Ti-Mo-O Nanotube Arrays Grown by Anodization of Magnetron Sputtered Films

Abstract

In this study, we introduced the method for the growth of titanium molybdenum oxide (TMO) nanotubes directly from metallic precursor solid state solution and provided their structural and chemical characterization. Precursor films with content of molybdenum from 32 to 82 at% were prepared using co-deposition magnetron sputtering. The optimization of deposition parameters allowed for the growth of a continuous nanotube array with a length up to 700 nm ± 10% by anodic oxidation. Scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS) revealed nanotube formation with Ti_1−xMo_xO₂ composition, where x can reach the value of 0.5. Scanning transmission electron microscopy combined with EDS (STEM-EDS) confirmed the incorporation of Mo into the TiO₂ lattice and uniform elemental distribution across the nanotube at the submicron level. The nanobeam electron diffraction (NBD) and X-ray diffraction analyses (XRD) did not show any notable crystal phase formation for the titanium molybdenum oxide phase.

1. Introduction

The development of energy-saving and renewable technologies is currently one of the main priorities of the power generation industry. The utilization of solar energy is a great opportunity to create clean and renewable energy sources. However, the direct conversion of solar light into electric power is not the only renewable energy source currently in the spotlight. Since the discovery of the electrochemical photolysis of water in 1972 [1], many studies have been focused on improving photocatalytic efficiency. The decomposition of molecules under the influence of sunlight is used in many areas, including ecology, energy generation, and medicine [2]. One of the most promising materials for practical applications is the titanium dioxide-based photo catalyst, which can be used effectively for environmentally friendly energy generation. However, pure TiO₂ shows satisfactory photocatalytic properties only under UV light due to its relatively large bandgap of 3.2 eV. Nevertheless, it possesses suitable band-edge positions for many redox reactions, a comparably high lifetime of excited electrons, and an exceptional photo corrosion resistance [3]. To broaden the use of titania nanostructures in the manufacture of chemical sensors and photocatalytic applications, a number of factors, including the conductance of TiO₂ in air, the sensing signal, the response and recovery durations, must be enhanced. One of the ways to improve the sensory and photocatalytic properties of titanium dioxide nanotubes is doping with metals such as Mo [4], Mn [5], Cr [6], V [7], Ni [8], Fe [9], and valve metals [10,11]. In the form of nanotube arrays, TiO₂-based materials are even more promising compared to particle-like structures due to their better incident light utilization and increased effective surface area. Therefore, they are now popular for use in electrochemical, catalytic, and sensory applications, and also as antimicrobial material [12].

As reported, metallic titanium and molybdenum form solid state solutions in a wide range of concentrations [13]. Molybdenum has also been found to be a promising dopant for improving the photocatalytic performance of TiO₂. Ti⁴⁺ in the TiO₂ structure can be replaced by Mo⁶⁺ because their ionic radiuses are similar (Mo⁶⁺ 0.62 Å, Ti⁴⁺ 0.68 Å) [4,14,15,16,17,18], which leads to a change in the lattice parameters and the bandgap structure. Additionally, Mo is expected to increase the mechanical and physicochemical stability of the material, allowing it to operate under harsher conditions [19].

Anodic oxidation nanotube growth is a well-developed method for aluminum and titanium [20]. The process is relatively inexpensive compared to other nanotube growth techniques. Moreover, the process can be used to create a variety of nanotube structures, such as single-walled nanotubes (SWNTs) [21], double-walled nanotubes (DWNTs) [22], and multi-walled nanotubes (MWNTs) [23]. Doping of titanium dioxide with a low percentage of various elements, including iron or molybdenum, has. Been investigated by a number of scientific groups [4,5,6,7,8,9,24,25,26,27,28,29]. However, it those publications, the value of the dopant typically did not exceed 3 at% In this paper, we used segmented target magnetron sputtering to prepare a metallic alloy with high molybdenum content. We decided to use molybdenum instead of iron, which may produce better photoreactive results (according to [30]) because the ferromagnetic properties of iron can significantly influence the geometry of the discharge and make deposition hardly repeatable. Anodic oxidation was used to prepare TiO₂-based nanotubes with high molybdenum alloying of up to 30 at%. We decided not to modify the grown TiO₂ nanotube but to directly prepare the alloyed nanotubes for a solid-state solution precursor. It is possible to expect changes to the bandgap value due to the significant presence of Mo in the lattice compared to that of TiO₂. This will lead to more efficient utilization of solar light and an improvement of photocatalytic and antimicrobic properties [30].

2. Materials and Methods

A combined method of DC magnetron sputtering and anodic oxidation was employed for the preparation of Mo-alloyed TiO₂ nanotubes (Figure 1). The overall process consisted of two main steps: precursor formation and anodic oxidation.

2.1. DC-Magnetron Sputtering

Precursor films were fabricated on a polished Si (100) substrate. Prior to the deposition process, the substrates underwent a cleaning procedure including ultrasonic cleaning in acetone, isopropanol, and distilled water. The base vacuum pressure inside the chamber was <5 × 10⁻⁴ Pa. During the sputtering process, argon (99.999% purity) pressure was maintained at 0.9 Pa. The deposition of Ti-Mo mixed films was performed utilizing unbalanced magnetron Torus 2” (Kurt J. Lesker, Jefferson Hills, PA, USA). Instead of using a compound Ti-Mo target, the Ti target (99.95% purity) was covered with a strip of molybdenum (99.5% purity). The amount of molybdenum in the resulting film was adjusted using strips (Figure 1A) with various widths from 10 to 16 mm.

2.2. Anodic Oxidation

The anodization was performed in an ethylene glycol-based electrolyte containing 0.5 wt% ammonium fluoride (97 wt% purity) under similar conditions to those described in [24]. The setup for anodic oxidation is shown in Figure 1B. To avoid rapid dissolution of nanotubes, at the initial phase of oxidation the current density was limited to 3 µA/cm², while the voltage was increased to the required value. Then, the voltage was kept constant for 30 min. The optimal oxidation parameters were obtained at room temperature with a voltage of 40 V and a constant distance of 20 mm between the sample and the cathode.

2.3. Characterization

The surface morphology of all samples was characterized using field emission scanning electron microscopy (FE-SEM, LYRA III, TESCAN s.r.o., Brno, Chech Republic) and the elemental composition was estimated using energy dispersive X-ray spectroscopy (EDS) (Bruker Nano, Bruker GmBH, Mannheim, Germany).

Lamellas for transmission microscopy were milled in a TESCAN LYRA III with a Ga+ ion beam. The maximum thickness of the TEM sample was 80 nm.

Microstructural characterization (STEM, STEM-EDS, and nanobeam electron diffraction (~100 nm beam width)) was performed using a probe-corrected FEI/TFS Titan Themis 300 electron microscope operated at 200 kV accelerating voltage.

The crystalline phases were identified (The PA Nalytical X’Pert Pro (Malvern Panalytical, Malvern, UK) with Cu Kα source, incidence angle −1.5° was used).

3. Results and Discussion

In our previous study [31], the optimal deposition parameters for pure titanium films were discussed, which served as the starting point for the Mo-alloyed precursor films deposition. These parameters included a magnetron voltage of 650 V and a current of 100 µA, with a target-to-sample distance of 20 mm. Initial Ti-Mo films prepared in the mentioned conditions exhibit a columnar structure that is typical for metal films produced by magnetron sputtering [32]. However, such a structure is not ideal for the subsequent anodic oxidation process. In the anodic oxidation of titanium and molybdenum, fluorine-based electrolytes were utilized, as described in Section 2.2. It is known that fluorine tends to travel along grain boundaries [33], which can interfere with the reaction between the metal (Ti, Mo) and oxygen. To mitigate the formation of columnar grains and promote a more favorable structure for anodic oxidation, we followed Anders’ structure zone model [34] and increased the energy flux during deposition. In this context, we refer to the well-known effect of nanopores suppression due to the local rearrangement of atoms by ion impact in metal layers [34,35,36].

To achieve this, we employed a combination of two methods: reducing the target-to-sample distance and increasing the magnetron power. Additionally, a bias voltage was applied. Figure 2A illustrates the influence of the sample–target distance on the deposition rate, as well as the effect of voltage bias on the Ti-Mo ratio. Optimization was performed on the setup with a 12 mm wide molybdenum strip.

By reducing the target-to-sample distance (Figure 2A,B) and increasing the energy of Ar ions accelerating towards the growing films, the deposition rate of the films can be adjusted. This allows for the control of film thickness and promotes the formation of a more favorable structure for subsequent anodic oxidation. Moreover, the application of a bias voltage during deposition (Figure 2C) can further influence the composition of the Ti-Mo films. These modifications in deposition parameters and energy flux management are crucial for tailoring the structure and composition of the Ti-Mo films to enhance their suitability for the desired applications, such as anodic oxidation and subsequent nanotube array growth.

It is possible to see a significant increase of deposition rate at short target–sample distances (Figure 2A) but even the highest deposition rates do not sufficiently inhibit the formation of columnar structures (see Figure 3A). As a result, a shapeless oxide—instead of nanotube array—was obtained later during oxidation.

The application to the sample of a negative bias up to 100 V, while the other sputtering parameters were kept unaltered, changed both the structure and composition (Figure 2C). The relatively light Ti atoms are more prone to re-sputtering [31,33] by impinging ions than the heavier Mo atoms; therefore, increasing the bias voltage could reduce the relative amount of Ti in the coating. However, the main purpose of bias application is to suppress the columnar structure, as described above. The optimal combination of composition, deposition rate, and film structure was obtained at −75 V bias. An indication of the difference in structure is shown in Figure 3A,B, where the layers without biasing show a sharper boundary between individual columnar grains compared to those with applied bias. Moreover, in the following text, we show that we were able to prepare nanotubes on precursor layers prepared with bias, which provides additional indirect evidence of nanopores suppression. The samples deposited in the set-up with a 12 mm Mo strip with applied bias shows a composition of Ti_0.5Mo_0.5 with negligible oxygen content, according to the EDS analysis.

Figure 3D shows the grazing incidence X-ray diffraction (GIXRD) pattern of the precursor film, where several peaks are visible. Blue vertical lines indicate the database position of peaks for molybdenum (COD 1512521) and the green lines for titanium (COD 96-151-2548). In both cases, it is a cubic system with space group Im–3m. The measured peaks are in between these two phases; this indicates the presence of a single phase of Ti_xMo_1−x, probably a solid solution, which corresponds with the binary phase diagram [11]. No observable peak splitting indicates the presence of a single phase. From the position of peaks and lattice parameters for Mo (a = 3.1472 Å) and Ti (a = 3.3065 Å), the lattice parameter of Ti_xMo_1−x can be determined according to Vegard’s law as:

$$a_{{Ti}_x{Mo}_{1-x}}=x{a}_{Ti}+\left(1-x\right)a_{Mo}=0.5\times 3.3065+(1-0.5)\times 3.1472=3.2269\AA .$$

(1)

This is in fair correspondence with the EDS analysis shown in Figure 3C and confirms the formation of the single phase in the binary alloy. On this basis, we deposited a set of precursors with various compositions for anodic oxidation. The modification of the metals ratio was achieved by changing the width of the molybdenum strip. For our particular system, the width range was 10 to 16 mm. The rest of the deposition parameters were kept unchanged. The precursors’ concentrations are shown in

Table 1. Composition of precursors and oxidized and nanotube arrays measured by SEM-EDS.

		Ti at%	Mo at%	O at%	C at%	F at%	Ti/Mo Ratio
sample A	precursor	65.8	34.2				1.9
	oxidized	19.0	9.6	57.9	6.5	7.0	2.0
sample B	precursor	47.3	52.7				0.9
	oxidized	13.6	14.5	55.8	12.7	3.1	0.9
sample C	precursor	36.3	63.7				0.6
	oxidized	17.2	16.5	56.1	6.3	3.6	1.0
sample D	precursor	26.8	73.2				0.4
	oxidized	14.1	25.1	49.7	9.4	1.2	0.6
Sample E	precursor	15.4	84.5				0.2
	oxidized	10.4	17.9	60.0	9.3	2.4	0.6

.

Precursor samples with concentrations of molybdenum of 34.2 at% (A), 52.7 at% (B), 63.7 at% (C), and 73.2 at% (D) showed nanotube formation after oxidation under the conditions described in Section 2.2. The top view and cross-sections are shown in Figure 4. The precursor films were not oxidized down to the substrate to avoid possible delamination; therefore, some residual Ti-Mo layer was still present under the nanotube array. As can be seen from Table 1, the Ti-Mo ratio remained almost unchanged after oxidation and the samples consisted of well-defined nanotubes with a diameter of 50 nm ± 5% (A, B, C). On the other hand, samples with high concentrations of Mo (D, E) showed significant changes. The mechanism of Ti-O nanotube formation is well known [37,38], and similar behavior is expected for nanotubes with lower concentrations of Mo. The growth includes two competitive processes:

Formation of oxide layer:

Ti + 2H₂O → TiO₂ + 4H

(2)

Dissolution of oxide layer:

TiO₂ + 6F + 4H → [TiF₆]²⁻ + 2H₂O

(3)

F-ions are attracted to the anode by applied positive voltage; therefore, reactions proceed in a defined direction, forming a nanotubular array. According to Figure 4, the Ti-driven mechanism works up to a concentration of approximately 60 at% of molybdenum. At higher concentrations of molybdenum, reactions with Mo become prevalent.

Mo + 3F₂ → MoF_6.

(4)

MoF₆ + 3H₂O → MoO₃ + 6HF

(5)

In the second case, a significant amount of volatile oxide MoO₃ was created, resulting in a decrease of Mo content, and well-defined nanotubes were not formed.

For a detailed investigation of nanotube structure, thin lamellas were prepared on sample B (with a thickness of less than 80 nm, aiming for single-nanotube thickness) using the FIB tool. Both micrographs in Figure 5A,B present a homogenous structure without any apparent grain formation The nanobeam electron diffraction (NBD) pattern, probing only the tube array, is dominated by the broad diffuse maxima typical of amorphous material (Figure 5C). The amorphous nature of the as-grown/oxidized nanotube array is also confirmed by the GIXRD pattern (Figure 6, black). The XRD pattern does not show any other notable maxima different from the precursor pattern (Figure 6, blue). A detailed study using STEM-EDS showed the uniform distribution of Ti and Mo across the nanotube, even at then a no meter scale (Figure 7). The presence of fluorine at the inner edges of the nanotubes is also visible. So, it is possible to assume that the influence of the bottom not-oxidized layer does not significantly contribute to the results of the SEM-EDS analysis.

4. Conclusions

In this study, we presented a new method for the fabrication of an Mo-alloyed TiO nanotube array on the basis of precursors deposited by magnetron sputtering with partial target covering. In our method, nanotubes were grown directly from a Ti-Mo solid state solution—contrary to the methods of other publications—which makes a high dopant content in the lattice possible.

As a precursor, 500 nm thick Ti-Mo films with Mo concentrations in the range of 32 at%–83 at% were prepared. Although the columnar character of the precursor was suppressed by applying a negative bias during deposition, X-ray analysis of the films confirmed the formation of a crystalline BCC phase.

Ti_1−xMo_xO₂ nanotubes with an average diameter of 50 nm ± 5% and an average length of up to 700 nm ± 10% were obtained during an anodization process that lasted 30 min and used ethylene glycol electrolyte containing 0.5 wt% NH₄F at 40 V.

The grown nanotubes kept the Ti-Mo ratio close to that of precursor values, up to Ti_0.5Mo_0.5O₂. Precursor samples with higher concentrations of Mo did not form nanotubular structures after the anodic oxidation process.

A deeper examination of the nanostructure showed a homogeneous distribution of Ti and Mo in the formed nanotubes, which confirmed the well-controlled synthesis processes. In addition, the mutual interaction between both metals and oxygen led to the formation of amorphous oxide phases.

In the case of films with a higher concentration of molybdenum (>72 at%), anodic oxidation processes did not lead to the formation of nanotubes.

NBD and XRD analysis did not show any notable formation of crystal structures of titanium molybdenum oxide phase.