Co3O4 Tri-component Heterojunction

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Construction of Au/CuO/Co3O4 Tri-component Heterojunction Nanotubes for Enhanced Photocatalytic Oxygen Evolution under Visible Light Irradiation Guowen Hu, Chen-Xia Hu, Zhi-Yuan Zhu, Lei Zhang, Qiang Wang, and Hao-Li Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01153 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Sustainable Chemistry & Engineering

Construction

of

Heterojunction

Au/CuO/Co3O4 Nanotubes

Tri-component

for

Enhanced

Photocatalytic Oxygen Evolution under Visible Light Irradiation Guowen Hu, Chen-Xia Hu, Zhi-Yuan Zhu, Lei Zhang, Qiang Wang*, and Hao-Li Zhang* State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Key Laboratory of Special Function Materials and Structure Design (MOE), Lanzhou University, Lanzhou, 730000, P. R. China Address: No. 222 Tianshui South Road, Lanzhou, Gansu, P. R. China Corresponding Author *H.-L. Zhang e-mail: [email protected]; *Q. Wang e-mail: [email protected].

KEYWORDS: Heterojunction, Nanotube, Photocatalysis, Oxygen evolution, Visible light irradiation.

ACS Paragon Plus Environment

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ABSTRACT: Semiconductor-metal heterojunctions are widely used in the design of photocatalyst for water splitting and organic compound degradation. In this work we introduced the design and fabrication of Au/CuO/Co3O4 tri-component heterojunction to produce oxygen efficiently through photocatalysis of water. A facile and universal strategy was employed to prepare Au/CuO/Co3O4 tri-component nanotubes. The CuO/Co3O4 nanotubes were firstly synthesized by electrospinning technique and subsequent thermal treatment. Upon visible light excitation, the CuO/Co3O4 nanotubes in the presence of HAuCl4.4H2O readily transformed into Au/CuO/Co3O4 heterojunction nanotubes. These tri-component heterostructures demonstrated high O2 generation rate of 2.92 mmol h-1g-1 during the photocatalytic process, a value much larger than that of CuO/Co3O4 nanotube control. The work suggests that the design of tricomponent heterojunctions integrating semiconductor-semiconductor and semiconductor-metal heterojunctions could be an effective route to raise photocatalytic oxygen production efficiency under solar light.


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INTRODUCTION

Intensive researches are being conducted in seeking clean energy production.1-4 Photocatalysis has emerged as a promising solution for energy shortage and deteriorating environmental problems,5-9 among which water splitting via photocatalysis to provide hydrogen and oxygen as new energy sources has attracted great attention.10-12 Photocatalytic water splitting in a general sense refers to reduction of water to hydrogen, or oxidization of it to oxygen, or simultaneous occurrence of both reactions for an “overall” splitting in the most desirable way. Compared to water reduction where two electrons are involved, the oxidization of water is a slow fourelectron transfer process, requiring simultaneous O-H bond breaking and O-O bond forming. Hence, highly efficient catalysts are essential to accelerate water oxidation processes.13 photocatalysts to oxidize water contain expensive and rare noble metals like Ru and Ir, which would radically hinder their wide application.14-16 Therefore, fabrication of highly efficient and low-cost photocatalysts are key issues in converting solar energy into chemical energy effectively.

Recent progress has turned to some transition metal-based catalysts because of their low cost, rich storage, and low toxicity.17-20 Among various semiconductors of transition metals oxides, like CuO21-22 and Co3O423-26 photocatalysts have attracted intensive interest. These oxides are very robust

and

can

result

in

narrow to

wide band

gap

depending on

their

dimensions/morphologies, and hence are promising substitute for the noble metals as photocatalysts. For instance, the CuO/Co3O4 sea anemone-like composite has been reported to enhance photocatalytic performance.27 However, the photocatalytic efficiency of bare semiconductor is limited owing to low mobility of charge carriers and serious electron–hole


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recombination as well as the lack of highly active catalytic sites on the semiconductor surface.28 To optimize the photocatalytic performance, the semiconductors have been integrated with plasmonic metal nanoparticles to form hybrid photocatalytic structures,29-31 as noble metal nanoparticles could extend light absorption, facilitate charge separation, transfer and augment the surface reactivity as well in most cases.32-34 Some recent works have shown that noble metalCo3O4 heterojunctions exhibited high catalytic activity.35-36

Scheme 1. The preparation process of the Au/CuO/Co3O4 heterojunction nanotubes.

Herein, we propose a tri-component heterostructure to further raise the efficacy of metaloxide-based photocatalysts. This strategy is based on noble-metal/CuO/Co3O4 tri-component heterojunction nanotubes. The spinning technique was used to fabricate Au/CuO/Co3O4 heterojunction nanotubes, in combination with thermal treatment and photoreduction. Such heterojunction structure integrated the advantages of interfacial sensitization and plasmonic effects, thus preventing electron-hole recombination and enhancing catalytic efficiency. Encouraged by the unique band structure and increased light absorption, the resultant heterojunction nanotubes significantly improved water oxidation efficacy, far exceeding those of the corresponding CuO/Co3O4 binary heterojunction nanotubes, and is also superior to other


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types of heterojunctions such as TiO2/C3N437, Au@TiO238 heterostructure and TiO2/Fe2TiO512 hollow microspheres. This work thus combined the benefits of metal-semiconductor heterojunction and plasmonic effects, shedding light on the design of photocatalysts with high O2 evolution efficiency for sustainable conversion of sunlight into clean energy. EXPERIMENTAL SECTION Materials. All the chemicals and reagents are of analytical grade and used without further purification. Polyvinylpyrrolidone (PVP), polyacrylonitrile (PAN), cupric acetate and cobalt (II) aetate tetrahydrate were from Macklin. Hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl4·4H2O) were purchased from Sigma-Aldrich. Purified water with resistivity of 18.2 MΩ.cm was from a Molecular Lab Water Purifier. Instrumentation. The field-emission scanning electron microscope (FE-SEM, Hitach, S-4800, Japan) and the TEM (FEI Tecnai F30, 200 kV) were employed to characterize the morphology of the heterostructures. The composition was analyzed via energy dispersive X-ray (EDX) spectroscopic equipment on the TEM. Other instruments include a Bruker AXS D8 Advance diffractometer with CuKα radiation (l=1.5418 Å) for X-ray powder diffraction pattern measuremnet; a PHI-5702 with AlKα radiation for X-ray photoelectron spectroscopy (XPS) measurements; an Agilent Cary 5000 UV-Vis-NIR Spectrophotometer to obtain the absorption spectra, where BaSO4 was used as a reference of 100% reflection; An inductively coupled plasma-atomic emission spectrometer (ICP-AES, Varian VISTA-MPX) for determination of Au contents; One ASAP2020 was used for the adsorption/desorption measurement; a CHI660E potentiostat (Chenhua, China) for the electrochemical tests.


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The synthesis of Au/CuO/Co3O4 heterojunction nanotubes. First, CuO/Co3O4 heterojunction nanotubes was fabricated according to the reference, except a heating rate of 1 0C min-1.39 Then 100 mg of the prepared CuO/Co3O4 heterojunction nanotubes were added into 25mL (1mM HAuCl4) ethanol solution. After being illuminated for 30 mins under a Xe arc lamp of 300 Watts, the product was obtained through centrifuging. After being washed with water and centrifuged for three times, the product was eventually dried in vacuum. Photoelectrochemical measurements. 5 mg of the as-prepared Au/CuO/Co3O4 heterojunction nanotubes was used to prepare the working electrode based on the procedure reported in reference.40 Pt wire was the counter electrode and Ag/AgCl (saturated KCl) was the reference electrode. Mott –Schottky measurement was carried out under the following condition: fixed frequency: 1000 Hz; amplitude: 5 mV; the electrolyte: 1 M NaOH aqueous solution at pH of ∼13.6. The transient photocurrent was measured at a bias voltage of 0.8 V in aqueous solution containing 0.1 mol/L Na2SO4 and 0.1 mol/L NaOH. The irradiation source was a Xe lamp (300 W) 17 cm away, equipped with a high-pass filter (> 420 nm). The apparent quantum efficiency (AQE) was obtained by following the procedure as reported in reference.41 Photocatalytic Oxygen production by Au/CuO/Co3O4 nanotubes. The photocatalytic water oxidation experiments were carried out based on the procedures reported in reference.41-43 Briefly, 10 mL aqueous solution containing 0.1 M Na2S2O8 (sacrificial electron acceptor) and 1.0 M NaOH at pH=8.8 was deaerated by N2, with addition of 10 mg Au/CuO/Co3O4 nanotubes as the photocatalyst. A Xenon lamp of 100 mW was employed as the light source. The produced O2 was detected by gas chromatography equipped with a thermal conductivity detector (TCD) (Shimadzu GC-2014C).


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RESULTS AND DISCUSSION The synthesis route of the Au/CuO/Co3O4 tri-component heterojunction nanotubes involves mainly two steps as shown in Scheme 1. First, 1D mesoporous CuO/Co3O4 heterojunction nanotubes were fabricated through the electrospinning technique combined with thermal treatment. Second, noble metal gold was decorated onto the CuO/Co3O4 heterojunction nanotubes by photoinduced reduction. Figure 1A showed the representative SEM images of the as-electrospun nanotubes with smooth surface, lengths of several micrometers and diameters around 390 nm. After calcination, well-ordered mesoporous hollow structure of CuO/Co3O4 nanotubes was formed, as evidenced from Figure 1B (SEM) and Figure 1C (TEM). The hollow structure of the CuO/Co3O4 nanotubes is consistent with previous literature.44 However, the average diameter of the tube dramatically decreased, which can be explained as a result of decomposition of the utilized acetate anions and elimination of the used polymer.


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Figure 1. (A) The SEM image of as-spun nanotubes, (B-C) SEM and TEM image of CuO/Co3O4, (D-F) SEM, TEM, SAED pattern and HRTEM images of Au/CuO/Co3O4 nanotubes, (G) EDS mapping of Au/CuO/Co3O4 nanotubes. Au nanoparticles (AuNPs) are subsequently loaded on the surface of CuO/Co3O4 heterojunction nanotubes by photoinduced reduction method. SEM (Figure 1D) and TEM


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(Figure 1E) images display the rough surface of the nanotubes, with pores of approximately 210 nm in diameter. However it is hard to identify the produced AuNPs. High-resolution images (HRTEM) of the Au/CuO/Co3O4 nanotubes in Figure 1F instead reveal the co-existence and intimate contact of Au, Co3O4 and CuO in the heterojunction nanotubes. The lattice space of 0.236 nm and 0.243 nm are ascribed to the Au (111) plane45 and Co3O4 (311) plane,46 respectively. The lattice spacing of 0.232 nm agrees well with the (111) plane of hexagonal wurtzite CuO.47 The EDX spectrum in Figure S1 also confirms the Au/CuO/Co3O4 nanotubes consist of Au, Cu, Co and O elements. In addition, the SAED patterns (the inset of Figure 1E) indicate the Au/CuO/Co3O4 nanotubes have polycrystalline configurations, consistent with the TEM results and further XRD analysis. Moreover, the EDAX elemental mapping demonstrates the homogeneous distribution of Co, Cu, and O and coexistence of Au on the nanotube surface (Figure 1G).

Figure 2. (a) X ray diffraction pattern of Au/CuO/Co3O4 and CuO/Co3O4 nanotubes, (b) N2 adsorption/desorption isotherm and pore diameter distribution (inset) of Au/CuO/Co3O4 nanotubes.


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X-ray diffraction (XRD) patterns of the CuO/Co3O4 and Au/CuO/Co3O4 nanotubes are displayed in Figure 2a. The peaks in the CuO/Co3O4 nanotubes could be perfectly indexed as the cubic spinel Co3O4 (PDF#43-1003) and CuO phase (CuO-PDF#45-0937). Meanwhile, compared with the XRD pattern of the CuO/Co3O4, the additional diffraction peaks of Au/CuO/Co3O4 at 77.6°, 64.6°, 44.4° and 38.2° correspond to the (311), (220), (200), and (111) crystal planes of Au (PDF#04-0784), respectively,45 which evidenced the successful in situ photodeposition of AuNPs onto the CuO/Co3O4 nanotubes. The low response to diffraction intensity can be contributed to the lower content of Au.48 Indeed, inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis showed a low loading Au of 2.05 wt% on the heterojunction nanotubes. In addition, as shown in Figure 2b, Brunauer-Emmett-Teller (BET) specific surface area of 20.7 m²/g for the Au/CuO/Co3O4 nanotubes is obtained through adsorption-desorption measurements. Meanwhile, the results indicate a typical shape of type-IV for mesoporous oxides. The pore size distribution curve of Au/CuO/Co3O4 nanotubes (inset in Figure 2b) exhibits a sharp peak centered at about 17.8 nm, further confirming the mesoporous structure of the heterojunction.


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Figure 3. The XPS spectra of Au/CuO/Co3O4 nanotubes: (a) Survey Spectrum, (b) Cu2p, (c) Co2p, (d) Au4f. X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition and surface state of the Au/CuO/Co3O4 heterojunction nanotubes. The survey spectrum of Au/CuO/Co3O4 nanotubes shows that the nanocomposite is composed of elements of C, O, Cu, Co and Au (Figure 3a). The Cu 2p spectra in Figure 3b showed two peaks at ∼933.9 and 953.8 eV, which correspond to Cu 2p3/2 and Cu 2p1/2, respectively. Together with an additional satellite peak at ~942.0 eV,49 elemental copper in the Au/CuO/Co3O4 nanotubes exist in the form of Cu2+ can be concluded. For elemental Co, two peaks at ~779.6 and 794.6 eV are assigned to Co 2p3/2 and Co 2p1/2 (Figure 3c), respectively. In addition, a spin-energy separation


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of ~15 eV is also observed. These facts indicate the elemental Co is present in the form of Co3O4.50 The spectrum of Au 4f (Figure 3d) shows two peaks at 84.0 eV and 87.4 eV, which are assigned to Au 4f7/2 and Au 4f5/2 bands, respectively and implies neutral Au atoms. The observation further proves AuNPs are successfully photodeposited on the CuO/Co3O4 nanotubes.51

Figure 4. (a) UV-visible diffuse reflectance spectra of Au/CuO/Co3O4 and CuO/Co3O4 nanotubes, (b) Tauc plots of CuO and Co3O4 nanotubes through Kubelka−Munk function transformation. The UV-vis diffuse reflectance spectra of the as-prepared CuO/Co3O4 and Au/CuO/Co3O4 nanotubes are displayed in Figure 4a, where two obvious bands are assigned to the absorption of CuO and Co3O4, respectively. After the introduction of AuNPs on the surface of CuO/Co3O4, the Au/CuO/Co3O4 exhibit clearly increased photoresponse across the visible light region. As Au nanoparticles absorb strongly in this region stemming from surface plasmon resonance, they can serve as effective light harvesting materials to promote the photoresponse of Au/CuO/Co3O4 and


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hence the catalytic efficiency.52-55 According to the Tauc’s equation,56 the band gap energies of CuO and Co3O4 can be extracted (Figure 4b): (αhv) = (Ahv-Eg)n/2

(1)

where α is the absorption coefficient, h the Plank’s constant, v the light frequency, A the constant and Eg band gap energy. The n number is dependent on the optical transition type of the semiconductors, and for CuO and Co3O4 with directly allowed transitions, the value of 1 is adopted.57-58 Accordingly, a band gap of 1.49 eV for CuO and 1.87 eV for Co3O4 nanotubes was estimated.59-60

Figure 5. Mott-Schottky curves of Co3O4 and CuO in aqueous solution containing 1 M NaOH. The Mott-Schottky (MS) plots of CuO and Co3O4 photoelectrodes in Figure 5 are used to extract the flat band potential of CuO and Co3O4. A linear relationship of 1/C2 versus applied potential displays a straight line with a negative slope, which corresponds to depletion regions typical of a p-type semiconductor. The flat band potentials are determined to be equivalent to 1.415 V and 1.233 V versus the reversible hydrogen electrode (RHE) for the CuO and Co3O4,


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respectively. For a p-type semiconductor (conductivity not measured), previous research proposed an empirical estimation of the energy difference of ~0.20 V between the top of the valence band (VB) and the Fermi level59, 61. That is, the VB is ~0.20 V more positive. Hence, the valence band edge of the CuO and Co3O4 are estimated to be 1.615 V and 1.433 V, respectively.62 Accordingly, considering the average band gap energy value (1.49 eV for CuO) obtained from the Tauc plots of UV/Vis spectra (Figure 4b), the conduction band (CB) and VB positions of CuO are calculated to be 0.125 V and 1.615 V, respectively. Similarly, the CB and VB positions of Co3O4 are calculated to be -0.437 V and 1.433 V, respectively.

Figure 6. (a) O2 generation of Au/CuO/Co3O4 and (b) cycling measurements for stability evaluation. The photocatalytic results were shown in Figure 6. Remarkably, the Au/CuO/Co3O4 nanotubes catalyst resulted in the highest O2 evolution rate (2.92 mmol h-1g-1), enhancing O2 evolution rate by 47 percent compared with that of the CuO/Co3O4 nanotubes (1.98 mmol h-1g-1). Obviously AuNPs played a crucial role in this situation, which were expected to act as electron trap centers to enhance charge separation during photocatalysis. As a control measurement, without light


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irradiation only negligible amount of O2 is generated (

Co3O4 Tri-component Heterojunction

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