Laser selective photo-activation of amorphous TiO2 films to anatase

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Laser selective photo-activation of amorphous TiO2 films to anatase and/or rutile crystalline phases Jaime A Benavides, Charles Trudeau, Luis Felipe Gerlein, and Sylvain G. Cloutier ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00171 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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ACS Applied Energy Materials

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Laser selective photo-activation of amorphous TiO2

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films to anatase and/or rutile crystalline phases

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Jaime A. Benavides, Charles P. Trudeau, Luis F. Gerlein, Sylvain G. Cloutier*.

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Département de génie électrique, École de Technologie Supérieure,

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1100 Notre-Dame Ouest, Montréal, Québec H3C 1K3, Canada

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E-mail: [email protected]

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Keywords: Nanoparticles, TiO2, sol-gel, anatase, rutile, oxygen vacancies, laser-assisted

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conversion, film patterning

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Abstract

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Titanium dioxide (TiO2) is a remarkable metal-oxide semiconductor with unique optoelectronic

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properties ideal for photovoltaics and photocatalytic conversion. The principal crystalline phases

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for TiO2 are anatase, rutile and brookite. The combination of both anatase and rutile crystalline

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structures can positively impact the optoelectronic properties of TiO2 films. With standard sol-

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gel processing, high-temperature conversion generally yields one dominant phase and limits the

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combined use of anatase and rutile TiO2 for optoelectronic devices. We report on a singular route

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to controllably-engineer hybrid nanocrystalline films of TiO2 at room temperature to

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synergistically exploit both anatase and rutile TiO2 phases. Relying on sol-gel chemistry, this

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approach starts from an amorphous film and uses photo-induced activation using a low-power

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laser to achieve specific spatially-controlled pattern consisting of different TiO2 crystalline

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phases within the same film. While achieving remarkable precision, reproducibility and control,

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we also avoid costly high-temperature, ion metal-assisted or specific atmospheric processing that

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currently prevents the integration of TiO2 in several optoelectronic platforms. In the future, we

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believe this unprecedented level of control and the ability to engineer the TiO2 crystalline

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structure at the microscopic scale will allow the design and fabrication of novel high-

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performance TiO2 hybrids for energy conversion and environmental applications.

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Introduction

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oxidizing power, high photochemical corrosive resistance and low cost1. Since the last decade, it

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also serves a wide variety of applications including sensing devices2, organic dye- and quantum

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dot-sensitized solar cells3, photo-electrochromics4, water-splitting5 and photocatalysis6 for

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extensive environmental applications, among others. All these applications of TiO2 stem from its

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unique optoelectronic properties, which strongly depend on the crystalline structure. Moreover, it

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is already established that synergistic effects from combining multiple crystalline structures can

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have positive consequences on these properties7–9. It has proven challenging to synthesize a

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particular crystalline phase while coexisting next to the other, considering the conditions

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necessary to achieve each one of them and take advantage of these synergistic effects.

Titanium dioxide is widely used for its non-toxicity, chemical and biological stability, strong


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Thermal annealing between 450°C and 1100°C is generally required to achieve the desired

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crystalline structure starting from amorphous TiO210. The anatase structure generally appears

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around 450ºC, while conversion to rutile appears between 800°C to 1100°C11. Anatase TiO2

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displays a tetragonal atomic structure with six atoms in each primitive cell. This phase is

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energetically metastable in bulk TiO212. Its relatively wide 3.2 eV electronic bandgap makes it a

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superb blocking layer for optoelectronic or photovoltaic devices13,14. In contrast, bulk rutile TiO2

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being also tetragonal, possesses 12 atoms in its primitive cell but this phase displays higher

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energetic equilibrium and it is the most common form of TiO2 found naturally. Rutile TiO2 has a

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lower electronic bandgap, 3.0 eV, and is better suited for photoelectrochemical applications.

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The Brookite phase is the least studied one because is difficult to synthesize pure16 and it is out

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of the scope of this paper. Although rutile is acknowledged as the most stable phase for bulk

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TiO2, anatase is more stable for TiO2 nanoparticles smaller than 50 nm17.

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High-temperature treatment is also generally used to sinter nanocrystalline TiO2 particles and

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form interconnected nanoparticle networks. This sintering process is generally performed around

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500ºC18. Given the wide range of possibilities that TiO2 has to offer for the development of new

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flexible, lightweight and wearable technologies requiring low-temperature substrates, researchers

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have generally tried to crystallize or sinter the TiO2 before its integration in the devices. As such,

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the high temperatures used to reach the anatase and rutile crystalline phases can also be most

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detrimental to temperature-sensitive devices. Different approaches have been explored to produce TiO2-based devices at low temperatures18–

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process requires a thin adhesion layer between the pre-sintered TiO2 film and the substrate,

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followed by an application of high pressures and moderate temperatures18.

. For example, pre-sintered TiO2 porous layers can be used, but the relatively complex transfer

Alternative


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approaches towards low-temperature device assembly at low temperature include compression

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methods and the use of conductive inks or electrolytes between the TiO2 particles19–22. While

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these approaches partially succeeded in avoiding high-temperature sintering, it remains that the

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degree of crystallinity and the optoelectronic properties of the resulting TiO2 films are far from

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ideal.

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Nevertheless, achieving high-quality crystalline TiO2 films at room-temperature remains one

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of today’s most important technical challenges towards low-cost optoelectronic devices

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including photovoltaic cells and photocatalytic device architectures. With the advent of laser-

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based particle sintering used for additive manufacturing, researchers have demonstrated

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optically-induced phase conversion from anatase-to-amorphous TiO2 using low intensity visible

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light under vacuum23. More recently, the same photo-activation process was reported under

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oxygen-poor conditions24. Finally, it was also shown that Fe and Al-doped TiO2 nanoparticles

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can respectively promote (Fe) or inhibit (Al) the anatase-to-rutile phase transition under UV-

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laser irradiation, taking the process one step further by achieving micropatterning of rutile

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phase25.

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In this paper, we present a much simpler method for triggering the crystallization of a TiO2

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sol-gel precursor at low-energies and achieve complete control of the crystalline phase at room-

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temperature and in ambient air using low-power laser-induced photo-activation of an amorphous

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TiO2 nanoparticle films. As we show, this well-controlled photo-activation can allow conversion

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from amorphous-to-anatase, amorphous-to-rutile, amorphous-to-mixture of anatase/rutile or from

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amorphous-to-anatase-to-rutile simply by adjusting the power density of the laser. This simple

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process is easily scalable to industrial-like environments and it is perfectly compatible with

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emerging laser-based 3D printing technologies.


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Figure 1. Characterization methods. a) TEM image of the pristine amorphous TiO2 nanoparticles as synthetized using a sol-gel route b) AFM image of a thin film of amorphous TiO2 nanoparticles produced by sol-gel chemistry. The average nanoparticle diameter is estimated around 10 nm. c) SEM image of the amorphous TiO2 films after focusing under SEM. Black arrows point out the

charging effect areas (where the electron beam was focused). White areas in the film correspond to crystallized areas where the focused electron beam triggered nucleation and transformation from amorphous to crystalline TiO2. 1 2 3

Fabrication of the patternable amorphous TiO2 nanoparticle films using sol-gel chemistry

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First, a glass substrate is placed at the bottom of a beaker filled with the TiO2 nanoparticles

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dispersion produced by sol-gel chemistry following the protocol described in the Methods

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section. The beaker is then covered with a perforated parafilm membrane to allow slow

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evaporation of the hexane solvent at room-temperature. When the solvent is completely

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evaporated (approx. 24 hours), we remove the glass substrate with a 23µm thickness film of

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amorphous TiO2 nanoparticles created over its surface. The amorphous nature of the TiO2


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nanoparticle film shown in Figure 1 was confirmed by XRD and Raman spectroscopy26 (see

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Figure S-1 in supplementary information).

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During the SEM observation of this amorphous nanoparticle film, we observed that the

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focused electron beam from the SEM was sufficient to initiate a nucleation and crystallization of

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the amorphous TiO2 film (Fig. 1c, see also movie S-1). The whiter areas indicate where the

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nucleation process occurred. From there, it spreads along the film following fractal patterns. This

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was the first indicator to confirm that this protocol required very low activation energy compared

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with previous results from the literature. Additionally, it was also noticed that thicker areas of the

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TiO2 film, require an increase of the irradiation voltage from 5kV to 15 kV to trigger nucleation

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under SEM. Unfortunately, it is not possible to obtain Raman signatures of those crystallized

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features since they are too small for the spatial resolution available in our confocal Raman

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microscope. To explore in more detail this phenomenon, we decided to test the behavior of the

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amorphous TiO2 films under direct laser-induced photo-activation.

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Patterning on-demand crystalline structure over amorphous TiO2 films

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As the SEM results successfully demonstrated the very low activation energy required to

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initiate the phase transformation, we used low-power laser-induced photoactivation to generate

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mixed structures of anatase and/or rutile simply by controlling the power density of the laser. To

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do so, we used a WITEC Alpha300 confocal Raman microscope equipped with a 60 mW fiber-

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coupled continuous-wave laser at 532 nm and a mechanical attenuator. The source beam is then

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coupled through a 10X objective for excitation of the sample, which is mounted on a motorized

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high-precision XYZ stage.


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Figure 2. Spatially-resolved laser-induced photoactivation process. a) Schematics of the experimental setup used for laser-induced photo-activation of the TiO2 structure.

b) Laser scanning microscope image (LEXT OLS4100; Olympus) of the amorphous TiO2 films after crystalline phase alteration. The patterned letters ÉTS consist of anatase-phase crystalline TiO2, while the underline consists of rutile-phase crystalline

TiO2. The darker background consists of amorphous TiO2. The different crystalline phases are confirmed by Raman micro-spectroscopy measurements as shown in c) and d), respectively.

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Our results shown in Figure 2 confirm we can controllably photo-activate the phase transition

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from the amorphous TiO2 to the anatase and/or rutile phase at room-temperature and in ambient

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environment simply by adjusting the excitation power density. A power density above 75W mm-

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transition to rutile TiO2 can be completed using an incident power density above 445 W mm-2

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(the horizontal line under “ÉTS” in Fig.2). Both crystalline phases are confirmed by the Raman

is required to complete the transition to anatase TiO2(the “ÉTS” letters in Fig.2), whereas the


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micro-spectroscopy results in Figure 2(c,d). Indeed, Figure 3 shows high-resolution Raman

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spectra of each crystalline structure obtained at these different laser power densities. When using

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an intermediate power density (i.e. 275 W mm-2) we observe a mixture of both anatase and rutile

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crystalline TiO2.

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Figure 3. Raman spectra of the various TiO2 crystalline structures obtained using different

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excitation laser power densities.

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To understand the influence of the incident laser power density over the crystallization process,

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we recorded the transient evolution of the Raman signature over a period of 1 minute after

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opening the laser shutter at the 5 second mark. Figure 4 shows the laser-assisted crystallization

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kinetics to reach anatase (Fig.4a,d), mixed anatase and rutile (Fig.4b,e) and rutile (Fig.4c,f)

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phases. Here, it is possible to observe that the crystallization process for all three TiO2

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polymorphs starts as soon as the amorphous film is exposed to the laser irradiation.

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Figure 4. Evolution of crystalline structure as function of time and Raman intensity. Figures

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a), b) and c) correspond to the Raman spectra evolution for the transition from amorphous to (a)

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pure anatase, (b) mixed anatase/rutile and (c) pure rutile observed at 75, 275 and 445 W mm-2

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power densities respectively taken over 1 minute. The laser shutter opens at the 5-second mark.

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The Raman peak intensity evolution for the dominant Raman peak is recorded over 1 hour for

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the conversion to (d) anatase, (e) mixed anatase/rutile and (f) rutile. The dashed line marks the

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time required to reach 90% of maximum Raman intensity.

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For the pure anatase crystalline phase obtained using a 75 W mm-2 power density, the

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identifying peak at 152cm-1 starts to appear during the first 6 seconds of laser exposure. The

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other characteristic peaks of the anatase phase at 395cm-1, 513cm-1 and 634cm-1

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defined after only 30 seconds of exposure. The other TiO2 crystalline phase transitions observed

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exhibit similar behavior shown in Figure 4(d).

(27)

are well

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In the mixed crystalline structure generated under 275 W mm-2 exposure, we observe well-

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defined rutile characteristic peaks such as 152cm-1, 261cm-1, 420cm-1 and 610cm-1 instantly after

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laser exposure. As expected, the intensity of all these peaks increases over time but, contrary to


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the characteristic peak ratios present in rutile, the peak at 152cm-1 still dominates, suggesting

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incomplete transition from anatase to rutile. Indeed, the transient evolution of the 152cm-1 peak

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behaves similarly to what can be observed for anatase, indicating the presence of a mixed,

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anatase and rutile, crystalline phase achieved at this power density. Figure 4(e), shows the

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evolution of the 152cm-1 rutile peak, which rapidly increases upon laser exposure before

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reaching saturation. On the other hand, the mix anatase/rutile phase behaves more as a

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transitional state, where contributions of both phases can be perceived simultaneously. As such,

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the mixture phase displays both strong 152 cm-1 (from anatase) and 420 cm-1 (from rutile) peaks.

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With higher portion of anatase phase dominating the Raman spectra, it is possible that the peak

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at 420 cm-1 displays different kinetics due to the contribution of the adjacent anatase peaks and

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displays its distorted and irregular behavior.

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For the rutile crystalline structure generated at higher power densities, it is possible to observe

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in Figure 4(f) the well-defined rutile peaks at 152cm-1, 261cm-1, 420cm-1 and 610cm-1

(28)

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immediately after laser exposure. The Figure 4(f) depicts the evolution of the dominant 420cm-1

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rutile peak.

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In all cases, we notice little changes in the intensity of the Raman signal after the first 600

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seconds of exposure, suggesting that conversion is complete after a short exposure time and that

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the process is not cumulative over time.

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To study in detail the patterning process and its effect on the amorphous TiO2 film structure,

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we decided to pattern both anatase and rutile crystalline structures in a checkered pattern. The

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dominant crystalline phase is confirmed by performing a Raman mapping of the photo-activated

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area (Figure 5). Again, each transition, from amorphous to anatase or rutile is selectively

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achieved by controlling the laser power density. It is important to note that patterning velocity is


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not uniform during the process since all the patterns that are shown in this work are produced by

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moving manually the XY positioner using the joystick. Nevertheless, we are certain that

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automation of this process will yield excellent outcomes. Since laser exposure induces crystalline

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conversion in the amorphous TiO2 films, the excitation power density within the confocal Raman

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system is kept below 75 W mm-2 for the acquisition of the Raman data.

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Anatase areas show a well-defined shape. Trace A (Fig. 5a) corresponds to the path left by the

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laser during the anatase patterning process. This path is narrow with approximately 9.3 µm of

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width. Rutile areas are also well defined but the crystalized area is bigger than intended. This

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comes as a result of the wider trace left by the laser during the rutile pattering process. The laser

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spot does not change while increasing the power density but the affected area in the film results

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larger. Indeed, wider trace R (Fig. 5a) has a width of approximately 17.2 µm.

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Figure 5. Patterning on-demand crystalline structure. a) Laser scanning microscope image

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(LEXT OLS4100; Olympus) of checkered pattern over amorphous TiO2 films. Anatase TiO2 is

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generated in squares 1 and 2, while rutile TiO2 is generated in the squares 3 and 4. b) Micro-

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Raman image of the pattern. The blue and red colors represent anatase and rutile TiO2

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respectively. c) Topographic 3D surface reconstructions for the checkered pattern. 3D image


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generated using software ImageJ based on the data given by the laser scanning microscope

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analysis.

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The visible difference between traces A and R in Figure 5a. arises from the different values of

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power density required to crystalize each phase; i.e. an increment in power density during the

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patterning process translated in an increment in the width of the trace left by the laser.

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It is possible to appreciate a clear contrast difference between the amorphous area and the

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crystallized regions in the patterned sample (Figure 5a). In the crystallized regions, the color

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changes from dark to light gray and cracks can be clearly observed in all crystallized regions,

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especially in the rutile TiO2 regions. These cracks appear as a direct consequence of the natural

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densification of the TiO2 during the crystallization process. Indeed, the atoms reorganize during

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the laser-induced photo-activation process. This reorganization at the atomic level results in an

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important reduction in volume due to a difference in packing density for each phase. Indeed, the

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density values reported in the literature are 3830 kg m-3 and 4240 kg m-3 for anatase and rutile

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respectively29. Such reduction generates cracks in the photo-converted regions. Cracks observed

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in the rutile TiO2 regions are more pronounced than cracks formed in anatase regions. This

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contrast is explained by the smaller unit-cell volume for rutile TiO2 and higher density due to the

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increased number of atoms. As shown in the Table S-1 of supplementary information, the

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volume of the anatase unit-cell is more than twice that of rutile. In other words, during rutile

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patterning, atoms get closer and the reorganization of its lattice generates more shrinkage,

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resulting in larger and deeper cracks when the material crystallizes. This loss of volume on the

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material becomes more obvious when we analyze the structure using a 3D topographic surface

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reconstruction of the checkered pattern. In Figure 5c, it is noticeable that the anatase tiles are


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shallower, smoother and more regular than rutile tiles. As expected, rutile regions also exhibit

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deeper cracks as stronger densification of the TiO2 occurs in rutile compared with anatase.

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Phase stability within nanocrystalline TiO2 is notorious for its great dependence on the particle

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size30,31. Under 14 nm, it is possible to achieve conversion to anatase at low-temperatures,

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starting from amorphous TiO2 nanoparticles32,33. This explains the results seen here, since the

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size measured for the amorphous TiO2 nanoparticles synthetized are around 10 nm. We believe

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that localized thermal effects caused by the focused laser are responsible for the crystalline phase

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transition.

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In this work the irradiation source is a 532nm laser, whilst smaller than the 3.2 eV bandgap of

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TiO2, the absorption of visible light is attributed to the presence of oxygen vacancies34 generated

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during the synthesis of these TiO2 nanoparticles. Additionally, phase transition from anatase to

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rutile can be induced by light irradiation in rich oxygen atmospheres23,35. The phase transition

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phenomenon relies on the ability of the material to absorb or desorb molecular oxygen29. In other

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words, the high concentration of molecular oxygen facilitates the phase transition due to the

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ionic mobility created in oxygen vacancies. Indeed, high concentration of oxygen vacancies

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creates band states within the TiO2 bandgap which effectively allows the absorption of the 532

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nm laser source while raising the energy of the system36. Also, it promotes the lattice relaxation,

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helping with the rearrangement process of the ions that participate in the phase transition.25,37


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Figure 6. Proposed Jablonski diagram for the laser-induced phase transition from anatase to

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rutile TiO2. The band states make possible the phase transition under visible light.

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In our particular case, the oxygen molecules act as very efficient photoexcited electron

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scavengers, trapping the excited electrons from the conduction band into the surface states of the

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TiO238. The oxygen molecules are then adsorbed at the surface of the TiO2 nanoparticles to

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partially compensate these oxygen vacancies39. The presence of oxygen vacancies promotes the

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formation of Ti3+ sites in the crystal structure as the electrons left behind by the vacancy are

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distributed on neighboring Ti sites, reducing them from Ti4+ to Ti3+.38,40,41 At this point, assisted

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by the continuous laser irradiation, the adsorbed oxygen molecule passivates the TiO2 by

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bridging the metallic ions42. As result of the phase transition and the relaxation of the electrons, a

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vibrational phonon is emitted as heat (Figure 6).

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Conclusions


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1

In this paper, we report low-power laser-assisted photoactivation of TiO2 films performed at

2

room-temperature in ambient environment. This simple approach allows to selectively and

3

controllably generate conversion anatase and/or rutile crystalline phases simply by controlling

4

the laser power density incident on a film of amorphous TiO2 nanoparticles synthesized through

5

an incomplete nonhydrolytic sol-gel ester elimination reaction of titanium isopropoxide and oleic

6

acid.

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Raman micro-spectroscopy is used to confirm the transition to pure anatase, mixed

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anatase/rutile and pure rutile crystalline phases generated using different laser power densities.

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Transient evolutions are also measured using time-resolved Raman micro-spectroscopy. The

10

results show that crystallization occurs in the very first seconds of irradiation, that this effect is

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permanent and non-accumulative. Laser scanning microscope images and 3D topographic

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surface reconstruction of the photo-activated areas evidence the loss in volume after

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crystallization due to denser atomistic reorganization after phase transition. The method

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presented in this work can be used to spatially-organize different crystalline phases with high

15

level of precision to study new synergistic effects7,8. Our results are consistent with the literature

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suggesting that amorphous TiO2 nanoparticles under 14 nm allow laser-assisted phase transition

17

to anatase. Regarding the subsequent transition from anatase to rutile, we confirm it relies on the

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high capacity of the synthetized TiO2 to absorb molecular oxygen23. These results are consistent

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with the state-of-the-art. Most importantly, all the phase transitions presented on this work are

20

achieved without requiring any kind of dopant in the TiO2 film prior photoactivation.

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Our confocal Raman system allows for scan speeds up-to 4000µm2 s-1 while allowing phase

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transitions. This potentially allows larger scale fabrication confined within the size constrains of

23

our system, 25 x25 mm2. Nonetheless, both phases can be patterned using this area for a usable


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device that can be readily used in optoelectronic devices. Other industrial systems that allow for

2

larger scale and faster fabrication are possible as long as the radiation power density condition is

3

met.

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Throughout this paper, we have discussed the importance of the easy integration of TiO2 in

5

many photo-electro-catalytic applications. We believe this accurate, reproducible and scalable

6

method of producing crystalline TiO2 at the microscopic level opens the door to novel pathways

7

of TiO2-based platforms, especially for energy conversion and environmental applications.

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Methods Chemicals. Oleic acid (90%, Aldrich) (OA), titanium (IV) isopropoxide (TTIP, Aldrich, 97%), ethanol (ACS reagent, ≥99.5% (200 proof, absolute) and hexane.

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Synthesis of TiO2 nanoparticles. TiO2 nanoparticles are synthesized through an incomplete

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nonhydrolytic sol-gel ester elimination reaction of titanium isopropoxide (TTIP) and oleic acid

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(OA)43. TTIP (3.36g) is added to OA (10 g) at room temperature under nitrogen atmosphere. The

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resulting mixture is heated to 280˚C in a period of 20 min (14˚C min−1) and it is kept at this

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temperature for 2h. The light-yellow solution gradually turns dark brown. At this point, the

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solution is cooled down to room temperature and ethanol in excess is added to yield a beige

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precipitate. The solution is centrifuged for 30 min to recuperate the nanoparticles. Finally, the

19

solution is re-dispersed in hexane and ready to use. The TiO2 nanoparticles obtained from this

20

synthesis are shown in Figure 1a), from the AFM image Figure 1b) an average particle diameter

21

of 10.5 nm was calculated.

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1 2


Corresponding Author *E-mail: [email protected] (S.G.C.)

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Authors contribution

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J.B. performed both, the concept and the synthesis of the TiO2 patternable films. C.T.

6

performed the Raman measurements, analysis and interpretation of Raman results. L.F.G.

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performed the 3D reconstruction of the sample and the time evolution analysis. Interpretations of

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the results and data analysis were discussed between all the authors. The first draft of the

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manuscript was written by J.B. and revised by C.T., L.F.G. and S.G.C. All authors have

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approved the final version of the manuscript before its submission.

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Acknowledgements

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Sylvain G. Cloutier thanks the Canada Research Chair and the NSERC Discovery programs

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for their support.

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Funding Sources

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S.G.C. is most thankful for the financial support from the Canada Research Chairs (Award

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231814) and NSERC (Award 491592).

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Abbreviations

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TiO2, titanium dioxide; OA, oleic acid; TTIP, titanium (IV) isopropoxide.


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Competing interests

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A patent application will be filed in the coming weeks and the content of this article draft shall

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be reviewed under confidentiality.

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Competing financial interests

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The authors declare that they have no competing financial interests.

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Laser selective photo-activation of amorphous TiO2 films to anatase

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