One-Step Synthesis of TiO 2 / Perlite Composites by Flame Spray Pyrolysis and Their Photocatalytic Behavior

TiO 2 /perlite composites were prepared via facile one-step flame spray pyrolysis (FSP) route. Titanium alkoxide (TIPO) and expanded perlite were used as Ti source and substrate, respectively. Precursor TIPO-ethanol solutions containing homogeneously dispersed perlite particles were processed through FSP setup at different experimental conditions regarding the gas flow and precursor supply rates. The structure, morphology, and the composition of the obtained powders were investigated. The porosity and the light absorbance of the TiO 2 /perlite composites were examined and their photocatalytic activity in NO oxidation was evaluated. Commercial titania powder P25 was also FSP processed and investigated for comparison.TheXRD analysis revealed that biphased titania with different anatase-rutile ratio and particles size 20–40 nm was synthesized onto the perlite which according to microscopy results was covered by neck-connected TiO 2 nanoparticles. The anatase-rutile interplay was also demonstrated by the Raman spectra where presence of Si-O-Ti vibrational modes was observed for some samples.TheUV-Vis diffuse reflectance spectra of the TiO 2 /perlite composites revealed up to 70% reflection that was connected to the presence of the gray perlite and superficial carbon.The best photocatalytic activity of the composites was connected to almost equal anatase-rutile ratio and possible synergetic effect of the two TiO 2 phases.


Introduction
The TiO 2 semiconductor is one of the most investigated materials during the last years.TiO 2 nanoparticles are used nowadays in an enormous spectrum of applications such as solar cells [1], gas sensors [2], photochromic devices [3], superhydrophilic surfaces [4] and photocatalysts [5].The above applications are related to the three different crystalline phases of TiO 2 , anatase, rutile, and brookite [6].Both anatase and rutile crystal structures are commonly used as photocatalysts with anatase showing a greater activity for most photocatalytic processes.This is attributed to anatase's slightly higher Fermi level, its lower capacity to adsorb oxygen, and its higher degree of hydroxylation [7,8].There are also studies which claim that a synergetic effect occurs and a mixture of anatase and rutile is more active than pure anatase [9].The enhanced activity arises from the increased efficiency of the electron-hole separation due to the multiphase nature of the photocatalyst [10].
There are several works investigating deposition of TiO 2 nanoparticles on various surfaces such as polymeric matrices [27], graphene oxide [28], carbon fibers [29], clays [30], and layered double hydroxide LDH [31][32][33].Systems of nanosized TiO 2 with highly adsorptive supports enhance the performance of the photocatalysts due to larger specific area, porosity, and higher adsorption capacity in comparison to the bare TiO 2 .Support limits the aggregation of TiO 2 nanoparticles, induces the microporosity into the materials [31], and contributes to the efficient spatial separation between the photogenerated electrons and holes.Glassy substrates were also used for deposition of TiO 2 photocatalysts [34][35][36][37].Such a glassy substrate is the perlite which is a natural amorphous material widely used in construction and industry.The use of natural perlite in blended cement production was investigated and the results revealed that perlite possesses sufficient pozzolanic characteristics [38].Its average chemical composition is approximately 75% SiO 2 with oxides of Al (14.8%),K (4.8%), Na (2.9%), Ca (0.9%), Mg (0.1%), Fe 2 O 3 (1.5%), and water (4.0%) [39].Crude perlite particles expand after heating to 870 ∘ C due to the presence of water and form innumerable sealed glassy cells in a manner similar to popcorn.These account for the excellent insulating properties and light weight of the expanded perlite.Being highly porous and chemically inert media, perlite granules can act as a good adsorbent.Therefore, expanded perlite can be selected as substrate for TiO 2 particles.TiO 2 /perlite composites prepared by various experimental methods were investigated aiming at enhanced photocatalytic activity in water and air decontamination.Thus, the TiO 2 -coated perlite granules were prepared using sol-gel method and tested for photocatalytic purification of furfural polluted waste water [40].Also, TiO 2 (Evonik-Degussa P25) immobilized on different supports (perlite granules, glass plates, and steel fiber) by sol-gel and investigated their photocatalytic oxidation of phenol [41].The preceding works on TiO 2 /perlite composite materials used mainly wet chemistry to immobilize the photocatalyst onto the substrate.To the best of our knowledge, there is only one report on preparation of Ag-coated perlite by the spray pyrolysis method in order to obtain a catalyst for ozone decomposition using a cheap natural glass as substrate [42].
In this paper we present for the first time the onestep synthesis of the TiO 2 /perlite composites by flame spray pyrolysis (FSP) processing.The structural and the morphological properties of the prepared materials as well as their photocatalytic performance in air pollutants oxidation are examined.

Experimental
2.1.Precursor Materials.The expanded perlite was provided by S&B Industrial Minerals S.A. to be used as a support for deposition of TiO 2 nanoparticles.Perlite was ground in a planetary mill for 2 h in order for fine granules to be obtained.The TiO 2 /perlite composite materials were prepared as follows.Initially, 5 mL titanium isopropoxide (TIPO), Aldrich 97%, was dissolved in 40 mL ethanol, Carlo Erba, and stirred for 30 min.Then, 5 g perlite was added and the dispersion was stirred for 60 min.The mixture was further homogenized using an ultrasound tip sonicator (Hielscher UIP 1000 hd).Total energy input of 1 Wh was applied for sonication time for 2 min.Finally, the slurry was diluted with ethanol to receive 100 mL suspension and the liquid obtained was nominated as precursor solution (PS).The prepared PS was ready to be processed through the flame spray pyrolysis system.It should be noted that water was not used in order to avoid hydrolysis.

Flame Spray Pyrolysis Device.
A homemade FSP setup was used to synthesize TiO 2 /perlite composites.In Figure 1 the schematic of the experimental set-up is shown.The FSP reactor consisted of four stainless steel concentric tubes.The PS was fed by a syringe pump (Inotec, IER-560) through the innest capillary tube (i.d.0.8 mm).Oxygen was used as dispersion gas fed through the surrounding annulus (i.d.The PS was processed through the FSP and the resulting powders were collected on glass microfiber filters (Whatman GF/D, 257 mm diameter) with the aid of a vacuum pump.Different TiO 2 /perlite samples were prepared by variation of the experimental conditions, namely, the gas flow rates (dispersion and sheath) and the syringe pump rate for the PS supply.Table 1 summarizes the above parameters used during the FSP process as well as the fraction of anatase and rutile phases of the flame made samples.

Characterization of TiO 2 /Perlite
Composites.The XRD patterns of the samples were obtained by X-ray diffractometer (SIEMENS D500) with secondary graphite monochromator and CuKa radiation operating in Bragg-Brentano geometry.
The measured 2 range between 10 ∘ and 90 ∘ was scanned in steps of 0.03 ∘ /2 s.The accelerating voltage and applied current were 40 kV and 35 mA, correspondingly.The crystalline phases were identified with reference to the PDF cards of the International Centre for Diffraction Data.The average crystallite sizes of the anatase and rutile phases were determined from the intensities of the primary peaks of anatase (101) reflection at 2  = 25.3 ∘ and rutile (110) reflection 2  = 27.5 ∘ using the Scherrer relation [43] as follows: where  = 0.15418 nm is the X-ray wavelength,  (rad) is the full width at the half maximum of the diffraction peak, and  Β is the Bragg angle.Since only the anatase and rutile phases are present in the examined TiO 2 /perlite composites, the weight content of these phases was derived from the area of the corresponding peak after the background substraction using (2) [44] as follows: where   and   are the surfaces of the primary rutile (110) and anatase (101) peaks, correspondingly.The specific surface areas (SSA) of the powders were determined via Brunauer-Emmett-Teller (BET) method.Standard device Sorptomatic 1990 FISONS was used for the nitrogen adsorption/desorption measurements at 77 ∘ K.All the samples were degassed at 453 ∘ K for 12 h.Assuming spherical and unagglomerated particles, the SSA of the TiO 2 nanoparticles (SSA TiO 2 ) was calculated from the average particle diameter estimated by XRD spectra,  XRD () , and the weighted density of the particles  () (anatase:   = 3.84 gr/cm 3 , rutile:   = 4.26 gr/cm 3 ) by (3) as follows: The SSA TiO 2 was calculated taking into account the mass fraction of anatase and rutile phases.The BET measured SSA of TiO 2 /perlite composites (SSA COMP ) and the calculated SSA TiO 2 are presented in Table 2.
The structural properties of the prepared composite materials were examined by Raman spectroscopy.The Raman spectra were measured with an inVia Raman microscope (Renishaw) using the 514.5 nm line of Ar+ laser emission with a resolution of 1.1 cm −1 .The laser power was 50 mW as an output and Raman emission signal at each measurement point was accumulated for 20 s for all samples.
UV-Vis diffuse reflectance of the powders in the wavelength range 350-800 nm as obtained using UV-2100 Shimadzu instrument.The measurements were performed using BaSO 4 etalon as a baseline.
The morphology of the powders was observed using scanning electron microscopy (SEM).An FEI Inspect microscope with tungsten filament operating at 25 kV was employed.In order to improve the surface conductivity of the samples standard gold deposition was applied through vacuum evaporation.The samples were also investigated using transmission electron microscopy (TEM).An FEI CM20 microscope operated at 200 kV was used.The samples for TEM observation were prepared as follows.Small amounts of the studied photocatalysts were dispersed in ethanol and the suspensions were treated in ultrasonic bath for 10 min.Then, a drop of the dilute suspension was placed on a carbon-coated grid and was allowed to dry by evaporation at ambient temperature.

International Journal of Photoenergy
Nitric oxide (NO) was chosen as representative airborne pollutant due to its potential health risks and ability to generate photochemical smog.The photocatalytic oxidation of NO by the prepared TiO 2 /perlite composite materials was investigated employing standard procedure based on ISO/DIS 22197-1 [45].The experimental set-up, the procedures of specimens' preparation, and photocatalytic activity measurement are described elsewhere [46][47][48][49].Briefly, the samples were placed in a flow-type photoreactor where model air containing 1 ppm NO was issued.Flow rate of 3 L/min and relative humidity of 50% were retained during the experiment.UV-A light illumination with intensity 10 W/m 2 for 90 min was applied.The concentrations of the NO, NO 2 , and NO  (NO  = NO − NO 2 ) were monitored in dark and under illumination.The photocatalytic activity of the samples was expressed by the calculated photonic efficiency  (mole/einstein) in NO and NO 2 oxidation as well as in NO  removal for the entire illumination period.

Results and Discussion
The measured XRD patterns of the prepared materials are shown in Figure 2. It can be observed that the samples were crystallized in the form of anatase and rutile.The position and the relative intensities of the peaks are consistent with the anatase TiO 2 JCPDS card number 21-1272 and rutile TiO 2 JCPDS card number 21-1276.The average crystallite size obtained from the most intensive diffraction peaks (101) and ( 110) at 2 = 25.28 ∘ and 2 = 27.45∘ ranged between 20 and 30 nm for anatase phase and 27 and42 nm for rutile phase, respectively (Table 1).Also, a broadened bump in the 2 range from 20 ∘ to 35 ∘ was observed originating from the amorphous nature of perlite.In the same time, the diffraction peaks (100), ( 101) and (110) were attributed to evident partial crystallization of the perlite during the flame processing procedure.The broadened part and quartz peaks are absent in the sample FSPA, which consist of P25 Degussa only.Finally, the interplay between the anatase and rutile phases in the composites (Table 1) is influenced by the PS and total O 2 flow rates [50], but the clear tendency in its behavior cannot be determined from our experiments.
In previous papers, it was demonstrated that the SSA of the particles decreased with the increase of the precursor flow rate [51,52] as well as with the increase of the oxygen flow rate [23].Higher precursor flow rate provokes higher frequency of the particle collision and sintering due to higher concentration of the forming powder in the flame.Also, higher oxygen flow rate leads to higher flame temperature encountered by the particles.As a result, the enlargement of particle size and reduction of their SSA were observed in both cases.
The BET measured SSA of the composites (SSA COMP ) and the calculated SSA of TiO 2 particles (SSA TiO 2 ) as well as total oxygen flow rate used in our experiments are given in Table 2.It can be perceived that the SSA COMP is lower than SSA TiO 2 .This can be explained by mixing of photocatalytically active TiO 2 component with porous but lower SSA perlite substrate.In addition, given that the precursor flow rate does not vary considerably, it seems that both SSA values decrease with the increase of the oxygen flow rate.This correlates with the literature results mentioned above.
The Raman spectra of the investigated composites as well as of the reference P25 sample are given in Figure 3.
The presence of the two anatase and rutile phases demonstrated by the XRD results above and their alteration can be observed also for the prepared composites.Thus, the Raman lines of the anatase phase [53] at 144 cm −1 (  mode), 197 cm −1 (  mode), 399 cm −1 ( 1 mode), 515 cm −1 ( 1 mode), and 639 cm −1 (  mode) can be observed in all the samples.Additionally, a weak peak at 450 cm −1 and a small shoulder around at 610 cm −1 were also found in the reference sample (Figure 3-inset).Both peaks correspond to the   and  1 modes of the rutile phase, respectively [54].These rutile peaks were changed in the spectra of the TiO 2 /perlite composites (samples FSPA, FSPB, and FSPD).More specifically, the shoulder at 610 cm −1 disappeared and the weak peak at 450 cm −1 caused a broadening of the anatase peak around 399 cm −1 .
It can be also observed that the lowest   anatase mode near 144 cm −1 is shifted to higher wavenumbers in the binary TiO 2 /perlite systems (samples FSPB, FSPC, and FSPD) in comparison to the bare TiO 2 nanoparticles (reference P25).The blue shift can be attributed to compressive stresses appeared in the samples during the flame processing procedure [55].This procedure is very abrupt and can evoke stresses in the flame-made nanoparticles produced using alkoxide precursor in comparison to the sample P25.
In addition, the peaks at 950 cm −1 and 1080 cm −1 related to the vibrational modes of Si-O-Ti [56] were observed for the samples FSPB, FSPC, and FSPD.This is an indication of the bond formation between TiO 2 photocatalysts and perlite substrate during the FSP processing.
The UV-Vis diffuse reflectance spectra for the prepared photocatalysts are given in the reflectance was high only for the reference sample P25.
The bare perlite and all the TiO 2 /perlite composites had a reflectance not higher than 70%.It was attributed to the presence of gray colored perlite in the prepared composites and probably to a large amount of residual carbon from the precursor and gas combustion.
The SEM images of the pure perlite and the perlite with immobilized titania nanoparticles are shown in Figures 5(a) and 5(b), respectively.It can be seen that bare perlite has a flake-like form.In the FSP composites, the flakes become rounded and are covered with TiO 2 agglomerates.This change in the perlite morphology is attributed to the high temperature FSP processing.The elemental mapping analysis presented in Figures 5(b) and 5(c) reveals that titania (Ti green colour) is homogenously dispersed on the surface of perlite (Si red colour).
Further morphological and structural characterization of the samples was carried out using TEM.As can be seen in Figure 6(a) the perlite flakes are almost entirely covered by TiO 2 nanoparticles that appear as an aggregation of small spherical neck-connected particles.It is suggested that these particle aggregations were created from large precursor solution droplets originated from 800 m nozzle diameter.The precursor rapid hydrolysis accompanied with droplets' abrupt shrinkage, during their passage through the flame zone, results in the formation of neck-connected spherical TiO 2 nanoparticles, as shown in Figure 6(b).Their mean size is about 20 nm in agreement with XRD results.
A typical HRTEM image of the TiO 2 nanocrystals on the perlite substrate is presented in Figure 6(c).It is evident that each nanoparticle is single crystalline.The lattice fringes shown in the HRTEM image correspond to the rutile and anatase crystal structure with -spacing  = 0.325 nm and  = 0.352 nm, respectively.This result is further confirmed by the fast Fourier transform of the HRTEM image shown as inset in Figure 6(c).
The calculated photonic efficiencies of the investigated samples are presented in Figure 7.The calculated values for the commercial Evonik-Degussa P25 are also given for comparison.It can be seen that the FSPA and FSPD TiO 2 /perlite composite samples exhibited improved photocatalytic activity in comparison to P25.In these samples, the anatase and rutile phase as well as TiO 2 are almost equal according to the XRD results.
Although the anatase is considered a more superior photocatalyst than rutile, the better photocatalytic performance of these composites can be related to possible synergistic effect between the two titania phases.Below is given a simple interpretation of this effect.Because of the higher position of the anatase's conduction band edge relative to that of rutile, transfer of the photogenerated electrons from the anatase to rutile takes place.This leads to reduction of the electronhole recombination rate and as a result to enhancement of the photocatalytic activity of the composite material.Furthermore, the observed enhanced photoactivity of the TiO 2 /perlite system can be also attributed to the presence of semitransparent porous perlite substrate that facilitates the dispersion of the TiO 2 particles as well as the penetration of UV-irradiation into the depth of the material [52,57].

Conclusions
TiO 2 /perlite composites were synthesized with the help of a homemade FSP reactor and using milled perlite flakes dispersed in TIPO and ethanol as precursor solution.The obtained TiO 2 consisted of interconnected spherical nanoparticles homogenously distributed on perlite flakes.The XRD analysis showed that titania crystallized in the form of anatase and rutile with nanoparticles' size ranging from 20 to 40 nm.Raman microscopy revealed attachment of titania nanoparticles onto perlite substrate via formation of the Si-O-Ti bond.The SSA of the TiO 2 /perlite composites decrease with the increase of the oxygen flow rate.Although the SSA of the TiO 2 /perlite composites was lower than that of P25 sample, their photocatalytic activity in NO oxidation was comparable and even higher for some of them.This result was attributed to the presence of perlite glassy substrate that facilitates photocatalyst dispersion as well as to the synergistic effect between two crystalline phases.

2 2 Figure 1 :
Figure 1: Schematic of the flame spray pyrolysis device.During spray procedure and after evaporation and combustion of precursor, TiO 2 nanoparticles are formed on perlite substrate.

Figure 2 :Figure 3 :
Figure 2: XRD patterns of TiO 2 /perlite correspond to the (101) reflection of the anatase and to the (110) of the rutile.For comparison, the XRD pattern of Degussa P25 is presented.

Figure 5 :Figure 6 :
Figure 5: Scanning electron microscopy (SEM) micrographs of pure perlite (a), composite TiO 2 /perlite (b), and elemental mapping for the FSPD sample using energy dispersive spectroscopy (c).Red colour corresponds to Si and green to Ti.

Table 1 :
Anatase (  %) and rutile (  %) compositions, crystallite sizes (  and   ) under different sheath, dispersion oxygen fractions, and syringe pump flow rates. of the precursor was supplied through a sinter metal ring (i.d.30 mm, o.d.38 mm).In this paper, the total O 2 flow rate represents the sum of dispersion and sheath O 2 supply.The gas flow rates were regulated by calibrated mass flow controllers (Bronkhorst EL-Flow).

Table 2 :
Measured BET SSA of the TiO 2 /perlite composites and calculated SSA of their TiO 2 component with respect to the total oxygen flow rate.