IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 23 (2012) 165603 (10pp) doi:10.1088/0957-4484/23/16/165603

Deposition of photocatalytically activeTiO2 films by inkjet printing of TiO2nanoparticle suspensions obtained frommicrowave-assisted hydrothermalsynthesisMelis Arin1, Petra Lommens1, Simon C Hopkins2, Glenn Pollefeyt1,Johan Van der Eycken3, Susagna Ricart4, Xavier Granados4,Bartek A Glowacki2 and Isabel Van Driessche1

1 SCRIPTS, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3),B-9000 Ghent, Belgium2 Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street,Cambridge CB2 3QZ, UK3 Laboratory for Organic and Bioorganic Synthesis, Department of Organic Chemistry, Ghent University,Krijgslaan 281 (S4), B-9000 Ghent, Belgium4 ICMAB-CSIC, Campus de la UAB, E-08193 Bellaterra, Spain

E-mail: Isabel.VanDriessche@UGent.be

Received 30 November 2011, in final form 24 February 2012Published 30 March 2012Online at stacks.iop.org/Nano/23/165603

AbstractIn this paper, we present an inkjet printing approach suited for the deposition of photocatalytically active,transparent titanium oxide coatings from an aqueous, colloidal suspension. We used a bottom-upapproach in which a microwave-assisted hydrothermal treatment of titanium propoxide aqueoussolutions in the presence of ethylenediaminetetraacetic acid and triethanolamine was used to createsuspensions containing titania nanoparticles. Different inkjet printing set-ups, electromagnetic andpiezoelectric driven, were tested to deposit the inks on glass substrates. The presence of preformedtitania nanoparticles was expected to make it possible to reduce the heating temperature necessary toobtain the functionality of photocatalysis which can widen the application range of the approach toheat-sensitive substrates. We investigated the crystallinity and size of the obtained nanoparticles byelectron microscopy and dynamic light scattering. The rheological properties of the suspensions wereevaluated against the relevant criteria for inkjet printing and the jettability was analyzed. Thephotocatalytic activity of the obtained layers was analyzed by following the decomposition of amethylene blue solution under UV illumination. The influence of the heat treatment temperature on thefilm roughness, thickness and photocatalytic activity was studied. Good photocatalytic performance wasachieved for heat treatments at temperatures as low as 150 ◦C, introducing the possibility of using thisapproach for heat-sensitive substrates.

(Some figures may appear in colour only in the online journal)

1. Introduction

Thin TiO2 coatings can be used to create transparent, pho-tocatalytically active, self-cleaning, antifogging and superhy-

drophilic surfaces [1–7]. Given its high refractive index andabsorption in the UV part of the electromagnetic spectrum, itis one of the most promising wide bandgap semiconductorsfor use in optoelectronics such as solar cells [1, 8–10].

10957-4484/12/165603+10$33.00 c© 2012 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 23 (2012) 165603 M Arin et al

In our previous work [11], thin, transparent andphotocatalytically active TiO2 films were prepared fromaqueous Ti4+ precursor solutions in which hydrolysis andthe resultant precipitation were avoided by blocking thehydrolysis reaction in pure aqueous media using complexingligands as stabilizing agents. Details on the stabilization ofdifferent metal ions in water-based precursors can be foundelsewhere [12–16]. Yet, to convert these kinds of precursorsinto active TiO2 films, sintering at temperatures above 500 ◦Cis still necessary with the methods described above. Todecrease the energy input and allow coatings on heat-sensitivesubstrates such as polymers, it is important to reduce theconversion temperature.

In this work, we present a low temperature method forproducing TiO2 coatings using inkjet printing of aqueoussuspensions containing TiO2 nanoparticles. This allows theheat treatment temperature to be reduced since the desiredphase is already present and only the removal of the solventand other chemicals is necessary to create the final, crystallinelayer. The TiO2 nanoparticles used in this work are obtainedfrom bottom-up, microwave-assisted hydrothermal synthesis.Microwave synthesis provides a more efficient way of heatingcompared to conventional hydrothermal synthesis, since theheat is immediately directed to the solution while, with aconventional furnace, heat needs to dissipate from the furnaceto the autoclave and then to the actual solution. Therefore, it isa fast method (taking a few minutes instead of 24 h for classichydrothermal synthesis) that allows the preparation of highquality nanoparticles [17–19].

As a precursor, we use environmentally friendly, aqueousprecursor solutions that contain Ti4+ ions which arestabilized by complexing agents and show no hydrolysis.This is different from most typical sol–gel-based synthesisapproaches where hydrolyzed and sometimes peptized Ti-alkoxide precursors are autoclaved to create nanoparticlesat higher temperatures and pressures [20–25]. To furtherincrease the efficiency of the coating procedure, we shiftedfrom the commonly used dip-coating and spin-coatingtechniques to inkjet printing to apply the coatings. Thisis a non-contact deposition method which can create largearea coverage by direct patterning on almost any substrate,allowing versatile deposition of thin films [26, 27]. In this way,both patterned deposits and completely covered surfaces canbe generated with the same equipment and precursors [28].This one-step process therefore results in simplicity, low cost,less material waste, scalability to large area manufacturingand convenient control of the thickness of the coating.Therefore, inkjet printing is in line with industrial needs forrobust, high volume and precise deposition of liquids [26].

2. Experimental procedure

2.1. Materials

Aqueous TiO2 precursor solutions were prepared usingtitanium propoxide (TIP) (Sigma-Aldrich, ≥97.0%) as theTi4+ source, with ethylenediaminetetraacetic acid (EDTA)(Acros Organics, 99.5%) and triethanolamine (TEA) (Acros,

99+%) as complexing agents. Small amounts of ethanol(EtOH) (absolute, Panreac) were used to dissolve the metalalkoxide to help control hydrolysis during water addition. Allmaterials were used without further purification.

2.2. Ti4+ precursor solution

Two different precursor solutions were used for this work.Solution Ti–E was prepared by dissolving TIP in EtOHin a 3.8:1 molar ratio to Ti4+. EDTA was added (0.5:1EDTA/Ti4+) to this solution with a small amount of TEA.After stirring for 1 h, H2O was added in a 100:1 molarratio to Ti4+ to the solution in order to obtain a final Ti4+

concentration of 0.4 M. Solution Ti–E + EG was identical tosolution Ti–E, except for the addition of ethylene glycol (EG)(Sigma-Aldrich, ≥97.0%) to the final precursor solution in a10 wt% range.

2.3. Microwave synthesis

A microwave furnace (CEM Discover) operating at afrequency of 2450 MHz was used to perform reactions underair at elevated temperatures and pressures. It was equippedwith a single-mode cavity in which the pressure can beincreased to 20 bar, and with a selectable power outputfrom 0 to 300 W. 3 ml of the aqueous precursor solutionwas poured into the 10 ml reaction vessel, which is placedinside the reactor. The temperature was set at 110–140 ◦Cand the reactor was exposed for 10–15 min. As soon as thedesired temperature and pressure (5–7 bar) were reached, thepower output from the microwave generator was reduced to5–10 W to keep the pressure constant. During the treatmentthe solution was stirred with a teflon-coated iron oxide barthat reflects the microwave energy. After treatment, the systemcooled down by directing cooled air onto the vessel. Theobtained solutions were filtered with a standard 3.1 µm glassmicrofiber syringe filter prior to inkjet printing.

2.4. Inkjet printing

For inkjet printing of the precursor solutions, both a singleelectromagnetic nozzle with a 90 µm diameter jewel orifice,modified from a Domino Macrojet printhead and mounted ona Roland pen plotter, and a Dimatix piezoelectric multi-nozzlefully integrated printer (Fujifilm Dimatix Inc. DMP-2831)with 23 µm diameter orifices were used.

The electromagnetic nozzle can dispense inks with awide range of viscosities, producing >1 nl droplets at jettingfrequencies of up to 1 kHz. The printing pressure is setbetween 0.3 and 0.8 bar and the printhead is fed through anexternal reservoir.

The piezoelectric-driven Dimatix printer can dispenseinks with a viscosity up to 10 cP, containing <1 µm particles,producing 28–75 pl droplets at jetting frequencies up to20 kHz. In this printer, a negative pressure is required tokeep the ink inside since the ink reservoir is essentially opento the air. During jetting, the ink acceleration comes fromthe deformation of the piezoelectric element. For printing,

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the sols were filled into disposable cartridges which have 16piezoelectric nozzles at the bottom, and placed on the plotter.

To assess jetting behavior from the electromagneticnozzle, a drop visualization system (purpose-built, Universityof Cambridge) was used to image the inkjet at 50 µs intervalsafter the start of ejection. The ink was back-lit with astrobed collimated LED light source and imaged using acamera (Allied Vision Technologies Inc.) with a telecentriczoom lens (Moritex). The camera shutter and strobe weresynchronized and triggered at a selectable delay after inkejection, and timed such that each acquired image was formedfrom a single ejection event: typical shutter times and flashdurations were 32 µs and 15 µs respectively for jetting at10 Hz. Semi-automated quantitative image analysis was usedto estimate the volume and position of each droplet in eachframe, and from that the net ink velocity.

All inkjet depositions were performed at room temper-ature. The glass substrates used for coating were standardmicroscope slides with dimensions 20 mm × 50 mm. Inthis work, we aimed at covering a 2 × 2 cm2 area with ahomogeneous TiO2 coating by generating droplets of the solsfrom the nozzle moving linearly in a raster pattern. For theelectromagnetic nozzle, a nominally rectangular pattern arraywith an inter-droplet distance of 400–500 µm was used; forpiezoelectric printing, a square pattern with an inter-dropletdistance of 20–40 µm is used to obtain optimum substratecoverage.

After printing, the layers are dried at 30 ◦C for 3–4 hunder air. Further heating was performed in a tube furnace(Carbolite, UK) at temperatures between 100 and 500 ◦C for2 h with a heating rate of 2 ◦C min−1 under air.

2.5. Analysis

The viscosity of each solution and sol was determined usinga Brookfield DVE viscometer. The wettability was studied bymeasuring the contact angle of a 10 µl droplet of precursorsolution on glass substrates, and the surface tension by thependant drop method using an optical tensiometer (KrussDSA30). The nanoparticle size distributions were determinedby a DLS (dynamic light scattering) technique using aMalvern Nano series Zetasizer. In order to determine the solidcontent in the sols, elementary analysis was carried out on anx-ray fluorescence (XRF) spectroscope (Rigaku NeX CG).

Transmission electron microscopy (TEM) (JEM-2200FS)was applied in order to measure the particle size and toinvestigate the crystallinity. Specimens for TEM studies wereprepared by depositing a drop of purified aqueous sol ontoa 300-mesh holey carbon copper grid. For purification, thesols obtained after microwave synthesis were destabilizedby adding ethanol. After centrifugation, the supernatant wasremoved and the precipitated nanoparticles were resuspendedin water by ultrasonification.

XRD analysis was performed with a 0.04◦ step sizeand a 2 s step time (Bruker D8, Cu Kα radiation). Thehomogeneity and smoothness of the films was investigatedby atomic force microscopy (AFM) (Molecular Imaging,PicoPlus). The results of AFM measurements were analyzed

using the WSxM 4.0 software program [29]. FIB-SEM (FEINova 600 Nanolab Dual-Beam FIB) was used to directlydetermine the thickness from cross-sectional views and tostudy the morphology inside the layer. The transparency ofthe films on glass substrates was determined using a UV–visspectrophotometer (Perkin Elmer Lambda 950).

The photocatalytic activity of the sol–gel films wasevaluated by following the degradation of methylene blueunder UV illumination. The experiments were carried outin a set-up prepared according to ISO 10678:2010(E) wherethe samples are placed in round-bottomed photocatalyticcells with a near-UV-transparent window (cutoff below340 nm). An irradiation box equipped with a VilberLourmat VL-315BLB blacklight blue fluorescent light tubewas used. The photon source has a maximum emissionat 365 nm and emits 10 W m−2. Aqueous methyleneblue solutions were prepared from Sigma-Aldrich powder(without further purification). The azo-dye solutions wereused without oxygen gas bubbling and their absorption wasset to be at 0.733. The concentration was correlated to theabsorption of the methylene blue solution at 664 nm (ελ =664). The titania-coated microscopy slides were accuratelyshaped at 4.0 cm2 and inserted into the photocatalytic cell.Photocatalysis experiments took place under stirring as closeas possible to the light source.

3. Results and discussion

3.1. Preparation and analysis of nanoparticle inks

As a first step in this research we aimed at preparing a stable,aqueous Ti4+ precursor solution suited for further treatment.Therefore we stabilized TIP by dissolving it first in EtOHand adding EDTA to prevent hydrolysis. TEA was also addedto the solution to increase the pH to neutral, thus inducingdissolution of the EDTA [30]. After this, a large excess ofwater (100:1 molar ratio to Ti4+) could be added to theprecursor. The pH of the solution was 5.2 and the viscosity2.9 cP. In order to increase the stabilizing of the particlesduring microwave treatment, ethylene glycol (EG) was addedto the reaction matrix and the viscosity increased to 3.3 cP.The solutions exhibit a slightly yellow color, which is typicalfor the presence of triethanolamine, which is being used asa complexing agent in our inks (figure 1(a)). When stored insealed beakers at room temperature, the solutions were stablefor several months. Heating of a small amount of the solutionspoured into a petri dish to 60 ◦C for 4 h led to the formationof stable and transparent gels (figure 1(c)).

Microwave treatment of the two solutions for 10 minat 140 ◦C resulted in a more intense yellowish color(figure 1(b)), related to changes in coordination duringprocessing, and precipitates free sols, slightly exhibitingthe Tyndall effect. Figure 2 shows DLS measurementsfor both sols as obtained after microwave treatment, afterkeeping them in a closed vessel in the dark for threemonths and after washing the freshly microwave-treated solby the precipitation/resuspension method. After microwavesynthesis, both sols showed a monodisperse but broad size

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Figure 1. Digital photographs taken for (a) solution Ti–E prior to microwave treatment, (b) after microwave treatment at 140 ◦C for 10 min,(c) gel obtained after drying the microwave-treated Ti–E solution at 60 ◦C and (d) heat-treated (to 150 ◦C) TiO2 layer on glass obtained byDimatix piezoelectric printing of the Ti–E+ EG ink, where the sample is placed on a paper on which geometric figures were printed toillustrate its transparency.

distribution, with an average particle size (Z average (d nm))of 37 nm and 43 nm for solution Ti–E and Ti–E + EG,respectively. The larger hydrodynamic radius found for thesample containing additional ethylene glycol might be relatedto a larger volume of organics being present on the surfaceof the particles. After storing this microwave-treated Ti–E solfor three months, the particle size for the sol was decreasedto 33 nm, without an obvious change in the shape of thesize distribution. This can be related to the slight dissolutionof the particles during aging. The suspensions were clearlyfree from agglomerates, even three months after synthesis,which is an important issue when using inkjet printing sinceagglomerated particles can block the printing nozzle. It hasbeen reported that, if the size of the particles is greater than 5%of the orifice diameter, jetting stability may be impaired [31].In Perelaer’s study [32], no clogging problems were observedprinting even larger particles. In our studies with differentinks, printing devices and concentrations, we have found ita practically useful rule of thumb that the particle size forprintable suspension inks should not exceed one-fiftieth of thediameter of the nozzle opening.

For some experiments, the sols were destabilized andthe precipitated particles resuspended in order to reduce theamount of organics remaining in the samples. This procedureapplied on Ti–E sols directly after microwave treatmentclearly caused a shift in the average particle size, from 37 to19 nm. This change in size can have different explanations:loss of surface-adsorbed organics, decreased agglomeration ora real decrease of the particle size.

XRF analysis was used to determine the Ti concentrationin the precursor solution before microwave treatment, inthe sol after microwave treatment and also on a washedsol. Although we had prepared the solution with a Ti4+

concentration of 0.4 M, the stoichiometric calculations fromXRF analysis showed that the solution before microwavetreatment contained 0.48 mol l−1 Ti4+, and after treatmentthe sol contained the same amount, giving a solid content of32 g l−1 TiO2. After washing of the sol by precipitation of thetitania powders by destabilization and resuspension in waterwe found that 1, 4 mass% of the Ti ions remains, which meanshalf of the titania ions are lost during washing.

TEM measurements were performed on sol Ti–E aftermicrowave treatment and purification. Figure 3(a) shows an

Figure 2. Size distribution curves obtained from DLSmeasurements shown in logarithmic scale for: (×) Ti–E sol aftermicrowave treatment at 140 ◦C for 10 min, (©) Ti–E+ EG sol aftermicrowaving at 140 ◦C for 10 min, (-) Ti–E sol (microwaved at140 ◦C for 10 min) after three months of storage and (M) Ti–E sol(microwaved at 140 ◦C for 10 min) after washing.

image of a cluster of agglomerated nanoparticles which isformed during drying of the drop on the grid. In figure 3(b),a HR-TEM image showing three agglomerated particles isshown. The cluster has a diameter of 45–75 nm. It consistsof at least four relatively large, polygon-shaped particleswith an average diameter of about 30 nm, in line with DLSmeasurements, and one small particle (6 nm in diameter) atthe top of the image. All particles are crystalline, as shownby the presence of clear lattice fringes. Measuring the latticespacing from the zoomed image in the inset in figure 3(b)results in an inter-planar spacing (d spacing) of 0.352 nm,typical of the (101) plane in anatase phase titania [33–35].The fringes corresponding to this lattice plane were the mostprominent in all images being collected. However, inter-planarspacings for the (120) and (111) planes of brookite are toosimilar to unambiguously distinguish between brookite andanatase from HR-TEM images alone. These results werefurther confirmed by collecting a SAED pattern for imagefigure 3(a). As can be seen from figure 3(c), the pattern shows

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Figure 3. TEM image of TiO2 nanoparticles inside Ti–E sol (microwaved at 140 ◦C for 10 min) with (c) selected-area electron diffractionpattern (SAED).

Figure 4. XRD patterns obtained for TiO2 powder: (a) as-obtainedafter washing and drying from a microwaved Ti–E ink, (b) aftercalcining for 2 h at 350 ◦C and (c) after calcining for 2 h at 500 ◦C.(hkl) indices are given for the anatase phase.

discrete spots indicative of the presence of single-crystalnanocrystalline material. By drawing circles through all themost intense spots, the corresponding inter-planar spacingscould be extracted, i.e. 0.355, 0.238, 0.188 and 0.169 nm.These are in good agreement with the expected spacings forthe (101), (004), (200) and (211) anatase planes [36].

In addition to TEM analysis, we also performed XRDanalysis of the obtained powders in order to analyze thecrystallinity of the as-synthesized nanoparticles and theinfluence of heat treatment. Washed powders obtained fromthe Ti–E+EG ink were dried at 60 ◦C and further heat-treatedup to 500 ◦C under air. For dried, as-obtained samples, wefind poorly defined and very broad anatase reflections at2θ = 25.3◦, 37.8◦, 48◦ and 54◦ (figure 4). Calcination ofthese samples at 350 and 500 ◦C increases the crystallinitywith obvious and sharp reflections related to the anatasephase. The decreasing peak width with increasing temperatureis obviously related to the increase in grain growth of thecrystalline material.

In order to turn the as-prepared sols into high qualitycoatings by inkjet printing, we also needed to ensure that therheology of the sol/ink fulfills the criteria for inkjet printing.

The energy for the acceleration of the droplets is viscous flowand the surface tension of the sols [37].

A first criterion to ensure proper jetting is the viscosityof the solution. Too high a viscosity impedes ink ejectionand acceleration: for piezoelectric printing, it damps thepropagating pressure wave created by the deformation ofthe piezoelectric actuator [38], and for electromagneticprinting it increases the required ink pressure for jetting,potentially beyond the range which the jetting device andtubing can withstand. Similarly, if the viscosity is too low(e.g. <1 mPa s), jetting behavior is readily influenced bypressure variations, and for electromagnetic printing ink may,under some circumstances, even leak out of the nozzle whileno print signal is given. We find that, after microwavesynthesis, the sols have a viscosity of 3.1 and 3.5 cP forTi–E and Ti–E + EG, respectively. Both values are withinthe specifications of the two different nozzles being used inthis work, although ideally for high-resolution piezoelectricprinting the viscosity should be a bit higher. Therefore, wechose to use only the ethylene glycol ink, exhibiting thehighest viscosity, with the piezoelectric printhead.

The generation of droplets in an inkjet printer is acomplex process, and the precise physics and fluid mechanicsinvolved in printing are the subject of much research [39–42].The behavior of inks in the printing system can bestbe quantified by a number of dimensionless groupings ofphysical constants, i.e. the Reynolds (Re), Weber (We) andOhnesorge (Oh) numbers:

Re = vrρ/η, We = v2rρ/σ and

Oh = We1/2/Re = η/(σρr)1/2 (1)

where σ , ρ, η and v are the ink surface tension, density,viscosity and velocity respectively and r is the radius ofthe orifice of the nozzle [26, 38, 40–42]. The Reynoldsnumber is a ratio of internal and viscous forces and theWeber number shows the ratio between internal and surfacetension forces. The inverse value of the Ohnesorge number isa characteristic dimensionless number which is independentof droplet velocity. Often it is written that Oh−1 should be>2 [38] for proper jetting properties. In Derby’s study [40],it was proposed that it should be 1 < Oh−1 < 10. If the ratiois too low, viscous forces become more dominant, preventingink drops from being ejected from the nozzle; conversely,

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Table 1. Fluid properties of Ti–E and Ti–E+ EG sols (after microwave synthesis).

SolSurface tensionσ (×10−2 J m−2)

Densityρ (kg m−3)

Dimension r(×10−5 m)

Viscosityη (×10−3 Pa s) RexWe−1/2

Ti–E 1.9 1184 Domino 4.5 3.1 10.3Dimatix 1.15 5.2

Ti–E+ EG 1.7 Domino 4.5 3.5 8.6Dimatix 1.15 4.4

if the ratio is too high the possibility for satellite dropletformation becomes high. In practice, even if the ratio is higherthan 10, the ink is reliably printable as long as the satellitesmerge with the main droplet in flight [38].

In table 1 we show the different numbers calculated forthe Ti–E and Ti–E + EG sols after microwave synthesisat 140 ◦C for 10 min. Since two different kinds of printerswith different nozzles were used, the calculations weredone using a radius of half of the orifice diameter of90 µm for the electromagnetic Domino nozzle and 23 µmfor the piezoelectric Dimatix nozzle. For piezoelectric andelectromagnetic printing, the inverse Ohnesorge numbers arewell in the defined range as seen in table 1.

3.2. Inkjet printing

A first series of printing tests were performed using anelectromagnetic Domino nozzle with 90 µm orifice. Adrop visualization study was performed in order to examinethe drop formation process, confirm reliable jetting andto establish the preferred pressure and opening time. Theevolution of the droplet shape during jetting at the optimumparameters for sol Ti–E + EG (0.6 bar and 400 µs) is shownin figure 5. The behavior of the droplets was examined,with representative points in the evolving ink stream shapeidentified in figure 5(a), and the changes in volume andcenter-of-mass position plotted on the graph in figure 5(b).Initially, the drops form a liquid column which transformsinto the actual droplet and an elongated tail. Here, break-upof this tail from the droplet leads to the formation of asatellite drop. The existence of satellite droplets at the timeof impact with the substrate should be avoided, as the keygoal is to leave a single, isolated droplet to optimize precision,resolution and accuracy during printing [43]. This wouldideally be achieved by preventing satellite droplet formation,but it is also sufficient for droplets to recombine in flight:as shown in figure 5, this latter condition was achieved afteroptimization of the printing parameters, resulting in formationof a single, spherical drop. The distance between the nozzleand the substrate should be sufficient to allow the droplets tomerge before impact. On the other hand, an increased standoffwill reduce the accuracy, because drag from air currents inthe printing chamber makes the droplets deviate from theirvertical trajectory, so the distance should be set as low aspossible [28]. In our case, the optimal distance between printhead and substrate was determined to be 4 mm. The dropletsrecombine into a single drop within 3 mm of the nozzle andwithin 2 ms of the first ink leaving the nozzle. As shownin figure 5(b), after the ink has left the nozzle, it moves at

Figure 5. Jetting analysis for the Ti–E+ EG sol at a pressure of600 mbar and an opening time of 400 µs: (a) sequence of images ofdrop formation, (b) images collected at 500 µs intervals and(c) graph representing the detailed analysis of the droplet volume,number of drops per image and position of the center of mass as afunction of time after the jetting trigger.

constant velocity (despite the shape changes). The resultingdrop had a volume of 11 nl and a velocity of 1.7 m s−1.As seen in figure 5(a), starting from the initial point, thedroplet follows a straight path and is rotationally symmetricabout the vertical axis, which are also parameters of goodjettability [44].

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Piezoelectric printing was performed with a DimatixMaterials Printer. This printer is equipped with a built-incamera for drop watching. The applied voltage waveformcauses deformation of the fluid chamber membrane, resultingin a pressure wave. The standard waveform for this printerconsists of three segments: a negative voltage period to drawink into the chamber, a positive voltage period to dispensethe fluid and a segment that allows the nozzle to recover toits original shape without drawing in air while doing so. Wefound that, for the Ti–E+EG ink, the best results are obtainedwith a slew rate of 0.65 V µs−1 and the duration for eachsegment is 3.584 µs with a maximum voltage of 18 V for all16 nozzles.

The next step in the printing optimization was the studyof the interaction between droplets and substrate. In order toform continuous coatings, we wanted to achieve controlledspreading of the droplets after impact on the substrate.Good wetting of the ink over the substrate combined witha well-chosen droplet pattern upon printing is important toobtain homogeneous coatings of the selected thickness. Wefound that a 2 µl droplet of the Ti–E + EG ink exhibited asmall contact angle of 11.2◦±0.1◦ although the same volumeof Ti–E ink displayed 23.1◦ ± 0.1◦, when placed on a glassmicroscopy slide.

The printing pattern finally used was then selected basedon the printing parameters and the wetting behavior of the ink.For electromagnetic printing we used a hexagonal pattern witha droplet spacing of 400 µm and for piezoelectric printing weused a square pattern with a droplet spacing of 35 µm.

Based on these results, we dried the printed layers at roomtemperature and finally heated them at temperatures varyingbetween 100 and 500 ◦C in air for 1–2 h. As can be seenfrom the digital picture shown in figure 1(d), the layers werevery transparent to visible light, even after heat treatment attemperatures as low as 150 ◦C.

Further characterization was done on Dimatix printedlayers since they were more homogeneous, as thin as possibleand exhibited high quality.

3.3. Transparency of TiO2 films

UV–vis spectroscopy was used to examine the transparencyof the films in a more quantitative way (figure 6). As areference, an uncoated microscopy slide glass was used.For the films printed with the Dimatix system from solTi–E + EG microwaved at 140 ◦C for 10 min and heatedat 100 and 200 ◦C, the transmittance at 500 nm droppedfrom 90% for pure glass to 89%, for the film heatedat 300 ◦C to 83%, and for the film heated at 500 ◦C to81%. For the sample heated at 500 ◦C, an obvious redshiftin absorption onset was visible. In none of the spectrawas there any sign of interference fringes, indicating thatthe layers were very thin (150 nm or less) [45, 46].

3.4. Thickness and morphology

FIB-SEM cross-sectional analysis was performed on thepiezoelectric printed layers as a function of heating

Figure 6. UV–vis optical transmittance spectra for TiO2 layerssintered at different temperatures prepared from sol Ti–Emicrowaved at 140 ◦C for 10 min.

temperature (figure 7) to quantify the thickness of thecoatings. For layers heated below 250 ◦C, the remainingvolume fraction of organic species was still large and thereforeit was not possible to analyze the layers by FIB-SEM.Figure 7(a) shows a cross-sectional view for a sample heatedat 250 ◦C. The TiO2 layer was 130–160 nm thick andcontained a small number of pores with a diameter of about10–20 nm. When the heating temperature was increased to350 ◦C, the layer thickness decreased to 90–110 nm. Thedecrease in thickness observed as a function of heatingtemperature is related to the loss of organics [47]. Increaseof the heating temperature can also lead to a decrease in thenumber of pores.

In order to determine the roughness of the films,non-contact AFM measurements were performed (figure 8).The reported RMS roughness values for uncoated soda-limemicroscope glass slides fall in the range of 0.2–0.3 nm asdetermined from AFM (on 50 µm× 50 µm surface area) [48,49], which shows that the surface of the glass is so smooththat it cannot be of any relevant influence on the top coating.We analyzed different samples, all obtained by piezoelectricprinting of a Ti–E+EG sol, microwaved at 140 ◦C for 10 minand heated at temperatures between 100 and 300 ◦C. Sincesample roughness is an important parameter when comparingphotocatalytic activities of different samples, we determinedfor each sample the RMS surface roughness on an area of5.0 µm×5.0 µm. For a sample heated at 100 ◦C, we found anRMS value of 3.6 nm; increasing the temperature to 200 and300 ◦C led to a decrease of the RMS roughness to 2.1 and to1.5 nm, respectively.

3.5. Photocatalytic activity

The photocatalytic activity of the TiO2 films obtained bypiezoelectric inkjet printing was studied by following thedegradation of a methylene blue dye in contact with theTiO2 as a function of exposure time to UV light. It is

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Figure 7. FIB-SEM cross-sectional micrographs of TiO2 films printed on glass with the multi-nozzle, piezoelectric Dimatix set-up,showing the thickness of the layers after heating for 2 h at (a) 250 ◦C and (b) 350 ◦C.

Figure 8. AFM topography images of TiO2 films obtained by piezoelectric printing of Ti–E+ EG sol (microwaved at 140 ◦C for 10 min),heated at (a) 100 ◦C, (b) 200 ◦C and (c) 300 ◦C.

generally accepted that the photocatalytic process followsa pseudo-first-order kinetic mechanism, as described by thefollowing equation [50–52]:

ln(C/C0) = −kt (2)

where C is the concentration of methylene blue afterphotocatalysis time t, C0 is the initial methylene blueconcentration and k is the rate constant of the reaction.The graphical representation of this equation, for TiO2

films printed from nanoparticle-containing sols and heatedat 150–500 ◦C compared with the photocatalytically active

commercial Saint-Gobain glass5 is depicted in figure 9. Fromthe graph, one can see that even a sample heat-treated attemperatures as low as 150 ◦C has high photocatalytic activity.The samples calcined at temperatures between 150 and 500 ◦Cshow higher photocatalytic activity than the commercial layer,which is known to be coated by chemical vapor deposition,and is a transparent, photocatalytically active layer with athickness of 50 nm (see footnote 5).

For each sample the rate constant k of photocatalysis wascalculated by performing a least-squares linear fit to all the

5 Saint-Gobain Glass UK Ltd, Eggborough. UK.

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Figure 9. Logarithmic plot of the decomposition of methylene blueas a function of UV exposure time for different samples of TiO2films obtained by piezoelectric printing of Ti–E+ EG sol(microwaved at 140 ◦C for 10 min) heat-treated at 150, 250 and500 ◦C. As a reference, a measurement with commercialSaint-Gobain Bioclean c© glass is included. A blank measurementon uncoated glass is also included in the graph. The quality of the fitis proved with the square of the correlation coefficient (R2) wherethe values are >0.9.

collected data points. We found that for the sample heated at150 ◦C, k150 is 11.0 × 10−4 min−1, for the film sintered at250 ◦C, k250 = 9.1× 10−4 min−1, k350 = 10.3× 10−4 min−1

and finally k500 = 11.6 × 10−4 min−1. The commercialtitania film was found to exhibit a rate constant ks of 5.0 ×10−4 min−1. From these rate constants calculated over thewhole duration of the experiment, the sample heated at 500 ◦Cshows the highest efficiency and the performance decreaseswith decreasing heating temperature. Against expectations,the layer that was only heated to 150 ◦C performs very wellat the beginning, but this clearly levels off at longer exposuretimes, resulting in a nonlinear response in the plot. This mightbe related to damage to the layers from being immersed inwater for longer times, due to dissolution or photocatalyzeddigestion of the residual organic content, resulting in alowly stable layer. The stability of the layers against specificconditions will therefore be the subject of another paper.Currently, we are performing weathering tests to evaluate theperformance of our layers as a function of exposure time tosimulate outdoor conditions.

Changes in specific surface and thus roughness, density,differences in crystallinity, organic content and surfacemorphology obviously play important roles in photocatalyticactivity. Also the layer thickness influences the maximumamount of UV light that can be absorbed [53–55]. From AFMwe could clearly detect a significant decrease of the surfaceroughness as a function of temperature, based on which weexpect a decreased activity for the samples treated at thehighest temperatures. Yet, on the other hand, their crystallinity(decreasing amorphous volume fraction) will certainly behigher, as can be seen from XRD analysis (shown in figure 4)and the organic content is reduced to the minimum afterheating at 500 ◦C.

4. Conclusions

Transparent, photocatalytically active TiO2 films have beenobtained by inkjet printing of aqueous TiO2 nanoparticlesuspensions obtained from bottom-up synthesis and lowtemperature heating. We developed a microwave-assisted,hydrothermal route for the preparation of colloidal suspen-sions containing titania nanoparticles of appropriate size. Byshifting from classical hydrothermal to microwave-assistedhydrothermal synthesis, we can reduce the complete synthesistime from over 24 h to 5 min, creating new possibilitiesfor industrial use of these kinds of synthesis approaches.Furthermore, the precursor solutions and the suspensionsobtained after synthesis are mainly aqueous, present long timestability and can be used without further processing for inkjetprinting. This relatively new technique in the production ofceramic thin films allows cheap, fast and extremely flexibleprocessing of almost any precursor ink, with a close to zerowaste of ink material.

In this work, we show that the combination of inkjetprinting with fast ink synthesis using microwave assistanceallows functional, high quality, photocatalytically active TiO2layers to be deposited on glass. The maximum temperaturesinvolved in the thermal treatment can be reduced to as low as150 ◦C, creating the possibility to use this protocol also forheat-sensitive substrates such as wood or polymers.

This work clearly shows that TiO2 layers prepared bycombining nanoparticle bottom-up synthesis with inkjet print-ing can be a suitable alternative to current photocatalyticallyactive self-cleaning glasses coated by TiO2 using expensivetechniques.

Acknowledgments

This research was carried out under the InteruniversityAttraction Poles Programme IAP/VI-17 (INANOMAT)financed by the Belgian State, Federal Science Policy Officeand EFECTS, a project funded by the European Union,FP7-NMP-2007-SMALL-1 grant no. 205854. One of theauthors (GP) is funded by a PhD grant of the Agency forInnovation by Science and Technology (IWT Flanders). Theauthors thank Bart Lucas (Ghent University—Department ofPharmaceutics) for the use and help with DLS measurementsand Jan Goeman (Ghent University—Department of OrganicChemistry) for helping with microwave synthesis.

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