Robotic Deposition of TiO2 Films on Flexible Substrates from Hybrid

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Robotic Deposition of TiO2 Films on Flexible Substrates from Hybrid Inks: Investigation of SynthesisProcessing-Microstructure-Photocatalytic Relationships Maria Alejandra Torres Arango, Alana Samara Valença de Andrade, Domenic Terrence Cipollone, Lynnora Olivia Grant, Dimitris Korakakis, and Konstantinos A. Sierros ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05535 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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ACS Applied Materials & Interfaces

Robotic Deposition of TiO2 Films on Flexible Substrates from Hybrid Inks: Investigation of Synthesis-Processing-Microstructure-Photocatalytic Relationships Maria A. Torres Arango1, Alana S. Valença de Andrade1, Domenic T. Cipollone1, Lynnora O. Grant1, Dimitris Korakakis2 and Konstantinos A. Sierros1* 1

Flexible Electronics for Sustainable Technologies Laboratory (FEST), West Virginia

University, Department of Mechanical & Aerospace Engineering, 395 Evansdale Drive, Morgantown, WV, 26505, USA 2

Material Growth and Characterization Lab, West Virginia University, Lane Department of

Computer Science and Electrical Engineering, 395 Evansdale Drive, Morgantown, WV, 26505, USA KEYWORDS TiO2, aqueous ink, direct writing, low temperature, UV treatment, photocatalysis.

ACS Paragon Plus Environment

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ABSTRACT

TiO2 is an important material widely used in optoelectronic devices due to its semiconducting and photocatalytic properties, non-toxicity, and chemically inert nature. Some indicative applications include water purification systems and energy harvesting. The use of solution, water-based, inks for the direct writing of TiO2 on flexible substrates is of paramount importance since it enables low cost and low energy intensive large-area manufacturing, compatible with roll-to-roll processing. In this work we study the effect of crystalline TiO2 and polymer addition on the rheological and direct writing properties of Ti-organic/TiO2 inks. We also report on the bridging crystallite formation from the Ti-organic precursor into TiO2 crystalline phase, under ultra violet (UV) exposure or mild heat treatments up to 150 oC. Such crystallite formation is found to be enhanced by polymers with strong polarity and pKα such as Polyacrylic Acid (PAA). X-ray diffraction (XRD) coupled with Raman and X-ray photoelectron (XPS) spectroscopy are used to investigate the crystalline phase transformation dependence based on the initial TiO2 crystalline phase concentration and polymer addition. Transmission electron microscopy imaging and selected area electron diffraction patterns confirm the crystalline nature of such bridging printed structures. The obtained inks are patterned on flexible substrates using nozzle based robotic deposition, a lithography-free, additive manufacturing technique that allows the direct writing of material in specific, digitally predefined, substrate locations. Photocatalytic degradation of methylene blue solutions, highlights the potential of the studied films for chemical degradation applications, from low-cost environment friendly materials systems.


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1. INTRODUCTION Polycrystalline TiO2 films have been studied in the last two decades due to their utilization in dye-sensitized solar cells (DSSC’s) and photocatalysis applications1–3. TiO2 is an important wide bandgap semiconductor (Eg ~3.2eV), it is abundant, chemically inert, and in thin film form it combines high surface area, non-toxicity and photo-catalytic properties4–7. However, like most of the metal-oxide compounds, high temperature processing is required in order to obtain specific TiO2 crystalline micro-structures. For crystalline TiO2, the most common annealing temperatures are 500°C and 800-1000°C yielding anatase and rutile polymorphs, respectively 8. For the brookite phase on the other hand, specific pH conditions should also accompany the process 9,10. These high temperature conditions limit the use of ceramics in flexible applications, which often rely on heat-sensitive plastic substrates. Alternative processes to obtain crystalline structures compatible with polymeric substrates include hydrothermal growth11,12, combustion annealing13, electrophoresis14, electrospinning 15, and vacuum based techniques such as chemical vapor deposition

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. All these however, exhibit their own limitations such as large-area

fabrication incompatibility or high specific deposition energy, leading to increased cost. Therefore, there is a great need for novel processing methods that can enable lithography-free, roll-to-roll compatible, large-area deposition on flexible plastics. Direct writing methods may be an alternative approach to mitigate the aforementioned challenges and enable large-area, lithography-free flexible applications. A robotically controlled direct writing technique, based on the pneumatic extrusion of ink from a nozzle on digitally predefined locations, that can mitigate challenges arising during ink-jet printing has been proposed in the past

17,18

. This recently resurfaced method, enables the

deposition of functional inks on a layer-by-layer fashion and allows for 2D film patterning on


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planar and arbitrary shaped surfaces, as well as fabrication of 3D structures from high viscosity and/or fast drying inks. Lateral dimensions that have been reported can be up to two orders of magnitude lower than those achieved by ink-jet printing19. Recent significant advancements using nozzle based robotic deposition (NBRD) include the demonstration of a Li-ion microbattery20, a bio-printed ear21, and a quantum-dot light emitting diode22. Relevant works on NBRD of TiO2 2D and 3D structures, include sol-gel and particle based formulations with promising results and applications23–26; nevertheless, these often implement high-temperature heat treatments that are incompatible with most polymer substrates. Polycrystalline films fabricated using nano/micro sized crystalline TiO2 particle inks/pastes that contain organic binders and Ti precursors

27,28

signify an important approach for low-thermal

energy budget post deposition processing, compatible with polymeric substrates for energy and photocatalysis applications29. Cold pressing of dried pastes has also been explored for flexible device manufacturing

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which in turn reduces cracking of the films; nevertheless, an inherent

loss in porosity of the film due to the applied mechanical stress has been reported. The latter may not be desirable in cases where high surface area is needed. The synthesis and utilization of TiO2 from biological sources is also an area that receives attention at an accelerated pace

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due to

the impact that environmentally-focused nanotechnology brings. Specifically for photocatalysis, the use of TiO2 films on flexible substrates is important for maximization of the active surface area and efficient degradation of chemicals; as well as for relatively simple replacement of the photocatalytic films in the reactors, extending the reactors’ operational life. Water compatible formulations are pivotal in the design of TiO2 inks since open-air, vacuum-free processing is a key component in low-cost manufacturing, aligned with NBRD. Moreover, from the environmental perspective, water-based solutions are desirable since they avoid the use of


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organic solvents, which may pose health risks and environmental damage. Titanium (IV) bis(ammonium lactato) dihydroxide (TALH), is a water compatible Ti precursor that hydrolyzes at slow rates in pH neutral solutions11,33 as opposed to Ti-alkoxides that exhibit instantaneous sedimentation once exposed to water, even in trace amounts. The use of TALH for TiO2 ink formulations for DSSCs

27,34

has opened new routes for low-temperature polycrystalline TiO2

film fabrication. However, further understanding of the TALH-TiO2 system is needed for its utilization with non-traditional, additive manufacturing techniques such as NBRD and in applications other than DSSC’s. Studies that integrate all these concepts: additive manufacturing, low thermal-budget synthesis methods, and environment friendly technologies are key for the next generation of flexible device design and manufacture. In this work, we investigate the TALH-TiO2 crystalline phase system, utilizing UV light and low temperature as a means to transform the amorphous phase. We study the effect of crystalline TiO2 and various polymers addition to the system on its rheological and NBRD properties, and moreover on the TiO2 crystallite formation from TALH precursor. Finally, we implement the TALH-TiO2 system for heterogeneous photocatalytic degradation of methylene blue MB, and discuss the role of polymers and processing on the photocatalytic properties of the obtained films. 2. MATERIALS AND EXPERIMENTAL METHODS 2.1. Ink Synthesis Appropriate amounts of TAHL solution (50 wt% in H2O) – Sigma Aldrich, DI water and Titanium (IV) oxide TiO2 nanopowder (~21 nm diameter) Aeroxide® P25 (70% Anatase, 30% Rutile) – Sigma Aldrich, were mixed and set to stir for about 10 min.


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Then, the inks were sonicated in a water/ice bath for 15 minutes while occasionally stirring to prevent sedimentation. The TALH concentration was kept to 0.4M for all formulations, whereas the crystalline TiO2 phase was varied in TALH:TiO2 molar ratios of (1:1, 1:3, 1:6 and 1:12). In parallel, DI water and Polyacrylic Acid (PAA) Product #323667 – Sigma Aldrich or Polyvinyl Pyrrolidone (PVP) Mw 8000 – Acros Organics, were mixed and stirred until becoming homogeneous with a magnetic stirrer. The TAHL:Polymer molar ratio was kept equal to 1 throughout the entire experiment. The polymer solutions were dropwise added to the titania solutions while stirring and set for sonication for 15 more minutes. The obtained inks were stored in the same vials used for preparation and were magnetically stirred right before deposition. 2.2. Film Deposition Two film deposition techniques were used for this study. Doctor Blading was used mainly for the characterization of the material system, its transformations and performance upon different energy treatments. NBRD of the films on flexible substrates was used to explore the inks’ printing properties and assess their potential and particular challenges. Titania films were deposited using the doctor blade technique with Scotch tape 3M 600 (58.4 µm thickness) used as spacer, on cleaned glass slides and on ITO/PET OC300/ST504/7mil Solutia TM

substrates. The films were allowed to dry for ~30 min.

Direct writing of films was performed through NBRD using a Nordson JR2300N robotic arm equipped with a Performus V pneumatic pressure ink dispenser system. During direct writing, the inks are loaded into syringe type cartridges and pneumatic pressure controllably extruded through a nozzle onto a substrate. Through movement/dispensing control software, specific amounts of ink are applied at digitally pre-defined substrate locations, enabling virtually no-


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waste during deposition of materials. The directly written samples were prepared on ITO/PET substrates. The direct writing parameters were varied to explore the printing ranges of the formulated inks as follows; pneumatic pressure from 6.89 kPa to 103.42 kPa, writing speed from 1 mm/s to 15 mm/s, distance to substrate from 5 to 150 µm. Stainless steel nozzles with inner diameter of 100 µm were used for the depositions. UV treatments were performed on the deposited films with a SpectroLINKERTM XL-1500 Spectroline® UV crosslinker machine with G15T8 bulbs of 15 watt – 254 nm radiation wavelength for 2h, 4h and 6h, respectively. For the (1:6) ink, additional doctor bladed films were fabricated on glass and heat treated at 150°C, 380°C and 420°C. Annealing was performed in a box furnace KSL 1100X, MIT Corporation; all annealing processes were held for 30 min and the heating rate was 5 °C min-1. 2.3.Characterization 2.3.1. Ink Characterization Thermogravimetric analysis (TGA) of the inks was performed with a Pyris 1 TGA PerkinElmer thermogravimetric analyzer from room temperature to 900°C and a heating rate of 5°C/min. Contact angle measurements of the inks and DI water were taken by image analysis of 2µl droplets (Thermo-Scientific Matrix 12.5 µl pipetting tool) on various substrates using ImageJ software (NIH). The substrates used for contact angle characterization consist of cleaned glass, PEN (TeonexQ65®), and ITO/PET OC300/ST504/7mil Solutia

TM

. Ink viscosity was measured

with a Brookfield DV-II + Pro rotational viscometer.


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2.3.2. Film Characterization X-ray diffraction (XRD) was performed with a Bruker D8 Discover XRD machine with power settings of 40 kV and 40 mA, and 1D detector mode. The data were analyzed with the aid of the X-Pert Highscore Plus PANalytical software. Raman spectra were obtained with a Reinshaw INVIA Raman spectrometer with a 532nm wavelength excitation source at 5% power and 50X magnification. The acquired spectra were analyzed with the WiRE 3.4 Renishaw software. Optical images were taken with a Dino Edge-Digital programmable optical microscope. Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4700 SEM machine at 5 kV accelerating voltage and 12 mm working distance. X-ray photoelectron spectroscopy (XPS) was conducted with a PHI 5000 VERSAPROBE 5700 XPS/UPS machine with 284.8 eV C1s (C-C binding energy) internal calibration standard. A monochromated 25 W, 15 kV Al Kα x-ray source (photon energy of 1486.6 eV) and a hemispherical analyzer were used, at ~ 5x10-10Torr pressure in the main chamber; the acquired spectra were analyzed using MultiPak v9 software. Transmission electron microscope (TEM) images and selected area electron-diffraction (SAED) patterns were taken with a JEOL JEM-2100 TEM machine at 200kV acceleration voltage equipped with a Gatan Erlangshen ES500W digital camera and a Gatan Orius SC600 high-resolution digital camera. Thickness measurements for the directly written samples were taken with a Bruker Dektak XT Profilometer, with a 2µm stainless steel tip. Roughness measurements of the samples were performed by profilometry and AFM. The roughness measurements from profilometry were obtained with the aid of the Vision64 profilometer controller software. AFM images were obtained with an Agilent 5500 SPM atomic force microscope using tapping mode. The AFM tip is has a radius of 2-5 nm and a resonant


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frequency of ~300 kHz, the tip is made of Si with a 100 nm Al coating on the back side. AFM image analysis was conducted using Gwyddion 2.45 Software. 2.3.3. Photocatalysis Measurements Heterogeneous photocatalysis degradation of a 10 µM methylene blue (MB) aqueous solution was performed by placing samples of 2 cm X 1 cm active area on ITO/PET substrates, from TALH:TiO2 (1:6) and (1:12) inks in identical beakers, each containing 20 ml of solution. A control solution was also placed in identical conditions and was labeled as Blank. Once immersed in the solution, the samples were left to stabilize for 30 minutes in the dark to allow for dye adsorption on the TiO2. The UV irradiation, in 15 min steps, was performed in the SpectroLINKERTM XL-1500 Spectroline® UV crosslinker machine at 254 nm radiation wavelength. The samples were place at 9.5 cm distance from the bulbs, which deliver an average intensity of ~6000µW/cm2. Light absorbance of MB solutions was measured through UV-Visible spectroscopy with a Lambda35TM UV-Vis spectrometer PerkinElmer. The spectra were taken from 196 nm to 1100 nm wavelength in polystyrene disposable cuvettes. 3. RESULTS AND DISCUSSION 3.1. TiO2 – TALH Material System The different constituents of the ink, strongly influence its functionality and its printability. The ink is composed of (1) the crystalline TiO2 phase, (2) the TALH organic metal-oxide precursor, (3) water as a solvent and (4) an additive polymer – either PAA or PVP. The TiO2 film is intended to exhibit high surface area while having a continuous-porous mesostructure. Here the crystalline TiO2 phase is intended to serve as the photocatalytic wide


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band-gap semiconductor, and the Ti-organic complex TALH, will serve as the bridging element between the already crystallized material to create a continuous structure. The porosity results from the volumetric contraction of the solidifying phase over the crystalline TiO2 which occurs due to evaporation of the solvent and decomposition of the organics from the metal-oxide precursor. Compared to denser microstructures, in such porous TiO2, improved photocatalytic 35 and DSSC performance have both been attributed to enhanced electron mobility 36. Increasing the amount of crystalline phase in the ink results also in an increase of its viscosity. As shown in Figures 1.a. and 1.b., all formulated inks exhibit shear thinning behavior that is more pronounced for the high crystalline phase concentration polymer additive formulations. Also, the addition of polymer has a viscosity increasing effect as compared to lower crystalline concentration (1:6) counterparts.


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Figure 1. Viscosity of (a) different particle loading no-polymer inks and (b) polymer formulated inks (1:12) filled markers, (1:6) open markers. And (c) contact angle of different particle concentration and polymer inks on ITO/PET substrates. When depositing the films onto the substrates, the lower TiO2 content inks bulged up right after being spread; the TiO2 particles concentrated at the center of the “bulged” material yielding


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uneven films, characteristic of Marangoni flow 37. As the inks were applied, drying of the (1:12) films was almost immediate; in contrast, for all lower crystalline TiO2 concentrations drying of the films was relatively slow (~5 min for (1:6) to ~1 h for (1:1)). Therefore, as the TiO2 content increased, the solvent evaporation rate was higher, and the films became more uniform. This result is expected because of the lower solvent content (i.e. for the higher TiO2 concentration inks) in proportion to the solids content, and concomitantly higher surface area (TiO2 particles surface area) available for the solvent to evaporate. Spreading/wetting of the inks is lower for higher crystalline TiO2 concentration inks as can be observed from Figure 1c and S1. As polymers were added to the formulations, the surface energy of the inks was modified as can also be observed from the contact angle measurements, Figure 1c. The polarity of the added polymers is thought to be directly related to the wetting behavior38 and can be observed from the change of the contact angles. When polymers with specific pKα are incorporated, the wetting properties of such inks are modified; the pH values of the individual precursor ink components in solution were measured to be ~2 for PAA, ~6-7 for PVP and ~7-8 for TALH 0.4M, respectively. Therefore, the noticeable change in the contact angle of the ink for the PAA solutions can be attributed to the lower pH of the final ink (~4-5), due to the contribution of the added PAA. The inclusion of PAA in our ink formulations is expected to result in a higher probability of TiO2 formation upon UV or thermal exposure, when compared to more pH-neutral formulations, due to partial neutralization of ammonium lactate ions by the acidic groups in PAA. Although TALH is a relatively stable compound in neutral solutions at room temperature, hydrolysis of TALH into TiO2 (and ammonium lactate as byproduct) can be achieved at relatively low temperature (~ 100°C) conditions as has been described by Mo ckel et al.

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. Basically, hydrothermal

decomposition of TALH will result in TiO2 crystallites, the size of which will be strongly


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influenced by the presence of ammonium lactate as a limiting factor in the crystal growth. If the ammonium lactate is removed or neutralized by incorporation of acids, the resulting TiO2 crystals will exhibit larger sizes. Similarly, sol-gel growth of thin films of TiO2 on polymeric substrates from TALH in strong acidic / basic conditions33 suggests that there should be a one-toone correspondence between H+ or OH- groups and the ammonium lactate ions present in TALH for TiO2 formation (precipitation) in TALH-Acid/Base solutions at 70°C. TGA of the different formulated inks is shown in Figure 2. TGA of the different crystalline concentration inks, as well as for the crystalline TiO2, and TALH is presented in Figure 2a. From the bare TiO2 (Aeroxide ®) curve, an important inflection point can be distinguished at around 400°C where the transition from amorphous titania to TiO2 in anatase phase has been reported in the past

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. However, it is interesting to analyze the nature of such transition since the added

crystalline phase is already in anatase or rutile phase. The latter being the most thermodynamically stable, thus coming in conflict with reversed transformation from rutile to anatase phase. To this respect, Gribb et al. 39 have found that for small enough crystallites (~ 2.8 nm), a reverse transition is possible. Ohtani et al. 2010

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have also reported that the TiO2 as

obtained from the Aerosil ® process, may contain amorphous TiO2 phase, the amount of which varied depending on the flame conditions

40,41

; leading to different anatase/rutile/amorphous

ratios from batch to batch and even further, vary within the same package of as received TiO2 Degussa (Evonik®) Aeroxide powder. There is therefore, a possibility that the initially added “crystalline phase” may contain a minimum amount of amorphous TiO2. In the case of the TGA curve for the 2.08M TALH, there are important inflection points at ~100°C, 150°C, 325°C and 450°C. They can be assigned in order to: end of water evaporation in the ink, organics decomposition onset, TAHL decomposition/amorphous titania formation, and


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amorphous titania/anatase TiO2 transformation. The TGA for the formulated inks, Figure 2(a), shows the same inflection points (although not as pronounced as for TALH 2.08M) which are observed to shift to slightly lower temperatures, see bands in Figure 2.a. These plots could be thought of as the superposition of their constituents. For example, for the TALH:TiO2 (1:1) with a 0.4 TALH initial concentration, the initial weight loss from the solvent evaporation is the most pronounced inflection; for the following transitions, the presence of already crystalline phase with almost negligible weight loss, dampens the weight change from the organics in the ink, resulting in less pronounced inflections for the transitions at ~150°C, 325°C and 450°C respectively. Similarly, for the increasing crystalline TiO2 concentration inks, these transitions show systematic attenuation. When polymer additives are included in the formulation, the TGA curves, Figure 2 (b), exhibit very little difference and no shifting of the final transition temperature is observed. However a slight delay in the transitions can be expected due to an amount of thermal energy that has to be used for the polymers’ decomposition and is evident from the slightly higher weight percentage for both PAA and PVP inks before the final transition.


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Figure 2. Thermo-gravimetric Analysis (TGA) of (a) different particle loading inks, TAHL 2.08M and TiO2 only. The TALH concentration for the formulated inks is 0.4M. And (b) Polymer added inks with TALH:TiO2 ratio (1:6). Heating rate of 5°C/min. The (1:6) ink is selected for further study of the TiO2 crystallite formation from TALH as an intermediate particle concentration formulation, being near the upper viscosity limit requirement for ink-jet printing and the lower limit for doctor blading, NBRD, and screen printing 23,42 (with the aid of polymer agents); it also exhibits the representative TGA profile of the TiO2-TALH systems. The XRD patterns of all the films obtained from the formulated inks exhibit TiO2 diffraction peaks characteristic of anatase and rutile phases. Figure 3, shows the main (011) – Anatase and (110) – Rutile peaks, in agreement with PDF codes 00-021-1272 and 00-021-1276, respectively. For comparison purposes, XRD of as received crystalline TiO2 was also taken. The effect of two different treatments on the obtained films is investigated: conventional annealing and UV exposure. Additionally, during the UV treatment, the temperature also increases to approximately 60°C after 2.5 hours. For all TiO2 concentrations the degree of crystallinity, represented by the peak relative intensity, increases slightly upon 2h UV exposure. However for the case of 4h UV exposure, the


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diffraction peaks broaden. These results may be considered as follows: (a) for the lower crystalline TiO2 concentration inks, there is more Ti-organic precursor available for crystallization and hence a higher probability of particle nucleation/growth upon energy input, and (b) for the higher crystalline phase concentration inks, there are more TiO2 nucleating sites available (larger TiO2 surface area); and equivalently more photocatalytic active sites for TALH transformation

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and polymer decomposition. These two considerations, thought as probability

factors, should be maximized to achieve bridging of the added particles through the organics with a minimum amount of energy. In other words, bridging will happen for all TALH concentrations with large enough energy input, but the optimally minimum energy input depends on the concentration. Guttierrez-Tauste et al. report the optimum TALH:TiO2 concentration to be (1:7.4) based on the mechanical endurance of their films under sonication in water 27.

Figure 3. XRD of (a) TiO2 Aeroxide ® and the different particle concentration inks and (b) of the TALH:TiO2 (1:6) inks with various polymers, after different UV treatments: No Treatment (thin-bottom line), 2h UV (medium) and 4hUV (dark-top line). (c) Crystallite size change upon different energy input conditions, variation calculated with respect to non-treated samples. The crystallite size, as calculated using Scherrer’s formula43, from the XRD peak broadening is an important indicator, and can be used to further understand how the probability factors affect


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the nucleation of TiO2 from TALH, see Figure 3. Upon UV exposure, except for the (1:6) nopolymer formulation, the average crystallite size decreases (with respect to non-treated samples) which may suggest that there is nucleation of new TiO2 particles from TALH (up to ~20% decrease is found). For the (1:6) no-polymer case, there is a systematic increase of 2.17% for 2h UV exposure and of 11.28% after 4h UV. Furthermore, when the obtained films are submitted to heat treatments the crystal size also increases. The growth trend is slightly different, with the highest crystallite size for 150°C (15.19% increase) and lower sizes for 380°C (4.93% increase) and 420°C (0.94% increase), respectively. The percentages are calculated with respect to the average crystallite size for the untreated ink formulation. In light of the Scherrer crystallite analysis, TiO2 formation from TALH as new crystallites or on the surface of already present TiO2 may suggest competing crystalline phase growth mechanisms. Also, these results suggest that the different treatment energy sources may have important implications on such crystalline formation. The crystallite structure in the (1:6) no-polymer formulation upon UV exposure, can be attributed to a maximization of the two aforementioned probability factors: Ti-organic availability, and photo-catalytic/nucleating sites for TiO2 formation. Compared to the other crystalline TiO2 content formulations, the (1:6) no-polymer formulation favors TiO2 nucleation on already present TiO2 crystals. For the temperature-induced crystallization of the (1:6) formulation, it is suggested that the initial mechanism is TiO2 growth on the surface of already present crystals and then subsequent nucleation of individual particles takes place. The latter mechanism is probably hindered from initiating crystallization on TiO2 surfaces due to the organics present in the system

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. In contrast, the addition of polymers with strong polarities,

favors the nucleation of individual particles upon UV exposure as indicated from the crystallite


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size decrease of 8.45% (2h) and 20.01% (4h) for PAA; and 21.56% (2h) and 8.06% (4h) for PVP. Raman spectra of the films as dried (No Treatment) are shown in Figure 4 (a-d). The films’ inorganic region 100 cm-1 to 700cm-1 shows the TiO2 characteristic pattern with the molecule’s different vibrational modes B1g – Rutile (145 cm-1), Eg – Anatase (145 cm-1, 199 cm-1), B1g – Anatase (398cm-1), Eg – Anatase (448 cm-1), A1g and B1g – Anatase (518 cm-1), Eg and B1g – Anatase (640 cm-1) and it is found to be in accordance with previous TiO2 Raman characterization

44,45

. Except for the case of the bare TiO2 particle film, identified as TiO2

Aeroxide (Figure 4d); all films show the characteristic pattern of ammonium lactate in the organic region of the spectra (700 cm-1 to 2000 cm-1), with peaks at 791 cm-1, 868 cm-1, a sharp peak at 932 cm-1, and broader bands/peaks centered at 1055 cm-1, 1095 cm-1, 1125 cm-1, 1285 cm-1, 1309 cm-1, 1393 cm-1, 1452 cm-1 and 1635 cm-1. It should be mentioned that the obtained spectra were slightly displaced toward higher energy shift values, this shift is attributed to the combination of the experimental error associated with the equipment and temperature fluctuations, and the effect of organic molecule presence on the TiO2 surface46.


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Figure 4. Visible Raman λ=532nm for the as dried (No treatment) TALH:TiO2 inks (a) inorganic region (c) organic region, as dried (No treatment) different polymer formulated inks (b) inorganic region and (d) organic region. Integrated Raman Intensity ratios for (e) different particle loading inks and (f) different polymer formulated inks as dried and after 2h UV


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treatment. Open Symbols (No Treatment), filled symbols (2hUV). Inset in (e) depicts the integration areas: Anatase (395 cm-1) and Rutile (445 cm-1) peaks. The anatase to rutile ratio, as calculated from Raman spectroscopy 8, shows that anatase formation is favored upon UV exposure for most of the TALH:TiO2 concentration inks without polymeric additives Figure 4e. When additives are incorporated however, the trend is opposite, suggesting a reduction in anatase phase in relation to rutile phase, Figure 4f. This change in the anatase/rutile ratio may evidence the formation of TiO2 in a different crystalline structure such as TiO2-(B)47 or even in the amorphous state. Nevertheless, since the added crystalline TiO2 is in anatase or rutile phase, which are stable TiO2 phases when compared to nucleating TiO2 from TALH, it is unlikely that the amount of anatase in the system decreases. Therefore, it is shown that the UV, and concomitantly developed heat, causes the crystalline phase amount to grow as (a) new anatase particles on the surface of existing TiO2, or (b) through existing rutile crystal growth from the organic Ti-precursor, at the interfaces between rutile/anatase particles; or at the interfaces between aggregated anatase particles as described by Zhang et al. 8. The hypothesis of formation of new crystallites is supported from the XRD analysis showing peak broadening, which we mainly attribute to crystallite size decrease. Furthermore, TEM images and selected area diffraction patterns of the specific samples with greater average particle size decrease (as calculated from XRD) were obtained to confirm the nucleation of new crystallites and are shown in Figure 5. The diffraction pattern shows combination of spot and ring patterns that correspond to anatase TiO2, in agreement with the single crystal character or the bigger (original) TiO2 added crystals and the polycrystalline bridging structures nucleating form TALH upon energy input.


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Figure 5. TEM images of the (a) as received TiO2 Aeroxide particles, (b) TALH:TiO2 (1:6) PAA film after 4h UV exposure, and (c) TALH:TiO2 (1:6) film after 150°C-30 min treatment. HR-TEM images of the enclosed regions show the lattice fringes of the crystals and the bridging formations from TAHL in (b) and (c). Diffraction pattern taken for the enclosed region in (c) characteristic of Anatase TiO2. Atomic content from XPS, indicates an O:Ti ratio greater than 2 for all fabricated films (the stoichiometric value for TiO2). A systematic decrease in this ratio towards the stoichiometric value was observed as the UV exposure time, and as the initial crystalline TiO2 content were increased. When polymers are incorporated to the inks, the O:Ti ratio shows dependence on the type of energy treatment employed during the film synthesis. Thus, the O:Ti ratio exhibits values


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closer to 2, for the PAA films when using 6h UV; and for PVP films when using the 150°C -30 min treatment. The presence of a shoulder in the detailed scan for the O1s peak at ~531.5eV is characteristic of a hydrated state of TiO2. It can be observed that such shoulder diminishes with increasing UV exposure, see Figure 6. Similar diminishing of such oxygen energetic state has been reported for TiO2(H2O) upon thermal annealing33. These results indicate the photocatalytic role of the TiO2 particles and how the formulation (crystalline phase incorporation) and processing (UV exposure) variations influence the final TiO2 chemical state. While the Ti2p peak position remains fixed for all no-polymer cases, with the characteristic ∆Eb of ~5.75 eV; the intensity ratio between the Ti2p spin-orbit splitting peaks Ti2p1/2 at 464.5 eV and Ti2p3/2 at 458.75 eV deviates from the theoretical 1:2 area ratio as the TiO2 crystalline phase and UV exposure time are increased. See Figure 6c. This result may be attributed to the increase in energy of the TiO2 system upon UV exposure due to its photocatalytic nature. On the other hand, a slight shifting (of ~ 0.5eV) of the Ti2p peak towards lower binding energy values is observed when polymers are added to the formulations. See supporting information Figure S2. In particular, for the PAA formulation the intensity ratio between the Ti2p spin-orbit splitting peaks Ti2p1/2 and Ti2p3/2 is very close to the theoretical 1:2 area ratio, independently of the energy treatment employed, and in contrast to the deviation trend displayed for the other formulations.


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Figure 6. X-ray photoelectron spectroscopy of the films exposed to different UV conditions (a) O1s detailed scan for (1:12) no-polymer films; (b) C1s and (c) Ti2p detailed scans. The cyan line in (c) corresponds to the experimental XPS data. 3.2 Direct Writing of Films The acquired viscosity range can be useful for the utilization of these inks in different printing techniques beyond direct writing, including ink-jet, gravure, screen printing, doctor blading and dip-coating. The viscosity of the inks, is a pivotal parameter for the deposition technique selection and for the resulting printed structures. In this work, the viscosity values of the different TALH:TiO2 inks, see Figure 1, and the intended application allow the printing of 2D film structures. In our case, reduction of clogging is found when direct writing the polymer formulated inks (1:12 and 1:6), and results in less defects such as discontinuities or uneven coverage of the patterned films (otherwise frequent). The use of polymeric additives is known to aid in controlling of the evaporation rate

42

, reduce cracking and improve substrate wetting


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27,48,49

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; and avoid nozzle clogging42. Another cause for clogging is the aggregation of particles

within the inks. 1:12 formulated inks exhibited the most kinetically stable behavior, resulting in better film printability. Inks with lower initial TiO2 content exhibit strong sedimentation and are not suitable for this technique without the use of stabilizing agents. Furthermore, for the implementation of the TALH:TiO2 ink system on 3D structures, further tuning of the viscoelastic properties of the inks is necessary but beyond the scope of this work. The direct writing NBRD parameters include ink extrusion pressure, inner diameter of the printing nozzle, dispensing height; i.e., the nozzle-substrate clearance, and speed at which the nozzle traverses the substrate surface50. For (1:12) no polymer added ink the following direct writing ranges were found to yield the best quality (i.e. continuous and uniform) and most reproducible films. These were 55 kPa to 70 kPa pneumatic pressure, 5 mm/s to 10 mm/s writing-speed, ~60 µm to 80 µm dispensing height using a 100 µm – nozzle size. For the (1:12) polymer added formulations the process parameters are summarized in Figure 7. Regarding the writing speed, it was observed that if deposition is relatively slow (~ 5 mm/s) the drying front of the films matches the deposition speed yielding uniform films. However, if the writing speed (NBRD case) is even slower, the drying front of the deposited material would be faster than the ink being deposited, resulting in clogging of the nozzles. The coupling between the writing speed and the applied pressure is very important since too much pressure at low speed results on overflow of material and thick films. Conversely, the same pressure at high speeds results in thinner films and eventually uncovered regions. Also, if too much material is extruded over a small area, cracking of the films occurs due to the strong TiO2 particle aggregation. For (1:6) inks, the pressure-speed printing ranges are smaller due to the lower


24

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viscosity of the inks. Thus, for low viscosity inks, lower pressures are required as compared to the printing of higher viscosity inks. As important as the aforementioned parameters is the distance to substrate; when printing the patterns (at a 90° angle between the nozzle axis and the surface) if the distance is too large, the surface tension between the nozzle tip and the inks may cause the inks not to fall onto the substrates, but instead to climb up the nozzle surface. In contrast, if the tips are too close to the substrates, the profile of the deposited material is mechanically altered by the nozzle and presents a “valley” like shape. This shape also depends on the elastic properties of the inks and recovery of the dome shape due to relaxation of the inks onto the substrate.

Figure 7. (a) Typical optical image of an as deposited film using (1:12) PVP inks on ITO/PET substrates at 10 mm/s and 34.47 kPa. (b) Schematic of the nozzle based robotic deposition (NBRD) direct writing method. (c) Direct writing parameter maps for the different TALH:TiO2 polymer added inks on ITO/PET substrates.


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There is a direct relation between the spreading of the inks and the thickness and micron-scale roughness of the films. For any fixed speed, as the pressure is increased the spreading of the ink increases since there is more material being deposited, the thickness of the resulting films increase, and similarly does the roughness. Spreading is highly dependent on the ink-substrate interactions as highlighted from the contact angle results, accordingly we observe more spreading for the lower initial TiO2 particle concentration inks. Briefly, 1:12 TALH:TiO2 inks yield thick films ~70 µm to 80 µm thickness and of ~10 µm to 19 µm roughness; and thin films of ~20 µm to 30 µm thickness and of ~4.5 µm to 6.5 µm roughness. Similarly, 1:6 TALH:TiO2 inks yield thicker films of ~ 50 µm thickness and ~20 µm roughness, and thinner films of ~20 µm and ~5 µm roughness. The addition of polymers with different chemistries greatly influences the films’ spreading, as can be observed from Figure 7c. When printing new materials systems, the assessment of the ink-substrate interactions is crucial to address potential problems such as ink-substrate incompatibility (non-stickiness) or delamination. Moreover, the direct writing parameters are affected by the choice of the polymers included in the formulation. From the two polymers included in this investigation, we observe larger printing ranges for the more acidic (PAA) formulations on ITO/PET substrates. This is in agreement with the contact angle measurements for the different ink systems as discussed in section 3.1 TiO2 – TALH Material System. SEM images of the films surface show porous films for all cases regardless of the particle concentration. See supporting information Figure S3. Similarly, TiO2 particle aggregates can also be observed as a typical feature for all films, irrespective of the deposition technique. The similarity of such film surface microstructure for different deposition techniques may be attributed to the strong particle aggregation in the colloidal systems.


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3.3. Photocatalytic Behavior The mechanism of degradation of MB involves direct and indirect photocatalysis pathways51,52. When the catalyst, in contact with the dye solution is excited by the light, electron-hole pairs are generated (Eq. 1), then the electrons and holes at the conduction and valence band respectively, migrate to the catalyst surface to further produce redox reactions. Generally, the holes react with water molecules adsorbed at the TiO2 surface producing *OH radicals (Eq. 2); and the electrons react with O2 producing superoxide radicals (Eq. 3). Subsequent reactions between the generated species and the dye (Eq. 4 – Eq. 7) result in the dye decolourization. + ℎ → + ℎ

Eq. 1

ℎ + → ∗ +

Eq. 2

+ → ∗

Eq. 3

∗ + →

Eq. 4

→ 2 ∗

Eq. 5

∗

+ →

→ +

Eq. 6 Eq. 7

For low dye concentrations (millimolar range) the rate of photocatalytic degradation can be fitted to a pseudo first-order kinetic model as follows:

!" !

# = %&''

Eq. 8

Where kapp is the apparent first-order rate constant, C0 is the initial dye concentration and C is the concentration of dye at a given time t. The concentration of a solution can be calculated from its light absorbance at a given wavelength (Eq. 9) according to the Beer-Lambert law. Similarly, the percentage of MB degradation can be calculated from Eq. 10


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!" !

# =

("

#

Eq. 9

( )

* +,-. (%) =

Page 28 of 40

(" ( ("

# × 100

Eq. 10

The degradation parameters from the different films are summarized in Table 1. In general the kapp values are comparable to similar TiO2 mesostructured films53,54. It is observed that the films obtained from the inks with lower initial crystalline TiO2 content, i.e. TALH:TiO2 (1:6) inks, show higher photocatalytic performance when compared to the films obtained from (1:12) formulations. Also, it is found that there are strong polymer-TiO2 interactions with direct effect on the photocatalytic performance. The addition of polymers to the ink formulations results in doubling the kapp values for the films with low initial particle concentration (1:6). For films from PAA formulations, an important increment of nearly two times in the kapp is observed for the (1:12) films. Once more, the different polymer chemistries contribute significantly on the TiO2 crystallite formation and growth from TALH, and it is evidenced by the enhancing effect of the photocatalytic activity for the PAA films with high TiO2 initial content. The carboxylic groups present in PAA, more prone to bonding to the TiO2 surface than the PVP molecules that do not have the OH side-groups, may further facilitate the bridging of neighboring TiO2 particles by forming organic scaffolds for the new TiO2 crystallite nucleation. Evidence of this, is the mechanical stability of the PAA films that showed no visible deterioration after the photocatalysis experiment as opposed to the PVP counterparts with rather poor stability. In general, the 1:12 films were more stable than the 1:6 films.


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Table 1. Apparent first-order degradation rate constant kapp and time for 50% MB degradation from different TiO2 films. AFM and Profilometry roughness measurements for the films.

Sample

kapp x 10-2(min-1)

Time for Degradation > 50% (min)

Roughness AFM (nm)

Profilometry (µ µm)

0.11 ----* 0.80 2.90 90 3.87 1:6 6hUV 0.62 1.97 105 5.01 1:6 150°C 0.52 1.53 120 3.46 1:12 6hUV 0.39 0.98 --2.47 1:12 150°C 1.19 12.27 75 1.73 PAA 1:6 6hUV 0.73 17.42 105 1.82 PAA 1:6 150°C 1.12 11.63 75 1.43 PAA 1:12 6hUV 0.60 --150 12.66 PAA 1:12 150°C 1.55 11.71 45 2.49 PVP 1:6 6hUV 0.89 10.22 75 6.61 PVP 1:6 150°C 0.56 8.18 120 2.47 PVP 1:12 6hUV 0.37 9.53 --3.53 PVP 1:12 150°C * The maximum degradation for the Blank (control) solution was 12% after 150 min UV exposure. Blank

A strong dependence on the photocatalytic activity from the energy treatment used for the films synthesis is also identified. In general, the UV curing treatment results in better photocatalytic performance than the mild annealing treatment, independently of the initial TiO2 particle concentration. Overall, the films with higher roughness from AFM exhibit the best photocatalytic performance. We believe that this characteristic behavior is related to the TiO2 crystallite size, favoring films with higher nano-structured features (nano-roughness) and resulting in more efficient degradation of the dye. An opposite trend is observed between the roughness values from profilometry and AFM, i.e. the samples with smoother features as measured with AFM, exhibit higher roughness values from profilometry. This difference in the results can be attributed to the mesoscopic nature of the obtained films and the different techniques’ resolution.


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AFM (scanning 1 µm X 1 µm areas) would detect surface details in the nano-scale, giving information about the crystallites formed from TALH; whereas profilometry would resolve for the “bulk” film roughness in the micron-scale, accounting for larger surface features such as the aggregates formed by the bigger, initially added TiO2 nanoparticles during the ink formulation. Additionally it is found that the incorporation of polymers in the ink formulations results in increase of the nano-roughness.

4. CONCLUSIONS Directly written TiO2 films were fabricated on flexible substrates from the studied aqueous system. Mild temperature and UV irradiation conditions were used to transform the amorphous/crystalline formulations to semi-crystalline/crystalline films which can allow for diverse applications on flexible substrates. It is found that the viscosity and therefore printing properties of the inks can be tailored through solvent amount, polymer addition and TALH:TiO2 ratio, to meet the requirements of specific printing techniques even beyond direct writing. It is also reported that the crystalline TiO2 initial concentration serves as (a) a provider of TiO2 nucleation sites, (b) a provider of photo-catalytic active sites for organics decomposition, (c) a solvent evaporation control parameter, (d) an ink rheology controlling parameter. Furthermore, the use of polymers is found to affect the formation of crystallites since the energy supplied to the system is used towards polymer decomposition in addition to the TiO2 nucleation from TALH. The different polymer chemistries and polarities also influence the wetting properties of the inks onto the substrates; and moreover, affect the crystallite formation by partial


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reaction with the ammonium lactate present in TALH. In general, for the temperature and UV ranges explored in this work, the upper limit for crystallite size is comparable to the initial TiO2 particle size while smaller particles nucleate from TALH. The direct written films are observed to be porous, being suitable for applications where high surface area is important. The porous structures obtained through Doctor Blading and NBRD show negligible variation of the microstructure and assembling of the colloidal system upon dying and curing. For NBRD, relationships between the different printing parameters and obtained macro/micro-structures were stablished. Further work on direct writing of patterns from the proposed ink system, and the optimization of the curing (heat/UV exposure) treatment in relation to the patterned material thickness, will provide additional means for the implementation of the TALH:TiO2 ink system in flexible electronic applications. Photocatalysis degradation of methylene blue indicates the potential of the fabricated films for organic chemicals decomposition. The TALH:TiO2 ratio and the type energy treatment employed for the film synthesis, i.e. UV exposure or mild annealing, are found to have great influence on the photocatalytic degradation rate. Additionally, it was found that the presence of carboxylic groups in the polymer additives for the inks, may favor the bridging of neighboring TiO2 particles. In summary, this work provides an insight on the effect of TiO2 crystalline phase and different polymer addition, to the crystallite formation from TALH, as metal-oxide precursor in environment friendly TiO2 ink formulations for direct writing and printing for flexible applications.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], phone: +1 304 293 3420, fax: +1 304 293 6689 Author Contributions All authors have given approval to the final version of the manuscript. ASSOCIATED CONTENT Supporting Information Contact angle of different TALH:TiO2 formulations; XPS Ti2p detailed scans of polymer-based formulations; SEM images of the films’ surface; Photocatalytic degradation and linearized MB concentration change in time. ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Science Foundation (award no. 1343726 and 1358137). We thank the helpful discussions and comments from Dr. Aaron J. Kessman. We also thank Dr. Charter Stinespring and Andrew Graves for their help with AFM characterization, and Ioannis Kortidis for the helpful discussion about photocatalysis. We acknowledge use of the WVU Shared Research Facilities. REFERENCES (1)

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Table of Contents Graphic


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Robotic Deposition of TiO2 Films on Flexible Substrates from Hybrid

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