La₂Sn₂O₇ enhanced photocatalytic CO₂ reduction with H₂O by deposition of Au co-catalyst

Abstract

La₂Sn₂O₇ (LSO) micro/nanospheres, synthesized by a hydrothermal method, exhibited photocatalytic performance for CO₂ reduction. The evolution rate of the main reduction products (CH₄ and CO) was 0.20 and 0.10 μmol h⁻¹, respectively. Loading of Au co-catalyst over La₂Sn₂O₇ efficiently enhanced the photocatalytic activity of CO₂ reduction. The highest photocatalytic efficiency was obtained over 1 wt% Au/La₂Sn₂O₇ with an apparent quantum yield (AQY) of 2.54%. Characterization techniques including SEM, TEM, XRD, XPS, DRS, PL, and electrochemical analysis were applied and the physicochemical properties of La₂Sn₂O₇ and Au/La₂Sn₂O₇ samples are discussed. A possible mechanism of photocatalytic CO₂ reduction over Au/La₂Sn₂O₇ is proposed.

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1. Introduction

Because of the growing consumption of fossil fuels, atmospheric levels of carbon dioxide are greatly increasing thus causing tremendous concerns about ensuing effects on the global climate and future energy supplies. Research activities on the potential conversion of CO₂ are thus considerably stimulated.^1,2 Photocatalytic transformation of CO₂ is one of the most promising routes for the utilization of CO₂ because it can reduce CO₂ to hydrocarbon fuels, such as CH₄ and CH₃OH, by solar energy in an environmentally friendly manner. Recently, various materials for photocatalytic CO₂-reduction such as La₂Ti₂O₇,³ LaPO₄,^4–6 TiO₂,^7–12 C₃N₄,^13–15 ZnO,¹⁶ Zn₂GeO₄,¹⁷ TaON,¹⁸ BaCeO₃,¹⁹ CdS^20–23 and metal complexes²⁴ have been reported, but with relatively low efficiencies. The synthesis of photocatalysts with high performance for CO₂ reduction has been a challenging – yet vital – research topic.

Semiconductors with a sufficiently more negative conduction band edge than the reduction potential of CO₂, and with superior CO₂ adsorption capability, favour photocatalytic CO₂ reduction.⁴ Lanthanum-based materials, due to a highly negative conduction band edge, may be suitable for efficient reduction of CO₂ with H₂O²⁵ and alkaline lanthanum cations are considered to have efficient CO₂ adsorption sites.²⁶ Many lanthanum-based materials have been reported with photocatalytic performance for hydrogen production and photocatalytic CO₂ reduction.^5,7 La₂Sn₂O₇, as a key member of the lanthanum-based stannate family, possesses tailorable and tunable pyrochlore properties with high possibilities of leading to a wide variety of chemical and physical properties which could be applied as luminescent materials²⁷ and catalysts for different reactions.^28,29 Therefore, La₂Sn₂O₇ materials are expected to be promising candidates for CO₂-reduction photocatalysts.

In the present study, a La₂Sn₂O₇ sample was prepared by a hydrothermal method and its photocatalytic performance for CO₂ reduction with H₂O was investigated. Au nanoparticles were deposited on La₂Sn₂O₇ by a NaBH₄ reduction method in order to acquire higher photocatalytic CO₂ reduction efficiency. Synthesized La₂Sn₂O₇ and Au/La₂Sn₂O₇ samples were characterized by physical and chemical measurements and the effects of Au doping on photocatalytic performance are discussed. A possible reaction mechanism for photocatalysis on the prepared La₂Sn₂O₇ and Au/La₂Sn₂O₇ samples under UV light irradiation is also proposed.

2. Experimental

2.1. Catalyst preparation

La₂Sn₂O₇ was fabricated via a hydrothermal method. In a typical experiment, 2 mmol La(NO₃)₃·6H₂O and 2 mmol Na₂SnO₃ were dissolved in 40 mL DI water at room temperature with stirring to form a homogeneous solution. Then, Na₂SnO₃ solution was added into the La(NO₃)₃ solution drop by drop with vigorous stirring. The resulting suspension was transferred into a 100 mL Teflon-lined stainless steel autoclave and hydrothermally treated at 200 °C for 12 h, and then cooled naturally. The precipitate was collected by centrifugation and washed several times with distilled water before drying at 60 °C to obtain the final product. Au nanoparticles were deposited on the surface of La₂Sn₂O₇ (Au/La₂Sn₂O₇) by a NaBH₄ reduction method. The La₂Sn₂O₇ powders were dissolved in 100 mL H₂O by ultrasonic dispersion and an appropriate volume of HAuCl₄ solution (5 mg Au mL⁻¹) was added. After vigorous stirring for 12 hours, the resulting mixture was reduced with NaBH₄ solution (0.1 M), then centrifuged and washed thoroughly with distilled water, and dried at 60 °C for 12 h. The gold contents of the samples were estimated by precursor feeding. The values for the actual Au loading of the synthesized catalysts were obtained by using an inductively coupled plasma mass spectrometer (ICP-MS, OPTIMA 8000), and summarized in Table S1.†

2.2. Characterization

Powder X-ray diffraction (XRD) data was collected using a Bruker D8 Advance instrument (Cu Kα1 irradiation, =1.5406 Å). Morphological images of the surfaces and transmission electron micrographs were obtained using a Nova NanoSEM 230 microscope (FEI Corp) and a JEOL JEM 2010F microscope. UV-vis diffuse reflectance spectra (DRS) were measured using a Varian Cary 500 Scan, with BaSO₄ as a reference. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 photoelectron spectroscope system. Brunauer–Emmett–Teller (BET) specific surface area and CO₂ adsorption were recorded using an ASAP2020M apparatus. Electrochemical experiments were performed in a three-electrode cell made of quartz. A Pt plate and Ag/AgCl electrode served as the counter electrode and reference electrode, respectively. The working electrode was made by dip-coating a catalyst slurry (10 mg mL⁻¹ in EtOH) on fluorine-doped tin oxide (FTO) glasses, whose area was set as ca. 0.5 cm × 0.5 cm, and then was air-dried naturally. Electrochemical experiments were determined by a Zennium electrochemical workstation (Zahner Elektrik, Germany). The Mott–Schottky experiments and photocurrent response behaviors were conducted on a Precision PARC workstation. A 0.2 M Na₂SO₄ aqueous solution was used as the electrolyte for the two experiments. A 300 W Xe lamp (CEL-S500/350) was utilized as the light source for the photocurrent response behavior test. Electrochemical impedance spectroscopy (EIS) experiments were conducted on a CHI660D workstation (CH instrument) in an electrolyte of 0.5 M KCl aqueous solution containing 0.01 M K₃[Fe(CN)₆]/K₄[Fe(CN)₆] (1 : 1). Photoluminescence (PL) spectra were carried out on an Edinburgh instrument FLS980 spectrophotometer at ambient conditions.

2.3. Photocatalytic reactions

Photocatalytic reactions for CO₂ reduction were carried out at 1 atm CO₂ partial pressure in a 200 mL reactor, with an irradiation quartz reaction cell inside of it. 50 mg photocatalyst was added into the quartz reactor and dispersed in 140 mL H₂O. A 125 W high-pressure mercury lamp (GGZ125, Shanghai Yaming Lighting Co, Ltd with a maximum emission at about 365 nm) was used as the light source and the reactor was kept invariably at 20 °C by flowing cooling water during the reaction process. The whole reaction system was first evacuated by a mechanical pump to remove air before irradiation, then CO₂ (99.999% purity) was introduced into the reactor for 40 minutes to establish an adsorption–desorption balance and provide the necessary reaction gas. Gas products were detected by GC (Agilent 6890N) and the apparent quantum yield (AQY) was also measured under the same photocatalytic reaction conditions. The incident light intensity of a high pressure mercury lamp was measured by Spectri Light ILT950. The total number of incident photons was collected by a calibrated silicon photodiode.

The apparent quantum yield (AQY) was calculated from the following equations:

3. Results and discussion

3.1. Characterization of La₂Sn₂O₇ and Au/La₂Sn₂O₇

XRD patterns of the La₂Sn₂O₇ and Au/La₂Sn₂O₇ samples are shown in Fig. 1. The four characteristic peaks located at 28.8°, 33.4°, 48.0°, and 57.0° are indexed to the (222), (400), (440), and (622) diffraction planes of cubic crystalline La₂Sn₂O₇ (JCPDS: 73-1686), respectively. The average crystal size of the La₂Sn₂O₇ sample is estimated to be about 14.5 nm from the (222) peak at around 28.8° by using the Scherrer equation. Diffraction peaks of the Au/La₂Sn₂O₇ sample ascribe only to the cubic phase of La₂Sn₂O₇, and no diffraction peaks for Au were observed. This is due to a lower amount of metal loading and homogenous dispersion of Au on the La₂Sn₂O₇.

Fig. 1 XRD patterns of the La₂Sn₂O₇ and Au/La₂Sn₂O₇ samples.

Morphology and sizes of the samples were characterized by SEM and TEM. The La₂Sn₂O₇ sample shows micro/nanospheres with diameters that varied from 300 to 500 nm, assembled by densely arranged nanoparticles (Fig. 2a). The TEM of the 1 wt% Au/La₂Sn₂O₇ sample indicates few Au NPs dispersed on the La₂Sn₂O₇ surface (Fig. 2b). The HRTEM image of the 1 wt% Au/La₂Sn₂O₇ sample (Fig. 2c) shows a lattice spacing of ca. 0.234 nm, corresponding to the (111) planes of Au NPs,^30,31 and the size of the Au particle is about 5–7 nm. Meanwhile, the lattice spacing of the La₂Sn₂O₇ structure is ca. 0.309 nm, corresponding to the (222) plane of cubic La₂Sn₂O₇. Furthermore, the obtained EDX pattern corresponding to Fig. 2d shows the presence of La, Sn, O, Au, and Cu elements (Cu peaks arise from TEM grid), and the obtained EDX-mapping (Fig. S1†) shows uniform distribution of the elements.

Fig. 2 (a) SEM image of the La₂Sn₂O₇ sample; (b) TEM image; (c) HRTEM image; (d) EDX pattern of the 1 wt% Au/La₂Sn₂O₇ sample.

An X-ray photoelectron spectroscopy (XPS) survey spectrum of the prepared 1 wt% Au/La₂Sn₂O₇ sample (Fig. S2†) indicates the presence of La, Sn, O, and Au. No other element signal is found except for a C signal (attributed to adventitious carbon), in accord with the results of EDX (Fig. 2d). In the high resolution XPS spectrum of Au 4f (Fig. 3a), two main peaks located at 82.7 and 86.4 eV correspond to Au⁰ 4f_7/2 and Au⁰ 4f_5/2.³² And the Au⁰ 4f_7/2 negative binding energy shift of 1.4 eV, compared with that of bulk metallic Au⁰ (84.1 eV), indicates strong interactions between Au and La₂Sn₂O₇.³³ With regard to La 3d (Fig. 3b), the binding energies of La 3d_5/2 and La 3d_3/2 peaks are present at 836.1 and 853.2 eV with the shake-up peaks located at 840.2 and 857.3 eV, respectively, consistent with the reported results of La³⁺ in a La(III) oxidation state.³⁴ The Sn 3d XPS spectra (Fig. 3c) show two main peaks at 486.2 eV (Sn 3d_5/2) and 494.7 eV (Sn 3d_3/2) with the shake-up peaks located at 488.3 and 496.7 eV,³⁵ which may be assigned to the characteristic of Sn⁴⁺. As for O 1s XPS spectra (Fig. 3d), the O 1s signal could be fit into two peaks, and the peaks located at 528.7 and 531.2 eV are ascribed to the crystal lattice oxygen and surface hydroxyl, respectively.³⁶

Fig. 3 XPS spectra of the 1 wt% Au/La₂Sn₂O₇ sample: (a) Au element; (b) La element; (c) Sn element; and (d) O element.

The UV-vis diffused reflection spectra of the samples are shown in Fig. 4. Onset of the absorption edge for the La₂Sn₂O₇ is at about 315 nm. The corresponding bandgap energy (E_g) is estimated to be 3.93 eV, as determined by equation E = 1240/λ. Compared to bare La₂Sn₂O₇ materials, the absorption edge of Au/La₂Sn₂O₇ samples does not change, suggesting that Au is only loaded on the surface of La₂Sn₂O₇ rather than being incorporated into the lattice of La₂Sn₂O₇.³⁷ Meanwhile, an intense absorption band is centered at 531 nm, which could be assigned to the SPR of the Au NPs.³⁸ It can be seen that Au loading enhances the light absorption of La₂Sn₂O₇ samples, and the absorption intensity heightens along with increased Au doping amounts.

Fig. 4 The UV-vis diffused reflection spectra of La₂Sn₂O₇ and Au/La₂Sn₂O₇ samples.

The BET surface area (S_BET) of the prepared samples was investigated by N₂ adsorption–desorption measurements. The S_BET of the as-prepared La₂Sn₂O₇ and 1 wt% Au/La₂Sn₂O₇ were measured to be 75.6 and 70.6 m² g⁻¹ (Fig. S3†), respectively. The wide pore size distribution curves (Fig. S4†) indicate stacking among the small La₂Sn₂O₇ nanoparticles. Meanwhile, CO₂ adsorption over the samples was also measured and results are shown in Fig. 5. The CO₂ adsorption amounts of the La₂Sn₂O₇ and 1 wt% Au/La₂Sn₂O₇ samples are 11.65 and 10.07 cm³ g⁻¹ (Fig. 5), respectively. 1 wt% Au/La₂Sn₂O₇ shows a lower CO₂ adsorption capability compared to La₂Sn₂O₇, which may be due to some surface CO₂ adsorption sites, such as La species over La₂Sn₂O₇, that are covered with deposited Au nanoparticles.³⁹

Fig. 5 CO₂ adsorption isotherms of La₂Sn₂O₇ and 1 wt% Au/La₂Sn₂O₇ samples.

As shown in Fig. 6, the flat-band potential of La₂Sn₂O₇ determined from Mott–Schottky plots is ca. −0.89 V vs. Ag/AgCl at pH 7 (equivalent to −0.69 V vs. NHE at pH 7). The positive slopes of these plots suggest that La₂Sn₂O₇ is an n-type semiconductor.⁴⁰ With regard to the n-type semiconductor, the conduction band lies very close to the flat-band potentials.⁴¹ Therefore, the conduction band (E_CB) of La₂Sn₂O₇ is −0.79 V vs. NHE at pH 7, which is more negative than the redox potential of CO₂/CO (−0.53 V vs. NHE), CO₂/CH₄ (−0.24 V vs. NHE), and H₂O/H₂ (−0.41 V vs. NHE). So, it is thermodynamically possible that the photogenerated electrons in the prepared La₂Sn₂O₇ reduce CO₂ with H₂O to CO, CH₄, and H₂.

Fig. 6 Mott–Schottky plots of La₂Sn₂O₇.

3.2. Photocatalytic activity and stability

Photocatalytic CO₂ reduction activities of the samples were investigated under mild conditions without any sacrificial reagents. As shown in Fig. 7a, the samples exhibited photocatalytic performance for CO₂ reduction with H₂O, and CH₄, CO, and H₂ are the main reduction products; other products such as CH₃OH, HCHO, and HCOOH were not detected during the reaction. The output products of CH₄, CO, and H₂, over bare La₂Sn₂O₇ were 0.60, 0.31, and 6.90 μmol after 3 h irradiation, corresponding to the evolution rates of 0.20, 0.10, and 2.30 μmol h⁻¹, respectively, with an apparent quantum yield (AQY) of 0.89%, which is superior to the P25 sample with a AQY of 0.68% (Table S2†). After loading Au, the samples exhibited better photocatalytic CO₂ reduction activities. The evolution rate of the main products increased with addition of Au. When the loading amount of Au reached 1.0 wt%, the evolution rate of CH₄, CO, and H₂ increased to the max of 0.43, 0.37, and 6.98 μmol h⁻¹, respectively (Fig. 7b), which was 2.2, 3.7, and 3.0 times as high as that of bare La₂Sn₂O₇. The generation of H₂, CH₄, and CO over 1.0 wt% Au/La₂Sn₂O₇ increased almost linearly with irradiation time (Fig. 7b), and their outputs are 2.05, 1.74, and 33.08 μmol after 5 h irradiation, respectively, with an apparent quantum yield (AQY) (2.54%) enhanced by 3 times more than the La₂Sn₂O₇ sample.

Fig. 7 (a) The output of CH₄, CO, and H₂ over La₂Sn₂O₇ and Au/La₂Sn₂O₇ samples under irradiation for 3 h; (b) CH₄, CO, and H₂ formed as a function of irradiation time over 1 wt% Au/La₂Sn₂O₇ samples.

The amount of O₂ formed was also measured to determine charge balance during the photocatalytic reaction. After a 5 h photocatalytic reaction over the 1.0 wt% Au/La₂Sn₂O₇ sample, the amount of O₂ was 21.08 μmol. Supposing that the oxidation of H₂O to O₂ is the only reaction to expend the photogenerated holes, and the photogenerated electrons are used to form CH₄, CO, and H₂, then the stoichiometric molar ratio of O₂/(4CH₄ + H₂ + CO) should be 0.5. The molar ratio of O₂/(4CH₄ + H₂ + CO) calculated from our results is 0.49, which is reasonably close to the stoichiometric ratio.

Stability is very important for practical photocatalytic systems; thus cyclic testing of the photocatalytic CO₂ reduction activities was performed to investigate stability. During the 4 times repeated reaction cycles of a 1 wt% Au/La₂Sn₂O₇ sample (Fig. S5†), there was no distinct decrease in H₂, CH₄, and CO evolution rates, thus demonstrating that the 1 wt% Au/La₂Sn₂O₇ sample shows considerable photostability.

Control experiments were also carried out. No product was detected in the reaction system in the dark or without catalyst, indicating that the CO₂ reduction requires presence of a photocatalyst. Only H₂ was detected when CO₂ was replaced by high purity N₂ in the reaction system. This result suggests that the formed reduced products originated from CO₂ and not from other sources of contamination.

3.3. Photocatalytic mechanism

Transient photocurrent responses of the La₂Sn₂O₇ and 1 wt% Au/La₂Sn₂O₇ samples were measured under intermittent UV-vis illumination. As shown in Fig. 8a, the transient photocurrent of a 1 wt% Au/La₂Sn₂O₇ sample was much higher than that of the bare La₂Sn₂O₇, indicating a more efficient separation and longer lifetime of photogenerated charge carriers with the loading of Au.⁴² It has been proven that a smaller arc radius in a Nyquist plot implies a more effective charge separation of electron–hole pairs and a faster interfacial charge transfer.^43,44 The impedance arc of 1 wt% Au/La₂Sn₂O₇ is much smaller than that of La₂Sn₂O₇ (Fig. 8b), corresponding to its higher speed of charge separation of electron–hole pairs and interfacial charge transfer, which agrees well with their photocatalytic performances as discussed above. Meanwhile, photoluminescence spectra of the La₂Sn₂O₇ and 1 wt% Au/La₂Sn₂O₇ samples with an excitation wavelength of 310 nm are shown in Fig. 9. A strong PL emission spectrum centered around 410 nm is observed over the La₂Sn₂O₇, which comes from a trap-state emission, originating from the anion (O²⁻) or cation (La³⁺ and Sn⁴⁺) vacancy defects, and other crystallographic defects.⁴⁵ After loading Au on La₂Sn₂O₇, the emission intensity remarkably decreases, indicating enhancement of the separation and transfer of photogenerated electrons and holes. This result also is in accordance with the photocurrent performance. All the observations above highlight the function of the Au co-catalyst for enhancing charge transfer, leading to a higher photocatalytic efficiency.

Fig. 8 (a) Transient photocurrent response; (b) Nyquist plots of La₂Sn₂O₇ and 1 wt% Au/La₂Sn₂O₇ samples.

Fig. 9 Photoluminescence (PL) spectra of La₂Sn₂O₇ and 1 wt% Au/La₂Sn₂O₇ samples collected at the excitation wavelength of 310 nm.

The mechanism for CO₂ reduction with H₂O over Au/La₂Sn₂O₇ is proposed as shown in Fig. 10. When light with photon energy higher or equal to the band gap of La₂Sn₂O₇ is incident on the sample, then electrons and holes are generated in the CB and VB, respectively. Photogenerated electrons on the CB are transferred to the absorbed CO₂ and H⁺ to produce CH₄, CO or H₂ on the surface of La₂Sn₂O₇. Simultaneously, holes on the VB edge of La₂Sn₂O₇ are positive enough to oxidize water to form O₂. The deposited Au acts as a co-catalyst which facilitates transfer of photoexcited electrons and prevents the recombination of photoexcited electrons and holes.

Fig. 10 Proposed mechanism of photocatalytic CO₂ reduction over La₂Sn₂O₇ with Au co-catalyst.

4. Conclusion

La₂Sn₂O₇ micro/nanospheres were successfully synthesized by a facile hydrothermal method and Au nanoparticles were deposited on La₂Sn₂O₇ via H₂AuCl₆ impregnation followed by NaBH₄ reduction. The prepared La₂Sn₂O₇ sample showed photocatalytic CO₂ reduction activities and CH₄, CO, and H₂ were the main reduction products. Deposition of an Au co-catalyst significantly improved photocatalytic performance. The highest evolution rates of CH₄, CO, and H₂ were observed over 1.0 wt% Au/La₂Sn₂O₇ samples with 0.43, 0.37, and 6.98 μmol h⁻¹, respectively. The apparent quantum yield (AQY) was 2.54%, which was enhanced by 3 times in contrast to bare La₂Sn₂O₇. An Au co-catalyst plays an excellent role as an electron transfer mediator. Our results indicate promising application of the La₂Sn₂O₇ photocatalyst for CO₂ reduction. Moreover, Au nanoparticles could be superior candidates for a co-catalyst to enhance CO₂ reduction activities.

Acknowledgements

This work was financially supported by the NSFC (Grants No. U1305242, 21373050), the National Key Basic Research Program of China (973 Program: 2013CB632405, 2014CB239303, 2014CB260410, 2014BAC13B03).

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Footnote

† Electronic supplementary information (ESI) available: XPS survey spectrum of the 1 wt% Au/La₂Sn₂O₇ sample; N₂ adsorption/desorption isotherms (77 K) of La₂Sn₂O₇ and 1 wt% Au/La₂Sn₂O₇ samples; the stability test of 1 wt% Au/La₂Sn₂O₇ photocatalyst for CH₄, CO, H₂ evolution rate in four repeats of reaction. See DOI: 10.1039/c7ra00765e

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