structure and their growth is perpendicular to the substrate’s surface. It has been shown that ripples on the walls of the NT are formed. The morphology of the nanotubes depends on voltage applied and duration of the anodi-zation. For higher voltage applied, the nanotubes diameter is higher. For layers obtained at 10 and 20 V, increase in anodization time re-sults in narrow diameter distribution and obtaining more homogene-ous oxide structure as well as decrease of characteristic dimensions values (wall thickness and diameter). The most uniform nanotubu-lar structure was fabricated by anodization at 20 V for 40 min. The chemical composition of the anodized nanotubes corresponds to the composition of the substrate Ti2448 alloy. Annealing in 450°C re-sults in obtaining crystalline structure in form of anatase.

ACKNOWLEDGMENTS

This work was funded by The National Centre for Research and Development within PBS III initiative under the project „Model obiektu wodnego typu “stealth” o innowacyjnych rozwiązaniach w zakresie kształtu, konstrukcji i materiałów decydujących o jego trudno-wykrywalności”.

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Table 2. Chemical composition of the alloy and oxide layer Tabela 2. Skład chemiczny stopu i warstwy tlenkowej

Element Ti2448 alloy Oxide layer

Ti 64 41.0

O — 28.5

Nb 24 20.6

Zr 4 3.1

Sn 8 6.8

Fig. 8. XRD spectra of nanotubular oxide layer obtained at 20 V after annealing at 450°CRys. 8. Dyfraktogram XRD nanorurkowej warstwy tlenkowej wyt-worzonej przy napięciu 20 V po wygrzewaniu w 450°C

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[8] Karlinsey R. L.: Preparation of self-organized niobium oxide microstruc-tures via potentiostatic anodization. Electrochem. Commun. 7 (2005) 1190÷1194.

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[10] Macak J. M., Tsuchiya H., Ghicov A., Yasuda K., Hahn R., Bauer S., Schmuki P.: TiO2 nanotubes: Self-organized electrochemical formation, properties and applications. Curr. Opin. Solid State Mater. Sci. 11 (2007) 3÷18.

[11] Macak J. M., Hildebrand H., Marten-Jahns U., Schmuki P.: Mechanistic aspects and growth of large diameter self-organized TiO2 nanotubes. J. Electroanal. Chem. 621 (2008) 254÷266.

[12] Macak J. M., Tsuchiya H., Taveira L., Aldabergerova S., Schmuki P.: Smooth anodic TiO2 nanotubes. Angew. Chemie — Int. Ed. 44 (2005) 7463÷7465.

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[14] Khudhair D., Bhatti A., Li Y., Hamedani H. A., Garmestani H., Hodgson P., Nahavandi S.: Anodization parameters influencing the morphology and electrical properties of TiO2 nanotubes for living cell interfacing and in-vestigations. Mater. Sci. Eng. C. 59 (2016) 1125÷1142.

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[20] Ghicov A., Aldabergenova S., Tsuchyia H., Schmuki P.: TiO2–Nb2O5 nanotubes with electrochemically tunable morphologies. Angew. Chemie — Int. Ed. 45 (2006) 6993÷6996.


Nanorurkowe warstwy tlenkowe wytwarzane na stopie Ti–24Nb–4Zr–8Sn w procesie elektrochemicznej anodyzacji

Anna Matras1*, Agata Roguska2, Małgorzata Lewandowska1

1Wydział Inżynierii Materiałowej, Politechnika Warszawska, Warszawa, Polska, 2Instytut Chemii Fizycznej, Polska Akademia Nauk, Warszawa, Polska, *[email protected]

Inżynieria Materiałowa 1 (215) (2017) 2÷7DOI 10.15199/28.2017.1.1© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: nanorurki tlenku tytanu, elektrochemiczna anodyzacja, stopy tytanu, analiza powierzchni.

1. CEL PRACY

Elektrochemiczna anodyzacja jest prostą i uniwersalną metodą po-zwalającą na otrzymywanie porowatych warstw tlenkowych o upo-rządkowanej strukturze na powierzchni różnych materiałów meta-licznych, zarówno czystych metali, jak i stopów, w szczególności stopów tytanu. Preferowanymi stopami do utleniania anodowego są materiały jednofazowe, ponieważ nie ulegają niekorzystnemu selektywnemu rozpuszczaniu, które zachodzi dla stopów dwufazo-wych. W ciągu ostatnich kilku lat jednofazowe stopy β tytanu zy-skiwały coraz większą popularność ze względu na połączenie dużej wytrzymałości, małej gęstości oraz dobrej odporności na korozję, wykazując przy tym bardzo dobre właściwości przetwórcze.

W artykule scharakteryzowano nanorurkowe warstwy tlenko-we o uporządkowanej strukturze wytworzone w procesie elektro-chemicznej anodyzacji na stopie Ti–24Nb–4Zr–8Sn w elektrolicie na bazie gliceryny zawierającym jony fluorkowe. Przeprowadzo-no charakterystykę fizykochemiczną w celu oceny właściwości strukturalnych i chemicznych otrzymanych warstw. Zastosowano zmienne parametry procesu wytwarzania, tj. napięcie i czas ano-dyzacji, w celu określenia ich wpływu na morfologię i właściwości nanorurkowych warstw tlenkowych.

2. MATERIAŁ I METODYKA BADAŃ

W pracy wykorzystano jednofazowy stop tytanu o strukturze β Ti2448 (Ti–24Nb–4Zr–8Sn, % mas.). Nanorurkowe warstwy tlen-kowe wytworzono na płytkach o grubości 0,4 mm w procesie elek-trochemicznej anodyzacji w organicznym elektrolicie zawierają-cym glicerynę, wodę dejonizowaną oraz fluorek amonu. Wykorzy-stano zasilacz prądu stałego, aby utrzymać napięcie stałe o zadanej wartości (10, 20, 30 V) przez określony czas (odpowiednio 10 i 40 minut). Po procesie anodowania próbki wypłukano w wodzie dejo-nizowanej i wysuszono w powietrzu w celu usunięcia pozostałości resztek elektrolitu. Tak przygotowane warstwy zostały wyżarzone w temperaturze 450°C przez 3 godziny, aby uzyskać stabilne me-chanicznie nanorurki dobrze zintegrowane z podłożem.

W celu oceny morfologii otrzymanych warstw zastosowano ska-ningową mikroskopię elektronową (mikroskop SU-8000, Hitachi). Skład chemiczny wytworzonych nanorurek zbadano metodą mikro-analizy rentgenowskiej (EDS). W celu oceny zmian strukturalnych warstw zastosowano dyfrakcję promieni rentgenowskich (XRD), stosując promieniowanie Cu Kα (λ = 1.54 Å) W pracy oszacowano również charakterystyczne wymiary nanorurek, to znaczy średnicę, grubość i długość, korzystając z oprogramowania ImageJ.

3. WYNIKI I ICH DYSKUSJA

Nanorurki wytworzone przy potencjale 10 V (rys. 1) mają regu-larny kształt i uporządkowaną strukturę, można jednak wyróżnić

obszary z w pełni wykształconymi nanorurkami, jak również miej-sca w początkowym etapie ich powstawania. Warstwy tlenkowe otrzymane przy napięciu 20 V (rys. 3) są w pełni uporządkowane na całej powierzchni próbki i na przekroju poprzecznym mają okrągły, regularny kształt. Nanorurki wytworzone przy 30 V (rys. 5) wciąż mają kształt regularny, okrągły na przekroju, jednakże tracą swoje uporządkowane ułożenie. Dla wszystkich zastosowanych napięć zaobserwowano występowanie pierścieni (charakterystycznych „żeberek”) na ściankach. Wydłużenie czasu anodyzacji z 10 do 40 minut powoduje zmniejszenie średniej średnicy nanorurek dla warstw wytworzonych przy potencjałach 10 i 20 V. Jest to związane z mechanizmem powstawania tlenkowych warstw nanorurkowych w procesie anodowego utleniania. W początkowym etapie two-rzenia się nanorurek średnia średnica i średnia grubość ścianki są większe (tab. 1), ponieważ wpływa na nie nanoporowata warstwa, która w następnych etapach ulega rozpuszczeniu, tworząc cienkie ścianki uporządkowanych nanorurek. Analiza składu chemicznego metodą EDS (tab. 2) ujawniła wzrost zawartości tlenu w anodyzo-wanych warstwach w porównaniu ze stanem wyjściowym podłoża. Wyniki wskazują, że otrzymana warstwa jest zbudowana z mie-szaniny tlenków metali występujących w podłożu (Nb, Zr i Sn). Analiza XRD (rys. 8) wykazała, że wygrzanie warstw w 450°C spowodowało przekształcenie amorficznych nanorurek w strukturę anatazu. Obecność w tym stopie Nb znacząco podnosi temperaturę przemiany anatazu w rutyl i zwiększa zakres temperatury, w której warstwa może być wygrzewana bez doprowadzenia do zniszczenia nanorurkowej struktury.

4. PODSUMOWANIE

Anodowe utlenianie jednofazowego stopu tytanu β Ti2448 w elek-trolicie na bazie gliceryny zawierającym odpowiednią ilość jonów fluorkowych prowadzi do uzyskania porowatej warstwy tlenku w postaci nanorurek. Tak wytworzone warstwy mają uporządkowa-ną strukturę, a ich wzrost jest prostopadły do podłoża. Zaobserwo-wano charakterystyczne dla nanorurek wytwarzanych w elektroli-tach o dużej zawartości wody obręcze („żeberka”) na ściankach na-norurek. Morfologia nanorurek jest zależna od przyłożonego napię-cia i czasu trwania procesu. Wraz ze zwiększeniem przyłożonego napięcia otrzymywane nanorurki charakteryzują się większą śred-nicą. W przypadku warstw wytwarzanych przy napięciu 10 i 20 V wydłużenie czasu anodyzacji z 10 do 40 minut doprowadziło do uzyskania bardziej jednorodnej struktury z wąskim rozkładem śred-nicy nanorurek. Najbardziej jednorodną i uporządkowaną strukturę otrzymano dla warstwy wytwarzanej przy napięciu 20 V przez 40 minut. Skład chemiczny nanorurek odpowiadania mieszaninie tlen-ków metali z podłoża (Nb, Zr i Sn). Wyżarzanie w temperaturze 450°C spowodowało zmianę amorficznych nanorurek w krystalicz-ną strukturę anatazu.

fabrication of nanotubular oxide layer on ti–24nb–4zr–8sn alloy … text.pdf · 2018. 1....

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