# Influence of the Synthesis pH on the Properties and  Activity of Sol-Gel TiO2 Photocatalysts

Authors: Javier Marugán; Paul Christensen; Terry  Egerton; Herry Purnama
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The activity of titania photocatalysts is highly dependent on the synthesis procedure. This work reports a study of the correlation between the photocatalytic activity and the physicochemical and photoelectrochemical properties of sol-gelTiO2 powders and electrodes synthesized using different pH values and temperatures. The activity of the materials has been evaluated using the decolorization of
Reactive Orange 16 (RO16)  as model reaction. In contrast with the large number of studies reporting the influence of the temperature, our results point out that preparation pH has even more influence on the crystalline phases and the photocatalytic activity of TiO2 powders and photocurrents of the electrodes. However, the effects on the activity and the photocurrents recorded after immobilization on a conducting support do not vary accordingly. Consequently, our results indicate that the measurement of the photocurrent in an electrolyte media is not a good indicator of the photocatalytic activity of the unsupported system.


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## Body

## 1. Introduction

The fundamentals of semiconductor photocatalysis and its application to
the removal of chemical pollutants have been extensively reviewed [1–5]. However, commercialization
of   photocatalysis for water treatment has been hindered both by difficulties of
postreaction catalyst recovery [6] and by low-quantum efficiencies [7], which means
that large reactors are required for small-capacity plants. Consequently,
although immobilized TiO2 has been shown to be less active than
slurry systems, much effort has been devoted to the development of highly
active fixed catalysts. One approach is to immobilize the TiO2 on a
conducting support and apply a potential bias. This, simultaneously, addresses
both problems [8].The activity of titania powders and electrodes is highly dependent on the
synthesis procedure [9, 10]. A common way of synthesizing TiO2 electrodes is to coat conducting materials with titania sol-gel suspensions, as
this procedure provides fine control of the physicochemical properties of the
TiO2 crystalline phase and hence of its photocatalytic activity [11].
The heat treatment temperature used to crystallize the titania has been shown
to be critical in the activity of sol-gel coatings [12]. However, to the best
of our knowledge, there are no reports about the influence of the synthesis pH or
of the pH of coating suspensions on the activity and photoelectrochemical
properties of the catalysts.In this work we have studied the correlation between the photocatalytic
activity and the physicochemical and photoelectrochemical properties of sol-gelTiO2 synthesized using two different pH routes. The activity of the
materials has been evaluated using the decolorization of Reactive Orange 16 as
model reaction.


## 2. Experimental

## 2.1. Synthesis of the Materials

Titanium dioxide  sols were prepared by hydrolysis and condensation of
titanium tetraisopropoxide (TTIP) (97%,
Sigma Aldrich, Gillingham, UK) at different pH values, following a procedure
based on that described by O’Regan et al. [13]. Powder photocatalysts were
obtained from the colloidal suspensions by increasing the pH to 7 with sodium
hydroxide and vacuum filtering through a 2.7 μm paper filter  (Whatman plc, Maidstone, UK). The filter cake was resuspended in deionized
water three times to remove the ions from the solution until the filtrate
conductivity fell below 200μS⋅cm−1, and finally was rinsed twice
with 2-propanol to minimize particle agglomeration. The solids were dried at
room temperature and then calcined at the desired temperature for two hours.To prepare the electrodes, the titania colloidal suspension was first
concentrated toca.150g⋅L−1, using a
vacuum rotary evaporator, to increase the viscosity. Thin-film electrodes were
fabricated by dropping suspension onto 1cm×1cm titanium plates (99.6%, Goodfellow, Huntingdon, UK) and spinning
at 2800 rpm prior to calcination for 10 minutes at the desired temperature. The
dropping, spinning, and heating sequence were repeated five times. The white
coating was more apparent on the neutral (pH 6.5) plates than on the acid (pH
1.5) electrodes.


## 2.2. Characterization Techniques

X-ray diffractograms (XRD) were collected
in the range10<2θ<90° in a Philips X’Pert diffractometer using
nickel-filtered monochromatic Cu Kα radiation. In all cases, only anatase and
rutile crystalline phases were detected. To investigate the morphology of the
TiO2 electrodes, scanning electron microscopy (SEM) micrographs were
taken on a JEOL JSM5300LV working at an acceleration voltage of 25 kV under
environmental conditions, without conductive coating on the sample.Diffuse reflectance spectra (DRS) of
the small plate electrodes were recorded from 200–500 nm relative to a polytetrafluoroethylene
reference with a Varian Cary 500 Scan U-VIS-NIR
spectrophotometer equipped with an integrating sphere diffuse reflectance
accessory. Reflectances were reported as the Kubelka-Munk function,
F(R), whose value, for constant scattering, is proportional to the absorption.Electrochemical
characterization of the small plate electrodes was carried out by cyclic
voltammetry, with a sweep rate of 100 mVs−1, using an Eco-Chemie
μAutolab Type II potentiostat. Potentials were quoted with respect to an
Ag/AgCl reference electrode and a 10
cm in diameter nickel gauze was employed as counter
electrode. Photocurrents were recorded under illumination by two 36 W actinic UV
lamps (Philips Lighting, Guildford, UK).


## 2.3. Photoreactions Procedure

The photoreactor was a cylindrical concentric
system with two axially located 8 W UV lamps (Sylvania: Light Bulbs Direct, Amersham, UK), two Pyrex
glass walls containing the suspension of catalyst in the dye solution, and a
sintered frit distributing the sparge oxygen. More details about the reactor
can be found elsewhere [12]. The photocatalytic activity of the photocatalysts
was evaluated using the decolorization of the azo dye Reactive Orange 16 (RO16), represented in
Scheme 1, as a model reaction. The initial concentration of dye was 0.05 mM. The pH obtained
after dissolution of the dye in deionized water was used without further
adjustment. The catalyst loading was 0.5g⋅L−1 in all the experiments.Scheme 1
Reactive Orange 16 (RO16).Prior to the start of the reaction,
the dye solutions were equilibrated in the dark for 30 minutes in contact with
theTiO2 material and the oxygen bubbles. After switching on the
lamps, the evolution of the reaction was followed by colorimetric measurements
(ε493nm=23.45L⋅mmol−1⋅cm−1) using a Shimadzu UV/vis spectrophotometer.


## 3. Results and Discussion

## 3.1. Characterization Results

The initial synthesis of the pure
titania powder photocatalysts followed the original acidic procedure of O’Regan
et al. [13], described above. The influence of the calcination temperature was
investigated first. As shown in Figure 1, the diffraction line breadth
demonstrates that the TiO2 existed as small crystallites of anatase
after calcination at 500°C but that after calcination at 600°C,
these crystals grew substantially and transformed to rutile.Figure 1
X-ray diffraction pattern
of pure titania powder photocatalysts.An equivalent set of materials was
then prepared following a sol-gel route at pH 6.5. According to Wang and Ying
[14], neutral aqueous media lead to anatase materials that undergo neither
phase transition to rutile nor significant grain growth, whereas the use of
nitric acid favours the formation of rutile. The XRD patterns (Figure 1) of
these pH 6.5 materials show that at both 500°C or 600°C, pure anatase materials were
obtained with only traces of rutile. Table 1 summarizes the composition calculated
from the areas of the 101 anatase peak (2θ∼25.3°) and the 110 rutile peak (2θ∼27.4°)
according to the procedure of Zhang and Banfield [15]: (1)Anatase(wt%)=0.884×Aanatase0.884×Aanatase+Arutile.The average crystallite sizes were estimated from the width of signals
using the Scherrer equation (Table 1). The average anatase size increased with
calcination temperature, especially for materials synthesized at acidic pH,
which favours the thermal transformation   to rutile.Table 1
Synthesis
conditions, crystallinity, and photocatalytic activity in terms of the first-order kinetic constant for
RO16 decolorization of powder photocatalysts.
NamepHTemp.  (C°)XRD DatakRO (h-1)Anatase  (%)Danat   (nm)Sg† (m2g-1)P-TiO2-11.5500100141100.22P-TiO2-21.56008942370.32P-TiO2-36.5500100121280.38P-TiO2-46.560010024641.09†Estimated
assuming nonporous spherical particles with a density corresponding to bulk anatase.


## 3.2. Photocatalytic Results

Figure2 shows the photocatalytic
activity of dispersions of these catalyst materials measured as the decrease
with irradiation time of the absorption at 493 nm. In all cases, a pseudo-first-order
kinetics model satisfactorily represents the results and the derived first-order
rate constants are shown in Table 1.
In both cases, an increase in the temperature of the
heat treatment from 500°C to 600°C leads to a significant increase in the activity even though the surface area, estimated
from the average anatase size, decreases. However, the clear and important
conclusion is that titania materials synthesized via the neutral sol-gel route
are more photoactive than samples prepared in acid media.Figure 2
First-order kinetics fit
of the RO16
photocatalytic decolorization.The lack of correlation between photocatalytic
activity and the specific surface area suggests that the reaction rate may be
more influenced by the generally assumed higher activity of anatase. Although
this would be consistent with the increased activity of P-TiO2-4
(100% anatase) relative to P-TiO2-2 (11% rutile), it is inconsistent
with the lower activity of P-TiO2-2 relative to (100% anatase)
P-TiO2-1. We cannot exclude the possibility that since the P-TiO2-1
was calcined at lower temperature, it may have significant amounts of amorphous
TiO2, which reduces the photocatalytic activity.


## 3.3. Photoelectrochemical Results

The striking increase
in photocatalytic activity associated with the particle preparation at pH 6.5
prompted a corresponding photoelectrochemical study on electrodes prepared by
coating sol-gelTiO2 coated onto 1 cm2 titanium plates.
Photocurrents were calculated as the difference between the current recorded in
a cyclic voltammetry experiments in which the electrode was irradiated with UV
light and the current recorded in the dark.Figure3(a) shows the influence of
the pH on the photocurrent recorded in 0.1 M Na2SO4. As can be
seen, the shape of the current voltage curve depended on the pH of the TiO2 coating suspension and on the treatment temperature. The curve for the
electrode coated with a neutral TiO2 solution and heated at 500oC
showed a progressive increase of the photocurrent with increase of the applied
potential, in agreement with classical semiconductor theory. In contrast, for
the electrode coated with an acidic TiO2 sol the photocurrent seems
to be independent on the voltage. This previously observed [9] behaviour is attributed
to particles, which are smaller than the thickness of the depletion layer.
Because of this, increasing the electric field does not increase charge
separation [16]. At higher temperatures, this effect disappears, probably by
growing of the titania particles, and the cyclic voltammetries of both kind of
electrodes are quite similar. In all cases, the photocurrents of neutral TiO2 electrodes at high potential values are higher than the photocurrent of acid
TiO2 electrodes at high potentials.Influence of pH and
temperature on the photocurrent density ofTiO2 electrodes in (a) 0.1 M Na2SO4 and (b) 0.1 M Na2SO4 + 1 M  CH3OH.
(a)(b)Figure3(b) also displays
the corresponding current voltage curves after addition of 1 M methanol to the electrolyte
solution. Whereas the behaviour of the neutral TiO2 electrodes is
quite similar to that in the absence of methanol, the photocurrents recorded
for the acid TiO2 electrodes increase dramatically. This methanol
enhancement has been previously reported by Christensen et al. [12] when
comparing the photoelectrochemical properties of sol-gel electrodes (synthesized
in acid media) with TiO2 electrodes obtained thermally from titanium
plates. Similarly, Mintsouli et al.  [10] have reported a photocurrent
enhancement in particulate electrodes (prepared from Degussa P25 suspensions)
when adding oxalate to the electrolyte. In both cases, the organic compound is
considered to scavenge photogenerated holes, decreasing charge recombination
and hence increasing the photocurrent. This effect is reported to be especially
marked in particulate electrodes of high roughness and titania surface area, but
almost negligible for  thermal electrodes [12]. The SEM micrographs, Figure 4,
shows that the acid TiO2 electrode presents a more fractured but less
rough surface, whereas the neutral TiO2 electrode calcined at 500°C consists of a
continuous but rough film. At higher temperature, the two electrode types are
more similar, although the neutral electrode seems to be more particulate.SEM micrographs spectra ofTiO2 electrodes calcined at 500°C: (a) pH = 1.5 and (b) pH = 6.5.
(a)(b)Figures5(a) and 5(b) show
the influence of the heat treatment temperature on the photocurrent, measured
at 1.0 V, with and without the addition of methanol for the pH 1.5 and pH 6.5
electrodes. In Na2SO4 (only) solution, both electrodes
show a maximum photocurrent at 650°C comparable with the maximum at 600–650°C reported by
Christensen et al. [17]. For the pH 6.5 electrodes, the methanol photocurrent
enhancement, calculated as the difference of the two plots, is vanishingly
small for all heat treatment temperatures. For the pH 1.5 electrodes, the
methanol photocurrent enhancement decreases to zero at ∼700°C,
which compares well with the 750°C reported in earlier work [17]. Consequently,
the differences between electrodes synthesized in acid and neutral media
disappear progressively when the calcination temperature is increased, probably
due to the growth of the titania particles and the increase in the density of
the titania film.Influence of temperature
on the photocurrent density ofTiO2 electrodes: (a) pH = 1.5 and
(b)  pH = 6.5. The error bars have been
estimated from the spread of 2 independent measurements.
(a)(b)Figure6 shows the X-ray diffraction patterns of the four electrodes. Acid sol-gel
electrodes show the presence of anatase crystals at 500°C. Narrowing of the
diffraction lines confirms the crystallite growth between 500°C and 650°C (The crystal sizes
calculated for the acid pH electrodes are 10.2, 23.0, 30.9, and 68.3 nm for
calcination temperatures of 500, 600, 650, and 700°C, resp.).
Although the anatase peak height is little altered, the integrated intensity
has reduced suggesting that anatase has transformed to rutile (A rutile phase
is detected above 600°C).
Remarkably, no anatase phase was detected in neutral pH TiO2 electrodes below a treatment temperature of 700°C. However, rutile
crystals were formed at lower temperatures, as was reported earlier for “thermal
electrodes” prepared by heating titanium metal [17].  Therefore, these results suggest that either,
unlike the pH 1.5 coating, the crystal growth of the pH 6.5 coating is
controlled by the titanium substrate, or the rutile phase mainly grows directly
from the oxidation of the titanium support.Figure 6
Influence of pH and
temperature on the X-ray diffraction pattern ofTiO2 small
electrodes.Finally, the diffuse reflectance
spectra of electrodes heated at500°C are shown in Figure 7. It is immediately obvious that these spectra are unlike
the “typical” spectra of TiO2 and it is probable that they include a
contribution from the underlying oxidized titanium metal. Separate studies [18]
show that uncoated titanium metal is blue when heated to 500°C but grey or black when
heated to 650°C.
The influence of the colored substrate is likely to be increased when the
particle size of the sol-gel-derived TiO2 is small, and because it
is not an efficient scatterer, it is not completely opaque. Despite these
complications, the spectra show that for electrodes calcined at 500°C the peak absorption of
the neutral pH electrode is ∼20nm below that of the acid electrode whose peak
is near 350 nm. This blue shift could be explained by the extremely small
crystallite size of the titanium dioxide particles (in agreement with the
almost negligible intensity of the X-ray diffraction signals). Small particles
can cause the apparent absorption maximum to shift to shorter wavelengths—as described by Mie theory [19]—and may also lead to an increased band gap in
quantum-size semiconductor particles [20].Figure 7
Influence of pH on the
diffuse reflectance spectra ofTiO2 electrodes.At650°C, the increased absorption
of the underlying oxidized titanium metal dominates the spectrum and its strong
visible absorption means that the normally observed absorption edge is
difficult to discern (therefore the 650°C spectra are not reported). However, the
difference between the neutral and acid electrodes appeared to have disappeared.Summarising,
the physicochemical characterization of the materials seems to point out that
the use of a neutral sol-gel route leads to titania catalysts constituted by
extremely small anatase crystals of higher activity in comparison with those
obtained at acidic pH values. However, despite the strong dependence of the
photoelectrochemical properties of the materials on the synthesis pH, the
photocurrents recorded under potentiostatic control do not correlate the
activity exhibited by the materials for the decolorization of an azo dye
solution.


## 4. Conclusions

The main conclusion of
this work is the significance of the synthesis pH of sol-gel titania powders
and electrodes for photocatalytic applications. In contrast with the large
number of studies reporting the influence of the temperature on the
physicochemical and photoelectrochemical properties of titania powders and coatings,
our results indicate that pH is even more influential in controlling both the
crystalline phases, and the photocatalytic activity ofTiO2 powders
and photocurrents of the electrodes. However, the photocatalytic activities of
the TiO2 slurries are not proportional to the photocurrents recorded
after immobilization on a conducting support. Consequently, these results indicate
that the measurement of the photocurrent in an electrolyte media is not necessarily
a good indicator of the photocatalytic activity of the unsupported system.



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