Introduction

The search for cheap, efficient and novel nanomaterial fabrication technology has been motivated intensively in various fields of nanotechnology. TiO2 has been extensively investigated as a key semiconductor material in water photosplitting, photocatalyst, lithium-ion batteries, gas sensor and photovoltaic cells due to its excellent properties, low cost, environmental friendliness and the fact that it is widely available1,2,3,4,5,6. The ability to grow oriented single crystal TiO2 nanowire/nanorod arrays is highly desirable for above applications because of their superior photoelectronic properties7,8,9,10,11. To date, the most effective strategies involve the template-assisted electrodeposition route12, chemical vapor deposition13,14 and hydrothermal/solvothermal methods11,15,16,17,18,19,20,21,22,23,24,25. However, these approaches are not very ideal considering their following disadvantages. For example, chemical vapor deposition method needs high temperature reactions, high energy cost and has low productivity. The template-assisted electrodeposition route needs complex and tedious multi-step procedures involving the preparation of anodic aluminum oxide (AAO) template, electrodepostion and removal of AAO template. In addition, heat treatment is also required for the amorphous to crystalline phase transformation. The hydrothermal/solvothermal synthesis are normally performed in high-pressure stainless steel vessel called an autoclave since they are usually conducted under a temperature higher than the boiling temperature of the solvent, which limit their safety and large-scale production. Furthermore, for the typical hydrothermal synthesis of TiO2 nanowires, the use of high concentration of strong alkaline (e.g. sodium hydroxide)15,16 or strong acid (e.g. hydrochloric acid)11,18,19,20,21,22,23,24,25 can cause corrosion of equipment and the industrial waste discharge. Therefore, it is highly desirable to develop simple, low cost, high yield and environmentally friendly synthesis methods for direct fabrication of oriented single crystal TiO2 nanowire arrays at low temperature and preferably under atmospheric conditions.

Results

Here we present for the first time a unified approach for the confined-space growth of the single crystal TiO2 nanowires using the saturated fatty acids such as lauric acid (C12H24O2), myristic acid (C14H28O2), palmitic acid (C16H32O2) and stearic acid (C18H36O2). The pictured fabrication process for the TiO2 nanowires can be readily seen in Fig. 1. The salient features of our synthesis strategy is that well crystal TiO2 nanowire structures can be easily fabricated in atmospheric glass vessel at low temperature by cleverly manipulating the unique physical properties of saturated fatty acids (Supplementary Table S1). Taking lauric acid as an example, firstly, its melting point must be very low (e.g. 44°C) so that we can melt the solid saturated fatty acids into liquid phase easily and more importantly, the hydrolysis of dropped Ti precursor can be avoided at such low temperature. Secondly, its density (e.g. 862 kg/m3) is necessary to be lower than the aqueous TiCl3 solution so as to float on the top of the aqueous TiCl3 solution. Thirdly, it must be water-insoluble so as to form liquid-liquid interface with the aqueous TiCl3 solution. Fourthly, the boiling temperature (e.g. 298.9°C) should be much higher than the reaction temperature (60–150°C) so that melted fatty acids can form the stable sealing layer to confine the produced water vapor around aqueous TiCl3 solution during the reaction. And then self-hydrolysis of the aqueous TiCl3 solution, TiO2 nucleation and subsequent crystallization can be realized in such sealed confined-space. Finally, saturated fatty acids should have straight chains and their molecules can be packed together very tightly, allowing TiO2 nuclei to grow within the saturated acid matrix and fused together into one-dimensional structures. Therefore, the saturated fatty acid plays a dual role in above TiO2 nanowire synthesis: on the one hand, it forms a “soft” sealing layer to confine the water vapor around the TiCl3 solution and then produce enough pressure to favor crystallization in this small confined space. On the other, it also acts as a soft template to induce the formation of one-dimensional structure. Therefore, our unique methods are different from the conventional approaches such as traditional calcination, chemical vapor deposition and hydrothermal methods which require high temperature treatment or high pressure vessel to complete crystallization.

Figure 1
figure 1

Schematic illustration for the confined-space synthesis of TiO2 nanowires in atmospheric glass vessel at low temperature.

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Figure 2 shows field emission scanning electron microscopy (FESEM) images of products fabricated by using different saturated fatty acids such as lauric acid (Fig. 2a and b), myristic acid (Fig. 2c and d), palmitic acid (Fig. 2e and f) and stearic acid (Fig. 2g and h), respectively. As expected, all the samples show morphologies of highly oriented TiO2 nanowire array since these saturated fatty acids have similar physical properties as described in Supplementary Table S1. Figure 2a, c, e and g are side view FESEM images of highly oriented TiO2 nanowire arrays and the length of the oriented nanowire arrays is ranging from 3 to 6 μm. Figure 2b, d, f and h are top view of the same samples, showing that the film is composed of uniform and well-separated nanowires. Figure 3 shows the X-ray diffraction (XRD) patterns of all above samples and all the diffraction peaks match well with those of the tetragonal rutile TiO2 phase (JCPDS no. 75–1750).

Figure 2
figure 2

FESEM images of the as-synthesized TiO2 nanowires grown in atmospheric glass vessel by using 0.3 mL of TiCl3 solution and 15 g of different saturated fatty acids at 150°C for 12 h.

(a) and (b) lauric acid, (c) and (d) myristic acid, (e) and (f) palmitic acid, (g) and (h) stearic acid.

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Figure 3
figure 3

XRD pattern of the TiO2 nanowires fabricated in atmospheric glass vessel by using different saturated fatty acids at 150°C for 12 h.

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Lauric acid was chosen as an example to synthesize TiO2 nanowires for further investigation of their characteristics and properties. Figure 4 shows transmission electron microscopy (TEM) image, selected area electron diffraction (SAED) and high resolution TEM image of as-prepared TiO2 nanowires. SAED confirms that TiO2 nanowires have the single crystal rutile structure (Fig. 4a). It is obtained from Fig. 4b that the diameter of nanowires is about 10 nm. Moreover, the distance between the adjacent lattice fringes is 0.325  nm, which can be assigned to the interplane distance of the rutile TiO2 (110) plane. The angle between the (110) plane and the growth direction [001] was measured to be 90°, indicating that the growth direction of the TiO2 nanowires is along the [001] direction.

Figure 4
figure 4

Representative TEM results of TiO2 nanowires produced with 0.3 mL of TiCl3 solution and 15 g of lauric acid in atmospheric glass vessel at 150°C for 12 h.

(a) TEM image and corresponding SAEDs of nanowires. (b) HRTEM image of the TiO2 nanowires.

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The TiO2 nanowires show a type IV isotherm with H3 hysteresis loop (Supplementary Fig. S1), which implies the existence of mesopores (2–50 nm). The pore size distribution was derived from the desorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) method (the inset in Supplementary Fig. S1). The corresponding pore size peak is 1.59, 2.73, 3.43, 6.85 and 8.63 nm, respectively, indicating the highly mesoporous nature of the TiO2 nanowires. As calculated by the Brunauer-Emmett-Teller (BET) method, such porous structures give rise to a high specific surface area of 60.6 m2 g−1. The thermal gravimetric analysis (TGA) were used to investigate the residual organic species existed in mesoporous structure of nanowires, as shown in Supplementary Fig. S2. The as-prepared sample contains only 7 wt% of organic species and the weight loss of 1.5 wt% around 100°C can be attributed to the evaporation of adsorbed moisture. Since the amount of residual organic species in nanowires is very small, we inferred that the formation of the mesoporous structure is attributed to the void of inter-nanowires instead of residual lauric acid.

To investigate the formation process of the TiO2 nanowires, time-dependent experiments were carried out, as shown in Supplementary Fig. S3. The morphology of products collected at early stage exhibited separated flower-like nanowire aggregates with a diameter of about 1.5 μm after 1 h of reaction (Supplementary Fig. S3a). Interestingly, prolonging the growth time to 2 h, these flowers were bound together and had a tendency to form oriented nanowire thin film (Supplementary Fig. S3b). As the reaction proceeded to 3 h, the ordered degree of nanowires increased further (Supplementary Fig. S3c). With an increase of reaction time up to 6 h, the oriented TiO2 nanowire arrays were finally formed (Supplementary Fig. S3d). When the growth time was further extended to over 12 h, the length and orientation of the TiO2 nanowire arrays were identical without any change (Fig. 2a). At that time, the total reaction reached the equilibrium.

Supplementary Figure S4 shows the SEM images of the products after reaction of 0.3 mL of TiCl3 precursor in 15 g of Lauric acid for 12 h at temperatures varying from 60°C to 120°C. At lower temperature of 60°C and 80°C, nanowire aggregates accompanied by many nanodots began to form (Supplementary Fig. S4a, b). When increasing the temperature to 100°C, the fully well separated nanowires were formed through the Ostward ripening process26 and the accompanying nanodots began to disappear (Supplementary Fig. S4c). When the temperature was further increased to 120°C, oriented nanowire arrays formed (Supplementary Fig. S4d). Similar to the time dependent experiments, there were almost no changes of the length and orientation of the TiO2 nanowire arrays while the reaction temperature was further increased up to 150°C (Fig. 2a).

We have also investigated the effect of various concentration of TiCl3 solution on the formation of TiO2 nanowire film. While the concentration of TiCl3 solution decreased from 9% to 6% and to 3%, the TiO2 nanowire array films were still obtained (Supplementary Fig. S5a, b and c). With further decrease of the concentration to 1%, only small amount of TiO2 nanowires formed on the partial area of film (Supplementary Fig. S5d). Therefore, the nanowire growth kinetics is also possibly governed by the concentration of TiCl3 solution and the low concentration of TiCl3 solution can increase their partial vapor pressure and evaporation rate. The fast evaporation caused relatively low pressure around the TiCl3 solution, which is unfavorable to the formation of nanowires. To investigate the relations between the evaporation rate and the confined vapor pressure around TiCl3 solution, we added the 15 g and 50 g lauric acid in the vessel without lid to seal TiCl3 solution, respectively. By adding 15 g lauric acid and with lid off, only irregular shaped structures were produced. We noticed that there are untrafine vapor bubbles around TiCl3 solution. These bubbles might slowly push their way out of the lauric acid and into the atmosphere, leading to relatively low pressure around TiCl3 solution, which is unfavorable to the TiO2 nanowire formation. In the our original experiments, with the lid on and 15 g lauric acid added, even small amount of vapor escaped from the sealing layer, but they could be confined in the upper space of the vessel by the lid, which would increase the pressure above lauric acid. Therefore, most of the vapor molecules colliding with lauric acid would not have sufficient kinetic energy to overcome this soft sealing layer and they would stick to the surrounding area of TiCl3 solution, thereby, the evaporation rate with lid is much slower than that without the lid, which would produce high pressure in small area around TiCl3 solution and thereby favor the formation of TiO2 nanowires. Remarkably, when the amount of lauric acid increased to 50 g, even with the lid off, the soft sealing layer was thick enough to confine vapor molecules around TiCl3 solution, which has the same effects as conditions with lid on and 15 g lauric acid added. As shown in Supplementary Fig. S6, well crystallized TiO2 nanowire film was obtained.

Considering the potential application in future, we also designed the following experiments. Firstly, our method only utilizes the physical properties of saturated fatty acid instead of chemical reaction to favor the formation of nanowires. From point of economic view, we also recycled the previously used lauric acids and covered onto fresh TiCl3 solution to grow nanowires in the vessel. Amazingly, perfect TiO2 nanowire film can also be grown by using the recycled lauric acid (Supplementary Fig. S7). Secondly, we added much more TiCl3 solution in the same vessel to grow nanowires and then the TiO2 nanowrie production and film thickness increased accordingly. As shown in Supplementary Fig. S8, the mass of obtained film is 0.026 g, 0.065 g, 0.194 g and 0.239 g after 0.3 mL, 1 mL, 3 mL and 5 mL TiCl3 solution reacted in the vessel, respectively. In addition, the thickness TiO2 nanowire film is up to 35 μm while the 5 mL of TiCl3 solution was added (Supplementary Fig. S9).

The above-mentioned TiO2 nanowire structures can also be synthesized on arbitrary substrates such as silicon substrate, fluorine doped tin oxide (FTO) glass, Ti foil and Teflon substrate. As shown in Fig. 5a and b, hierarchical TiO2 microspheres assembled from oriented nanowires were formed on silicon substrate. In addition, the oriented TiO2 nanowire arrays can also be grown on FTO glass substrate coated with thin TiO2 seed layer (Fig. 5c), which is similar to that fabricated by using hydrochloric acid assisted hydrothermal method18. While this synthetic method was applicable to the Ti foil, flower-like nanowire aggregates were formed (Fig. 5d, e). Interestingly, a two-side oriented nanowire arrays can be produced on Teflon substrate (Fig. 5f), which was similar to that obtained by reacting titanium powder with a mixture solution of H2O2 and HCl under hydrothermal condition27. The different morphology of nanowire aggregates formed on arbitrary substrates are due to the different surface property of the substrates, which are also of great importance to nanowires in wider application such as water photosplitting, photocatalyst, gas sensor and photovoltaics.

Figure 5
figure 5

Confined synthesis of TiO2 nanowires on arbitrary substrates by using 0.3 mL of TiCl3 solution and 15 g of lauric acid in atmospheric glass vessel at 150°C for 12 h.

(a) and (b) silicon substrate, (c) FTO glass, (d) and (e) Ti foil, (f) Teflon substrate.

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Discussion

As we described before, the confined growth of the single crystal TiO2 nanowires are conducted by using the saturated fatty acids such as lauric acid, myristic acid, palmitic acid and stearic acid, which is different from the conventional hydrothermal method. Then, there are three issues need to be addressed. One is why the TiO2 nanowires possess rutile structure instead of anatase structure. The second question then arises what is the underlying factor that affects the formation of nanowire structures and whether unsaturated fatty acids (e.g. oleic acid) can give rise to the same results since they also have the similar physical properties with the above saturated fatty acids? The third issue is why the low reaction temperature can lead to high crystallinity in atmospheric glass vessel. For the first issue, TiCl3 solution can easily be hydrolyzed and oxidized to octahedral monomer ([TiO(OH2)5]2+) (rutile nuclei)28. According to the crystal structure of anatase and rutile TiO2, the rutile TiO2 crystalline structure is the linear chains of TiO6 octahedra by sharing equatorial edges, while anatase is the spiral chain of apical edge sharing TiO6 octahedra. In the interfacial region of saturated fatty acid and TiCl3 solution, linear chain arrangement of [TiO(OH2)5]2+ octahedral monomers (rutile nuclei) can be easily shared equatorial edges than apical edge sharing spiral structures due to the geometric confinement of fatty acid molecular with similar line chain structure. For the second issue, the boiling temperature of oleic acid is 360°C (at 760 mmHg), in addition, it is water-insoluble and lighter than water (Supplementary Table S1) and it also seems to be suitable for growing TiO2 products by our confined-space synthesis method in atmospheric glass vessel at low reaction temperature. To appreciate this, we also investigated the reaction of the TiCl3 solution confined by oleic acid in atmospheric glass vessel at temperature of 150°C for 12 h. As we expected, the crystallization phase of obtained sample was also rutile (Fig. 6a). However, the as-prepared aggregates were distributed in random, as shown in Fig. 6b. Oleic acid is typical unsaturated fatty acid, therefore, we infer that the differences in geometry between saturated and unsaturated fatty acids play an important role in the construction of nanostructures. For saturated fatty acid, it is an amphiphilic linear monomer, containing a hydrophobic alkyl group (-CH3) at one end, a long hydrocarbon chain and a terminal carboxylate group (-COOH) at the other end. There are no double bonds between the carbon atoms in saturated fatty acid chain as the carbons are fully “saturated” with hydrogen atoms. As a result, saturated fatty acids form straight chains and can be packed together very tightly. In addition, TiCl3 precursor itself contains an amount of water and the hydrolysis can easily occur. Accordingly, with the increase of temperature and time, TiO2 nuclei can diffuse within the saturated fatty acid matrix and fuse together into the one-dimensional nanowires during the continuous reaction process. However, Oleic acid (18:1 n−9) has 18 carbon atoms with the first double bond occurring at the ninth carbon atom away from the carboxylic acid group and thus creating bent chains (Fig. 6c). Therefore, their molecules cannot be packed tightly together and the formed TiO2 products are particle aggregates. For the third issue, both the saturated fatty acid and unsaturated acid can play a role of “soft” sealing layer to confine the water vapor around the TiCl3 solution and then produce enough pressure to favor crystallization in the small confined space. However, their different is that saturated fatty acid with straight line chains also acts as a soft template to induce the formation of one-dimensional structure. The same concepts can also be applied to icosane (CH3(CH2)18CH3) in our method by considering its straight-chain saturated hydrocarbon structure and similar physical properties to saturated fatty acids (Supplementary Table S1). As shown in Figure 7, the TiO2 nanowire arrays were also obtained by using icosane as soft sealing layer to confine the TiCl3 solution.

Figure 6
figure 6

The confined-synthesis of TiO2 samples in 0.3 mL of TiCl3 solution and 15 g of oleic acid in atmospheric glass vessel at 150°C for 12 h.

(a) XRD measurement. (b) FESEM images. (c) Schematic illustration for molecular structure of oleic acid. Here, the yellow, blue and red balls represent C, H and O atoms, respectively.

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Figure 7
figure 7

The TiO2 nanowire arrays obtained by using 15 g of icosane to confine 0.3 mL of TiCl3 solution in atmospheric vessel at 150°C for 12 h.

(a) Cross-sectional FESEM images of the TiO2 nanowire arrays. (b) Top view FESEM images of the TiO2 nanowire arrays. The inset is corresponding high magnification FESEM image.

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Conclusions

In summary, we have presented for the first time the confined-space synthesis of oriented single crystal TiO2 nanowires in atmospheric glass vessel at low temperature by cleverly manipulating the unique physical properties of saturated fatty acids and icosane. Moreover, the synthetic methodology we have developed is scalable, long-chain saturated fatty acids or n-alkane, such as lauric acid, myristic acid, palmitic acid and stearic acid and icosane, could provide a general confined-space strategy for the growth of TiO2 nanowires in atmospheric glass vessel at low temperature. Our unique procedure is performed under mild conditions without the use of high temperature, high pressure vessel and strong acids, thus saving energy and cost. In addition, we demonstrate the growth of TiO2 nanowires on arbitrary substrates. We believe that these findings are of great importance for nanowires in wider applications. We found that the unsaturated fatty acids can also facilitate the crystallization of products, but their bent chains lead to the formation of TiO2 particle aggregates. The unified approach strategies presented here are designed for TiO2 materials but would be in fact general for all other inorganic materials. We thus anticipate our method to be a possible starting point for non-classical crystallization strategies and provide a new and deeper understanding of the crystal growth.

Methods

Sample preparation

In a typical synthesis, 15 g saturated fatty acids (e.g. lauric acid, myristic acid, palmitic acid and stearic acid) were melted to form liquid phase in a glass vessel at 50–70°C. Then, 0.3 mL Titanium (III) chloride solution (≥12% TiCl3 basis, 5–10% free aicd as HCl, Sigma-Aldrich) was dropped into the above melted acids. After that, the glass vessel was covered with the lid and kept at 150°C for 12 h for the confined-space synthesis of nanowires. Afterward, the glass vessel was taken out and cooled naturally to room temperature and solid mixture of TiO2 nanowire film and saturated fatty acids was obtained. Then the obtained mixture was heated at 70°C to melt the saturated fatty acids again. After the melted acids were removed, the nanowire films were collected and washed with deionized water and ethanol for several times prior to be dried in air at 60°C. The growth temperature, reaction time, substrate, concentration and the amount of TiCl3 solution and other long-chain molecules (i.g. icosane, oleic aicd) were independently varied to investigate the effect of growth parameters on product formation according to above similar procedure. The TiCl3 solution was diluted to 1-9% by using 10% hydrochloric acid aqueous solution to avoid hydrolysis. When more TiCl3 solution was added (e.g. 1–5 mL), 50 g lauric acid was added to form the thick sealing layer to confine them in the vessel with the lid on.

Structural characterization

The surface morphologies of the samples were studied using FESEM (JEOL, JSM6300, Japan). The crystal structure was performed by XRD (PanAnalytic X'Pert Pro, Phillips) using Cu-Kα radiation (λ = 0.15405 nm, at 40 kV and 25 mA). The TEM, high-resolution TEM images and SAED were obtained with a JEOL JEM- 2010F microscopy operating at 200 kV. Nitrogen adsorption-desorption isotherms were measured on a NOVA1200 instrument (Quantachrome Corporation) at 77 K. Prior to the measurement, the samples was degassed in a vacuum at 200°C for 12 h. The thermal properties of the samples were studied with thermogravimetric analysis (TG209, Netzsch Instruments, Germany) from 25 to 800°C.