International Journal of Photoenergy

Volume 2008, Article ID 827605, 7 pages

http://dx.doi.org/10.1155/2008/827605

## Studies of Dye Sensitisation Kinetics and Sorption Isotherms of Direct Red 23 on Titania

^{1}School of Chemistry, Bangor University, Gwynedd LL57 2UW, UK^{2}Corus Colors, Shotton Works, Flintshire CH5 2 NH, UK^{3}School of Engineering, Swansea University, Swansea SA2 8PP, UK

Received 27 March 2008; Revised 27 March 2008; Accepted 19 May 2008

Academic Editor: Russell Howe

Copyright © 2008 Peter J. Holliman et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Sorption kinetics and isotherms have been measured for a commercial dye (Direct Red 23) on different samples of powdered Titania, and the data were analysed to better understand the dye sensitization process for dye sensitised solar cells (DSSCs). For the sorption kinetics, the data show rapid initial sorption (<1 hour) followed by slower rate of increasing uptake between 1 and 24 hours. While higher initial concentrations of dye correspond to higher sorption overall, less dye is absorbed from higher initial dye concentrations when considered as percentage uptake. The correlation between the sorption data and model isotherms has been considered with time. The Langmuir model shows better correlations compared to the Freundlich isotherm. The dye uptake data has also been correlated with Titania characterization data (X-ray diffraction, scanning electron microscopy, BET and zero point charge analysis). Kinetic data show significantly better fits to second-order models compared to first order. This suggests that chemisorption is taking place and that the interaction between the dye sorbate and the Titania sorbent involves electron sharing to form an ester bond.

#### 1. Introduction

The need for commercially viable renewable energy
sources such as photovoltaic (solar cell) devices is increasing in line with
concerns over climate change resulting from fossil fuel combustion
[1]. Dye
sensitised solar cells (DSSCs) have the advantage that this is a relatively
low-cost photovoltaic
technology [2]. A DSSC device uses a dye chemisorbed
to an oxide semiconductor such as Titania and operates by the dye reaching an
excited state by absorbing photons in the visible region and injecting these
electrons into the oxide. The electrons then pass between TiO_{2} particles to an electrode and around a circuit to a second electrode finally passing
via an electrolyte back to the
dye to continue the cycle.

A major breakthrough in DSSC
technology (solar efficiency 7%) was achieved by O’Regan and Grätzel
in 1991 [3]
using a Ru bipyridyl complex (the N3 dye) as the dye sensitizer along with
high-surface area
nanoparticulate Titania. Subsequent work by Grätzel and coworkers to develop
new dyes has resulted in increased solar efficiencies for DSSC (up to 10.4%)
using N719 [4] and Ru terpyridyl “black dye” [5]. Since 1991, a great deal of
work has been carried out on studying new dyes as sensitizers
to optimise extinction coefficient, rates of electron injection into TiO_{2}, and rates of regeneration of the
ground state dye by interaction with the electrolyte [6, 7]. While ruthenium(II)bipy
complexes have by far been the most widely studied dyes for DSSC, more recently,
indoline [8], unsymmetrical phthalocyanines
[9],
and squaraine dyes [10]
have also been reported as showing promise.

Typically, Titania is sensitized by soaking the oxide in a dye solution for either a period of hours or even overnight. However, to scale up the manufacture of DSSC, dyes must meet materials processing criteria, one of which is that the dye sensitisation process needs to take place more rapidly. In this context, the adsorption and desorption behavior of the Ru dye N3 has been studied stressing that dye coverage above 0.3 of a monolayer is necessary to increase current efficiency [11]. Also working on Ru dyes, Nazeeruddin et al. have reported a swift uptake procedure for the N719 dye [12]. More recently, it has been reported that electron injection rates into the metal oxide are an order of magnitude slower for aggregated dye compared with monomeric sensitizer dye emphasising the need for efficient monolayer sensitization [13].

This paper investigates the sorption isotherms and kinetics for the commercially available azo dye Direct Red 23 on different samples of Titania and correlates these data with both model isotherms and the physicochemical characteristics for the Titania samples. These data have been used to develop a clearer understanding of the dye sensitization process when considering time and dye concentration as two key variables.

#### 2. Experimental

Three variables were
explored using the commercial azo dye Direct Red 23, C.I. 29160 (Aldrich);
TiO_{2} grade (four samples labelled A to D were used), dye concentration
(from 163 mg l^{−1} to 650 mg l^{−1}), and time (up to 24 hours).

Sorption experiments were carried out using two replicates. The data were
averaged and the standard errors are shown in Figures 2(b)
and 2(c). In a
typical experiment,
methanol (50 mL, HPLC grade, Aldrich) containing the appropriate dye
concentration was added to titanium dioxide powder (1 g) in a 50 mL flask,
shaken for 1 minute and left to stand. Aliquots (1.5 mL) were taken after 15, 30,
and 60 minutes, then every 1 hour up to 8 hours and then at 23 and 24 hours. These were centrifuged for 10 minutes
at 8000 rpm before
the solution dye concentration was measured by UV-visible spectroscopy at nm. If necessary, dilution was carried
out (1 mL in 10 mL of methanol). The amount of sorption
(mg kg^{−1}) was calculated as the difference between the
amount of dye initially added and that measured at each measurement time.
Langmuir (1) and Freundlich
(2) and (3) sorption isotherm
equations were fitted to the experimental data by a computerized least-squares
optimization routine, where

where is the concentration of dye
in the solution (mg l^{−1}) at equilibrium, is the dye
concentration adsorbed onto the TiO_{2} (mg g^{−1}) at
equilibrium, and (mg g^{−1}) and (l mg^{−1})
are constants related to the adsorption capacity and to the energy of
adsorption, respectively [14]. For the Freundlich
isotherm, is a constant which measures the adsorption capacity
and is a measure of the adsorption intensity [15]. The values for and were calculated from the intercept and slope of the plots. The
solid-to-solution partition coefficient () was determined from the
following equation

where is the total dye ().

For TiO_{2} samples A and C, first- and second-order
kinetic equations were fitted to
the data according to the equations proposed by Kannan and Sundaram (5) [16]
and Ho and McKay (6) [17], respectively,

For the first-order model (5), is the amount of dye
adsorbed in mg g^{−1} at various times (), is the maximum adsorption capacity, and
is the first-order rate constant in min^{−1} [16]. Linear correlation
of versus was made to samples A and C at 3 different dye
concentrations. For the second-order
model, the initial adsorption rate in mg g^{−1} min^{−1} can be defined as ,
where is the equilibrium adsorption capacity in mg g^{−1} and is the second-order constant in
g mg^{−1} min [17]. Both variables were
calculated from the slope and intercept, respectively.

##### 2.1. Characterisation Techniques

X-ray powder diffraction (XRD) data was measured between 5 and 75 degrees two theta using Ni-filtered Cu 1 radiation ( Å) on an X’Pert PRO theta-theta diffractometer (PANalytical Ltd) at 45 kV and 35 mA.

Gold sputtered samples were analyzed by scanning electron microscopy (SEM) at 11 kV on a Hitachi S-520 SEM. EDAX was measured on the same instrument at 14 kV with an Oxford Instruments 7497 EDAX with Link ISIS computer software.

The surface areas
of the TiO_{2} powders were measured at 77 K on a Micrometrics Gemini
III 2375 and the data analyzed using BET (Braumer, Emmet, and Teller)
isotherms.

Zero point charge (ZPC) was determined by the salt-addition method [18] using three
replicates. In the procedure, ten aliquots of 0.1 M (40 mL,
Aldrich) were adjusted to varying pH between 2 and 11 using NaOH_{(aq)} (99.99%, Aldrich) or HNO_{3(aq)} (Aldrich). TiO_{2} (0.2 g) was
added to each aliquot, thoroughly mixed, and the pH measured after 24 hours.
The difference between final pH and initial pH (pH) was plotted against initial pH. ZPC was then calculated from this graph at the point where .

UV-visible spectroscopy was
measured in silica cuvettes (1 cm path length) on an ATI UNICAM UV4
spectrometer and the data analyzed using VisionScan software. The instrument
was calibrated using inhouse standards from 0 to 200 mg l^{−1} (typical
equation was with ).

#### 3. Results and Discussion

##### 3.1. TiO_{2} Powder Characterisation

Four commercial samples of Titania powder have been investigated. Sample A is Degussa P25, an uncoated nanoparticulate Titania; sample B is Kronos 1001 an uncoated, anatase pigment grade Titania; sample C is Kronos 2063; and sample D is Kronos 2220, a coated rutile pigment grade Titania.

X-ray
powder diffraction data (Table 1) show that
Sample A is a mixture of *ca.* 80% anatase [19] and 20%
rutile [20].The diffraction peaks are broader than the other samples
reflecting the smaller particle size range which is confirmed by electron
microscopy. Sample B shows only anatase along with narrower diffraction lines suggesting
larger particles. Samples C and D also exhibit narrower diffraction peaks more
characteristic of larger particles. However, for both samples, rutile is the
major phase.

Scanning electron
microscopy shows that all the samples are made up of spherical particles with the
main morphological difference between samples being particle size. Thus, sample
A has the smallest particles (20–50 nm), the
particles in samples B and C are 200–300 nm while those
in D are 300–500 nm. Table 1
shows that the surface areas of samples B, C, and D are all broadly similar at
11 to 15 m^{2} g^{−1} while sample A has a much higher surface
area at *ca.* 50 m^{2} g^{−1}.
Similarly, sample A differs from the others with a value for the zero point
charge (ZPC) of 4.50 compared to samples B and C (both *ca.* 6.50)
while sample D could not be measured due to problems
dispersing this powder in water.

##### 3.2. Sorption Kinetics and Isotherms

Direct Red 23 is a commercial, azo dye containing two
sulfonate groups (Figure 1) which can interact with O–H groups on the surface
of Titania. The % sorption against
time for an initial concentration of 330 mg l^{−1} of
Direct Red 23 onto four samples of Titania are shown in Figure 2(a). For sample
A, the data show very fast uptake of the majority of the dye during the first
15–30 minutes. This
is followed by a much more gradual uptake of dye between 0.5 and 24 hours. Sample
C shows the next fastest and highest dye uptake but this is much more gradual.
However, there is more rapid dye uptake initially (in this case, 0–5 hours) and then
a slower rate up to 24 hours. Finally, Samples B and D show little dye uptake
(%) with sample B showing evidence of sorption in the first few hours followed
by desorption between 3 and 24 hours.

Figure 2(b) shows the effect on the rate of dye uptake of changing the initial dye
concentration for Titania sample A. For 163 mg l^{−1}, almost all the dye is sorbed after 1 hours whilst for 330 mg l^{−1} and 650 mg l^{−1} dye
uptake is still increasing up to *ca.* 8
hours. By comparison, Figure 2(c) shows the corresponding data for sample C. Here, the larger
errors reflect the lower dye uptake compared to sample A and indeed the low
uptake and large errors make further analysis of samples B and D impossible.
However, for sample C, the data do show similar trends to sample A of slower
and proportionally lower uptake of dye with increasing initial dye
concentration. These data would seem to indicate that dye uptake becomes less
favourable as the oxide surface becomes increasingly populated with sorbed dye
molecules and solution dye concentrations decrease as might be expected.

Table 2 shows the results of fitting the sorption data to the Langmuir and Freundlich isotherm models after 1, 5, and 24 hours of sorption. The calculations are only shown for samples A and C because, as mentioned above, the dye uptake on samples B and D is so low. For both samples A and C, the values are generally better after 1 hour and then 24 hours; the increased scatter observed after 5 hours suggesting some surface reorganisation after initial dye sorption. For sample A, in all cases, there is a closer fit from the regression lines to the Langmuir isotherm (e.g., ranges from 0.94 to 0.99 for Langmuir but from 0.55 to 0.93 for the Freundlich isotherm). By comparison, sample C shows slightly better fits to the Freundlich isotherm at 1 and 5 hours ( ranging from 0.55 to 0.87 compared to 0.17 to 0.74 for Langmuir) but, conversely, a better fit to Langmuir at 24 hours ( is 0.97 to 0.98 compared to 0.77 to 0.91 for Freundlich). The Langmuir isotherm assumes monolayer coverage of sorbent but also that all the sorption sites are identical and that there is no interaction between sorbent molecules [21]. By comparison, the Freundlich isotherm assumes that the enthalpy of adsorption decreases exponentially with coverage [22]. The data show that the Langmuir model fits much more closely to the experimental data for this dye for sample A and sample C at equilibrium.

Thus a mechanism of sorption can be envisaged whereby, for the pristine
oxide surface, the largest number of vacant sorption sites are available and
the initial rate of dye uptake is quite rapid (Figure 2). For sample A, this
results in almost quantitative dye sorption in equilibrium with very low dye
concentration in the surrounding solution. However, as the sorption sites are
filled, fewer vacant sorption sites remain and relatively more dye molecules remain
in solution. This can be represented quite clearly when considering how the partition
coefficient () varies with time. For instance, at lower dye
concentrations, there is a trend towards increasing with time.
Thus, for sample A, at 200 mg l^{−1}, increases from 278
to 423 to 1085. This corresponds to an increasing proportion of dye sorbing to Titania
compared to that remaining in solution. By comparison, although there is still
an increase, remains more constant at the higher dye
concentrations (640 mg l^{−1}) for sample A. Overall, comparing samples
A and C after 24-hour
equilibration time, ranges from 51 to 1085 for sample A but from
1 to 2.7 for sample C emphasizing that the equilibrium between sorbed and dissolved
dye is hugely in favor of sorption for sample A across a range of dye
concentrations but is much more evenly balanced for sample C (particularly at
higher initial dye concentrations). Table 2 also shows that the Langmuir coefficients
( and ) are larger for samples A compared with sample C which corresponds
with the much greater dye uptake for the former over the latter.

Comparing all the sorption data with the characterisation data for the different Titania samples (Table 1) suggests that dye uptake is strongly influenced by metal oxide surface area and zero point charge. Most importantly, the higher surface area of sample A appears directly related to the increased uptake of this oxide because this presents a larger number of sorption sites which would be expected to lead to increased dye uptake. In addition, although the number of samples is relatively small, the data suggest that a lower value of zero point charge (ZPC) such as in sample A is an advantage. This implies that Titania sample A has a deprotonated surface at lower pH than Titania samples B and C. This is important if the mechanism of dye uptake is ascribed to chemisorption of the dyes to the Titania via the formation of sulfonate ester linkages between dye and surface O–H groups on the oxide with associated loss of water. Thus, the chemisorption process can be envisaged as occurring by nucleophilic attack of O–H groups from the Titania at the sulphur atom of the sulfonate group of the dye to forming the ester linkage. This suggests that deprotonation of the surface would lead to stronger nucleophiles which would enhance this process leading to more favourable uptake for sample A.

#### 4. Dye Sorption Kinetics

The results of the modelling of the kinetic data are shown in Table 3 with selected graphs in Figure 3. The data show that the linear regressions are poorer for the first-order kinetic model compared to the second-order data particularly for sample A (first-order ranges from 0.09 to 0.90 compared to 0.99 to 1.00 for second order) across the three different concentrations studied. This suggests a second-order kinetic process dominates Direct Red 23 adsorption onto these Titania samples. This suggests that the rate-limiting step is a chemisorption process where sharing or exchange electrons between sorbent and sorbate takes place. In this case, the likelihood is that a sulfonate ester bond forms between the sulfonate groups on the dye and surface hydroxyl groups in the Titania.

Sample A produced the highest values for the parameters in the second-order kinetics equation (Table 3). Comparing the data for the initial adsorption rate (hour), these are the highest at the lowest dye concentration, decreasing dramatically as the initial dye concentration increases. This suggests that the kinetics of adsorption to the most energetically available adsorption sites are much faster and that as coverage increases, the rate of adsorption slows when only less energetically favorable sites remain. This gives a different insight into the adsorption process compared to the sorption isotherms (Langmuir) which reflect much more an equilibrium situation.

#### 5. Conclusions

Dye sensitization is a key process in the manufacture of dye sensitized solar cell devices with very rapid dyeing uptake leading to full-monolayer coverage representing optimum objectives. Closely linked to these are the requirements for large amounts of dye uptake to maximize light absorption allowing for thinner devices along with a need for strongly chemisorbed dye molecules which do not desorb under device working conditions. The data presented for sorption of Direct Red 23 onto Titania sample A (Degussa P25) do show some of these characteristics and this is believed to be related to the higher surface area of this material along with a lower value of ZPC which offer a greater number of sorption sites and a more favourable sites for chemisorption of the dye, respectively.

Finally, whilse Direct Red 23 is not used in the currently, most efficient dye sensitized solar cells, this commercially available dye does possess similar anionic groups for adsorption onto Titania compared to Ru complexes and organic dyes. Thus, the data presented have important implications for dye sensitisation especially in the context of growing interest in organic dyes for DSSC.

#### Acknowledgments

We gratefully acknowledge EU (ESF) Objective One funding and Corus support for BVV, Dr. Callum Hill for BET data, and Steve Jones for collecting the XRD data.

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