Hybrid Lithographic Arbitrary Patterning of TiO2 Nanorod Arrays.

In this work, we report a hybrid lithographic method that combines the top-down soft lithography and the bottom-up hydrothermal approach for growing single-crystalline TiO2 nanorod arrays with arbitrary patterns. The arbitrary patterns of TiO2 seeds were obtained through the microcontact printing of the TiO2 seed precursor onto Si substrates using prepatterned poly(dimethylsiloxane) (PDMS) as stamps, followed by a baking process. Afterward, TiO2 nanorod arrays were selectively grown on patterned TiO2 seeds through conventional hydrothermal methods. By controlling the TiO2 seed precursor concentration, the hydrothermal reaction time and temperature and the patterns, the morphology and density of the TiO2 nanorods can be tuned in a controllable manner. Overall, this work provides a new strategy for the low-cost and facile preparation of patterned TiO2 nanorod arrays that has potential applications in micro-nano-optoelectronic devices and other fields.

Introduction

Titanium dioxide (TiO2)[1] is an important n-type semiconductor material with a wide band gap between 3.0 and 3.2 eV.[2] It is an earth-abundant and nontoxic material with excellent photocatalytic properties.[3] Various TiO2 nanostructures including nanorods,[4,5] nanowires[6,7] nanotubes,[8,9] nanosheets,[10,11] etc. have been reported to be used for applications including solar cells,[12−14] lithium-ion batteries,[15,16] supercapacitors,[17] photocatalysts,[18,19] photochemical sensor,[20] biomedical applications,[21] etc. Commonly reported methods for the fabrication of TiO2 nanostructures include chemical vapor deposition (CVD),[22] hydrothermal method,[23] sol–gel method,[24−26] etc. Among them, the hydrothermal method is a simple, economical, and high-throughput bottom–up method for growing single-crystal TiO2 nanorods. The TiO2 nanorod crystals grown by the hydrothermal method have high crystallinity and uniform size distribution. However, the number, distribution, and crystal orientations of TiO2 nanorods fabricated by the hydrothermal method are quite random, preventing conventional hydrothermal methods from making ordered TiO2 nanostructures for more sophisticated applications, such as device integration.

One strategy to solve this challenge is to develop a hybrid patterning approach, i.e., a combination of the bottom–up hydrothermal method and the top–down patterning approaches, that is capable of generating arbitrary patterns, e.g., photolithography. This is critical for microelectronic devices that require patterning strategies.

Conventional top–down approaches, which are widely used in the semiconductor industry, include photolithography,[27] ion-beam lithography,[28] and electron-beam lithography.[29] They have been used to pattern different materials including TiO2. However, these patterning approaches are commonly considered to have a high cost, as they either require expensive equipment or have a low throughput (such as focused ion-beam lithography and e-beam lithography). As a result, they are not suitable for scalable patterning of TiO2 crystal for broad applications.

Recently, various low-cost alternate top–down patterning strategies, such as soft lithography, nanoimprint,[30−33] and nanosphere lithography, have been developed due to their simplicity, low cost, high throughput, and versatility (nonplanar surfaces).[34−37] Soft lithography uses elastomer such as poly(dimethylsiloxane) (PDMS) as stamps to realize microscale pattern transfer to planar or nonplanar surfaces through processes such as replica molding, imprinting, or microcontact printing. For instance, Checcucci[38] et al. fabricated multifunctional metasurfaces based on TiO2 microarrays through the direct nanoimprint of titania sol–gel coatings for application as dielectric Mie resonators. For another example, Huh[39] et al. reported using a hybrid approach that combines nanoimprint lithography and hydrothermal growth to selectively grow single-crystalline TiO2 nanorods with patterns. The patterned TiO2 nanoarrays were found to increase the optical path via scattering effects and enable a smooth coating of the active layer, thereby improving the performance of organic–inorganic hybrid perovskite solar cells. Xu[40] et al. obtained microcolumn (MP) and microcavity (MC) arrays of TiO2 thin films by the sol–gel thermal nanoimprint method. The rutile-type TiO2 nanorod three-dimensional (3D) microcolumn/microcavity arrays were prepared using them as seed layers in hydrothermal growth. The photochemical decomposition performance of the rutile-type TiO2 nanorods was greatly improved compared with that of TiO2 nanorods grown on flat seed layers.

In this work, we reported a hybrid method that combines microcontacting printing lithography and hydrothermal method to grow and pattern single-crystalline TiO2 nanorod arrays with arbitrary features. Compared with the sol–gel method, the synthesis time of nanomaterials prepared by the hydrothermal method was shorter, the conditions were easier to control, and the size of nanomaterials was smaller. Arbitrary patterns of TiO2 seeds were obtained through microcontact printing. It is more convenient to prepare two-dimensional patterned nanomaterial arrays. Afterward, TiO2 nanorod arrays are selectively grown on the patterned TiO2 seeds through the hydrothermal reaction. By controlling the TiO2 seed precursor concentration, the hydrothermal reaction time and temperature, and the patterns, the morphology and density of the TiO2 nanorods can be tuned in a controllable manner.

Results and Discussion

The overall schematic fabrication process of TiO2 nanorod arrays with patterns through the hybrid lithographic approach is illustrated in Scheme . First, we prepared a Si master (Figure S1) with linear array features by photolithography. Subsequently, the inverted features on the Si master were transferred to a PDMS stamp through replica molding (Scheme a). Then, the PDMS stamp was inked with a Si substrate with a precoated TiO2 seed precursor (Scheme b). The patterned TiO2 precursor was then transferred to a clean Si substrate through a microcontact printing process, followed by a postbake process (Scheme c,d). Finally, after the hydrothermal reaction, TiO2 nanorod arrays seeded from the patterned TiO2 seeds were formed (Scheme e).

Scheme 1

Fabrication Scheme of Linear TiO2 Nanorod Arrays by Soft Lithography Hydrothermal Growth. (a) PDMS Stamp with Inverted Features on the Si Master. (b) “Linking” of Precoated TiO2 Seed Precursor to the PDMS Stamp. (c) Microcontact Process for Transferring TiO2 Seed Precursor to Si. (d) Postbake Process Converting the TiO2 Seed Precursor to Crystalized TiO2 Seeds. (e) Linear TiO2 Nanorod Arrays Grown by the Hydrothermal Method

We characterized the morphology of the obtained TiO2 nanostructures with a scanning electron microscope (SEM). As shown in Figure a, the patterned TiO2 seeds by microcontact printing soft lithography have well-defined linear array features that match those on the PDMS stamp. Note that TiO2 seed linear arrays have linewidths <1 μm, which match to the channel widths of the Si master (Figure S1a). As shown in Figure b, ordered arrays of TiO2 nanorods are formed on Si substrates, and the pattern matches that of the TiO2 seeds in Figure a.

Figure 1

Time-driven growth of TiO2 nanorod arrays. SEM images of TiO2 seed layers (a) and TiO2 nanorods grown at (b) 1 h, (c) 2 h, and (d) 3 h. (e) Diagram of the average diameters and lengths of TiO2 nanorods. (f) X-ray diffraction (XRD) result of TiO2 nanorod arrays on a silicon substrate. Note, TiO2 is falsed colored in (a).

Figure b–d shows the typical SEM images of the TiO2 nanorods grown at different reaction durations from 1 to 3 h at the same temperature (160 °C). Their lengths and diameters are measured and the results are summarized in Figure e. As expected, the dimensions of TiO2 nanorods gradually increase due to the crystal growth with the increase in the reaction duration. After 1 h hydrothermal reaction, the average lengths and diameters of the nanorods are 200 ± 40 and 80 ± 20 nm, respectively. When the reaction duration reaches 2 h, their lengths and diameters increase to 1.8 ± 0.2 μm and 200 ± 30 nm, respectively. When the duration increases to 3 h, the lengths and diameters of nanorods further increase to 3.1 ± 0.3 μm and 260 ± 30 nm, respectively (Figure d). Thus, the dimensions of the TiO2 nanorods can be tuned by varying the reaction duration.

Next, we characterized the crystal structure of the fabricated TiO2 nanostructures by XRD. The XRD result in Figure f confirms that the obtained TiO2 has a rutile structure, i.e., the peaks in the XRD pattern can be attributed to the (110), (101), (200), (111), (210), (211), and (220) facets of rutile-type TiO2.

We also studied the TiO2 seed density-dependent growth of TiO2 nanorods. The TiO2 seed density was controlled by tuning the TiO2 seed precursor concentration. TiO2 seed concentration from 0.03 to 0.9 M and Si master (Figure S1b) were chosen in this study. After growing TiO2 nanorods, the representative results are provided in Figure . When the TiO2 seed precursor concentration is as low as 0.03 M, the formed TiO2 nanorods are loosely grown on the surface with a low density (Figure a). When the concentration of the precursor solution increased to 0.3 M, the density of TiO2 increased obviously to fully fill the linear array patterns (Figures b and S2). As the TiO2 seed precursor concentration continued to increase, the density of TiO2 nanorods increased as well, as shown in Figure c,d. This phenomenon is understandable that an increase in the TiO2 seed precursor would lead to an increase in the density of TiO2 seeds, thus resulting in the increased density of TiO2 nanorods growth on the seeds. It is worth noting that despite the increased density of nanorods, the diameters and lengths of the nanorods did not change much due to the same hydrothermal growth conditions.

Figure 2

Seed density-dependent growth of TiO2 nanorods. SEM images of TiO2 nanorod arrays grown under different seed precursor concentrations: (a) 0.03 M, (b) 0.3 M, (c) 0.6 M, and (d) 0.9 M.

Moreover, we also investigated the temperature effect on the hydrothermal growth of the TiO2 nanorods. Meanwhile, to confirm that the geometry of the patterned features would not affect the conditions of the hydrothermal growth, we chose a dot array instead of a linear array in this study (Figure S3). The dot array patterns of the TiO2 seeds were formed on a Si substrate and then used as a template to grow TiO2 nanorods. Typical SEM images of the TiO2 nanorod array obtained at different reaction temperatures from 140 to 180 °C are provided in Figure a–c. The dimensions of the nanorods obtained at different temperatures were measured and are summarized in Figure d. When the reaction temperature was increased from 140 to 180 °C, the lengths and diameters of the nanorods increased from 1.7 ± 0.2 μm and 150 ± 20 nm to 3.9 ± 0.4 μm and 300 ± 30 nm, respectively. Likewise, the pattern size also increased compared to the initial pattern due to the excessive hydrothermal growth of the nanorods. However, when a low growth temperature of 130 °C was used (2 h), tiny TiO2 nanocrystals were formed instead of a nanorod (Figure S4), suggesting that a minimum temperature of ∼140 °C was necessary to obtain a nanorod structure of TiO2 with limited reaction time. In short, by simply adjusting the hydrothermal temperature and time of growth and the TiO2 seed precursor concentration, one can readily obtain TiO2 nanorod arrays with desirable lengths, diameters, and nanorod density.

Figure 3

Temperature-dependent growth of TiO2 nanorod arrays. SEM images of TiO2 nanorods grown at different reaction temperatures: (a) 140 °C, (b) 160 °C, and (c) 180 °C. (d) Diagram of the average diameters and lengths of TiO2 nanorods.

We further demonstrated the versatility and diversity of this hybrid fabrication patterning approach. We successfully realized arbitrary patterning of TiO2 nanorod arrays with different features, such as characters of “SCI”, “WINTER”, and “CATION”, as shown in Figure . The arbitrary patterning capability was useful for device integration purposes. We would integrate this method into device applications in the ongoing study.

Figure 4

(a–c) SEM images of TiO2 nanorod arrays with different features.

Conclusions and Prospects

In summary, we reported a hybrid lithographic method that combines the top–down soft lithography and the bottom–up hydrothermal approach to grow single-crystalline TiO2 nanorod arrays with arbitrary patterns. The arbitrary patterns of TiO2 seeds were obtained through the microcontact printing of TiO2 seed precursor onto the Si substrates using prepatterned poly(dimethylsiloxane) (PDMS) as stamps, followed by a baking process. Afterward, TiO2 nanorod arrays were selectively grown on patterned TiO2 seeds through conventional hydrothermal methods. By controlling the TiO2 seed precursor concentration, the hydrothermal reaction time and temperature, and patterns, the morphology and density of the TiO2 nanorods could be tuned in a controllable manner. Overall, this work provides a new strategy for the low-cost and facile preparation of patterned TiO2 nanorod arrays that has potential applications in optoelectronic devices, photocatalysis, and biomedicine in the future.

Materials and Methods

Materials and Characterizations

Positive photoresist BCI 3511 was purchased from Suzhou Research Materials Microtech Co., Ltd. Negative photoresist SUN-9i was purchased from Suntific Materials (Weifang, China), Ltd. Anhydrous potassium hydroxide (KOH) was purchased from Sinopharm Chemical Reagent Co., Ltd. A scanning electron microscope (Zeiss Sigma 300VP) was used for the microscale morphology characterization of the obtained TiO2 nanostructures. The TiO2 crystal structure was characterized by X-ray diffraction (XRD), which was performed on a Bruker D8 Advance powder diffractometer (operating at 40 kV, 40 mA, with a Cu Kα source).

Fabrication of PDMS Stamps

Before the fabrication of the PDMS stamps, their masters with designed patterns were first fabricated through conventional UV photolithography and wet etching of Si(100) as reported in our previous work.[41] The fabricated Si masters with different microscale features including line/dot arrays and characters were used to prepare PDMS stamps through a replica molding process. Typically, a 10:1 mass ratio of silicone elastomer base and curing agent was mixed thoroughly, degassed under vacuum, then poured into a plastic Petri dish of suitable size with Si masters placed at the bottom. Then, the Petri dish containing the mixture and the Si master was placed in an oven with a temperature of 80 °C for 3 h to facilitate the polymerization of PDMS. Next, the Petri dish was taken out of the oven and cooled to room temperature. Finally, the PDMS stamp with desirable patterns was separated from the Si masters (Figure S5). These PDMS stamps are used to pattern TiO2 seeds as in the following sections.

Patterning of TiO2 Seeds via Microcontact Printing

We first prepared a 0.3 M TiO2 seed precursor solution in a 25 mL round-bottom flask. In a typical experiment, 150 μL of tetrabutyl titanate (C16H36O4Ti, 99%, Sigma Aldrich) and 600 μL of hydrochloric acid (30% aq. Sigma Aldrich) were added to a flask and mixed through magnetic stirring at 1500 rpm for several minutes. After the mixture was cooled down to room temperature, 15 mL of isopropanol (99%, Sigma Aldrich) was added and mixed through magnetic stirring for another 30 min.

In a typical microcontact printing experiment, a PDMS stamp and two pieces of Si substrates were treated with 2 min of oxygen plasma (13.56 MHz, 60 W, ∼6 Pa) to yield hydrophilic surfaces. The prepared TiO2 seed precursor solution was spin-coated on one of the two Si substrates at 1500 rpm to form a uniform wet coating. Then, the PDMS stamp was lightly placed on it and kept for 10 s to absorb adequate “inks” (TiO2 seed precursor solution) only on the extruded microstructures. Subsequently, the “inked” PDMS stamp was removed from the first Si substrate and then contacted with the other Si substrate for 30 min. Upon removal of the PDMS stamps, the TiO2 seed precursor solution was successfully printed on the second Si substrate. Next, the Si substrate with a patterned TiO2 seed precursor was heated at 450 °C for 2 h to convert the TiO2 seed precursor to crystalized TiO2 seeds (Figure a).

Controlled Growth of TiO2 Nanorod Arrays with Patterns

After successfully patterning the TiO2 seeds by microcontact printing soft lithograph, we selective grew TiO2 nanorod arrays on those seeds by hydrothermal reactions in a controllable manner. A typical experiment is described below.

First, the precursor solution for the TiO2 nanorods was prepared by mixing 60 mL of hydrochloric acid (30% aq.) and 60 mL of isopropanol (stirred at 1200 rpm for 30 min). Then, 2 mL of tetrabutyl titanate and 40 mL of ethyl acetate were instilled in turns.

Next, 20 mL of the TiO2 precursor solution and the Si substrate with patterned TiO2 seeds were placed and sealed in a 50 mL Teflon-lined high-pressure reactor. The reactor was heated at 140–180 °C for 1–3 h in a convection oven; then, it was removed from the oven and naturally cooled to room temperature. The Si substrate was taken out from the Teflon containers and then rinsed with deionized water and ethanol three times to remove the freestanding TiO2 particles formed in the solution and reaction solution residue. Finally, the Si substrate with selectively grown TiO2 nanostructures was blow-dried in the air.