# Development of Photocatalytic Active TiO2 Surfaces by Thermal Spraying of Nanopowders

Authors: Filofteia-Laura Toma; Ghislaine Bertrand; Didier Klein; Cathy Meunier; Sylvie Begin
Journal: Journal of Nanomaterials
 (2008)

---

## Abstract

Titanium dioxide is a very useful photocatalyst for the decomposition and diminution of environmental water and air pollutants. In such applications, it can be used as slurry or as immobilized coating obtained by different deposition methods. The studies performed in the last years showed that thermal spraying could be employed to elaborate TiO2 coatings with high performance for the decomposition of organic compounds.
This manuscript presents a comparative study on the microstructure and photocatalytic performance of titania coatings obtained by different thermal spray techniques: atmospheric plasma spraying (APS), suspension plasma spraying (SPS) and high-velocity oxygen fuel spray process (HVOF). Different titania powders and suspensions were used to study the influence of the feedstock materials on the coating characteristics. The deposits were mainly characterised by SEM and X-ray diffraction. The photocatalytic performance was evaluated from the removal of nitrogen oxides.
The experimental results showed that a drastic reduction of the pollutant concentration was obtained in presence of coatings elaborated by suspension plasma spraying. TiO2 coatings resulting from the spraying of agglomerated powder presentd less efficiency. That was mainly explained by the significant phase transformation from anatase to rutile that occurred in the enthalpic source during the spray processes.


---

## Body

## 1. Introduction

Titanium dioxide (TiO2)
is an attractive material for numerous technological processes.
It finds
applications as gas sensors, as pigments in foodstuffs, paints, cosmetics, or
pharmacology,
as a corrosion resistant coating, in heterogeneous catalysis and
photocatalysis, in solar cells for the production of hydrogen and electric
energy, in electronic devices, and so on.
After the initial work by Fujishima and
Honda [1] on the photolysis of water on TiO2 electrodes, extensive
studies on titania photochemical applications have been carried out in the last
few decades. As a photocatalyst, titanium dioxide can be used in the
degradation of air and water pollutants, in medical sterilization, and even in
cancer therapy [2–4]. Among the two TiO2 crystalline phases, anatase
and rutile that can contribute to the photocatalysis, it is generally assumed
that the anatase—the metastable phase, which by thermal
treatment irreversibly turns into rutile—allows a higher-photocatalytic
degradation of the pollutants. However, some anatase powders
containing small quantities of rutile present a better efficiency than those of
pure anatase [5, 6]. For the photocatalytic applications, titanium dioxide can
be used in powder form (slurry) or immobilized in thin film or coating form obtained
by different deposition techniques (sol-gel, dip-coating, and physical or
chemical vapour deposition, etc.). TiO2 powders have an outstanding photocatalytic efficiency compared with their coating counterparts due to their
higher specific surface area. However, the separation of powder from the liquid state used in water treatments and recycling processes
is troublesome, mainly because of the formation of aggregates. On the other
hand, it is considered that the aforementioned difficulties concerning films
and coatings can be overcome and their applications can also be expanded by
employing various materials as substrates.Thermal spraying is
widely used to prepare titania coatings for mechanical and biomedical
applications due to their hardness, wear and corrosion resistance, and
biocompatibility [7–12]. The studies
performed in the last years showed that the thermal spray technique could be
employed to obtain TiO2 coatings with an effective photocatalytic
performance for the decomposition of organic compounds [13–16].In thermal spraying, the
material feedstock (commonly a powder with typical particles size distribution
ranging from 10 to 100μm) is injected in an enthalpic source (thermal plasma
or flame obtained by ionization-excitation of inert gases or combustion, resp.).
The particles are melted (or partially melted), accelerated by a gas flow, and
crushed on the target substrate where rapid solidification and deposit built-up
occur. In the last years, a new technique, called liquid thermal spraying, with
liquid materials as feedstock precursors was developed [17–24]. The liquid
precursors can be (i) organic (ethoxide, butoxide, acetate, etc.) or inorganic (nitrate,
chloride, etc.) solutions, and (ii) aqueous or alcoholic suspensions of the
material to be sprayed. The suspension allows the feeding of particles with
diameters ranging from several nm to 1 μm into the plasma plume. To be used as
a precursor in thermal spraying, the slurry (suspension of fine particles in a
solvent) has to contain a high-powder load with a low viscosity and a high
stability. The deposition depends on different parameters such as the
hydrodynamic properties of the liquid which depend on the solid content, droplet
diameters, and thermal and kinetics history of the droplets before impinging on
the substrate [25].In this paper, a comparative
study on the microstructure and photocatalytic performance of the nanostructuredTiO2 coatings obtained by different thermal spray techniques:
atmospheric plasma spraying (APS),
suspension plasma spraying (SPS), and high-velocity oxygen fuel (HVOF) spray process is proposed.
The coating microstructure was mainly
characterized by scanning electron microscopy and X-ray diffraction. The
photocatalytic behavior of sprayed coatings was evaluated by a degradation of
gaseous nitrogen oxide pollutants (NO, NOx), major air pollutants that cause
serious problems on the environment and human health.


## 2. Experimental Procedure

### 2.1. Material Feedstock

Three commercialTiO2 powders were used in this study: a pure anatase nanopowder, with
7 nm crystalline size, namely, TiO2-ST01 (Ishihra Sangyo, Japan)
agglomerated by spray-drying in order to prepare spherical microsized anatase
particles (with a size distribution ranging from 10 to 50 μm); a 23 nm anatase nanopowder TiO2-PC105
(Millennium Inorganic Chemicals, France) and TiO2-P25 powder (Degussa
AG, Germany) generally considered as a reference in the photocatalytic
applications, containing 80 vol.% anatase (25 nm) and 20 vol.% rutile (50 nm).Distilled water and
ethylic alcohol were used as solvents in the preparation of suspensions, in
which the powder load was set to 20–25 wt%.No
dispersing agent was added in the composition of the suspensions.


### 2.2. Thermal Spraying

Different thermal spray techniques
were used to elaborate nanostructured coatings. Moreover, the spray conditions
had been adjusted in order to minimize the particle heat input and to maintain
the original anatase phase in the elaborated coating.Atmospheric plasma
spraying (APS) was carried out with a Sulzer-Metco PTF4 plasma gun (6 mm nozzle
torch). Ar at 40 L·min-1 was used as primary plasma gas; H2 (3 L·min-1) and He (20 L·min-1) were added as secondary
plasma gases. The arc intensity was about 400 A. Stainless steel substrates
(60×70×2 mm3) previously sand-blasted were set to 80 and 100 mm from the
exit of the nozzle torch. The powder rate was 14 g·min-1 and carried
by Ar at 3 L·min-1.A special device was
designed for the injection of liquid in the plasma [26]. The suspensions were
introduced by a peristaltic pump, at a feed rate of about 20–25 mL·min-1,
in the system that ensured the atomization of the liquid and radial injection
of the resulting drops in the plasma jet
(Figure 1). Argon at a flow rate of 3–5 L·min-1 allowed the atomization of the liquid before entering in the plasma.Figure 1
Suspension plasma spraying setup.High-velocity oxygen flame spraying (HVOF spraying)
was performed with a Sulzer-Metco CDS 100 gun (with a 3-inche nozzle) using
natural methane, as fuel gas. The flame was obtained by the combustion of 100 L·min-1CH4 with 400 L·min-1O2. N2 at a flow rate of 50 L·min-1 was also added to the gas mixture to
decrease the flame temperature. The powder feed rate was 20 g·min-1 and was carried by N2 at a flow rate of 9.5 L·min-1. The
spray distance was set to 150 mm.


### 2.3. Characterisation

The coatings morphology was examined using a JEOL JSM-5800 LV scanning
electron microscopy. X-ray diffraction (X’Pert MPD Philips diffractometer) withCu Kα radiation was used to assess the anatase to rutile ratio. Scan step was
0.02° s-1 with a step time of 0.5 second in the 20–90° 2θ range. The
volume percentage of anatase was determined according to the relation developed
by Berger-Keller et al.
[27]. CA=8IA/(8IA+13IR), where IA and IR
are the X-ray intensities of the anatase (1 0 1) and rutile (1 1 0) peaks,
respectively. The
crystallite size was evaluated from the X-ray diffraction based on the Scherrer
formula.The photocatalytic
performance ofTiO2 was evaluated by the
decrease of nitrogen oxides (NO,NOx) concentration in a purpose-built test chamber
described elsewhere [28]. A 15-W daylight lamp with a fraction of 30% UVA and 4% UVB ensured the irradiation of the TiO2. The
NO and NOx concentrations were measured continuously using an
AC-30M NOx dual chamber analyzer (environmental SA, France)
and recorded with a data acquisition system.The photocatalytic tests were performed over the
sprayed coatings and the initial powders. The photocatalytic performances were
evaluated as the ratio of the photocatalytic conversion of nitrogen oxides and determined by the following relations:(1)NOconversion(%)=[NO]initial−[NO]UV[NO]initial×100,NOxconversion(%)=[NOx]initial−[NOx]UV[NOx]initial×100,
where NO conversion (%) and NOx conversion (%) were the photocatalytic removal of NO and NOx concentrations in
the presence of the catalyst and UV irradiation; [NO]initial and [NOx]initial represented the
values of the NO and NOx concentrations without UV irradiation; [NO]UV and [NOx]UV were the values of the NO and NOx
concentrations under UV irradiation.


## 2.1. Material Feedstock

Three commercialTiO2 powders were used in this study: a pure anatase nanopowder, with
7 nm crystalline size, namely, TiO2-ST01 (Ishihra Sangyo, Japan)
agglomerated by spray-drying in order to prepare spherical microsized anatase
particles (with a size distribution ranging from 10 to 50 μm); a 23 nm anatase nanopowder TiO2-PC105
(Millennium Inorganic Chemicals, France) and TiO2-P25 powder (Degussa
AG, Germany) generally considered as a reference in the photocatalytic
applications, containing 80 vol.% anatase (25 nm) and 20 vol.% rutile (50 nm).Distilled water and
ethylic alcohol were used as solvents in the preparation of suspensions, in
which the powder load was set to 20–25 wt%.No
dispersing agent was added in the composition of the suspensions.


## 2.2. Thermal Spraying

Different thermal spray techniques
were used to elaborate nanostructured coatings. Moreover, the spray conditions
had been adjusted in order to minimize the particle heat input and to maintain
the original anatase phase in the elaborated coating.Atmospheric plasma
spraying (APS) was carried out with a Sulzer-Metco PTF4 plasma gun (6 mm nozzle
torch). Ar at 40 L·min-1 was used as primary plasma gas; H2 (3 L·min-1) and He (20 L·min-1) were added as secondary
plasma gases. The arc intensity was about 400 A. Stainless steel substrates
(60×70×2 mm3) previously sand-blasted were set to 80 and 100 mm from the
exit of the nozzle torch. The powder rate was 14 g·min-1 and carried
by Ar at 3 L·min-1.A special device was
designed for the injection of liquid in the plasma [26]. The suspensions were
introduced by a peristaltic pump, at a feed rate of about 20–25 mL·min-1,
in the system that ensured the atomization of the liquid and radial injection
of the resulting drops in the plasma jet
(Figure 1). Argon at a flow rate of 3–5 L·min-1 allowed the atomization of the liquid before entering in the plasma.Figure 1
Suspension plasma spraying setup.High-velocity oxygen flame spraying (HVOF spraying)
was performed with a Sulzer-Metco CDS 100 gun (with a 3-inche nozzle) using
natural methane, as fuel gas. The flame was obtained by the combustion of 100 L·min-1CH4 with 400 L·min-1O2. N2 at a flow rate of 50 L·min-1 was also added to the gas mixture to
decrease the flame temperature. The powder feed rate was 20 g·min-1 and was carried by N2 at a flow rate of 9.5 L·min-1. The
spray distance was set to 150 mm.


## 2.3. Characterisation

The coatings morphology was examined using a JEOL JSM-5800 LV scanning
electron microscopy. X-ray diffraction (X’Pert MPD Philips diffractometer) withCu Kα radiation was used to assess the anatase to rutile ratio. Scan step was
0.02° s-1 with a step time of 0.5 second in the 20–90° 2θ range. The
volume percentage of anatase was determined according to the relation developed
by Berger-Keller et al.
[27]. CA=8IA/(8IA+13IR), where IA and IR
are the X-ray intensities of the anatase (1 0 1) and rutile (1 1 0) peaks,
respectively. The
crystallite size was evaluated from the X-ray diffraction based on the Scherrer
formula.The photocatalytic
performance ofTiO2 was evaluated by the
decrease of nitrogen oxides (NO,NOx) concentration in a purpose-built test chamber
described elsewhere [28]. A 15-W daylight lamp with a fraction of 30% UVA and 4% UVB ensured the irradiation of the TiO2. The
NO and NOx concentrations were measured continuously using an
AC-30M NOx dual chamber analyzer (environmental SA, France)
and recorded with a data acquisition system.The photocatalytic tests were performed over the
sprayed coatings and the initial powders. The photocatalytic performances were
evaluated as the ratio of the photocatalytic conversion of nitrogen oxides and determined by the following relations:(1)NOconversion(%)=[NO]initial−[NO]UV[NO]initial×100,NOxconversion(%)=[NOx]initial−[NOx]UV[NOx]initial×100,
where NO conversion (%) and NOx conversion (%) were the photocatalytic removal of NO and NOx concentrations in
the presence of the catalyst and UV irradiation; [NO]initial and [NOx]initial represented the
values of the NO and NOx concentrations without UV irradiation; [NO]UV and [NOx]UV were the values of the NO and NOx
concentrations under UV irradiation.


## 3. Results and Discussion

### 3.1. Coating Microstructure

#### 3.1.1. Plasma Spraying

The morphology of
the sprayed coating was mainly dependent on the thermal spray technique. The
deposits obtained by the plasma spraying of the agglomeratedTiO2-ST01
nanopowder were characterized by a bimodal microstructure (Figure 2): on the
one hand, a thin lamellar morphology commonly observed with sprayed coatings
that contained fully melted particles, and on the other hand, a structure of
densely agglomerated grains that contained partially melted particles.SEM-microstructures of
nanostructuredTiO2-ST01 coating prepared by APS technique: (a)
fracture; (b) surface.
(a)(b)When the agglomerated ST01
nanopowder was dispersed in distilled water (25 wt% powder loading) and
injected in the plasma jet as a suspension, the microstructure of the resulting
coatings did not present the lamellar structure, which is a characteristic of
the thermal sprayed coatings. TheTiO2-ST01 suspension-sprayed
coatings were relatively porous and constituted of partially melted or unmelted
fine particles as depicted in Figure 3. The different microstructure of the
ST01 coatings elaborated by suspension plasma spraying compared with their counterparts
obtained from conventional APS was explained by the fact that the thermal
transfer between the plasma jet and the material was less important when the
suspension was used as feedstock material. In this case, a part of the plasma
energy was used to evaporate the solvent of the suspension droplets; the flying
time of the resulting particles (free of liquid) was not enough to ensure a
complete melting of the particles before impact on substrate.SEM-microstructures
ofTiO2-ST01 coating prepared by suspension plasma technique: (a)
cross-section; (b) surface.
(a)(b)A nonlamellar microstructure was also
observed in the coatings obtained by the plasma spraying of the suspensions (20 wt% powder loading) of nonagglomerated titania powders P25 and PC105. The
coatings formed from aqueous suspensions exhibited relatively dense structures
(see Figure4). Moreover, it seemed, at high-microscopic scale, that the
coatings were built up by the dense stacking up of fine spherical nanoparticles
[29].SEM-morphology of coatings starting from
aqueous suspensions of (a)TiO2-P25; (b) TiO2-P25 at high-microscopic
scale; (c) TiO2-PC105 [29].
(a)(b)(c)Different
morphologies were observed in the case of the deposits elaborated from
alcoholic suspensions of the two titania nonagglomerated powders.TiO2-P25
alcoholic suspension-sprayed coating contained
melted and nonmelted particles as shown in Figure 5(a). The presence of the melted zones was
explained by the fact that the enthalpy of the evaporation of ethanol was lower
(0.8·106 J/kg) than that of water (2.3·106 J/kg).
Moreover, it was claimed that the injection of an alcoholic suspension in the
plasma plume was assumed to increase the jet temperature as well as the gas
speed [25]. In this case, the thermal transfer between the plasma and the
particles was more important so the particles were easily heated and then
melted. Thus, the molten particles had enough momentum to impinge on the
substrate and spread out in splat forms like usually achieved in thermal spray
process.SEM-morphology of coatings starting from
alcoholic suspensions of (a)TiO2-P25, cross-section (1: unmelted
regions; 2: melted zones); (b) TiO2-PC105, cross-section; (c) TiO2-PC105,
surface.
(a)(b)(c)The coatings produced from an
alcoholic suspension of PC105 particles were porous; different microstructural
features were distinguished on the cross-section of the coating: fully melted
zones, nonmelted regions, and spherical particles (Figures5(b), 5(c)). These
features were explained by the large distribution of the liquid droplets
injected in the plasma as well as the different thermal history of the
particles in the enthalpic source before impinging on the substrate. It was
interesting to notice the very different microstructures of these coatings
compared to that resulting from the P25 alcoholic suspension. This structure was
probably due to the different physical properties of the suspensions such as
viscosity or surface tension.


#### 3.1.2. HVOF Spraying

TheTiO2 deposits elaborated by the HVOF spraying of agglomerated TiO2-ST01
nanopowder were porous and characterized by a layered structure (Figure 6). The
coatings were mainly built up by the melted nanoparticles impacting on the
substrate that flatten to form splats which are consecutively piled on top of the
others. Some partially molten particles embedded in the coating structure were
also observed. The surface analysis showed the presence of fine particles which
conserved the initial features of the agglomerated TiO2-ST01
nanopowder.SEM-microstructures
ofTiO2-ST01 coating prepared by HVOF technique: (a) cross-section;
(b) surface.
(a)(b)


### 3.2. Coating Crystalline Structure

X-ray diffraction was
performed to assess the anatase to rutile ratio. The analysis showed that the
passage of the feedstock material (as particles or liquid suspension) in the
enthalpic source (plasma or flame) involved the modifications of the chemical
state of titanium dioxide compared to that of the initial powders. These
structural transformations depended on the thermal spray method used in the
elaboration ofTiO2 coatings.Table1 resumes the
crystalline phases and the crystalline size of the particles determined in
different sprayed coatings. The coating elaborated by the plasma spraying of
agglomerated ST01 powder contained the rutile structure as a major phase; the
anatase content was estimated between 10.9 and 12.2 vol.%. Some traces of TiOx
or Magnéli phases were also identified. The crystallite sizes of the anatase increased
from 7 nm to 34 nm. An important phase transformation from anatase to rutile
was also observed in the coating obtained by HVOF process. In contrast, when
the agglomerated ST01 powder was injected in the plasma in form of aqueous suspension,
the content of anatase phase was higher than 96 vol.%; furthermore, the crystalline
size of anatase was preserved (7.0–7.3 nm),
practically identical to that of the original powder.Table 1
Phases and crystallites size in the thermal sprayed coatings.
Ref.TiO2 coatingAnatase (vol.%)Anatase crystallites average size (nm)Rutile crystallites average size (nm)Observations (other phases)ST01-APS10.9–12.219.2–34.867.0–78.6Traces of Magnéli phases (Ti3O5; Ti6O11)ST01-WSC*>
96.07.0–7.30–25.1—ST01-HVOF12.680.090.0—P25-WSC77.6–81.427.1–29.251.7–60.5—P25-ASC**23.046.0120.0—PC105-WSC91.9–95.134.9–38.7140.9–194.9—PC105-ASC36.2151.199.2—*WSC-coating resulted from plasma spraying of aqueous suspension.
**ASC-coating resulted from plasma spraying of alcoholic suspension.The crystalline
structure of the coating was relative to the melting degree of the particles
during spraying. It is generally accepted that the rutile phase corresponds to
the fully melted particles, whereas the anatase structure was mostly presented
in the partially and nonmelted particles [13].With an aqueous
suspension of the agglomerated ST01 nanopowder, the phase transformation was
avoided because a part of the plasma energy was used to vaporize the water
contained in the liquid droplets. So, the flight time in the plasma jet was not
long enough so that the anatase to rutile structural transformation occurred
before the particle impact on the substrate.In the case of
suspension spraying of nonagglomeratedTiO2 nanopowders, the XRD
analysis showed that the anatase ratio depended on the nature of the solvent
(water or alcohol) used in the suspension preparation. The plasma spraying of an aqueous slurry led to coatings where the anatase
content and the average crystallite size were almost the same as those
identified in the raw powders. On the other hand, the injection of alcoholic
suspensions involved the structural transformation from anatase to rutile
(about 23 vol.% for P25-coatings and 36 vol.% for PC105 coatings), as well
as the increase of the crystallite
size of the anatase from 23–25 nm to 150 nm.
In this case, the phase transformation was attributed to an easier
fragmentation of the alcoholic slurry (due to a lower surface tension of the
ethanol), followed by a rapid vaporization of the ethanol. Thus, the flight
time of the resulting particles had to be long enough to ensure the partial
structural transformation from anatase to rutile.


### 3.3. Photocatalytic Performance for the Degradation of
Nitrogen Oxides

The photocatalytic
properties of the sprayed coatings were evaluated by the diminution of gaseous
nitrogen oxides and compared with those obtained in the presence of the
corresponding powders. 0.4 g of each
powder was uniformly distributed over the entire geometric surface of a Petri
dish with 54 cm2 surface area.The photocatalytic
activities ofTiO2 coatings are represented in Figure 7. Different behaviors in the
photocatalytic performances were noticed depending on the spraying parameters
and nature of the material feedstock.Photocatalytic
degradation of nitrogen oxides in presence of coatings elaborated from (a)
agglomerated ST01 nanopowder; (b) nonagglomerated P25 and PC105 nanopowders.
(a)(b)A very low degradation
of nitrogen oxide pollutants (<5%) was obtained in presence of the TiO2 surfaces produced by APS and HVOF sprayings of agglomerated ST01 nanopowder. Other authors
reported good photocatalytic responses of APS- or HVOF-sprayed TiO2 coatings in the
decomposition of different organic compounds such as phenol, acetaldehyde, or
methylene blue [13–16]. Moreover, the photocatalytic properties of sprayed
coatings were improved by the doping of TiO2 with Pt [30], metallic
oxides such as Al2O3, Fe3O4, ZnO,
SnO2 [31–33], or hydroxyapatite [34]. In our case, the lower-photocatalytic
efficiency of APS- and HVOF-sprayed coatings was also correlated with the
chemistry and reactivity of nitrogen oxides that are more complex than that of
other molecules [35].When the ST01 powder was
injected in aqueous suspension form, the sprayed coating ensured a substantial
removal of the nitrogen oxides (around 52% forNO and 34% for NOx), much higher
than that of the powder (31% for NO and 18% for NOx). High rates of
degradation of gaseous pollutants were also recorded in the presence of aqueous
suspension plasma sprayed coatings of nonagglomerated nanopowders P25 (around
36% for NO and 15% for NOx) and PC105 (around 18% for NO and 8% for NOx),
comparable with that measured in the presence of the powders. Meanwhile, a lower
degradation of pollutants (8–10%) was
obtained for the coating resulting from the plasma spraying of alcoholic
suspensions.The different
photocatalytic performances were principally correlated with the crystalline
structures of the sprayed coating. Anatase phase was a key parameter in the
photocatalytic activity and a higher-anatase ratio with reduced crystallite
size allowed a better degradation of nitrogen oxides. Besides, the titania deposits
elaborated by the spraying of aqueous ST01 and P25 suspensions provided a
higher performance than that of the raw powders. In these cases, the enhanced
photocatalytic performance could not be explained considering only the anatase
ratio. It was assumed that the removal of the impurities coming from the raw
powder elaboration and a cleaning up of the free-water particles when crossing
the plasma could explain such behavior. Different techniques of surface
investigations (infrared spectroscopy and X-ray photoelectrons spectroscopy)
performed on the surface of P25 aqueous suspension sprayed coating [29] showed
the presence of a higher hydroxylation compared to that on the raw powder
surface. The hydroxyl groups played a beneficial role and enhanced the
photocatalytic activity of titania as already reported in the literature [36, 37].


## 3.1. Coating Microstructure

### 3.1.1. Plasma Spraying

The morphology of
the sprayed coating was mainly dependent on the thermal spray technique. The
deposits obtained by the plasma spraying of the agglomeratedTiO2-ST01
nanopowder were characterized by a bimodal microstructure (Figure 2): on the
one hand, a thin lamellar morphology commonly observed with sprayed coatings
that contained fully melted particles, and on the other hand, a structure of
densely agglomerated grains that contained partially melted particles.SEM-microstructures of
nanostructuredTiO2-ST01 coating prepared by APS technique: (a)
fracture; (b) surface.
(a)(b)When the agglomerated ST01
nanopowder was dispersed in distilled water (25 wt% powder loading) and
injected in the plasma jet as a suspension, the microstructure of the resulting
coatings did not present the lamellar structure, which is a characteristic of
the thermal sprayed coatings. TheTiO2-ST01 suspension-sprayed
coatings were relatively porous and constituted of partially melted or unmelted
fine particles as depicted in Figure 3. The different microstructure of the
ST01 coatings elaborated by suspension plasma spraying compared with their counterparts
obtained from conventional APS was explained by the fact that the thermal
transfer between the plasma jet and the material was less important when the
suspension was used as feedstock material. In this case, a part of the plasma
energy was used to evaporate the solvent of the suspension droplets; the flying
time of the resulting particles (free of liquid) was not enough to ensure a
complete melting of the particles before impact on substrate.SEM-microstructures
ofTiO2-ST01 coating prepared by suspension plasma technique: (a)
cross-section; (b) surface.
(a)(b)A nonlamellar microstructure was also
observed in the coatings obtained by the plasma spraying of the suspensions (20 wt% powder loading) of nonagglomerated titania powders P25 and PC105. The
coatings formed from aqueous suspensions exhibited relatively dense structures
(see Figure4). Moreover, it seemed, at high-microscopic scale, that the
coatings were built up by the dense stacking up of fine spherical nanoparticles
[29].SEM-morphology of coatings starting from
aqueous suspensions of (a)TiO2-P25; (b) TiO2-P25 at high-microscopic
scale; (c) TiO2-PC105 [29].
(a)(b)(c)Different
morphologies were observed in the case of the deposits elaborated from
alcoholic suspensions of the two titania nonagglomerated powders.TiO2-P25
alcoholic suspension-sprayed coating contained
melted and nonmelted particles as shown in Figure 5(a). The presence of the melted zones was
explained by the fact that the enthalpy of the evaporation of ethanol was lower
(0.8·106 J/kg) than that of water (2.3·106 J/kg).
Moreover, it was claimed that the injection of an alcoholic suspension in the
plasma plume was assumed to increase the jet temperature as well as the gas
speed [25]. In this case, the thermal transfer between the plasma and the
particles was more important so the particles were easily heated and then
melted. Thus, the molten particles had enough momentum to impinge on the
substrate and spread out in splat forms like usually achieved in thermal spray
process.SEM-morphology of coatings starting from
alcoholic suspensions of (a)TiO2-P25, cross-section (1: unmelted
regions; 2: melted zones); (b) TiO2-PC105, cross-section; (c) TiO2-PC105,
surface.
(a)(b)(c)The coatings produced from an
alcoholic suspension of PC105 particles were porous; different microstructural
features were distinguished on the cross-section of the coating: fully melted
zones, nonmelted regions, and spherical particles (Figures5(b), 5(c)). These
features were explained by the large distribution of the liquid droplets
injected in the plasma as well as the different thermal history of the
particles in the enthalpic source before impinging on the substrate. It was
interesting to notice the very different microstructures of these coatings
compared to that resulting from the P25 alcoholic suspension. This structure was
probably due to the different physical properties of the suspensions such as
viscosity or surface tension.


### 3.1.2. HVOF Spraying

TheTiO2 deposits elaborated by the HVOF spraying of agglomerated TiO2-ST01
nanopowder were porous and characterized by a layered structure (Figure 6). The
coatings were mainly built up by the melted nanoparticles impacting on the
substrate that flatten to form splats which are consecutively piled on top of the
others. Some partially molten particles embedded in the coating structure were
also observed. The surface analysis showed the presence of fine particles which
conserved the initial features of the agglomerated TiO2-ST01
nanopowder.SEM-microstructures
ofTiO2-ST01 coating prepared by HVOF technique: (a) cross-section;
(b) surface.
(a)(b)


## 3.1.1. Plasma Spraying

The morphology of
the sprayed coating was mainly dependent on the thermal spray technique. The
deposits obtained by the plasma spraying of the agglomeratedTiO2-ST01
nanopowder were characterized by a bimodal microstructure (Figure 2): on the
one hand, a thin lamellar morphology commonly observed with sprayed coatings
that contained fully melted particles, and on the other hand, a structure of
densely agglomerated grains that contained partially melted particles.SEM-microstructures of
nanostructuredTiO2-ST01 coating prepared by APS technique: (a)
fracture; (b) surface.
(a)(b)When the agglomerated ST01
nanopowder was dispersed in distilled water (25 wt% powder loading) and
injected in the plasma jet as a suspension, the microstructure of the resulting
coatings did not present the lamellar structure, which is a characteristic of
the thermal sprayed coatings. TheTiO2-ST01 suspension-sprayed
coatings were relatively porous and constituted of partially melted or unmelted
fine particles as depicted in Figure 3. The different microstructure of the
ST01 coatings elaborated by suspension plasma spraying compared with their counterparts
obtained from conventional APS was explained by the fact that the thermal
transfer between the plasma jet and the material was less important when the
suspension was used as feedstock material. In this case, a part of the plasma
energy was used to evaporate the solvent of the suspension droplets; the flying
time of the resulting particles (free of liquid) was not enough to ensure a
complete melting of the particles before impact on substrate.SEM-microstructures
ofTiO2-ST01 coating prepared by suspension plasma technique: (a)
cross-section; (b) surface.
(a)(b)A nonlamellar microstructure was also
observed in the coatings obtained by the plasma spraying of the suspensions (20 wt% powder loading) of nonagglomerated titania powders P25 and PC105. The
coatings formed from aqueous suspensions exhibited relatively dense structures
(see Figure4). Moreover, it seemed, at high-microscopic scale, that the
coatings were built up by the dense stacking up of fine spherical nanoparticles
[29].SEM-morphology of coatings starting from
aqueous suspensions of (a)TiO2-P25; (b) TiO2-P25 at high-microscopic
scale; (c) TiO2-PC105 [29].
(a)(b)(c)Different
morphologies were observed in the case of the deposits elaborated from
alcoholic suspensions of the two titania nonagglomerated powders.TiO2-P25
alcoholic suspension-sprayed coating contained
melted and nonmelted particles as shown in Figure 5(a). The presence of the melted zones was
explained by the fact that the enthalpy of the evaporation of ethanol was lower
(0.8·106 J/kg) than that of water (2.3·106 J/kg).
Moreover, it was claimed that the injection of an alcoholic suspension in the
plasma plume was assumed to increase the jet temperature as well as the gas
speed [25]. In this case, the thermal transfer between the plasma and the
particles was more important so the particles were easily heated and then
melted. Thus, the molten particles had enough momentum to impinge on the
substrate and spread out in splat forms like usually achieved in thermal spray
process.SEM-morphology of coatings starting from
alcoholic suspensions of (a)TiO2-P25, cross-section (1: unmelted
regions; 2: melted zones); (b) TiO2-PC105, cross-section; (c) TiO2-PC105,
surface.
(a)(b)(c)The coatings produced from an
alcoholic suspension of PC105 particles were porous; different microstructural
features were distinguished on the cross-section of the coating: fully melted
zones, nonmelted regions, and spherical particles (Figures5(b), 5(c)). These
features were explained by the large distribution of the liquid droplets
injected in the plasma as well as the different thermal history of the
particles in the enthalpic source before impinging on the substrate. It was
interesting to notice the very different microstructures of these coatings
compared to that resulting from the P25 alcoholic suspension. This structure was
probably due to the different physical properties of the suspensions such as
viscosity or surface tension.


## 3.1.2. HVOF Spraying

TheTiO2 deposits elaborated by the HVOF spraying of agglomerated TiO2-ST01
nanopowder were porous and characterized by a layered structure (Figure 6). The
coatings were mainly built up by the melted nanoparticles impacting on the
substrate that flatten to form splats which are consecutively piled on top of the
others. Some partially molten particles embedded in the coating structure were
also observed. The surface analysis showed the presence of fine particles which
conserved the initial features of the agglomerated TiO2-ST01
nanopowder.SEM-microstructures
ofTiO2-ST01 coating prepared by HVOF technique: (a) cross-section;
(b) surface.
(a)(b)


## 3.2. Coating Crystalline Structure

X-ray diffraction was
performed to assess the anatase to rutile ratio. The analysis showed that the
passage of the feedstock material (as particles or liquid suspension) in the
enthalpic source (plasma or flame) involved the modifications of the chemical
state of titanium dioxide compared to that of the initial powders. These
structural transformations depended on the thermal spray method used in the
elaboration ofTiO2 coatings.Table1 resumes the
crystalline phases and the crystalline size of the particles determined in
different sprayed coatings. The coating elaborated by the plasma spraying of
agglomerated ST01 powder contained the rutile structure as a major phase; the
anatase content was estimated between 10.9 and 12.2 vol.%. Some traces of TiOx
or Magnéli phases were also identified. The crystallite sizes of the anatase increased
from 7 nm to 34 nm. An important phase transformation from anatase to rutile
was also observed in the coating obtained by HVOF process. In contrast, when
the agglomerated ST01 powder was injected in the plasma in form of aqueous suspension,
the content of anatase phase was higher than 96 vol.%; furthermore, the crystalline
size of anatase was preserved (7.0–7.3 nm),
practically identical to that of the original powder.Table 1
Phases and crystallites size in the thermal sprayed coatings.
Ref.TiO2 coatingAnatase (vol.%)Anatase crystallites average size (nm)Rutile crystallites average size (nm)Observations (other phases)ST01-APS10.9–12.219.2–34.867.0–78.6Traces of Magnéli phases (Ti3O5; Ti6O11)ST01-WSC*>
96.07.0–7.30–25.1—ST01-HVOF12.680.090.0—P25-WSC77.6–81.427.1–29.251.7–60.5—P25-ASC**23.046.0120.0—PC105-WSC91.9–95.134.9–38.7140.9–194.9—PC105-ASC36.2151.199.2—*WSC-coating resulted from plasma spraying of aqueous suspension.
**ASC-coating resulted from plasma spraying of alcoholic suspension.The crystalline
structure of the coating was relative to the melting degree of the particles
during spraying. It is generally accepted that the rutile phase corresponds to
the fully melted particles, whereas the anatase structure was mostly presented
in the partially and nonmelted particles [13].With an aqueous
suspension of the agglomerated ST01 nanopowder, the phase transformation was
avoided because a part of the plasma energy was used to vaporize the water
contained in the liquid droplets. So, the flight time in the plasma jet was not
long enough so that the anatase to rutile structural transformation occurred
before the particle impact on the substrate.In the case of
suspension spraying of nonagglomeratedTiO2 nanopowders, the XRD
analysis showed that the anatase ratio depended on the nature of the solvent
(water or alcohol) used in the suspension preparation. The plasma spraying of an aqueous slurry led to coatings where the anatase
content and the average crystallite size were almost the same as those
identified in the raw powders. On the other hand, the injection of alcoholic
suspensions involved the structural transformation from anatase to rutile
(about 23 vol.% for P25-coatings and 36 vol.% for PC105 coatings), as well
as the increase of the crystallite
size of the anatase from 23–25 nm to 150 nm.
In this case, the phase transformation was attributed to an easier
fragmentation of the alcoholic slurry (due to a lower surface tension of the
ethanol), followed by a rapid vaporization of the ethanol. Thus, the flight
time of the resulting particles had to be long enough to ensure the partial
structural transformation from anatase to rutile.


## 3.3. Photocatalytic Performance for the Degradation of
Nitrogen Oxides

The photocatalytic
properties of the sprayed coatings were evaluated by the diminution of gaseous
nitrogen oxides and compared with those obtained in the presence of the
corresponding powders. 0.4 g of each
powder was uniformly distributed over the entire geometric surface of a Petri
dish with 54 cm2 surface area.The photocatalytic
activities ofTiO2 coatings are represented in Figure 7. Different behaviors in the
photocatalytic performances were noticed depending on the spraying parameters
and nature of the material feedstock.Photocatalytic
degradation of nitrogen oxides in presence of coatings elaborated from (a)
agglomerated ST01 nanopowder; (b) nonagglomerated P25 and PC105 nanopowders.
(a)(b)A very low degradation
of nitrogen oxide pollutants (<5%) was obtained in presence of the TiO2 surfaces produced by APS and HVOF sprayings of agglomerated ST01 nanopowder. Other authors
reported good photocatalytic responses of APS- or HVOF-sprayed TiO2 coatings in the
decomposition of different organic compounds such as phenol, acetaldehyde, or
methylene blue [13–16]. Moreover, the photocatalytic properties of sprayed
coatings were improved by the doping of TiO2 with Pt [30], metallic
oxides such as Al2O3, Fe3O4, ZnO,
SnO2 [31–33], or hydroxyapatite [34]. In our case, the lower-photocatalytic
efficiency of APS- and HVOF-sprayed coatings was also correlated with the
chemistry and reactivity of nitrogen oxides that are more complex than that of
other molecules [35].When the ST01 powder was
injected in aqueous suspension form, the sprayed coating ensured a substantial
removal of the nitrogen oxides (around 52% forNO and 34% for NOx), much higher
than that of the powder (31% for NO and 18% for NOx). High rates of
degradation of gaseous pollutants were also recorded in the presence of aqueous
suspension plasma sprayed coatings of nonagglomerated nanopowders P25 (around
36% for NO and 15% for NOx) and PC105 (around 18% for NO and 8% for NOx),
comparable with that measured in the presence of the powders. Meanwhile, a lower
degradation of pollutants (8–10%) was
obtained for the coating resulting from the plasma spraying of alcoholic
suspensions.The different
photocatalytic performances were principally correlated with the crystalline
structures of the sprayed coating. Anatase phase was a key parameter in the
photocatalytic activity and a higher-anatase ratio with reduced crystallite
size allowed a better degradation of nitrogen oxides. Besides, the titania deposits
elaborated by the spraying of aqueous ST01 and P25 suspensions provided a
higher performance than that of the raw powders. In these cases, the enhanced
photocatalytic performance could not be explained considering only the anatase
ratio. It was assumed that the removal of the impurities coming from the raw
powder elaboration and a cleaning up of the free-water particles when crossing
the plasma could explain such behavior. Different techniques of surface
investigations (infrared spectroscopy and X-ray photoelectrons spectroscopy)
performed on the surface of P25 aqueous suspension sprayed coating [29] showed
the presence of a higher hydroxylation compared to that on the raw powder
surface. The hydroxyl groups played a beneficial role and enhanced the
photocatalytic activity of titania as already reported in the literature [36, 37].


## 4. Conclusion

This paper proposed a
comparative study on the microstructure and photocatalytic properties of
titanium dioxide coatings obtained by different thermal spray methods:
atmospheric plasma spraying, suspension plasma spraying, and high-velocity oxygen
fuel process using agglomerated and nonagglomeratedTiO2 nanopowders
as feedstock materials. The coatings elaborated by APS and HVOF techniques were
characterized by the common lamellar microstructure observed in all thermal
sprayed deposits. Besides, the use of agglomerated nanoparticles as feedstock
materials involved an important phase transformation from anatase to rutile.
The photocatalytic ability for nitrogen oxides was lower than 5% and was
correlated principally with the reduced anatase content.The coatings
elaborated by suspension plasma spraying presented a specific structure that
depended on the nature of the solvent used in the preparation of the suspensions.
The anatase crystalline structure and the crystallite size were preserved in
the coatings resulting from an aqueous suspension, whereas a significant phase
transformation occurred with alcoholic suspensions. Furthermore, the aqueous
suspension sprayed coatings presented a remarkable photocatalytic efficiency
that in some conditions was higher than that of the corresponding powder.It can be concluded
that the suspension plasma spraying is an appropriate technique to elaborate
active titania surfaces for the photocatalytic reduction of reactive gaseous pollutants
such as nitrogen oxides.



---

*Source: 384171-2008-04-07.xml*