Facet-selective growth of Cu–Cu₂O heterogeneous architectures

Abstract

We have demonstrated a facile protocol for the synthesis of facet-selective growth of low-cost metal Cu nanoparticles on {111} facets of polyhedral 26-facet Cu₂O architectures. The novel Cu–Cu₂O heterogeneous architectures show better adsorption and photodegradation of methyl orange than those of the original Cu₂O architectures.

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Facet-selective growth of heterogeneous nanostructures with new functionalities is a significant frontier in design and synthesis of advanced materials.¹ As one type of attractive building blocks with unique physical and chemical properties, metal–semiconductor heterogeneous structures have been extensively investigated due to their excellent photocatalytic activities.² It is believed that the recombination of the electrons and holes can be prevented due to the formation of a Schottky barrier at the metal–semiconductor interface, and the photocatalytic efficiency of semiconductors can be obviously improved.³ To precisely control the structure and shape of metal–semiconductor heterogeneous structures, thorough understanding of the formation mechanism and the growth process are imperative. Recently, a large number of multifunctional metal–semiconductor heterogeneous architectures have been synthesized by various wet chemical methods, including core–shell structures such as Au@PbS,⁴ Au@ZnS,⁵ Au@CdS,⁶ Au@Ag₂S,⁷ and Au@Cu₂O,⁸ as well as by loading metal particles onto semiconductors, tuning materials such as Ag–ZnO,^1,9Au–ZnO¹⁰ and so forth. However, these studies have mainly focused on loading the high-cost noble metals.

Various facets in a crystal usually exhibit different physical and chemical properties that are deeply facet-dependent because of the fundamental anisotropy of crystals, such as {001} facets of anatase TiO₂ showed higher activity than {101} facets,¹¹ {0001} facets of ZnO displayed high activity than that of other facets, and tetrahexahedral Pt and Au nanocrystals enclosed by 24 {037} or {122} facets possessed excellent electro-oxidation activity.^12,13 Hence, the study of the properties of definite crystal facets is not only helpful for us to synthesize novel heterogeneous structures, but also to provide a new chance for constructing multifunctional materials with potentially unique and exciting properties.

Polyhedral Cu₂O crystals, one of the typical p-type semiconductors, are usually enclosed by low-index {111}, {100} or {110} facets. An essential difference between these facets is the surface atom structures. Importantly, evidence for selective crystal-facet-dependent properties of Cu₂O crystals have been found their catalytic application. For example, octahedral Cu₂O with exposed four pairs of {111} facets exhibited much higher photocatalytic activity than the cubic one enclosed by six {100} facets.¹⁴ Therefore, it is believed that the facet-selective growth of metal materials on a specific crystal surface of Cu₂O crystal to form novel heterogeneous nanostructures through reasonable design and control of the growth environment is significant.

Herein, we present the first evidence on the achievement of the facet-selective growth of low-cost metal Cu nanoparticles on {111} facets of polyhedral 26-facet Cu₂O architectures (Cu–Cu₂O heterogeneous architectures). To our best knowledge, there are no reports on the one-pot synthesis of the polyhedral 26-facet Cu–Cu₂O heterogeneous architectures selectively exposed with copper nanoparticles building blocks on their {111} facets. The formation of these novel Cu–Cu₂O heterogeneous architectures not only makes the purposive crystal design of spatial arrangement possible, but also offers a good opportunity to understand the foundational significance of metal nanoparticles for enhancing the photocatalytic activity of semiconductors. In the present work, it was found that the as-prepared Cu–Cu₂O heterogeneous architectures showed better adsorption and photodegradation of the methyl orange (MO) dye than that of the original 26-facet Cu₂O architectures.

Our synthetic strategy to prepare the purposive well-defined Cu–Cu₂O heterogeneous architectures is based on a selective replacement route, which can be attributed to the reduction of Cu₂O precursors.¹⁵ Experimental details are shown in the ESI†. It can be synthesized by a two-step process, i.e. the formation of uniform and monodisperse Cu₂O templates via a template-free complex precursor solution route,¹⁶ then the synthesis of Cu–Cu₂O heterogeneous architectures in ultra-pure aqueous solution at appropriate reaction temperature with the aid of hydrazine hydrate. The formation of Cu–Cu₂O heterogeneous architectures in our experiment is based on the following reaction:

2Cu₂O + N₂H₄ → 4Cu + N₂↑ + 2H₂O (1)

The polyhedral 26-facet Cu₂O template is chosen here because it is enclosed by 8 {111} facets, 6 {100} facets, and 12 {110} facets.¹⁷ The surface atom structures of {110}, {100} and {111} facets are different in Cu₂O crystal lattice,¹⁸ which can be in favor of the facet-selective growth of metal Cu nanoparticles on Cu₂O crystals. The X-ray diffraction (XRD) pattern of both cubic Cu₂O (JCPDS file no. 05-0667) and cubic Cu (JCPDS file no. 04-0836) could be indexed for these as-prepared products (see ESI†, Fig. S1), suggesting the formation of metal copper in the as-prepared products. In order to further demonstrate the presence of copper in the as-prepared products, the surface state was investigated by X-ray photoelectron spectroscopy (XPS). Fig. S2† displays the corresponding XPS spectrum of the Cu–Cu₂O heterogeneous architectures (see ESI†). Peaks of Cu 2p_3/2, Cu 2p_1/2, O 1s and C 1s can be identified. According to the position of the Cu 2p core level peak,¹⁹ it is obviously seen that the Cu(I) oxidation state (932.4 eV) and Cu (952.2 eV) exist in the as-prepared products, indicating the presence of new products of Cu in original Cu₂O templates. Therefore, it is deduced that a proportion of Cu₂O was reduced to Cu by hydrazine hydrate.

The morphology of the products was observed by field-emission scanning electron microscopy (FESEM). Fig. 1a is the typical low-magnification FESEM image of the polyhedral Cu₂O crystals, showing that these highly symmetric polyhedral microcrystals are uniform and monodisperse. Fig. 1b displays a typical individual Cu₂O particle, and it can be seen that this particle is a micrometre scale 26-facet polyhedron with three pairs of {100} facets, four pairs of {111} facets and six pairs of {110} facets, and the facets are all smooth surfaces. However, after addition of Cu₂O precursors into the ultra-pure aqueous solution composed of hydrazine hydrate, an obvious conversion of Cu₂O crystals to Cu nanoparticles could be observed with the reaction temperature increase to 60 °C. Fig. 1c and d show the typical FESEM images of the as-synthesized Cu–Cu₂O heterogeneous architectures at different magnifications, respectively. From the low-magnification FESEM result (Fig. 1c), it can be found that these products are almost the excellent 26-facet architectures in our synthesis. A typical individual 26-facet Cu–Cu₂O heterogeneous architecture is shown in Fig. 1d, which obviously shows that the formation of selectively rough and smooth surfaces can be successfully achieved. The architecture fully reproduces the shape of the original 26-facets Cu₂O crystal. Moreover, the facet-selective growth of new nanoparticle building blocks on {111} facets of the original 26-facet polyhedral Cu₂O templates is occurring, but the smooth rectangular {110} facets and square {100} facets do not change. The attempted morphology and structural characterization of the as-prepared Cu–Cu₂O heterogeneous architecture was performed on transmission electron microscopy (TEM). However, as for our Cu₂O template, it was difficult to characterize by TEM because the samples were strongly distorted as the electron beam focused on them. Similarly, this phenomenon has also occurred in the previous report on Cu₂O crystals,²⁰ but the exact reason is as yet uncertain. To demonstrate the formation of new metal copper nanoparticles, intense ultrasonic treatment in water was employed to break these as-prepared products. Some partially separate nanoparticles are shown in Fig. S3a†. Fig. S3b† shows the corresponding electron energy dispersive X-ray (EDX) spectrum of the residual nanoparticles, and it can be found that only copper and carbon elements are detected in the spectrum (the carbon element comes from the carbon film), but no oxygen peak is seen, confirming that the obtained nanoparticles are metal copper. Therefore, it is proposed that the facet-selective growth of metal Cu nanoparticles on {111} facets of polyhedral 26-facet Cu₂O architectures is successfully achieved in our synthesis.

Fig. 1 (a and b) Typical FESEM images of the original polyhedral 26-facet Cu₂O crystals. (c and d) Typical FESEM images of the as-prepared polyhedral 26-facet Cu–Cu₂O heterogeneous architectures.

The reduction of a copper always initiates from the surface. In order to shed light on the facet-dependent reducibility of Cu₂O crystals, the density functional theory (DFT) calculations (see ESI†) were performed to understand the structures of Cu₂O(111), Cu₂O(110) and Cu₂O(100). Fig. 2 shows the optimized (110), (111) and (100) surface structures, and it can be seen that the bond length between the oxygen atom and copper atom in the (111) facet is 1.876 Å, and it is larger than that in (110) (1.831 Å) and (100) facets (1.818 Å). The reducibility of a copper can be uncovered by the bond length between the oxygen atom and copper atom. It means that the shorter bond length suggests stronger interaction, and the copper in this facet is difficult to be reduced. In contrast, Cu₂O facets with longer bond length between the oxygen atom and copper atom are facile to be reduced. Therefore, the results demonstrate that the (111) surface of Cu₂O is much more facile to be reduced than the (110) and (100) surfaces, leading to the formation of the facet-selective growth of Cu–Cu₂O heterogeneous architectures.

Fig. 2 Optimized structures of Cu₂O(111), (110) and (100) surfaces: (a) side view of Cu₂O(111) surface; (b) side view of Cu₂O(110) surface; and (c) side view of Cu₂O(100) surface. The red and blue balls represent oxygen and copper atoms, respectively.

To demonstrate the potential application of the as-synthesized Cu–Cu₂O heterogeneous architectures in degradation of organic contaminants, we have investigated their adsorption abilities and photocatalytic activities by choosing the photocatalytic degradation of the MO dye as a model reaction (see ESI†). UV-Vis spectra were used to investigate the adsorption and photocatalytic degradation activity of the MO dye. The characteristic absorption peak at 465 nm of MO was used to monitor the photocatalytic degradation process. It can be obviously seen that the 26-facet Cu–Cu₂O heterogeneous architectures have a much better adsorption capacity (Fig. S4, see ESI†). The results show that 26-facet Cu–Cu₂O heterogeneous architectures adsorb about 13.4% of MO in the solution, while 26-facet polyhedral Cu₂O adsorbs about 1.6% in 0.5 h. A better photocatalytic activity of 26-facet Cu–Cu₂O heterogeneous architectures than that of the original Cu₂O polyhedral crystals can be directly determined in the curve shown in Fig. 3. The ratio C/C₀ was used to describe the degradation, which stands for the concentration ratio before and after a certain reaction time. Fig. S4a† is the optical absorption spectra of the MO tested at different durations with the polyhedral 26-facet Cu₂O crystals. The intensity of the absorption peak at 465 nm of MO decreased slowly with the increase of the reaction time, and about 20% of MO (Fig. 3, line A) was degraded after 90 min, which indicates that the photocatalytic efficiency was low. The control experiment was carried out to compare the catalytic activity of the Cu–Cu₂O heterogeneous architectures. It was found that the intensity of the absorption peak at 465 nm of MO decreased faster than that of the above 26-facet Cu₂O crystals (Fig. S4b†), and the intense yellow color of the starting solution gradually disappeared with increasing exposure time to the UV light. About 80% of the MO (Fig. 3, line B) was degraded after 90 min, so the decomposition of the MO aqueous solution at 90 min in the presence of above two samples is as follows: Cu–Cu₂O heterogeneous architectures (80%) > original Cu₂O (20%). The above results indicate that the as-prepared Cu–Cu₂O heterogeneous architectures show much better adsorption ability and photocatalytic degradation activity of the MO dye than those of the original 26-facet Cu₂O architectures. Note that the photodegradation experiment of the MO dye here was carried out using low-cost Cu–Cu₂O heterogeneous architectures, which suggests that the potential application is quite feasible in practice.

Fig. 3 A plot of the extent of adsorption and photodegradation of MO by different catalysts. Line A: the original polyhedral 26-facet Cu₂O crystals. Line B: the as-prepared polyhedral 26-facet Cu–Cu₂O heterogeneous architectures.

Based on the above photocatalytic investigation, it is believed that better degradation of MO by Cu–Cu₂O heterogeneous architectures can be attributed to the introduction of Cu nanoparticles on the original {111} facets of polyhedral 26-facet Cu₂O crystals, which leads to the synergetic effect and specific charge-transfer kinetics in the as-prepared Cu–Cu₂O heterogeneous architectures.^21–24 The photocatalytic mechanism of the as-synthesized Cu–Cu₂O products under UV irradiation might be explained as follows. After the facet-selective growth of Cu nanoparticles on the {111} facets of 26-facet polyhedral Cu₂O crystals, a metal–semiconductor heterostructure was formed. When the Cu–Cu₂O heterogeneous architectures were irradiated with UV light, electrons in the valence band (VB) were excited to the conduction band (CB), with simultaneous formation of holes in the VB.³ Due to the Schottky barrier formed at the Cu–Cu₂O interface, Cu nanoparticles might act as electron sinks, decrease the recombination of photoinduced electrons and holes, and increase the lifetime of the electron pairs, thus obviously improving the photocatalytic efficiency.^3,25 The holes can be captured by OH⁻, leading to the formation of hydroxyl radical species (˙OH).²⁶ Moreover, the electrons can be trapped by the adsorbed O₂ and form the superoxide anion radicals (˙O₂⁻), which can finally be reduced to ˙OH radicals.³ It has been demonstrated that the ˙OH radicals favor oxidizing organic contaminates because of their high oxidative capacity.²⁷ Thus the photocatalytic capacity of the {111} facets could be obviously improved compared to that of the original templates. Similarly, the special effect of the external introduction of noble metal nanoparticles on Cu₂O for enhancing the photocatalytic degradation of organic contaminates has been well discussed by Wang and co-workers.³ Therefore, the MO dye can be effectively degraded by the Cu–Cu₂O photocatalysts under the irradiation of UV light.

In summary, facile synthesis of polyhedral 26-facet Cu–Cu₂O heterogeneous architectures selectively exposed with Cu nanoparticles building blocks on their {111} facets was successfully achieved via a facet-dependent reduction of Cu₂O templates solution route. The photocatalytic superiority of these novel Cu–Cu₂O heterogeneous architectures can be attributed to the introduction of beneficial components of copper nanoparticles, which exhibited higher adsorption ability and photocatalytic activity for enhancing the degradation of the MO dye under the irradiation of UV light. These results provide the convincing evidence for the facet-selective growth of metal–semiconductor heterogeneous structures with new functionalities.

Acknowledgements

We thank the support from the National Basic Research Program of China (no. 2010CB635101), National Science Foundation of China (NSFC no. 51071116 and 50871081) and National High Technology Research and Development Program of China (863 Program, no. 2009AA03Z320).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and theoretical calculation, XRD pattern, XPS spectrum, EDX spectrum, and UV-vis absorption spectra. See DOI: 10.1039/c1ce05743j

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