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Synthesis and Characterization of ZnO/CuO Verticallyaligned Hierarchical Tree-like Nanostructure Zhengxin Li, Meng Jia, Baxter Abraham, Jolie Blake, Daniel Bodine, John T. Newberg, and Lars Gundlach Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02840 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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Synthesis and Characterization of ZnO/CuO Vertically-aligned Hierarchical Tree-like Nanostructure Zhengxin Li,

†

Meng Jia,

†

Baxter Abraham,

Newberg,

†

†

Jolie Blake,

and Lars Gundlach

†

Daniel Bodine,

†

John

∗,†,‡

†Department of Chemistry and Biochemistry, University of Delaware, 109 Lammot DuPont

Laboratory, Newark, Delaware 19711, United States ‡Department of Physics and Astronomy, University of Delaware, 109 Lammot DuPont

Laboratory, Newark, Delaware 19711, United States E-mail: [email protected]

Phone: +1 (302) 831-2331

Abstract Vertically-aligned ZnO nanowire-based tree-like structures with CuO branches were synthesized based on a multi-step seed-mediated hydrothermal approach. The nanotrees form a p-n junction at the branch/stem interface that facilitates charge separation upon illumination. Photo-electrochemical measurements in dierent solvents show that ZnO/CuO hierarchical nanostructures have enhanced photocatalytic activity compared to non-hierarchical structure of ZnO/CuO, pure ZnO and pure CuO nanoparticles. The combination of ZnO and CuO in tree-like nanostructures provides opportunities for the design of photo-electrochemical sensors, photocatalytic synthesis, and solar energy conversion. 1

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Keywords ZnO, CuO, nanostructure, p-n junction, and photoelectrochemistry

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Introduction Ordered arrays of semiconducting tree-like hierarchical nanostructures are of considerable interest in photo-catalysis, photo-electrochemical catalysis, energy storage and photovoltaic conversion because of their larger surface-area to volume ratio, more ecient photocurrent conversion, and enhanced quantum eciency. 14 The photocatalytic activity of heterogeneous hierarchical semiconductors depends strongly on the composition, the structural design, and the electronic properties of the interface. 2 A heterojunction formed at the interface between a p-type and n-type semiconductor nanoparticle can facilitate charge separation and reduce electron/hole recombination dramatically by spatially separating the charge carriers. 4 ZnO is an n-type semiconductor, with a direct band gap of around 3.4 eV depending on doping, vacancies, and morphology. 5,6 ZnO can be synthesized in a large number of nanostructures with dierent morphologies. It has been considered as an alternative material to TiO2 in solar cell construction due to its higher electron mobility. 7,8 However, the application of ZnO in visible light conversion is limited by its large band gap that restricts the absorption to the UV-range and a high charge carrier recombination rate. 9 Thus the combination of ZnO with a narrow band-gap p-type semiconductor is a promising approach to extend the light absorption range and increase charge carrier lifetimes at the same time. CuO is a p-type semiconductor with a tunable band-gap between 1.2 eV - 2.1 eV. 10 It shares its low cost, earth abundance and high stability with ZnO. Numerous studies showed that photoelectrochemical eciency can be improved by the formation of a p-n heterojunction between ZnO and CuO. 4,5,11,12 Several studies reported on the synthesis of ZnO/CuO nanocomposites, 13,14 decoration of ZnO nanostructures with CuO nanoparticles, 15 synthesis of ZnO/CuO core-shell nanowire arrays, 16 and ZnO/CuO nanotrees. 4,5 While the later morphology, that results in ZnO branches exposed from the CuO stem, is advantages for solar energy conversion and hydrogen production, the inverse morphology with exposed CuO branches allows to utilized the photocatalytic properties of CuO. Epitaxial growth of CuO(100) on ZnO(0001) has been observed as a strain reducing layer at the Cu 2 O/ZnO interface. 17 It is well known, 3


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that ZnO nanowires grow along the [0001] direction while CuO nanowires show growth along [010]. 18 Comparing the lattice parameters at the ( 1 0 1 0) ZnO facet and at the CuO(010) facet suggests that CuO nanowires can grow as branches attached to ZnO nanowire stems. 1921 ZnO nanowires (NWs) were synthesized by modifying an established hydrothermal procedure. 22 Reducing the starting concentration and growing the wires upside down resulted in long straight wires. Subsequently, a seed-growth sequence was applied to grow CuO branches attached to the ZnO wires. The conditions for CuO NWs synthesis were modied from known hydrothermal procedures 23 and optimized for achieving controlled growth of wire-like branches instead of core-shell structures. The resulting nanotrees (NTs) were characterized by UV-Vis, SEM and XRD for electronic and structural properties. Cyclic voltametry (CV) and electrochemical impedance spectroscopy (EIS) in the dark and under illumination were applied to characterize electrochemical and photo-electrochemical properties. Finally, the photocatalytic properties were investigated by photo-degradation of toluene and ascorbic acid in aqueous solutions.

Experimental Materials

Zinc acetate dihydrate (Zn(CH 3 COOH)2 · 2H2 O, 98.0% -101.0%, Alfa Asear) and copper acetate monohydrate (Cu(CH 3 COOH)2 · H2 O, 98+%, Acros Organics) were used as ZnO and CuO seeding precursors, respectively. Zinc nitrate hexahydrate (Zn(NO 3 )2 · 6H2 O, 99+%, Fisher), copper nitrate trihydrate (Cu(NO 3 )2 · 3H2 O, 99%, ACROS Organics) and hexamethylenetetramine ((CH 2 )6 N4 , 99+%, Alfa Asear) were used for ZnO and CuO hydrothermal growth. Potassium Chloride (KCl, 99%, Alfa Aesar) and tetrabutylammonium hexauorophosphate (BuNF4 , 99%, Sigma-Aldrich) were used as supporting electrolytes for aqueous solutions and acetonitrile (MeCN) solutions, respectively. Potassium ferricynide(III) (K 3 Fe(CN)6 , 4


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99+%, Acros Organics), potassium hexacynanoferrate(II) trihydrate (K 4 Fe(CN)6 , 98+%, Alfa Asear), ferrocene (99%, Alfa Aesar), L-(+)-ascorbic acid (99+%, Alfa Aesar), toluene (99.0%, Sigma-Aldrich) were used to characterize the photo-electrochemical performance of the electrodes. Indium tin oxide (ITO) coated glass slides were purchased from Yingkou Opv Tech New Energy Co. Ltd.

Preparation of ZnO/CuO NTs

For the formation of the ZnO seed layer, a 5 mM solution of Zn(CH 3 COOH)2 in ethanol was prepared and sonicated for 5 min. ITO coated glass slides ( 2 × 1 × 0.1 cm) were cleaned and sonicated in ethanol for 5 min. Preparation of the ZnO seed layer involved ve cycles of the following procedure. The clean ITO glass was immersed into 10 mL of 5 mM Zn(CH3 COOH)2 for 3 s, rinsed with ethanol, and dried under N 2 at room temperature. Subsequently. the seeded sample was annealed in a furnace at 350 °C for 45 min in air. Two cycles of this procedure were conducted to produce a uniform seed layer on the surface of the ITO slide. For hydrothermal growth of ZnO NWs, a 5 mM solution of Zn(NO 3 )2 and (CH2 )6 N4 in water was prepared with a mole ratio of 1:1. A ZnO-seeded ITO glass was placed on a silica ring with the seed layer facing down in a Teon liner. The Teon liner was placed in an autoclave and lled with 10 mL of the Zn(NO 3 )2 /(CH2 )6 N4 solution. The set-up is shown in Scheme 1 (a). Hydrothermal growth was preformed in the sealed autoclave at 90 °C. The autoclave was removed after 5 h. The growth procedure was repeated three times to achieve the desired length of the NWs. For generating the CuO seed layer on the ZnO NWs, a 5 mM solution of Cu(CH 3 COOH)2 in ethanol was prepared and sonicated for 10 min. The as-prepared ZnO NW substrate was immersed in 10 mL of 5 mM Cu(CH 3 COOH)2 solution for 10 s, rinsed with ethanol, and dried with N2 at room temperature. This procedure was repeated 4 times followed by annealing in a furnace at 250 °C for 2.5 h in air. 5


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For growing the CuO NW branches, a 2 mM solution of Cu(NO 3 )2 and (CH2 )6 N4 in water was prepared with a mole ratio of 1:1. The CuO-seeded ZnO slide was positioned on the silica support ring with the seeded side down inside a Teon liner. The liner was placed in an autoclave and lled with the prepared precursor solution. The sealed autoclave was heated at 90 °C for 5 h. Finally, the sample was annealed for 30 min at 250 °C exposed to air for converting remaining Cu(OH) 2 into CuO. 24

Structural and optical characterization

The morphology of the hierarchical structures was characterized by a Hitachi S5700 scanning electron microscope (SEM) at 20 kV and 25 µA. The crystal structure and composition was measured with a Bruker D8 X-Ray powder diractometer (XRD) using ltered Cu K α1 radiation coupled with a LynxEye position sensitive detector, and with a Thermo Fisher K-Alpha+X-ray photoelectron spectrometer (XPS). UV-vis transmission measurements of ZnO, CuO, and ZnO/CuO were carried out with a photon control ber probe spectrometer (SPM-002).

Electrochemical and photoelectrochemical characterization

All measurements were performed using a Bio-logic SP-300 potentiostat with an EIS module (Bio-logic Science Instruments). The as-prepared ZnO/CuO samples on ITO glass were used as the working electrode with platinum wire as the counter electrode and Ag/AgCl (sat. KCl) as the reference electrode for all measurements. 0.1 M KCl and 0.1 M BuNF 4 were employed as supporting electrolytes in aqueous solutions and in acetonitrile solutions, respectively. All photo-electrochemical measurements were taken with illumination by an 8-Watt 365 nm UV light source (Thermo Scientic 3UV-38 3UV) that resulted in a uence of 1020 µW/cm2 .

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Results and discussion Hexamethylene-tetramine-assisted thermolysis of ZnO/CuO NTs

Scheme 1: The set-up of the autoclave liner for ZnO and CuO hydrothermal growth is shown in (a). The sequence for ZnO/CuO NTs synthesis is shown in (b). The schematic diagram of the experimental set-up for the synthesis and the steps that are involved in the synthesis of ZnO/CuO NTs are shown in Scheme 1 (a) and (b), respectively. The reactions leading to ZnO NWs are shown in equations 1 to 3. 25 To reduce the formation of random ZnO deposition, the sample was placed with the seed layer facing down on a silica ring as support. For the same reason, the synthesis was performed with lower concentration of precursors compared to other established procedures. To obtain the desired NW length, the low concentration growth was repeated three times. During the hydrothermal growth, (CH2 )6 N4 releases OH- and precipitates Zn 2+ as Zn(OH)2 . Zn(OH)2 is deposited on the ZnO seed layer and is partly converted to ZnO. However, it is well known that the conversion is not complete and residual zinc hydroxide is converted into ZnO by a subsequent annealing step in air. 25 This step can be combined with the annealing of the rst CuO seed layer. 7


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− (CH2 )6 N4 + 10H2 O → 6HCHO + 4NH+ 4 + 4OH

(1)

Zn2+ + 2OH− → Zn(OH)2

(2)

Zn(OH)2 → ZnO + H2 O

(3)

Figure 1 shows a comparison between SEM images of wires prepared via the new approach and an established method with higher concentration precursors and the seed layer facing up during hydrothermal growth. Figure 1(a), 1(b), and 1(c) show images after the rst, the second and the third cycle of hydrothermal growth with 5 mM precursor solution. It can be seen that the wires retain their ordered vertical alignment on the substrate during the synthesis and grow in length. ZnO NWs prepared with the seed layer facing up and 20 mM precursor solution are shown in Figure 1(d). In addition to thin vertically aligned wires the sample shows much thicker randomly oriented wires. A sample grown with 3 cycles at low concentration (5 mM) and seed layer facing up is shown in Figure 1(e), exhibiting some randomly oriented NWs. The new procedure resulted in the highest density of ordered NWs and a high degree of homogeneity. The wires from this procedure were around 5 µm in length compared to 2 µm obtained by the established method (see Figure S1). After preparation of the ZnO NWs that represent the stems for the NTs, the CuO branches are grown following the reaction steps in equations 4 to 8. 23 The reaction of (CH 2 )6 N4 produced OH- and chelated Cu2+ to form the intermediate complex [Cu(NH 3 )4 ]2+ that is converted into Cu(OH) 2 and CuO. Residual Cu(OH) 2 was converted to CuO by annealing at 250 °C in lab air. For comparison, CuO NWs were also grown following the same procedure on a clean ITO glass substrate. An SEM image of pure CuO NWs is sown in Figure 1(f).

− NH3 + H2 O → NH+ 4 + OH

8


(4)

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2+ Cu2+ + 4NH+ + 4H + 4 → [Cu(NH3 )4 ]

(5)

[Cu(NH3 )4 ]2+ + 4OH− → [Cu(OH)4 ]2− + 4NH3

(6)

[Cu(OH)4 ]2− → Cu(OH)2 + 2OH−

(7)

Cu(OH)2 → CuO + H2 O

(8)

Figure 1: SEM images after 1st (a), 2nd (b), and 3 rd (c) cycle of hydrothermal growth of ZnO nanowires with seeded side facing down, (d) traditional high concentration hydrothermal growth of ZnO with seeded side facing up, (e) three cycles of low concentrtion hydrothermal growth of ZnO nanowires with seeded side facing up, and (f) hydrothermal growth of CuO nanorods prepared with the same amount of precursors used in ZnO/CuO NTs.

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Structural Characterizations of ZnO/CuO NTs

SEM images of ZnO/CuO NTs with dierent magnications are shown in Figure 2 and Figure S1a. The measurements were performed in the areas indicated in Figure S2. CuO branches show the parallelepipedal shape that is compatible with a monoclinic crystal structure. The branches are about 200 nm in length. The composition and crystal structure are determined by XPS and XRD, respectively. Figure 2(b) suggests that the ZnO stems are not fully covered by CuO branches. The majority of branches is attached to the upper part of the ZnO wire. This can be explained by limited precursor diusion in the ZnO NW array during the hydrothermal growth.

Figure 2: SEM images of ZnO/CuO NTs with dierent magnications. XPS was used to measure the surface composition of the ZnO/CuO NT sample shown 10


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in Figure 3 with C 1s (C-C binding energy: 284.5 eV) 26 as charge correction reference. As expected, the main components of the NTs are Zn, O, and Cu (Figure 3 (a)). The Zn 2p region shown in Figure 3 (c) reveals peaks at 1044.6 eV and 1021.4 eV that are assigned to Zn 2p1/2 and Zn 2p3/2 levels in Zn2+ in the wurtzite ZnO structure, respectively. 26 The Cu 2p region in Figure 3 (d) shows the expected Cu 2+ peaks and satellites of CuO: Cu 2p 1/2 (s) (963.3 eV), Cu 2p1/2 (953.4 eV), Cu 2p3/2 (s) (941.7 eV), and Cu 2p 3/2 (932.8 eV). 15,27 Cu1+ in Cu2 O peaks, on the other hand, would be expected to show much weaker satellites 28 and an overall shift to lower energies. 26 Contributions at 531.1 eV can be attributed mostly to OH- groups from dissociated water which cannot be completely removed by annealing at a temperature of 250 °C, given that the sample is exposed to water in laboratory air. 29,30 The peaks at 530.0 eV, and 528.9 eV can be assigned to O 2− in ZnO and in CuO, respectively according to XPS spectra of pure ZnO and CuO in S3. 15,2933 It should be noted that neither indium nor tin was observed in the survey and the ITO substrate is not expected to contribute to the O1s spectrum. Contributions from carbon are contaminants mostly in the form of adventitious carbon due to bench top synthesis and exposure to laboratory air.

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Figure 3: XPS spectrum of ZnO/CuO NTs (a) and high-resolution spectrum of Zn 2p, O 1s, and Cu 2p peaks, respectively (b-d).

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SEM images of the trees suggested that CuO branches are attached preliminary to the top of the stems. To conrm this morphology we compared standard XRD and high-resolution thin-lm pattern. Standard 2θ/θ XRD mode shown in Figure 4 probes the whole depth of the ZnO/CuO NT lm. Consequently, the signal is dominated by contributions from wurtzite ZnO involving (100), (002), (101), and (102) peaks [JCPDS le No.36-1451] and high-resolution XRD spectrum of pure ZnO NWs is shown in S4. In high-resolution thin-lm mode (inset in Figure 4) the detection depth can be estimated to be 1 µm and the XRD pattern shows (111) and (11 ¯ 1) peaks from the monoclinic CuO branches [JCPDS le No. 45-0937] in addition to the ZnO peaks and high-resolution XRD spectrum of pure CuO NWs is shown in S4. The XRD measurements conrm that the NTs consist of wurtzite ZnO NW stems decorated on the top with monoclinic CuO branches. The cubic structure of Cu 2 O is not observed.

Figure 4: XRD spectrum of ZnO/CuO NTs in coupled 2 θ/θ mode, inset: XRD spectrum of ZnO/CuO NTs in high-resolution mode.

Optical characterizations of ZnO/CuO NTs

UV-vis absorption spectroscopy was employed to determine the band gap of the pure ZnO NW sample, the pure CuO NW sample and the ZnO/CuO NTs (Figure 5). The measurements were performed in the areas indicated in Figure S2. Band gap values were 13


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calculated via the Kubelka-Munk (K-M) method (see calculation in S5). 34 The resulting band gaps are 3.36 eV for pure ZnO and 1.21 eV for pure CuO. These band gaps agree well with reported bulk values. 35,36 For the NTs the corresponding band gaps are very similar, 3.25 eV for ZnO stems and 1.21 eV for CuO branches as shown in the inset in Figure 5. This conrms that the NTs are bi-material structures and that the synthesis of the CuO branches does not lead to major doping of the ZnO stems, since Cu doping has been reported to shift the absorption onset of ZnO nanoparticles to 3.55 eV upon 5% doping. 37

Figure 5: UV-vis transmittance spectra of CuO, ZnO, and ZnO/CuO NTs (inset: K-M analysis with normalized UV-vis transmittance spectra).

Electrochemical and photoelectrochemical properties of ZnO/CuO NTs

Cyclic voltammetry measurements were carried out in the dark with a scan rate of 40 mV/s on ZnO, CuO, and ZnO/CuO NTs in 1 mM Fe(CN) 6 3- /Fe(CN)6 4- in 0.1 M KCl aqueous solution (Figure S6a) and in 1 mM ferrocene in 0.1 M BuNF 4 acetonitrile solution (Figure S6b). In both systems the NTs displayed the highest oxidation and reduction peak currents. This indicates that the interfacial charge transfer is more ecient for ZnO/CuO NTs compared with pure ZnO and CuO NWs and that the addition of the CuO branches did 14


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not decrease the conductivity. This can either be explained by the increase in surface area in the hierarchical NTs and/or by the additional internal eld due to the p-n heterojunction between CuO and ZnO. Scan rate dependent cyclic voltammograms were collected for the ZnO/CuO NTs in 1 mM Fe(CN)6 3- /Fe(CN)6 4- in 0.1 M KCl aqueous solution (Figure 6(a)), and in 1 mM ferrocene in 0.1 M BuNF4 acetonitrile solution (Figure 6(b)). The inset in Figure 6(a) and (b) show the shift of anodic and cathodic peak currents with the square root of scan rate. The peaks shift linearly with a tted R 2 of 0.9987 (oxidation) and 0.9972 (reduction) for water, and 0.9976 (oxidation) and 0.9869 (reduction) for acetonitrile. This indicates that the reaction is diusion-controlled and that charge transfer across the ZnO/CuO interface is not rate limiting. 3840 EIS was performed in the dark and under illumination in Fe(CN) 6 3- /Fe(CN)6 4- at the open circuit voltage. Nyquist plots of the measurements on pristine ZnO NWs and CuO/ZnO NTs in the two solvents are shown in Figure 8(a) and Figure 7(a). The diameter of the semicircle along the real axis corresponds to the charge transfer resistance and serves as an indicator for the electrochemical activity of the electrode. ZnO/CuO NTs have smaller arc radius corresponding to smaller charge transfer resistance at the interface compared to pristine ZnO in both solvents. The charge transfer resistance is a complex quantity that includes interface and bulk properties. However, since CuO is not in direct contact with the ITO electrode in the NT samples, it can be concluded that the interface between ZnO and CuO does not deteriorate the electrochemical activity of the sample. Under illumination the radii of the semicircle are smaller for the NT samples compared to the NW samples. This is an indication for the formation of a p-n hetero-junction that allows charge separation, increases charge carrier lifetime, induces an additional potential under illumination and reduces the charge transfer resistance. 41,42 It should be noted that this measurement is independent of the total surface area. Mott-Schottky (MS) analysis was performed to investigate the semiconducting properties

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Figure 6: Scan rate-resolved CV curves of ZnO/CuO NTs in 1 mM Fe(CN) 6 3- /Fe(CN)6 4- in 0.1 M KCl aqueous solution, and in 1 mM ferrocene in 0.1 M BuNF 4 in acetonitrile. (insets: current vs square root of scan rate plots).

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of the NTs. MS analysis gives qualitative and quantitative information about the type of semiconductor, the donor concentration, and the at band potentials. The Mott-Schottky equation has been used to analyze the data. 43 The variation of the capacitance of the space charge region versus the applied bias potential (Mott-Schottky plot) for the NTs is shown in Figure 7(b) and 8(b) for pristine CuO, pristine ZnO, and ZnO/CuO NTs in the dark at 1 kHz with 0.1 M KCl and 0.1 M BuNF 4 in water and acetonitrile, respectively. The positive slope of pristine ZnO is characteristic for the intrinsic n-type character of ZnO. MS analysis measured in water results in a donor density of 7.31×1019 cm−3 which is comparable to previously reported values, 37 and a at band potential of -0.7417 V. The corresponding values for pristine CuO are 1.34×1019 cm−3 which again compares well to published values, 44 and +0.3373 V. Values for the measurement in acetonitrile are very similar (Table S7) The Mott-Schottky plot for the hetero-structured NTs shows the characteristic feature of a p-n junction with positive and negative slopes. It should be noted that in the NTs the electrolyte is in contact with the CuO branches, as well as with the ZnO stem which makes further analysis of the MS plot complicated. However, a direct comparison of the at band potential with the band gaps of the two materials would suggest that CuO and ZnO form a type II heterojunction at the interface. In Figure 7(c) and 8(c), transient photocurrents of ZnO, CuO, and ZnO/CuO NTs in 0.1 M KCl under 365-nm illumination at 0-V bias potential are shown. Photocurrents of pristine CuO and ZnO show the expected negative and positive photocurrent, respectively, according to their majority charge carriers. The addition of CuO branches to the ZnO NW stems leads to a two-fold increase of the photocurrent. This is another strong indication for the formation of a p-n heterojunction between ZnO and CuO that leads to enhanced absorption and to the formation of spatially separated electrons and holes. The EIS measurements in the dark and under illumination support this nding. The stability of ZnO/CuO NTs was monitored by storing a sample in a glass bottle exposed to air and repeatedly recording the photocurrent in 0.1 M KCl aqueous solution

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Figure 7: Nyquist plots measured in 1 mM Fe(CN) 6 3- /Fe(CN)6 4- in 0.1 M KCl aqueous solution (a), Mott-Schottky plots measured under dark condition in 0.1 M KCl aqueous solution at 1 kHz (b), transient photocurrents at 0-V bias potential in 0.1 M KCl aqueous solution under illumination (c).

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Figure 8: Nyquist plots measured in 1 mM ferrocene in 0.1 M BuNF 4 in acetonitrile (a), Mott-schottky plots measured under dark condition in 0.1 M BuNF 4 in acetonitrile at 1 kHz in (b), transient photocurrents at 0-V bias potential in 0.1 M BuNF 4 in acetonitrile under illumination (c).

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under identical conditions (Figure S8). After 6-month the photocurrent decreased less than 5% with an RSD of 2.37%.

Photocatalytic Activity of ZnO/CuO NTs

Toluene and ascorbic acid are used as model compounds in evaluating photocatalytic activity of ZnO/CuO NTs. Toluene has a high oxidation potential of 2.26 eV and is hard to oxidize with electrochemical methods. 45 However, degradation of toluene has been shown to be very ecient in the present of photo-generated electrons and holes. 46,47 Ascorbic acid, on the other hand, is a common electrochemical active acid that has served as a model system for photo-degradation. Therefore, toluene and ascorbic acid are used for evaluating photocatalytic activity of ZnO/CuO NTs. The CV curve of ascorbic acid in 0.1 M KCl aqueous solution and toluene in 0.1 M BuNF4 /MeCN in the dark are shown in Figure S9 and S10. Both systems show very small dark current that was subtracted in the following measurements under illumination and no degradation of ascorbic acid or toluene in the dark is expected. Transient photocurrent measurements were taken at a series of concentrations of ascorbic acid in 0.1 M KCl aqueous solution with 0-V bias potential and with the background current subtracted (Figure 9(a)). Three repeated transients were recorded. Prompt and reversible photocurrent response is observed for each on-o cycle. The current at the end of the rst 50 s illumination period was plotted as a function of the concentration in Figure 9 (b) and (d). The current was tted as a double logarithmic function of the concentration

C

I

for low concentrations

(0 < C ≤ 2.08 µM ) : log I = A log C + B , and resulted in an R2 of 0.986. For high concentrations above 2.08 µM a linear function, I = D ∗ C + E , was tted and resulted in an R2 of 0.996. The same procedure applied to the photocurrent measurements of toluene yielded an R2 of 0.978 below 5 µM , and an R2 of 0.995 above 5 µM . The transition from an exponential to a linear relationship between

I and C conrms that heterogeneous photo-

catalysis is established and the photocurrent is only related to the concentration of molecules 20


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rather than the eciency of catalyst. This kinetic is typical for a simple decomposition reaction of toluene and ascorbic acid on the surface in water. In case of the NTs it most likely involves generation and separation of electrons and holes in the catalyst, followed by the generation of reactive species. If we assume that the NTs indeed form a p-n heterojunction with type II level alignment, holes would accumulate in the CuO branches and electrons in the ZnO stems. Accordingly, O•2 would be generated at the ZnO surface and OH• at the CuO surface. Finally, the reactive species lead to oxidation of the adsorbed target molecule to innocuous nal products like CO2 and water that desorb from the surface. 48 At low concentration of the target molecule, the interface concentration is considered very small and the reaction rate is limited by the bulk concentration, resulting in rst order kinetics. At high concentration of target molecule, on the other hand, the reaction shows zero order kinetics and the reaction rate is a constant limited by only the constant number of reaction sites on the surface. For comparison, the photo-current measured with nontreelike randomly oriented ZnO/CuO composites that were grown without the seeding step is shown in Figure S11. The much smaller photo-current is another indication that the treelike hetero-nanostructures with exposed CuO branches combine ecient charge separation due to the p-n heterojunction with the photocatalytic activity of CuO. However, it should be noted that the photocurrent depends strongly on the accessible surface area that is very hard to determine and this comparison should be taken with care. Further proof of the proposed mechanism will involve additional investigations.

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Figure 9: Transient photocurrents of ascorbic acid in 0.1 M KCl aqueous solution in (a). I vs. C curve for (a) in (b), inset: t curve in the concentration range from 0 to 2.08 µM. Transient photocurrents of toluene in 0.1 M BuNF 4 in acetonitrile in (c). I vs. C curve for (c) in (d), inset: t curve in the concentration range from 0 to 5 µM.

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Conclusion In summary, a new synthetic route was developed for the synthesis of novel tree-like nanostructures with ZnO NWs as stems and CuO nanorods as branches. The morphology of the structure was characterized with SEM, XRD, and XPS, and compared to nanostructure obtained by an established growth method. The electrochemical and photoelectrochemical properties are investigated in acetronitrile and water solutions. For both systems NTs show high conductivity, fast carrier transport and eective electron-hole separation. Therefore, ZnO/CuO NTs are a promising semiconductor material for future applications in solar energy conversion devices, photo-catalysis and photocatalytic sensors.

Acknowledgement The authors gratefully acknowledge the funding of NSF 1428149 for Thermo Scientic K-Alpha instruments.

Supporting Information Available SEM image of a single ZnO/CuO NT, photos of CuO, ZnO, and ZnO/CuO NTs electrodes, XPS and XRD spectra of ZnO NWs and CuO NWs, Kubelka-Munk analysis, CV curves of CuO, ZnO, and ZnO/CuO NTs in 1 mM Fe(CN) 6 3- /Fe(CN)6 4- in 0.1 M KCl aqueous solution and 1 mM ferrocene in 0.1 M BuNF 4 in acetonitrile at band voltages and carrier densities of ZnO, CuO, ZnO/CuO NTs in 0.1 M KCl aqueous solution and 0.1 M BuNF4 in acetonitrile, transient photocurrent curves(a) and photocurrents(b) in 0.1 M KCl aqueous solution recorded in continuous months, CV curve of 1 mM ascorbic acid in 0.1 M KCl aqueous solution, CV curve of 1mL Toluene in 0.1 BuNF 4 in acetonitrile, SEM image of nontree structure of ZnO/CuO prepared without seeding process, transient photocurrent in 0.1 M KCl aqueous solution, and c:transient photocurrent in 0.1 BuNF 4 in acetonitrile. 23


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