Hydrothermal synthesis of delafossite CuFeO₂ crystals at 100 °C

Abstract

In this work, we first report a fast, facile hydrothermal method to synthesize delafossite CuFeO₂ crystals through low-temperature (100–160 °C) reaction. In detail, CuFeO₂ crystals with submicron-sized (100–300 nm) could be obtained based on hydrothermal reaction from the starting materials of Cu(NO₃)₂, FeCl₂ and NaOH at 100 °C for 12 h. Furthermore, the TG curve of CuFeO₂ is similar to other copper based delafossite oxides, and CuFeO₂ tend to be oxidized by O₂ in air at a higher sintering temperature (>400 °C). The obtained CuFeO₂ film has an optical transmission of 50–60% in visible region, and the optical bandgap for the direct band transition was estimated to be 3.18 eV. Finally, the magnetic hysteresis loops measured at room temperature show CuFeO₂ crystals exhibit antiferromagnetic behavior, the Néel temperature (T_N) about 12 K and the biggest magnetic susceptibility around 0.168 emu g⁻¹. This quick and facile method maybe opens a new route for preparation of CuFeO₂ crystals, due to the low reaction temperature and short reaction time.

---

1. Introduction

The crystal structure of delafossite oxides, deriving their name from the mineral CuFeO₂, was first confirmed by Pabst in 1946. The delafossite structure is constructed from alternate layers of two-dimensional close-packed copper cations with linear O–Cu⁺–O bonds and slightly distorted edge shared Fe³⁺O₆ octahedras.^1,2 Nowadays, CuFeO₂ is a p-type semiconductor of delafossite oxides and it has attracted much attention as a p-type TCO (Transparent Conducting Oxide) used for several applications such as transparent diodes, photocatalysts, photovoltaics, ferroelectrics and so on.^3–7 Generally, the CuFeO₂ powders could be synthesized through high temperature solid state reactions under an inert gas environments (Ar or N₂) at 900–1200 °C, because of Cu⁺ is more stable than Cu²⁺ at high temperature. However, the solid state reaction or sol–gel method need an post-treatment at high temperature,^8,9 and the high temperature is a serious defect for these preparation methods. Unfortunately, the preparation of powder is a required initial step, especially for several thin film deposition methods require target material, just like magnetron sputtering or pulsed laser deposition.¹⁰ Therefore, it remains a great challenge to fabricate CuFeO₂ powders by a low temperature method, especially desirable to obtain crystals with nano-sized, owing to the semi-conductive nano-materials were widely used in optoelectronic devices. For example, as reflected in previous works,^11,12 crystal size control of the p-type delafossite oxides (such as CuAlO₂, CuGaO₂ and CuCrO₂) are critical for the high performance of p-type dye-sensitized solar cells (DSSCs).

As we know, hydrothermal method (involves chemical reactions) relies on liquid phase transport of reactants to nucleate formation of the desired product, which is occurred in water above ambient temperature and pressure in a closed reaction system.^10,13 Many works reported the preparation of delafossite oxides through hydrothermal method, and the synthesized temperature can be dramatically reduced from the traditional synthesized temperature around 1000 °C to below of 400 °C.^13–17 Particularly, In 2006, K. R. Poeppelmeier and coworkers developed low temperature hydrothermal method to synthesize a serials of delafossite-type oxides at 210 °C, including CuMO₂ (M = Al, Sc, Cr, Mn, Fe, Co, Ga, and Rh).¹⁸ Later in 2012, M. Miyauchi and coworkers demonstrated a lower temperature hydrothermal method to synthesize CuFeO₂ crystals at 180 °C, but the crystal size is bigger than 4 μm.² Recently, M. M. Moharam and coworkers demonstrated hydrothermal synthesis of p-type transparent conducting oxides CuFeO₂ crystals at 280 °C, and the crystal size is also much bigger than 1 μm (1–5 μm).¹⁵ However, until now, we didn't find any reports focused on the hydrothermal synthesis of CuFeO₂ crystals with nano-sized. On the other hand, several groups including ours started the applications of other delafossite structure oxides with nano-sized, which were used as photocathodes in p-type DSSCs since 2012, such as CuAlO₂,^10,13 CuGaO₂,^19–21 CuCrO₂,^22–24 and AgCrO₂,²⁵ most of these delafossite oxides were synthesized through a facile hydrothermal method, and the synthesized temperature around 200 °C. On the basis of our previous works,^22–25 we attempted to synthesis the delafossite CuFeO₂ oxides through low temperature hydrothermal method, especially for CuFeO₂ with nano-sized.

Herein, we describe a facile, one step hydrothermal synthesis of delafossite oxides CuFeO₂ crystals through low-temperature (100–160 °C) hydrothermal methods in this work. We first synthesized submicron-sized (100–300 nm) CuFeO₂ crystals through hydrothermal reaction at 100 °C for 12 h. As far as we know, no prior studies were carried out for synthesis of CuFeO₂ crystals at this low reaction temperature and short reaction time. Moreover, the crystal phases and morphologies, compositions, and chemical states of elements, thermal stability, optical properties and magnetic properties of CuFeO₂ crystals have been studied, which is the first to elucidate the effects of the critical hydrothermal synthesis parameters, is of great significance.

2. Experimental sections

All of the chemicals in these experiments without specially notification were purchased from Sigma Aldrich with analytical grade and used without further purification. In a typical hydrothermal process, a certain amounts of reactants were dissolved in deionized water, and the obtained solution was transferred into Teflon-lined autoclave. The sealed autoclave was maintained at 100–160 °C for reaction. After the reaction finished, the autoclave was naturally cooled to room temperature. Finally, the obtained precipitate was washed for several times in a centrifugal cleaning machine and then stored in absolute alcohol solution for further use.

Powder X-ray diffraction patterns were collected at room temperature by using a Panalytical X'pert Pro diffractometer (XRD, Cu Kα radiation). A field emission scanning electron microscope (FESEM) system (FEI-Nova NanoSEM 450) coupled with energy dispersive X-ray spectroscopy (EDX) were used to observe the microstructure and determine the composition of the as-synthesized CuFeO₂ oxides. The thermal stability of CuFeO₂ oxides were investigated by a differential scanning calorimeters-thermo gravimetric analyzer (DSC-TG, Diamond TG/DTA, Perkin-Elmer Instruments) at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C. X-ray photoelectron spectroscopy measurements (XPS) were performed with a physical electronics surface analysis equipment (Model PHI 5600), and the C (1s) line (at 285.0 eV) corresponding to the surface adventitious carbon (C–C line bond) has been used as the reference binding energy. The ultraviolet-visible-near infrared (UV-vis-NIR) spectra of films were recorded on a Perkin-Elmer UV-vis spectrophotometer (UV-vis, Model Lambda 950) in the wavelength range of 300–800 nm. The magnetic hysteresis loops and magnetic susceptibility were measured by a magnetic property measurement systems (MPMS-XL-7, Quantum Design, Inc.).

3. Results and discussion

3.1 Hydrothermal synthesis of CuFeO₂ crystals

CuFeO₂ crystals were prepared according to our previously hydrothermal procedure for other copper based delafossite oxide, CuCrO₂ nanocrystals.^22,23 At first, 15 mmol Cu(NO₃)₂·3H₂O and 15 mmol FeCl₂ were dissolved in 70 mL deionized water at room temperature, 2.4–10.4 g NaOH was added to the above solution and stirred for 10 minutes. After reaching a homogeneous state, the solution was loaded into a 100 mL Teflon-lined autoclave, which was sealed and maintained at 100–160 °C for 12–48 h. After the reaction finished, the obtained black gray precipitate was washed with diluted ammonia solution (1 mol L⁻¹), deionized water and absolute alcohol in sequence for several times, and then stored in absolute alcohol solution. Different reaction parameters, such as the synthesis temperature (80, 100, 120, 140, and 160 °C), the mineralizer quantity (NaOH, 2.4, 3.4, 4.4, 6.4, and 10.4 g) and the reaction times (12 h, 24 h, 36 h and 48 h) have been examined to find out the optimal conditions, as summarized in Table 1.

Table 1 Details of the reactions conditions employed to synthesize CuFeO₂ crystals

Temp. (°C)	NaOH (g)	Time (h)	Phase composition
a Majority phase.b Minor phase.
80	4.4	24	Cu2O, Fe2O3
100	4.4	24	CuFeO2 (39-0246,^a 79-1546^b)
120	4.4	24	CuFeO2 (39-0246,^a 79-1546^b)
140	4.4	24	CuFeO2 (39-0246,^a 79-1546^b)
160	4.4	24	CuFeO2 (39-0246,^a 79-1546^b)
100	2.4	24	Cu2O, Fe2O3
100	3.4	24	CuFeO2 (39-0246,^a 79-1546^b)
100	6.4	24	CuFeO2 (39-0246,^a 79-1546^b)
100	10.4	24	CuFeO2 (39-0246,^a 79-1546^b)
100	4.4	12	CuFeO2 (39-0246,^a 79-1546^b)
100	4.4	36	CuFeO2 (39-0246,^a 79-1546^b)
100	4.4	48	CuFeO2 (39-0246,^a 79-1546^b)

---

To clarify the effect of hydrothermal temperature, a series of experiments have been carried out at different synthesis temperatures (80, 100, 120, 140, and 160 °C) for preparing CuFeO₂ crystals. For example, 15 mmol Cu(NO₃)₂, 15 mmol FeCl₂, 4.4 g NaOH were dissolved in 70 mL deionized water and kept the same, and the reaction time was set unchanged at 24 h. From Fig. 1a, when the reaction temperature decreases from 160 °C to 100 °C, it can be identified from the XRD patterns that the reaction products remained almost unchanged as CuFeO₂ crystals, and two structural polytypes of CuFeO₂ have been formed (see Table 1). Particularly, when the synthesis temperature was set at 100 °C, most of the diffraction peaks of products could be indexed as 3R-CuFeO₂ (rhombohedral, R-3m, JCPDS card no. 39-0246) from Fig. 1a, and very little 2H-CuFeO₂ (hexagonal, P63/mmc, JCPDS card no. 79-1546) also could be detected with a weak diffraction peak at 34.998° (denoted by “+”).

Fig. 1 (a) XRD patterns of products freshly obtained from the hydrothermal synthesis of CuFeO₂ crystals at different reaction temperatures (100–160 °C). Also shown are SEM images of products freshly obtained at different reaction temperatures ((b) 160 °C and (c) 100 °C).

But when further decreasing the reaction temperature to 80 °C, the obtained reaction products were in red color, may be due to the formation of Cu₂O and Fe₂O₃ during the reaction process, and no CuFeO₂ crystal phase can be identified from the XRD pattern of reaction product. It seems 100 °C is a critical temperature for the phase formation of CuFeO₂. In addition, Fig. 1b shows that both hexagonal and rhombohedral morphologies of CuFeO₂ crystals appear at the reaction temperature of 160 °C, the size of rhombohedral CuFeO₂ crystals are around 300–500 nm, and the CuFeO₂ crystals with hexagonal structure about 50 × 500 nm also appearing in the reaction products (Fig. 1b). Moreover, in comparison to the products generated at 160 °C and 100 °C, both the morphology and crystal size of CuFeO₂ crystals show a little difference (Fig. 1c). This result is consistent with the XRD patterns are shown in Fig. 1a, when the reaction temperature increased from 100 °C to 160 °C, the difference of diffraction peak's intensity for 3R-CuFeO₂ and 2H-CuFeO₂ can be ignored, indicating that the products have a similar morphology and crystal size. In addition, the integrated intensity of the diffraction peaks becomes more stronger with increasing the reaction temperature, which is also suggested that the CuFeO₂ crystals grew up slowly at a higher reaction temperature.

In the previous reports on the synthesis of other copper based delafossite oxides, such as CuGaO₂ (ref. 20 and 21) and CuAlO₂,¹⁰ the NaOH/KOH concentration (pH value) in hydrothermal precursor has a strongly impact on the morphology/crystal structure (3R or 2H) of the final reaction products. In order to obtain a single phase of 3R-CuFeO₂ crystals, a series of experiments have been carried out by adding different quantity of NaOH (2.4, 3.4, 4.4, 6.4, and 10.4 g) in the precursor, which contains 15 mmol Cu(NO₃)₂ and 15 mmol FeCl₂ and kept the same. Moreover, all these hydrothermal reactions were undertaken at 100 °C for 24 h. By comparing the XRD patterns in Fig. 2a, it is found that the CuFeO₂ crystals could be obtained at various addition amounts of NaOH (3.4–10.4 g), and two structural polytypes (3R and 2H) of CuFeO₂ have been formed with different NaOH addition. When the NaOH quantity was further adjusted to more than 3.4 g (such as 6.4 g and 10.4 g), the XRD diffraction peaks of these products could be similarly assigned to CuFeO₂. Furthermore, the diffraction peak at 34.998° owing to 2H-CuFeO₂ crystal phase (denoted by “+”) does not show a clear change (Fig. 2a), indicating that the content of 2H-CuFeO₂ in reaction products have a little change with increasing the addition amount of NaOH. These XRD results are in good agreement with the SEM results as shown in Fig. 2c–e. All CuFeO₂ crystals displayed with hexagonal and rhombohedral morphologies, and the content of two structural polytypes (3R and 2H) in reaction products have no obviously change. Moreover, the sizes of 3R-CuFeO₂ and 2H-CuFeO₂ crystals grew up gradually, and the size of main crystal phases (3R-CuFeO₂) increases accordingly from ∼200 nm to ∼500 nm while as increasing the addition quantity of NaOH from 3.4 g to 10.4 g. The result demonstrated that the alkaline condition of precursor is in favour of CuFeO₂ crystal growth, which is consistent with our previous study of another delafossite oxide AgAlO₂.¹⁰ But when further decreasing the NaOH addition to 2.4 g, no CuFeO₂ but a mixture of Cu₂O (JCPDS card no. 78-2076) and Fe₂O₃ (JCPDS card no. 89-0597) can be identified from XRD patterns. The XRD result is consistent well with SEM image as shown in Fig. 2b. The crystal size of Cu₂O and Fe₂O₃ nanoparticles are less than 100 nm, and it's clear that the morphology of Cu₂O and Fe₂O₃ (Fig. 2b) is quite different from the others crystals (see Fig. 2c–e). Therefore, it suggests that the quantity of NaOH in precursors have a bigger effect on the morphology than crystal structure, but it is difficult to get pure 3R-CuFeO₂ crystal phase through the adjustment of the quantity of NaOH mineralizer. Thus, it seems not consistent with previous report on the role of NaOH mineralizer in synthesis reaction for delafossite oxides.¹⁰ One possible reason might be due to the reaction temperature as low as 100 °C in this work, which caused a limited activation energy for the crystal transformation, leading to both 3R-CuFeO₂ and 2H-CuFeO₂ crystal phases co-exist in reaction products.

Fig. 2 (a) XRD patterns of freshly obtained CuFeO₂ products from precursors with different NaOH quantities. Also shown are SEM images of freshly obtained CuFeO₂ products from precursors with different NaOH quantities: (b) 2.4 g, (c) 3.4 g, (d) 6.4 g, and (e) 10.4 g.

In addition, to study the effect of reaction time to reaction products, a series of experiments have been carried out at different reaction time (12, 24, 36, and 48 hours) for preparing CuFeO₂ crystals. 15 mmol Cu(NO₃)₂, 15 mmol FeCl₂ and 4.4 g NaOH were dissolved in 70 mL deionized water and kept the same, the reaction temperature was set unchanged at 100 °C. According to the XRD patterns in Fig. 3a, the similar diffraction peaks revealed that the reaction products remained unchanged while reaction time prolongs from 12 h to 48 h, and all these XRD patterns could be identified as CuFeO₂ crystals with two structural polytypes. For example, when the reaction time was set at 12 h, most of the diffraction peaks of products could be indexed as 3R-CuFeO₂ (rhombohedral, R-3m, JCPDS card no. 39-0246) from Fig. 3a, and very little 2H-CuFeO₂ (hexagonal, P63/mmc, JCPDS card no. 79-1546) also could be detected with a weak diffraction peak at 34.998° (denoted by “+”). Fig. 3b shows that both hexagonal and rhombohedral morphologies of CuFeO₂ crystals appear after 12 h reaction, the crystal size of rhombohedral and hexagonal CuFeO₂ around 100–300 nm and 50 × 500 nm were presented in the products, respectively. Moreover, in comparison to the products generated after 48 h reaction, both the morphology and crystal size of CuFeO₂ crystals show no obvious difference (Fig. 3c), this result is consistent with XRD results. However, the integrated intensity of these diffraction peaks become a little stronger when the reaction time prolonged from 12 h to 48 h (see Fig. 3a), which could be attributed to the CuFeO₂ crystals grew up slowly with a longer reaction time.

Fig. 3 (a) XRD patterns of freshly obtained CuFeO₂ products from different reaction time. Also shown are SEM images of freshly obtained CuFeO₂ products from different reaction time: (b) 12 h, (c) 48 h.

3.2 The chemical compositions of CuFeO₂ crystals

To analyze the chemical compositions of CuFeO₂ crystals, SEM-EDS mapping scan was employed to characterize the powders deposited on a Si wafer substrate. The measurement results are shown in Fig. 4. It can be observed that all of the Cu, Fe and O elements are homogeneously distributed, the element percentages of Cu (23.30 at%), Fe (24.08 at%), O (52.61 at%) are nearly consistent with the concentrations of their source materials in the hydrothermal precursor and match well with the stoichiometric proportion of CuFeO₂ (1 : 1.03 : 2.26).

Fig. 4 (a–c) EDS elemental mapping, (d) SEM image, and (e) elemental analysis report of the CuFeO₂.

Furthermore, the element chemical states of the CuFeO₂ crystals have been investigated by XPS. The corresponding results are shown in Fig. 5, the full-scale XPS spectrum shows only the Cu, Fe, O, and C at the sample surface (Fig. 5a), indicating the formation of single phase oxide as CuFeO₂. Fig. 5b show that the peaks located at approximately 932.5 eV and 952.4 eV are corresponding to the binding energies of Cu 2p_3/2 and Cu 2p_1/2, and Fig. 5c shows the signal of Cu⁺ peak (closed to 917 eV), all these signals could confirm the monovalent state of copper in the sample.^2,10 The peaks close to 711.1 eV and 725.6 eV (Fig. 5d) are corresponding to the binding energies of Fe 2p_3/2 and Fe 2p_1/2, which confirm the trivalent state of iron (Fe³⁺).²

Fig. 5 Typical XPS spectra of CuFeO₂: (a) survey spectrum, (b) Cu 2p, (c) Cu LMM Auger spectrum; (d) Fe 2p.

As mentioned above, we described a low temperature (100–160 °C) hydrothermal method for preparing submicron-sized (<300 nm) CuFeO₂ crystals. In comparison to recently reported hydrothermal synthesis of CuFeO₂ (>180 °C),^2,15,16 the reaction parameters in this work are much more gentle and energy saving. Though, after roughly optimizing the reaction temperature, NaOH quantity and reaction time in the precursors, the smallest size of as-synthesized CuFeO₂ around 300 nm could be achieved at the low temperature of 100 °C, which consists of both 3R-CuFeO₂ and 2H-CuFeO₂ crystal phases. However, the crystal size of CuFeO₂ oxides is much bigger than 100 nm, so it still has much room for further optimizations on the parameters such as concentrations of starting reactants, pH values of precursor, additives, etc.

3.3 Thermal stability of CuFeO₂ crystals

From the thermo-gravimetric (TG) curve of CuFeO₂ powders is shown in Fig. 6a, the TG result of CuFeO₂ is similar to other copper based delafossite oxides, such as CuAlO₂ and CuGaO₂.^9,10 The initial weight loss may be due to the evaporation of chemically combined water of crystallization and the variation of oxygen vacancy in the sample. Fig. 6a shows that the mass of CuFeO₂ increases sharply above 500 °C in air, which should be due to the oxidation of CuFeO₂ (the monovalent copper (Cu⁺) was oxidized into divalent copper (Cu²⁺)).^9,10 The XRD patterns (Fig. 6b) of the samples sintered at different temperatures (such as 400 °C, 500 °C and 600 °C) support this point. For example, all of the diffraction peaks could be indexed as pure CuFeO₂ after the sample was sintered at 400 °C in air for 1 hour, and no impurity phase could be detected. After the sample was sintered at 500 °C, besides CuFeO₂, the diffraction peaks owing to new crystal phases of CuO (JCPDS #65-2309, denoted by “*”) and CuFe₂O₄ (JCPDS #34-0425, denoted by “#”) could be detected, which is suggested to be derived from partial oxidization of CuFeO₂ crystals. Moreover, the diffraction peaks intensity of CuO and CuFe₂O₄ phases became much stronger after CuFeO₂ sample was sintered at 600 °C, suggesting that the oxidization reaction of delafossite CuFeO₂ has been almost completed. On the basis of the consistent XRD and TG results, it is suggested that the following chemical reaction should be involved during the high temperature (>400 °C) sintering:

4CuFeO₂ + O₂ = 2CuFe₂O₄ + 2CuO (1)

Fig. 6 Thermogravimetric (TG) curve of (a) CuFeO₂ at a heating rate of 10 °C min⁻¹ in air. Also shown are the corresponding XRD patterns for (b) CuFeO₂ powder after sintering in air at different temperatures.

According to eqn (1), the full oxidization of CuFeO₂ should lead to theoretical mass increase of 5.28%, but the tested mass increase was only 4.26% (Fig. 6a). This issue may be due to the fast temperature ramping process (10 °C min⁻¹) was applied during TG analysis, resulting in a small difference between underestimate on the mass changes and calculated theoretical values on the basis of the chemical reaction.¹⁰

3.4 Optical properties of CuFeO₂ crystals

In order to study the optical properties of CuFeO₂ crystals, uniformity films were deposited on the glass slide through spray deposition method according to our previous works.^10,25 After heated in air at 300 °C for 1 h, the deposited films were examined by the UV-vis spectroscopy. The optical bandgaps are estimated by the following equation:^26,27

(αhν)^1/n = A(hν − E_g) (2)

where α, h, ν, A, and E_g are the absorption coefficient, Planck constant, the frequency of light, a constant, and the band gap, respectively. Moreover, the exponent n depends on the type of transition, for direct-allowed transition, n = 1/2; for indirect-respectively. Moreover, the exponent n depends on the type of transition, for direct-allowed transition, n = 1/2; for indirect-allowed transition, n = 2.^24,25 Fig. 7 show the optical transmittance spectra within the wavelength range of 300–800 nm and the calculated bandgap of CuFeO₂ film. In detail, the average optical transmittance of black gray CuFeO₂ film (the thickness about 1.0 μm) is around 50% to 60% in visible range. The direct bandgap energy of CuFeO₂ is around 3.18 eV (inset in Fig. 7), and this calculated value is very close to the earlier reports of CuFeO₂ (3.10–3.38 eV).^28,29

Fig. 7 The optical transmittance and the calculated bandgap energy of CuFeO₂ films.

3.5 Magnetic properties of CuFeO₂ crystals

At last, the magnetic properties of CuFeO₂ powder were measured on the superconducting quantum interference device (MPMS-XL-7). Both the M–H curve measured under 300 K and temperature dependences were measured under an applied field of 1000 Oe of CuFeO₂ oxides are shown in Fig. 8, and the sharp peak near 10 K and small divergence at low temperatures are visible. According to the M(T) form this magnetic transition can be identified as an antiferromagnetic one, it is in good agreement with others reports.^30,31 The antiferromagnetic transition of as-prepared CuFeO₂ occurs at ∼12 K (Néel temperature, T_N),³² and the biggest value of magnetization around 0.168 emu g⁻¹. The high-temperature (>25 K) susceptibility is well fitted by Curie–Weiss equation: χ_m = C/(T − θ), with θ = 110 K indicating on the predominance of ferromagnetic interactions in the CuFeO₂ system.

Fig. 8 M–H curve for CuFeO₂ powder measured at 300 K, the lower inset shows the magnetization curves measured under an applied field of 1000 Oe.

4. Conclusions

In summary, we first report a fast, facile preparation method of delafossite oxides CuFeO₂ crystals through a low-temperature (100–160 °C) hydrothermal synthesis from all inorganic starting materials. The synthesis conditions such as reaction temperature, pH value (NaOH amount) and reaction time, have a significant effect on the crystal size and morphology of CuFeO₂. In particular, the CuFeO₂ crystals could be obtained based on the hydrothermal reaction from the starting materials of Cu(NO₃)₂, FeCl₂ and NaOH, and the synthesize temperature for CuFeO₂ have been expanded to as low as 100 °C. It is noteworthy that we have successful synthesized submicron-sized (100–300 nm) CuFeO₂ crystals through one single step hydrothermal reaction at 100 °C for 12 h for the first time. The TG result show that the CuFeO₂ oxides tend to be oxidized by O₂ in the air at a higher sintering temperature (>400 °C). Moreover, the obtained CuFeO₂ film was transparent, presents an optical transmission around 50–60% in visible range, and the optical bandgap energy was estimated to be 3.18 eV. Finally, the magnetic measurements revealed CuFeO₂ oxides exhibit antiferromagnetic behaviour. This quick and facile method may be opens a new route for the preparation of submicron-sized CuFeO₂ crystals, and the significant advantages in this method are lower reaction temperature and shorter reaction time. But there is a big problem to obtain a pure single CuFeO₂ crystals phase with nano-sized, such as 3R-CuFeO₂ or 2H-CuFeO₂. Thus, there is a lot of works need to be done in this fields, and some related works is under carrying out in our laboratory.

Acknowledgements

The authors would like to express sincere thanks for the financial support by the National Natural Science Foundation of China (nos 21103058, 51402223, 51461135004), 973 Program of China (no. 2011CBA00703), Doctoral Fund of Ministry of education priority development projects (no. 20130143130002), China Postdoctoral Science Foundation (no. 2014M552098) and Fundamental Research Funds for the Central Universities (WUT: 2014-IV-091).

References

1. R. Seshadri, C. Felser and K. Thieme, et al., Chem. Mater., 1998, 10(8), 2189–2196.
2. X. Qiu, M. Liu and K. Sunada, et al., Chem. Commun., 2012, 48(59), 7365–7367.
3. S. Kato, R. Fujimaki and M. Ogasawara, et al., Appl. Catal., B, 2009, 89(1), 183–188.
4. L. Zhang, P. Li and K. Huang, et al., Mater. Lett., 2011, 65(21), 3289–3291.
5. M. S. Prévot, N. Guijarro and K. Sivula, ChemSusChem, 2015, 8(8), 1359–1367.
6. Y. Tanaka, N. Terada and T. Nakajima, et al., Phys. Rev. Lett., 2012, 109(12), 127205.
7. C. G. Read, Y. Park and K. S. Choi, J. Phys. Chem. Lett., 2012, 3(14), 1872–1876.
8. L. Lu, J. Z. Wang and X. B. Zhu, et al., J. Power Sources, 2011, 196(16), 7025–7029.
9. A. P. Amrute, Z. Łodziana and C. Mondelli, et al., Chem. Mater., 2013, 25(21), 4423–4435.
10. D. Xiong, X. Zeng and W. Zhang, et al., Inorg. Chem., 2014, 53(8), 4106–4116.
11. F. Odobel and Y. Pellegrin, J. Phys. Chem. Lett., 2013, 4, 2551–2564.
12. M. Yu, T. I. Draskovic and Y. Wu, Phys. Chem. Chem. Phys., 2014, 16, 5026–5033.
13. S. Gao, Y. Zhao and P. Gou, et al., Nanotechnology, 2003, 14(5), 538.
14. T. Sato, K. Sue and H. Tsumatori, et al., J. Supercrit. Fluids, 2008, 46(2), 173–177.
15. M. M. Moharam, M. M. Rashad and E. M. Elsayed, et al., J. Mater. Sci.: Mater. Electron., 2014, 25(4), 1798–1803.
16. Y. Dong, C. Cao and Y. S. Chui, et al., Chem. Commun., 2014, 50(70), 10151–10154.
17. T. W. Chiu and P. S. Huang, Ceram. Int., 2013, 39, S575–S578.
18. W. C. Sheets, E. Mugnier and A. Barnabe, et al., Chem. Mater., 2006, 18(1), 7–20.
19. M. Yu, G. Natu and Z. Ji, et al., J. Phys. Chem. Lett., 2012, 3(9), 1074–1078.
20. Z. Xu, D. Xiong and H. Wang, et al., J. Mater. Chem. A, 2014, 2(9), 2968–2976.
21. M. Yu, T. I. Draskovic and Y. Wu, Inorg. Chem., 2014, 53(11), 5845–5851.
22. D. Xiong, Z. Xu and X. Zeng, et al., J. Mater. Chem., 2012, 22(47), 24760–24768.
23. D. Xiong, W. Zhang and X. Zeng, et al., ChemSusChem, 2013, 6(8), 1432–1437.
24. X. Xu, B. Zhang and J. Cui, et al., Nanoscale, 2013, 5(17), 7963–7969.
25. D. Xiong, H. Wang and W. Zhang, et al., J. Alloys Compd., 2015, 642, 104–110.
26. Y. F. Wang, Y. J. Gu and T. Wang, et al., J. Sol-Gel Sci. Technol., 2011, 59(2), 222–227.
27. R. S. Yu and C. P. Tasi, Ceram. Int., 2014, 40(6), 8211–8217.
28. H. Y. Chen and J. H. Wu, Appl. Surf. Sci., 2012, 258(11), 4844–4847.
29. Z. Deng, X. Fang and S. Wu, et al., J. Alloys Compd., 2013, 577, 658–662.
30. V. Eyert, R. Frésard and A. Maignan, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78(5), 052402.
31. D. Zhang, J. Zhu and N. Zhang, et al., Sci. Rep., 2015, 5, 8737.
32. T. Zhao, M. Hasegawa and H. Takei, J. Cryst. Growth, 1996, 166(1), 408–413.

---