Author’s Accepted Manuscript
Atmospheric synthesis of superhydrophobic TiO2
nanoparticle deposits in a single step using liquidflame spray
Mikko Aromaa, Anssi Arffman, Heikki Suhonen,Janne Haapanen, Jorma Keskinen, Mari Honkanen,Juha-Pekka Nikkanen, Erkki Levanen, Maria E.Messing, Knut Deppert, Hannu Teisala, MikkoTuominen, Jurkka Kuusipalo, Milena Stepien, JarkkoJ. Saarinen, Martti Toivakka, Jyrki M. Makela
PII: S0021-8502(12)00075-4DOI: http://dx.doi.org/10.1016/j.jaerosci.2012.04.009Reference: AS4546
To appear in: Journal of Aerosol Science
Received date: 20 December 2011Revised date: 12 April 2012Accepted date: 19 April 2012
Cite this article as: Mikko Aromaa, Anssi Arffman, Heikki Suhonen, Janne Haapanen,Jorma Keskinen, Mari Honkanen, Juha-Pekka Nikkanen, Erkki Levanen, Maria E.Messing, Knut Deppert, Hannu Teisala, Mikko Tuominen, Jurkka Kuusipalo, MilenaStepien, Jarkko J. Saarinen, Martti Toivakka and Jyrki M. Makela, Atmosphericsynthesis of superhydrophobic TiO2 nanoparticle deposits in a single step using liquidflame spray, Journal of Aerosol Science, http://dx.doi.org/10.1016/j.jaerosci.2012.04.009
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Atmospheric Synthesis of Superhydrophobic TiO2 Nanoparticle Deposits in a Single Step using Liquid Flame Spray
Mikko Aromaaa, Anssi Arffmana, Heikki Suhonena, Janne Haapanena, Jorma Keskinena, Mari Honkanenb, Juha‐Pekka Nikkanenb, Erkki Levänenb, Maria E. Messingc, Knut Deppertc, Hannu Teisalad, Mikko Tuominend, Jurkka Kuusipalod, Milena Stepiene, Jarkko J. Saarinene, Martti Toivakkae and Jyrki M. Mäkeläa
a Department of Physics, Tampere University of Technology, P.O. Box 692, FI‐33101 Tampere, Finland
b Department of Materials Science, Tampere University of Technology, P.O. Box 589, FI‐33101 Tampere, Finland
c Solid State Physics and the Nanometer Structure Consortium, Lund University, Box 118, SE‐22100 Lund, Sweden
d Department of Energy and Process Engineering, Tampere University of Technology, P.O. Box 541, FI‐33101 Tampere, Finland
e Laboratory of Paper Coating and Converting, Center for Functional Materials, Abo Akademi University, Porthansgatan 3, FI‐20500 Turku, Finland
Corresponding author: Mikko Aromaa, [email protected], +358‐40‐8490370
Abstract
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.
Keywords: Liquid Flame Spray; titanium dioxide; nanoparticle deposition; functional coating
1. Introduction
Nanoparticles are synthesised using several different methods (Kodas and Hampden‐Smith 1999), by liquid, solid and gaseous routes. Gaseous aerosol routes are highly promising for a number of reasons. Compared
to liquid chemistry and milling of bulk materials, aerosol techniques are simple and offer high production rates (Strobel and Pratsinis 2007). There are various different aerosol reactors including hot wall (Backman et al. 2004), laser (Cannon et al. 1982a and b and Morita et al. 1999), plasma (Pfender 1999) and flame reactors (Pratsinis 1998). Flame synthesis has been used for a long time in material science (Ulrich 1971) and offers multicomponent and composite nanoparticle synthesis in a single step (Jud et al. 2006).
Liquid Flame Spray has been widely used in various applications e.g. glass colouring and synthesis of anti‐microbial deposit layers (Gross et al. 1999, Keskinen et al. 2006). Recently, we have shown that titania nanoparticles can be deposited directly from the flame onto a paperboard substrate. This process is done in roll‐to‐roll fashion and in a pilot scale with line speeds up to 150 m/min. The nanoparticle deposits have highly hydrophobic properties (Mäkelä et al. 2011, Stepien et al. 2011 and Teisala et al. 2010). However, there has been speculation about the mechanism of obtaining hydrophobicity, as titanium dioxide is, by nature, hydrophilic (Takeda et al. 1999). Several other groups have also synthesised superhydrophobic titanium dioxide structures such as nanorods, nanotubes, composites etc. (Borras et al. 2008 and Feng et al. 2005) as well as combinations of different structures (Lai et al. 2009). Also other hydrophilic materials have been turned hydrophobic due to the creation of nanosized features using techniques such as deep reactive ion etching of e.g. Si surfaces (Song and Zou 2007). The cause of the superhydrofobicity is claimed to be due to the so called lotus structure (Barthlott and Neinhuis 1997). This means basically, that there are micron sized features together with nanosized characteristics on top of lotus structured surfaces, which cause the superhydrophobicity. Similar phenomenon to 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 organic molecules (Balaur et al. 2005). The aim of this paper is to further investigate the particles in order to reveal the causes of hydrophobicity on the LFS nanoparticle deposits on paperboard surfaces which is left open in previous papers (Mäkelä et al. 2011, Stepien et al. 2011 and Teisala et al. 2010).
Recently, core‐shell particles have attracted great attention. Core materials can be encapsulated and functionalised by different shell materials. One of the shell materials of interest is carbon because of its great potential with properties achieved with attaching functional groups on the surface (Athanassiou et al. 2006). A carbon shell can be easily produced in aerosol reactors. Carbon is also stable and it can be functionalised with additive molecules (Grass et al. 2007). The formation 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 is sufficient, the particles are encapsulated with a graphene shell. The graphene shell can afterwards be functionalised with liquid chemistry. Also, some precursor materials and solvents for flame spray synthesis assist the formation of the carbon shell. Ernst et al. (2008) noticed that usage of xylene as a solvent for platinum precursor in platinum particle synthesis embeds 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 to control the surface properties. Numerous technologies, especially plasma techniques, have been used to fabricate state of the art coatings on paper and board surfaces. Superhydrophobic surfaces have been created by fluorinating the paper using grafting and post‐functionalisation (Nyström et al. 2006) or by silane coating the paper through solution immersion processes (Li et al. 2008). Plasma‐assisted deposition of thin fluorocarbon (Mukhopadhyay et al. 2002, Pykönen et al.
2010 and Vaswani et al. 2005), organosilicon (Pykönen et al. 2010) and hydrocarbon coatings (Pykönen et al. 2010) have also resulted in hydrophobic paper surfaces. Balu et al. (2009) created a superhydrophobic paper surface by combining plasma etching with plasma enhanced chemical vapour deposition (PECVD) of fluorocarbon film. Generally, manufacturing and converting processes of paper and board are usually continuous, high volume operations where the line speeds can vary from hundreds up to two thousand meters per minute. This sets high demands also for the surface modification and deposition techniques used in the further processing of paper and board substrates. Therefore, the utilisation of batch‐ and wet 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 and subsequent nucleation of the precursor material followed by condensation growth. Precursor materials may also undergo some chemical reactions prior to nucleation. Liquid Flame Spray (LFS) is one type of apparatus for aerosol synthesis of nanoparticles. It has been described in several papers (Aromaa et al. 2007 and Mäkelä et al. 2004). In this study, the substrate travels 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 flow atomizes the precursor flow into micron‐sized droplets (Keskinen et al. 2008) which are subsequently evaporated in the flame. The liquid flow rate and combustion gas flow rates are controllable. In this study, we use titanium(IV) isopropoxide (TTIP, 97+% 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):
2 3 6 2TTIP TiO 4C H 2H O→ + +
This reaction takes place even at temperatures as low as 250 °C (Chen et al. 1993). Propene is an additional product for thermal decomposition of TTIP. When burning the flame with stoichiometric gas flows of oxygen and hydrogen, the surrounding oxygen is also available for isopropanol and propene to combust into carbon dioxide and water. Propene might also react with hydrogen to form propane and combust after the reduction. However, if the gas flows for the flame are oxygen lean, there is insufficient amount of oxygen for isopropanol, propene and propane to combust and partial oxidation 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 the differences between the deposits as the synthesis is done in a same way and the particle deposits are supposed to have similar 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 is C2H4 instead of C3H6. However, Cho et al. (2009) have studied the formation of SiO2 from TEOS in flame reactors and found out that in flame spray reactors precursor volatility is playing a role in particle formation via gas phase instead of
intradroplet reaction and SiO2 formation through liquid‐to‐solid conversion. Thermal decomposition of TEOS usually occurs at temperatures above 600 °C (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 (Mäkelä et al. 2011, Stepien et al. 2011 and Teisala et al. 2010) located at the Tampere University of Technology, Finland. The line speed and burner distance can be controlled in the machine environment together with other parameters of the process such as precursor 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 the hood. The fume hood is closed from three sides and open in the front. The dimensions of the hood in mm are 800 × 400 × 800 (height × width × length). The hood lies 20 mm above the substrate and the gap is closed from the three sides with a piece of flame‐proofed cloth. From the ceiling of the hood, exhaust fumes are ventilated away using a tube with 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 cm × 29.7 cm) samples of paper and paperboard together with other materials can be used as substrates for nanoparticle deposits in order to do quick tests with several parameters. The conveyor line makes it easier to prepare samples because the length of one sample in the paper converting machine is in the order of hundreds of meters. The conveyor line is illustrated in Fig 2. It contains a stainless steel sample carriage travelling along a track. The track is oval shaped with a straight part in the deposition area. The carriage is pushed by arm from an electric motor which controls the speed of the carriage. A sample holder is attached on the carriage. The height of the holder can be altered in order to alter the distance between the burner and the sample. Also the speed of the sample running under the deposition process can be adjusted portably from 0 to 150 m/min. It has been showed previously that the orientation of the burner does not play a role in nanoparticle formation (Arabi‐Katbi et al. 2002) and thus the flame can also direct the particles upwards 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. These parameter 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 have hydrophobic nature with a water contact angle up to 160 °. However, as mentioned in the introduction part, titanium dioxide, as well as other ceramic materials, is supposed to be hydrophilic by nature (Takeda et al. 1999). The hydrophilicity is induced by hydroxyl groups that attach onto the titania surface. Stoichiometric gas flows of 20/40 (O2/H2) slpm have been used otherwise previously with LFS studies (Keskinen et al. 2006). Thus, some of the samples are also synthesised here with stoichiometric flow ratio as they act as a reference parameter set as the oxygen lean process might have some special feature in forming the superhydrophobic deposits.
Samples of paperboard (pigment coated Natura paperboard 210 g/m2, Stora Enso, Skoghall, Sweden) were used as substrates for nanoparticle deposition with both a converting machine and a specially built laboratory scale track. Also samples for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were prepared with both devices. Several SEM and TEM samples were prepared in order to verify the similarity of the deposits so that the deposits can be reproduced with the laboratory scale conveyor line with the same parameters in both. TEM samples were prepared so that the TEM grid (Agar, Holey Carbon, 300 mesh, Cu) was attached onto the paperboard sample by attaching a mask of the same material with an opening on top of which the TEM grid peeked out. The mask was large enough to avoid edge effects. The studied TEM and SEM samples were similar with similar process parameters on the paper converting machine and 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 TEM samples. The sheet replaced the paperboard in order to investigate whether the paperboard as a substrate affects the deposition process. The TEM sampling was performed in the same way with metal sheets as it was performed with the paperboard.
Powder samples of the LFS synthesised nanoparticles were collected using a custom‐made plate‐type electrostatic precipitator (ESP). The charging and collecting plates are combined. There are two steel plates with a distance of 8 cm and the voltage between the plates is 20 kV. On the negative plate, there are corona needles to charge the particles before the collection 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 were also analysed using X‐Ray photoelectron spectroscopy (XPS; PHI Quantum 2000). TEM grids were studied with JEOL JEM‐2010 and high resolution TEM JEOL, JEM‐3000‐F, equipped with an energy dispersive spectrometry, EDS, for elemental analysis. The crystallinity of the powder samples was analysed with X‐ray diffraction (XRD, Kristalloflex D‐500, Siemens) and the specific surface 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 a coupled 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 were solved and the effects of turbulence on reaction rates and transfer phenomena are modelled solving two additional partial differential equations that describe the state of turbulence. In addition, the radiation heat transfer was simulated using the discrete ordinates model.
Combustion of hydrogen and isopropanol with oxygen from the spraying nozzle and surrounding air were the modelled reactions. For the isopropanol a single step overall reaction scheme was used and the combustion of hydrogen was assumed to be purely mixing limited. Stainless steel walls were assumed to be in constant temperature but the moving paperboard was simulated in more detail. The paperboard was assumed to have a finite thickness and to radiate and convect 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 more thorough description of the simulations and the results can be found from Arffman et al. (2012). A schematic figure of the simulated case geometry is shown in the 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 of molar amounts between fuel and oxidant is compared in the real situation against the stoichiometric one. The equivalence ratio is defined as:
fuel
oxidant real
fuel
oxidant stoichiometric
nn
Φn
n
⎛ ⎞⎜ ⎟⎝ ⎠=
⎛ ⎞⎜ ⎟⎝ ⎠
where n is the molar concentration. The ratio means that if Φ < 1, the system is oxidant rich and if Φ > 1, the system is fuel rich, i.e. oxygen lean. The ratio easily defines whether there is enough oxygen for all the compounds to oxidize.
3. Results and discussion
In previous studies, we have synthesised titanium dioxide nanoparticle deposits on various substrates using the stoichiometric gas flow rates (Keskinen et al. 2006 and Mäkelä et al. 2004). In a previous paper (Mäkelä et al. 2011) we introduced 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 developed for that system, and will be reported later in a full paper (Arffman et al. 2012). Here, we report a limited set of the data on the temperature and concentration profiles to assist the consideration on nanoparticle and deposit morphology. Especially comparison between stoichiometric and oxygen lean parameters will be relevant. We have previously synthesised hydrophobic surfaces with the oxygen lean gas flows (Mäkelä et al. 2011, Stepien et al. 2011 and Teisala et al. 2010), but the reason for hydrophobicity has not been clear. The aim for the results is to show the reason for different wetting behaviour of the titanium dioxide and silicon dioxide deposits. The titanium dioxide and silicon dioxide deposits on paperboard have water contact angle values of 151 ± 1 ° and 21 ± 1 ° respectively, the WCA for the reference paperboard is 60 ± 1 °.
3.1 Simulation model
The CFD‐simulation of the process was used to clarify the differences between the two processes presented in the experimental 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‐ or superstoichiometric the process is. When feeding precursor solution of 50 mg(Ti)/ml (TTIP and isopropanol) with 32 ml/min into the flame of 15 slpm of oxygen and 50 slpm of hydrogen, the ratio between the amount of oxygen in a process that would be stoichiometric and the amount of oxygen in the real process is 4.6: 1 being stoichiometric and over 1 being oxygen lean environment. Of course, as the deposition is performed in open atmosphere, the ambient oxygen decreases the calculated equivalence ratio to some extent, but not the amount needed. The calculation allows a possibility of insufficient 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 value of 3.3. The vast amount of precursor solution increases the amount of reducing components in the flame and keeps the ratio high. Therefore, no major conclusions can be drawn from the synthesis conditions for the nanoparticles to be different in both cases.
Fig. 3 shows the simulated temperature contours from the cross‐section of the deposition process and from the surface of the paperboard with different hydrogen‐oxygen ratios. Fig. 3 (a) shows that temperatures in the stoichiometric case are much higher than in the oxygen lean case b). All images show the temperature of the gas in the process. The upper image is a view from the side (white square being the burner) and the image below is the temperature of the gas right above the substrate. The maximum temperature of the flame is approximately 2300 °C with the stoichiometric gas flows and around 1700 °C with the oxygen lean gas flows. This means that turbulent mixing cannot provide enough oxygen from the surroundings for the complete combustion of both isopropanol and hydrogen in the hydrogen rich flame. The conditions are 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 oxidize. The incomplete combustion is also modelled. The reference system with stoichiometric conditions does not have such strong incomplete oxidation of the precursor material according to the model leaving the possibility of unreacted precursor material to cause the hydrophobic effect. However, TTIP decomposes thermally at very low temperatures. Here, the temperature of the flame does not limit the formation of titanium 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 powder material that is being deposited on the substrate between the studied parameters. XRD analysis was performed for both of the powders. The analysis spectra are shown in Fig. 4. The powder is mainly composed of anatase with some peaks of rutile present in the spectra. This is in line with previous studies dealing with formation of rutile as residual particles which appear 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 have an effect on the particle crystallinity. BET analysis was also performed for the samples in order to measure the specific 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 values are quite close to each other and the small difference can be explained by the difference in primary particle formation and aggregation 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 XPS spectrum is shown in Fig. 5. XPS is essential in order to find out whether there is unburned precursor material left in some form. The oxidation state of the deposited titanium oxide can be concluded from the XPS spectrum. The spectrum shows the peaks for oxygen and titanium and thus no other oxidation states than TiO2 are virtually detectable in the deposit. This clearly demonstrates that there is no possibility of a titanium suboxide or other titanium species on the deposit. The XPS spectrum also
shows a peak for calcium which comes from the substrate. There is a large peak for carbon visible which can be from the substrate or it can be a result of incomplete combustion of the precursor solution, which is justified with the calculation of equivalence ratio in the simulation chapter. The calculation leaves a possibility of insufficient combustion of isopropanol and propene which may result in carbon formation. Also one possible source for carbon peak is slight combustion 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 differences between the materials. Images show lotus‐like structures as there are micron sized features and nanosized characteristics as well. Stepien et al. (2011) refer to a petal effect which is similar to lotus effect, but there is additionally high adhesion between the water droplet and the surface. However, TiO2 deposit has a water contact angle of 156.2 ± 1.1 ° and SiO2 deposit has a water contact angle of 12.2 ± 1.2 ° with the reference being 76.8 ± 0.7 °. Also, the surface roughness values presented 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 particle morphologies of the deposits have any differences in smaller scale. TEM images acquired at different magnifications for both 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 from Fig. 6(a), where a coating or a shell of quite a light material (less contrast in the TEM images) is visible in the TiO2 sample on 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 conclusion that the coating shell on the particle deposits consists of carbon. Carbon is highly hydrophobic, which supports the water contact angle test results. On the other hand, TiO2 and SiO2 particles look quite alike in SEM. The particle size for both materials is similar and thus the hydrophobic effect might not be alone due to lotus‐shaped surface but also from the coating 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 it would 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 is formed directly from deposited liquid precursor droplets which have reacted and formed SiO2. Similar deposition of liquid in flame reactor has been previously observed by Tricoli et al. (2010). Camenzind et al (2008) published similar formation of silicon dioxide from unreacted precursor in diffusion flames. Another alternative for the deposit formation would be chemical vapour deposition, if the gaseous compounds do not have time to nucleate and the formation of solid deposits takes place on the surface. Both of the presented alternatives may happen at the same time because some of the precursor material is for sure evaporated from the initial droplets but possibly not all. XPS analysis of the deposit confirms the material being SiO2. Cho et al. (2009) studied the particle formation of SiO2 from TEOS in a similar aerosol flame spray generator and found out that the temperature of the flame is limiting the particle formation. They found out that if the flame 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‐solid route and the residence
time in the hot region is small. Thus, it is quite possible that there is deposition of tiny liquid precursor droplets onto the substrate and nanoparticle deposit is formed from liquid‐to‐solid conversion via hydrolysis of the precursor or thermal decomposition of the precursor. The lack of similar shell as there is with the TiO2 particles could be explained with the temperature difference. The liquid layer formed from precursor droplets is absorbing the heat of the flame 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 using the same conveyor line that was used for the paperboard samples. The water contact angle tests were performed on selected reference substrates and the contact angles were (WCA for the fresh reference surface in brackets after the WCA value for TiO2 nanoparticle deposit sample): aluminium 49 ± 8 ° (89 ± 1 °); glass, matt 0 ° (24 ± 3 °); glass, glossy 17 ± 1 ° (26 ± 2 °). The WCA of TiO2 deposited on the matt glass sample is marked as 0 ° as the water droplet spread over the width of the camera screen immediately after the droplet touched the surface. All the samples obtain hydrophilic behaviour and all the samples have a lower WCA as the reference and even the rough structure in matt glass does not offer a platform for lotus‐like effect. TEM images of the set show no evidence of the shell on top of the particles with either of the deposition parameters. Some of the images of particles from TEM analysis on copper surface without the shell are shown in Fig. 7. The water contact angle of the nanoparticle layer deposited on the reference materials is relatively low, which is in agreement with the findings of ceramic materials being hydrophilic (Takeda et al. 1999) if there is no lotus or petal effect. XPS analysis left a possibility of insufficient combustion of isopropanol and propene from precursor solution. These results together rule out the possibility of formation of the carbon from precursor material and are strongly suggesting that the formation 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 the hole in holey carbon grid. Analysis shows that the shell does not have significant order between the atoms. An example of HR‐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 mainly consist 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 no signal in the region outside the particle, where the shell is present. Therefore, the elemental mapping rules out unreacted precursor and titanium carbide as part of the shell, in the limit of detection. The finding supports the results from XPS. The EDS 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 temperature from the maximum. Therefore, slight combustion of the substrate may occur and the material is able to burn and form carbon as a product of incomplete combustion in oxygen lean atmosphere. Also the substrate material plays a role. Is seems that the substrate needs to contain organic material and a
carbon source must be available and in such a form that it can evaporate and form the shell on the titania deposit as a result of the combustion. The temperature of the paperboard is around 100‐300 °C (Teisala et al. 2010) 0.5 m after the deposition process. Thus, the temperature is suitable for evaporation and even combustion of hydrocarbons that are in the substrate material. Oxygen lean process promotes carbon formation over carbon monoxide and carbon dioxide formation. The source of carbon is lacking from the metal sheets and no deposition parameters are found to form hydrophobic deposits on the metal sheets. SiO2 deposits on the other hand have a hydrophilic nature. The deposits form via different processes. However, the fact that SiO2 deposits are hydrophilic together with the coating shell on TiO2 particle deposits do not rule out the possibility of hydrocarbon contaminants as analysed by Takeda et al (1999) or the petal effect being the key factor in superhydrophobic behaviour of TiO2 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 functionalise the surface of nanoparticles. In this study, we have synthesised TiO2 nanoparticle aerosol and deposited it onto a paperboard substrate. The nature of the substrate promotes formation of carbonaceous coating on top of the deposited particles, when using specific parameters for nanoparticle deposit synthesis. The temperature of the process needs to be controlled so that carbon containing materials are evaporated from the substrate, combusted and re‐condensed onto the particles. The resulting nanoparticle deposit has a uniform carbonaceous shell. TEM images confirm the shell consisting of carbon and explain the origin of the carbon peak in the XPS results. The deposit may also have adsorbed hydrocarbon contaminants on the surface. Altogether, the deposit gives hydrophobic properties for the substrate (or transforms the hydrophilic particles into hydrophobic). The particles do not have other changes in crystalline form depending on the process 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 stable hydrophilic deposit on paper and paperboard. The particles are produced in large scale and the deposition can be done in a roll‐to‐roll process in a pilot scale.
Acknowledgements
This work was performed under the Nanorata 2 project funded by The Finnish Funding Agency for Technology and Innovation, Tekes.
References
Arabi‐Katbi, O.I., Wegner, K. and Pratsinis, S.E. (2002). Aerosol synthesis of titania nanoparticles: effect of flame orientation and configuration. Annales de Chimie Science des Matériaux, 27, 37‐46.
Arffman, A., Aromaa, M., Haapanen J., Keskinen, J., Teisala, H., Tuominen, M., Kuusipalo, J., Stepien, M., Saarinen, J.J., Toivakka M. and Mäkelä, J.M. (2012). Simulation of Deposition Rates of LFS Made Nanoparticles on Moving Surfaces. Submitted to Journal of Industrial and Engineering Chemistry.
Aromaa, M., Keskinen, H. and Mäkelä, J.M. (2007). The effect of process parameters on the Liquid Flame Spray generated titania nanoparticles. Biomolecular Engineering, 24, 543‐548.
Athanassiou, E.K., Grass R.N. and Stark W.J. (2006). Large‐scale production of carbon‐coated copper nanoparticles for sensor applications. Nanotechnology, 17, 1668‐1673.
Backman, U., Tapper, U. and Jokiniemi, J.K. (2004). An aerosol method to synthesize supported metal catalyst nanoparticles. Synthetic Metals, 142, 169‐176.
Balaur, E. Macak, J.M., Taveira, L. and Schmuki P. (2005). Tailoring the wettability of TiO2 nanotube layers. Electrochemistry Communications, 7, 1066‐1070.
Balu, B., Kim, J.S., Breedveld, V. and Hess, D.W. (2009). Tunability of the adhesion of water drops on a superhydrophobic paper surface via selective plasma etching. Journal of Adhesion Science and Technology, 23, 361‐380.
Barthlott, W. and Neinhuis, C. (1997). The purity of sacred lotus or escape from contamination in biological surfaces. Planta, 202, 1‐8.
Borras, A., Barranco, A. and Gonzáles‐Elipe, A.R. (2008). Reversible Superhydrophobic to Superhydrophilic Conversion of Ag@TiO2 Composite Nanofiber Surfaces. Langmuir, 24, 8021‐8026.
Camenzind, A., Schulz, H., Teleki, A., Beaucage, G., Narayanan, T. and Pratsinis, S.E. (2008) Nanostructure evolution: From aggregated to spherical SiO2 particles made in diffusion flames. European Journal of Inorganic Chemistry, 6, 911‐918.
Cannon, W.R., Danforth, S.C., Flint, J.H., Haggerty, J.S. and Marra, R.A. (1928a). Sinterable Ceramic Powders from Laser‐Driven Reactions: I, Process Description and Modeling. Journal of American Ceramic Society, 65, 324‐330.
Cannon, W.R., Danforth, S.C., Haggerty, J.S. and Marra, R.A. (1982b). Sinterable Ceramic Powders from Laser‐Driven Reactions: II, Powder Characteristics and Process Variables. Journal of American Ceramic Society, 65, 330‐335.
Chen, S., Mason, M.G., Gysling, H.J., Paz‐Pujalt, G.R., Blanton, T.N., Castro, T., Chen, K.M., Fictorie, C.P., Gladfelter, W.L., Franciosi, A., Cohen, P.I., and Evans, J.F. (1993). Ultrahigh Vacuum Metalorganic Chemical Vapor Deposition Growth and In‐Situ Characterization of Epitaxial TiO2 Films. Journal of Vacuum Science and Technology A, 11, 2419‐2429.
Cho, K., Chang, H., Kil, D.S., Park, J., Jang, H.D. and Sohn, H.Y. (2009). Mechanism of the Formation of Silica Particles from Precursors with Different Volatilities by Flame Spray Pyrolysis. Aerosol Science and Technology, 43, 911‐920.
Courtecuisse, V.G., Chhor, K., Bocquet, J.‐F. and Pommier, C. (1996). Kinetics of the titanium isopropoxide decomposition in supercritical isopropyl alcohol. Industrial and Engineering, Chemistry Research, 35, 2539‐2545.
Ernst , F.O., Büchel, R., Strobel, R. and Pratsinis, S.E. (2008). One‐Step Flame Synthesis of Carbon‐Embedded and ‐Supported Platinum Clusters. Chemistry of Materials, 20, 2117‐2123.
Feng, L. Zhang, Y., Xi, J., Zhu, Y., Wang, N., Xia, F. and Jiang, L. (2008). Petal Effect: A Superhydrophobic State with High Adhesive Force. Langmuir, 24, 4114‐4119.
Feng, X., Zhai, J. and Jiang, L. (2005). The Fabrication and Switchable Superhydrofobicity of TiO2 Nanorod Films. Angewandte Chemie International Edition, 44, 5115‐5118.
Grass, R.N., Athanassiou, E.K. and Stark, W.J. (2007). Covalently functionalized cobalt nanoparticles as a platform for magnetic separations in organic synthesis. Angewandte Chemie International Edition, 46, 4909‐4912.
Gross, K.A., Tikkanen, J., Keskinen, J., Pitkänen, V., Eerola, M., Siikamäki, R. and Rajala, M. (1999). Liquid flame spraying for glass coloring. Journal of Thermal Spray Technology, 8, 583‐589.
Hart, H., Craine, L.E. and Hart, D.J. (2003). Organic Chemistry. Houghton Mifflin Company: Boston.
Jud, E., Gauckler, L., Halim, S. and Stark, W. (2006). Sintering Behavior of In Situ Cobalt Oxide‐Doped Cerium–Gadolinium Oxide Prepared by Flame Spray Pyrolysis. Journal of the American Ceramic Society, 89, 2970‐2973.
Keskinen, H., Mäkelä, J.M., Aromaa, M., Keskinen, J., Areva, S., Teixeira, C.V., Rosenholm, J.B., Pore, V., Ritala, M., Leskelä, M., Raulio, M., Salkinoja‐Salonen, M.S., Levänen, E. and Mäntylä, T. (2006). Titania and titania‐silver nanoparticle deposits made by liquid flame spray and their functionality as photocatalyst for organic‐ and biofilm removal. Catalysis Letters, 111, 127–132.
Keskinen, H., Mäkelä, J.M., Aromaa, M., Ristimäki, J., Kanerva, T., Levänen, E., Mäntylä, T. and Keskinen, J. (2007). Effect of silver addition on the formation and deposition of titania nanoparticles produced by Liquid Flame Spray. Journal of Nanoparticle Research, 9, 569‐588.
Keskinen, H., Aromaa, M., Heine, M.C. and Mäkelä, J.M. (2008). Size and velocity measurements in sprays and particle‐producing flame sprays. Atomization and Sprays, 18, 619‐644.
Kodas, T.T. and Hampden‐Smith, M.J. (1999). Aerosol processing of materials‐ Wiley‐VHC: New York.
Lai, Y., Gao, X., Zhuang, H, Huang, J., Lin, C. and Jiang L. (2009). Designing Superhydrophobic Porous Nanostructures with Tunable Water Adhesion. Advanced Materials, 21, 3799‐3803.
Li, S., Zhang, S. and Wang, X. (2008). Fabrication of cellulose‐based materials through a solution‐immersion process. Langmuir, 24, 5585‐5590.
Morita, H., Kihou, Y., Semba, K. and Taibu, T. (1999). Laser synthesis of organic materials in the gas phase: A noble method to prepare nanoparticles. Journal of Photopolymer Science and Technology, 12, 95‐100.
Mukhopadhyay, S.M., Joshi, P., Datta, S. and MacDaniel, J. (2002). Plasma Assisted Surface Coating of Porous Solids Applied Surface Science, 201, 219‐226.
Mäkelä, J.M., Keskinen, H., Forsblom, J. and Keskinen, J. (2004). Generation of metal and metal oxide nanoparticles by liquid flame spray process. Journal of Materials Science, 39, 2783‐2788.
Mäkelä, J.M., Aromaa, M., Teisala, H., Tuominen, M., Stepien, M., Saarinen, J.J., Toivakka M. and Kuusipalo, J. (2011). Nanoparticle Deposition from Liquid Flame Spray onto Moving Roll‐to‐Roll Paperboard Material. Aerosol Science and Technology, 45, 827‐837.
Nyström, D., Lindqvist, J., Östmark, E., Hult, A. and Malmström, E. (2006). Superhydrophobic bio‐fibre surfaces via tailored grafting architecture. Chemical Communications, 34, 3594‐3596.
Pfender, E. (1999). Thermal Plasma Technology: Where Do We Stand and Where Are We Going?. Plasma Chemistry and Plasma Processing, 19, 1‐31.
Pratsinis, S.E. (1998). Flame aerosol synthesis of ceramic powders. Progress in Energy and Combustion Science, 24, 197‐219.
Pykönen, M., Johansson, K., Dubreuil, M., Vangeneugden, D., Ström, G., Fardim, P. and Toivakka, M. (2010). Evaluation of plasma‐deposited hydrophobic coatings on pigment‐coated paper for reduced dampening water absorption. Journal of Adhesion Science and Technology, 24, 511‐537.
Song, Y. and Zou, M. (2007). Superhydrophobic surfaces by dynamic nanomasking and deep reactive ion etching. Proceedings of the Institution of Mechanical Engineers, Part N: Journal of Nanoengineering and Nanosystems, 221, 41‐48.
Stepien, M., Saarinen, J.J., Teisala, H., Tuominen, M., Aromaa, M., Kuusipalo, J., Mäkelä, J.M. and Toivakka, M. (2011). Adjustable wettability of paperboard by liquid flame spray nanoparticle deposition. Applied Surface Science, 257, 1911‐1917.
Strobel, R, and Pratsinis, S.E. (2007). Flame aerosol synthesis of smart nanostructured materials. Journal of Material Chemistry, 17, 4743‐4756.
Takeda, S., Fukawa, M., Hayashi, Y. and Matsumoto, K. (1999). Surface OH group governing adsorption properties of metal oxide films. Thin Solid Films, 339, 220‐224.
Teisala, H., Tuominen, M., Aromaa, M., Mäkelä, J.M., Stepien, M., Saarinen, J.J., Toivakka, M. and Kuusipalo, J. (2010). Development of superhydrophobic coating on paperboard surface using the Liquid Flame Spray. Surface and Coatings Technology, 205, 436‐445.
Tricoli, A., Righettoni, M., Krumeich, F., Stark, W.J. and Pratsinis, S.E. (2010). Scalable flame synthesis of SiO2 nanowires: dynamic of growth. Nanotechnology, 21, 465604.
Turns, S. A. (1996). An Introduction to Combustion; Concepts and Applications. McGraw‐Hill: New York.
Ulrich, G.D. (1971). Theory of particle formation and growth in oxide synthesis flames. Combustion Science and Technology, 4, 47–57.
Vaswani, S., Koskinen, J. and Hess, D.W. (2005). Surface modification of paper and cellulose by plasma‐assisted deposition of fluorocarbon films. Surface Coating Technology, 195, 121‐129.
Xia L.‐Q., Lee, P.W., Chang, M., Latchford, I., Narwankar, P.K., Urdahl, R., Nickles, A.S., Achutharaman, R. and Lewis, C.B. (2000) Chemical Vapor Deposition. In: Y. Nishi and R. Doering (Eds.), Handbook of Semiconductor Manufacturing Technology. Marcel Dekker Inc.: New York, pp. 309‐356.
Figure captions:
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.
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.
Fig. 5. XPS sample of the hydrophobic paperboard on which titania nanoparticles have been deposited.
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.
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)
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 Fig. 8(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 colours for individual elements: titanium, red; oxygen, blue and carbon, green.
Highlights:
‐ atmospheric synthesis and deposition of functional nanoparticles
‐ superhydrophobic coating on paperboard
‐ shell formation on the particles from the substrate fumes
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