Elsevier

Applied Catalysis A: General

Volume 504, 5 September 2015, Pages 238-247
Applied Catalysis A: General

Tailoring assemblies of plasmonic silver/gold and zinc–gallium layered double hydroxides for photocatalytic conversion of carbon dioxide using UV–visible light

https://doi.org/10.1016/j.apcata.2014.12.042Get rights and content

Highlights

  • Assemblies of layered double hydroxide with Ag or Au nanoparticles were prepared.

  • Assemblies with Ag/Au via LDH reconstruction were compared to that via ion exchange.

  • Surface plasmon resonance of Ag enhanced the CO2 photoreduction on LDH to CH3OH/CO.

  • Au promoted the CO2 photoreduction on LDH but switched to CO selective.

  • Au worked as electron trap due to greater work function and the SPR did not work.

Abstract

In the search for novel efficient photocatalysts for the conversion of CO2 into fuels, plasmonic photocatalysts based on the self-assemblies of silver or gold nanoparticles with [Zn3Ga(OH)8]2CO3·mH2O layered double hydroxide (Zn3Ga|CO3 LDH) were prepared and tested for the photoreduction of CO2 by H2 under irradiation with UV–visible light. Ag and Au nanoparticles were obtained directly on the LDHs via the ion-exchange method or the reconstruction method of the LDHs. The catalysts exhibited intense surface plasmon resonance (SPR) effect at 411 and 555 nm attributable to Ag and Au nanoparticles, respectively. The rate of CO2 photoreduction on Ag/Zn3Ga|CO3 increased by a factor of 1.69 than that of Zn3Ga|CO3 while the methanol selectivity also increased from 39 to 54 mol%. On Au/Zn3Ga|CO3, the reduction rate of CO2 was 1.78 times higher than on Zn3Ga|CO3 LDH whereas the methanol selectivity decreased from 39 to 13 mol%. Electron microscopy and UV–visible and X-ray spectroscopy detected particular interactions of the cationic layers of Zn3Ga|CO3 with Ag and Au nanoparticles. Results show that for Ag/Zn3Ga|CO3 catalysts, CO2 photoreduction by H2 under visible light was promoted by the SPR effect of Ag nanoparticles while for Au/Zn3Ga|CO3 catalysts Au nanoparticles might act as electron-trapping active sites.

Introduction

The increasing concentration of CO2 in the atmosphere, as a result of the combustion of the carbon-based fuels, is predicted to result in unacceptable changes in the Earth's climate [1], [2]. The photoreduction of CO2 to fuels using light energy can contribute simultaneously to reduction of the major greenhouse gas and the development of sustainable energy. A key technological target to reach efficient photoconversion of CO2 to fuels (i.e. artificial photosynthesis) is to develop an efficient and robust photocatalyst [3]. An important factor limiting the conversion efficiency of almost every active photocatalyst is the high rate of charge carrier recombination. Recently, Ingram and Linic [4] demonstrated that the recombination problem was significantly alleviated by assembling plasmonic nanoparticles and semiconductor supports. Moreover, it has been recently reported that in plasmonic nanoparticles/support co-catalytic systems the photo-responsive features of the metal nanoparticles that manifest the surface plasmon resonance (SPR) effect are able to tune and assist the photocatalytic properties of the support. The particular features of the plasmonic nanostructures at the support interface might reduce the charge carrier recombination rate, and thereby enhance the visible-light-induced photocatalytic activities.

Layered double hydroxides (LDHs) or hydrotalcite-like materials are layered porous matrices belonging to the class of anionic clay (cationic layers intercalating anions) with many actual and potential applications in catalysis [5]. Recently, García group introduced a novel concept of Ti, Ce, or Cr-doped semiconductors based on Zn-containing LDHs [6]. LDHs can be defined by a versatile elemental composition and the ratio, and have basic properties and a high adsorption capacity for CO2 [7]. LDH photocatalysts comprising Zn and Ga have been reported, in our previous work, to convert CO2 into methanol or CO using H2 [8], [9], [10]. The photocatalytic reduction of CO2 using water and LDHs [11] and the combination of photooxidation catalyst and LDHs were also reported [12]. However, the band gap of these LDHs, e.g. 5.6 eV for [Zn3Ga(OH)8]2CO3·mH2O, corresponded to ultraviolet (UV) region and accordingly these LDHs were primarily responsive to UV light [10]. The relatively wide band gap was advantageous to set the conduction band (CB) minimum enough negative for these LDHs compared to the reduction reaction potentials ofCO2 + 2H+ + 2e  CO + H2O (−0.11 V)andCO2 + 6H+ + 6e  CH3OH + H2O (−0.32 V)versus standard hydrogen electrode (SHE) [10]. It is essential to utilize visible light, 97% of the spectrum of solar radiation [13], while utilizing the enough negative potential of CB minimum.

In this work, [Zn3Ga(OH)8]2CO3·mH2O was combined with a sensitizer of silver or gold nanoparticles that are responsive to visible light. For Ag nanoparticles dispersed in liquid, the corresponded SPR peak progressively shifted from 405 nm to 510 nm as the Ag particles grows from 10 nm to 100 nm [14]. Further, the SPR peak progressively shifted from 515 nm to 572 nm as the Au nanoparticles grows from 5 nm to 100 nm [15]. The hybridization of SPR of 100 nm of Ag cube and silicon nitride substrate at the interface was visualized by electron energy loss spectroscopy in a monochromated scanning transmission electron microscopy (TEM) [16]. Plasmon-induced excited electron injection from nanoparticles, e.g. Ag and Au, to semiconductors, e.g. TiO2, exceeding the Schottky barrier, has been reported [17], [18], [19], [20]. Mean 7 nm of Ag nanoparticles were combined with a LDH material of [Zn2Al(OH)6]2CO3·mH2O by the reaction of aqueous solution of Ag+ salt with the LDH powder freshly calcined at 823 K [21] while mean 2.9 and 3.4 nm of Au nanoparticles were combined with a LDH of [Zn2Al(OH)6]2CO3·mH2O and [Zn2Al0.7Ce0.3(OH)6]2CO3·mH2O by the reaction of aqueous solution of Au3+ with the LDH calcined at 823 K [22]. Based on these procedures, the anionic clay was transformed into metal oxides upon calcination at 823 K. The layered structure LDH was then reconstructed when the calcined LDH at 823 K was introduced into the aqueous solutions of silver or gold salts [21], [22]. In this work, assemblies of Ag and Au nanoparticles and [Zn3Ga(OH)8]2CO3·mH2O were obtained via ion-exchange method at 290 K or via the structural reconstruction of the LDHs in the specific aqueous solutions of Ag or Au. It is noteworthy that Ag and Au nanoparticles were obtained directly on the LDH platelets and no organic products were used during the preparation of Ag/LDH and Au/LDH assemblies via the reconstruction method. We present here the performances of Ag/[Zn3Ga(OH)8]2CO3·mH2O and Au/[Zn3Ga(OH)8]2CO3·mH2O for the photoconversion of CO2 using H2 and UV and visible light.

Section snippets

Sample synthesis

LDH compound of [Zn3Ga(OH)8]2CO3·mH2O was synthesized using a reported procedure from metal nitrates, Na2CO3, and NaOH in aqueous solutions controlled at pH 8 [10]. This compound is abbreviated as Zn3Ga|CO3.

For the ion exchange method, we followed the procedure presented in Ref. [23]. Firstly, 0.50 g of Zn3Ga|CO3 powder was immersed in an aqueous solution of AgNO3 (>99.8%, Wako Pure Chemical; 0.050 M, 10 mL) in a flask and refluxed at 353 or 373 K for 15 min, magnetically stirred at 900 rotations

Characterization of the Ag/LDH and Au/LDH assemblies

DR UV–visible spectra for as-synthesized Ag/LDHs and Au/LDHs composites are summarized in Fig. 1A. The absorption edge was in UV light region for all the samples. The absorption edge was extrapolated to give the Eg value of 5.6–5.1 eV for Zn3Ga|CO3, Ag/Zn3Ga|CO3-IE, and Au/Zn3Ga|CO3-IE, and the Eg value of 3.1–3.0 eV for Zn3Ga|CO3 reconstructed with Ag or Au (Table 1A). The former values were consistent with previous study for the LDHs [10], but the latter values should be affected by the

Structure of Ag/LDH and Au/LDH catalysts

Regular layered structures with the interlattice distance between 0.752 and 0.754 nm were confirmed for Zn3Ga|CO3 and all the assemblies of Zn3Ga|CO3 LDH with Ag or Au nanoparticles tested in this study (Table 1). The interlattice distance corresponds to the LDH containing carbonate and water molecules in the interlayers [10] revealing that Ag and Au nanoparticles might be located only on the surface of the platelets of Zn3Ga|CO3 (that are defined by sizes between 200 and 700 nm and the thickness

Conclusions

The present results illustrate the potential of the plasmonic nanostructured assemblies of Au/Zn3Ga(OH)8]2CO3·mH2O and Ag/Zn3Ga(OH)8]2CO3·mH2O as photocatalysts for CO2 reduction by H2 using both UV and visible light. The CO2 photoreduction rate using Ag/Zn3Ga|CO3-IE catalyst was higher (220 nmol h−1 gcat−1) by a factor of 1.69 in comparison to that of Zn3Ga|CO3. Under visible light irradiation, the photoreduction rate of Ag/Zn3Ga|CO3-IE decreased to 56% of corresponding rate under irradiation

Acknowledgments

The authors are grateful for the financial supports from the Grant-in-Aid for Scientific Research C (26410204, 22550117) from Japan Science Promotion Agency and from the Romanian National Authority for Scientific Research, CNCS-UEFISCDI (PN-II-IDEI-PCE-75/2013). X-ray absorption experiments were conducted under the approval of the Photon Factory Proposal Review Committee (2014G631, 2011G033).

References (39)

  • A. Corma et al.

    J. Catal.

    (2013)
  • Y. Izumi

    Coord. Chem. Rev.

    (2013)
  • F. Cavani et al.

    Catal. Today

    (1991)
  • Z. Yong et al.

    Energy Convers. Manag.

    (2002)
  • M. Morikawa et al.

    Appl. Catal.

    (2014)
  • N. Ahmed et al.

    Catal. Today

    (2012)
  • N. Ahmed et al.

    J. Catal.

    (2011)
  • G. Carja et al.

    Int. J. Antimicrob. Agent

    (2009)
  • L. Wang et al.

    Catal. Today

    (2011)
  • X. Liu et al.

    J. Alloys Compd.

    (2007)
  • Y. Yoshida et al.

    J. Catal.

    (2012)
  • B.J. Kip et al.

    J. Catal.

    (1987)
  • S.C. Roy et al.

    ACS Nano

    (2010)
  • D.B. Ingram et al.

    J. Am. Chem. Soc.

    (2011)
  • C. Gomes Silva et al.

    J. Am. Chem. Soc.

    (2009)
  • K. Teramura et al.

    Angew. Chem. Int. Ed.

    (2012)
  • M. Morikawa et al.

    Catal. Sci. Technol.

    (2014)
  • R.E. Blankenship et al.

    Science

    (2011)
  • Cited by (75)

    View all citing articles on Scopus
    View full text