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PAPER

# Effect of dopant loading and calcination conditions on structural and optical properties of ZrO₂ nanopowders doped with copper and yttrium

L Khomenkova¹,²,* ①, O Marchylo¹, Yu Polishchuk¹, S Ponomaryov¹, O Isaieva¹①, I Vorona¹, L Melnichuk³, X Portier⁴, O Melnichuk³① and N Korsunska¹①

¹ V. Lashkaryov Institute of Semiconductor Physics of the NAS of Ukraine, 41 Prospekt Nauky, Kyiv 03028, Ukraine

² National University 'Kyiv-Mohyla Academy', 2 Skovorody str., Kyiv 04070, Ukraine

³ Nizhyn Mykola Gogol State University, 2 Grafska str., Nizhyn 16600, Ukraine

⁴ CIMAP—CEA, UMR CNRS 6252, UNICAEN, ENSICAEN, 6 Boulevard Marechal Juin, Caen 14050, France

* Author to whom any correspondence should be addressed.

E-mail: khomen@ukr.net and mov310310@gmail.com

Keywords: ZrO₂, doping, crystalline structure, UV-vis diffused reflectance, Raman scattering, infrared reflection spectroscopy

## Abstract

Undoped, Cu and/or Y doped ZrO₂ nanopowders were synthesized with Zr, Y, and Cu nitrates using a co-precipitation approach. Their structural and optical properties were examined regarding dopant content (0.1–8.0 mol.% of CuO and 3–15 mol.% of Y₂O₃) and calcination conditions (400 °C–1000 °C and, 1, 2 or 5 h) through Raman scattering, XRD, TEM, EDS, AES, EPR, UV–vis and FTIR diffused reflectance methods. The results showed that both Cu and Y dopants promoted the appearance of additional oxygen vacancies in ZrO₂ host, while the formation of tetragonal and cubic ZrO₂ phases was primarily influenced by the Y content, regardless of Cu loading. The bandgap of most of the powders was observed within the 5.45–5.65 eV spectral range, while for those with high Y content it exceeded 5.8 eV. The (Cu,Y)-ZrO₂ powders with 0.2 mol.% CuO and 3 mol.% Y₂O₃ calcined at 600 °C for 2 h demonstrated nanoscaled tetragonal grains (8–12 nm) and a significant surface area covered with dispersed CuₓO species. For higher calcination temperatures, the formation of Cu₂Zr⁺ EPR centers, accompanied by tetragonal-to-monoclinic phase transformation, was found. For fitting of experimental FTIR reflection spectra, theoretical models with one, five, and seven oscillators were constructed for cubic, tetragonal, and monoclinic ZrO₂ phases, respectively. Comparing experimental and theoretical spectra, the parameters of various phonons were determined. It was found that the distinct position of the high-frequency FTIR reflection minimum is a unique feature for each crystalline phase. It was centered at 700–720 cm⁻¹, 790–800 cm⁻¹, and 820–840 cm⁻¹ for cubic, tetragonal, and monoclinic phases, respectively, showing minimal dependence on phonon damping coefficients. Based on the complementary nature of results obtained from structural and optical methods, an approach for monitoring powder properties and predicting catalytic activity can be proposed for ZrO₂-based nanopowders.

## 1. Introduction

Zirconia (ZrO₂) is a versatile material available in various forms, such as crystals [1, 2], ceramics [3, 4], films [5], and (nano)powders [6, 7]. ZrO₂ possesses unique mechanical, electrical, thermal, and optical properties, making it suitable for a wide range of applications, including high-temperature and corrosion-resistant coatings, radiation detectors, biological labeling, catalysts, oxygen sensors, and solid-oxide fuel cells [[7] and references therein]. However, for many critical applications, pure ZrO₂ may be insufficient, and its doping or alloying with other metal oxides can be required [1–10]. The addition of complementary cations to the crystal lattice of zirconia can significantly alter its properties [1, 5–7].
One of the crucial parameters affecting ZrO₂ applications is its crystal structure. Pure zirconia in bulk form typically adopts a monoclinic (m) crystal structure at room temperature. It transforms into tetragonal (t) phase upon heating (around 1460 °C) and into cubic (c) phase at higher temperatures. However, it is unstable upon cooling since reverse c-t-m transformation occurs.

For pure ZrO₂ in powder form, especially nanopowders, heating at moderated temperature (400 °C–500 °C) favors formation of c- and t-phases [6] while further temperature rise (up to 1100 °C) converts them in m-phase due to grain growth [3, 6, 7]. This phase is stable upon powders cooling. Since application of powders requires stability of the t- and c-phases not only at room temperature but also at higher temperatures, doping ZrO₂ with subvalent cations is employed [1–14].

Subvalent additives such as yttrium [1, 2, 6, 11, 12], calcium [8], scandium [9], cerium [6, 10], terbium [6], etc., are typically used to stabilize the t- and c-ZrO₂ [7, 8, 11]. Yttrium is the most common stabilizing agent because it can remain in ZrO₂ host upon annealing at high temperatures (around 1400 °C). The stabilization of both phases is achieved through the formation of oxygen vacancies required for dopant charge compensation [6].

Besides zirconia stabilization via the formation of oxygen vacancies, subvalent impurities are able to enhance other properties [8–14]. In particular, doping with rare-earth ions results in the appearance of intense luminescence in specific spectral ranges [6, 13, 14]. Doping with copper ions favors the appearance of specific green luminescence [15].

In recent years, ZrO₂ powders with stable t- and c-phases have attracted considerable attention due to their catalytic properties and larger amount of oxygen vacancies compared to those of m-ZrO₂ [16, 17].

To achieve high catalytic activity of the powders, several parameters must be considered, including the surface/volume localization of the dopants, their chemical forms and powder surface area [18, 19]. In this regard, in some studies, the attention also focus on amorphous zirconia because of its superior catalytic performance resulting from a higher surface area [17].

Zirconium oxide powders can be produced using a variety of precursor materials and methods. Compounds containing zirconium, such as Zr hydroxide (Zr(OH)₄), Zr oxychloride (ZrOCl₂), Zr oxynitrate (ZrO(NO₃)₂), Zr sulfate (Zr(SO₄)₂) can undergo various chemical reactions [7] or thermal treatments [20] to yield ZrO₂ powders. Additionally, Zr alkoxides and organometallic compounds are also attractive for synthesizing not only ZrO₂ [21] but also ZrC powders [22] via sol–gel processes.

Doping of ZrO₂ with multiple additives (referred to as ‘co-doping’) becomes more attractive for enhancing catalytic activity. For instance, it has been reported for Y-stabilized ZrO₂ (YSZ) powders doped additionally with Pt [19], Ag [23] or Cu [24–26]. It’s worth noting that among metallic dopants, copper is more commonly addressed due to its abundance and low cost.

The simultaneous incorporation of several dopants into the zirconia host induces complex structure transformation. For instance, incorporating copper into Y-doped ZrO₂ powders promotes a reduction of grain size and stabilizes t- and c-ZrO₂ polymorphs [25]. Additionally, it lowers the temperature of the t-m phase transformation [26]. Similar effects have been observed for ZrO₂ co-doped with Y and Tb³⁺ [2], Nb and Er³⁺ [27], and V and Y [28].

As mentioned above, precise control over the content, spatial distribution and chemical forms of Cu dopants is crucial for Cu-doped ZrO₂-based catalysts. Copper dopants can be introduced in ZrO₂ powders through impregnation [18, 19] and chemical precipitation [29], enriching mainly the grain surface or near-surface region by copper. Alternatively, co-precipitation allows for simultaneous Cu presence throughout the volume of the grains and on their surface [7, 15, 19, 30, 31].

The effect of copper content on the properties of catalysts is influenced by the nature of Zr precursor. The comparison of the (Cu,Y)-ZrO₂ powders prepared from ZrOCl₂·nH₂O [30] and ZrO(NO₃)₂·nH₂O salts [15, 31] revealed fewer oxygen vacancies in those from ZrOCl₂·nH₂O. This was attributed to the simultaneous incorporation of chlorine from the Zr precursor via the formation of Cu-Cl complexes and their integration in Cu-Y-ZrO₂ network during sediments precipitation [15]. An increase of Cu loading leads to the pronounced formation of the Cu-Cl complexes, which act as barriers to the appearance of additional oxygen vacancies because no charge compensation is required for Cu dopant [30]. It is interesting that the presence of residual Cl and C even in pure ZrO₂ was reported in [32] and attributed to incomplete hydrolysis reaction of zirconium ethoxide. This drawback can be overcome by using of ZrO(NO₃)₂·nH₂O precursor. The powders prepared for this salt exhibited higher amount of oxygen vacancies, which increased with higher Cu loading [15, 31]. These powders showed the high catalytic activity in CO-PROX reaction attributed to the formation of tetragonal ZrO₂ grains covered by dispersed CuₓO oligomers. However, the focus of these reports was on the (Cu,Y)-ZrO₂ powders with specific dopant content, namely 3 mol.% of Y₂O₃ and 1 or 8 mol.% of CuO [30, 31]. Nevertheless, a more precise variation of Cu and Y content across a wide range was desirable.
It is evident that a comprehensive investigation into the effect of Cu loading (at fixed Y content) and Y loading (at fixed Cu content) on the various properties of the powders, calcined at different temperatures and durations, requires the preparation and characterization of significant number of samples.

When analyzing powders, various methods are employed, including Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Energy Dispersive x-ray (EDX), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES), as well as Fourier-transform Infrared (FTIR), Raman scattering, and Ultraviolet-Visible (UV-vis) spectroscopies. Each method serves distinct aspects of powder characterization, offering unique insights that often complement one another when combined. Indeed, to obtain information about the crystalline structure and chemical composition of doped ZrO₂ grains, TEM, STEM-EDX, XPS, and AES methods can be employed. However, such analyses are time-consuming and costly. Simultaneously, correlating their results with those obtained by other methods, including widely available and low-cost optical methods, can be advantageous for efficient optimization of powder properties. Leveraging established correlations between results from structural and optical methods can facilitate the rapid monitoring of powder properties, with a particular emphasis on utilizing optical methods for further analysis that can provide additional benefits in powder diagnostics.

The objective of this study is twofold. Firstly, to investigate the influence of varying Cu and Y content over wide ranges (0.1–8.0 mol.% CuO and 3–15 mol.% Y₂O₃, respectively), along with calcination temperature (from 400 up to 1000 °C) and duration (from 1 to 5 h), on the structural and optical properties of Cu-ZrO₂, Y-ZrO₂ and (Cu,Y)-codoped ZrO₂ powders prepared from Zr, Y and Cu nitrates by co-precipitation method. Secondly, to introduce infrared reflection spectroscopy as a non-destructive method for express material diagnostics, employing its sensitivity to the crystalline structure and surface complexes. The study differentiates the contributions of m-, t- and c-ZrO₂ phases using developed theoretical models, showcasing their effectiveness in fitting experimental spectra and evaluating the structure of powders as well as the nature of surface complexes.

## 2. Experimental details

### 2.1. Reagents

Zirconyl (IV) nitrate hydrate (ZrO(NO₃)₂·6H₂O, 99.5% (contains up to 4% Hf), Thermo Scientific Chemicals), Yttrium (III) nitrate hexahydrate (Y(NO₃)₃·6H₂O, 99.9%, Thermo Scientific Chemicals), and copper (II) nitrate trihydrate (Cu(NO₃)₃·3H₂O, 99.0%, for analysis, Thermo Scientific Chemicals), Ammonium hydroxide solution 25% (NH₄OH, for analysis EMSURE® ISO, Reag. Ph Eur, Merck) were used in the synthesis of undoped and doped ZrO₂-based powders.

### 2.2. Powder synthesis and calcination

Undoped ZrO₂ powders, as well as their counterparts including Cu-ZrO₂, Y-ZrO₂ and (Cu,Y)-codoped ZrO₂, were synthesized. The quantities of Zr, Cu and Y precursors were adjusted to vary the content of Y₂O₃ and/or CuO in the doped powders across wide ranges, specifically, 3–15 mol. % of Y₂O₃ and 0.1–8.0 mol.% of CuO. The precursors taken in required ratio were dissolved in 100 ml of double-distilled water and magnetically stirred for 2 h at 60 °C until a transparent homogeneous solution was obtained. To induce precipitation, this solution was mixed with a 25% ammonium hydroxide solution, resulting in a gel-like substance.

The chemical reaction for Zr precursor depends on the pH of solution and can follow two pathways:

$$ \mathrm{ZrO(NO_3)_2 + H_2O + 2NH_4OH \rightarrow Zr(OH)_4 + 2NH_4NO_3} $$

and

$$ \mathrm{ZrO(NO_3)_2 + 2NH_4OH \rightarrow Zr(OH)_2 + 2NH_4NO_3} $$

For Y and Cu precursors the reactions are [33]:

$$ \mathrm{Y(NO_3)_3 + NH_4OH \rightarrow Y(OH)_3 + 2NH_4NO_3} $$

and

$$ \mathrm{Cu(NO_3)_2 + 2NH_4OH \rightarrow Cu(OH)_2 + 2NH_4NO_3} $$

For the production of (Cu,Y)-codoped powders, the global reaction is:

$$ \mathrm{ZrO(NO_3)_2 + Y(NO_3)_3 + Cu(NO_3)_2 + 15H_2O + 7NH_4OH \rightarrow Zr(OH)_4 + 2Y(OH)_3 + Cu(OH)_2 + 7NH_4NO_3} $$

However, the balanced reaction depends on the desired composition of the final materials. In our case, the amount of each solution was adjusted to achieve the required composition of the final product as mentioned above.
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When the gel-like substance was obtained, it was dried at 80 °C for 48 h to remove water molecules and subsequently heated at 150 °C for 24 h to complete the process. The resulting sediments underwent further calcination at temperatures ranging from $T_c = 400$ to 1000 °C for durations of $t_c = 1, 2$, or 5 h.

The calcination at high temperatures of Zr, Y and Cu hydroxides results in the formation of corresponding oxides

$$Zr(OH)_4 \xrightarrow{T_c, t_c} ZrO_2 + 2H_2O\uparrow$$

$$2Y(OH)_3 \xrightarrow{T_c, t_c} Y_2O_3 + 3H_2O\uparrow$$

$$Cu(OH)_2 \xrightarrow{T_c, t_c} CuO + H_2O\uparrow$$

For (Cu,Y)-codoped ZrO₂ the calcination results in the formation of complex oxide via reaction

$$xZr(OH)_4 + 2yY(OH)_3 + zCu(OH)_2 \xrightarrow{T_c, t_c} (Cu, Y)-ZrO_2 + H_2O\uparrow,$$

where final product (Cu,Y)-ZrO₂ may be considered as a network with zCuO-yY₂O₃-xZrO₂ composition. As for the ZrO₂ powders doped with one element, either Cu or Y, the final reactions are similar. The calcination conditions determined spatial distribution of these elements.

In this study, ZrO₂ and Y-doped ZrO₂ powders were colorless, while Cu-doped powders were of different blue-green colors under daylight illumination as shown in figure 1 for some powders. Their coloration enhanced after the calcination procedure.

## 2.3. Methods for materials characterization

The structural and optical properties of the powders were analyzed by means of Raman scattering, X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Energy dispersive spectroscopy (EDS), Auger electron spectroscopy (AES), Electron Paramagnetic Resonance (EPR), UV-vis diffused reflectance, and FTIR reflection spectroscopy.

Raman scattering spectra were obtained in backscattering geometry using a 785-nm Preconfigured Raman Spectrometer System (StellarNet Inc., USA). The system included a 785 nm Ramulaser Raman laser with
adjustable power (0–499 mW), the Raman-HR-TEC-785 thermo-electrically cooled Raman spectrometer with 4 cm⁻¹ resolution and high sensitivity, and a Raman probe that allowed a laser spot of 0.15 mm in diameter on the sample surface. The laser power density on the sample was always kept low to prevent property modifications. Spectra were recorded with an integration time of 15 s and 5 times averaging for each spectrum. The polarization of the scattered light was not analyzed.

X-ray Diffraction (XRD) patterns were acquired in Bragg-Brentano geometry using a Philips X'Pert-MRD diffractometer equipped with CuKα₁ (λ = 0.15406 nm) radiation source. The data were collected in the range of 2θ = 20°–80° with a scanning step of 0.03° to ensure accurate measurements.

Conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were performed using a double-corrected cold FEG JEOL ARM 200 F microscope operated at 200 kV. The powder was stirred with n-butanol, and a drop of the resulting liquid was deposited on a 3 mm diameter copper grid (300 mesh) covered by a holey carbon film. The digitized images were acquired and processed using the commercial DigitalMicrograph software from GATAN.

Auger electron spectroscopy (AES) study was performed using an Auger microprobe JAMP 9500 F (JEOL) tool, provided a 3 nm resolution in secondary electron image mode. The microprobe features a sensitive hemispheric Auger spectrometer with energy resolution (ΔE/E) ranging from 0.05 to 0.6%, along with an ion etching gun for layer-by-layer analysis. The Ar⁺ ion beam diameter is 120 µm, capable of rastering at 1 × 1 mm. The Ar⁺ ion energy range varies from 0.01 up to 4.0 keV, with a minimum beam current of 2.0 µA at 3.0 keV. The specimen chamber maintains a vacuum better than 5 × 10⁻⁷ Pa. Additionally, the procedure of thermal drift correction was used during Auger spectra recording because of dielectric nature of the powders. Quantitative element analysis was conducted by calibrating relative sensitivity factors (RSF) on homogeneous reference samples of known compositions.

UV-vis diffuse reflectance spectra ranging from 220 to 1000 nm were recorded using a SilverNova spectrometer equipped with TE-cooled CCD detector (StellartNet Inc., USA) with respect to Spectralon® White Diffuse Reflectance Standard (99%) (Edmund Optics®). The recorded spectra were transformed to the absorption spectra using Spekwin32 software associated with this setup and further analyzed with SpectraGryph software, both created by Dr Friedrich Menges (http://spectroscopy.ninja).

Electron paramagnetic resonance spectra were obtained using the upgraded X-band Varian E-12 spectrometer (~9.5 GHz) with the sensitivity limit of about 10¹² EPR centers. The spectra were recorded in respect to the signal intensity from a MgO:Mn standard. All EPR spectra were normalized on the mass of the investigated powders.

Infrared reflections spectra were collected over the 200–4000 cm⁻¹ range (with a 1 cm⁻¹ resolution) using an IRTracer-100 Fourier Transform Infrared Spectrophotometer equipped with a DRS-8000A diffuse reflectance tool (Shimadzu Company, Japan).

All experiments were performed at room temperature.

## 3. Results and discussion

This section examines the structural properties of both undoped and doped ZrO₂ powders versus dopant content and calcination conditions. Subsequently, their correlation with the optical absorption spectra is explored. Finally, theoretical modelling of the infrared reflection spectra of various ZrO₂ phases is introduced to demonstrate its utility for fitting experimental data as a non-destructive approach for materials characterization.

### 3.1. Raman scattering and XRD study

#### 3.1.1. *Pure ZrO₂ powders*

The shape of Raman scattering spectra of ZrO₂-based powders significantly depends on their crystalline structure. Monoclinic ZrO₂ exhibits multiple narrow phonon bands corresponding to nine A_g modes, centered at 302, 344, 473, 553, 633, and 706 cm⁻¹, and six B_g modes, peaking at 219, 330, 378, 499, 533, and 610 cm⁻¹ [34, 35]. Tetragonal ZrO₂ shows phonon modes centered at 259, 330, 612, and 636 cm⁻¹ [36], while cubic ZrO₂ has broader phonons peaked at 263, 450, and 615 cm⁻¹. The spectral positions of all these phonons may vary depending on the Zr precursor and fabrication procedure [20, 22, 29, 32], while being in the vicinity of numeric values already mentioned.

Typically, undoped ZrO₂ has a monoclinic structure. However, some studies on undoped ZrO₂ nanopowders have reported the presence of a tetragonal and/or cubic structure at room temperature [6, 37]. In this case, ZrO₂ particles were smaller than 10 nm, and the stabilization of both phases was achieved due to high surface/volume ratio and high surface-induced tensions. Annealing of such powders at high temperatures caused grain sintering, followed by an increase in sizes and a decrease of their surface/volume ratio, resulting in the transformation of tetragonal structure to monoclinic [37].
The Raman scattering spectra of pure ZrO₂ powders prepared in this study are presented in figure 2(a). It is seen that after drying at 150 °C for 24 h, only a peak near 1040–1050 cm⁻¹ was observed, which was attributed to amorphous Zr hydroxide. The intensity of this peak decreased significantly after powder calcination at T_c = 400 °C–500 °C for 2 h, and for higher T_c, no such signal was observed. The powders calcined at T_c = 600 °C–700 °C exhibited a significant contribution of Rayleigh scattering, potentially caused by the presence of softly-bound agglomerates of the particles. This hindered the registration of the Raman signal from ZrO₂ grains. In contrast, the Raman signal of the ZrO₂ powder calcined at T_c = 800 °C has well-defined phonons. Most of these phonons coincided with those mentioned above for tetragonal phase, while the presence of phonons at 533 and 553 cm⁻¹ indicated the appearance of the monoclinic phase. Additionally, the significant presence of Rayleigh scattering suggested the nanocrystalline nature of these powders.

The increase in calcination temperature up to T_c = 900 °C promotes pronounced crystallization of the powder. Both tetragonal and, in larger extent, monoclinic phases are observed. Subsequently, calcination at T_c = 1000 °C results in complete *t-m* transformation. These findings are corroborated by XRD analysis of the same samples.

For the powders calcined at T_c < 600 °C, no XRD reflexes were detected, indicating their amorphous or nanocrystalline nature. Powders calcined at T_c = 600 °C exhibited XRD peaks at 2θ ≈ 30.3° and 35.2°, and, with lower intensity, at 2θ ≈ 28.2° and 31.5°. The two first peaks are considered a characteristic of cubic and tetragonal ZrO₂ phases, while two others are attributed to the monoclinic one.

To distinguish the contributions of cubic and tetragonal phases, analysis of the XRD patterns in the range of 2θ = 72°–76° can be conducted. A single symmetric peak is indicative of the formation of the cubic phase, whereas two peaks near 2θ = 73.2° and 74.6° (or one asymmetric broad peak covering this range) are characteristic of the tetragonal phase.

As depicted in figure 2(c), for T_c = 600 °C, two XRD peaks are detected in the range of 2θ = 72–76°, providing evidence of the formation of tetragonal ZrO₂. The analysis of lattice parameters yielded values of *a* = 0.3592 nm, *b* = 0.3592 nm, and *c* = 0.5179 nm for *t*-ZrO₂ coinciding with the tabulated data (ICDS card 96–153–9832). The peaks at 2θ ≈ 28.2° and 31.5° belong to the monoclinic phase, but its contribution is significantly lower, allowing us to conclude that the main contribution is given by the tetragonal phase in the powders calcined at 600 °C.

At higher T_c, the intensities of the peaks at 2θ ≈ 30.3°, 35.2°, 73.2° and 74.6° decrease, indicating a reduction in the contribution of tetragonal phase at T_c = 700 and 800 °C (figures 2(b), (c)). Additionally, the magnitudes of the peaks at 2θ ≈ 28.2° and 31.5° increase (figure 2(b)), along with those appearing in the range of 2θ = 68–76° (figure 2(c)).

All these peaks belong to the monoclinic ZrO₂ phase with lattice parameters *a* = 0.5120 nm, *b* = 0.5216 nm, *c* = 0.5281 nm, α = 90.000°, β = 99.010° and γ = 90.000° (ICDS card 96-900-5834). This phase becomes noticeable at T_c = 800 °C. Its contribution continues to increase with the rise in calcination temperature, and for T_c = 1000 °C, the monoclinic ZrO₂ phase with lattice parameters *a* = 0.5149 nm, *b* = 0.5202 nm, *c* = 0.5323 nm, α = 90.000°, β = 99.164° and γ = 90.000° (card 96-230-0545) becomes dominant. It is worth pointing out that the half-width of the reflexes decreases with T_c rise, indicating the enlargement of the coherent domain size (*d*) and the growth of the ZrO₂ grains from *d* ~ 10 nm (T_c = 600 °C) up to *d* ~ 27 nm (T_c = 1000 °C). Thus, these XRD data show the transformation of the tetragonal phase to the monoclinic one (figures 2(b), (c)) and support the conclusion drawn from Raman scattering data (figure 2(a)).

### 3.1.2. *Doped ZrO₂ powders*

Raman scattering spectra of Y-doped ZrO₂ powders with various Y content are depicted in figure 3. These spectra reveal that in Y-ZrO₂ powders with low Y loading (3 mol.% Y₂O₃) calcined at T_c = 700 °C–1000 °C, the tetragonal ZrO₂ phase is present and remains stable at high temperatures (figures 3(a), (b)). Interestingly, for the powders calcined at T_c < 700 °C, the cubic ZrO₂ phase was detected.

At higher Y loading (8–15 mol.% Y₂O₃), only the cubic phase is observed regardless of calcination temperature and duration (up to 5 h) (figure 3(b)). However, a peak at 485 cm⁻¹ is still detected, indicating a mixture of cubic and, at a lesser extent, tetragonal phases similar to data reported in [8]. For Y-ZrO₂ powders with 15 mol.% Y₂O₃ calcined at high T_c (900 °C–1000 °C), broad Raman peaks were observed at about 275–280 cm⁻¹ and 615–618 cm⁻¹ that corresponds to cubic ZrO₂ phase. These findings are similar to those reported in [2, 38].

The Raman spectra of Cu-doped ZrO₂ powders prepared with low Cu loading (0.1–0.5 mol.% CuO) showed shapes similar to those of pure ZrO₂ counterparts. Significant Rayleigh scattering was detected and explained by the nanocrystalline nature of the powders. An increase of Cu loading up to 1 mol.% CuO resulted in the appearance of near-infrared luminescence in Cu-doped ZrO₂ powders. The increase of calcination temperature from 600 to 1000 °C enhances further this luminescence. This can be caused by the formation of copper-related luminescent centers sensitive to a 785-nm excitation light used for our Raman scattering experiment. This effect
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Raman scattering spectra (a) and XRD patterns (b) for pure ZrO₂ powders calcined at 600 °C–1000 °C during 2 h. In (a), the spectra for 600 °C–800 °C were multiplied by factor 5. In both figures the curves were shifted in vertical direction for clarity.</figcaption></figure>

is the subject of another work and will not be discussed in detail here. Analysis of XRD patterns of Cu-ZrO₂ powders prepared at low Cu loading (0.1–0.5 mol.% of CuO) revealed their similarity to those reported for pure ZrO₂ powders in figure 2. Calcination at T_c = 600 °C resulted in the formation of the t-ZrO₂ phase. This
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Raman scattering spectra for: (a) pure ZrO₂ (1) and Y-doped ZrO₂ (2,3) powders calcined at 1000 °C for 2 h, with Y content of 3 mol.% Y₂O₃ (2) and 8 mol.% Y₂O₃ (3); (b) Y-doped ZrO₂ powders calcined at 800 °C (1,2) and 1000 °C (3–5) during 1 h (1,3), 2 h (2,4) and 5 h (5), with Y content of 15 mol.% Y₂O₃; (c) (Cu,Y)-codoped ZrO₂ powders calcined at 600 °C–1000 °C for 2 h, with Cu and Y content of 8 mol.% CuO and 3 mol.% Y₂O₃, respectively. The spectra in the figures were vertically shifted for clarity.</figcaption></figure>

statement is supported by TEM and EDX examination of these powders. A high-resolution TEM image of few powder grains lying on the holey carbon film of a TEM copper grid is shown in figure 4(a). Some crystalline grains with the sizes of 8–12 nm can be seen. STEM EDX images for the distribution of oxygen (figure 4(b)),
<figure><img src="image_8.png" /><figcaption>**Figure 4.** High-resolution TEM image of the few grains in Y-doped ZrO₂ powder (a) and STEM-EDX (b)–(d) chemical maps recorded for oxygen (b), zirconium (c) and yttrium (d).</figcaption></figure>

zirconium (figure 4(c)) and yttrium (figure 4(d)) elements were recorded. The comparison of these chemical maps revealed the uniform distribution of yttrium dopant in zirconium dioxide grains.

Annealing of Cu-ZrO₂ powders at higher temperatures causes the grain growth and structural transformation to the monoclinic phase (figure 3). This phenomenon is significant for the powders with high Cu content (8 mol. % CuO). This transformation occurs via Cu outward diffusion from the grain volume to their surface and formation of CuO inclusions. These latter can be revealed in Raman spectra via the presence of the phonons appearing in the interval of 269–296, 316–343 and 604–627 cm⁻¹ [39] which positions shift to the higher wavenumbers for powders with larger grains. The formation of crystalline CuO phase was also indicated by XRD study showing CuO related peaks at 2θ ~ 68° [37] as well as at 2θ ~ 35.5 and 46.4° [39].

The Raman study of (Cu,Y)-ZrO₂ powders revealed the presence of the tetragonal phase, which remained stable up to 1000 °C for powders with 3 mol.% Y₂O₃ and 0.1–1.0 mol.% CuO. However, the increase in Cu loading led to the destabilization of this phase. Although for T_c = 900 °C the phonon mode peaked near 260 cm⁻¹ is still present (figure 2(c)), the main contribution is given by the monoclinic phase indicating transformation of both *t*- and *c*-phases to the *m*-ZrO₂. This could be attributed to copper and, in large extent, yttrium out-diffusion from the grains. Similar results were described in the [31]. It was reported that the calcination at high T_c decreases the Cu content in the grains, facilitating its outward diffusion from the grain volume to the surface. Subsequently, this opens the ‘channels’ for Y out-diffusion, leading to the destabilization of *t*- and *c*-phases [31]. Thus, one can assume that similar mechanism of structural transformation can occur in the powders investigated here.

The TEM examination of (Cu,Y)-ZrO₂ powders in initial state revealed their amorphous nature. Calcination at 600 °C for 2 h results in crystallization and the formation of the tetragonal phase. For the powders with low Cu content (0.1 mol.% CuO) the grain sizes were found to be similar to those in figure 4. For higher Cu loading (0.2 mol.% CuO), the grain growth was also revealed by TEM examination.

Figure 5(a) shows a selected area electron diffraction (SAED) pattern of a region containing tens of grains. The diffraction rings fit perfectly with the tetragonal phase of ZrO₂ with a = b = 0.364 nm and c = 0.527 nm. Its spatial group is P4₂/nmc. Figure 5(b) is a typical HRTEM image showing a tetragonal ZrO₂ grain along the [−101] direction. The average size of these grains is about d = 15 nm.

The structural characterization of the powders with higher Cu content (1–8 mol.% CuO) revealed a microstructure similar to that of the powders studied earlier and described in [37]. It turned out that with the T_c rise, the coherent domain sizes of ZrO₂ grains varied from d ~ 8 nm (T_c = 600 °C) up to d ~ 15 nm (T_c = 1000 °C), being smaller than in undoped powders. This indicates that the additional incorporation of copper results in the structural distortion and smaller coherent domain size. It is interesting that for higher Y content (8–15 mol.% Y₂O₃), both *t*- and *c*-phases were stable up to 1000 °C regardless of the Cu content. This
<figure><img src="image_9.png" /><figcaption>**Figure 5.** SAED image (a) and typical HRTEM image (b) of the (Cu,Y)-ZrO₂ powder (with 3 mol.%Y₂O₃ and 0.2 mol.%CuO) demonstrated a tetragonal structure of ZrO₂ grain along the [−101] direction.</figcaption></figure>

suggests that the Y dopant plays a primary role in stabilizing the *t*- and *c*-phases of zirconia host. The transformation of structural and optical properties revealed the complexity of variations in powder properties when the ZrO₂ host is simultaneously doped with several additives.

## 3.2. UV-vis absorption spectra

To study the optical properties of the powders, the UV-vis diffuse reflectance method was used. The recorded diffuse reflectance spectra were converted to the absorption spectra using software associated with the setup. Their analysis was performed with the respect to dopant loading and calcination regimes.

### 3.2.1. *Pure and Y-doped ZrO₂ powders*

The UV-vis absorption spectra of pure ZrO₂ powders exhibit a typical broad band in UV spectral range centered near 300 nm and increasing in intensity towards the shorter-wavelength side. Sometimes a peak near 225 nm is detected [6, 29, 33]. The spectra of Y-doped powders have similar shape (figure 6). In initial state (just after drying) the broad band peaked near 290–300 nm is present (figure 6(a)) along with an increasing in intensity towards the shorter-wavelength side. For higher Y content, this band tends to broaden with a slight shift of its maximum down to 290 nm (figure 6). Additionally, the band near 240 nm can also be observed, in particular in powders with high Y loading (10–15 mol.% Y₂O₃). Taking into account that all the powders are amorphous in initial state, both bands can be ascribed to the Zr hydroxide and/or amorphous ZrO₂ powder, if any (figure 6(a)).

Powder calcination induces a transformation of the UV absorption band. The contribution of the 300-nm band decreases with an increase in calcination temperature, but some signal is still observable at 300–310 nm, becoming broader towards the longer-wavelength side (figure 6(a)). Additionally, the signal intensity increases towards deeper ultraviolet. As a result of powder calcination, which leads to their crystallization, the UV band peaked at 300–310 nm is considered to belong to crystallized ZrO₂. Typically, the bandgap of crystallized ZrO₂ exceeds 5.5 eV (~225.4 nm) [2], and therefore, it can be assumed that UV absorption is more likely caused by energy levels formed by some defects inside the bandgap of the ZrO₂ host than band-to-band absorption. For Y-doped ZrO₂ powders, the UV band has higher intensity for higher Y content. Since, Y-doping is accompanied by the formation of oxygen vacancies, the UV band one can ascribe to newly formed oxygen vacancies.

### 3.2.2. (Cu,Y)-ZrO₂ powders

The absorption spectra of Cu-doped powders consist of two main absorption bands (figure 7). One of them is a UV band, observed at about 300 nm and 230 nm, close to the ZrO₂ band edge in initial powders. This band is similar to the UV band described earlier for Y-doped ZrO₂ powders. An increase in Cu loading is accompanied by band broadening towards the longer-wavelength side, which may be caused by the appearance of additional oxygen vacancies due to charge compensation of the Cu dopants. Annealing of Cu-doped powders at 600 °C results in further broadening of the UV band, as well as in the shift of its peak position up to 380 nm (figure 4). The higher the Cu loading, the more pronounced this shift becomes. A comparison of absorption spectra for Cu-free and Cu-doped samples with the same Y content reveals that the observed behavior of the absorption spectra is attributed to Cu doping, i.e., its incorporation in ZrO₂ host and the formation of additional vacancies required for Cu charge compensation. This effect is considerable since the powders were synthesized from Zr, Y and Cu nitrates in present study.

One more absorption band was observed in the 500–1000 nm spectral range (figure 7). Its significant contribution was detected for the powders Cu content higher than 0.2 mol.% CuO. This band is broader, and its contribution increases with Cu content rise. This band can be ascribed to the *d-d* transitions of the Cu²⁺ ions. These ions can be situated in octahedral or tetragonal distorted octahedral surrounding and be associated with
<figure><img src="image_10.png" /><figcaption>**Figure 6.** UV-vis absorption spectra of: (a) ZrO₂ powders doped with 10 (1,2) and 15 (3,4) mol.% of Y₂O₃, before (1,3) and after calcination at 800 °C for 2 h (2,4); (b) Y-doped ZrO₂ powder with 15 mol.% Y₂O₃ before (1) and after calcination at 800 °C (2,3) and 1000 °C (4–6) for 1 h (2,4), 2 h (3,5) and 5 h (6). The spectra were vertically shifted for clarity.</figcaption></figure>

dispersed CuO on the grain surface or with Cu<sub>Zr</sub><sup>2+</sup> substitutional atoms located in the near-surface region
[6, 18, 19].

The absorption spectra of all Cu-doped powders calcined at 600 °C, and those with low Cu loading calcined at 800 °C demonstrate the similar shape (figure 7). The comparison of obtained results with those reported in the [38] permits to conclude that the powders with low Cu loading contain the grains covered by dispersed Cu<sub>x</sub>O species. Since no signal from crystalline CuO was detected, one can assume that no deformation of CuO chains occurs. Based on the similarity of the spectra presented in [38–41] with those from present study, the Cu-related absorption band can be described rather to Cu<sub>2</sub>O or Cu<sub>x</sub>O than CuO species.

This indicates significant amount of extra-framework Cu sites and distinct nature of Cu<sub>x</sub>O<sub>y</sub> species present
at the grain surface.

The spectra of (Cu,Y)-ZrO₂ powders with high Y loading (up to 10 mol.% Y₂O₃) also showed two main
bands: one in the UV range related to oxygen vacancies and another CuO-related absorption in visible spectral
range (figure 7(d)). The curves 4 and 6 in figure 7(c) and curve 2 in figure 7(d) have similar shape. This indicates
that similar Cu<sub>x</sub>O species can be observed on the ZrO₂ grain surface in such powders. This finding is supported
by the examination of the same powders with electron paramagnetic resonance.

The EPR spectra were recorded for all powders and some of them, recorded for (Cu,Y)-codoped ZrO2
powders, are depicted in figure 8. Generally, the powders in the initial state show clear asymmetric shape. This
feature is indicative of surface Cu-related centers like Cu-OH. Similar spectra were reported in [29, 30]. The
<figure><img src="image_11.png" /><figcaption>**Figure 7.** UV-vis absorption spectra of (Cu,Y)-ZrO₂ powders in initial state (a) and after calcination at 600 °C for 2 h (b); Cu content varies from 0 to 8 mol.% CuO; (c) effect of calcination temperature on optical absorption of Cu-free (1,3,5) and (Cu,Y)-ZrO₂ (2,4,6) powders in initial state (1,2) and after calcination at 600 (3,4) and 800 (5,6) °C for 2 h; (d) the effect of Cu content on optical absorption spectra of Y-free (1,3,5) and (Cu,Y)-ZrO₂ (2,4,6) powders calcinated at 600 for 2 h; the content of dopants is mentioned in the figure.</figcaption></figure>

higher the Cu loading, the higher intensity of this signal becomes (figure 8(a)). Thus one can assume the increase of Cu content in the powders is accompanied by the increase in intensity of this signal.

The calcination of the powders at 600 °C for 2 h causes the transformation of EPR signal (figure 8(b)) that can be caused by the decomposition of some Cu-OH complexes or their reconstruction at the grain surface. The appearance of the signal related to Cu<sub>Zr</sub><sup>2+</sup> defects is seen. This effect is more pronounced after calcination at 800 °C caused significant transformation of EPR signal. Its shape becomes nearly the same as that of the signal detected for Cu<sub>Zr</sub><sup>2+</sup> site defects situated in monoclinic ZrO<sub>2</sub> [6]. At a glance, the parameters of observed EPR centers are similar to those reported in [6], however, this issue needs more study. At the same time, one can conclude that upon calcination at 800 °C for 2 h, a pronounced appearance of monoclinic phase in (Cu,Y)-ZrO<sub>2</sub> powders occurred. This is in the agreement with the Raman scattering data (figure 3).

Such powder transformation and redistribution of Cu and, consequently, Y dopants can stimulate the formation of additional phases. However, neither Raman scattering, nor TEM observation revealed the formation of such phases at 600 and 800 °C (figures 3–5). The distribution of chemical elements in these powders was studied with AES spectroscopy.
<figure><img src="image_12.png" /><figcaption>**Figure 8.** EPR spectra of novel (Cu,Y)-ZrO₂ powders recorded in initial state (a) and after calcination (b) for the samples with different Cu content mentioned in the figures. Y content was 3 mol.% Y₂O₃, being the same for (a) and (b).</figcaption></figure>

The AES spectra of the (Cu,Y)-ZrO₂ powders (3 mol.% Y₂O₃ and 8 mol.% CuO) studied earlier, along with the AES spectra for the powders obtained in the present study are shown in figure 9. The Auger signals for O<sub>KLL</sub> (around 509 eV), Cu<sub>LMM</sub> (around 915 eV), Y<sub>LMM</sub> (around 1746 eV), and Zr<sub>MNN</sub> (around 141 eV) were analyzed.

It is worth to point out that the AES spectrum for reference (Cu,Y)-ZrO₂ powder (curve 1), investigated earlier using TEM, EDS, Raman scattering and XRD methods, shows no specific phases. The AES spectrum of the powder with the same chemical composition prepared in this work (curve 2) is similar to that of reference sample. The same similarity is seen for curve 3, however, Cu<sub>LMM</sub> signal has lower intensity due to low Cu loading. Since the AES spectra are similar and, in this study, TEM observation revealed similar powder structures, it is hardly possible to expect the formation of different phases and substances than ZrO₂ grains doped with Cu and Y and/or covered with CuₓO substances.

Analysis of the Cu<sub>LMM</sub> AES signal showed that the Cu<sub>LMM</sub> peak position is in the range of 909–916 eV where the peaks from metallic Cu, CuₓO and Cu₂O species can be detected. Thus, one can assume that in our powders, Cu dopants present rather as surface complexes CuₓO and Cu-OH than CuO. This conclusion is in agreement with results reported in [30, 39–41] as well as with EPR data and UV-vis absorption spectra.

We have performed analysis of the UV-vis absorption spectra of the powders using Tauc plot approximation. The spectra of all samples were transformed using $(\alpha\hbar v)^n = f(\hbar v)$ relationship. However, only for some powders, the linear behavior of this relationship in UV range was seen (figure 10). By extending the absorption edge to the energy axis, the bandgap value $E_g$ was determined by extrapolating a straight line at $\alpha\hbar v = 0$. It turned out that for most powders the bandgap value is in the range $E_g = 5.45–5.65$ eV. For the powders, which showed dependence similar to the curve 4 in figure 10, one can conclude that their bandgap exceeds 5.8 eV (detection limit of our setup). This can be a case for the powders with cubic structure that is known to demonstrate the largest bandgap among different ZrO₂ phases [6, 42].

Apart from this, pure ZrO₂ powders tend to demonstrate larger bandgap than their Y-doped and (Cu,Y)-codoped counterparts (figure 10, curves 1–3) that is in agreement with the results reported earlier [29, 43]. For
<figure><img src="image_13.png" /><figcaption>**Figure 9.** AES spectra (convoluted with a hardware function) for the reference (1) and novel (Cu,Y)-ZrO₂ (2,3) powders calcined at 600 °C for 2 h. Accelerating voltage – 10 kV, probe current – 5·10⁻⁸ A.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 10.** Tauc plot of UV-vis absorption spectra for the powders calcined at 600 °C for 2 h: 1 - pure ZrO₂, 2 - Y-ZrO₂ (3 mol.% Y₂O₃), 3 - (Cu,Y)-ZrO₂ (0.2 mol.% CuO and 3 mol.% Y₂O₃), and 4—Y-ZrO₂ (10 mol.% Y₂O₃).</figcaption></figure>

the powders calcined at 800 °C–1000 °C, no linear relationship in UV spectral range was observed. However, for Y-doped ZrO₂ with high Y content, the cubic structure was detected assuming to enlarge the bandgap. In this regard, the plateau in the range of 5.2–5.8 eV (figure 10, curve 4) can be caused by the overlapping of several absorption bands, for instance, the contribution of O 2p → Zr 4d charge transfer transitions and newly formed host defects as oxygen vacancies or defect complexes. Besides, it can originate from the lowering of the coordination number of oxygen with zirconium. In doped materials, it is effectively 7 rather than 8 in pure tetragonal ZrO₂ that can also lead to some narrowing of the bandgap of doped tetragonal ZrO₂[43].

Thus, the study of UV-vis absorption spectra of (Cu,Y)-ZrO₂ powders allows assuming that the powders with a Cu loading of 1 mol.% CuO calcined at 600 °C and those with lower Cu loading (0.2–0.5 mol.% CuO) calcined at 600 and 800 °C show an intense Cu-related absorption band that is a signature of the presence of large amount of dispersed CuₓO complexes at particle surface. Furthermore, due to Y doping, these particles exhibit stabilized *t*- and *c*-phases, which are known to show higher catalytic activity compared to *m*-ZrO₂. Additionally, these powders consist of small ZrO₂ particles (*d* = 8–15 nm) with large surface area and can hold promise for catalytic tests, for instance, for the CO-PROX reaction that are in progress.

### 3.3. Infrared reflection spectra

As mentioned earlier, Raman scattering is sensitive to crystal symmetry and has been successfully applied to characterize phase transformations in the ZrO₂ host. However, distinguishing the phonons for the *c*-ZrO₂ phase
<table><caption>**Table 1.** Self-consistent parameters for bulk ZrO₂ with different crystalline structure.</caption><thead><tr><th>Parameters</th><th>ε<sub>0</sub></th><th>ε<sub>∞</sub></th><th>ν<sub>T</sub>, cm<sup>-1</sup> (γ<sub>β</sub><br>cm<sup>-1</sup>)</th><th>ν<sub>L</sub>, cm<sup>-1</sup> (γ<sub>β</sub><br>cm<sup>-1</sup>)</th></tr></thead><tbody><tr><td>ZrO<sub>2</sub>, cubic [45]</td><td>27</td><td>4.8</td><td>375 (10)</td><td>695 (10)</td></tr><tr><td>ZrO<sub>2</sub>, tetragonal [46]</td><td>25</td><td>4.8</td><td>164 (97)<br>339 (69)<br>467 (106)<br>580 (35)<br>672 (172)</td><td>232 (119)<br>354 (95)<br>650 (139)<br>581 (39)<br>734 (113)</td></tr><tr><td>ZrO<sub>2</sub>, monoclinic [47]</td><td>16</td><td>4.8</td><td>92 (130)<br>330 (55)<br>410 (20)<br>480 (30)<br>534 (100)<br>570 (60)<br>740 (110)</td><td>104 (260)<br>381 (80)<br>423 (28)<br>515 (60)<br>556 (138)<br>615 (60)<br>760 (120)</td></tr></tbody></table>

in a mixed-phase sample is challenging because they are broader compared to those of m- and t-ZrO₂. Additionally, for powders with smaller grains, Rayleigh scattering is significant and can dominate in Raman scattering spectra. In such cases, infrared reflection spectroscopy proves to be sensitive to both the structure of the powders and the nature of surface substances. Developing theoretical models for fitting experimental data can be helpful for characterizing nanopowders with mixed crystalline phases. In this section, we discuss the results obtained for the same powders using infrared reflection spectroscopy.

### 3.3.1. *Theoretical models and spectra simulation*

To model infrared reflection spectra of ZrO₂ powders, a mathematical model was built based on the frequency dependence of the dielectric constant given by the well-known Helmholtz–Kettler formula

$$ \varepsilon(\nu) = \varepsilon_{\infty} + \sum_{i=1}^{n} \frac{\varepsilon_f \nu_i^2}{\nu_i^2 - \nu^2 + i\gamma_f \nu} \quad (1) $$

where $\varepsilon_\infty$ is the high-frequency dielectric permittivity, $\varepsilon_f$ is the contribution of active i-th phonon with the frequency $\nu_i$ and $\gamma_f$ is corresponding damping coefficient, and $\nu$ is the frequency of infrared irradiation [44]. In other words, the dielectric constant can be presented as

$$ \varepsilon(\nu) = \varepsilon_1 + i\varepsilon_2 = \varepsilon_{\infty} + \varepsilon_f, \quad (2) $$

and the reflection coefficient $RT(\nu)$ can be calculated as

$$ RT(\nu) = \left[ \frac{\sqrt{\varepsilon(\nu)} - 1}{\sqrt{\varepsilon(\nu)} + 1} \right]^2. \quad (3) $$

The simulation of the IR reflection spectra for different ZrO₂ phases in the range of ‘residual rays’ was performed using the formula (2) and taking into account the interaction of infrared radiation with the phonon subsystem only because of dielectric ZrO₂ nature. For the cubic ZrO₂ phase, a single-oscillator model was applied [45], while for tetragonal and monoclinic phases—five [46] and seven [47] oscillators were considered, respectively.

It is worth pointing out that, typically, the frequencies of different Zr-O and Zr-O-Zr vibrations are reported in the literature [20, 22, 32] while the damping coefficients of these phonons are less addressed [45–47].

Self-consisted parameters of the oscillators are presented in table 1. The value of static dielectric constant was determined from the Lydden–Sachs–Teller ratio [44]. The simulation was carried out using the Kramers–Kronig ratio.

By applying the developed mathematical model and considering the additive and phenomenological contribution of phonon subsystem into the dielectric permittivity, the infrared reflection spectra for different ZrO₂ phases were simulated (figures 11(a), (b)). Obtained results were found to be in a good agreement with experimental data reported in [45–47].

Analysis of the peculiarities of the theoretical spectra simulated for three ZrO₂ phases revealed a distinct minimum in the reflection spectra. This unique spectral feature can be used for the express identification of the crystalline phase. Specifically, a reflection minimum occurs at approximately 700–720 cm⁻¹ for c-ZrO₂,
<figure><img src="image_15.png" /><figcaption>**Figure 11.** (a) Simulation of the IR reflection spectra using the parameters of phonon subsystem for the *m*-, *t*- and *c*-ZrO₂ phases from table 1; (b) Modelling of IR reflection spectra for Y-doped ZrO₂ with 3, 6, 20 and 46 mol.% of Y₂O₃; (c) experimental spectrum for tetragonal 3Y-ZrO₂ sample (blue symbols) and its fitting (red line).</figcaption></figure>

790–800 cm⁻¹ for *t*-ZrO₂ and 820–840 cm⁻¹ for *m*-ZrO₂ phase. In addition, the simulation of the spectra revealed minimal effect of phonon damping coefficient γf on the spectral position of the reflection minimum for each phase.

This approach was initially employed to fit an experimental spectrum obtained for a Y-doped ZrO₂ sample (with 3 mol.% Y₂O₃) (figure 11(c)). The reflection minimum position is around 800 cm⁻¹. To fit this spectrum, a five-oscillator model and self-consistent parameters, collected in table 2, were used. The result is present in figure 11(c) by the red curve, showing good agreement with experimental data. However, the best match was achieved when additional oscillator (with the frequency ~375 cm⁻¹) was included. Finally, the fitting indicated
<figure><img src="image_16.png" /><figcaption>**Figure 12.** Experimental IR reflection spectra of (a) ZrO₂ (1) and 3Y-ZrO₂ (2) calcined at 600 °C for 2 h; (b,c) (Cu,Y)-ZrO₂ powders calcined at 600 °C (b) and 800 °C (c) for 2 h; Cu and Y loading are mentioned in figures. The spectra are shifted vertically for clarity.</figcaption></figure>

<table><caption>**Table 2.** Self-consistent parameters for fitting of experimental data.</caption><thead><tr><th>Spectrum</th><th>ε<sub>0</sub></th><th>ε<sub>∞</sub></th><th>ν<sub>T</sub>, cm<sup>-1</sup><br>(γ<sub>δ</sub> cm<sup>-1</sup>)</th><th>ν<sub>L</sub>, cm<sup>-1</sup><br>(γ<sub>δ</sub> cm<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="5">Figure 11(c) (mixture of<br>tetragonal and cubic<br>phases)</td><td rowspan="5">25</td><td rowspan="5">4.8</td><td>164 (2)</td><td>232 (2)</td></tr><tr><td>339 (120)</td><td>354 (110)</td></tr><tr><td>450 (70)</td><td>666 (130)</td></tr><tr><td>600 (35)</td><td>601 (39)</td></tr><tr><td>672 (140)</td><td>734 (80)</td></tr><tr><td></td><td></td><td></td><td>375 (150)<sup>a</sup></td><td></td></tr></tbody></table>

that the primary phase is tetragonal, while some distortion of tetragonal lattice, for instance, due to dopants, cannot be ruled out.

### 3.3.2. Analysis of experimental IR reflection spectra

The spectra were recorded and analyzed for all powders. Figure 12 shows the spectra for the pure doped ZrO₂ powder (curve 1) and Y-doped ZrO₂ powder (curve 2). The corresponding XRD data are shown in figure 2. By comparing experimental and theoretical IR spectra, we can establish the correlation between crystalline structure and shape of the IR reflection spectra can be obtained and employed for further analysis.

As shown in figure 12(b), the spectra of the (Cu,Y)-codoped ZrO₂ powders calcined at 600 °C are similar regardless of Cu loading. The broad peak at about 450 cm⁻¹ and shoulders near 360–380 cm⁻¹ and 660–680 cm⁻¹ are clearly visible (figure 12(b)). The spectral position of the reflection minimum is about 800 cm⁻¹. In contrast, the spectra in figure 12(c) demonstrate significant shift of minimum position towards the lower wavenumbers with increasing Y content in the powders. Indeed, starting from about 830 cm⁻¹ for Y-free powder it shifts towards 770 cm⁻¹ for 3 mol.% Y₂O₃ and remains nearly the same for 10 mol.% Y₂O₃ (figure 12(c)). The change in spectrum shape also indicates a structural transformation from monoclinic ZrO₂ to tetragonal one. It is worth to point out that the effect of the Cu loading on such transformation is negligible. These findings support our statement regarding the main role of Y dopant in formation of tetragonal (cubic) structure of doped ZrO₂, which remains stable at elevated calcination temperatures.

Figure 12(c) shows that IR reflection spectra of Cu-ZrO₂ powders calcined at 800 °C are similar to those of m-ZrO₂ (figure 12(a)). Consequently, the Cu-ZrO₂ powders calcined at 800 °C have mainly monoclinic structure while the increase in Cu loading leads to the broadening of the vibration modes. The results support also the EPR data, showed the formation of Cu<sub>2</sub>Zr<sup>2+</sup> defects in monoclinic structure. Based on the results described in this section, one can conclude that the developed models for simulating IR reflection spectra give a hand to determine the type of crystalline structure of the powders (both for single and mixed-phase cases) as well as to track structural transformation caused by the doping and/or thermal treatment.

<footnote>a the additional phonon is employed to fit experimental data.</footnote>
## 4. Conclusions

This study had two primary objectives: to investigate the effect of Cu and/or Y dopant content and calcination regimes on the structural and optical properties of Cu-ZrO₂, Y-ZrO₂ and (Cu,Y)-ZrO₂ nanopowders, and to develop an approach that integrates various optical methods for express monitoring of powder properties. Undoped and doped ZrO₂ nanopowders were synthesized using a co-precipitation method with Zr, Y and Cu nitrates. The dopant content was varied across a wide range (0.1–8.0 mol.% of CuO and 3–15 mol.% of Y₂O₃). The powders underwent calcination at temperatures ranging from 400 to 1000 °C for durations of 1, 2 and 5 h in an air atmosphere, followed by analysis using different methods. Comparing the results obtained for Y-ZrO₂, Cu-ZrO₂ and (Cu,Y)-ZrO₂ powders revealed that both impurities stimulates the appearance of additional oxygen vacancies in the powders. However, the formation of the tetragonal and cubic ZrO₂ phases is primarily influenced by Y content, regardless of the Cu loading. It was found that the powders with 0.2–1.0 mol.% CuO content and calcined at 600 °C, exhibited tetragonal grains with the sizes of about 8–10 nm and dispersed CuₓO species on their surface, which may be attractive for catalytic applications. Calcination at higher temperatures resulted in grain growth (up to about 15 nm), reduced surface area, the formation of crystalline CuO, and appearance of Cu₂⁺ site defects in monoclinic ZrO₂ phase. The bandgap value of the powders was found to be in the range 5.45–5.65 eV. Doping with Cu and Y results in the appearance of additional oxygen vacancies that form energy levels inside bandgap and manifest in the absorption spectra as UV band with peak position around 300–320 nm (3.8–4.1 eV). The Y-ZrO₂ powders with high Y content (8 mol.% Y₂O₃ and higher) calcined at elevated temperatures show a cubic structure with the bandgap larger than 5.8 eV. Additionally, this study demonstrated the effectiveness of developed theoretical models for identification of different ZrO₂ crystalline phases by means of infrared reflection spectroscopy. For cubic, tetragonal and monoclinic phases, theoretical models with one, five and seven oscillators were built, respectively. Simulated theoretical spectra revealed a distinct position of the high-frequency reflection minimum, correspondingly observed at 700–720 cm⁻¹, 790–800 cm⁻¹ and 820–840 cm⁻¹ for cubic, tetragonal and monoclinic phases. This unique feature for each crystalline phase depends only slightly on phonon damping coefficients. By comparing experimental and theoretical reflection spectra, the parameters of various phonons were determined. Additionally, the potential for identifying single and mixed-phase ZrO₂ structures was demonstrated. Integrating the aforementioned methods, an approach for monitoring powder properties is proposed, aiding in understanding the characteristics of ZrO₂ powders doped with several impurities and predicting their catalytic behavior.

## Acknowledgments

This work was supported by the National Research Foundation of Ukraine from the state budget, project 2020.02/0380 «Structure transformation and non-equilibrium electron processes in wide bandgap metal oxides and their solid solutions» as well as by the National Academy of Sciences of Ukraine (projects III-10-21 and III-10-24). All the TEM results reported in this manuscript fall within the scope of the research foundation IRMA (FR 3095).

## Data availability statement

The data cannot be made publicly available upon publication due to legal restrictions preventing unrestricted public distribution. The data that support the findings of this study are available upon reasonable request from the authors.

## ORCID iDs

L Khomenkova https://orcid.org/0000-0002-5267-5945

O Isaeva https://orcid.org/0000-0003-1313-5409

O Melnichuk https://orcid.org/0000-0002-6768-8765

N Korsunska https://orcid.org/0000-0002-4778-5074

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