SrNb2O6 nanoplates as efficient photocatalysts for

Electronic Supplementary Material (ESI) for ChemComm. This journal is © The Royal Society of Chemistry 2015

SrNb2O6 nanoplates as efficient photocatalysts for the preferential reduction of CO2 in the presence of H2O Shunji Xie, Yu Wang, Qinghong Zhang,* Weiping Deng and Ye Wang* State Key Laboratory of Physical Chemistry of Solid Surfaces, Innovation Center of Chemistry for Energy Materials, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

Electronic Supplementary Information 1. Experimental details (1) Material synthesis The solid-state reaction (SSR) between an alkali or alkaline metal oxide or carbonate and Nb2O5 was carried out at 1473 K in air for the synthesis of NaNbO3−SSR, KNbO3−SSR, MgNb2O6−SSR, CaNb2O6−SSR, SrNb2O6−SSR and BaNb2O6−SSR. SrNb2O6 samples with different morphologies were mainly synthesized by a hydrothermal method, and were denoted as SrNb2O6−X−Y, where X is the morphology and Y is the value of surface area (m2 g-1). For the hydrothermal synthesis, the amorphous Nb2O5·nH2O was first synthesized with a procedure reported in our previous paper.1 In brief, NbCl5 was dissolved in ethanol, and then, an NH3·H2O aqueous solution was added into the NbCl5 ethanol solution under continuous stirring. After the mixture was aged for 2 h, the Nb2O5·nH2O precipitates were separated by centrifugation. Then, the freshly prepared Nb2O5·nH2O was transferred to a Teflon-lined autoclave pre-charged with an aqueous solution of Sr salts for hydrothermal treatment. For the synthesis of SrNb2O6 nanoplates, typically, Sr(OH)2 was used as the precursor of Sr. The molar ratio of Nb/Sr was 1:1, and the pH of the suspension was adjusted to 13 by the addition of NaOH. The hydrothermal treatment was performed at 493 K for 24 h. After the hydrothermal treatment, the solid product was recovered by centrifugation, washed repeatedly with distilled water and finally dried at 353 K under vacuum. The obtained sample had a specific surface area of 105 m2 g-1 and was denoted as SrNb2O6−plate−105. When the molar ratio of Nb/Sr was changed to 1:0.5, the obtained sample possessed a lower surface area (50 m2 g-1) and was denoted as SrNb2O6−plate−50. To study the effects of synthetic parameters in the hydrothermal synthesis, we changed the precursor of Sr, the pH of the suspension from 8 to 13 adjusted by NaOH or NH3·OH, the hydrothermal time from 0 to 48 h and the hydrothermal temperature from 453 to 513 K. The SrNb2O6−rod−8.5 with a nano-rod morphology and a surface area of 8.5 m2 g-1 was synthesized by the hydrothermal treatment of a mixture of Nb2O5·nH2O and Sr(AC)2 at 493 K for

S1

24 h (Nb/Sr = 1:1, pH = 10), followed by washing and drying. The SrNb2O6−particle−3.7 was synthesized by the hydrothermal treatment of a mixture of Nb2O5·nH2O and Sr(NO3)2 at 513 K for 24 h (Nb/Sr = 1:1, pH = 8), followed by washing and drying. The SrNb2O6−particle−46, which had a larger surface area (46 m2 g-1) but particulate morphology, was synthesized by a polymerizable complex (PC) method.2,3 In brief, anhydrous citric acid (CA) and ethylene glycol (EG) were added into a methanol solution of NbCl5 with a ratio of Nb/CA/EG = 1/15/60. After stirring at 373 K, SrCO3 was added into the mixture, and the stirring continued until SrCO3 dissolved completely. The mixed solution was further kept at 453 K for several hours, forming a polymeric gel. The SrNb2O6−particle−46 was finally obtained after calcination of the gel at 873 K in air for 6 h. TiO2 (P25), a reference catalyst, was purchased from Degussa. The Pt−TiO2 catalyst was prepared by a photodeposition method with the procedure described in our previous paper.4

(2) Photocatalyst characterization The materials were characterized by Powder X-ray diffraction (XRD), N2 physisorption, scanning

electron

ultravolet-visible

microscopy (UV-vis)

(SEM),

spectroscopy,

transmission CO2

electron

chemisorption,

microscopy

(TEM),

photoelectrochemical

measurements and photoluminescence (PL) spectroscopy. XRD patterns were collected on a Panalytical X’pert Pro diffractometer using Cu Kα radiation (40 kV, 30 mA). N2 physisorption was carried out with a Micromeritics Tristar 3020 surface area and porosimetry analyzer. Diffusion reflectance UV-Vis spectra were recorded on a Varian-Cary 5000 spectrometer equipped with a diffuse reflectance accessory. The spectra were collected with BaSO4 as a reference. SEM was carried out using LEO1530 or S-4800 (ZEISS SIGMA) scanning electron microscope with 10 kV or 20 kV accelerating voltage. TEM and high-resolution TEM (HRTEM) measurements were performed on a Tecnai F20 electron microscope (Phillips Analytical) operated at an acceleration voltage of 200 kV. The amount of CO2 chemisorption was measured at room temperature with a ASAP2020C Micromeritics apparatus by adopting the procedure reported by Teramura et al.5 Photoelectrochemical measurements were carried out with an Ivium CompactStat (Holland) using a standard three-electrode cell with a working electrode, a Pt wire as the counter electrode and an SCE electrode as the reference electrode. A 0.5 M solution of Na2SO4 was used as the electrolyte. The working electrode was prepared by cleaning an F-doped SnO2-coated glass (FTO glass, 1 cm × 2 cm). The photocatalyst was dispersed in ethanol, and the suspension was added dropwise directly onto the FTO by microsyringe with a gentle stream of air to speed drying. The film was dried at 393 K for 1 h, and the typical surface density of the photocatalyst was 1 mg cm-2. The PL spectroscopic measurements were performed with an Edinburgh Analytical

S2

Instrument FLS 920 spectrophotometer with an excitation wavelength of 296 nm at room temperature.

(3) Photocatalytic reaction Photocatalytic reduction of CO2 with H2O vapour was carried out in a stainless-steel reactor (volume, ~100 mL) with a quartz window on the top of the reactor (Fig. S1). The photocatalyst was placed on a Teflon catalyst holder. The light source was 100 W Xe lamp (Beijing Trusttech Co., Ltd.) at UV-vis (λ = 300-780 nm). Typically, 0.010 g of catalyst was used. Liquid water with a volume of 4 mL was pre-charged in the bottom of the reactor. It should be noted that the catalyst was not immersed into the liquid water. Instead, the catalyst was surrounded by H2O vapour and CO2. The pressure of CO2 was typically regulated to 0.2 MPa. The temperature of the reactor was kept at 323 K, and the vapour pressure of H2O was 12.3 kPa under such a circumstance. The photocatalytic reaction was typically performed for 10 h.

Fig. S1 Reactor used for the photocatalytic reduction of CO2 with H2O vapour. 1 pressure digital meter, 2 Xe lamp, 3 O-ring quartz window, 4 stainless steel reactor, 5 magnetic stirrer. V1 and V2: valves. The amounts of CO, CH4 and H2 formed were analyzed by gas chromatography (GC). The reaction system was connected to an online GC through valves, and the gaseous products could be directly introduced to the GC for analysis. After the effluents containing CO2, CO and CH4 were separated by a carbon molecular sieve (TDX-01) column, CO and CO2 were further converted to CH4 by a methanation reactor and were then analyzed by an FID detector. The detection limits of our analytic method for CH4 and CO were both 0.002 µmol, corresponding to ~0.5 ppm in concentration. H2 and O2 were analyzed by an Agilent Micro GC3000 (Micro GC) equipped with a Molecular Sieve 5A column and a high-sensitivity thermal conductivity detector (TCD). Argon was used as the carrier gas for H2 analysis and helium was used as the carrier gas for O2 analysis. The Micro GC was connected directly to the reactor by a stainless-steel tube and the gaseous

S3

products were introduced into the Micro GC by a self-suction type injection pump. The detection limits for H2 and O2 were 0.004 µmol (~1 ppm) and 0.04 µmol (~10 ppm), respectively. The relative errors for the products except for O2 were typically lower than 5%. Liquid products such as CH3OH, HCHO and HCOOH possibly formed and dissolved in water were also analyzed carefully by gas chromatography or high-performance liquid chromatography, but no such products were detected under our reaction conditions. We performed the same experiment (including both reaction and analysis) for at least 3 times for each catalyst and the relative standard deviations for the amounts of H2, CO, and CH4 formed were

SrNb2O6−particle−3.7

>

SrNb2O6−particle−46

>

SrNb2O6−rod−8.5 > SrNb2O6−plate−50 > SrNb2O6−plate−105. The higher intensity of the luminescence band reflects the higher probability of recombination of the photogenerated electron-hole pairs. Thus, the result from the PL spectroscopic studies agrees well with that from the transient photocurrent response measurements (Fig. 2). Both results demonstrate that the SrNb2O6 nanoplates favour the separation of photoexcited charge carriers, leading to higher photocatalytic activity.

S12

Fig. S13 Photoluminescence (PL) spectra. (a) PL excitation spectrum for SrNb2O6−SSR−2.2 monitored at 564 nm; (b)-(g) PL emission spectra under 296 nm excitation: (b) SrNb2O6−SSR−2.2, (c) SrNb2O6−particle−3.7, (d) SrNb2O6−particle−46, (e) SrNb2O6−rod−8.5, (f) SrNb2O6−plate−50, (g) SrNb2O6−plate−105.

References 1

W. Fan, Q. Zhang, W. Deng and Y. Wang, Chem. Mater., 2013, 25, 3277-3287.

2

K. Yoshioka, V. Petrykin, M. Kakihana, H. Kato and A. Kudo, J. Catal., 2005, 232, 102-107.

3

M. Yoshino, M. Kakihana, W. Cho, H. Kato and A. Kudo, Chem. Mater., 2002, 14, 3369-3376.

4

S. Xie, Y. Wang, Q. Zhang, W. Fan, W. Deng and Y. Wang, Chem. Commun., 2013, 49, 2451-2453.

5

K. Teramura, S. Okuoka, H. Tsuneoka, T. Shishido and T. Tanaka, Appl. Catal. B, 2010, 96, 565-568.

6

Y. Lan, M. Yang and Y. Xu. Nanoscale, 2014, 6, 6335-6345.

S13