The effect of chemical states of dopants on the microstructures and band gaps of metal-doped ZrO2 thin films at different temperatures

Article abstract of DOI:10.1021/jp047440n

The microstructures and band gaps of sol-gel-derived ZrO₂ thin films incorporating seven-transition-metal ions (Cr³⁺, Mn ²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu ²⁺, and Zn²⁺) were investigated at 550 and 950?°C. Results obtained in this study indicate that the chemical states of the dopants play pivotal roles in controlling the physicochemical properties of the metal-doped ZrO₂ thin films at different temperatures. The reduction of Cr³⁺ and Co²⁺ to zerovalent metals expanded the d spacings and enlarged the crystallite sizes in the doped ZrO₂ thin films at 550?°C, while the incorporation of Mn²⁺, Fe³⁺, Ni²⁺, Cu²⁺, and Zn²⁺ contracted the d spacings of the ZrO₂ lattices in the solid solutions. The solubility of the dopants in the ZrO₂ lattices decreased at 950?°C, and this phenomenon induced an m-tetragonal-to-monoclinic phase transformation in the doped ZrO₂. The phase transformation was suppressed in the Cr-, Fe-, and Cu-doped ZrO₂ thin films as a result of the reduction of Cr ³⁺ and Fe³⁺ to species having larger sizes than Zr ⁴⁺. Band gaps of ZrO₂ were reduced in the doped ZrO ₂ thin films. The newly generated band gaps of these solid solutions at 550?°C were highly dependent on the d-electronic configurations of the dopants. On the other hand, the segregated metal oxides or zerovalent metals from the ZrO₂ lattice at 950?°C dominated the band gaps of the doped ZrO₂ thin films. Dehydroxylation and deoxygenation processes drove the reduction of metal ions during the thermal treatment. The metal ions having d⁵ and d¹⁰ configurations in the ZrO₂ lattice possessed high stability against thermally induced reductions, while the other ions tended to be reduced in the sol-gel-derived ZrO₂ matrix.

Full text of DOI:10.1021/jp047440n

18098
J. Phys. Chem. B 2004, 108, 18098-18103
The Effect of Chemical States of Dopants on the Microstructures and Band Gaps of
Metal-Doped ZrO2 Thin Films at Different Temperatures
Sue-min Chang and Ruey-an Doong*
Department of Atomic Science, National Tsing Hua UniVersity, 101, Sec. 2,
Kuang Fu Road, Hsinchu, 30013, Taiwan
ReceiVed: June 14, 2004; In Final Form: September 2, 2004
The microstructures and band gaps of sol-gel-derived ZrO2 thin films incorporating seven-transition-metal
ions (Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+) were investigated at 550 and 950 °C. Results obtained
in this study indicate that the chemical states of the dopants play pivotal roles in controlling the physicochemical
properties of the metal-doped ZrO2 thin films at different temperatures. The reduction of Cr3+ and Co2+ to
zerovalent metals expanded the d spacings and enlarged the crystallite sizes in the doped ZrO2 thin films at
550 °C, while the incorporation of Mn2+, Fe3+, Ni2+, Cu2+, and Zn2+ contracted the d spacings of the ZrO2
lattices in the solid solutions. The solubility of the dopants in the ZrO2 lattices decreased at 950 °C, and this
phenomenon induced an m-tetragonal-to-monoclinic phase transformation in the doped ZrO2. The phase
transformation was suppressed in the Cr-, Fe-, and Cu-doped ZrO2 thin films as a result of the reduction of
Cr3+ and Fe3+ to species having larger sizes than Zr4+. Band gaps of ZrO2 were reduced in the doped ZrO2
thin films. The newly generated band gaps of these solid solutions at 550 °C were highly dependent on the
d-electronic configurations of the dopants. On the other hand, the segregated metal oxides or zerovalent
metals from the ZrO2 lattice at 950 °C dominated the band gaps of the doped ZrO2 thin films. Dehydroxylation
and deoxygenation processes drove the reduction of metal ions during the thermal treatment. The metal ions
having d5 and d10 configurations in the ZrO2 lattice possessed high stability against thermally induced reductions,
while the other ions tended to be reduced in the sol-gel-derived ZrO2 matrix.
Introduction
versa.14 Ionic sizes of dopants also influence crystallite sizes in
doped ZrO2 samples. Small crystallite sizes of ZrO2 are usually
obtained when the ionic sizes of dopants are largely different
Zirconium dioxide (ZrO2) is one of the most promising
catalysts widely used in the petroleum industry for the deforma-
tion, dehydrogenation, and isomerization of organic com-
pounds.1,2 The wide band gap and suitable redox potentials in
bands also make ZrO2 an attractive photocatalyst for the
decomposition and mineralization of a wide variety of pollut-
ants.3,4 Several studies have reported that incorporation of
transition-metal ions in the first series (including Fe, V, and
Ni) can effectively enhance the efficiency of ZrO2-based
catalytic systems.5-7 The effects that dopants have on the
performance of ZrO2 are mainly associated with modifying the
microstructures including phases and crystallite sizes as well
as the reducing band gaps.5-8
ZrO2 exists in three polymorphs: monoclinic, tetragonal, and
cubic.9 Incorporation of metal ions into the ZrO2 lattice can
stabilize the tetragonal phase, which is only stable above 1200
°C, at room temperature because of introduced lattice de-
fects.10,11 The types of dopants determine the stability of
m-tetragonal phase in the metal-doped ZrO2 at elevated tem-
peratures. Previous studies have demonstrated that m-tetragonal
ZrO2 is maintained at up to 950 °C in Sc-doped ZrO2 film,12
while the high stability of m-tetragonal ZrO2 is observed at 1050
°C in lanthanide-doped ZrO2 thin films.13 Such effects may be
related to different distortion in the metal-doped ZrO2 micro-
structures when the ionic sizes of the dopants are different from
Zr4+. The unit cell undergoes a contraction when Zr4+ is
replaced by metal ions that have smaller ionic radii, and vice
from Zr4+ 13
. In addition to the influence on the microstructures,
the electronic structures of ZrO2 are also altered by dopants
because of introduced energy levels.15 The energy levels
between the intrinsic bands of ZrO2 can significantly reduce
the band gap.6,16 ZrO2 is a direct-transition material having the
band gap of about 5.0 eV.17 The wide band gap has been reduced
to 2.3 and 2.8 eV, respectively, when 5 wt % of Fe3+ or V5+ is
doped.6,18 Depending on the types and chemical states of metal
ions, the band gaps of metal-doped ZrO2 thin films varied. The
differences in the band gaps are mainly associated with the
electronic configurations of the ions, which control the energy
levels in the ZrO2.
To facilitate catalytic systems in various systems, metal-doped
ZrO2 is usually fabricated in thin films. A sol-gel process is
one of the methods most frequently used for the fabrication of
metal-doped ZrO2 films because it is simple and available for
various dopants in different amounts. Thermal treatment is a
necessary step to stabilize and crystallize sol-gel-derived ZrO2
films. Calcination temperatures have great influence on the
microstructures and band gaps of metal-doped ZrO2 films.19,20
Such effects include inducing m-tetragonal-to-monoclinic phase
transformation and increasing crystallite sizes. A red-shift of
0.25 eV in the band gap of a Y-doped ZrO2 film has been
reported when the calcination temperature increases from 400
to 1200 °C.21 In addition, studies have reported that the chemical
states of metal ions in the ZrO2 lattice are converted during
thermal treatment. The reduction of Zr4+ to Zr3+ or Zr2+ has
* Corresponding author. E-mail: radoong@mx.nthu.edu.tw; tel: +886-
3-5726785; fax: +886-3-5718649.
10.1021/jp047440n CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/16/2004

Microstructures and Band Gaps of Metal-Doped ZrO2
J. Phys. Chem. B, Vol. 108, No. 46, 2004 18099
been found to occur after calcination.22,23 The changes in the
chemical states of dopants such as V5+, Cu2+, and Ru3+ in the
ZrO2 lattice have been also identified after thermal treat-
ment.18,24,25 The final chemical states of dopants are temperature-
dependent. Indovina et al.24 reported that Cu2+ in the ZrO2 lattice
was reduced to Cu+ at 473 K and then to Cu0 at a high
temperature. The mechanism of the change in the chemical states
of the dopants is related to the reagents used and to the
atmosphere during thermal treatment. The redox properties of
the metal ions in the metal oxide lattice are governed by their
types and chemical states.26 The thermally induced conversions
of chemical states can result in different ionic sizes and
electronic configurations of dopants. However, such effects on
the microstructures and band gaps of metal-doped ZrO2 films
receive less concern so far.
In this study, the microstructures and band gaps of the sol-
gel-derived ZrO2 thin films doped with seven-transition-metal
ions (Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+) are
systematically investigated at different temperatures. We em-
phasize mainly the effect of the chemical states of the dopants
on these physicochemical properties of doped ZrO2 thin films.
In addition, the mechanism of the conversions of the chemical
states and the redox properties of the seven transition metals in
the ZrO2 lattice are discussed.
Figure 1. XRD patterns of pure and doped ZrO2 thin films that had
been calcined at 550 °C in air for 12 h.
of 150.0 mA. XRD patterns were obtained using an X-ray
incident angle of 1.0° and the diffraction signal at 2θ was
detected from 25 to 70° using a sampling width of 0.020° and
a scanning speed of 4.0°/min.
UV-Visible Spectroscopy. The absorption measurements
were performed using a UV-vis spectrometer (Shimadzu, S
1600). Thin film samples were fixed on the light source path.
The absorption spectra were recorded in the range from 800 to
200 nm using quartz as the reference. The scanning wavelengths
(λ) were then converted into photon energies (E, eV) using the
relationship of E ) 1240/λ. The differences in absorbance (∆A)
between the doped and pure ZrO2 films were calculated (∆A )
Adoped - Apure) to characterize the influence that the dopants
had on the optical properties of the ZrO2 film.
XPS Measurements. The X-ray photoelectron spectroscopy
(XPS) measurements were performed using a Physical Electron-
ics ESCA PHI 1600 instrument equipped with an Al KR X-ray
source (1486.6 eV). The spherical capacitor analyzer equipped
with a multichannel detector had a takeoff angle of 70° relative
to the horizon of the sample plane. The photoelectrons were
collected into the analyzer at a passing energy of 23.5 eV and
the collection step was 0.1 eV. All of the analytical processes
were performed under ultrahigh vacuum conditions (maintained
<1.4 × 10-8 Torr). The XPS spectra were acquired after using
soft argon etching to remove a 2-nm thickness from the top of
the thin films. The binding energy was referenced to the Zr(3d5/2)
at 182.5 eV. The oxidation states were identified according to
the binding energy, the doublet separation energy (∆E ) BEp
2p3/2 - BEp 2p1/2), and the auger parameter (R ) BEp - BEa).
Experimental Section
The Preparation of Metal-Doped ZrO2 Thin Films. Pure
and metal-doped zirconium dioxide (ZrO2) thin films were
prepared by a sol-gel method. The sol solution was first
obtained by dissolving ZrCl4 (4 g; Fluka) by stirring it
vigorously in 2-propanol (IPA; 10 mL; Riedel-de Hae¨n, 99.8%,
water content < 0.1%) in an ice-water bath, and then diluting
the solution with IPA to get a molar ratio of IPA/Zr of 90. The
solution was then hydrolyzed fully using a stoichiometric
amount of water. Six metallic saltssCr(NO3)3‚9H2O (Riedel-
de Hae¨n), Mn(NO3)2‚4H2O (Merck), FeCl3‚6H2O (Ishizu),
CoCl2‚6H2O (Merck), CuCl2‚2H2O (Merck), and Zn(NO3)2‚
4H2O (Acro¨s)swith appropriate amounts were dissolved in the
sol solution to obtain doping levels from 2 to 20 atomic
percentage (at. %) relative to zirconium. Because Ni is readily
precipitated in IPA solution, NiCl2 was first dissolved in a small
amount of ethanol (Fluka, 99.8%, water content < 0.05%) and
then mixed with the sol solution. All the metal-doped ZrO2 sol
solutions were aged for 24 h at room temperature in sealed
centrifuge tubes. Thin ZrO2 films were obtained by spin-coating
a portion of the sol solution (20 µL) onto a 1.4-cm2 quartz slide
at 3000 rpm for 30 s. Prior to the addition of the sol solution,
the quartz slides were cleaned using K2C2O7/H2SO4 solution to
remove traces of organic contaminants on the surface. After
coating, the thin films were heated at 80 °C for 5 min to
evaporate the solvent, and then they were calcined at 550 or
950 °C in air for 12 h.
SEM Characterization. Scanning electron microscopy (SEM;
Hitachi S-4700 Type II) was used to determine the morphology
and thickness of each thin film. The microscope was operated
at an accelerating voltage of 25 kV and the images were
collected under a vacuum of 10-8 Torr. All samples for SEM
analysis were coated with platinum. The ZrO2 thin films
obtained in this study were smooth and had thicknesses of ca.
20 nm (see Supporting Information, Figure S1). The metal-
doped ZrO2 thin films had similar characteristics.
XRD Identification. The crystalline properties of the thin
films were identified by X-ray diffraction (XRD; MAC Science,
MXP18) using Cu KR radiation (λ ) 1.5405 Å) and operating
at an accelerating voltage of 40.0 kV and an emission current
Results
Crystalline Properties of Metal-Doped ZrO2 Films at
Different Temperatures. Figure 1 presents the XRD patterns
of the pure and 20 at. % metal-doped ZrO2 thin films calcined
at 550 °C. The peaks of the (101)t, (111)m, and (1h11)m profiles
centered at 30.2°, 28.2°, and 31.4° 2θ positions clearly indicate
the coexistence of metastable tetragonal and monoclinic phases
in the pure ZrO2 film. Only the metastable tetragonal phase was
identified, however, in the metal-doped ZrO2 samples. No
diffraction peaks corresponding to the metal oxides of the
dopants were observed in the XRD spectra, suggesting that the
dopants were dispersed well in the ZrO2 matrix. To further
identify whether a solid solution was formed, the changes in
the d spacing of the (101)t profile in the ZrO2 thin film doped
with various amounts of Fe and Mn were examined (see

18100 J. Phys. Chem. B, Vol. 108, No. 46, 2004
Chang and Doong
Figure 2. XRD patterns of pure and doped ZrO2 thin films that had
been calcined at 950 °C in air for 12 h.
TABLE 1: The d spacings of the (101)t Profile and the
Crystallite Sizes of the Pure and Doped ZrO2 Thin Films
sample
d-spacing (pm)
crystallite size (nm)
Figure 3. UV-vis absorption spectra of the ZrO2 thin film and the
difference absorption spectrum between the pure and metal-doped ZrO2
thin films calcined at 550 °C.
ZrO2
295
304
289
288
290, 303
293
290
15.6
27.1
14.2
10.5
12.5, 17.5
14.0
10.4
Cr-ZrO2
Mn-ZrO2
Fe-ZrO2
Co-ZrO2
Ni-ZrO2
Cu-ZrO2
Zn-ZrO2
transformation was suppressed in the presence of dopants. The
XRD patterns indicate that m-tetragonal phases remained in the
doped ZrO2 thin films after calcination at 950 °C and only a
small fraction of crystals existed as the monoclinic phase in
the Mn-, Co-, Ni-, and Zn-doped ZrO2 thin films. The d spacings
of the (111)m profiles in the doped ZrO2 calcined at 950 °C
ranged between 315 and 317 pm; these values are close to that
of pure ZrO2 (316 pm). Similarly, the d spacings of the (101)t
profiles in the doped ZrO2 thin films (294-295 pm) were also
close to that of the pure ZrO2 film (295 pm). These results reveal
that the dopants segregate from the ZrO2 lattices at the elevated
temperature.
Spectroscopic Properties of Metal-Doped ZrO2 Films at
Different Temperatures. The band gaps (Eg) of the pure and
doped ZrO2 thin films were determined from their UV-vis
absorption spectra. Figure 3 displays the absorption curves of
the pure ZrO2 film and the difference in absorbance (∆A )
Adoped - Aundoped) between the pure and 20 at. % doped ZrO2
films calcined at 550 °C. The pure ZrO2 thin film showed a
steep increase in the absorption at the energy region >5.5 eV.
The direct band gap of 5.7 eV for the ZrO2 film was determined
from the onset of the absorption (see Supporting Information,
Figure S3). With the exception of the Zn-doped ZrO2 thin film,
the increase in ∆A at the energy region <5.7 eV was observed
in all of the doped ZrO2 thin films, indicating that red-shifts
exist in the doped ZrO2 thin films. On the basis of these
absorption edges of the red-shifts, the reduced band gaps in the
doped ZrO2 thin films were determined. The absorption edges
of the Cr-, Mn-, Fe-, Co-, Ni-, and Cu-doped ZrO2 samples
calcined at 550 °C were located at the excitation energies of
3.4, 3.1, 3.0, 3.7, 4.1, and 3.3 eV, respectively. No obvious
difference in absorbance between the Zn-doped and pure ZrO2
films was observed, depicting that the impurity level of Zn2+
is close to the conduction or valence band of ZrO2.
290
13.4
Supporting Information, Figure S2). Linear relationships existed
between the d spacings and the concentrations of the dopants,
which follow Vegard’s law,14 in the Mn- and Fe-doped ZrO2
films, indicating that the substitutional solid solutions were
formed at 550 °C.
Table 1 lists the d spacings of the (101)t profile and the
crystallite sizes in the pure and doped ZrO2 films at 550 °C.
The d spacing of the pure ZrO2 was 295 pm and it varied when
doped with different metal ions. The d spacings of the Mn-,
Fe-, Ni-, Cu-, and Zn-doped ZrO2 thin films were in the range
of 288-291 pm, which indicate that shrinkage occurs after
doping. In contrast, an expansion of d spacing to 304 pm
occurred when Cr was doped. Two (101)t peaks with the d
spacings of 303 and 290 pm, respectively, appeared in the Co-
doped ZrO2 thin film, depicting that both shrinkage and
expansion in the d spacing concurrently occurred. The different
changes in the microstructure infer that two kinds of Co species
might exist in the doped ZrO2. Similar to the changes in the d
spacings, the crystallite sizes in the ZrO2 thin films also
increased or decreased depending on the nature of the dopants.
The crystallite sizes in the pure and metal-doped ZrO2 thin films
were estimated from the full width at half-maximum (fwhm)
of the (101)t peak using Scherrer’s formula.27 The crystallite
size of the (101)t profile in pure ZrO2 thin film was calculated
to be 15.6 nm. Incorporation of Mn2+, Fe3+, Ni2+, Cu2+, and
Zn2+ reduced the crystalline sizes to 10.4-14.2 nm, whereas
larger crystallite sizes, ranging between 27.1 and 17.5 nm, were
obtained in the Cr- and Co-doped ZrO2 thin films. In addition,
preferred orientations of the (101)t profiles with d spacings of
304 and 303 pm, respectively, were also observed in the Cr-
and Co-doped ZrO2 films. Such preferred orientation did not
appear, however, in the other metal-doped ZrO2 thin films.
Figure 2 presents the XRD patterns of the pure and doped
ZrO2 films calcined at 950 °C. The pure monoclinic phase,
which was transformed from the m-tetragonal phase, was
observed in the pure ZrO2 thin film at 950 °C, but this phase
Figure 4 presents the absorption curve of the pure ZrO2 film
and the difference in absorbance (∆A ) Adoped - Aundoped
)
between the pure and metal-doped ZrO2 films calcined at 950
°C. The ZrO2 film calcined at 950 °C showed two direct band-
to-band transitions with the band gaps of 5.1 and 5.9 eV (see
Supporting Information, Figure S3). The Cr-, Mn-, Fe-, and Ni-
doped ZrO2 films showed red-shifts in the absorption spectra;

Microstructures and Band Gaps of Metal-Doped ZrO2
J. Phys. Chem. B, Vol. 108, No. 46, 2004 18101
the suitable redox conditions (see Supporting Information, Table
1S). After calcination at 550 °C, Cu+ was still the major
oxidation state, while Fe2+ was oxidized and reduced to Fe3+
and Fe0, respectively, in an atomic ratio of 75:25. The fraction
of Co0 increased from 36 to 49% after calcination implies that
some Co2+ was reduced upon thermal treatment. Moreover, the
existence of Cr0 in the Cr-doped ZrO2 film calcined at 550 °C
also suggests that the reduction of Cr3+ to Cr0 resulted during
the calcination process. In contrast, the Mn2+ and Zn2+ ions
maintained their original oxidation states after calcination at 550
°C.
To further understand whether the reducing capacity toward
the doped metal ions increased at elevated temperatures, the
chemical states of the dopants in the doped ZrO2 films calcined
at 950 °C were identified. The results indicate that the Fe3+
ions were reduced to Fe2+/Fe0 in the doped ZrO2 film at 950
°C. This phenomenon is attributed to the presumed electron
transfer that occurs upon the elimination of oxygen in the ZrO2
lattice.28 Concurrently, the contents of Cr0, Co0, and Ni0
decreased as the temperature increased from 550 to 950 °C.
This situation indicates that oxidation occurred upon calcination
at 950 °C. Mn2+, Zn2+, and Cu+ were not detected in the ZrO2
thin films after the calcination at 950 °C. However, the atomic
ratios of Mn/Zr and Zn/Zr detected in the surface layer were
0.5 and 0.7, respectively, which are higher than their stoichio-
metric ratio of 0.2 (20 at. %). This observation reveals that the
Mn2+ and Zn2+ ions were segregated from the ZrO2 lattice and
accumulated at the surface of the thin films at 950 °C. The
accumulated Zn2+ ions retained their original chemical states,
whereas some of the Mn2+ ions (33%) were oxidized to Mn4+
in the surface layer upon exposure to oxygen in air. Such surface
accumulation of dopants did not occur in the Cu-doped ZrO2
thin film because no photoelectrons of Cu were detected at the
surface. Presumably, this phenomenon is due to the reduction
to Cu0, which has a strong ability to diffuse into the substrate.29
Figure 4. UV-vis absorption spectra of the ZrO2 thin film and the
difference absorption spectrum between the pure and metal-doped ZrO2
thin films calcined at 950 °C.
however, no obvious shifts in photon energies were observed
in the Co-, Cu-, and Zn-doped ZrO2 thin films. The red-shifts
in the Cr-, Fe-, and Ni-doped ZrO2 thin films began at 2.8, 2.0,
and 3.5 eV, respectively, and two absorption bands having the
onsets at 3.1 and 1.8 eV were observed in the spectra of the
Mn-doped ZrO2 sample. The metal-doped ZrO2 thin films
calcined at 950 °C display smaller band gaps than those calcined
at 550 °C. This discrepancy may be attributed to conversions
in the chemical states of the dopants upon thermal treatment.
In addition, two absorptions in the Mn-doped ZrO2 film also
infer that possibly two kinds of Mn species are formed at 950
°C. Therefore, the chemical states of the seven dopants in the
ZrO2 thin films at different temperatures were further examined
by using XPS.
Discussion
Chemical States of the Dopants at Different Tempera-
tures. The chemical states of the dopants in the thin films were
identified after removing a 2-nm thickness from the top of each
thin film. Table 2 summarizes the chemical states of the dopants
in the doped ZrO2 films at 80-950 °C. The percentages of the
species were calculated from the ratios of the peak areas of each
In this study, we have found that the conversions in the
chemical states of the dopants at different temperatures play
important roles in the crystalline properties and band gaps of
the metal-doped ZrO2 thin films. Depending on the ionic sizes
of dopants after calcination, the dopants have two effects on
the d spacings of the ZrO2 films. The introduction of dopants
that have ionic sizes smaller than that of Zr4+ (98 pm), which
includes Mn2+, Fe3+, Ni2+, Cu+, and Zn2+, contracted the d
spacings of the doped ZrO2 lattices. In contrast, expansion of
peak in the XPS spectra. Results indicate that Fe3+, Co2+, Ni2+
,
and Cu2+ had already reduced to Fe2+, Co0, Ni0, and Cu+,
respectively, prior to calcination. These reductions occurred in
conjunction with the oxidation of 2-propanol to acetone under
TABLE 2: The Binding Energy (BE), Doublet Separation (∆E), and Chemical States of Dopants in the ZrO2 Thin Films at
Different Temperatures
as-dried
∆Eb (eV)
calcined at 550 °C
calcined at 950 °C
dopant
Cr
BEa (eV)
576.6
chemical states
Cr3+
BEa (eV)
∆Eb (eV)
chemical states
BEa (eV)
∆Eb (eV)
chemical states
9.8
11.4
13.0
574.4
575.8
641.5
9.2
9.8
11.4
Cr0 (75)c
Cr3+ (25)
Mn2+
574.4
576.8
641.3
642.5
707.0
709.5
777.5
780.4
851.8
854.0
9.2
9.8
Cr0 (54)
Cr3+ (46)
Mn2+ d (67)
Mn4+ d (33)
Fe0 (35)
Mn
Fe
641.5
709.5
Mn2+
Fe2+
11.4
11.6
12.6
13.0
15.0
15.0
17.1
18.2
707.5
710.9
777.1
780.4
851.7
852.8
932.2
1021. 8
13.0
13.2
15.1
15.4
17.1
18.2
19.9
23.3
Fe0 (25)
Fe3+ (75)
Co0 (49)
Co2+ (51)
Ni0 (53)
Ni2+ (47)
Cu+
Fe2+ (65)
Co0 (13)
Co
Ni
777.2
780.3
852.0
855.2
932.0
1022. 2
15.1
15.4
17.1
17.8
19.7
23.1
Co0 (36)
Co2+ (64)
Ni0 (58)
Ni2+ (42)
Cu+
Co2+ (87)
Ni0 (39)
Ni2+ (61)
Cu
Zn
Zn2+
Zn2+
1021.8
23.0
Zn2+ d
a The binding energy of 2p3/2 line. b ∆E ) BE 2p3/2 - BE 2p1/2 c Values in parentheses are the percentages of the species. d The species accumulated
.
in the surface layer.

18102 J. Phys. Chem. B, Vol. 108, No. 46, 2004
Chang and Doong
Figure 6. Schematic impurity levels of the transition-metal ions in
the ZrO2 thin films.
Figure 5. The d spacings of the (101)t profile in the metal-doped ZrO2
thin films plotted as a function of the radius of the dopants.
volume during the transformation from m-tetragonal phase to
monoclinic phase, and thus the stability of the m-tetragonal
phase at high temperatures is enhanced. A similar phenomenon
has been reported for lanthanide-doped ZrO2 thin films that
stabilize the m-tetragonal phase at temperatures up to 1050 °C.13
the d spacings in the Cr- and Co-doped ZrO2 lattices was due
to the reduction of Cr3+ and Co2+ to the large-sized metallic
elements (Cr0: 118 pm; Co0: 116 pm). The appearance of both
contracted and expanded d spacings in the Co-doped ZrO2
suggests that the Co2+ and Co0 species are included in separated
crystal domains. Although about half of the Ni2+ ions were
reduced to Ni0 at 550 °C, only one contracted d spacing was
obtained for the Ni-doped ZrO2 thin film. This finding indicates
that both the Ni2+ and Ni0 species are included in the same
crystallite domain and that the averaged dopant size determines
the d spacing. A positive correlation exists between the dopant
sizes and the d spacings of ZrO2 when the solid solutions were
formed at 550 °C (Figure 5). In addition, the preferred
orientation of the (101)t profile and the large crystallite sizes in
the Cr- and Co-doped ZrO2 thin films may possibly be due to
the expansion of d spacings caused by the large metallic sizes.
In addition to the changes in the microstructures, the
electronic structures of the metal-doped ZrO2 thin films also
vary at different temperatures. The band gap of the ZrO2 thin
film calcined at 550 °C was 5.7 eV, which is larger than the
reported value (5.0 eV).17 This increase in the band gap is due
primarily to a quantum confinement effect when the crystallite
size is down to the nanoscale.21 Incorporation of the metal ions
into the ZrO2 thin films resulted in red-shifts in the absorption
spectra. This observation suggests that impurity levels are
introduced between the intrinsic bands of the ZrO2 films, which
subsequently generate new band gaps. Photon-induced electron
transitions in the doped ZrO2 can be either transitions between
the localized impurity levels and the delocalized conduction
band or d-d transitions.31 In this study, only continuous
absorption bands were observed in the difference absorbance
(∆A) spectra, revealing that the transitions between the impurity
levels and the conduction band are dominant and the band gaps
of the doped ZrO2 films are dependent on the positions of the
impurity levels in the intrinsic band gap. Substantial amounts
of metallic Cr, Co, and Ni existed in the ZrO2 lattices at 550
°C. However, the red-shifts of the Cr-, Co-, and Ni-doped ZrO2
thin films were inconsistent with the atomic absorption of Cr
(357.9 nm), Co (240.7 nm), or Ni (232.0 nm),32 respectively.
This finding suggests that the red-shifts in these thin films are
associated mainly with electron transitions from the impurity
levels of the remaining Cr3+ (25%), Co2+ (51%), and Ni2+
(47%) to the conduction band. Figure 6 illustrates the energy
levels of the impurity ions in the ZrO2 intrinsic bands at 550
°C. The band gaps of the doped ZrO2 films are dependent on
the d-electronic configurations of the impurity ions. The red-
shifts indicate that Mn2+ and Fe3+, which have d5 configurations,
have the highest energy levels.
Unlike the d spacings present in the samples calcined at 550
°C, no obvious difference in d spacings between the pure and
doped ZrO2 thin films was observed after calcination at 950
°C, which implies that the dopants are segregated from the ZrO2
lattice. This segregation is attributed to decreases in the solubility
of the dopants at elevated temperatures. However, the peaks
corresponding to the metal oxides of the dopants were not
observed in the XRD patterns, reflecting the fact that the
crystallite sizes of the segregated metal oxides in the ZrO2 matrix
was too small to be detected by XRD. Ram30 investigated the
structural properties of Cr-doped ZrO2 nanoparticles and found
that the segregation of dopants could trigger the phase trans-
formation of ZrO2 from m-tetragonal to monoclinic form. In
this study, however, only small fractions of m-tetragonal phase
were converted into monoclinic phase in the Mn-, Co-, Ni-,
and Zn-doped ZrO2 films. The fact that the m-tetragonal phases
are maintained in the doped ZrO2 thin films reveals that trace
amounts of the dopants are still included in the ZrO2 lattice
even though most of the dopants have segregated. The different
stabilities of the m-tetragonal phases in the doped ZrO2 films
are controlled by the conversions of the chemical states of the
dopants in the ZrO2 lattices. When large fractions of dopants
Compared to the situation at 550 °C, the band gaps of the
metal-doped ZrO2 thin films decreased at 950 °C. The changes
in the band gaps are related to the segregation and reduction of
the dopants. The Fe3+ ions, which were stable in the ZrO2 lattice
at 550 °C, were reduced to Fe2+ and Fe0 when the temperature
increased to 950 °C. Moreover, the solubility of the dopants
decreased and metal oxides or metals segregated from the ZrO2
lattice at 950 °C. The accumulated Mn at the film surface
comprised Mn2+ and Mn4+ species. The band gaps of 1.8 and
3.1 eV in the Mn-doped ZrO2 correspond to the band gaps of
MnO2 and MnO, respectively.33,34 In addition, the Cr- and Fe-
doped ZrO2 thin films had band gaps of 2.8 and 2.0 eV,
are reduced to species that have radii larger than that of Zr4+
,
such as Cr0 (118 pm), Fe0 (117 pm), and Fe2+ (106 pm), the
stability of the m-tetragonal phase can be maintained. In contrast,
the phase is easily transformed to monoclinic form in the metal-
doped ZrO2 thin film when the doped ions have smaller ionic
radius, such as those of Mn2+, Co2+, Ni+, and Zn2+. This
phenomenon is attributed to the fact that large-sized dopants
can assist the ZrO2 structure to bear the contraction in lattice

Microstructures and Band Gaps of Metal-Doped ZrO2
J. Phys. Chem. B, Vol. 108, No. 46, 2004 18103
respectively; these values are similar to those expected for Cr2O3
and FeO.35,36 On the other hand, the absorption edge at 355 nm
for the Ni-doped ZrO2 is close to the value of the atomic
absorption of Ni (352.4 nm). In contrast to the solid solution
forms, these results reveal that the band gaps of the dopants in
their oxides or metallic forms dominate the new band gaps of
the doped ZrO2 thin films at 950 °C.
Supporting Information Available: Descriptions of the
morphology and thickness of the sol-gel-derived ZrO2 thin film,
the relationship between d spacings and concentrations of the
dopants, the direct band gaps of ZrO2 thin films at different
temperatures, and the reducing behavior of the metal ions in
the as-dried ZrO2 thin film. This material is available free of
charge via the Internet at http://pubs.acs.org.
When the microstructures and the band gaps of the doped
ZrO2 thin films with respect to the conversions of the chemical
states of the dopants have been demonstrated, the mechanisms
of the conversions of the chemical states during thermal treat-
ment require further clarification. From the XPS results obtained
at different temperatures, the reducing behavior of dopants in
the ZrO2 thin film was observed. The reduction of the doped
ions is associated initially with the oxidation of 2-propanol to
acetone in the as-dried samples. Afterward, dehydroxylation and
deoxygenation processes drive the reduction of the metal ions
in the sol-gel-derived ZrO2 thin films during thermal treatment.
The further reduction of Fe3+ to Fe2+ and Fe0 from 550 to 950
°C indicates the fact that the reducing capacity to cations was
enhanced with increasing temperatures. In contrast to these
reductions, oxidations of the dopants also occurred at 950 °C,
in which the m-tetragonal phase was converted to the monoclinic
phase. This phenomenon is presumably due to this phase
transformation being triggered by an intake of oxygen from air.37
The systematic identification of the chemical states of the
dopants depicts that the redox properties of the dopants in the
ZrO2 matrix are highly dependent on their d-electronic con-
figurations. At 550 °C, the dopants that have half-filled (d5)
and closed-shell (d10) d-electron configurations, including Mn2+
and Fe3+, Cu+, and Zn2+, have specially high stability against
thermally induced reductions. On the other hand, metal ions
that lack these stable electronic structures, such as Cr3+(d3) and
Co2+(d7), are reduced to metallic elements upon calcination.
These results clearly indicate that the conversions of the
chemical states change the ionic sizes and electronic configura-
tions of the dopants, which control the microstructures and band
gaps of the doped ZrO2 at different calcination temperatures.
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Acknowledgment. We thank the National Science Council,
Taiwan, R.O.C. for the financial support under grant No. NSC
92-2113-M007-068.