Introduction

Transparent conductive oxides (TCOs) play an important role in transparent electrodes and photovoltaic industry. They are widely used in flat panel display, touch panel, solar cell, and light-emitting diode [1, 2], due to their functional combination of optical transparency and electrical conductivity in visible band. It is well known that most TCOs are n-type semiconductors, such as In2O3 [3], ZnO [4], and SnO2 [5]. However, in most applications, such as light-emitting diode and functional window, both n-type and p-type transparent conductive materials are required. Therefore, p-type TCO has been a worldwide hot research issue. In 1997, Kawazoe et al. first found that copper aluminum oxide (CuAlO2) was a kind of p-type TCO material with a direct energy gap of 3.5 eV and positive Seebeck coefficient, and they also showed that CuAlO2 possesses an electrical conductivity in the level of 1 S/cm at room temperature [6]. The discovery of p-type TCO makes it possible for people to fabricate transparent oxide optoelectronic devices, such as transparent p–n junction diode and transistor using an appropriate combination of p-type and n-type TCO complementary techniques of Si-based electronics [7]. However, the conductivity of p-type TCO compounds is much lower than that of the more commonly used n-type TCOs, like Sn-doped In2O3. For the practical applications of p-type TCOs, the improvement of electrical property seems to be very exigent demanding and necessary, in order to achieve similar properties with n-type oxides [8].

At present, CuXO2 (X = Al, Fe, Ga, Cr and In) has become an important kind of p-type TCOs, and both theoretical and experimental researches have been carried out to explore high-performance and low-cost delafossite-type TCOs. Using the first-principle calculation, Nakanishi et al. revealed that CuAlO2 has great potential to be p-type transparent superconductors [9]. With solid-state reaction method, Dong et al. synthesized CuNdO2 delafossite, which is a p-type transparent conductive oxide with an optical bandgap of 3.14 eV [10]. In our previous work, using sol–gel method, we prepared CuFe1−x Sn x O2 films on Al2O3 (001) substrates for growing orientations [11]. Delafossite compound belongs to the family of ternary oxide, whose general and chemical formulas are ABO2 and A+B3+O2, respectively. Compared with the delafossite compounds CuXO2 (X = Al, Ga) [12], CuCrO2 has drawn more attention owning to low synthesis temperature and good thermal stability. The structure of delafossite CuCrO2 can be treated to be consisted of two alternative layers: a planar layer of Cu cation in a triangular pattern and a CrO2 layer with Cr3+-centered edge-sharing octahedra. Each Cu atom is linearly coordinated with two oxygen atoms to form an O–Cu–O dumbbell unit which is parallel to c-axis [13]. The crystal structure of CuCrO2 can be described by a rhombohedral spatial group \( R\overline{3} m \), and there are only four atoms (one Cu, one Cr, and two O atoms) in the primitive rhombohedral cell. In the close energy proximity with O 2p levels, the hybridization of Cu 3d10 energy levels increases the energy of the valence band maximum value, and also it delocalizes the hole state to form an intrinsic p-type semiconductor [14]. For CuScO2 and CuYO2 compounds, the intercalation of oxygen to form CuMO2+x phases will improve their conductivity [15]. However, because of small ionic radius of Cr (0.615 Å), the conductivity of CuCrO2 cannot be easily improved by the intercalation of oxygen [16]. Therefore, the intrinsic resistance of CuCrO2 films is too large to be suitable for practical application. Up to now, it is well known that the introduction of some metal ions into TCOs could effectively increase both carrier density and electrical conductivity. Though this type of carrier doping has been widely used in n-type semiconductors, only a few studies have reported successful p-type doping for oxide semiconductors [17–19].

In this article, in order to improve the conductivity of CuCrO2 compound, we reported our effort to synthesis polycrystalline delafossite oxides of CuCr1−x Zn x O2 (x = 0, 0.03, 0.05, 0.07, 0.1) by the general chemical solution process. We also investigated the effects of Zn2+-doping on the structure and electrical properties of such compound. The rest of this paper is organized as follows. First in “Experiment and measurement” section, experimental details for the synthesis and measurement of the CuCr1−x Zn x O2 oxides are described. Then in “Results and discussion” section, measurement results as well as related analysis and discussion are given, including field emission scanning electron microscope (FESEM) characterization, crystalline structure, X-ray photoelectron spectroscopy (XPS), optical property based on Fourier transform infrared spectrometer (FTIR), thermal and electrical properties. Finally, some conclusions are reached in “Conclusion” section.

Experiment and measurement

Preparation of CuCr1−x Zn x O2 powder

Polycrystalline delafossite oxides CuCr1−x Zn x O2 (x = 0, 0.03, 0.05, 0.07, 0.1) were prepared by sol–gel method. Analytically pure copper acetate [Cu(AcO)2·H2O], chromium nitrate [Cr(NO3)3·9H2O], and zinc acetate [Zn(AcO)2·2H2O] were used as starting materials. Specifically, copper acetate (1.9965 g, 0.01 mol) was dissolved in 30 mL anhydrous ethanol and stirred for about 30 min, yielding a blue transparent solution. After the solution gets stable and homogeneous, the required chromium nitrate hydrate and zinc acetate hydrate were added into the solution. The mass was counted by the mole ratio of Zn/Cr. The solution was stirred for more than 5 h at 45 °C in order to get a well-mixed precursor solution. The resulting solution was baked and fully grinded in an agate mortar and sintered in air at 1000 °C for 10 h.

Preparation of CuCr1−x Zn x O2 pellets

The CuCr1−x Zn x O2 powder was pressed into pellets with 15 mm diameter and 2.5 mm thickness under 21 Mpa using a powder tableting machine (769YP-40C). The resulting pellets were sintered in air at 1000 °C for another 24–48 h until the samples with pure phase were obtained. Furthermore, ohmic contact was formed on a pallet for Hall measurement. On four symmetric positions of the circle pellet, four electrodes were made with Indium. Following that, put the pallet into a short annealing furnace for half an hour to form ohmic contact finally.

Characterization and measurement

The morphology of CuCr1−x Zn x O2 samples was observed by a JSM-7500F FESEM. Crystal structure was analyzed using the conventional X-ray diffractometer (XRD) technique with a Bruker D8 Advance X, Pert diffractometer (Cu Ka = 1.540 Å), and the scanning speed was 4°/min from 10° to 70°. Raman spectra were recorded at room temperature with the 514.5 nm line of an Ar+ laser, excitation from a Spectra Physics krypton ion laser. XPS (ESCALAB 250) was used to determine the chemical composition and valence of the elements. FTIR spectra for bonding information were investigated on the powder samples using an FTIR (Perkin-Elmer Spectrum GX FTIR/FT-Raman spectrometer) in the range of 400–4000 cm−1. Thermogravimetry and differential thermal analysis (TG–DTA) of the precursor material was carried out with a SDT Q600 V20.9 model. Electrical properties were determined using a Hall-effect measurement system (ACCENT HL55OOPC) at room temperature and ULVAC-RIKO ZEM-2 thermoelectric property measurement system under a low-pressure helium atmosphere.

Results and discussion

FESEM analysis

The SEM observation in Fig. 1 reveals the morphologies of the sol–gel-derived delafossite-CuCr1−x Zn x O2-polycrystalline post-annealed at 1000 °C in air for 24 h. As can be seen from Fig. 1f, the rhombohedral R-3 m CuCr1−x Zn x O2 crystal appears to be hexagonal slices. It is also found that as Zn-doping ratio increases, the grain size is enlarged within a wide range of particle size distribution varying from 0.8 to 18 µm.

Fig. 1
figure 1

FESEM images of the CuCr1−x Zn x O2 samples. a–e are corresponding to the Zn-doping ratios of x = 0, 0.03, 0.05, 0.07, 0.1, respectively. f is the amplified CuCr0.9Zn0.1O2 crystal grain

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Structure characterization

Figure 2a shows the XRD patterns of the derived sol–gel CuCr1−x Zn x O2 samples (x = 0, 0.03, 0.05, 0.07, and 0.1) post-annealed at 1000 °C in air. The XRD peaks of a CuCrO2 sample will display the phase of delafossite structure (JCPDS file: 39-0247) and the impurity phase of CuO (ICSD Card File:01-089-5897). The obtained XRD patterns show all the peaks of CuCrO2 sample and those of CuO, which confirms that the formed CuCr1−x Zn x O2 possesses delafossite structure. The XRD peaks of the CuCr1−x Zn x O2 solid solution shift toward lower angles with Zn-doping, as can be seen in the magnified XRD patterns of the (018) peak displayed in Fig. 2b. The lattice spacing parameters of the CuCr1−x Zn x O2 samples (x = 0, 0.03, 0.05, 0.07, and 0.1) are exhibited in Table 1. The lattice parameters along a-axis and c-axis are 2.9762 and 17.1024 Å, respectively, which are in good agreement with CuCrO2 (JCPDS No. 39-0247 a = 2.976 Å and c = 17.102 Å). Also, the two parameters rapidly increase as increasing the Zn content. This effect occurs due to the partial substitution of Zn2+ (0.74 Å) with large ionic radii into Cr3+ (0.615 Å) sites with small radii, leading to local lattice distortion [20].

Fig. 2
figure 2

a XRD patterns of the CuCr1−x Zn x O2 (x = 0, 0.03, 0.05, 0.07, 0.1) samples, where black circle represents CuO. b Enlarged XRD pattern of CuCr1−x Zn x O2 (x = 0, 0.03, 0.05, 0.07, 0.1) samples at (018) peak (Color figure online)

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Table 1 The lattice parameters of the CuCr1−x Zn x O2 (x = 0, 0.03, 0.05, 0.07, 0.1) samples
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To further study the doping effect on the structure, Raman spectroscopy was measured for these samples. As shown in Fig. 3, the Raman spectrum of CuCr1−x Zn x O2 (x = 0, 0.03, 0.05, 0.07, 0.1) shows three typical vibrational bands of delafossite structure. Similar results can be found in the earlier results for CuGaO2 [21] and CuCrO2 [22]. These bands are identified as σ(A1g) at 691 cm−1, σ(E g) at 444 cm−1 and σ(Ag) at 207 cm−1. It suggests that these vibrations may be associated with the spectral features of the edge-sharing CшO6 octahedra and possibly the O–CuI–O linear bond. The substitution of Cr by Zn induces abrupt changes in frequency and inline width of the A 1g and E g modes. The shift of the two frequencies to shorter wave numbers, indicating a weak (Cr, Zn)–O bonding, is consistent with the observed lattice expansion along different axes and with the difference in ionic radii between Cr3+ and Zn2+. The A1g mode, in particular, is shifted by up to 33 cm−1, and the A1g mode frequency is very sensitive to (Cr, Zn)-site atom-oxygen bonding characteristics.

Fig. 3
figure 3

Raman spectra of the CuCr1−x Zn x O2 (x = 0, 0.03, 0.05, 0.07, 0.1) samples at room temperature (Color figure online)

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For these Zn-doped samples, we also note a strong decrease in the intensity of the initially strong band around 535 cm−1, and also the A 1g–E g mode magnitude ratio is reverted. We suggest that the decrease in intensity is related to the decreased disorder. For the delafossite CuCrO2 structure, the Cu cation is too small as compared with Cr cation, which results in tilt and twist of the CrO6 octahedral. Then, the frame around the Cu cation collapses, and local symmetry may be severely damaged. Since Cu ion and Zn ion are with a similar size, the doping of Zn ions maintains the delafossite structure, and the disorder of CrO6 octahedral is weakened. When the doping ratio reaches a certain value, local disorder disappears completely. Therefore, the doping of Zn ions in CuCrO2 structure plays an important role in stabilizing the phase structure.

The above results confirm that the substitution of Cr by Zn in the investigated concentration range will possibly maintain the delafossite structure of the oxides. However, local changes do arise because of doping. These changes are much more obvious in Raman spectroscopy than in XRD due to good local probe property of the former characterization method.

XPS analysis

Figure 4 shows the Zn-2p and Cr-2p spectra of the CuCr1-x Zn x O2 oxides. In the Zn-2p spectrum, two distinct peaks with two corresponding binding energies of Zn–2p3/2 = 1021.4 and Zn–2p1/2 = 1044.5 eV are observed, which indicates that the valence state of Zn ions is +2 through the comparison with the National Institute of Standards and Technology (NIST) X-ray photoelectron spectroscopy database. As shown in Fig. 4b, the two strong peaks in the Cr–2p spectrum at 576 and 585.7 eV for 2p3/2 and 2p1/2 peaks, respectively, indicate that the valence state of Cr ions is +3 [23]. Consequently, the oxidation state of Zn ions for the CuCr1−x Zn x O2 samples stably exists in divalent state as Zn2+ oxidation.

Fig. 4
figure 4

XPS of a Cr-2p, b Zn–2p for the CuCr1−x Zn x O2 samples (x = 0, 0.03, 0.05, 0.07, 0.1) (Color figure online)

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Besides, in the Zn-doped CuCrO2 system, the charge balance can be explained as below. Because of Zn-doping, the substitutional defect of “ZnCr” will be generated, which has a negative charge. At the same time, an oxygen atom vacancy defect of “Vo” will also be produced. Therefore, the overall charges are balanced in Zn-doped CuCrO2.

FTIR analysis

FTIR absorption band in the range of 4000–400 cm−1 is usually assigned to the vibration of ions in crystal lattice. As shown in Fig. 5, the bonding vibration of CuCr1−x Zn x O2 delafossite is within the range of 540–731 cm−1. For the five samples, the apparition of the peaks centered at 540 cm−1 is assigned to the absorption of Cu–O stretching vibration [24]. The peak around 704 cm−1 presents the Cr–O stretching vibrations in CrO6 distorted octahedral [25]. This is the energy of phonon vibration to cooperate with photon energy for electron transition in inter-band of allowed indirect gap. Besides, the peaks of the Cr–O stretching are 712, 718, 724, and 731 cm−1 for the CuCr1−x Zn x O2 samples with x = 0.03, 0.05, 0.07, and 0.1, respectively. The shift of the peak towards higher wavenumber is due to the Cr(Zn)–O stretching vibrations in weakened distorted octahedral as the substitution of Zn for Cr. In addition, the absorption of water stretching vibration peak around 3430 cm−1 can be observed. The intensity of the deformation vibration absorption peak near 1627 cm−1 slightly decreases with the increase of particle size. The reason is that, as the increase of the ratio between CuCr1−x Zn x O2 particle size and its specific surface area, the absorbed water volume increases.

Fig. 5
figure 5

FTIR spectra of CuCr1−x Zn x O2 (x = 0, 0.03, 0.05, 0.07, 0.1) at room temperature (Color figure online)

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Thermal analysis

Thermal analysis was carried out to measure the process of the stable product formation. The CuCrO2 powder was preheated at 300 °C in air in order to remove organic material, and thermo grams were recorded using a slowly heated sample with 10 °C/min from the ambient temperature to 1000 °C. Figure 6 shows the TG–DTA result of the CuCrO2 sample. It can be seen that there is a 4 % weight gain at 430 °C, accompanied with an exothermic process. This abrupt increase and oxide exothermic peaks between 430 and 700 °C are probably due to the chemical reaction between Cu, Cr, and O to form CuCr2O4 and CuO. The final 4 % weight loss occurs around 850 °C, along with an endothermic process. This abrupt decrease is probably caused by the transformation of the phase CuCr2O4 to the delafossite phase as the following reaction

Fig. 6
figure 6

TG–DTA pattern of the prepared CuCrO2 sample (Color figure online)

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$$ {\text{CuCr}}_{2} {\text{O}}_{4} + {\text{CuO}} \to 2{\text{CuCrO}}_{2} + \, 1/2{\text{O}}_{2} \uparrow $$

The TG–DTA pattern indicates that the sintering temperature for obtaining stable phase should be higher than 850 °C.

Electrical properties analysis

A Hall measurement was carried out on the CuCr1−x Zn x O2 (x = 0, 0.03, 0.05, 0.07, 0.1) pellets at room temperature for deciding carrier type and concentration. The results show that the Hall constant of the samples are given positive sign, indicating the carriers are holes and the samples are p-type conductors. The measured values of Hall constants (R H) are 15.36, 11.41, 9.81, 7.12, 5.94 (m3/C) for the samples with x = 0, 0.03, 0.05, 0.07, and 0.1, respectively. Then carrier concentration can be calculated from Hall constant by the formula p = 1/e R H, where e is electron charge. The hole carrier concentrations are calculated to be 4.07 × 1017, 5.48 × 1017, 6.37 × 1017, 8.78 × 1017, 1.05 × 1018 cm−3 for the five samples. The results show that the hole concentration gets higher with the increase of Zn-doping ratio.

The Seebeck coefficient (S) and the temperature-dependent conductance were simultaneously measured on the 2.5 mm × 2.5 mm × 15 mm pellets for CuCr1−x Zn x O2 (x = 0, 0.05, 0.1). As shown in Fig. 7a, the Seebeck coefficients of the CuCr1−x Zn x O2 samples are positive over the measured range. This confirms that the samples are p-type conductors and the major conductive carriers are holes. The Seebeck coefficient increases as temperature gets higher, which probably results from some effects such as thermal expansion, grain boundary barrier effect or the change of bandwidth and gap energy. The relationship between the Seebeck coefficient and temperature is related by S = (k B/e)[(E as /k B T) + A], where k B is Boltzmann’s constant, E as is the energy difference between the upper valence-edge energy (E v ) and the Fermi energy level (E F), T is absolute temperature, and A is a constant [26]. The value of E as is obtained by the linear plotting of S as a function of 1000/T, which corresponds to the Fermi energy level in semiconductor. The results of E as for x = 0, 0.05, 0.1 are approximately 53, 71 and 80.5 meV, respectively, which reveals that as the Zn content increases, the Fermi energy level of CuCr1−x Zn x O2 also increases.

Fig. 7
figure 7

Curves of a Seebeck coefficient and b logσ versus 1000/T for the CuCr1−x Zn x O2 (x = 0, 0.05, 0.1) samples

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For p-type CuCrO2 conductor, electron transportation is controlled by the polaron hopping mechanism of hole carriers caused by the mixed valence Cu+/Cu2+ on Cu–Cu layer [27]. The mixed valence Cu+/Cu2+ results from the oxygen non-stoichiometry value (y) intercalating into CuCrO2 structure, and thus CuCrO2+y is generated with a valence formula of \( \left[ {{\text{Cu}}^{ + }_{{1 - 2{\text{y}}}} {\text{Cu}}^{2 + }_{{2{\text{y}}}} } \right]{\text{Cr}}^{3 + } {\text{O}}_{{2 + {\text{y}}}} \). For p-type CuCr1−x Zn x O2, the electrical conduction is affected by the substitution of Zn2+ for Cr3+, producing the mixed copper valence [Cu+/Cu2+] as well as the valence formula of \( \left[ {{\text{Cu}}^{ + }_{1 - 2x} {\text{Cu}}^{2 + }_{2x} } \right]\left[ {{\text{Cr}}^{3 + }_{1 - x} {\text{Zn}}^{2 + }_{x} } \right]{\text{O}}_{2} \). Also, the polaron carriers can hop between Cu+ and Cu2+ sites on Cu–Cu plane and between Zn2+ and Cr3+ sites in octahedral layer for electrical conduction.

The electrical conductivity for a semiconductor can be described by the equation of σ = σ 0exp(−E aσ/k B T), where E aσ is the activation energy of conduction, T is the absolute temperature, and σ 0 is a constant [27]. For p-type CuCr1−x Zn x O2 (x = 0.03, 0.05, 0.07, 0.1), the activation energy corresponds to the energy for polaron used to hop between Zn2+ and Cr3+ sites, which can be directly obtained by the slope of the curve of logσ versus 1000/T. The curves of logσ versus 1000/T for the CuCr1−x Zn x O2 (x = 0, 0.05, 0.1) samples are shown in Fig. 7b. The negative slopes of the straight lines indicate positive value of activation energy of conduction. The obtained values of E aσ are 54, 41.5, and 32 meV for CuCr1−x Zn x O2 with x = 0, 0.05, 0.1, respectively. Also, the value of E aσ is found to decrease with the increase of Zn-doping concentration. The decrease of activation energy indicates that the polarons have low energy for hopping and thus for easy conduction.

Therefore, compared with the activation energy for polaron hopping between Cu+ sites and Cu2+ sites before Zn-doping, the energy between Zn2+ sites and Cr3+ sites after Zn-doping becomes lower. Also, the activation energy can be decreased by increasing the Zn-doping concentration. The observed decrease of activation energy suggests an increase of conductivity, which may be interpreted by oxygen vacancy migration as reported in other studies on delafossite materials [28–30]. Furthermore, due to the decrease of resistivity with the increase of temperature, we can draw the conclusion that the conductivity of the CuCrO2 and CuCr1−x Zn x O2 samples is thermally activated. A similar phenomenon can be found in the reported Ti-doped Fe2O3 by Shawn Chatman et al. [31]. Also, L. Farrell et al. found that the measured activation energy in Mg-doped Cr2O3 powder decreases even for low Mg-doping [32].

Conclusion

P-type delafossite CuCr1−x Zn x O2 (x = 0, 0.03, 0.05, 0.07, 0.1) oxides have been synthesized by sol–gel method. XRD and Raman spectra reveal the delafossite structure of the samples, and XPS results confirm that the chemical valence states of Cr and Zn ions are +3 and +2, respectively. FTIR spectra show the absorption of Cu–O and Cr–O stretching vibration. Seebeck and Hall coefficient reveal p-type conductive characteristics of the samples. Moreover, with the increase of Zn content, Fermi energy level increases, while the activation energy decreases. Therefore, the CuCr1−x Zn x O2 delafossite compound can be a good candidate for p-type transparent conductive oxide semiconductor materials.