Controlled and stepwise generation of Cu₂O, Cu₂O@Cu and Cu nanoparticles inside the transparent alumina films and their catalytic activity

Abstract

A stepwise generation of Cu₂O, Cu₂O@Cu core-shell and Cu nanoparticles (NPs) inside the alumina films has been accomplished. First, CuO–Al₂O₃ films were prepared on glass substrates by dip-coating method using alumina sol derived from partially acetylacetonate (acac) chelated aluminium-tri-sec-butoxide doped with copper(II) chloride, followed by heat-treatment at 450 °C in air. Heat-treatment of CuO–Al₂O₃ films at 450–500 °C in reducing atmosphere resulted in the formation of stable Cu₂O NPs which can be subsequently converted to Cu₂O@Cu core-shell NPs with tunable Cu-shell thickness, and followed by pure Cu NPs by further heat-treatment at 550–700 °C. UV–visible, transmission electron microscopy (TEM) and grazing incidence X-ray diffraction (GIXRD) studies of the films in different stages confirmed the generation of Cu₂O, Cu₂O@Cu core-shell and Cu NPs. It has been understood that the chemical structure and property of matrix alumina facilitated the stabilization of Cu₂O NPs inside the films. As a result, the formation of Cu₂O@Cu NPs can be achieved easily by controlled heat-treatment in reducing atmosphere. The catalytic properties of Cu₂O, Cu₂O@Cu and Cu NPs incorporated alumina films were studied by monitoring the degradation of an azo dye congo-red. All films showed very good catalytic activities; among these Cu₂O NPs doped films showed excellent stability, reusability and high rate constant values.

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Introduction

As a p-type semiconductor (direct band gap 2.17 eV) with unique optical and electrical properties,^1,2 cuprous oxide (Cu₂O) is a promising material with potential applications in solar energy conversion,³ magnetic storage devices,⁴ biosensors,⁵ catalysis,⁶ antibacterial activity⁷ and gas sensors.⁸ Cu₂O micro and nanostructures with various shapes such as cubic⁹ and hollow cubic,¹⁰ hollow spherical,¹¹ octahedral,¹² triangular platelike¹³ and pyramidal¹⁴ as well as core-shell nanocomposites^15–17 following different synthetic protocols such as electrodeposition,¹⁸ arc discharge,¹⁹ sonochemical²⁰ and polyol reduction methods²¹ have been synthesized. A few reports are available in the literature regarding the synthesis of Cu₂O@Cu core-shell NPs in solution.^15–17 It is noteworthy that generation of a Cu₂O shell on Cu core (Cu@Cu₂O) is quite trivial due to the easy generation of an oxide shell when Cu metal NPs are exposed to the atmosphere. However, the reverse type of core-shell, i.e. Cu₂O@Cu composite particles in the nano regime, is rare in the literature. Synthesis of few micron sized Cu₂O@Cu particles was reported, where the outer surface of Cu₂O had been partly converted to Cu after addition of strong reducing agent like hydrazine.^22,23 Another report showed the conversion of hydrothermally synthesized Cu₂O to Cu₂O@Cu particles having size of 5–6 microns by increasing the aging time of the reaction mixture.²⁴ Besides these solution based synthetic protocols, formation of Cu₂O and Cu₂O@Cu NPs in a dielectric film matrix is not reported in the literature. The semiconductor core-metal shell composite NPs are of increasing scientific and technological interest due to an expanding and wide range of applications.^25,26 The optical properties of the nano-shell are often influenced by the properties of the core material.²⁵ Efficiency of photocatalytic reactions can be maximized by promoting inter-facial charge transfer processes in these composite systems.^26b Tuning the sensing properties and tailoring the photoelectrochemical properties of the core materials remain a major challenge of designing semiconductor core-metallic shell composite systems. It can be noted that, in general, bulk cupric oxide (CuO) transforms to metallic Cu when heat-treated in reducing atmosphere, without the formation of stable intermediate phases such as Cu₂O.²⁷ Further, in SiO₂ matrix, Cu₂O NPs are not found to be formed as a stable intermediate phase,²⁸ as Cu²⁺ reduces mainly to Cu⁰. We found that the Cu⁺ state can be stabilized inside the alumina film matrix and it is possible to generate subsequently a core-shell structure comprising Cu₂O as core and Cu as shell (Cu₂O@Cu) from the initially formed Cu₂O NPs by controlled heat-treatment in reducing atmosphere. Finally this core-shell structure can be transformed to pure Cu NPs inside the alumina film. In this paper we report an easy and step-by-step synthesis of Cu₂O (semiconductor), Cu₂O@Cu (semiconductor core and metal shell) with tunable Cu-shell thickness and Cu (metal) NPs inside Al₂O₃ film matrix starting from a sol–gel derived Cu²⁺ doped Al₂O₃ film. As these NPs are embedded in the transparent alumina film matrix, use of the films as catalyst could be advantageous over bare NPs, thus ensuring all the unique characteristics of catalyst film.²⁹ Keeping in view the cost-effectiveness compared to the use of expensive NPs for catalytic process,³⁰ involvement of complicated photocatalytic,³¹ sonophotocatalytic³¹ and γ-ray irradiation processes,³² the catalytic behaviour of Cu₂O, Cu₂O@Cu and Cu NPs embedded alumina films towards the decomposition of congo-red dye has been studied and reported as one of the applications.

Experimental

Materials

All chemicals were used as received. Aluminum-tri-sec-butoxide Al[OCH(CH₃)C₂H₅]₃ (ASB) and congo-red (C₃₂H₂₂N₆Na₂O₆S₂) were supplied by Sigma-Aldrich; while acetylacetone (acac), 1-propanol and 2-butanol were obtained from s.d. fine-chem limited. CuCl₂·2H₂O was obtained from Qualigens fine chemicals. Mili-Q (Millipore) water (18 MΩ) was used throughout the study. Compressed gas mixture 10% hydrogen, balance argon was obtained from BOC India Ltd.

Preparation of ASB Stock Solution and Cu doped ASB sol

The ASB stock solution was prepared following the procedure reported in our previous paper with modifications.³³ First, a partially acetylacetonato complexed ASB stock solution was prepared in the following way: the required amount of ASB was quickly transferred to a 1-propanol–2-butanol (3 : 2 w/w ratio) solution of acac with stirring at room temperature and the resulting mixture was stirred for another 1 h in a sealed container. The molar ratio of acac/ASB used was 0.4, and this acac modified sol is designated as ASB_0.4acac. The concentration of ASB in this partially acetylacetonato complexed ASB stock solution was about 35 wt%. The required amount of CuCl₂·2H₂O dissolved in water and 1-propanol mixture was mixed with the sol with stirring. Subsequently two drops (∼0.0012 g) of concentrated HNO₃ was added in the sol and the stirring was continued for another 30 min. The Cu concentration was maintained at 10 mol% with respect to the equivalent amount of AlO_1.5 present in the sol. In the final sol, the required amount of 1-propanol was added to adjust the AlO_1.5 concentration to 4.5 equivalent wt%.

Preparation of Cu₂O, Cu₂O@Cu and Cu NPs doped alumina films

The above-mentioned sol was used for film deposition on glass substrates. Prior to film deposition, the soda-lime and silica glass slides were thoroughly cleaned as usual. The films were prepared from Cu doped sol by a single dipping technique with a withdrawal velocity of 20 cm min⁻¹. The as prepared films were dried at 60 °C for 30 min. After that the films were heat-treated at 450 °C (ramp at 2 °C min⁻¹) in air with a holding time of 1 h. For the preparation of Cu₂O NPs embedded film, these films were heat-treated at 450 °C in 10% H₂ and 90% Ar (will be designated as H₂–Ar hereafter) atmosphere for 30 min with a gas flow rate of 45 bubbles per min. The number of gas bubbles was counted at the gas outlet using a water filled Dreschel bottle. Further heat-treatment of this film at 460, 480 and 500 °C with 30 min holding time in each step in a cumulative heating procedure in H₂–Ar with the same gas flow rate resulted in the formation of Cu₂O@Cu NPs inside the film. Cu NPs doped films were obtained by heat-treating these Cu₂O@Cu NPs doped films at 550–700 °C in H₂–Ar with a holding time of 1 h.

Characterization techniques

The UV–vis spectra of coatings deposited on glass substrates were obtained using a Cary 50 scan spectrophotometer. Infrared absorption spectra of the sols (by placing the liquid sample between two KBr windows) and films (deposited on Si-wafers) were recorded by FTIR spectrometer (Nicolet 380) with 200 scans for each sample. Thickness and refractive index measurement of the films deposited on Si wafer was done using a J. A. Woollam Co. M 2000 spectroscopic ellipsometer. GIXRD patterns of the films were recorded by using Rigaku SmartLab operating at 9 kW (200 mA and 45 kV) using Cu-Kα (λ = 1.5406 Å) radiation. 0.3° grazing incidence angle was used for all the measurements. Field emission scanning electron microscopic (FESEM) images were taken using ZEISS SUPRA 35VP. TEM measurements were carried out with a Tecnai G² 30ST (FEI) operating at 300 kV. For the TEM study scratched off film samples were placed on the carbon coated Mo grids followed by a drop of methanol and dried at room temperature. The energy dispersive X-ray scattering (EDS) analyses of the Cu doped films were obtained by the EDS facilities attached with TEM.

Catalytic activities

Cu₂O, Cu₂O@Cu and Cu NPs doped alumina films were used to study the catalytic activities for the decomposition of congo-red dye. The catalytic studies were performed using one piece of both side coated doped alumina film deposited on soda lime glass substrate of approximate dimensions 1.25 cm × 0.8 cm × 260 nm (film thickness) and placing them in the reaction mixture of 2.95 ml of 0.5 × 10⁻⁴ M congo-red dye solution and 0.05 ml of 0.15 M NaBH₄ solution in the cuvette cell in such a way that the surfaces of the catalysts should not touch or overlap each other. The reaction was studied at 25 °C with constant stirring to ensure the homogeneous mixing of the reaction mixture. The evolution of absorption spectra due to the decomposition of congo-red dye was recorded using a Cary 50 scan UV–visible spectrometer attached with a Peltier temperature controller in kinetic mode operation. Spectra were also recorded in absence of catalysts. To test the reusability of the films, the used films were washed with distilled water and then dried at 60 °C for 10 min. These films were then ready for next time use. Reaction conditions were maintained similarly in all cases using Cu₂O, Cu₂O@Cu and Cu NPs embedded alumina films.

Results and discussion

Stepwise generation of Cu₂O, Cu₂O@Cu and Cu NPs inside the alumina films

10 mol% CuCl₂ doped alumina sol was used to prepare coatings on soda-lime and silica glass substrates. As prepared and 60 °C dried films were transparent and faintly greenish in colour. Films were first heat-treated in air at 450 °C to remove the organics. At this stage Cu remains as Cu²⁺ (CuO) in the alumina film. These air-annealed films (CuO/Al₂O₃) were, when heat-treated at 450 °C in H₂–Ar, transformed to yellow-greenish, and Cu₂O NPs are supposed to be formed (designated as Film A). Heat-treatment of this film at 460, 480 and 500 °C in H₂–Ar led to the formation of reddish-yellow films. These alumina films are expected to contain Cu as well as Cu₂O species and are designated as Films B (460 °C), C (480 °C) and D (500 °C). Further heat-treatment of the film at 550–700 °C in H₂–Ar for 1 h yielded pure Cu NPs inside the alumina film (550 °C and 700 °C heat-treated films have been designated as Films E and F). All the heat-treated films were visually transparent and crack free.

UV–visible studies

Formation of the Cu₂O, Cu₂O@Cu and Cu NPs inside the alumina film was first indicated by the UV–visible spectrometry. UV–visible spectra of the films along with the air-annealed CuO/Al₂O₃ film are shown in Fig. 1a. The air-annealed film (CuO/Al₂O₃) shows a very weak and broad peak centred at ∼700 nm (Fig. 1a, curve i, enlarged portion shown in the inset as ‘A’) due to the d–d transition of Cu²⁺ (3d⁹ system). The observed d–d transitions are in the range expected for Cu²⁺ species in an axially distorted octahedral (due to Jahn–Teller effect) environment of O-containing ligands.^28a Film A (annealed at 450 °C in H₂–Ar) shows weak absorption peak at ∼350 nm which is attributed to the band-to-band transition,¹⁵ indicating the presence of Cu₂O (Fig. 1a, curve ii). Besides this, two intrinsic band-gap absorptions of Cu₂O can be observed near 450 nm and 585 nm (weak).³⁴ Films C and D (annealed at 480 and 500 °C in H₂–Ar) show almost similar absorption behaviour as that of Film A, but a weak absorption peak can be observed near 575 nm (Fig. 1a, curves iii and iv). This gives an indication that some Cu NPs have been formed at these stages because the 575 nm band can be assigned as the surface plasmon resonance (SPR) band of Cu NPs. Film F shows an intense peak at 575 nm due to Cu-SPR indicating the presence of Cu NPs in the film (Fig. 1a, curve v). In this case absence of lower wavelength peaks (as observed in cases of Films A, C and D) indicates that all Cu₂O NPs have been converted to Cu NPs.

Fig. 1 (a) Optical absorption spectra of CuCl₂ doped alumina films after heat-treatment at different temperatures in cumulative heating procedures: (i) 450 °C in air, (ii) 450 °C in H₂–Ar/30 min (Film A), (iii) 480 °C in H₂–Ar/30 min (Film C), (iv) 500 °C in H₂–Ar/30 min (Film D) and (v) 700 °C in H₂–Ar/1 h (Film F). A magnified view of graph ‘i’ has been shown in inset (marked by A) indicating the presence of peak originated from Cu²⁺. (b) Band gap evaluation from the plot of (αE_photon)²vs. E_photon for Cu₂O NPs embedded in alumina film.

Fig. 1b shows the absorption band-gap (E_g) values of the Cu₂O NPs embedded alumina film calculated from the absorption curve of Film A (Fig. 1a, curve ii). Band gap energy was estimated using the following equation for semiconductor¹⁶

αhν = α₀(hν − E_g)^1/n

The term hν represents the photon energy, α is the absorption coefficient, α₀ is a constant and n has a value of either 2 for a direct transition or ½ for an indirect transition. The optical band-gap is the extrapolated value (the straight line to the X-axis of Fig. 1b) of E_photon at α = 0 from the plot of (αE_photon)²vs. E_photon, and gives the absorption edge energy corresponding to E_g = 2.19 eV. The evaluated band gap is little higher than the direct band gap value of bulk Cu₂O (2.0–2.17 eV).

FTIR study

FTIR spectrum of ASB_0.4acac shows peaks at ∼1597 (ν_C–C + ν_C–O) and 1533 cm⁻¹ (ν_C–O + ν_C–C) corresponding to the chelated acac groups attached to Al (Fig. 2).³⁵ After doping of CuCl₂ into ASB_0.4acac, the C–C + C–O combined peak is slightly shifted to lower wavelength. This shifting, evident from both the sol (Fig. 2a) and the film (Fig. 2b), indicates the possibility of formation of Cu–acac linkage to some extent. FTIR spectra of ASB_0.4acac as well as CuCl₂ doped sol and film confirm the absence of any peak in the 1700 cm⁻¹ region due to free acetylacetone. It indicates that ring opening of acac chelates has not occurred due to the addition of catalytic amount of HNO₃ during CuCl₂ doping. The spectra of the sol also show the presence of an H–O–H bending vibration at 1645 cm⁻¹ which is due to the water introduced to dissolve CuCl₂ and also from HNO₃. FTIR spectra of the 450 °C heat-treated undoped alumina and the Film A (Cu₂O/Al₂O₃) (Fig. 2c) show complete absence of organic peaks and the signature of γ-Al₂O₃.^29a,35a In the case of Film A, the peak at 617 cm⁻¹ (see the magnified region in Fig. 2c) corresponds to the vibrational mode of Cu–O in Cu₂O phase.³⁶

Fig. 2 FTIR spectra of (a) ASB_0.4acac and Cu doped ASB_0.4acac sols diluted with 1-propanol; (b) corresponding coatings derived from these sols dried at room temperature. In both cases, similar mol% of AlO_1.5 content was maintained. (c) FTIR spectra of undoped alumina film heat-treated at 450 °C in air (1 h) and Film A revealing the formation of γ-Al₂O₃ and presence of Cu₂O in γ-Al₂O₃, respectively. The Y-axis has been shifted for clarity reason.

GIXRD study

The CuO/Al₂O₃ film heat-treated at 450 °C in air shows no crystalline CuO related peaks (Fig. S1; ESI†) indicating existence of either amorphous CuO³⁷ or Al–O–Cu species. The formation and structural evolution of the NPs in alumina film matrix were observed when the film was heat-treated in H₂–Ar. This can be nicely followed by GIXRD as shown in Fig. 3. GIXRD of Film A (Fig. 3a) shows peaks which match well with the Bragg reflections of the standard cubic cuprite (Cu₂O) structure (Pn3m, lattice constant, a = 4.267 Å, JCPDS No. 00-005-0667). Film B (Fig. 3b) and C (Fig. 3c) show both diffraction peaks corresponding to Cu₂O and Cu (JCPDS no. 00-004-0836) indicating the conversion of some amount of Cu₂O to Cu NPs. GIXRD of Film D (Fig. 3d) also shows both diffraction peaks due to Cu₂O and Cu but the intensity of Cu peak is greater compared to the Film C (Fig. 3c) indicating much more conversion of Cu₂O to Cu. This result indicates that by altering the temperature, and keeping all other parameters unchanged, the reduction of Cu₂O to Cu can be controlled; thereby the amount of Cu in the Cu₂O/Cu composite NPs can be tuned. When the film was further heat-treated at 550 °C and above in H₂–Ar atmosphere, only Cu peaks were observed in the GIXRD. Fig. 3e and f show the GIXRD patterns of Film E (550 °C/H₂–Ar) and F (700 °C/H₂–Ar), respectively. Thus the XRD results confirm a systematic and step wise generation of Cu₂O followed by Cu₂O/Cu and finally Cu NPs inside the alumina film. It can be pointed out here that, in the case of Film E (550 °C/H₂–Ar) the intensity of Cu peaks (Fig. S2a; ESI†) decreased significantly after 1 h of storing at ambient conditions; however, on the contrary, under similar conditions the Film F (700 °C/H₂–Ar) showed no such deterioration of Cu peak (Fig. S2b; ESI†). This result indicates more air stability of Film F compared to Film E, so high temperature annealing causes stability of Cu NPs. It should be noted that increased densification of film matrix and growth of small Cu NPs would be expected at high temperature which can prevent air-oxidation of Cu NPs. We did not observe any prominent peak due to the crystalline form of γ-Al₂O₃ even after annealing at 700 °C in the XRD patterns. This suggests formation of very small crystallites of γ-Al₂O₃ (confirmed by FTIR; see Fig. 2c) as also observed by other workers.^35a

Fig. 3 GIXRD patterns of Cu doped alumina films after heat-treatment at different temperatures in H₂–Ar atmosphere in cumulative heating procedures (a) 450 °C/30 min (Film A); (b) 460 °C/30 min (Film B); (c) 480 °C/30 min (Film C), (d) 500 °C/30 min (Film D), (e) 550 °C/1 h (Film E) and (f) 700 °C/1 h (Film F). Bragg reflection positions of Cu (JCPDS no. 00-004-0836) and Cu₂O (JCPDS no. 00-005-0667) with relative intensities are shown as vertical red (Cu) and blue (Cu₂O) dash lines, respectively.

FESEM study

Fig. 4 shows the cross-section of the Cu₂O NPs embedded alumina film observed by the field emission scanning electron microscope. This study revealed that the film had almost uniform thickness close to 260 nm which matched well with the thickness value measured by the ellipsometer. Relatively big NPs can be seen clearly in the cross-section portion.

Fig. 4 FESEM cross-sectional image of the Cu₂O NPs embedded alumina film (Film A) showing the presence of NPs along the cross-section. The double-headed arrow represents the film thickness.

TEM study

TEM study of CuO/Al₂O₃ film shows that no discrete CuO NP is forming at this stage (Fig. S3a; ESI†) which is consistent with the GIXRD results (Fig. S1; ESI†). The corresponding EDS pattern (Fig. S3b; ESI†) shows the presence of Al, Cu and O in the sample. To observe the morphology of the NPs formed in the alumina films heat-treated in reducing atmosphere, systematic TEM studies of films at three different stages i.e. Film A (Cu₂O), D (representative Cu₂O/Cu) and F (Cu) were undertaken. Fig. 5 represents the TEM image of Cu₂O NPs embedded in alumina film (Film A). Mostly spherical NPs having size range of 100–150 nm as well as smaller NPs of dimension 15–20 nm are found to co-exist in the alumina matrix at this stage. The magnified view of some of these Cu₂O NPs has been shown in Fig. 5b. TEM also gives an indication that the matrix Al₂O₃ has porous structure as evident from Fig. 5b. The EDS pattern (Fig. 5c) taken from the TEM image (Fig. 5a) confirms the presence of Cu, O and Al. Peaks of Mo are from the Mo grid used for TEM study. The corresponding selected area electron diffraction (SAED) pattern (Fig. 5d) obtained from the low resolution TEM image (Fig. 5a) shows the spots characteristics of Cu₂O. High resolution TEM image (Fig. 5e) of one portion of a Cu₂O NP shows the clear fringes attributed to the (111) planes of Cu₂O. TEM images of Film D are shown in Fig. 6. From the low resolution image (Fig. 6a) a core-shell type NP formation can be detected. The magnified image of a single NP (marked as A) is shown in the inset of Fig. 6a. As shown in the figure, the darker contrast developed on the particle surface and the light contrast of the particle interior are composed of Cu and Cu₂O, respectively. This is confirmed further from the SAED pattern, which was taken from the marked NP of Fig. 6a, showing the spots corresponding to both Cu and Cu₂O. The HRTEM image of the dark region shows characteristic lattice fringes of Cu (111) (Fig. 6c), whereas the inner lighter region shows lattice fringes corresponding to Cu₂O (111) (Fig. 6d). Thus the TEM study confirms the formation of Cu as shell on Cu₂O core. The TEM image of Film F (Fig. 7) shows the presence of spherical NPs embedded in alumina film. SAED pattern (shown in inset of Fig. 7a) shows the spots corresponding to Cu NPs only at this stage. HRTEM image shows the characteristic lattice fringes of Cu (Fig. 7b).

Fig. 5 (a and b) TEM images of the Cu₂O doped alumina film (Film A) with different magnifications; (c) EDS spectrum showing the presence of different elements in the film, Mo is from the Mo grid used for TEM study; (d) SAED pattern obtained from (a); (e) high resolution TEM image of a portion of one Cu₂O NP.

Fig. 6 (a) TEM images showing the existence of Cu₂O@Cu NPs in the alumina film; magnified image of one NP (marked as A) has been shown in the inset indicating the core (Cu₂O) and shell (Cu); (b) SAED pattern taken from the NP (shown in the inset of ‘a’, marked as A) confirms the co-existence of Cu and Cu₂O in a single NP; (c) HRTEM image of the outer darker portion of the NP shows the lattice fringes corresponding to Cu (a magnified portion has been shown in the inset); (d) HRTEM image taken from the inner lighter portion of the NP showing lattice fringes corresponding to Cu₂O NPs (inset shows a magnified portion).

Fig. 7 (a) TEM image of Cu NPs embedded alumina film (Film F); SAED pattern with labeling of different crystalline planes has been shown in the inset and (b) HRTEM image of one small Cu NP (inset shows a magnified view) showing lattice fringes corresponding to the Cu(111) plane.

The UV–visible study at the first instance gave an indication of the formation of Cu₂O NPs in the alumina matrix, then the co-existence of Cu and Cu₂O and finally its transformation to Cu NPs. The GIXRD study strongly supports the UV–visible study. Firstly GIXRD confirms the formation of Cu₂O NPs and subsequently the Cu₂O/Cu NPs after controlled reduction of initially formed Cu₂O NPs. The ratio of Cu₂O/Cu can be tuned by controlling the heat-treatment conditions. At the last stage of reduction the film showed only the diffraction peaks corresponding to Cu. The TEM studies also confirmed the formation of Cu₂O@Cu NPs in case of Film D. All these results suggests that the Film B and C have also Cu₂O@Cu type structure with expected less shell (Cu) thickness because GIXRD patterns of these films show Cu peaks with relatively low intensity (Fig. 3b and c). Hence the outer surface of the Cu₂O NPs (formed in Film A) can be reduced to Cu in a controlled way to obtain Cu₂O@Cu core-shell NPs with the variation of core/shell dimensions.

It is noteworthy here that the alumina matrix plays an important role in the formation of Cu₂O NPs. After heat-treatment of Cu salt doped alumina film at 450 °C in air, Cu remains as Cu²⁺ which can be evident from the UV–visible study. When this film is annealed in H₂–Ar at 450 °C, Cu²⁺ partially reduces to the intermediate Cu⁺. It can be expected that during the heat-treatment of alumina films, very small cubic γ-Al₂O₃ crystallites would be formed (supported by FTIR studies; see inset of Fig. 2c). As Cu₂O has also cubic structure, the embedding cubic γ-Al₂O₃ host can facilitate the generation of Cu₂O and its stabilization. Further, the oxidizing character of γ-Al₂O₃³⁸ can also help to stabilize the intermediate oxidation state of Cu i.e. Cu⁺. We observed that in other dielectric film matrices such as SiO₂, the Cu⁺ phase can not be stabilized.³⁹ So it can be concluded that the γ-Al₂O₃ matrix directs the stabilization of the cubic Cu₂O (Cu⁺ state) inside it. Further, controlled thermal treatment in reducing atmosphere causes the reduction of the outer portion of Cu₂O into Cu NP, forming a core-shell type structure. Reduction at higher temperature (550–700 °C) converts all the Cu₂O to Cu NPs inside the alumina matrix.

Congo-red degradation catalyzed by the films

Cu based catalysts, both metallic and oxides, have been widely used as powerful heterogeneous catalysts.^6,40 The Cu-embedded alumina films synthesized in this work can be useful as catalysts because the matrix alumina film is porous in nature.⁴¹ From TEM studies (Fig. 5–7 and S3a; ESI†) the porous structure of the embedding alumina matrix can be observed. This was further confirmed from the ellipsometric measurements. Refractive indexes (RI) of the undoped film matrix (at 633 nm) were found to be in the range of 1.505–1.502 after heat-treatment at 450–700 °C under similar experimental conditions as those of Cu incorporated films. The experimentally observed RI values of these heat-treated films are found to be less compared to the dense (1.65) γ-Al₂O₃⁴² indicating presence of about 20% porosity.⁴³ So these Cu embedded porous alumina films could be useful as catalysts because the porous structure of the alumina films will cause easy accessibility of the solution through the pores, thereby facilitating the catalytic process. As an initial experiment we employed the Cu₂O, Cu₂O@Cu and Cu NPs embedded alumina films to catalyse the degradation reaction of azo dyes. Azo dyes contribute almost 70% of all used dyes and their degradation is difficult due to their complex structure and synthetic nature. As a representative, congo-red dye, which is considered as a primary toxic pollutant in water resources and having two –N=N– groups, was chosen. The spectrum of congo-red in distilled water exhibits the maximum absorption band centred at 498 nm (Fig. 8a) which could be assigned to the conjugated system formed by the –N=N– bonds of congo-red. The red colour of congo-red is due to this maximum absorption band. When an aqueous solution of NaBH₄ is added to congo-red solution, no colour change occurs indicating that the degradation of congo-red proceeds very slowly in the presence of NaBH₄. The degradation reaction does not proceed also in the presence of undoped alumina film (see inset of Fig. 8a). However, in presence of Cu₂O (Film A), Cu₂O@Cu (Film D was taken as representative) and Cu NPs embedded films (Film F was taken as representative) the absorption band at 498 nm is found to be decreased and disappeared, which can be ascribed to the breakage of –N=N– bonds. Compared to congo-red, its degradation products become inclined to biodegradation and mineralization, largely reducing the harm to the environment.⁴⁴ UV–visible spectral evolution of the degradation of congo-red dye by Cu₂O film (Film A) is shown in Fig. 8a. It can be noted here that the reaction proceeds through a clear isosbestic point at 306 nm indicating that this degradation reaction is proceeding smoothly. The catalytic mechanism involves efficient NP mediated electron transfer from borohydride ions to azo bonds.³⁰ The depletion of 498 nm peak has been used to study the rate of such catalysed reaction. Since the concentration of NaBH₄ was taken in excess the reaction was assumed to be of pseudo first order and the rate constant of the reaction was calculated by plotting lnA₄₉₈vs. time (Fig. 8b). The rate constant value has been determined from the slope of the straight line and found to be 0.567 min⁻¹. After the degradation reaction is over, the film catalysts are simply taken out from the reaction mixture, washed with water and dried at 60 °C for 10 min and they were used next time. Five consecutive cycles were performed to check the reusability of the film catalyst using the same film and only a minute decrease of reaction rate was observed after 5 such cycles. The rate constant values of the degradation reaction catalysed by Cu₂O NPs embedded alumina film with respect to the number of cycles are shown in Fig. 8c. It can be seen from Fig. 8c that the film catalyst remains fairly active at least up to 5 cycles of reactions. GIXRD of this film, taken before and after 5 cycles of catalytic reaction, shows no deterioration of Cu₂O NPs inside the film (see Fig. S4; ESI†). This reveals that, under the present experimental condition, no degradation or reduction of Cu₂O occurred in presence of NaBH₄. The rate of degradation of congo-red in five consecutive cycles was found to be lower in cases of Cu₂O@Cu and Cu NPs embedded films. Fig. 9 shows the rate constants for the five consecutive cycles catalysed by Cu₂O, Cu₂O@Cu and Cu NPs embedded alumina films. The above systematic catalytic experiments suggest that the catalytic efficiency of Cu₂O NPs is superior than Cu₂O@Cu and Cu NPs. Thus the Cu₂O NPs embedded alumina film could find important applications in the treatment of waste water, because it has the advantages of being easy to handle, easily separable from the reaction mixture, reusable for several cycles and having non-aggregation of NPs (which causes deterioration of catalytic activity). Moreover, the synthesis process of this film is easy and scalable and would be a cost-effective substitute for the noble metals and other complex processes.^30–32 So these film catalysts can be useful as highly efficient reusable catalysts for toxic dye degradation.

Fig. 8 (a) Successive UV-visible absorption spectra of the degradation of congo-red in the presence of Cu₂O embedded alumina film (Film A) at 25 °C, isosbestic point is shown with asterisk. Similar reaction in the presence of undoped alumina film has been shown in the inset; (b) pseudo first order plot of −lnA (absorbance intensity at 498 nm) vs. time for the above reaction; (c) rate constant values (k) of the catalytic reduction reaction in five consecutive cycles using the same catalyst.

Fig. 9 Rate constant values of the congo-red degradation reaction in five consecutive cycles using Cu₂O (Film A), Cu₂O@Cu (Film D) and Cu NPs (Film F) embedded alumina films.

Conclusions

In summary, we have developed a simple and effective approach for the preparation of Cu₂O NPs in an alumina matrix film. From structural analysis, it can be concluded that the Cu₂O phase is highly stabilized in alumina film matrix. After the formation of Cu₂O NPs in alumina, controlled reduction by slight modulation of the heat-treatment temperature allows the outer surface of Cu₂O to be transformed to Cu to generate Cu₂O@Cu NPs with tunable core/shell structure. Optical, GIXRD and electron microscopic studies unequivocally confirm the formation of Cu₂O as core and Cu as shell. Further reduction transforms Cu₂O@Cu NPs to Cu NPs. Therefore, following this simple and inexpensive approach, copper based NPs with different oxidation states and geometry can be synthesized inside the transparent alumina film matrix in a systematic stepwise manner. It is noteworthy that all these products synthesized in a cumulative way, especially the Cu₂O@Cu NPs incorporated alumina film, deserve attention for different important technological applications e.g. tuning of linear and non-linear optical properties, optical limiting applications to control high intensity laser lights, solar energy conversion, tailoring the photoelectrochemical, sensing and catalytic properties.^25,26 As one of the representative applications, we found that these films can be useful as film-catalysts to degrade azo dyes in aqueous solution. Among these, Cu₂O NPs embedded films showed excellent stability and reusability which can find application for treatment of waste water.

Acknowledgements

Financial support from the Department of Science and Technology (DST), Government of India under the Nano Mission program (Project No. SR/S5/NM-17/2006) is thankfully acknowledged. D. J. thanks CSIR, India for providing a fellowship.

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Footnote

† Electronic supplementary information (ESI) available: GIXRD patterns of CuO/Al₂O₃ (Fig. S1) and Cu/Al₂O₃ (Films E and F) at different conditions (Fig. S2), TEM of CuO/Al₂O₃ film (Fig. S3) and GIXRD of Cu₂O/Al₂O₃ (Film A) before and after catalytic reaction (Fig. S4). See DOI: 10.1039/c2ra21662k

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