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Journal of Aerosol Science

Journal of Aerosol Science 52 (2012) 57–68

0021-85

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journal homepage: www.elsevier.com/locate/jaerosci

Atmospheric synthesis of superhydrophobic TiO2 nanoparticledeposits in a single step using Liquid Flame Spray

Mikko Aromaa a,n, Anssi Arffman a, Heikki Suhonen a, Janne Haapanen a, Jorma Keskinen a,Mari Honkanen b, Juha-Pekka Nikkanen b, Erkki Levanen b, Maria E. Messing c, Knut Deppert c,Hannu Teisala d, Mikko Tuominen d, Jurkka Kuusipalo d, Milena Stepien e, Jarkko J. Saarinen e,Martti Toivakka e, Jyrki M. Makela a

a Department of Physics, Tampere University of Technology, PO Box 692, FI-33101 Tampere, Finlandb Department of Materials Science, Tampere University of Technology, PO Box 589, FI-33101 Tampere, Finlandc Solid State Physics and the Nanometer Structure Consortium, Lund University, PO Box 118, SE-22100 Lund, Swedend Department of Energy and Process Engineering, Tampere University of Technology, PO Box 541, FI-33101 Tampere, Finlande Laboratory of Paper Coating and Converting, Center for Functional Materials, Abo Akademi University, Porthansgatan 3, FI-20500 Turku, Finland

a r t i c l e i n f o

Article history:

Received 20 December 2011

Received in revised form

12 April 2012

Accepted 19 April 2012Available online 15 May 2012

Keywords:

Liquid Flame Spray

Titanium dioxide

Nanoparticle deposition

Functional coating

02/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.jaerosci.2012.04.009

esponding author: Tel.: þ358 40 8490370.

ail address: mikko.aromaa@tut.fi (M. Aromaa

a b s t r a c t

Titanium dioxide nanoparticles are synthesised in aerosol phase using the Liquid Flame

Spray method. The particles are deposited in-situ on paperboard, glass and metal

surfaces. According to literature, titanium dioxide is supposed to be hydrophilic.

However, hydrophobic behaviour is observed on paperboard substrates but not on

metal or glass substrates. Here, the water contact angle behaviour of the deposits is

studied along with XRD, XPS, BET and HR-TEM. The deposits are compared with silicon

dioxide deposits having, as expected, hydrophilic properties synthesised with the same

method. It seems probable that the deposition process combusts some substrate

material from the paperboard substrate, which later on condenses on top of the deposit

to form a carbonaceous layer causing the hydrophobic behaviour of the TiO2 deposit.

The similar layer does not form when depositing the nanoparticles on a metal or glass

surfaces. The observations are more than purely aerosol phenomena. However, they are

quite essential in nanoparticle deposition from the aerosol phase onto a substrate

which is commonly utilised.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Nanoparticles are synthesised using several different methods (Kodas & Hampden-Smith, 1999), by liquid, solid andgaseous routes. Gaseous aerosol routes are highly promising for a number of reasons. Compared to liquid chemistry andmilling of bulk materials, aerosol techniques are simple and offer high production rates (Strobel & Pratsinis, 2007). Thereare various different aerosol reactors including hot wall (Backman et al., 2004), laser (Cannon et al., 1982a & b & Moritaet al., 1999), plasma (Pfender, 1999) and flame reactors (Pratsinis, 1998). Flame synthesis has been used for a long time inmaterial science (Ulrich, 1971) and offers multicomponent and composite nanoparticle synthesis in a single step (Jud et al.,2006).

ll rights reserved.

).

M. Aromaa et al. / Journal of Aerosol Science 52 (2012) 57–6858

Liquid Flame Spray has been widely used in various applications e.g., glass colouring and synthesis of anti-microbialdeposit layers (Gross et al., 1999; Keskinen et al., 2006). Recently, we have shown that titania nanoparticles can bedeposited directly from the flame onto a paperboard substrate. This process is done in roll-to-roll fashion and in a pilotscale with line speeds up to 150 m/min. The nanoparticle deposits have highly hydrophobic properties (Makela et al.,2011; Stepien et al., 2011; Teisala et al., 2010). However, there has been speculation about the mechanism of obtaininghydrophobicity, as titanium dioxide is, by nature, hydrophilic (Takeda et al., 1999). Several other groups have alsosynthesised superhydrophobic titanium dioxide structures such as nanorods, nanotubes, composites etc. (Borras et al.,2008; Feng et al., 2005) as well as combinations of different structures (Lai et al., 2009). Also other hydrophilic materialshave been turned hydrophobic due to the creation of nanosized features using techniques such as deep reactive ionetching of e.g., Si surfaces (Song & Zou, 2007). The cause of the superhydrofobicity is claimed to be due to the so calledlotus structure (Barthlott & Neinhuis, 1997). This means basically, that there are micron sized features together withnanosized characteristics on top of lotus structured surfaces, which cause the superhydrophobicity. Similar phenomenonto lotus effect is a petal effect in which the superhydrophobic surface has also a high adhesive force (Feng et al., 2008).Another method for tuning a hydrophilic material into hydrophobic is to modify the surface with a layer of organicmolecules (Balaur et al., 2005). The aim of this paper is to further investigate the particles in order to reveal the causes ofhydrophobicity on the LFS nanoparticle deposits on paperboard surfaces which is left open in previous papers (Makelaet al., 2011; Stepien et al., 2011; Teisala et al., 2010).

Recently, core-shell particles have attracted great attention. Core materials can be encapsulated and functionalised bydifferent shell materials. One of the shell materials of interest is carbon because of its great potential with propertiesachieved with attaching functional groups on the surface (Athanassiou et al., 2006). A carbon shell can be easily producedin aerosol reactors. Carbon is also stable and it can be functionalised with additive molecules (Grass et al., 2007). Theformation of carbon shells has been performed in flame reactors by adding acetylene after formation of the core particles.Thus, the incomplete combustion of acetylene forms carbon structures on top of the particles. If the acetylene feed issufficient, the particles are encapsulated with a graphene shell. The graphene shell can afterwards be functionalised withliquid chemistry. Also, some precursor materials and solvents for flame spray synthesis assist the formation of the carbonshell. Ernst et al. (2008) noticed that usage of xylene as a solvent for platinum precursor in platinum particle synthesisembeds the platinum particles in a carbon shell resulting in inactive platinum catalyst particles.

Papers and boards are highly versatile materials with various beneficial properties, e.g., biodegradability, renewability,mechanical flexibility and affordability. The complete utilisation of the versatility of paper and board requires the ability tocontrol the surface properties. Numerous technologies, especially plasma techniques, have been used to fabricate state ofthe art coatings on paper and board surfaces. Superhydrophobic surfaces have been created by fluorinating the paper usinggrafting and post-functionalisation (Nystrom et al., 2006) or by silane coating the paper through solution immersionprocesses (Li et al., 2008). Plasma-assisted deposition of thin fluorocarbon (Mukhopadhyay et al., 2002; Pykonen et al.,2010; Vaswani et al., 2005), organosilicon (Pykonen et al., 2010) and hydrocarbon coatings (Pykonen et al., 2010) have alsoresulted in hydrophobic paper surfaces. Balu et al. (2009) created a superhydrophobic paper surface by combining plasmaetching with plasma enhanced chemical vapour deposition (PECVD) of fluorocarbon film. Generally, manufacturing andconverting processes of paper and board are usually continuous, high volume operations where the line speeds can varyfrom hundreds up to two thousand metres per minute. This sets high demands also for the surface modification anddeposition techniques used in the further processing of paper and board substrates. Therefore, the utilisation of batch- andwet chemical-type processes is limited in paper and board industry.

2. Experimental

2.1. Synthesis of the nanoparticles

Flame spray methods introduce precursor material into a flame and produce nanoparticles by evaporation andsubsequent nucleation of the precursor material followed by condensation growth. Precursor materials may also undergosome chemical reactions prior to nucleation. Liquid Flame Spray (LFS) is one type of apparatus for aerosol synthesis ofnanoparticles. It has been described in several papers (Aromaa et al., 2007; Makela et al., 2004). In this study, the substratetravels through the flame causing the nanoparticles to deposit onto the substrate forming the nanoparticle layer. The term‘deposit’ is used throughout the text as a reference to the nanoparticles deposited on the surface of the substrate. The term‘coating’ is reserved here for a later use.

The main principle in LFS is that the liquid precursor is fed into a turbulent oxygen–hydrogen flame. The hydrogen flowatomises the precursor flow into micron-sized droplets (Keskinen et al., 2008) which are subsequently evaporated in theflame. The liquid flow rate and combustion gas flow rates are controllable. In this study, we use titanium(IV) isopropoxide(TTIP, 97þ% Alfa Aesar) in isopropanol solution as precursor material for titanium dioxide particles. In the flame,evaporated TTIP molecules decompose thermally according to the following overall reaction (Courtecuisse et al., 1996):

TTIP-TiO2þ4C3H6þ2H2O

This reaction takes place even at temperatures as low as 250 1C (Chen et al., 1993). Propene is an additional product forthermal decomposition of TTIP. When burning the flame with stoichiometric gas flows of oxygen and hydrogen, the

M. Aromaa et al. / Journal of Aerosol Science 52 (2012) 57–68 59

surrounding oxygen is also available for isopropanol and propene to combust into carbon dioxide and water. Propenemight also react with hydrogen to form propane and combust after the reduction. However, if the gas flows for the flameare oxygen lean, there is insufficient amount of oxygen for isopropanol, propene and propane to combust and partialoxidation forms other carbon species such as CO or elemental C (e.g., Hart et al., 2003).

In this study, silicon dioxide deposits are only synthesised as a reference to titanium dioxide in order to study thedifferences between the deposits as the synthesis is done in a same way and the particle deposits are supposed to havesimilar physical characteristics. In earlier studies SiO2 deposits have shown hydrophilic properties (Stepien et al., 2011).Tetraethyl ortosilicate (TEOS, 98% Alfa Aesar) in isopropanol solution is used as precursor for synthesis of silicon dioxide.The reactions for synthesis of SiO2 from TEOS are similar to reactions for TTIP with the exception that the by-product isC2H4 instead of C3H6. However, Cho et al. (2009) have studied the formation of SiO2 from TEOS in flame reactors and foundout that in flame spray reactors precursor volatility is playing a role in particle formation via gas phase instead ofintradroplet reaction and SiO2 formation through liquid-to-solid conversion. Thermal decomposition of TEOS usuallyoccurs at temperatures above 600 1C (Xia et al., 2000).

2.2. Devices for depositing material on the substrate

LFS nanoparticle deposits have been synthesised in a paper converting machine in a roll-to-roll process (Makela et al.,2011; Stepien et al., 2011; Teisala et al., 2010) located at the Tampere University of Technology, Finland. The line speed andburner distance can be controlled in the machine environment together with other parameters of the process such asprecursor concentration and burner gas flow rates. A simplified illustration of the deposition process is presented in Fig. 1.The burner is installed into a fume hood facing downwards in the middle of the substrate which is rolling underneath thehood. The fume hood is closed from three sides and open in the front. The dimensions of the hood in mm are800�400�800 (height�width� length). The hood lies 20 mm above the substrate and the gap is closed from the threesides with a piece of flame-proofed cloth. From the ceiling of the hood, exhaust fumes are ventilated away using a tubewith a diameter of 100 mm. The system is also simulated as such.

A laboratory scale conveyor line was built to mimic the on-line deposition process in the paper converting machine.With the conveyor line A4-sized (21.0�29.7 cm) samples of paper and paperboard together with other materials can beused as substrates for nanoparticle deposits in order to do quick tests with several parameters. The conveyor line makes iteasier to prepare samples because the length of one sample in the paper converting machine is in the order of hundreds ofmetres. The conveyor line is illustrated in Fig. 2. It contains a stainless steel sample carriage travelling along a track. Thetrack is oval shaped with a straight part in the deposition area. The carriage is pushed by arm from an electric motor whichcontrols the speed of the carriage. A sample holder is attached on the carriage. The height of the holder can be altered inorder to alter the distance between the burner and the sample. Also the speed of the sample running under the depositionprocess can be adjusted portably from 0 to 150 m/min. It has been showed previously that the orientation of the burnerdoes not play a role in nanoparticle formation (Arabi-Katbi et al., 2002) and thus the flame can also direct the particlesupwards in the laboratory scale conveyor.

2.3. Sample preparation

All the samples were synthesised using the same parameters. Precursor concentration 50 mg(Atomic metal)/ml;precursor feed rate 32 ml/min; gas flow rates 15/50 (O2/H2) slpm; line speed 50 m/min; burner distance 150 mm. Theseparameter sets have been used previously on paperboard and other materials (Teisala et al., 2010). Teisala et al. (2010)reported that the TiO2 nanoparticle deposit on paperboard synthesised with the oxygen lean process parameters havehydrophobic nature with a water contact angle up to 1601. However, as mentioned in the introduction part, titaniumdioxide, as well as other ceramic materials, is supposed to be hydrophilic by nature (Takeda et al., 1999). The hydrophilicityis induced by hydroxyl groups that attach onto the titania surface. Stoichiometric gas flows of 20/40 (O2/H2) slpm have beenused otherwise previously with LFS studies (Keskinen et al., 2006). Thus, some of the samples are also synthesised here with

Fig. 1. The deposition unit in the paper converting machine with the fume hood which is modelled. The fume hood is closed from three sides and open in

the front. Exhaust fumes are ventilated from the top of the hood.

Fig. 2. An illustration of the laboratory scale machine mimicking the deposition process performed in the paper converting machine. A view of the track

from (a) above and (b) side. The carriage is moving along the track and passing by the deposition point.

M. Aromaa et al. / Journal of Aerosol Science 52 (2012) 57–6860

stoichiometric flow ratio as they act as a reference parameter set as the oxygen lean process might have some special featurein forming the superhydrophobic deposits.

Samples of paperboard (pigment coated Natura paperboard 210 g/m2, Stora Enso, Skoghall, Sweden) were used assubstrates for nanoparticle deposition with both a converting machine and a specially built laboratory scale track. Alsosamples for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were prepared with bothdevices. Several SEM and TEM samples were prepared in order to verify the similarity of the deposits so that the depositscan be reproduced with the laboratory scale conveyor line with the same parameters in both. TEM samples were preparedso that the TEM grid (Agar, Holey Carbon, 300 mesh, Cu) was attached onto the paperboard sample by attaching a mask ofthe same material with an opening on top of which the TEM grid peeked out. The mask was large enough to avoid edgeeffects. The studied TEM and SEM samples were similar with similar process parameters on the paper converting machineand with the conveyor line so the results presented here are not differentiated by the method of the deposit synthesis.

In the conveyor line, also glass and metal sheets (steel, aluminium and copper) were prepared together with TEMsamples. The sheet replaced the paperboard in order to investigate whether the paperboard as a substrate affects thedeposition process. The TEM sampling was performed in the same way with metal sheets as it was performed with thepaperboard.

Powder samples of the LFS synthesised nanoparticles were collected using a custom-made plate-type electrostaticprecipitator (ESP). The charging and collecting plates are combined. There are two steel plates with a distance of 8 cm andthe voltage between the plates is 20 kV. On the negative plate, there are corona needles to charge the particles before thecollection section. The collection plate is grounded.

2.4. Analysis

Paperboard and metal sheet samples were studied using a water contact angle (WCA) measurement device (KSV CAM200).SEM samples were cut from the paperboard and studied with Field Emission SEM (Zeiss ULTRAplus). Paperboard sheets werealso analysed using X-ray photoelectron spectroscopy (XPS; PHI Quantum 2000). TEM grids were studied with JEOL JEM-2010and high resolution TEM JEOL, JEM-3000-F, equipped with an energy dispersive spectrometry, EDS, for elemental analysis. Thecrystallinity of the powder samples was analysed with X-ray diffraction (XRD, Kristalloflex D-500, Siemens) and the specificsurface area of the powders with the Brunauer–Emmett–Teller (BET) method (Coulter Omnisorp 100cx).

2.5. Modelling tool

The LFS deposition process was simulated employing CFD package of Ansys Fluent 12.1. The flow field was solved in acoupled manner with the chemical species transport and combustion reactions in a 3D-model. RANS-based (Reynolds-averaged Navier–Stokes) modelling approach was used. That is, equations describing the time averaged flow field weresolved and the effects of turbulence on reaction rates and transfer phenomena are modelled solving two additional partial

M. Aromaa et al. / Journal of Aerosol Science 52 (2012) 57–68 61

differential equations that describe the state of turbulence. In addition, the radiation heat transfer was simulated using thediscrete ordinates model.

Combustion of hydrogen and isopropanol with oxygen from the spraying nozzle and surrounding air were the modelledreactions. For the isopropanol a single step overall reaction scheme was used and the combustion of hydrogen wasassumed to be purely mixing limited. Stainless steel walls were assumed to be in constant temperature but the movingpaperboard was simulated in more detail. The paperboard was assumed to have a finite thickness and to radiate andconvect the heat to the surroundings. This way, the surface temperature of the paperboard was possible to be solved.Motion of the paperboard has also an effect on the flame tip shape and this motion is also included in the model. A morethorough description of the simulations and the results can be found from Arffman et al. (2012). A schematic figure of thesimulated case geometry is shown in Fig. 1.

Together with the CFD simulation, the equivalence ratio of the flame is calculated. Referring Turns (1996), Ernst et al.(2008) define the equivalence ratio as a measure for stoichiometricity of the process. In the equivalence ratio, the ratio ofmolar amounts between fuel and oxidant is compared in the real situation against the stoichiometric one. The equivalenceratio is defined as:

F¼ðnfuel=noxidantÞreal

ðnfuel=noxidantÞstoichiometric

where n is the molar concentration. The ratio means that if Fo1, the system is oxidant rich and if F41, the system is fuelrich, i.e., oxygen lean. The ratio easily defines whether there is enough oxygen for all the compounds to oxidise.

3. Results and discussion

In previous studies, we have synthesised titanium dioxide nanoparticle deposits on various substrates using thestoichiometric gas flow rates (Keskinen et al., 2006; Makela et al., 2004). In a previous paper (Makela et al., 2011) weintroduced an application with non-stoichiometric gas flow rates of O2–H2 along with a simple model calculation.A thorough CFD model on the nanoparticle deposition system from the flame on a moving substrate is presently developedfor that system, and will be reported later in a full paper (Arffman et al., 2012). Here, we report a limited set of the data onthe temperature and concentration profiles to assist the consideration on nanoparticle and deposit morphology. Especiallycomparison between stoichiometric and oxygen lean parameters will be relevant. We have previously synthesisedhydrophobic surfaces with the oxygen lean gas flows (Makela et al., 2011; Stepien et al., 2011; Teisala et al., 2010), but thereason for hydrophobicity has not been clear. The aim for the results is to show the reason for different wetting behaviourof the titanium dioxide and silicon dioxide deposits. The titanium dioxide and silicon dioxide deposits on paperboard havewater contact angle values of 151711 and 21711 respectively, the WCA for the reference paperboard is 60711.

3.1. Simulation model

The CFD-simulation of the process was used to clarify the differences between the two processes presented in theexperimental section. Based on simple estimations on material balances, the processes do not seem to be much different.Both of the parameter sets used in this study are highly oxygen lean. Equivalence ratio describes how sub- orsuperstoichiometric the process is. When feeding precursor solution of 50 mg(Ti)/ml (TTIP and isopropanol) with 32 ml/mininto the flame of 15 slpm of oxygen and 50 slpm of hydrogen, the ratio between the amount of oxygen in a process thatwould be stoichiometric and the amount of oxygen in the real process is 4.6: 1 being stoichiometric and over 1 beingoxygen lean environment. Of course, as the deposition is performed in open atmosphere, the ambient oxygen decreasesthe calculated equivalence ratio to some extent, but not the amount needed. The calculation allows a possibility ofinsufficient combustion of isopropanol and propene.

In the case of stoichiometric gas flows (20 slpm O2 and 40 slpm H2) the equivalence ratio still remains high with a valueof 3.3. The vast amount of precursor solution increases the amount of reducing components in the flame and keeps theratio high. Therefore, no major conclusions can be drawn from the synthesis conditions for the nanoparticles to bedifferent in both cases.

Fig. 3 shows the simulated temperature contours from the cross-section of the deposition process and from the surfaceof the paperboard with different hydrogen–oxygen ratios. Fig. 3(a) shows that temperatures in the stoichiometric case aremuch higher than in the oxygen lean case (b). All images show the temperature of the gas in the process. The upper imageis a view from the side (white square being the burner) and the image below is the temperature of the gas right above thesubstrate. The maximum temperature of the flame is approximately 2300 1C with the stoichiometric gas flows and around1700 1C with the oxygen lean gas flows. This means that turbulent mixing cannot provide enough oxygen from thesurroundings for the complete combustion of both isopropanol and hydrogen in the hydrogen rich flame. The conditionsare oxygen lean and as combustion of hydrogen consumes oxygen, there is not enough left for the solvent (isopropanol)and by-products of decomposition of TTIP (propene) to oxidise. The incomplete combustion is also modelled. The referencesystem with stoichiometric conditions does not have such strong incomplete oxidation of the precursor material accordingto the model leaving the possibility of unreacted precursor material to cause the hydrophobic effect. However, TTIP

Fig. 3. Simulated temperature contours from the flame cross-section and from the paperboard surface. (a) Stoichiometric gas flows (20/40) slpm (O2/H2)

(b) Oxygen lean gas flows (15/50) slpm (O2/H2).

Fig. 4. XRD spectra for TiO2 powder produced with (a) oxygen lean combustion gas flows of 15/50 slpm (O2/H2) and (b) stoichiometric gas flows of

20/40 slpm (O2/H2). The peaks for anatase (A) and rutile (R) are labelled in the figure.

M. Aromaa et al. / Journal of Aerosol Science 52 (2012) 57–6862

decomposes thermally at very low temperatures. Here, the temperature of the flame does not limit the formation oftitanium dioxide.

3.2. Crystallinity and chemical composition of the synthesised TiO2 nanoparticles

Powder collection of the nanoparticles was carried out in aerosol phase in order to study the difference of the powdermaterial that is being deposited on the substrate between the studied parameters. XRD analysis was performed for both ofthe powders. The analysis spectra are shown in Fig. 4. The powder is mainly composed of anatase with some peaks of rutilepresent in the spectra. This is in line with previous studies dealing with formation of rutile as residual particles whichappear in small amounts (Keskinen et al., 2007). With both parameter sets the graphs look alike and the peaks are similar.Thus the nanoparticles are in both cases mainly anatase and the differences in gas flows and flame temperature do not

Fig. 5. XPS sample of the hydrophobic paperboard on which titania nanoparticles have been deposited.

M. Aromaa et al. / Journal of Aerosol Science 52 (2012) 57–68 63

have an effect on the particle crystallinity. BET analysis was also performed for the samples in order to measure thespecific surface area (SSA). The SSA of powder samples collected with ESP is 48.92 m2/g for the oxygen lean 15/50 slpm(O2/H2) gas flow sample and 56.86 m2/g for the stoichiometric gas flow 20/40 slpm (O2/H2) sample. The surface area valuesare quite close to each other and the small difference can be explained by the difference in primary particle formation andaggregation processes because of the significant temperature difference in the flame shown by the model.

XPS spectrum is shown here only for the TiO2 deposits on paperboard with the gas flows of 15 slpm O2 and 50 slpm H2

as XRD results indicate that the gas flow ratio of the burner does not have an impact on the particle crystallinity. The XPSspectrum is shown in Fig. 5. XPS is essential in order to find out whether there is unburned precursor material left in someform. The oxidation state of the deposited titanium oxide can be concluded from the XPS spectrum. The spectrum showsthe peaks for oxygen and titanium and thus no other oxidation states than TiO2 are virtually detectable in the deposit. Thisclearly demonstrates that there is no possibility of a titanium suboxide or other titanium species on the deposit. The XPSspectrum also shows a peak for calcium which comes from the substrate. There is a large peak for carbon visible which canbe from the substrate or it can be a result of incomplete combustion of the precursor solution, which is justified with thecalculation of equivalence ratio in the simulation chapter. The calculation leaves a possibility of insufficient combustion ofisopropanol and propene which may result in carbon formation. Also one possible source for carbon peak is slightcombustion of the substrate which forms carbon soot.

3.3. Deposit morphology

SEM images, already published in Stepien et al. (2011), of the deposited TiO2 and SiO2 particles show no differencesbetween the materials. Images show lotus-like structures as there are micron sized features and nanosized characteristicsas well. Stepien et al. (2011) refer to a petal effect which is similar to lotus effect, but there is additionally high adhesionbetween the water droplet and the surface. However, TiO2 deposit has a water contact angle of 156.271.11 and SiO2

deposit has a water contact angle of 12.271.21 with the reference being 76.870.71. Also, the surface roughness valuespresented by Stepien et al. (2011) are quite alike with the RMS roughness being 70.3 nm (reference paperboard), 94.3 nm(TiO2 deposit on paperboard) and 86.0 nm (SiO2 deposit on paperboard).

TEM sampling was performed for both deposited TiO2 and SiO2 particles in order to examine if the particlemorphologies of the deposits have any differences in smaller scale. TEM images acquired at different magnifications forboth titania and silica are presented in Fig. 6. The TEM images reveal an unexpected difference between the deposits.Fig. 6(a) is showing the magnification set for TiO2 and Fig. 6(b) a set for SiO2. A surprising observation can be made fromFig. 6(a), where a coating or a shell of quite a light material (less contrast in the TEM images) is visible in the TiO2 sampleon top of the deposited particles. Similar coating is not seen in any of the previous studies of TiO2 synthesised with LFS(e.g., Aromaa et al., 2007 and Keskinen et al., 2006). Summing up the XPS results with the TEM images leads to a conclusionthat the coating shell on the particle deposits consists of carbon. Carbon is highly hydrophobic, which supports the watercontact angle test results. On the other hand, TiO2 and SiO2 particles look quite alike in SEM. The particle size for bothmaterials is similar and thus the hydrophobic effect might not be alone due to lotus-shaped surface but also from thecoating on top of the particles.

Another unexpected result is seen in Fig. 6(b), with SiO2 particles. Visually, the shape of the particles is not round as itwould be for air-borne particles. With incomplete sintering in aerosol phase, the particles would still have round shapes,but here the shape is rougher. Instead, the shape of the particles would rather indicate that the nanoparticle deposit isformed directly from deposited liquid precursor droplets which have reacted and formed SiO2. Similar deposition of liquid

Fig. 6. A set of TEM images with different magnifications for (a) TiO2 nanoparticles and (b) SiO2 nanoparticles. The scale bars for the images: on left

100 nm and on right 20 nm.

M. Aromaa et al. / Journal of Aerosol Science 52 (2012) 57–6864

in flame reactor has been previously observed by Tricoli et al. (2010). Camenzind et al. (2008) published similar formationof silicon dioxide from unreacted precursor in diffusion flames. Another alternative for the deposit formation would bechemical vapour deposition, if the gaseous compounds do not have time to nucleate and the formation of solid depositstakes place on the surface. Both of the presented alternatives may happen at the same time because some of the precursormaterial is for sure evaporated from the initial droplets but possibly not all. XPS analysis of the deposit confirms thematerial being SiO2. Cho et al. (2009) studied the particle formation of SiO2 from TEOS in a similar aerosol flame spraygenerator and found out that the temperature of the flame is limiting the particle formation. They found out that if theflame temperature is low, the particles form before the evaporation of the precursor. As the model for the flame shows,here, the temperature of the flame is as low as in the study of Cho et al., where particles formed through liquid-to-solidroute and the residence time in the hot region is small. Thus, it is quite possible that there is deposition of tiny liquidprecursor droplets onto the substrate and nanoparticle deposit is formed from liquid-to-solid conversion via hydrolysis ofthe precursor or thermal decomposition of the precursor. The lack of similar shell as there is with the TiO2 particles couldbe explained with the temperature difference. The liquid layer formed from precursor droplets is absorbing the heat of theflame and the thermal conductivity is not big enough to heat up the substrate in order to combust it sufficiently.

3.4. Possibilities for the formation of the coating shell

As a reference, a set of TiO2 deposits was prepared on inorganic substrates with various thermal conductivities: glass(both matt and glossy), copper, aluminium and stainless steel sheets. The deposits were synthesised on the plates usingthe same conveyor line that was used for the paperboard samples. The water contact angle tests were performed onselected reference substrates and the contact angles were (WCA for the fresh reference surface in brackets after the WCAvalue for TiO2 nanoparticle deposit sample): aluminium 49781 (89711); glass, matt 01 (24731); glass, glossy 17711(26721). The WCA of TiO2 deposited on the matt glass sample is marked as 01 as the water droplet spread over the width

Fig. 7. HR-TEM image of the TEM grid attached on copper sheet. There are no signs of carbon film on top of the particles as there is when the TEM grid is

attached on the paperboard. Gas flow rate 15/50 slpm (O2/H2).

M. Aromaa et al. / Journal of Aerosol Science 52 (2012) 57–68 65

of the camera screen immediately after the droplet touched the surface. All the samples obtain hydrophilic behaviour andall the samples have a lower WCA as the reference and even the rough structure in matt glass does not offer a platform forlotus-like effect. TEM images of the set show no evidence of the shell on top of the particles with either of the depositionparameters. Some of the images of particles from TEM analysis on copper surface without the shell are shown in Fig. 7. Thewater contact angle of the nanoparticle layer deposited on the reference materials is relatively low, which is in agreementwith the findings of ceramic materials being hydrophilic (Takeda et al., 1999) if there is no lotus or petal effect. XPSanalysis left a possibility of insufficient combustion of isopropanol and propene from precursor solution. These resultstogether rule out the possibility of formation of the carbon from precursor material and are strongly suggesting that theformation of the carbonaceous shell is due to the combustion of hydrocarbons in the substrate material.

TEM sample grids collected on paperboard were also analysed in HR-TEM. Analysis was performed on the edge of thehole in holey carbon grid. Analysis shows that the shell does not have significant order between the atoms. An example ofHR-TEM image is shown in Fig. 8(a). Therefore, the shell is not clearly graphene or graphite but more amorphous.

Elemental mapping was also performed for the HR-TEM samples. Elemental map shows that the particles mainlyconsist of titanium and oxygen. There is also carbon and copper present which are the peaks originating from the TEM grid.Furthermore, one can see sharp edges on the peak of titanium when scanning across the particle. There is virtually nosignal in the region outside the particle, where the shell is present. Therefore, the elemental mapping rules out unreactedprecursor and titanium carbide as part of the shell, in the limit of detection. The finding supports the results from XPS. TheEDS line scan of the particle is shown in Fig. 8(b).

The oxygen lean flame with flow rates of 15 and 50 slpm of O2 and H2, respectively, is reducing the flame temperaturefrom the maximum. Therefore, slight combustion of the substrate may occur and the material is able to burn and formcarbon as a product of incomplete combustion in oxygen lean atmosphere. Also the substrate material plays a role. Isseems that the substrate needs to contain organic material and a carbon source must be available and in such a form that itcan evaporate and form the shell on the titania deposit as a result of the combustion. The temperature of the paperboard isaround 100–300 1C (Teisala et al., 2010) 0.5 m after the deposition process. Thus, the temperature is suitable forevaporation and even combustion of hydrocarbons that are in the substrate material. Oxygen lean process promotescarbon formation over carbon monoxide and carbon dioxide formation. The source of carbon is lacking from the metalsheets and no deposition parameters are found to form hydrophobic deposits on the metal sheets. SiO2 deposits on theother hand have a hydrophilic nature. The deposits form via different processes. However, the fact that SiO2 deposits arehydrophilic together with the coating shell on TiO2 particle deposits do not rule out the possibility of hydrocarboncontaminants as analysed by Takeda et al. (1999) or the petal effect being the key factor in superhydrophobic behaviour ofTiO2 deposits as there is more to the micro- and nanosized patterning than only the RMS roughness value.

4. Conclusions

In literature, carbon coating of particles has been created in specially designed aerosol reactors in order to functionalisethe surface of nanoparticles. In this study, we have synthesised TiO2 nanoparticle aerosol and deposited it onto apaperboard substrate. The nature of the substrate promotes formation of carbonaceous coating on top of the depositedparticles, when using specific parameters for nanoparticle deposit synthesis. The temperature of the process needs to becontrolled so that carbon containing materials are evaporated from the substrate, combusted and re-condensed onto theparticles. The resulting nanoparticle deposit has a uniform carbonaceous shell. TEM images confirm the shell consisting ofcarbon and explain the origin of the carbon peak in the XPS results. The deposit may also have adsorbed hydrocarbon

Fig. 8. (a) HR-TEM image on the edge of the hole in the holey carbon TEM grid containing a particle. The image shows a high magnification of the shell.

No clear atomic layers are visible in the shell. (b) Line scan elemental analysis of one particle and the surrounding shell (different from (a)).

The sharp edges of the material peaks show that there are no titanium atoms in the shell. Also no other materials are detected. The peak of carbon cannot

be analysed with the method because of the carbon film of the grid. The plotted signals in the image (from upper left to lower right): titanium, oxygen

and carbon.

M. Aromaa et al. / Journal of Aerosol Science 52 (2012) 57–6866

contaminants on the surface. Altogether, the deposit gives hydrophobic properties for the substrate (or transforms thehydrophilic particles into hydrophobic). The particles do not have other changes in crystalline form depending on theprocess parameters. Produced titania nanoparticles can be deposited from aerosol on a surface to obtain white, stable,hydrophobic deposit on paper or paperboard. Silica deposits, on the other hand, produce a white and also stablehydrophilic deposit on paper and paperboard. The particles are produced in large scale and the deposition can be done in aroll-to-roll process in a pilot scale.

M. Aromaa et al. / Journal of Aerosol Science 52 (2012) 57–68 67

Acknowledgements

This work was performed under the Nanorata 2 project funded by The Finnish Funding Agency for Technology andInnovation, Tekes.

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