# Electrochemical Investigation of Doped Titanium Dioxide

Authors: J. W. J. Hamilton; J. A. Byrne; C. McCullagh; P. S. M. Dunlop
Journal: International Journal of Photoenergy
 (2008-02-18)

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## Abstract

Thin films of transition-metal doped (0.2, 1.0, and 5.0 atom%)TiO2 were prepared on titanium foil using a sol-gel route catalyzed by ammonium acetate. Dopants investigated were the fourth-period transition metals. The prepared films were characterised by Raman spectroscopy, Auger electron spectroscopy, and photoelectrochemical methods. The films doped with transition metals showed a lower photocurrent response than undoped samples. No major red shift in the photocurrent response spectra of the doped films was observed. A photocurrent response was observed under visible light irradiation of the samples and was potential dependent peaking around −0.3 V (SCE), which is indicative of electron promotion from a filled defect level. Examination of the defect level potential dependence by analysis of the current-time response under chopped illumination at fixed potential (−0.8 V–+1.07 V) gave a good correlation with the potential dependence observed in the visible light irradiation studies.


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## Body

## 1. Introduction

The photoactive properties of titanium
dioxide (TiO2) have been utilised for two main fields of
application: (i) environmental remediation including water/air purification, and
selfcleaning surfaces; (ii) solar energy conversion through photoelectrolytic
water splitting to yield hydrogen and oxygen [1–4]. Nanoparticle TiO2 electrodes have
also found application in dye sensitised photovoltaic cells [5, 6]. The reported
efficiency of TiO2 for solar applications in (i) and (ii) above is
low due to its wide band-gap (3.0–3.2 eV) requiring
ultraviolet irradiation (UV). In attempts to increase the solar efficiency by
utilising visible light, TiO2 has been doped with other elements
with the aim of producing a red shift in the absorbance spectrum. Transition
metal ions are commonly employed dopants due to
their strong absorption of visible light and their similar ionic radius to the
parent titanium ion. However, studies into the effectiveness of transition
metal ions as dopants to improve solar efficiency have reported conflicting
results. For example, chromium doping of TiO2 has been reported to
yield both an increase in activity [7–10] and a decrease in activity [11–16].
The majority of dopant studies do report a red shift in the absorption spectra
of the doped material, however, this does not necessarily correlate with an
increase in photoactivity [13, 17–20]. Unfortunately, there are few
publications which report both the photocurrent response and absorption spectra
for doped TiO2. Where they exist, the results are conflicting, with
some researchers reporting a good correlation between the photoactive spectrum
and the absorption spectrum [8, 11, 17] while others have not found a
correlation [21].The direct comparison of published
results for the efficiency of doped TiO2 photocatalysts is not
straightforward as reported materials have been produced using different
methods and have been tested under different conditions. For example, if the
photocatalytic degradation of a model pollutant is used as the measure of
efficacy then changes in surface adsorption, due to changes in surface PZC
resulting from doping, may result in an increase in photocatalytic degradation
rate of certain organics but not others [15, 22].Only a few papers discriminate between
the effects of the dopant ion energy levels and the dopant ion-induced oxygen
defect energy level within band structure. An oxygen defect level is introduced
when the dopant has a lower valence than titanium thus leaving oxygen bonding
unsatisfied and a new energy level is produced.The effect of this induced defect level
is difficult to study within a single dopant regime. Analysis of the dopant
induced oxygen level within the band structure of platinised TiO2 was studied by Karakitsou and Verykios using dopants of different valences to titanium
[23]. Using hydrogen evolution as the measure of photoactivity, they reported
that dopants with a valence greater than 4 gave higher activity, and dopants
with valence less than 4 had a lower activity, as compared to the undoped TiO2.
The decrease in the photoactivity with dopants of valence less than 4 was
attributed to an increased rate of charge carrier recombination.The effect of oxygen defect levels in
the TiO2 band structure have previously been reported in literature
[24, 25]. Siripala and Tomkievicz were the first to examine the effect of these
defects on charge transfer at the TiO2 electrolyte interface [26].
They reported that oxygen defect levels were responsible for observed
photocurrent using sub-band gap illumination. The sub-band gap photocurrent was
only observed between applied potentials of −0.6 V and +0.6 V (SCE). Other
workers have reported the generation of oxygen defect states via plasma
modification of TiO2 which yielded a visible photocurrent response
[27].In this work, we report the effect of
doping with transition metal ions on changes in UV/Vis absorption spectra,
crystal phase, current-potential response, and spectral photocurrent response.


## 2. Experimental

A sol gel procedure, adapted from the
work of Murakami et al. [28] using
ammonium acetate as a catalyst, was used to prepare TiO2 films. A
solution of 0.0973 g ammonium acetate, in 0.2 cm3 water/15 cm3 butan-1-ol, was added drop-wise to solution of 0.377 g titanium (IV) butoxide
in 10 cm3 butan-1-ol. For doped samples, the relevant dopant acetate
salt was added along with the ammonium acetate to give 0.2, 1.0, or 5.0 atom%
(with respect to titanium). Dopants
investigated were V, Cr, Mn, Fe, Co, Ni, and Cu. The resulting transparent sol
gel was stirred for a further 1 hour before dip coating onto previously cleaned
2 × 2 cm2 titanium foil coupons (1.27 × 10−2 cm thick,
Aldrich). Films were deposited in five coat cycles, drying between coats with
an infrared lamp, before annealing at 500°C in air for 1 hour with a
temperature ramp rate 1°C min−1. Electrical contact was
made to the coupons by attaching copper wire with conductive silver epoxy
(Circuit Works, Chemtronics). The contact and coupons were painted with a
negative photoresist (KPR resist, Casio Chemicals) and UV cured, leaving a
defined area of TiO2 exposed.Electrochemical characterisation of
samples was performed under potentiostatic control (Autolab PGSTAT30) in a one-compartment
cell with quartz window, platinum basket counter electrode, and a saturated
calomel reference electrode (SCE). The supporting electrolyte was 0.1 M sodium
perchlorate, and irradiation was provided from a 1 kW Xe arc lamp (ss1000
Spears Robinson) either; unfiltered, with an AM1 filter (to simulate solar
conditions) or with a sodium nitrite filter (λ > 420 nm) for high-intensity visible light.
For monochromatic irradiation, a high-intensity monochromator (GM252, set for
10 nm band pass) was positioned between the source and the photoreactor. Light
intensity was measured using a calibrated spectral radiometer (Jobin Yvonne
Gemini spectral radiometer). Linear sweep voltammetry (LSV) was carried
scanning from –1.0 V to +1.5 V
with a scan rate of 10 mV s−1. The current-time response was
measured at fixed potentials using chopped irradiation (Uniblitz vmm-t1,
Vincent Associates). The cell temperature was maintained at 25°C ± 2°C. The
presence or absence of dissolved oxygen was controlled by sparging with air or
oxygen free nitrogen (OFN), respectively. All potentials are reported versus
SCE.Elemental analysis was carried out by
Auger electron spectroscopy (AES) (KratosΦ660) in a vacuum of 10−6 torr with
electrons accelerated from a field-emission source at 4 kV accelerating voltage
to give a beam current of 2 μA. Depth
profiling was achieved using an argon etch.UV-Vis transmission spectra of films
deposited on quartz were measured using a spectrophotometer (Lambda 11, Perkin
Elmer). Optical band-gap values were calculated by extrapolation of the
absorption band edge. Crystal phase was analysed by Raman
spectroscopy (LabRam 300, ISA instruments) by comparing to commercial anatase
(99.5%, Aldrich) and rutile (created from predominantly rutile sample (Aldrich)
by heating to 800°C). The source was an argon laser λ=514 nm.
Optimum analysis was performed with an acquisition time of 5 seconds, averaged
over 10 accumulations, with the neutral density filter set to 50%.


## 3. Results and Discussion

AES
measurements indicated that not all of the corresponding dopant was incorporated at the hydrolysis stage of sol-gel procedure.
Determination of dopant concentrations at initial precursor levels below 1.0 atom% was not possible with the AES system used in this study. Analysis of
doped samples containing 1.0 atom% and 5.0 atom%, as precursor, confirmed the
presence of the dopants (Table1), however, they were at much lower levels (ca.
40%) than that of the initial precursor dopant level.Table 1
Elemental analysis by AES for Cu and Cr doped TiO2 films.
SampleCu
doped sample 1 atom% precursorElementCarbonOxygenTitaniumCopperBefore
etch8.65%75.17%15.79%0.39%After
etch22.86%52.42%24.64%0.09%Cr
doped sample 5 atom% precursorElementCarbonOxygenTitaniumChromiumBefore
etch13.57%66.60%17.63%2.20%After
etch11.44%66.75%20.21%1.59%For
simplicity, we will continue to refer to percentage doping of samples as the
initial concentration of dopant added to the sol precursor. Depth profiling of
the thin films showed that the dopant species were preferentially located on
the surface with lower concentrations present in the bulk. Additionally, a high
proportion of carbon was measured which originated from the alkoxide precursor
used in the sol-gel process. Carbon was present in all samples including the
undoped sample.Figure1 shows the Raman for a TiO2 film and a
TiO2:V film, compared with anatase and rutile powders. Analysis of the powder
samples showed all relevant Raman vibration modes of anatase and rutile
correlating with previous literature on Raman analysis of TiO2 [29].
Comparison of the sol-gel derived thin films showed all samples to contain the Eg vibrational mode at 144 cm−1 common to both anatase and rutile. However, the rutile signal at 144 cm−1 is much weaker compared to the anatase signal. Since the sol-gel film shows a
large peak at 144 cm−1 but no other rutile peaks, then we can assume
it was predominantly anatase. The TiO2:V sample showed peaks present
at 447 cm−1 (Eg)
and 612 cm−1 (A1g)
representative of rutile and 519 cm−1 (B1g) representative of an anatase crystal
structure. The spectra of the undoped sample and all other doped samples were
almost identical showing a predominantly anatase crystal phase, with the 399 cm−1 and 519 cm−1B1g peaks,
along with the 639 cm−1Eg peak associated with anatase.Figure 1
Raman spectra of (a) undoped TiO2 film, (b) vanadium doped TiO2, (c) commercial anatase powder, (d) commercial rutile powder (following high-temperature anneal).UV-Vis
absorption spectroscopy of the doped films, deposited on quartz, showed a minor
red shift in the absorbance spectra as compared to the undoped film, however,
this was mainly confined to the region from 250 nm to 350 nm (examples in Figure2).Figure 2
UV/Vis absorption spectra of doped TiO2 films (a) V 0.2 atom%, (b) Cr 0.2 atom%, (c) Mn 0.2 atom%, (d) undoped TiO2.With all dopants investigated, an
increase in the dopant concentration resulted in an increase in the absorbance
between 290 nm and 350 nm with a small increase in absorbance at wavelengths
greater than 350 nm. For example, the change in absorbance with concentration
of dopant using TiO2:Mn is given in Figure 3. Changes in the absorption
band edge, obtained by extrapolation of the UV/Vis absorption data, were in the
range of 0.01–0.25 eV. The
error in these calculations was not acceptable and therefore we do not report
the band-gap values.Figure 3
UV/Vis absorbance versus dopant concentration for TiO2:Mn (a) 5 atom%, (b) 1 atom%, (c) 0.2 atom%, (d) undoped.The
photoelectrochemical characterisation of the films using linear sweep
voltammetry (LSV) showed a typicaln-type semiconductor response with
negligible anodic current in the dark and a significant increase in the anodic
current under AM1 solar simulated irradiation (Figure 4).Figure 4
Linear sweep voltammograms under AM1 solar simulated irradiation for doped TiO2 films (0.5 atom% dopant) (a) undoped, (b) Fe, (c) Co, (d) Ni, (e) dark response for undoped film.The
photocurrent was dependent on the applied potential, and the undoped TiO2 sample showed a larger photocurrent response than any of the doped samples.
Furthermore, a decrease in photocurrent with increasing dopant concentration
was observed with all doped samples. Air sparging of the electrolyte prior to
LSV resulted in a positive shift in the onset potential for anodic current (Figure 5 and Table 2).Table 2
Anodic
photocurrent onset potential for different preparations in the presence of
oxygen (the undoped sample gave an onset potential of –0.8 V).
Onset
potential (V)Dopant0.2 atom%1.0 atom%5.0 atom%Vanadium−0.49−0.47−0.46Chromium−0.48−0.48−0.45Manganese−0.47−0.45−0.41Iron−0.56−0.45−0.49Cobalt−0.58−0.44−0.45Nickel−0.52−0.49−0.44Copper−0.67−0.56−0.54Figure 5
Effect of electrolyte air sparging on theI-E response for TiO2:Cr (0.2 atom%) under unfiltered irradiation (a) light, following air sparging, (b) light, following nitrogen sparging, (c) dark, following air sparging, (d) dark, following nitrogen sparging (SR = 10 mV s−1).The
photocurrent response spectra of these films were obtained using chopped
monochromatic illumination (10 nm band pass). The spectral current-time
response for the undoped TiO2 electrode is given in Figure 6. The
incident photon conversion efficiency (IPCE) was calculated byFigure 6
Current-time response under chopped irradiation for undoped sample as function of irradiation wavelength from 250 to 430 nm (left to right) in 10 nm intervals. Measured in a one-compartment cell with applied potential of +1.0 V and 0.1 M sodium perchlorate as a backing electrolyte with air sparging.(1)%IPCE=JI0F×100, where J is the photocurrent density, I0 is the incident light
intensity, and F is Faraday’s
constant.The %IPCE
versus wavelength
is given in Figure7. The monochromatic light intensity was a limiting factor
in these experiments yielding very low photocurrents. A small photocurrent
response at wavelengths above 400 nm was observed for a number of samples (see Figure
6), however, a change in current less than three times the deviation of the
background current (i.e., 30 nA) was not taken as significant. The
doped samples exhibited no red shift in the %IPCE compared to the undoped TiO2.Figure 7
Incident photon to current efficiency for samples doped to 0.5 atom%. Dopant: (a) none, (b) Fe, (c) Cu, (d) V, (e) Co, (f) Cr, (g) Ni, (h) Mn (measured in a one-compartment cell at an applied potential of +1.0 V and 0.1 M sodium perchlorate electrolyte with air sparging).To overcome the limitations of the low light intensity of the monochromatic source, the photocurrent response was measured under
visible excitation using a NaNO2 filter (λ ≥410 nm)
with the Xe source. All samples showed a photocurrent response under sub-band gap
irradiation which was dependent on applied potential with the photocurrent
maxima observed between −0.5 and −0.1 V (see Figure 8). The peak potentials and
onset potentials for anodic current are given in Table 3. AES analysis
confirmed the presence of carbon impurities at high levels and this is common
with sol-gel routes employing alkoxide precursors. It has been previously
reported that carbon doped TiO2 gives a visible response [30]. All samples contained carbon impurities and there was a small visible
response observed for the undoped TiO2 sample. This visible photocurrent for the undoped sample, which may be due to carbon impurities, is much less than that observed
for the metal ion dopants (see Figure 8 and Table 3). Recently, Asahi et
al. [31] reported the carbon defect level to be too low to give an optical
transition in the visible. Changes in the optical or photoactive spectra of TiO2 due carbon doping have been correlated to an increase in the
concentration of Ti3+ stabilised by carbon impurities [32, 33].Table 3
Photoelectrochemical
data for undoped and 0.5 atom% doped TiO2 films under visible
irradiation.
DopantOnset
potential for anodic current (V)Peak
potential (V)Photocurrent
at peak potential (mA)None−0.71−0.190.038Vanadium−0.51−0.240.049Chromium−0.49−0.260.051Manganese−0.53−0.300.070Iron−0.51−0.220.044Cobalt−0.60−0.310.076Nickel−0.53−0.350.066Copper−0.62−0.300.062Figure 8
Visible (λ>410 nm) photocurrent response (a) TiO2:Co 0.5 atom%, (b) undoped TiO2, and (c) dark current for undoped TiO2.The presence of metal ion dopants will
result in oxygen vacancy states or single-bonded oxygen (possibly hydroxyl) due
to the lower coordination of the dopant with respect to the parent Ti4+.
Nakamura et al. [27] reported that H2 plasma treatment of TiO2 resulted in the formation of oxygen vacancy states located around 2.02–2.45 eV above the
valence band corresponding to a wavelength between 506 and 614 nm. Electrons
may be promoted from the valence band to the vacancy states by visible light
excitation. They reported that plasma modification of TiO2 allowed visible-light-driven
oxidation of NO.Siripala and Tomkievicz [26]
investigated interactions between photoinduced and dark charge transfer at
single crystal TiO2-aqueous electrolyte interfaces. They reported a sub-band gap irradiation
resulted in electron injection from the valence band to states in the gap which
lie around 0.6 eV below the bottom of the conduction band. Carrier excitation
through such surface states should be much less potential dependent than
carrier excitation through the bulk. Indeed they report that sub-band gap photocurrent is observed only
between −0.5 and +0.4 V (SCE) at pH 6.5 (reported flat band potential was ca.
−0.8 V). They proposed that the surface
state was an intermediate of the dark reaction forming TiO2–H, the
energy for which lies within the band-gap.The
photocurrent under visible light excitation observed at negative potentials is
consistent with previously published results which attribute this visible
response to donor/acceptor energy levels below the conduction band caused by
surface defects [24–27]. Photocurrent under sub-band gap illumination is only
observed in the potential region where donor/acceptor energy levels are filled,
that is, more negative than −0.1 V in this case. The presence of defect states
is usually reported to have a negative effect on photoactivity because they act
as recombination centres for charge carriers. However, under appropriate
applied potential, these defect levels can act as electron donor states
yielding a photocurrent response under sub-band gap irradiation.To
investigate the potential dependence of these defect levels, the current-time
response of TiO2 electrodes under chopped illumination was examined.
Salvador [34] investigated the kinetics of recombination involving
defects, surface groups, and absorbed species, by photocurrent transient
measurements. Using single crystal TiO2 electrodes, they reported
that the initial photocurrent produced in the instant of illumination (Iph in)
decayed with time to a steady state (Iph st) due
to recombination on defects and surface states. By plotting the ratio of Iph st/Iph in versus
potential, the critical band bending was determined.Figures9 and 10 show examples of transients produced in this work for the undoped and
doped samples, respectively. For the undoped samples, the initial photocurrent
spike is absent at potentials more positive than +0.6 V whilst for the doped
samples, the initial spike was always present. Additionally, photocurrent
showed n-type to p-type conversion at negative potentials close the Efb. Matsumoto et
al. [21] reported increasing p-type character at negative potentials
following doping with transition metals which resulted in a lowering of the
anodic photocurrent in comparison to the undoped sample, with higher p/n ratios
correlating to higher defect densities.Figure 9
Changes in photocurrent transients as a function applied potential. Example of undoped sample under polychromatic illumination (applied bias from 1.0 V to −1.0 V in 0.2 V steps left to right).Figure 10
Changes in photocurrent transients as a function applied potential. Example of 0.2% chromium doped sample under polychromatic illumination (applied bias from +1.0 V to −1.0 V in 0.2 V steps left to right).The
films used in this study were made using a sol-gel route which gave oxide films
consisting of nanoparticles with a primary particle size around 38 nm [35]. For
nanoparticles in this size range, band bending is not significant. However,
initial and steady state photocurrent measurements can still be used to examine
recombination as a function of potential by subtracting Iph st from Iph in. Figure 11 shows the potential dependence of recombination in these electrodes. The
peaks in recombination (Iph in − Iph st) with
respect to potential are tabulated in Table 4.Table 4
Potential
dependence of recombination.
DopantPeak
inIph in − Iph st (V)Undoped0.0Vanadium−0.2Chromium−0.2Manganese−0.2Iron−0.2Cobalt−0.4Nickel−0.4Copper−0.4Figure 11
Iph in − Iph st as a function of potential from 0.2 atom% chromium doped sample.The
magnitude ofIph in − Iph st is
observed to follow the same potential dependence as the sub-band gap
photocurrent response. Under band-gap irradiation, the defect states act as
recombination centres lowering the photocurrent response at negative potentials;
however, under sub-band gap irradiation, the defect states act as electron
donors showing a visible photocurrent response. This is an agreement with other
reported results [36].There is a small visible photocurrent
response is observed at potentials more positive than −0.6 V due to defect
states. Nevertheless, it is clear that the primary band-gap photocurrent
response is decreased by the addition of metal ion dopants, which act as charge-carrier
recombination centres, and that the sub-band gap photocurrent is only a fraction
of the band-gap photocurrent and, therefore, may not be relevant for practical
applications including the photoelectrolytic
splitting of water under solar irradiation.


## 4. Conclusions

Thin films of transition metal doped
(0.2, 1.0, and 5.0 atom%) TiO2 were prepared on titanium foil using
a sol-gel route catalyzed by ammonium acetate. Raman spectroscopy confirmed
anatase crystal phase in all samples except in the case of the TiO2:V
sample where a mixture of anatase and rutile was obtained.The films doped with transition metal
ions showed lower photocurrent response than undoped samples under simulated
solar irradiation. The photocurrent response spectra of the doped films were
similar to the undoped sample with no major red shift. Sub-band gap irradiation of the samples gave a
small photocurrent at negative potentials which is indicative of electron
promotion from a filled defect level, the
occupation of which is potential dependant. Examination of defect level
potential dependence by analysis of photocurrent transients (from −0.8 to +1 V)
showed a good correlation with the potential dependence in visible illumination
studies.The primary band-gap photocurrent
response is decreased by the addition of metal ion dopants, which act as
charge-carrier recombination centres, and the sub-band gap photocurrent is only
a very small fraction of the band-gap photocurrent.



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*Source: 631597-2008-05-21.xml*