Cu2O Film via Hydrothermal Redox Approach: Morphology and

Article pubs.acs.org/JPCC

Cu2O Film via Hydrothermal Redox Approach: Morphology and Photocatalytic Performance Lun Pan,† Ji-Jun Zou,*,† Tierui Zhang,‡ Songbo Wang,† Zhe Li,† Li Wang,† and Xiangwen Zhang† †

Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ‡ Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: A hydrothermal approach is developed to fabricate Cu2O film via in situ redox reaction between Cu2+ and Cu plate. The crystallization process under different conditions was demonstrated, and the crystal structure of Cu2O was verified by XRD, Raman and XPS characterizations. Simply tuning the anionic groups of Cu2+ can generate different morphologies including rod-like arrays, cross-linked and truncated octahedrals. Mott−Schottky plots and PL spectra indicate that the rod-like arrays possess more copper vacancies than the other two morphologies. In photodegradation, the rod arrays exhibit much better performance, following by truncated and then cross-linked octahedrals. The photostability of the three morphologies was also determined. Although different surface reconstructions occur for the films owing to different charge transfer and consumption pathway, their photoactivities are all enhanced after the first run. Then rod arrays and cross-linked octahedrals show very stable activity, but truncated octahedrals show a gradually decreased activity. This work may be helpful for rationally modulating Cu2O-based materials and understanding their deactive mechanism in photocatalysis.

1. INTRODUCTION

Previously, Jayewardena et al. and Fernando et al. treated Cu foil in HCl or CuSO4 solution at 40 °C and obtained Cu2O film via the redox reaction between Cu and Cu2+ ions (eq 1).8b,c In principle, this is a great idea because no complex agents are required. However, the Cu2O layer they obtained grows loosely and randomly.8 Hydrothermal treatment at elevated temperature can enhance the uniformity of film, and the orientation and morphology can be modulated by adjusting the treatment conditions. With this consideration, we modify the redox reaction by adopting hydrothermal treatment, and develop a facile and template-free synthesis of Cu2O film. In addition, the effect of different anionic groups (Cl−, NO3−, and SO42−) on the shape transformation of Cu2O has been reported.12−15 Therefore in this work, these anionic groups were applied to control the surface morphology. By simply adjusting the anionic groups of Cu2+, rod-like arrays, cross-linked octahedrals and truncated octahedrals can be easily fabricated, respectively. Importantly, the photodegradation of organic dyes indicates that the photoactivity and photostability of Cu2O are closely dependent on Cu-vacancy concentration and surface morphology.

As an abundant and nontoxic material, Cu2O (a typical p-type semiconductor) has attracted increasing attention in photocatalysis, photoelectrochemical water splitting, photocurrent generation, and so on.1 Cu2O nanocrystals with different structures and surface morphologies have been synthesized by various methods such as wet chemical, electrodeposition, and solvothermal methods, and their surface-dependent catalytic, electrical and other properties have been studied.1−16 A widely used approach to synthesize Cu2O film is electrodeposition,1a in which some strategies such as utilizing amphiphiles and ligating agents and tuning pH value have been used to control the orientation and surface morphology. Other methods (such as polyol methods) are also used to produce oriented Cu2O films.8,9 Nevertheless, during these synthesis, surface capping agents, oxidants, reductants, and other additives must be introduced to trigger and control the growth of crystals and further the surface morphology of the films. So it is desirable to develop a simple and benign synthetic approach for Cu2O film with tunable morphologies. Herein, we develop a hydrothermal approach to prepare Cu2O film (CF) with tunable morphologies and Cu-vacancy concentrations, based on in situ redox reaction between Cu plate and Cu2+ ions (eq 1). Cu + Cu

2+

+ H 2O → Cu 2O + 2H

+

© XXXX American Chemical Society

Special Issue: Michael Grätzel Festschrift Received: August 12, 2013 Revised: October 23, 2013

(1) A

dx.doi.org/10.1021/jp408056k | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. SEM images of as-prepared Cu2O films. (a), A150-1; (b), A150-3; (c), B150-1; (d), B150-3; (e), C150-1; (f), C150-3. Particles in rectangles are Cu2O cubes.

2. EXPERIMENTAL SECTION 2.1. Materials. CuCl2·2H2O, Cu(NO3)2·3H2O, CuSO4· 5H2O, Na2SO4·10H2O, ethanol and rhodamine B (RhB) were all reagent grade and purchased from Tianjin Guangfu Fine Chem. Res. Ins. Cu foils were high-purity with the size of 20 mm ×20 mm ×1 mm. Its photograph, SEM image, and XRD pattern are shown in Figure S1 (Supporting Information, SI). Deionized water was used in all experiments. All the reagents were used as received. 2.2. Sample Preparation. Cu foil was polished with sandpaper, ultrasonically cleaned in ethanol and deionized water, dried at room temperature, and immediately used to avoid the surface oxidation. In a typical synthesis, 80 mL Cu2+ aqueous solution with concentration of cx mol/L (c1, 7.33 × 10−4; c2, 2.93 × 10−3; c3, 5.13 × 10−3; c4, 7.33 × 10−3; c5, 1.10 × 10−2) and clean Cu foil were sealed in a 100 mL Teflon-lined autoclave, and hydrothermally treated at temperature of T °C (T = 100, 150 or 180) for 12 h. The obtained samples were washed with deionized water and ethanol several times, and dried at room temperature. The films were named as A(or B, C)T-x, in which A, B and C refer to anionic groups (Cl−, NO3− and SO42−) present in the synthetic solution, respectively. 2.3. Characterizations. XRD characterization was conducted using Bruker AXS D8 focus (D8-S4) X-ray diffractometer equipped with a nickel-filtered Cu K radiation at 40 kV and 40 mA and operated in a 2θ range of 20−80° at a scanning rate of 5°/min. SEM images were observed using a field-emission scanning electron microscope (Hitachi S-4800) equipped with Thermo Scientific energy-dispersion X-ray fluorescence analyzer. UV−vis diffuse reflectance spectra (UV−vis DRS) were recorded with a Hitachi U-3010 spectrometer equipped with a 60 mm diameter integrating sphere using BaSO4 as the reflectance sample. Raman measurements were carried out at room temperature with a resolution of 1 cm−1, and the signals were recorded by a DXR Microscope Raman Microscope. The 532nm line of an Nd:YAG laser was used as excitation source. Steady-state photoluminescence (PL) spectra were measured by a RPM2000 (ACCENT) with He−Cd laser excitation (λ= 532 nm, 1.6 W). Mott−Schottky (MS) plots were obtained by capacitance measurement in a standard three-electrode setup10,11 (the electrolyte was 0.2 mol/L Na2SO4 solution buffered at pH

value of 6.59; the reference electrode was Ag/AgCl in 3.5 mol/L KCl solution; and a Pt wire was used as the counter electrode), where Cu2O electrode was used as the working electrode. The sinusoidal perturbation is 10 mV along with a frequency of 1 kHz, with a linear scan from 0.3 to 1.0 V vs RHE. 2.4. Photocatalytic Degradation. Photodegradation of rhodamine B (RhB) was conducted in a quartz chamber (150 mL) vertically irradiated by a 300 W high-pressure xenon lamp (PLS-SXE300UV, Beijing Trusttech. Co. Ltd.; its output spectrum is shown in Figure S2, Supporting Information (SI)). The as-prepared Cu2O film was laid at the middle of the reaction solution. Reaction conditions: temperature, 25 ± 0.2 °C; C0(RhB)= 20 μmol L−1. After stirring for 60 min in the dark to achieve adsorption equilibrium, the reaction was conducted by the magnetic stirring. Samples were withdrawn, centrifuged, and analyzed using Hitachi U-3010 UV−vis spectrometer.

3. RESULTS AND DISCUSSION 3.1. Morphology of Cu2O Films. CuCl2 was first used as the precursor of Cu2+ in the hydrothermal synthesis. At low hydrothermal temperature (T = 100 °C), the Cu2O crystals grow loosely and randomly with anisotropic orientation (Figure S3, SI), similar to the previous reports.8 Then the temperature was increased to make the Cu2O film dense and uniform. When T reaches 150 °C, at low Cu2+ concentration of c1, rod-like Cu2O arrays begin to crystallize on Cu plate (Figure 1a). Also, longer and denser rod arrays are formed when Cu2+ concentration is increased to c3 (Figure 1b) and c5 (Figure 2a-c). The formation of rod arrays may be attributed to the presence of Cl− anions, because other Cu2+ salts, like Cu(NO3)2 and CuSO4, cannot lead to this structure (see below). To confirm this deduction, NaCl (Na+ ions show no effects on the morphology of Cu2O14,15) was added to the synthetic solution [Cu(NO3)2 or CuSO4 solution]. As expected, rod-array structure appears (Figure S4, SI). Therefore, Cl− is the key factor for the formation of rod-like morphology in this hydrothermal process. Higher temperature (180 °C) and Cu2+ concentration (>c5) were also applied to obtain well-crystallized and longer Cu2O rods. However, many white particles in the shape of face-center-etched tetrahedral and airplane are formed on the surface, which are CuCl as determined by XRD and EDX elementary mapping (Figure S5, SI). B



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From XRD patterns (Figure 3a), the diffractive peaks located at (111) index becomes more predominant with the growth of Cu2O rods (from A150-1 to A150-5). It indicates that the rods crystallize along [111] axis. To confirm this, high-resolution TEM observation of the rods was conducted (Figure 4). The lattice spacing of 0.246 nm in the growth direction does correspond to the [111] planes of Cu2O.

Figure 4. TEM and HR-TEM images of a Cu2O rod stripped from A150-5.

Cu(NO3)2 was then utilized to synthesize CFs. Similarly, relatively high temperature (>100 °C) is necessary to obtain well-grown films. At T = 150 °C and c1, Cu2O particles in morphology of “lying” octahedrals begin to nucleate (Figure 1c). The particles grow up and interweave with each other along with the increase of Cu2+ concentration (c3−c5, Figure 1d and Figure 2d−f), and B150-5 has the best crystallization and smoothest surface. Since Cu2O octahedrals are mainly exposed with {111} facets,1 the growth direction of “lying” Cu2O octahedrals should be the [110] axis, which is verified by XRD patterns with (110) and (220) diffraction peaks more intense (Figure 3b). However, high temperature (180 °C) produces many CuO particles (Figure S6, SI) on Cu2O films based on eq 2. Cu 2 + + H 2O → CuO + 2H+

(2)

In addition, CuO crystals with different morphologies (bud, truncated tetragonal bipyramid, and 3D flower assembled by spindles) can be obtained by tuning cx. When the cuprous salt is changed to CuSO4, Cu2O films consisting of edge- and corner-truncated octahedrals and cubes exposed with {100} facets are produced at T = 150 °C, and the crystallization and thickness of film can be promoted with the increase of Cu2+ concentration (Figure 1e,f, Figure 2g−i and Figure 3c), especially for C150-5. Similarly, high hydrothermal

Figure 2. Top-view (a, b, d, e, g, h) and side-view (c, f, i) SEM images of as-prepared Cu2O films. (a−c), A150-5; (d−f), B150-5; (g−i), C150-5. Particles in rectangles are Cu2O cubes.

Figure 3. XRD patterns of Cu2O films prepared under different conditions. C



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temperature (180 °C) produces impurities on Cu2O films, which are confirmed as brochantite Cu4(SO4)(OH)6 by EDX elementary mapping and XRD patterns (Figure S7, SI) via eq 3. 4Cu 2 + + SO4 2 − + 6H 2O → Cu4(SO4 )(OH)6 ↓ +6H+ (3)

On the basis of the above, we can conclude that relatively high hydrothermal temperature (>100 °C) and Cu2+ concentration (c5) are necessary to produce well-crystallized Cu2O film using in situ hydrothermal redox approach. However, a high temperature of 180 °C introduces bulk impurities on the films. Importantly, three representative well-defined Cu2O films with different morphologies (A150-5, B150-5, and C150-5 in Figure 2) but similar thickness (ca. 4.5 μm) are fabricated (Scheme 1), which Scheme 1. In-Situ Hydrothermal Redox Strategy for Fabrication of Cu2O Films Using Cl−, NO3−, and SO42− Anions As the Morphology-Controlling Agents

Figure 5. Raman spectra of as-prepared Cu2O films and pure Cu plate.

this way because their binding energies are very close. However, it is reported that the LMM Anger transition can distinguish them with 568 and 570 eV for Cu and Cu2O, respectively.20 From Figure 6b, the sharp peak at 570 eV verifies the presence of Cu2O. In addition, EDX analysis (Figure S8, SI) detects only Cu and O elements with identical atomic composition for the three CFs, suggesting the CFs have no surface impurities. In UV−vis DRS (Figure S9, SI), the absorption at ca. 550 nm of Cu2O crystals is in close agreement with previous reports.24 Accordingly, the above characterizations demonstrate the high purity of Cu2O films. In Figure 6c−e, XPS O1s peaks can be fitted into two peaks located at ca. 530.3 and 531.7 eV, which are attributed to the lattice oxygen of Cu2O and surface-adsorbed oxygen species (O2 or H2O), respectively.23 The results indicate that ACF has large amount of oxygen species adsorbed on the surface, followed by CCF, and BCF is the least. The rod arrays should possess many corners and steps, which offers a large surface area and abundant adsorption sites. Additionally, the truncated octahedrals of CCF show more facets and sites for adsorption than the intercrossed “lying”octahedrals on BCF. 3.3. Defect Chemistry of Cu2O Films. The lattice defect of p-type Cu2O caused by copper vacancy (VCu) is a key factor affecting its photoperformance.1,10,11,25−27 The defect chemistry of as-prepared CFs was determined via the electrochemical impedance measurement in the dark (based on the capacitance of Cu2O electrode/electrolyte).10,11 The carrier density (Nd) was calculated based on eq 4 in a Mott−Schottky (M−S) plot with 1/ C2 versus potential.

allows us to study their structure- and morphology-dependent photocatalytic behaviors in detail. The synthesis conditions and the abbreviations of three CFs are listed in Table 1. Table 1. Hydrothermal Conditions and the Abbreviations for A150-5, B150-5, and C150-5 samples A150−5 B150−5 C150−5

abbreviation ACF BCF CCF

anions −

Cl NO3− SO42−

cxa/(mol/L) −2

1.10 × 10 1.10 × 10−2 1.10 × 10−2

Tb/°C

tc/h

150 150 150

12 12 12

a

Concentration of Cu2+. bHydrothermal temperature. cHydrothermal time.

3.2. Structure of Cu2O Films. The well crystallization of three CFs is also indentified by Raman spectra. As shown in Figure 5, pure Cu shows no Raman signals. After hydrothermal treatment, all samples exhibit only characteristic phonon frequency of Cu2O: Γ(1)15 (LO) at 154 cm−1, 2Γ−12 at 218 cm−1 and 308 cm−1, 4Γ−12 at 436 cm−1, Γ+25 at 510 cm−1, Γ(2)15 (TO) at 639 cm−1 and Γ(1)15(LO) +Γ(2)15 at 830 cm−1, respectively.17 Moreover, Cu2p XPS spectra were employed to investigate the valence of Cu element. The peaks located at 932.2 and 952.6 eV (Figure 6a) are referred to Cu 2p3/2 and Cu 2p1/2 binding energy of Cu+ or Cu0,11,18−23 while no peaks assigned to Cu2+ (located at around 934.4 and 954.0 eV) are observed, indicating the absence of CuO or Cu(OH)2 on the surface.11,20,23 It should be mentioned that Cu and Cu2O cannot be resolved in

1/C 2 = (2/Nde0ε0ε)[(V − VFB) − kT0/e0]

(4)

C, space charge capacitance; e0, electron charge; ε0, permittivity of vacuum; ε, dielectric constant of electrode (7.60 for Cu2O11); V, applied potential; T0, temperature; k, Boltzmann constant. The M−S plots of CFs are exhibited in Figure 7. The negative slope of the linear part confirms the characteristic of p-type semiconductor. The linear part of the curve is extrapolated to 1/ C2 = 0, and the values of VFB are estimated to be 0.66−0.69 V vs RHE for the three Cu2O films, which are similar to the previous reports.11,27 The carrier density of ACF is 1.22 × 1020 cm−3, 1−2 orders of magnitude higher than that of CCF (1.01 × 1018 cm−3) and BCF (2.75 × 1018 cm−3), respectively. This indicates that the rod arrays possess more VCu than the octahedrals and cubes. D



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Figure 6. Cu2p XPS spectra (a), Cu LMM Auger transition (b) and O1s peak fitting (c−e) of as-prepared Cu2O films.

emissions from β band to VCu.28−31 For ACF, the emissions mainly occur at the range of 1.23−1.41 eV (the series related to VCu), while those related to free emissions (direct band gap, n = 1 and β band emissions) are very low. Compared with ACF, the free emissions of CCF are increased with those related to VCu suppressed. For BCF, the free emissions overwhelm the βluminescence, and a new emission related to VO is found. Therefore, ACF possesses more VCu than CCF and BCF, but BCF has more oxygen vacancies. In addition, the intensity of direct band−band emissions located at 1.82−2.15 eV suggest that the photoinduced hole−electron-separation efficiency is in the order of ACF > CCF > BCF. 3.4. Photocatalytic Performance of Cu2O Films. As exhibited in Figure 9a, no photodegradation of RhB is observed in the blank test, while it happens in the presence of Cu2O photocatalysts. The first-run photoactivity order of CFs is ACF > CCF > BCF. In the preadsorption test, the adsorption over ACF is still higher than that over BCF and CCF (Figure S11, SI), attributed to the relatively high surface area and abundant defects and steps of rod-array structures. However, dye adsorption is trace compared with photocatalysis, indicating the photodegradation is mainly resulted from the photocatalytic effects of Cu2O. The high activity of ACF should be attributed to the rod arrays that offers fast charge separation (based on PL spectra), enhanced optical absorption (Figure S9, SI) and more adsorption active sites (based on XPS O1s analysis and preadsorption test). The higher activity of CCF compared with

Figure 7. Mott−Schottky plots of as-prepared Cu2O films.

The vacancy of CF was also testified by photoluminescence (PL) spectra in Figure 8. Although the PL signals of the three samples are similar, the intensities of the peaks are quite different from each other in the range of 550−700 nm and 800−1100 nm. Generally, the peaks can be classified into two main groups: the band-to-band transition (free exciton emissions), and the impurity transition (bound exciton emission related to impurities).28−33 In order to determine the peak position of emissions, the Gaussian multipeak fitting was conducted, as shown in Figure 8 and Figure S10 (SI). The peak at emitting energy of ca. 2.15 eV corresponds to direct-band gap transition. The peaks at 1.82−2.15 eV are related to the emission back to valence band, while those of 1.23−1.41 eV are related to the E



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Figure 8. PL spectra and their Gaussian fittings of as-prepared Cu2O films in the range of 550−700 nm and 800−1100 nm.

photoinduced electrons and holes on the rod arrays are effectively separated and quickly consumed via hole-induced (direct oxidation) and electron-induced (oxidized species produced by the electron capture of adsorbed oxygen) photodegradation at different active sites. Generally, Cu2O is prone to be reduced or oxidized in photocatalysis. The accurate reason for the quick consumption of electrons and holes on Cu2O rod arrays is not clear. The rods facilitate the transfer of photoinduced electrons to the top surface ({111} facet with relatively lower conduction band4) and the substrate (Cu plate),9 as shown in Scheme S1 (SI). So the surrounding surface of rods is very active for photooxidation that consumes holes, while the top {111} facet and the Cu plate are active for photoreduction.1 For BCF, porous particles are gradually formed during the photodegradation (Figure 9d,e), which are determined as CuO by XPS characterization (Figure 10b) and XRD pattern (Figure 10e). The result suggests that photoinduced holes on BCF are not so effective in the direct oxidation of reactant, and the accumulated holes oxidize Cu2O to CuO. The presence of trace CuO helps to produce Cu2O−CuO heterojunctions, which facilitates the charge separation and suppresses the charge recombination.11,36 Thus the photoactivity is enhanced after the

BCF is also attributed to the more effective charge-separation. In fact, Vo-luminescence related to oxygen vacancies is responsible for the fast decay component of β-luminescence,31 so the presence of abundant oxygen vacancies on BCF prevents the charge separation. In addition, it is reported that the holes on {111} facets can transfer to {100} facets, and in turn, electrons can move to {111} facets.34,35 Therefore, the facet junction of {111} and {100} facets on CCF can also improve the photoactivity. It is noted that the photocatalytic performances of all CFs are enhanced after the first run, and ACF and BCF show very good stability but CCF does not. This suggests that the CFs undergo different surface-structure evolution in the photocatalytic reaction. Then, the morphology evolution and phase alteration of Cu2O surface during the photodegradation were investigated. For ACF, after 1 run, a surface reconstruction of rod arrays happens with many Cu2O nanosheets formed on the surface (Figure 9b,c). Such nanosheets (mainly exposed with {111} facets9) make the photoexcited electrons have a high transportation velocity, leading to the improved performance in later runs. No Cu or CuO formations are observed as confirmed by XPS spectra and XRD patterns (Figure 10a,d), suggesting the F



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Figure 9. Activity and stability of Cu2O films in RhB photodegradation (a) and SEM images (b−g) of Cu2O film after 5 runs. (b,c) ACF, (d,e) BCF, (f,g) CCF.

Figure 10. Cu2p XPS (a−c) and XRD patterns (d−f) of Cu2O films after photodegradation for 5 runs.

first run. Importantly, the coverage of porous CuO on Cu2O can improve the photostability. As for CCF, it is reported that the

{100} facets are unstable and easy to become CuO.4,35 After 5run photodegradation, the surface of CCF is almost destroyed G



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and many compact CuO particles are formed (Figure 9f,g, Figure 10c,f), which suppress the light transmission and reactant diffusion and thus decrease the photoactivity.11,36

4. CONCLUSIONS A simple hydrothermal strategy has been developed to fabricate Cu2O film via in situ redox reaction between Cu2+ and Cu foil. Tuning the anions (Cl−, NO3− and SO42−) of Cu2+ produces Cu2O films composed of rod arrays, cross-linked and truncated octahedrals, respectively. {111} oriented Cu2O rod arrays, possessing abundant copper vacancies, show high photodegradation activity, following by truncated octahedrals possessing {111} and {100} facets, and octahedrals with {111} facet is the least. During the photodegradation, surface reconstruction occurs for rod arrays, while photocorrosion via oxidation happens to cross-linked and truncated octahedrals. Such surface changes improve their photoactivities to some degree. The rod arrays and “lying” octahedrals show very stable photoactivity, whereas truncated octahedrals show a gradually decreased activity.

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ASSOCIATED CONTENT

* Supporting Information S

The XRD patterns, SEM images, EDX elementary mapping, EDX, PL spectra, and UV−vis DRS of the as-prepared samples. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the supports from the National Natural Science Foundation of China (21222607, 51173083), the Foundation for the Author of National Excellent Doctoral Dissertation of China (200955), and the Program for New Century Excellent Talents in Universities (NCET-09-0594). L. Pan thanks the support from the Scholarship Award for Excellent Doctoral Student granted by the Ministry of Education of China (2011).

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REFERENCES

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Cu2O Film via Hydrothermal Redox Approach: Morphology and

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