Article pubs.acs.org/IECR
Cobalt Phosphate−ZnO Composite Photocatalysts for Oxygen Evolution from Photocatalytic Water Oxidation Yabo Wang,† Yongsheng Wang,‡ Rongrong Jiang,† and Rong Xu*,† †
School of Chemical & Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459 School of Mechanical & Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
‡
S Supporting Information *
ABSTRACT: Cobalt based oxygen evolution catalysts (Co−Pi) were loaded on the surface of ZnO by photochemical deposition in a neutral phosphate buffer solution containing Co2+ ions. Structural, morphological, and optical properties of the samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy, X-ray photoelectron spectroscopy, and UV−vis diffuse reflectance spectra techniques. The Co−Pi phase formed was amorphous and was deposited on the surface of ZnO uniformly as a layer of nanoparticles. The enhanced activity for oxygen evolution was directly observed from photocatalytic water oxidation over Co−Pi loaded ZnO. The oxygen produced in the first hour was more than 4 times of that obtained over ZnO alone. The results suggest that Co−Pi played the role of cocatalyst, which can trap photogenerated holes, leading to the enhancement of electron and hole separation efficiency. Further studies showed that the mixture of cobalt phosphate and ZnO exhibited similar enhancement in activity for oxygen evolution which could be due to the oxidation of nonactive cobalt(II) phosphate to active Co−Pi with higher oxidation states of cobalt upon light illumination during photocatalytic water oxidation process. In both systems, ZnO photocorrosion was observed based on inductively coupled plasma, XRD, and FESEM analyses.
1. INTRODUCTION Conversion of solar energy to chemical energy by water splitting represents an attractive process to solve the global energy and environmental challenges, as sunlight and water are abundant and renewable. Many efforts have been devoted to the searching of efficient photocatalysts to produce hydrogen and oxygen from water under solar energy in the past few decades.1−5 So far, only a limited number of semiconductor photocatalysts have been reported capable for overall water splitting,6−9 because semiconductor materials with suitable redox potentials and band gap for simultaneous water reduction and oxidation are not commonly available. Most research activities are focused on half reactions, that is, separate water reduction and water oxidation involving sacrificial reagents as electron donors and acceptors, respectively. The combination of a hydrogen evolution photocatalyst and an oxygen evolution photocatalyst may lead to overall water splitting, such as the construction of a Z-scheme reaction system.10,11 Compared with hydrogen production from water reduction which requires two electrons, water oxidation for oxygen production is more challenging since it requires four holes for generation of two oxygen−oxygen bonds in one oxygen molecule.12 To date, many efficient photocatalysts for hydrogen production have been developed, including doped and undoped metal oxides, metal sulfides, and their solid solutions.13−20 It has been generally found that cocatalysts on the surface of base semiconductor particulates improve the separation and transportation of photogenerated electrons and holes which leads to improved photocatalytic activities.16,17 Compared with hydrogen evolution photocatalysts, the reported active photocatalyst systems for oxygen evolution from water oxidation are much fewer. So far, WO321,22 and BiVO423 are typical photocatalysts © XXXX American Chemical Society
for water oxidation. Recently, some new semiconductor photocatalysts were reported to be active for oxygen evolution from water, such as BiCu2VO6,24 BiZn2VO6,25 TiNxOyFz,26 and nitrogen-doped CsCa2Ta3O10,27 with the oxygen evolution rates ranging from about 3−70 μmol h−1. Layered double hydroxides (LDHs) were also reported capable of photocatalytic water oxidation.28 Ag3PO4 crystals developed by Ye’s group showed rather good activity (1.6 and 7.8 times higher than that of BiVO4 and WO3, respectively) and a high quantum efficiency (about 90% at 420 nm) for oxygen evolution in the presence of AgNO3 as a sacrificial reagent.29 Nevertheless, it is still a great challenge in developing efficient and stable photocatalysts and understanding the mechanisms for water oxidation. A novel earth-abundant cobalt-based oxygen evolution catalyst was first reported by Kanan and Noreca in 2008. This amorphous catalyst can be readily formed in neutral phosphate buffer solution containing Co2+.12 Although the exact structure and composition of the cobalt based phosphate catalyst (Co−Pi) has not been fully understood, it is suggested that Co2+ was oxidized to Co3+ during photodeposition and precipitated in the forms of amorphous oxide and hydroxide containing phosphate.30 Electrochemical, extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR) results indicated that the catalytic ability of Special Issue: APCChE 2012 Received: November 25, 2011 Revised: March 24, 2012 Accepted: April 3, 2012
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dx.doi.org/10.1021/ie2027469 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Co−Pi is related to the oxidation of Co2+ to Co3+ and Co4+, leading to the formation of high valence Co4+−O intermediates.31−34 Furthermore, when deposited on photoelectrodes, such as ZnO, WO3, and α-Fe2O3, Co−Pi nanoparticles enhanced photocurrent and photoanode efficiencies.30,35−39 It was commonly suggested that photogenerated holes can be trapped by Co−Pi catalyst where water oxidation occurs, leading to the efficient charge separation and enhanced photoelectrochemical activities. Barroso et al. employed transient absorption spectroscopy to study Co−Pi deposited α-Fe2O3 photoelectrode. It was observed that the lifetime of photogenerated holes can be increased by more than 3 orders of magnitude with retarded electron−hole recombination because of the formation of a Schottky-type heterojunction.39 Despite the above progress achieved, almost all the studies were focused on the photoelectrochemical performances by using photoelectrode devices. Herein, we for the first time employed photocatalytic water oxidation process to evaluate the activities of Co−Pi/ZnO photocatalysts for oxygen evaluation. The photocatalytic reaction was carried out in a closed circulation system with online gas chromatography for an accurate measurement of the oxygen gas produced. In addition, it was observed that the mixture of cobalt phosphate and ZnO exhibited similar enhancement in activity for oxygen production as compared to Co−Pi/ZnO composites.
and 10 mA) in the range of 10°−70° with a scan speed of 0.04° s−1. UV−visible diffuse reflectance spectra (UV−vis DRS) were recorded on a UV−visible spectrophotometer (UV-2450, Shimadzu). The morphologies of samples were observed on field emission scanning electron microscopy (FESEM, JEOL JSM 6700F) equipped with an energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM, JEOL 3010). The concentrations of Zn2+ and Ag+ in the reaction solution were determined using inductively coupled plasma (ICP) optical emission spectrometer on a Perkin-Elmer ICP Optima 2000DV machine. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an Axis Ultra spectrometer (Kratos Analytical) using a monochromated Al Kα X-ray source (1486.7 eV) operating at 15 kV. The binding energy was calibrated using the C1s peak at 284.6 eV. 2.3. Photocatalytic Reaction. The photocatalytic water splitting reaction was conducted in a top window Pyrex cell connected to a closed gas circulation and evacuation system as shown in Scheme 1. A 300 W xenon lamp (Newport) was Scheme 1. Schematic Diagram of Photocatalytic Reaction System for Water Oxidation
2. EXPERIMENTAL SECTION 2.1. Co−Pi Loading on ZnO. All reagents were of analytical grade and used without any purification. Co−Pi nanoparticles were deposited on the surface of ZnO particulates by a similar photochemical method as described by Steinmiller and Choi.30 Typically, 0.3 g of ZnO (>99.0%, KANTO Chemical) was dispersed in 50 mL of sodium phosphate buffer solution (0.01 M, pH = 7.0) prepared from sodium phosphate monobasic (NaH2PO4·H2O, ∼99%, USB corporation) and disodium hydrogen phosphate (Na2HPO4, > 99.0%, KANTO Chemical) followed by ultrasonication for 3 min. To this suspension, 1.25 mL of 0.02 M cobalt nitrate (Co(NO3)2·6H2O, > 97.0%, GCE) aqueous solution was added under stirring. The mixture was subjected to UV light illumination for 30 min by using a UV lamp (8 W, λ = 302 nm, UVP). The precipitate was collected by centrifugation and washed with deionized water thoroughly before drying at 60 °C overnight. This sample was named as Co−Pi/ZnO. For comparison purposes, cobalt phosphate (CoP) and zinc phosphate (ZnP) in the absence of ZnO were prepared by slowly adding 18 mL of 0.5 M Co(NO3)2 or Zn(NO3)2 aqueous solution to 60 mL of phosphate buffer solution (0.1 M, pH = 7.0). After stirring for 30 min at room temperature, the precipitates were collected, washed with deionized water, and dried at 60 °C overnight. Silver phosphate (AgP) was also prepared according to the method reported in the literature.29 In a typical synthesis, 3 mmol of Na2HPO4 was thoroughly mixed with 9 mmol of AgNO3 (>99.0%, Sigma) in a rotator (labroller II, Labnet corporation) overnight. Then the mixture was washed with deionized water thoroughly and dried at the same condition. The samples consisting of a physical mixture of CoP and ZnO (CoP+ZnO) were prepared by grinding a certain amount of as-prepared CoP and the commercial ZnO powder in an agate mortar. 2.2. Characterization. The X-ray diffraction (XRD) patterns of the as-prepared samples were obtained on a X-ray diffractometer (Bruker D2 Phaser, Cu Kα, λ = 1.5406 Å, 30 kV
applied as the light source. In a typical run, 0.1 g of the catalyst was well dispersed with constant stirring in 100 mL of AgNO3 aqueous solution at 5 mM. Before the reaction, the system was evacuated and refilled with argon gas for several times to remove the air inside and finally filled with argon to approximately 30 Torr. The temperature of photoreaction cell was kept at about 20 °C by a cooling water jacket. The produced oxygen gas was analyzed with an online gas chromatography (Agilent 6890N, TCD detector, argon as carrier gas, 5 Å molecular sieve column). Before and after the reaction, the pH of the solution was measured to be almost constant and close to neutral at 6.6.
3. RESULTS AND DISCUSSION 3.1. Materials Properties of Co−Pi/ZnO. Figure 1 displays the XRD patterns of cobalt phosphate (CoP), Co− Pi loaded ZnO (Co−Pi/ZnO), mixture of CoP and ZnO (CoP +ZnO), and pristine ZnO. The pristine ZnO (curve d) shows good crystallinity with a hexagonal structure (JCPDS card no. 36-1451). The characteristic peaks shown in curve a can be well indexed to Co3(PO4)2·8H2O (JCPDS card no. 33-0432) which B
dx.doi.org/10.1021/ie2027469 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Figure 1. (A) Wide-range and (B) narrow-range powder XRD patterns of (a) cobalt phosphate (CoP), (b) Co−Pi loaded ZnO, Co−Pi/ZnO, (c) CoP and ZnO physical mixture, CoP+ZnO, and (d) ZnO. The peaks labeled with the asterisk (∗) in curve B-b can be indexed to Zn3(PO4)2·4H2O (JCPDS card no. 33-1474). The peaks labeled with the triangle (Δ) in curve B-c can be indexed to Co3(PO4)2·8H2O (JCPDS card no. 33-0432).
was formed due to precipitation of Co2+ ions by the phosphate anions. The grain size of this sample was estimated to be about 54 nm by using the Scherrer equation and on the basis of the strongest peak at 2θ of 13.25°. As shown in Figure 1B, some weak diffraction peaks can be observed in curves b and c in the range of 10°−25°, which are labeled by an asterisk (*) and triangle (Δ), respectively. The peaks present in curve b can be assigned to Zn3(PO4)2·4H2O (JCPDS card no. 33-1474). During the photochemical deposition of Co−Pi on the surface of ZnO, UV light was applied. Photocorrosion of ZnO may occur to a certain extent due to UV illumination. The leached Zn2+ was precipitated by the phosphate ions out in the form of zinc phosphate. The presence of Zn3(PO4)2·4H2O was further confirmed by FESEM results, which will be discussed shortly. The small peaks in curve c should be ascribed to the presence of a small portion of Co3(PO4)2·8H2O (CoP) as this sample is a physical mixture of CoP and ZnO. The above results indicate that the Co−Pi phase photochemically deposited on ZnO is amorphous, and its presence in sample Co−Pi/ZnO is not detectable by XRD as shown in Figure 1b. This is consistent with the findings reported by others about the formation of the amorphous Co−Pi thin film on various electrodes.12,30,31,33,40 In fact, the formation of cobaltate clusters of molecular dimension have been proposed in Co−Pi structures based on EXAFS studies by Nocera and co-workers.33 XPS spectra of Co 2p and P 2p of sample Co−Pi/ZnO are shown in Figure 2. The signal of Co 2p is rather weak due to its low percentage in the sample (
