Zn Doping-Induced Shape Evolution of Microcrystals: The Case of

To understand how the concentration and distribution of dopants affects the properties of an oxide, many studies using first principles total energy c...
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Zn Doping-Induced Shape Evolution of Microcrystals: The Case of Cuprous Oxide Bojun Heng,†,‡ Ting Xiao,† Wei Tao,† Xiaoyan Hu,† Xinqi Chen,† Bixiao Wang,† Daming Sun,† and Yiwen Tang*,† †

Institute of Nano-science and Technology, Central-China Normal University, Wuhan, 430079, People’s Republic of China Department of Applied Physics, Wuhan University of Science and Technology, Wuhan, 430065, People’s Republic of China

‡

ABSTRACT: Zn-doped Cu2O polyhedrons with various crystal morphologies, from 50-facet and 26-facet to 8-facet, were synthesized via a mild, low-temperature process based on the hydrothermal method. Addition of zinc salt to the reaction mixture might allow the introduction of Zn ions into the Cu2O crystal lattice, which is shown by X-ray photoelectron spectroscopy and energy dispersive spectroscopy. Doping of Cu2O crystals with Zn affects not only their morphologies but also significantly influences their optoelectronic properties. UV−visible and photoluminescence (PL) tests showed that the Zn-doped Cu2O system displays an increased band gap and enhanced photoluminescence properties. The open circuit potential-time (Ocp-t) method shows that a pure Cu2O crystal is an n-type semiconductor, while the Zn-doped Cu2O crystal shows p-type characteristics.

1. INTRODUCTION Transition metal oxides (TMOs) doped using different elements have attracted intense interest because of their enhanced electric, optical, catalytic, and magnetic properties stemming from possible spin states, their similarities in ionic radius, and the tendency to occupy the same sites in the crystalline structure. A number of doped TMOs and their optimized features have been reported, including Cu-doped ZnO with ferromagnetic properties,1 iodine-doped ZnO with enhanced photocatalytic performance,2 and Mg-doped ZnO with tunable optoelectronic properties.3 There are also some reports about Fe-doping of Cu2O leading to paramagnetic and diamagnetic behavior at different temperatures,4 or SnO2doped Cu2O crystals being applied to the photocatalytic degradation of trifluralin.5 Despite these efforts, the interactions between the dopant ions and the host lattices during the growth of doped crystals are not fully understood. Studies on the effects of dopants in determining the morphology of the crystals are rare. Cu2O has drawn attention as a promising material for its catalytic properties,6 for cheap photovoltaic power generation because of its theoretical solar cell efficiency, the fact that it is a relatively abundant material, and because of its simple © 2012 American Chemical Society

processing for semiconductor layer formation. As noted in previous reports,7 because the electronic quality of a semiconductor is largely determined by the nature of its impurities, Cu2O can be p- or n-doped. Zn ions are readily incorporated into Cu2O lattices by substitution, because the radius of Zn2+ is close to that of Cu+. To understand how the concentration and distribution of dopants affects the properties of an oxide, many studies using first principles total energy calculations have been carried out. Alejandro et al. have reported that doping Cu2O with Zn resulted in an n-type semiconductor with impurity levels above the conduction band minimum.5 Michael et al. applied first-principles density functional theory to show that doping Cu2O with Zn can strongly affect the formation of defects such as vacancies and thus affect the electrical characteristics of the entire system.8 Doping Cu2O microcrystals with Zn is therefore expected to be a straightforward process for the generation of microcrystals with tunable optoelectronic properties and which are promising for use in solution-processable devices. The photoelectric properties of Received: April 9, 2012 Revised: June 7, 2012 Published: July 9, 2012 3998

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Figure 1. SEM images of the Cu2O system products obtained from the reactions with different molar ratios of Cu(OAc)2 and Zn(NO3)2 in the reagent: (a) 4:0, (b) 4:1, (c) 4:2, (d) 4:3. The insets are magnified images of a single crystal. The right side denotes the schematic models of the corresponding products (b−d). microcrystals follows: 4 mmol of Cu(OAc)2 and 2 mmol of Zn(NO3)2 were dissolved in 100 mL of deionized water mixed with 20 mL of absolute ethanol under sonication. Subsequently, 30 mL of an aqueous solution of sodium hydroxide (5 M) was added to the above solution and kept in a water bath at a temperature of 70 °C for 5−15 min. Then, 50 mL of an aqueous solution of glucose (0.12 M) was added to the above solution and kept under the previous conditions for 2 h. After the reaction, the brick red precipitates were collected by centrifugation, washed with deionized water and absolute ethanol several times, and were finally dried in a vacuum oven at 60 °C for 8 h. To disclose the effects of the acetate and zinc ions in the reaction, we replaced Zn(NO3)2 with zinc acetate (Zn(OAc)2) and ensured that the other conditions were the same, or increased the amount of Cu(OAc)2 and kept the Zn(NO3)2 content constant. 2.2. Morphology and Structural Characterization. The particle size and morphologies of the powder were evaluated by scanning electron microscopy (SEM, JEOL JSM-6700F, 5.0 kV). The crystalline structure was determined by X-ray diffraction (XRD) on a Bruker D8 Advance powder X-ray diffractometer using Cu Kα (λ = 1.5418 Å) radiation. X-ray photoelectron spectroscopy (XPS) was a large area XPS (LAXPS) equipped with an Al K Alpha 300 W X-ray radiation (hν = 1486.6 eV) source for the excitation and energy dispersive spectra obtained using X-ray dispersive spectrum spectroscopy (SUTW-SAPPHIRE) with 20.0 kV accelerating voltage. 2.3. Optical Properties Characterization. UV/visible (UV/vis) spectra were measured and the absorption spectra were recorded using a Perkin-Elmer Lambda UV/vis 35 spectrophotometer The wavelength range was 400−800 nm. For photoluminescence (PL) characterization, the sample was optically pumped at 388 nm and 520 nm with the Perkin-Elmer Lambda 55 spectrometer. In each type of test, 0.2 g of powder was extruded in an oblate cylindrical metal sample cell equipped with a glass bottom. 2.4. Photoelectrochemical Properties Characterization. Photoelectrochemical experiments were conducted using a PARSTAT 2273 electrochemical station (Princeton Applied Research) in a 1.0 × 10−3 M air-saturated aqueous Na2SO4 solution with a three-electrode system (Cu2O system/Ni sheet electrodes as working electrodes, with platinum wire and a saturated calomel electrode as counter electrode

Cu2O are mainly controlled by the intrinsic defects, such as copper and oxygen vacancies. Copper vacancies are believed to be the cause of the p-type conductivity in cuprous oxide, while the conductivity of Cu2O is observed to be n-type because the density of O vacancies is higher than that of Cu vacancies.9 In Cu2O, aliovalent dopant compensation is achieved by forming Cu vacancies. For a dopant of oxidation state n+, we require (n − 1) Cu vacancies to compensate. The subsequent, nth, Cu vacancy dopes the system as p-type.8 To the best of our knowledge, experiments on Zn-doped Cu2O are seldom reported, especially in terms of the evolution of their crystal morphology with different doping levels and their tunable optoelectronic properties. Here, we show that in the synthesis of Cu2O microparticles, the introduction of Zn dopants led to a dramatic shape evolution in addition to the compositional variation of the resulting microcrystals. Depending on the relative concentrations of the dopant precursors, Zndoped Cu2O microparticles with well-defined shapes, from 50facet and 26-facet to 8-facet, which exhibited tunable optoelectronic properties, were obtained for the first time. The shape evolution mechanism of the Zn-doped Cu2O microcrystals was also explored.

2. EXPERIMENTAL METHODS 2.1. Preparation of Zn-Doped Cu2O under Hydrothermal Conditions. All reagents were of analytical grade and were used in their as-received state. We carried out a set of reactions, starting with different relative concentrations of the Zn precursor, zinc nitrate hexahydrate (abbr. Zn(NO3)2), in the reagents. In these reactions, the Cu precursor, cupric acetate monohydrate (abbr. Cu(OAc)2), was maintained at 4 mmol. The molar ratios of Cu(OAc)2:Zn(NO3)2 were varied between 4:0 (neat Cu(OAc)2), 4:1, 4:2, 4:3, and 4:4. For simplicity, these reactions were named after the mole content of the Zn precursor. For example, the 2 mmol Zn(NO3)2 reaction represents the reaction starting with 2 mmol of Zn(NO3)2 and 4 mmol of Cu(OAc)2. An example of the synthetic protocol for Zn-doped Cu2O 3999

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and reference electrode, respectively). The light source was a xenon lamp (300 W). The (0.5 × 2) cm2 Cu2O system/Ni sheet electrodes were cut out from dense sheets. The sheets were made through extruding 0.06 g of different sample powder on nickel foam under the same pressure on a wheel axle. Before been extruded on nickel foam, the powder was mixed with isopropanol and 0.05 mL of 50% (wt) PVDF, extruded several times and became a membrane.

ZnO or Zn peaks. When the Zn(NO3)2 mole content increased to 2 mmol in the reaction, the product became 26-facet polyhedron bounded with three pairs of {100} facets, four pairs of {111} facets, and six pairs of {110} facets, as shown in Figure 1c11 and can also be viewed as octahedral crystal with truncated corners and edges. Figure 1d shows that the morphology of the crystals obtained from the 3 mmol Zn(NO3)2 reaction turned out to be octahedral and a very small amount of octahedral with truncated corners, which presents a stable crystallization of the Cu2O cubic structure identified by the XRD pattern (Figure 2d). The 4 mmol Zn(NO3)2 reaction generates phase separated products which are obviously composed of two different morphologies: micro-polyhedrons and nanorods (graph not offered here), and the XRD pattern (Figure 2e) shows two phase peaks from Cu2O and ZnO. If we compare Figure 2b−d, no trace of zinc metal, oxides, or any binary zinc copper phases are observed within the sensitivity range of the XRD, which indicates the doping of the Zn ions into the Cu2O host to a large extent. From the XRD patterns, we also observe that the peak positions of the samples obtained from 1, 2, and 3 mmol Zn(NO3)2 reactions do not show an obvious shift when compared with undoped Cu2O crystals. This indicates that the crystal lattice shows no obvious change after Zn doping because of the similar radius of Cu+ and Zn2+. Briefly, with increasing Zn(NO3)2 mole content, the shape of the doped crystals changed from 50-facet to 26-facet and then to 8-facet, which can also be seen from the XRD patterns, while the intensity ratios between the {111} and {200} facets increase from Figure 2b to 2c and then to 2d gradually. Also, to identify whether the Zn ions are doped in the Cu2O lattice in the case where no ZnO peaks are found in the XRD patterns, we chose a sample obtained from the 1 mmol Zn(NO3)2 reaction to perform XPS testing, because the least amount of Zn(NO3)2 reactions in precursor could better explain the final doping effect. Figure 3d shows the XPS spectrum of this sample, calibrated using the carbon C 1s peak (C 1s = 284.6 eV). All indexed peaks can be ascribed to C, O, Cu, and Zn, and no transition metal ions other than Zn could be detected. Gaussian-Lorentz fitting curves for O, Cu, and Zn are displayed in Figure 3a−c, and the results of curve fitting of the XPS spectra are summarized in Table 1. Two species of O 1s were found, centered at ca. 527.7 eV (O1) and 530.8 eV (O2). O1 is located at the lower binding energies (BEs) of O 1s and probably belongs to oxygen-defect regions caused by oxygen interstitials. The high binding energy species of O2 indicates the lattice oxygen O2− of Cu2O, which is consistent with the literature data of 530.2−531.1 eV.12 The Cu 2p3/2 and Cu 2p1/2 core level lines could be Gaussian fitted with 929.2 eV (Cu1), 932.4 eV (Cu2), 949.3 eV (Cu3), and 952.2 eV (Cu4). Cu2 and Cu4 at the main peaks are attributed to the lattice Cu+, which is consistent with previously reported values for bulk Cu2O.13 The red shift peaks of Cu1 and Cu3 correspond to copper-deficient regions caused by Cu−O−Zn bonds or oxygen interstitials. From the Gaussian-Lorentz fitting curves of Zn 2p, we can see that the main peaks at 1021.9 eV (Zn2) and 1044.2 eV (Zn4) correspond to Zn 2p3/2 and Zn 2p1/2 of Zn2+,14 which reveals some zinc ions adsorbed on the surface. Otherwise the lower BEs of 1019.0 eV (Zn1) and 1041.2 eV (Zn3) should belong to Zn interstitials or Zn−O− Cu bonds in Cu2O lattice. The atomic ratio of O/Cu/Zn in this sample has also been calculated from the XPS survey spectra to be approximately 13:8:1, which implies oxygen enrichment and copper vacancies in the surface of this sample. We also

3. RESULTS AND DISCUSSION 3.1. Shape Evolution of Zn-Doped Cu2O Microcrystals. Sets of reactions with different molar ratios of Zn(NO3)2 in the reagents were conducted to systematically study the effects of doping on the resulting microcrystals. As shown in Figure 1a−d, the shape evolution of the doped microcrystals is evident. The initial synthesis method using pure Cu2O was referenced to the reports from Leng and coworkers.10 Taking the conditions that are conducive to the doping reaction into account, we elevated the reaction temperature to 70 °C and prolonged the reaction time to 2 h. Scanning electron microscopy (SEM, Figure 1a) shows that the 0 mmol Zn(NO3)2 reaction generates quasi-multifaceted particles with a size of 3−4 μm. Obviously, the longer reaction time caused etching of the particles and resulted in increased roughness of the plane, making it difficult to distinguish the shape and number of the facets in individual particle, while its XRD pattern (Figure 2a) shows that all of the diffraction peaks

Figure 2. XRD patterns of polyhedral crystals of the Cu2O system, corresponding to the samples obtained from (a) 0 mmol, (b) 1 mmol, (c) 2 mmol, (d) 3 mmol, and (e) 4 mmol Zn(NO3)2 reactions.

of Cu2O are indexed according to the standard cubic structure (space group: pn3m̅ , lattice constant a = 0.427 nm, JCPDS File No. 05-0667). Perfect well-defined 50-facet crystals with a diameter of about 3−4 μm were obtained from the 1 mmol Zn(NO3)2 reaction (Figure 1b). The crystallographic structures can be defined as 50-facet Cu2O microcrystals as reported by Leng and coworkers, which were covered by 6 squares ({100} facet), 8 H33 hexagons ({111} facet), 12 H24 hexagons ({110} facet), and 24 trapezoids ({311} facet). Also, H33 denotes connection with three hexagons and three trapezoids, while H24 denotes connection with two hexagons and four trapezoids.10 Interestingly, all of the {100} facets of these 50-facet crystals have been etched into pits, and this etching phenomenon can also be found in Figure 1a,c, which we attribute to the ease of etching of the {100} facets. In the XRD pattern (Figure 2b) for the 1 mmol Zn(NO3)2 reaction, we do not see some of the 4000

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Figure 3. (a−d) XPS spectra of selected sample from the 1 mmol Zn(NO3)2 reaction. (Peak positions are referenced to the adventitious C 1s peak taken to be at 284.6 eV.) (e) EDS patterns of the same sample from the 1 mmol Zn(NO3)2 reaction.

Cu2O microcrystal was octahedral, and further increase in the amount of Zn-doping resulted in a mixed phase. 3.2. Optical Properties of the Zn-Doped Cu2O Microcrystals. The doping of Zn ions into the Cu2O significantly modified the optoelectronic properties of the microcrystals. A comparison of the UV−vis diffuse reflectance spectra of the Zndoped Cu2O crystals is shown in Figure 4. The absorption

Table 1. Comparison of the XPS Data of O 1s, Cu 2p, and Zn 2p Peaks of the Sample from 1 mmol Zn(NO3)2 Reaction species

peak

at. (%)

O1 O2 Cu1 Cu2 Cu3 Cu4 Zn1 Zn2 Zn3 Zn4

527.7 530.8 929.2 932.4 949.3 952.2 1019.0 1022.0 1041.2 1044.2

30.5 69.5 17.1 65.1 4.7 13.1 23.6 25.4 4.5 46.5

considered a relatively better lattice atomic ratio of Cu/Zn, which was calculated to be about 31:1. Furthermore, EDS was employed to clarify the content of zinc ions in bulk phase of this kind of microcrystal. As shown in Figure 3e, in addition to the carbon from substrate of conductive adhesive, the microcrystal is mainly composed of Cu, O, and a small amount of Zn. The atomic ratio of Cu/Zn offered in Figure 3a is about 35:1. From the EDS on single particle, similar Zn content can also be observed, which shows that the doping is uniform throughout the system. Obviously, the atomic ratio of Cu and Zn from EDS is close to the conclusion of XPS. For the case of more Zn doping, such as the 2 mmol Zn(NO3)2 or 3 mmol Zn(NO3)2 reactions, we also can deduce that some amount of zinc ions was doped into Cu2O lattice. In conclusion, different relative concentrations of Zn(NO3)2 in the starting materials led to microcrystals that exhibited dramatic morphological and compositional changes. When the relative concentration of the Zn precursor in the reagents was less than 4 mmol, microcrystals that incorporated the Zn ions into the Cu2O lattice were obtained. The shape evolution of the Zn-doped Cu2O microcrystals with increasing relative concentrations of the Zn precursor followed an order of decreasing numbers of facets. The final morphology of the Zn-doped

Figure 4. Band gap calculation of Zn-doped Cu2O crystals obtained from 0 mmol, 1 mmol, 2 mmol, and 3 mmol Zn(NO3)2 reactions; inside was their UV−visible diffuse reflectance spectra.

spectra of the entire samples are dominated by strong light scattering bands attributed to the relatively large sizes of these crystals.15 Plots of (αEphoton)2 versus the energy (Ephoton) of absorbed light afford the band gap of the samples as shown in Figure 4 (where α and Ephoton are the absorption coefficient and the discrete photon energy, respectively). The extrapolated values (the straight line to the X-axis) of Ephoton at a = 0 give absorption edge energy; that is, band gap Eg = 2.10 eV, 2.16 eV, 2.13 eV, and 2.17 eV, corresponding to the samples from 0 mmol, 1 mmol, 2 mmol, and 3 mmol Zn(NO3)2 reactions, 4001

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3.3. Possible Formation Mechanism of Zn-Doped Cu2O Crystals. To understand the underlying mechanism of the observed shape evolution of the Zn-doped Cu2O microcrystals, the critical issues that govern the formation of these doped microcrystals with defined morphologies must be explored. The formation of the selected products of 26-facet Cu2O crystals from the 2 mmol Zn(NO3)2 reaction and 8-facet Cu2O crystals from the 3 mmol Zn(NO3)2 reaction at various growth stages were examined (Figure 6). It was observed immediately that after the addition of the NaOH solution, a black flocculent precipitate appeared, which was composed of CuO nanobelts curled together, as shown in Figure 6a,d. The CuO phase was identified from the XRD pattern in the inset of Figure 6a. Soon after the glucose solution was added, the nanobelts quickly aggregated into particles with sizes of 2−3 μm, approaching the final crystal profile, but their surfaces were rough and were strapped by some broken nanobelts (Figure 6b,e). When the reaction was extended to 20 min after addition of the glucose solution, uniform 26-facet crystals and octahedrons were formed. Interestingly, from the SEM images of Figures 1a−c, we find that the final crystals grow to 3−4 μm, and the {100} facets of the 50-facet and 26-facet crystals have obviously been etched into pits, which indicates that a longer reaction time would cause a second reconstruction, including oriented attachment growth and an etching reaction. Dissolved oxygen in the reaction solution from the air might promote this etching reaction. Too much oxygen in the reaction solution not only caused oxygen etching, but also formed more oxygen interstitials. The etching reaction was prone to occur on {100} facets,20 and thus the 26-facet crystal with its larger area of {100} facets was prone to generate pits, and more defects, which was perfectly consistent with the rich oxygen content of the XPS results and the higher intensity of the PL results. It had also been reported previously that the catalytic ability of the {100} planes could be improved by oxygen-assisted selective etching of Cu2O,21 just as its PL intensity has been improved here. In order to understand the zinc doping of the whole process, we also performed EDS on the intermediates product of copper oxide from 3 mmol Zn(NO3)2 reaction. Figure 7 shows that the Zn atomic number percent is 2.73%, slightly higher than previously mentioned cuprous oxide from 1 mmol Zn(NO3)2 reaction as Figure 3e. This shows that the soluble Zn2+ ions were wrapped into the formation reaction of CuO and Cu2O randomly during the whole process. The chemical reactions involved were fairly simple. Initially, when OH− was added to the mixed solvent containing copper salt, Cu(OH)2 was first precipitated from the solution and decomposed into black CuO at 70 °C. Upon the introduction of the glucose solution, the CuO nanobelts were quickly reduced into the different morphology of Cu2O particles, most likely via the soluble Cu(OH)42−. The conversion mechanism had been quoted as a reconstructive transformation from CuO in the aqueous solution, involving a dissolution reaction followed by the crystallization of Cu2O, in which hierarchically oriented growth rates and oriented attachments of Cu(OH)42− went through all of these processes.22 Herein, with the dopant content increasing, the plane number of the polyhedral microcrystals decreased from 50-facet to 26-facet and finally to 8-facet. Zn(NO3)2 as a strong acid and weak alkali salt would decrease the concentration of OH− in the reaction solution, which might result in fewer Cu(OH)42− and more Zn ions in the solution, as well as anisotropic growth rates on the various crystallographic

respectively. The evaluated band gap are near the direct band gap of 2.0−2.17 eV for bulk Cu2O. Obviously, there were blue shifts in the absorption edge energies of the Zn-doped Cu2O crystals in comparison to that of the undoped Cu2O, so doping leads to an increased band gap. The blue shift in the absorption edge was likely to be a result of an increase in the sp-d exchange interaction, which becomes a dominant factor over the Moss-Burstein type shift at higher doping levels.16 The bule shift also confirmed that the Zn ions are indeed incorporated into the Cu2O crystal lattice, which produced hybrid energy levels above the Cu2O semiconductor band gap. The PL spectra of the Cu2O crystals originate from either photoinduced electron−hole recombination or the intrinsic defects. We therefore used PL spectroscopy methods to study the defect structures of the prepared samples. Taking previous reports into account, photoluminescence was observed in both the undoped and doped Cu2O because of inherent defects such as cationic deficiencies present in the material. The intensity of the PL peak was therefore found to increase as the number of dopants increased, as the defect formation increased at the same time.17 Obviously, Figure 5a indicates that higher Zn

Figure 5. Photoluminescence spectra of as-prepared Zn-doped Cu2O crystals obtained from 0 mmol, 1 mmol, 2 mmol, and 3 mmol Zn(NO3)2 reactions. The excitation wavelengths are (a) 388 nm, (b) 520 nm.

doping levels lead to higher PL intensities, and the highest PL intensity come from the sample obtained from the 2 mmol Zn(NO3)2 reaction, which is probably because it has more defects on the {100} facets. A series of emission peaks occurred in the spectral range around 585 nm, which could be assigned to 1 s excitons,18 or band edge emissions. Figure 5b shows the 520 nm wavelength excitation PL spectra, where the emission peaks are present at a wavelength of around 695 nm. This wave band is most likely attributable to acceptor−related luminescence,19 which may be ascribed to intrinsic lattice defects caused by Zn2+ substitution (ZnCu). Overall, the sample obtained from the 2 mmol Zn(NO3)2 reaction demonstrates the most enhanced PL intensity across these two excitation wavelengths, and considering the morphology evolution shown in Figure 1, this suggests some causal relationship with the etched {100} facets. The product of 2 mmol Zn(NO3)2 reaction had 26 facets, which included six etched {100} facets with larger areas than the others and which induced more defects. Achieving doped Cu2O microcrystals with the desired shapes in conjunction with the ability to tailor their optoelectronic properties is attractive and exciting for the use of these materials in various applications. For instance, the enhanced PL intensity ensured the strengthening of potentially useful photoelectrochemical properties for fabrication of optical devices such as photoelectrochemical (PEC) cells.4 4002

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Figure 6. SEM images of the growth progress of (a−c) 26-facet Zn-doped Cu2O crystals and (d−f) 8-facet Zn-doped Cu2O crystals. (a, d) CuO nanobelts formed after adding NaOH solution and aging for 10 min; inset of (a) is its XRD pattern. (b, e) Zn-doped Cu2O system obtained after adding glucose solution and aging for 3 min; insets are their amplification images. (c, f) Crystals obtained after aging for 20 min.

final morphology was octahedral. On the other hand, when Zn(NO3)2 was maintained at 2 mmol invariably in the precursor, while increasing the Cu(OAc)2 content to 5 and 6 mmol, the well-defined 26-facet crystal morphology obtained remained unchanged. The above phenomenon indicates that acetate radical ions in the reaction solution have not played important roles in the growth rates on different planes, but too many Zn ions induce the formation of more Zn(OH)42−, which hinders the diffusion rate of the Cu(OH)42− ions and even affects the oriented growth rate of the Cu2O crystal in the reaction. We also see that the formation of the different Zn-doped Cu2O crystal shapes should be affected by the relative concentrations of Zn(NO3)2, but the effect of the Zn ions on the crystal shape evolution is less significant than that of the nitrate ions. It has also been reported that the presence of the nitrate ions could stabilize the {100} facets,24 and we believe that the presence of the Zn ions could stabilize the {111} facets in our experiments, and all of these factors in collaboration affect the anisotropic growth in the Cu2O crystallization process. 3.4. Photoactivity Properties of Zn-Doped Cu2O. The photoactivity of as-prepared samples was studied after pasting the powder onto nickel foam and rolling it into a sheet, which acted as the working electrode in a three-electrode system. The open circuit potential-time (Ocp-t) method using dark and light responses was applied to this study, using a 1.0 × 10−3 M air-saturated aqueous Na2SO4 solution. Different types of semiconductors have different photoelectrochemical responses to light. For an n-type semiconductor under illumination, the electron moves from the valence band to the conduction band, leaving a hole at the surface. The hole is a high energy species

Figure 7. EDS patterns of intermediate products of copper oxide from the 3 mmol Zn(NO3)2 reaction.

planes with different free surface energies.23 In our experiments, the growth rates along the perpendicular directions of the four planes followed a sequence: {311} > {110} > {100} > {111}. The {111} facet was the first stabilized plane, and this ratedetermining phenomenon was identified by the fact that the final morphology of the incorporated crystal was octahedral for increasing Zn(NO3)2 content. However, 4 mmol Zn(NO3)2 doping increased the Zn(OH)42− concentration in the reaction solution, which could reach the crystalline concentration of the ZnO crystal, and then induce the appearance of the ZnO phase. In our experiments, if we substituted for Zn(NO3)2 with 1 or 2 mmol of Zn(OAc)2 and kept 4 mmol Cu(OAc)2 invariably, no significant changes in the crystal shape or significant reductions in surface numbers were found; all of the morphologies of the obtained crystals were approximately 50facet, until the amount of Zn(OAc)2 reached 3 mmol, and the 4003

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Figure 8. The photoelectrochemical activity of Zn-doped Cu2O crystals produced from 1 mmol, 2 mmol, 3 mmol, and 0 mmol Zn(NO3)2 reactions.

Zn-doped Cu2O crystals. The different defects and opposing characteristics between pure Cu2O and Zn-doped Cu2O were also reflected in the PL patterns.

that can extract an electron from the solution species. Conversely, the open circuit potential of an n-type semiconductor shows a negative value. Compared with n-type semiconductors, a p-type semiconductor shows the reverse photoelectrochemical behavior: the open circuit voltage (VOC) is shifted toward a more positive potential upon illumination.9 Therefore, the VOC of the semiconductor could be used to determine the conduction types of our as-prepared samples of the Cu2O system. Figure 8 shows the photoelectronic activity of as-prepared samples from our experiments; the VOC of the samples obtained from 1 mmol, 2 mmol, and 3 mmol Zn(NO3)2 reactions show more positive properties as in ptype behavior under illumination, while pure Cu2O crystals show the reverse n-type behavior. By comparing their potential curves, we find that the Zn-doped Cu2O crystals produced from the 1 mmol, 2 mmol, and 3 mmol Zn(NO3)2 reactions have more stable photoelectronic potentials than the pure Cu2O crystals, which indicates that optical erosion occurs more easily on pure Cu2O crystals than on the Zn-doped Cu2O crystals. In a semiconductor, the quantity of the carriers decides the conductance and thermal conductivity of the substance. When the number of free electrons is greater than the number of holes, the free electrons are the dominant factors, giving an ntype semiconductor, and indicating that the key defects of pure Cu2O might be oxygen vacancies (VO) or copper interstitials (Cui) in the crystal lattice. However, when the number of holes is greater than the number of free electrons, the holes are dominant, giving a p-type semiconductor. Our experiment just confirmed the theoretical results: For a dopant of oxidation state Zn2+, we require 1 Cu vacancy to compensate. The subsequent, 2th, Cu vacancy dopes the system as p-type.8 The key defects of Zn-doped Cu2O crystals from the 1 mmol, 2 mmol, and 3 mmol Zn(NO3)2 reactions might therefore be copper vacancies (VCu), zinc substitution (ZnCu), or oxygen interstitials (Oi), which is consistent with the XPS results of oxygen enrichment and copper vacancies in the surface of the

4. CONCLUSIONS The introduction of Zn dopants was critical to the dramatic shape evolution of the Zn-doped Cu2O microcrystals. Zndoped Cu2O crystals with well-defined shapes, from 50-facet and 26-facet to 8-facet crystals, and exhibiting tunable optoelectronic properties, were obtained for the first time. Reaction mechanism studies showed that the relative concentration of Zn(NO3)2 in the reagents was a key factor which caused a lower concentration of Cu(OH)42−, an oriented growth rate, oriented attachment of Cu(OH)42−, and eventually led to fewer plane numbers in the doped microcrystals. The final morphology of the Zn-doped Cu2O crystals was uniformly octahedral, and increased Zn-doping led to a mixed phase. It was also interesting that pure Cu2O crystals showed n-type semiconductor characteristics, but the doped Cu2O crystals showed p-type characteristics in our Ocp-t tests.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 27 67867947. Fax: +86 27 67861185. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

Financial supported by self-determined CCNU basic research and operation research funds for colleges from the China Ministry of Education (CCNU09A02011). 4004

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dx.doi.org/10.1021/cg3004799 | Cryst. Growth Des. 2012, 12, 3998−4005