Strontium Titanate Based Artificial Leaf Loaded ... - ACS Publications

Article

Strontium titanate-based artificial leaf loaded with reduction and oxidation cocatalysts for selective CO2 reduction using water as an electron donor Shusaku Shoji, Akira Yamaguchi, Etsuo Sakai, and Masahiro Miyauchi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 2, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Strontium titanate-based artificial leaf loaded with reduction and oxidation cocatalysts for selective CO2 reduction using water as an electron donor Shusaku Shoji, Akira Yamaguchi, Etsuo Sakai, Masahiro Miyauchi*. Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan Keywords: photocatalyst, thin film, strontium titanate, CO2 reduction, water oxidation

* Corresponding author: M. Miyauchi E-mail address: [email protected]

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

Abstract

Thin film of SrTiO3 nanorods loaded with reduction and oxidation cocatalysts drove the selective reduction of carbon dioxide (CO2) into carbon monoxide (CO), as well as caused the production of equivalent oxygen molecules through water oxidation under UV irradiation. The described film functioned as a free-standing plate without any bias potential application, similar to a natural leaf. The film was facilely fabricated by a simple hydrothermal and annealing treatment of a titanium substrate to produce the SrTiO3 nanorod film (STO film) followed by two steps of loading the reduction and oxidation cocatalysts onto the surface of the STO. As a reduction cocatalyst, a CuxO nanocluster was chosen to achieve selective reduction of CO2 into CO, whereas a cobalt- and phosphate-based cocatalyst (CoPi) facilitated oxidation on the STO surface to promote oxygen generation. For the photocatalysis test, a wireless film was simply set into an aqueous solution bubbled with CO2 in a reactor, and CO production was observed in the headspace of the reactor under UV irradiation. Compared to the bare STO film, the dual cocatalyst-loaded STO film exhibited 2.5 times higher CO generation. H2 production was very limited in our system, and the amount of molecules generated by the reduction reaction was almost twice that of the generated oxygen molecules, proving that water molecules acted as electron donors. Our artificial leaf consists of abundant and non-toxic natural elements and represents the first achievement of stoichiometric CO2 reduction using water as an electron donor by a free-standing natural leaf-like plate form.


2

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Photocatalytic CO2 reduction, which converts CO2 into usable products such as CO, HCOOH, HCHO, CH3OH, and CH4, is one of the most attractive strategies to solve the global warming and fossil fuel shortage problems.1-2 Therefore, CO2 reduction by photocatalysis has been extensively studied, and metal oxides or chalcogenide-based photocatalysts such as TiO2, ZnO, CdS, GaP, and SiC have shown photocatalytic activity for CO2 reduction in an aqueous solution.3 For example, Sahara et al. reported a TaON with metal complex system,4 while Morikawa et al. reported the combination of a semiconductor light harvester with a metal complex system.5-6 In contrast to systems containing organic molecules, Kudo et al. established an inorganic hybrid system for CO2 photoreduction based on the ALa4Ti4O15 (A=Ca, Sr or Ba)7 and KCaSrTa5O15 powder system;8-9 and Zn-doped Ga2O3 powder system has also been reported.10 However, there are very few studies that show equivalent oxygen production through CO2 photoreduction, which indicates an uphill reaction similar to natural plants' photosystems I and II. It is very difficult to cause CO2 photoreduction in aqueous media since the potential of CO/CO2 redox reaction (-0.53 V vs. NHE at pH 7) is more negative than that of H+/H2 (-0.41 V vs. NHE at pH 7),2 resulting in H2 as the main reduction product. Thus, increasing the selectivity of CO2 reduction products over H2 evolution is highly demanding. The most effective approach to improving the selectivity of products is to load a reduction cocatalyst that drives the multi-electron reduction reaction and imparts selectivity. Previous studies have reported efficient and selective CO2 photoreduction by using a combination of ruthenium or rhenium complex cocatalysts and semiconductors to generate HCOOH and CO.4-6, 11 In addition to metal complex cocatalysts, silver (Ag) nanoparticles7,10 and Au-Cu alloys also promoted CO2 reduction into CO or CH4.12 Besides noble metal complexes


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 23

and noble metal particles, metallic copper or copper oxides are composed of ubiquitous and nontoxic elements from the earth and have appropriate chemisorption-desorption of CO2 and CO,1314

resulting in high CO2 reduction activity with high selectivity for CO or HCOOH.15-16 Further,

the copper based nano-scalecocatalysts increases the electron density to promote multi-electrons reaction.17-19 Recently, our group reported copper oxide nanocluster cocatalysts loaded onto niobate nanosheets20 in which the copper oxide nanoclusters’ valence states were a mixture of Cu(I) and Cu(II), designated as CuxO (x = 1 or 2). More recently, we demonstrated that the CuxO nanoclusters behaved as a general cocatalyst on various semiconductors.21 However, to achieve CO2 reduction with stoichiometric oxidation and reduction products, we must develop and confirm equivalent oxygen evolution from water to prove that the reaction proceeds uphill, like a natural plant. The loading of oxygen evolution cocatalysts is also very important to effectively promote oxygen generation from water. The oxygen evolution reaction involves four electrons and requires a high overpotential. Several kinds of electrocatalysts have been developed, such as IrOx,22 CoOx,23 and NiOx,24 that exhibit high oxygen generation performance under dark conditions. Kanan and Nocera et al. discovered an amorphous cobalt-based cocatalyst called “CoPi” that exhibited high activity and durability.25-28 It is noteworthy that the CoPi cocatalyst can also be loaded onto the active site of a photoelectrode to significantly improve oxygen generation, enhancing the photocurrent of the photoelectrode.28-31 In addition, the design of a device structure for artificial photocatalytic photosynthesis is also very important for practical applications. Most previous research studies used a dispersed powder form or a photoelectrochemical two-electrode system with application of a bias potential. However, the former is difficult to operate, and collecting the dispersed particles is often


4

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


problematic, while the latter consists of a complicated two-electrode system requiring a potentiostat. For construction of an efficient artificial photosynthesis system, a simple sole plate structure, similar to a natural leaf, is desirable. Nocera et al. developed a wireless artificial leaf for water splitting to generate H2 and O2 in high efficiency using a three-junctioned silicon solar cell.32 On the other hand, Srinivasan et al. constructed a layered leaf structure with a facile coating method to achieve overall water splitting.33 Apart from the photocatalytic powder-based and two-electrodes systems, the construction of a wireless film that functions as an artificial leaf for CO2 reduction is still challenging and highly desirable. However, very few studies have achieved stoichiometric CO2 reduction, even in powder systems, and there are not any known reports of stoichiometric CO2 reduction with high selectivity by an artificial leaf system.

We constructed an artificial leaf for CO2 reduction using an SrTiO3 (STO) film, since the chemical stability of STO is very high, and its conduction band is high enough to cause CO2 reduction. Scaife reported the relationship between bandgap and flat band potential values on various metal oxide semiconductors, and the flat band potential of STO is located more negative than that of hydrogen production.34 These results indicate that STO preferably drives CO2 reduction. Further, the STO crystal is cubic perovskite phase (ABO3), and A and B sites' doping is feasible under maintaining its charge neutrality.35-36 Herein, we have focused on the STO nanorod thin film to harvest UV-light photon energy, because of its high crystallinity and large surface area with porous structure, which are favorable for the loading of cocatalysts. CuxO and CoPi cocatalysts were deposited onto the STO nanorod films to achieve wireless CO2 photoreduction utilizing water molecules as electron donors through an uphill reaction. We optimized the loading amount of both cocatalysts to achieve high selectivity and efficiency. In the present study, we discuss the charge transport between the light-harvesting STO and the


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

cocatalysts and describe the strategic development of a selective CO2 reduction based on the wireless film system.

2. EXPERIMENTAL 2.1. Fabrication of STO nanorod thin films The STO nanorod thin film was coated over a metal titanium substrate according to our previous reports.21,

37

Briefly, STO nanorod thin films were fabricated by three steps; i. e.

hydrothermal treatment on a titanium substrate to form protonated titanate nanotubes array,38 ion exchange treatment of strontium ions under room temperature, and annealing in air to form crystalline STO nanorods array onto a substrate. We also prepared powder form STO for optical absorption analysis, since our film was coated over a metallic titanium substrate and its reflection spectrum was quite complicated because of the overlap with a substrate. STO nanorod powder was synthesized in the same manner with the thin film fabrication, using titanate nanotube powder prepared by the hydrothermal treatment on a commercial TiO2 powder.

2.2. Loading of CuxO and CoPi cocatalysts onto STO light harvester CuxO nanoclusters were deposited onto STO nanorod films by a simple impregnation method in an aqueous media containing Cu2+ ions.39 The weight fraction of Cu was 0.0005 wt.% versus water, which was the optimized condition described in the previous study.21


6

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Next, CoPi nanoparticles were deposited onto the CuxO-STO films by a photoelectrochemical method. Our films enables the loading of CoPi nanoparticles because the backside of the Ti substrate serves as a contact for the photoelectrochemical deposition of CoPi. A three-electrode cell was used with CuxO-STO as the working electrode, Ag/AgCl as the reference electrode, and Pt as the counter electrode. A bias potential of 1.0 V vs. Ag/AgCl was applied to the CuxO-STO film in a solution of 0.5 mM cobalt nitrate containing 0.1 M potassium phosphate buffer at pH 7 during the UV light irradiation. The amount of loaded CoPi was controlled by the loading time.

2.3. Characterization X-ray diffraction (XRD; Rigaku, SmartLab) was used to characterize the crystal structures of films by a grazing angle method. The sample nanostructures were analyzed by scanning electron microscopy (SEM; JSM-7500F, JEOL, Ltd.) and transmission electron microscopy (TEM; JEM-2010F, JEOL, Ltd.). X-ray spectroscope (EDX: PHI5000) was equipped in TEM apparatus, which was used to determine chemical compositions of our samples. The UV-Vis spectra of powder were recorded by a spectrophotometer (JASCO) using diffuse reflectance method. The valence states of copper nanoclusters were investigated by an electron spin resonance (ESR; EMX Plus, Bruker) under a N2 atmosphere. X-ray photoelectron spectroscopy (XPS; model ESCA-5500MT, Perkin Elmer Instruments) was also used to characterize the surface chemical composition (especially P and Cu species) using Mg Kα Xrays.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

2.4. Photocatalytic CO2 reduction The prepared sample was immersed into a sealed quartz glass reactor (500 mL) with an electrolyte solution (20 mL) consisting of 0.5 M KHCO3, 0.025 M KH2PO4, and 0.025 M K2HPO4. Gaseous CO2 was bubbled into the electrolyte solution to purge the air in the reactor. As a control experiment, Ar bubbling was conducted into an electrolyte solution, in which 0.5 M KCl, 0.025 M KH2PO4, and 0.025 M K2HPO4 were contained. UV light irradiation was performed using Hg-Xe lamp (LA-410UV; Hayashi Watch Works, Ltd.). Produced gas in the headspace of the reactor was investigated by a gas chromatograph with a barrier ionization detector (BID, Shimazu Ltd.).

3. RESULTS & DISCUSSION 3.1 Structure of thin film Figure 1 shows the XRD pattern of our STO film, which consists of pure perovskite phase. Similar to the previous research, we also confirmed that the hydrothermal and ionexchange processes converted the metal Ti to protonated titanate and, ultimately, to a perovskite structured STO.21 The optical absorption properties of the CuxO-STO nanorod and STO nanorod powders were shown in our supporting information (Fig. S1). UV light absorption in both spectra is originated in the interband transition of STO, since its bandgap is 3.2 eV. On the other hand, visible light absorption was seen in the CuxO-STO nanorod. The absorption at 430 nm of the CuxO-STO nanorod is attributed to interfacial charge transfer (IFCT),40 while the broad absorption around 580–800 nm is assigned to the d-d transition of the Cu(II) species.41 The SEM


8

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


image of our STO film reveals that the film surface is porous, and numerous rods uniformly cover the Ti foil substrate (Figure 2). Figure 3 shows the TEM images of CuxO/CoPi-STO. Lattice fringes are observed in rod like strucutures, but grain boundaries are not observed, indicating the high crystallinity of the STO nanorods. The TEM image reveals relatively large sized amorphous nanoclusters (4–5 nm) highly dispersed on the STO nanorods and attached to relatively small amorphous nanoclusters (1–3 nm). EDX spectroscopy was conducted at the points denoted "a" and "b" in Fig. 3, corresponding to points with and without a CoPi particle, respectively. The EDX spectra are shown in Figs. 3a and 3b; a signal corresponding to Co is clearly observed at point "a," which is focused at the particles of ~5 nm. On the other hand, the Co signal at point "b" is smaller than that at "a," indicating that the CoPi particles are attached and highly dispersed on the STO nanorods ~5 nm in size with a wide particle size distribution. Furthermore, a Cu signal is also detected at each point, indicating that CuxO is highly dispersed on the STO nanorods. Previous research revealed that the size of CuxO nanoclusters was less than 3 nm, which was smaller than the CoPi particle.39 In the EDX spectrum, the Sr and P peaks overlap; thus, XPS analysis was conducted to investigate the existence of phosphorous. Figure 4 shows the XPS spectrum of CuxO/CoPi-STO. Similar to our previous report, two peaks of Cu 2p1/2 and Cu 2p3/2 are observed around 930 to 955 eV.42 A clear P peak is also observed. These results indicate that the CuxO nanocluster and CoPi were successfully dual-loaded onto the STO nanorod.

3.2 Electrochemical and photo-electrochemical properties of CuxO/CoPi-STO Next, we investigated the effect of cocatalyst loading by monitoring the electro- or photoelectrochemical current densities in order to optimize the conditions for cocatalyst loading.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

The film structure is very useful to investigating its electrochemical and photoelectrochemical properties, since the backside of the titanium film is used as a contact for the wired electrode. First, we investigated the cathodic properties of bare STO and CuxO/CoPi-STO under dark conditions in the presence of CO2, since STO is a n-type semiconductor with electron carriers that cause reduction, even without photon irradiation. As shown in Fig. 5a, an increasing current density under cathodic bias potential is obviously observed with CuxO loading, indicating that CuxO works as an efficient cocatalyst toward electrochemical CO2 reduction. The thermodynamical redox potential of Cu2+/Cu1+ is 0.16 V (vs. normal hydrogen electrode (NHE) at pH= 0), which is more positive than the conduction band minimum of STO. Thus the excited electrons in STO can be injected into CuxO clusters. Figure 5b shows the photoelectrochemical properties of the optimized CoPi- and CuxOdual loaded STO, as well as the CuxO-loaded STO, under chopped UV irradiation. Typical anodic photocurrent behaviors are observed in both cases because of the n-type STO character, and the anodic photocurrent was greatly improved with the loading of the CoPi catalyst, which accelerates the water oxidation. Water oxidation in CoPi proceeds through Co3+/Co2+ redox couple, and its potential is located at +1.8 V (vs. NHE at pH= 0), which is more negative than the valence band maximum of STO. Therefore, excited holes in STO can be injected into the CoPi catalyst. Figure S2 in the supporting information shows the relationship between the CoPi loading amount on the CuxO-STO and the observed photocurrent, and a 30 s loading time is shown achieve the highest photocurrent. According to Figs. 5a and 5b, both the reduction and oxidation reactions are promoted by the loading of the CuxO and CoPi cocatalysts onto the surface of the STO.


10

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.3. Photocatalytic CO2 reduction in wireless film To conduct the photocatalysis test, the wireless film was simply immersed into an aqueous solution bubbled with CO2 or Ar in a glass reactor, and CO production in the headspace of the reactor was measured under UV irradiation. Figure 6 shows the time course of photocatalytic CO generation for the bare STO, CuxO-STO, CoPi-STO film and CuxO/CoPiSTO under UV irradiation with CO2 bubbling. In this experiment, we used the optimized thin films with the above mentioned electrochemical and photoelectrochemical properties. The CO generation rate was highest for CuxO/CoPi-STO under CO2 bubbling conditions, while that of CoPi-STO and CuxO/STO was higher than for bare STO. Under Ar bubbling, CO generation for all films was limited, but a small amount of CO was detected (see our supporting information, Figure S3). This very limited CO generated under Ar bubbling was attributed to the oxidation of organic contaminants on the film surfaces, even though the samples were cleaned by preirradiation treatment in air. However, the CO production under CO2 bubbling is much more obvious than that under Ar bubbling. We also investigated formic acid (HCOOH) generation in an aqueous solution by ion-chromatography with a sensitivity of < 0.1 µmol/L, but no HCOOions were detected under UV irradiation with CO2 bubbling. We also investigated the wavelength dependence of the CO2 photoreduction property. Although our cocatalyst-loaded STO samples absorbed visible light, as shown in the optical absorption spectrum, the IFCT absorption and d-d transition in the visible light region did not contribute to the CO2 photoreduction, as evidenced by the experiment using a UV cut-off filter (< 420 nm). These results reveal that the bandgap excitation of STO with electron-hole pairs is indispensable for driving CO2 reduction in our system.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

To prove the stoichiometric CO2 reduction in the present study, we also conducted the CO2 reduction in a tightly sealed glass reactor to eliminate atmospheric oxygen contamination, and simultaneously evaluated the reduction products, including H2 and O2, with a BID detector. Figure 7 shows the detected products from the reduction and oxidation reactions. Because oxygen evolution involves the transfer of 4 electrons, whereas hydrogen generation from H+ and CO generation from CO2 are each 2-electron reactions, the ratio of H2 and CO versus O2 should be 2:1, as per the following reduction and oxidation equations: 2H+ + 2e- → H2

(1)

CO2 + 2H+ + 2e- → CO + H2O

(2)

2H2O + 4h+ → O2 + 4H+

(3)

As shown in Fig. 7, the total amount of reduction products (H2 and CO) was approximately twice that of the oxidation product (O2). Furthermore, our previous research using isotope tracer analysis revealed that even the STO film loaded with only CuxO evolved CO and oxygen from CO2 and H2O. These results prove that our wireless film selectively produced CO under UV irradiation coupled with water oxidation, and the selectivity of CO production versus H2 was 80%. Water oxidation and CO2 reduction are multi-electron reactions, so the accumulation of electron and hole densities at the reaction sites is critical for stoichiometric CO2 reduction. The highly dispersed loading of nanocluster cocatalysts on the light harvester improves the electron and hole densities for CO2 reduction and water oxidation, respectively. We anticipate that our two-step cocatalyst loading method would be very effective for separating the reduction and oxidation sites on the STO surface. In our sample preparation, CuxO nanocluster reduction sites were firstly loaded over the STO surface by a simple impregnation method. Next,


12

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CoPi cocatalysts were loaded by a photoelectrochemical oxidation process in which Co2+ ions in the solution were oxidized by the photo-generated holes of STO and immobilized. Under these conditions, CoPi cocatalysts were deposited onto the oxidation sites of STO, while CuxO clusters acted as reduction sites. This cocatalyst loading process also contributes to the stoichiometric CO2 reduction through water oxidation by improving the multi-electron reaction and inhibiting charge recombination. To elucidate the stability of our photocatalyst, we estimated the turnover number (TON) of our film based on the results of Figure 6, and the estimated TON exceeded 4 even under the assumption of dense STO film (see our supporting information). CO generation rate was linearly increased even after 24 hours irradiation, suggesting that the TON is higher than the estimated value. Further, our STO film is porous and actual reaction sites are mostly located at the nano-sized cocatalysts, then the expected TON would be much higher than the estimated value. These results strongly imply the high stability of our film. In the present study, the selectivity for CO generation of our CuxO/CoPi-STO film was approximately 80% versus H2 production. This is the first known report of such high CO2 reduction selectivity, proving the utility of water molecules as electron donors through a wireless film system. Kortlever et al.43 suggested that CO2 reduction proceeds under two alternate pathways; one involving the production of formate species, and the other leading to the generation of CO, subsequently resulting in methane and hydrocarbons as final products. As mentioned above, we found that no formic acid was produced in the electrolyte solution. The reaction pathway strongly depends on the adsorption strength of CO2 anion radicals on catalyst surface. It is also reported that weak adsorption causes formate formation, while strong adsorption induces CO generation .44 Copper species would have relatively high adsorption strength with CO2 anion radicals, because of their d-electron back donation.13 Azuma et al.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

reported that copper catalyzed the reduction of CO2 molecules to CO rather than HCOOH, which is consistent with our present results.45

4. CONCLUSION Selective CO2 reduction was achieved by an artificial leaf that consists of a CuxO/CoPiSTO nanorod film on a titanium substrate. This film works as a sole wireless plate and could generate CO as well as O2 under UV irradiation with no bias application in an aqueous media without any sacrificial agents. The photo-generated reduction products were twice that of the produced O2 molecules, proving water molecules act as the electron donors. Dual loading of CuxO and CoPi onto STO enhanced the reaction rate, and its CO generation selectivity reached approximately 80%. The STO nanorod film was fabricated by a simple hydrothermal and ion impregnation treatment, and the CuxO and CoPi cocatalysts were facilely loaded onto the STO nanorod through a simple impregnation and photoelectrochemical deposition. The present paper provides a strategy for the development of a wireless artificial leaf system, in which the dual loading of cocatalysts onto a nano-structural light harvester greatly improves CO2 photoreduction activity and selectivity.

5. ACKNOWLEDGEMENT This research was supported by a grant from the Advanced Catalytic Transformation Program for Carbon Utilization (ACT-C, Grant No. JPMJCR12ZG) of the Japan Science and Technology Agency (JST) and JSPS KAKENHI Grant No. 26410234. We thank Mr. K. Hori at the Center for Advanced Materials Analysis of the Tokyo Institute of Technology for his assistance with the TEM characterization.


14

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information: UV-Vis spectra for powder (S1), photoelectrochemical property (S2), photocatalytic activity (S3), and TON estimation (S4).


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

REFERENCES 1. Hansen, J.; Nazarenko, L.; Ruedy, R.; Sato, M.; Willis, J.; Del Genio, A.; Koch, D.; Lacis, A.; Lo, K.; Menon, S.; Novakov, T.; Perlwitz, J.; Russell, G.; Schmidt, G. A.; Tausnev, N. Earth's Energy Imbalance: Confirmation and Implications. Science 2005, 308, 1431-1435. 2. Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors. Angew. Chem. Int. Ed. 2013, 52, 7372-7408. 3. Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Photoelectrocatalytic Reduction of Carbon Dioxide in Aqueous Suspensions of Semiconductor Powders. Nature 1979, 277, 637638. 4. Sahara, G.; Kumagai, H.; Maeda, K.; Kaeffer, N.; Artero, V.; Higashi, M.; Abe, R.; Ishitani, O. Photoelectrochemical Reduction of CO2 Coupled to Water Oxidation Using a Photocathode with a Ru(II)–Re(I) Complex Photocatalyst and a CoOx/TaON Photoanode. J. Am. Chem. Soc. 2016, 138, 14152-14158. 5. Arai, T.; Sato, S.; Kajino, T.; Morikawa, T. Solar CO2 Reduction Using H2O by a Semiconductor/metal-complex Hybrid Photocatalyst: Enhanced Efficiency and Demonstration of a Wireless System Using SrTiO3 Photoanodes. Energy Environ. Sci. 2013, 6, 1274-1282. 6. Sato, S.; Morikawa, T.; Saeki, S.; Kajino, T.; Motohiro, T. Visible-light-induced Selective CO2 Reduction Utilizing a Ruthenium Complex Electrocatalyst Linked to a P-type Nitrogen-doped Ta2O5 Semiconductor. Angew. Chem. Int. Ed. 2010, 49, 5101-5105. 7. Iizuka, K.; Wato, T.; Miseki, Y.; Saito, K.; Kudo, A. Photocatalytic Reduction of Carbon Dioxide over Ag Cocatalyst-loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) Using Water as a Reducing Reagent. J. Am. Chem.Soc. 2011, 133, 20863-20868. 8. Takayama, T.; Tanabe, K.; Saito, K.; Iwase, A.; Kudo, A. The KCaSrTa5O15 Photocatalyst with Tungsten Bronze Structure for Water Splitting and CO2 Reduction. Phys. Chem. Chem. Phys. 2014, 16, 24417-24422. 9. Takayama, T.; Iwase, A.; Kudo, A. Photocatalytic Water Splitting and CO2 Reduction over KCaSrTa5O15 Nanorod Prepared by a Polymerized Complex Method. Bull. Chem. Soc. Jpn. 2015, 88, 538-543. 10. Yamamoto, M.; Yoshida, T.; Yamamoto, N.; Nomoto, T.; Yamamoto, Y.; Yagi, S.; Yoshida, H. Photocatalytic Reduction of CO2 with Water Promoted by Ag Clusters in Ag/Ga2O3 Photocatalysts. J. Mater. Chem. A 2015, 3, 16810-16816. 11. Sato, S.; Arai, T.; Morikawa, T.; Uemura, K.; Suzuki, T. M.; Tanaka, H.; Kajino, T. Selective CO2 Conversion to Formate Conjugated with H2O Oxidation Utilizing Semiconductor/complex Hybrid Photocatalysts. J. Am. Chem. Soc. 2011, 133, 15240-15243. 12. Kang, Q.; Wang, T.; Li, P.; Liu, L.; Chang, K.; Li, M.; Ye, J. Photocatalytic Reduction of Carbon Dioxide by Hydrous Hydrazine over Au-Cu Alloy Nanoparticles Supported on SrTiO3/TiO2 Coaxial Nanotube Arrays. Angew. Chem. Int. Ed. 2015, 54, 841-845. 13. Copperthwaite, R. G.; Davies, P. R.; Morris, M. A.; Roberts, M. W.; Ryder, R. A. The Reactive Chemisorption of Carbon Dioxide at Magnesium and Copper Surfaces at Low Temperature. Catal. Lett. 1988, 1, 11-19. 14. Kuhl, K. P.; Hatsukade, T.; Cave, E. R.; Abram, D. N.; Kibsgaard, J.; Jaramillo, T. F. Electrocatalytic Conversion of Carbon Dioxide to Methane and Methanol on Transition Metal Surfaces. J. Am. Chem. Soc. 2014, 136, 14107-14113.


16

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15. Hara, K.; Kudo, A.; Sakata, T. Electrochemical Reduction of Carbon Dioxide under High Pressure on Various Electrodes in an Aqueous Electrolyte. J. Electroanal. Chem. 1995, 391, 141-147. 16. Reske, R.; Mistry, H.; Behafarid, F.; Roldan Cuenya, B.; Strasser, P. Particle Size Effects in the Catalytic Electroreduction of CO2 on Cu Nanoparticles. J. Am. Chem. Soc. 2014, 136, 6978-6986. 17. Yu, H.; Irie, H.; Hashimoto, K. Conduction Band Energy Level Control of Titanium Dioxide: Toward an Efficient Visible-Light-Sensitive Photocatalyst. J. Am. Chem. Soc. 2010, 132, 6898-6899. 18. Wang, P.; Xia, Y.; Wu, P.; Wang, X.; Yu, H.; Yu, J. Cu(II) as a General Cocatalyst for Improved Visible-Light Photocatalytic Performance of Photosensitive Ag-Based Compounds. J. Phys. Chem. C 2014, 118, 8891-8898. 19. Miyauchi, M.; Irie, H.; Liu, M.; Qiu, X.; Yu, H.; Sunada, K.; Hashimoto, K. VisibleLight-Sensitive Photocatalysts: Nanocluster-Grafted Titanium Dioxide for Indoor Environmental Remediation. J. Phys. Chem. Lett. 2016, 7, 75-84. 20. Yin, G.; Nishikawa, M.; Nosaka, Y.; Srinivasan, N.; Atarashi, D.; Sakai, E.; Miyauchi, M. Photocatalytic Carbon Dioxide Reduction by Copper Oxide Nanocluster-grafted Niobate Nanosheets. ACS Nano 2015, 9, 2111-2119. 21. Shoji, S.; Yin, G.; Nishikawa, M.; Atarashi, D.; Sakai, E.; Miyauchi, M. Photocatalytic Reduction of CO2 by CuxO Nanocluster Loaded SrTiO3 Nanorod Thin Film. Chem. Phys. Lett. 2016, 658, 309-314. 22. Zhao, Y.; Hernandez-Pagan, E. A.; Vargas-Barbosa, N. M.; Dysart, J. L.; Mallouk, T. E. A High Yield Synthesis of Ligand-Free Iridium Oxide Nanoparticles with High Electrocatalytic Activity. J. Phys. Chem. Lett. 2011, 2, 402-406. 23. Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780-786. 24. Lin, F.; Boettcher, S. W. Adaptive Semiconductor/electrocatalyst Junctions in Watersplitting Photoanodes. Nat. Mater. 2014, 13, 81-86. 25. Kanan, M. W.; Nocera, D. G. In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072-1075. 26. Kanan, M. W.; Surendranath, Y.; Nocera, D. G. Cobalt-phosphate Oxygen-evolving Compound. Chem. Soc. Rev. 2009, 38, 109-114. 27. Kanan, M. W.; Yano, J.; Surendranath, Y.; Dincă, M.; Yachandra, V. K.; Nocera, D. G. Structure and Valency of a Cobalt−Phosphate Water Oxidation Catalyst Determined by in Situ X-ray Spectroscopy. J. Am. Chem. Soc. 2010, 132, 13692-13701. 28. Pilli, S. K.; Furtak, T. E.; Brown, L. D.; Deutsch, T. G.; Turner, J. A.; Herring, A. M. Cobalt-phosphate (Co-Pi) Catalyst Modified Mo-doped BiVO4 Photoelectrodes for Solar Water Oxidation. Energy Environ. Sci. 2011, 4, 5028-5034. 29. Zhong, D. K.; Cornuz, M.; Sivula, K.; Gratzel, M.; Gamelin, D. R. Photo-assisted Electrodeposition of Cobalt-phosphate (Co-Pi) Catalyst on Hematite Photoanodes for Solar Water Oxidation. Energy Environ. Science 2011, 4, 1759-1764. 30. Steinmiller, E. M. P.; Choi, K.-S. Photochemical Deposition of Cobalt-based Oxygen Evolving Catalyst on a Semiconductor Photoanode for Solar Oxygen Production. Proc. Natl. Acad. Sci. USA 2009, 106, 20633-20636.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

31. McDonald, K. J.; Choi, K.-S. Photodeposition of Co-Based Oxygen Evolution Catalysts on α-Fe2O3 Photoanodes. Chem. Mater. 2011, 23, 1686-1693. 32. Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J. H.; Nocera, D. G. Wireless Solar Water Splitting Using Silicon-Based Semiconductors and EarthAbundant Catalysts. Science 2011, 334, 645-648. 33. Srinivasan, N.; Sakai, E.; Miyauchi, M. Balanced Excitation between Two Semiconductors in Bulk Heterojunction Z-Scheme System for Overall Water Splitting. ACS Catal. 2016, 6, 2197-2200. 34. Scaife, D. E. Oxide Semiconductors in Photoelectrochemical Conversion of Solar Energy. Sol. Energy 1980, 25, 41-54. 35. Miyauchi, M.; Tokudome, H. Super-hydrophilic and Transparent Thin Films of TiO2 Nanotube Arrays by a Hydrothermal Reaction. J. Mater. Chem. 2007, 17, 2095-2100. 36. Konta, R.; Ishii, T.; Kato, H.; Kudo, A. Photocatalytic Activities of Noble Metal Ion Doped SrTiO3 under Visible Light Irradiation. J. Phys. Chem. B 2004, 108, 8992-8995. 37. Miyauchi, M. Thin Films of Single-Crystalline SrTiO3 Nanorod Arrays and Their Surface Wettability Conversion. J. Phys. Chem. C 2007, 111, 12440-12445. 38. Miyauchi, M.; Tokudome, H.; Toda, Y.; Kamiya, T.; Hosono, H. Electron Field Emission from TiO2 Nanotube Arrays Synthesized by Hydrothermal Reaction. Appl. Phys. Lett. 2006, 89, 043114-1-3. 39. Liu, M.; Qiu, X.; Miyauchi, M.; Hashimoto, K. Cu(II) Oxide Amorphous Nanoclusters Grafted Ti3+ Self-Doped TiO2: An Efficient Visible Light Photocatalyst. Chem.Mater. 2011, 23, 5282-5286. 40. Qiu, X.; Miyauchi, M.; Sunada, K.; Minoshima, M.; Liu, M.; Lu, Y.; Li, D.; Shimodaira, Y.; Hosogi, Y.; Kuroda, Y.; Hashimoto, K. Hybrid CuxO/TiO2 Nanocomposites as RiskReduction Materials in Indoor Environments. ACS Nano 2012, 6, 1609-1618. 41. Dias Filho, N. L. Adsorption of Cu(II) and Co(II) complexes on a Silica Gel Surface Chemically Modified with 2-Mercaptoimidazole. Microchim. Acta 130, 233-240. 42. Hayez, V.; Franquet, A.; Hubin, A.; Terryn, H. XPS Study of the Atmospheric Corrosion of Copper Alloys of Archaeological Interest. Surf. Interface Anal. 2004, 36, 876-879. 43. Kortlever, R.; Shen, J.; Schouten, K. J. P.; Calle-Vallejo, F.; Koper, M. T. M. Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide. J. Phys. Chem. Lett. 2015, 6, 4073-4082. 44. Chaplin, R. P. S.; Wragg, A. A. Effects of Process Conditions and Electrode Material on Reaction Pathways for Carbon Dioxide Electroreduction with Particular Reference to Formate Formation. J. Appl. Electrochem. 2003, 33, 1107-1123. 45. Azuma, M.; Hashimoto, K.; Hiramoto, M.; Watanabe, M.; Sakata, T. Electrochemical Reduction of Carbon Dioxide on Various Metal Electrodes in Low-temperature Aqueous KHCO3 Media. J. Electrochem. Soc. 1990, 137, 1772-1778.


18

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. XRD patterns of STO nanorod thin film

Figure 2. SEM image of the surface of STO nanorod thin film


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

Figure 3. Structural analysis of CuxO/CoPi-STO. TEM image of CuxO and CoPi dual loaded STO nanorods (left). Right panels (a) and (b) show the EDX spectrum at the point marked by a circle in the TEM image.


20

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. XPS spectra of the CuxO/CoPi-STO: Cu 2p (a) and P 2p (b)

Figure 5. (a) Cyclic voltammograms of bare STO film (black line) and CuxO/CoPi-STO (green line) under dark condition. (b) Photocurrent-potential characteristics of CuxO-STO (blue) and CuxO/CoPi-STO (green) under chopped UV irradiation.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

Figure 6. Time course of photocatalytic carbon monoxide (CO) generation by CuxO/CoPi-STO, CuxO-STO and bare STO films under UV irradiation.

Figure 7. Total reduction product amount (left) and oxidation product (O2) amount. The ratio of them is approximately 2:1. Products were linearly generated and the products amount was measured after 16 hours UV irradiation.


22

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for TOC graphic 254x107mm (96 x 96 DPI)


Strontium Titanate Based Artificial Leaf Loaded ... - ACS Publications

Recommend Documents