Enhanced Solar-Driven Gaseous CO2 Conversion by CsPbBr3

Enhanced Solar-Driven Gaseous CO2 Conversion by CsPbBr3 Nanocrystal/Pd Nanosheet Schottky-Junction Photocatalyst. Yang-Fan Xu , Mu-Zi Yang ...
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Enhanced Solar-driven Gaseous CO2 Conversion by CsPbBr3 Nanocrystal/Pd Nanosheet Schottky-junction Photocatalyst Yang-Fan Xu, Mu-Zi Yang, Hong-Yan Chen, Jin-Feng Liao, Xu-Dong Wang, and Dai-Bin Kuang ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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Enhanced Solar-driven Gaseous CO2 Conversion by CsPbBr3 Nanocrystal/Pd Nanosheet Schottky-junction Photocatalyst Yang-Fan Xu,† Mu-Zi Yang,† Hong-Yan Chen, Jin-Feng Liao, Xu-Dong Wang, and Dai-Bin Kuang*

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P. R. China

Corresponding Author * E-mail: [email protected]

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ABSTRACT: In this report, a novel zero-dimensional CsPbBr3 nanocrystal (CsPbBr3 NC)/two dimensional Pd nanosheet (Pd NS) composite photocatalyst is prepared to afford efficient and stable photocatalytic gaseous CO2 reduction in the presence of H2O vapor under visible light illumination. Pd NS herein acts as an electron reservoir to quickly separate the electron-hole pairs in CsPbBr3 NC through a Schottky contact, and provides an ideal site for CO2 reduction reactions. A highest electron consumption rate of 33.79 μmol/g h is achieved by CsPbBr3 NC/Pd NS composite, which corresponds to a 2.43-fold enhancement over pristine CsPbBr3 NC (9.86 μmol/g h), thus providing a practical and universal solution for the halide perovskite materials to boost the photocatalytic performance through semiconductor/metal design.

KEY WORDS

photocatalysis; CO2 reduction; Palladium; CsPbBr3; nanocrystal

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As a notorious greenhouse gas, CO2 in nature is conjectured to lose its balance due to the immoderate consumption of fossil fuels.1-2 Thus, searching for valid approaches to maintain atmospheric CO2 level and close the CO2 cycle appears to be an urgent task.3-5 Since the pioneering works reported in 1970s,6-7 artificial photosynthesis which is actuated by the inexhaustible solar energy, has attracted tremendous attentions for CO2 conversion into solar fuels.8-12 It is well-perceived that pronounced optoelectronic properties are pivotal for a robust photocatalyst to construct a high-efficiency artificial photocatalysis system.11-17 Stimulated from the remarkable photovoltaic performance of lead halide perovskite,18-22 our group have successfully applied the halide perovskite nanocrystals (e.g. CsPbBr3, Cs2AgBiBr6) to conduct the CO2 photoreduction recently.23-25 Considering the instability of halide perovskite in aqueous,

26-27

the low polar

nonaqueous ethyl acetate has been selected as solvent to warrant a long-term stability for halide perovskite and a high CO2 solubility in our previous report.23,

28

Another potential method to

overcome the instability issue is to alter the CO2 reduction reactions from solid/liquid interface to solid/vapor interface for the so-called solid-vapor system construction. The solid-vapor CO2 reduction system is quite attractive because of several advantages such as reduced formation of the less active CO2 hydration products and higher selectivity towards CO2 reduction reactions (i.e. the hydrogen evolution side reactions can be suppressed).29 Another issue limiting the performance of the halide perovskite photocatalyst lies in the charge separation and transfer, especially for perovskite nanocrystals which can achieve over 90% photoluminescence quantum yields (PL QYs).30-32 Such high PL QYs also can be interpreted as that 3 ACS Paragon Plus Environment

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the excited electron-hole pairs have an intense propensity to be consumed in radiative charge recombination before taking part into the chemical reactions, since the former process occurred much more faster.33 Thus, efficient separation of the electron-hole pairs timely and spatially is of critical importance for perovskite nanocrystals with regard to obtaining high efficiency in photocatalysis. The most commonly used strategy to resolve this concern is to construct a close contacted interface by coupling with another material. Accordingly, our group has successfully constructed the CsPbBr3 QD/GO composite for a better charge separation and transfer, which leaded to a 25 % improved photocatalytic performance when compared to individual CsPbBr3 QD. However, further enhancements on the activity or selectivity of CO2 reduction reaction are still expectable if one can find a more suitable material beyond GO. Specifically, incorporation semiconductor with two dimensional metal not only can build a Schottky junction at the interface to accelerate the photo-induced electrons transfer from semiconductors to metal, but also introduce catalytic reaction sites to facilitate the transferred electrons’ injection into the subsequent chemical reactions.34-35 Herein, we report a novel CsPbBr3 nanocrystal/Palladium nanosheet (CsPbBr3 NC/Pd NS) Schottky junction composite for improved light-driven CO2 reduction reaction in the presence of H2O vapor. The Pd NS not only stands for an electron acceptor to quickly separate the photoexcited electron-hole pairs in CsPbBr3 NC and suppress the undesired radiative charge recombination, but also catalyze the CO2 reduction reaction with modified kinetics. As a result, the introduction of Pd NS boosted the photoelectron consumption rate from 9.86 μmol/g h to 33.09 μmol/g h under visible light illumination (>420 nm), revealing a promising and efficient strategy for future photocatalytic studies on halide perovskite materials. 4 ACS Paragon Plus Environment

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Figure 1. The (a) TEM and (b) HR-TEM images of the CsPbBr3 NC/Pd NS (600); (c-d) EDX mappings of the CsPbBr3 NC/Pd NS (600). The CsPbBr3 NC was synthesized in ambient atmosphere at room temperature. As confirmed by the XRD patterns (Figure S1), the as-prepared CsPbBr3 NC crystallized in pure orthorhombic phase and the broadened peak width indicated the shrunken particle size. The transmission electron microscopy (TEM) images showcased the CsPbBr3 NC had a cubic shape with size mainly dropped into the range of 10-16 nm (Figure S2). Prior to loading the CsPbBr3 nanocrystals, Pd nanosheets colloids in isopropanol were prepared according to the published method.36 XRD peaks (Figure S3) can be indexed to the metallic Pd, while the TEM image demonstrated that the as-prepared Pd exhibited nanosheet morphology with hexagonal shape (Figure S4a-b), and an average thickness of 2.53 nm was demonstrated by the corresponding height profiles in atomic force microscopic (AFM) scanning (Figure S4c), corresponding to ultrathin few layered Pd NS. The CsPbBr3 NC/Pd NS 5 ACS Paragon Plus Environment

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composites were denoted as CsPbBr3 NC/Pd NS (V), where the V was the volume of the added Pd NS colloid solution (100, 300, 600, 900 μL). The XRD patterns of the CsPbBr3 NC/Pd NS composites were analogous to that of the CsPbBr3 nanocrystals (Figure S1), indicating the constant states of the CsPbBr3 NC. However, the XRD peaks of Pd NS cannot be traced in the composites, which may originate from the low concentration of the Pd. To further acquire the real composition of the composites, we measured the Energy-dispersive X-ray (EDX) spectra on a scanning electron microscope (SEM), which indicate the weight percentage of Pd in the composite is in the range of 0.27-1.22% (Table S2). The X-ray photoelectron spectroscopy (XPS) was also employed (Figure S5), in which the existence of metallic Pd (i.e. not Pd2+) in the composite material was confirmed by the survey spectra accompanied with the high-resolution scan. The successful synthesis of the CsPbBr3 NC/Pd NS composite material was further witnessed with TEM images (Figure 1a-b), which clearly demonstrated that CsPbBr3 NCs were deposited onto the 2D Pd nanosheets. EDX mapping (Figure 1c-d) indicated Cs, Pb, Br, Pd elements were homogeneously distributed in the CsPbBr3 NC/Pd NS composite. Thermogravimetric analysis (TGA) revealed the inconspicuous weight losses in the samples below 300 oC (Figure S7), which was consistent with the largely lowered N-H, C-H, COOsignals in Fourier transform infrared (FTIR) spectra (Figure S8). These results indicated that the surface-adsorbed organics were removed, which would improve the electronic contact between CsPbBr3 NC and Pd NS,37 and eliminate the influence of the surface adsorbate to the CO2 reduction reactions.38 The UV-vis spectra of CsPbBr3 NC film (Figure S9) clearly demonstrated that the CsPbBr3 NC had an absorption band edge at 530 nm (2.33 eV), similar to the published results.39 As for the composite ones, their absorption behaviors retained analogical except that the absorbance at 6 ACS Paragon Plus Environment

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longer wavelength region (above 535 nm) significantly increased, which was derived from the addition of metallic Pd NS. EDX spectra

Figure 2. (a) Steady-state PL spectra, inset is the corresponding expanded view, the λexcitation=406.2 nm; (b) PL decay spectra, inset is the corresponding expanded logarithmic plot; (c) Transient absorption kinetic plots, λexcitation=400 nm. (d) The sketch of the composite material and their corresponding band alignments (band structure of CsPbBr3 NC obtained from ref. 23, 39).

Table 1. The time-resolved photoluminescence decay parameters of the CsPbBr3 NC and CsPbBr3 NC/Pd NS. Sample

τ1/ns

%

τ2/ns

%

τ3/ns

%

τaverage/ns

χ

CsPbBr3 NC CsPbBr3 NC/Pd NS (100) CsPbBr3 NC/Pd NS (300) CsPbBr3 NC/Pd NS (600) CsPbBr3 NC/Pd NS (900)

1.96

5.75

10.70

20.41

67.35

73.84

52.03

1.109

1.32

24.38

6.37

42.44

31.21

33.18

13.38

1.282

0.98

40.41

4.61

41.40

22.50

18.20

6.40

1.229

0.74

42.70

3.59

41.49

18.72

15.81

4.76

1.210

0.73

59.72

3.23

33.00

16.31

7.28

2.71

1.381 7

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The charge carrier dynamics of individual CsPbBr3 NC and CsPbBr3 NC/Pd NS composite had profound significance for the optoelectronic and photoelectrochemical performances. On account of this point, photoluminescence (PL) spectra and femtosecond transient absorption spectroscopy (fs-TAS) characterizations were performed. As shown in Figure 2a, the PL emission was sharply quenched to 0.5-8.6 % of its initial intensity after incorporating with the Pd NS, revealing the radiative charge recombination was largely restrained. Such conclusion was also provided in the time-resolved PL decay plots, where accelerated PL decays were obviously exhibited for CsPbBr3 NC/Pd NS samples (Figure 2b). All the PL decay was fitted with triple-exponential decay kinetic, resulting three components with different lifetime. As summarized in Table 1, in CsPbBr3 NC/Pd NS composite the lifetime of all decay components were shortened when compared to the individual CsPbBr3 NC. Moreover, the contribution of the fast decay component (τ1), which was related to the charge transfer processes, was significantly raised as increasing the Pd NS amounts, indicating more facile interfacial charge transfer.40 As a result, the CsPbBr3 NC film exhibited an average decay lifetime of 52.03 ns, which was consistent with the previous reports.41 While for the CsPbBr3 NS/Pd NS, the PL decay times were substantially shortened to 2.71-13.38 ns and were inversely proportional to Pd NS amounts. As a further step to clarify the charge transfer properties, the fs-TAS tests were performed on CsPbBr3 NC and CsPbBr3 NS/Pd NS films, as depicted in Figure S10. The most pronounced negative feature was witnessed at 522 nm, which related to ground-state bleaching (GSB) of CsPbBr3 NC. Tracing the TAS decays the negative peaks intensity of the composites were largely decreased at the same delay time compared with that of the CsPbBr3 NC (Figure 2c), agreed well with the steady-state PL results that the Pd NS facilitated the charge extraction from CsPbBr 3 8 ACS Paragon Plus Environment

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NC.31 Thus, it is clear that the electron-hole pairs of the CsPbBr3 were efficiently separated timely and spatially by the Pd NS prior to recombination, which would facilitate the subsequent photochemical processes, such as CO2 reduction as schemed in the Figure 2d. Table 2. Summary of the photocatalytic CO2 reduction performances after 3 h of constant illumination.

Sample

R(H2)/ R(CO)/ R(CH4)/ Relectrona / μmol g-1 μmol g-1 μmol g-1 μmol g-1

Select. for RCO2 reduc.a / CO2 μmol g-1 reduct.b/%

CsPbBr3 NC

0.000

3.623

2.794

29.598

29.598

100

CsPbBr3 NC/Pd NS (100) 0.502

7.921

3.067

41.382

40.378

97.6

CsPbBr3 NC/Pd NS (300) 1.167

12.633

3.935

59.079

56.746

96.0

CsPbBr3 NC/Pd NS (600) 3.286

5.768

10.411

101.394

94.824

93.5

CsPbBr3 NC/Pd NS (900) 1.401 3.898 5.258 52.662 49.86 94.7 a Relectron is the electron consumption for the reduced product, Relectron = 2R(CO) + 8R(CH4) + 2R(H2). b Selectivity for CO2 reduction = [2R(CO) + 8R(CH4)]/Relectron × 100%.

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Figure 3. Photocatalytic CO2 reduction performances: (a) The electron consumption rates under visible light illumination (>420 nm). (b) The quantum efficiency plots as function of various wavelengths measured under monochromatic illumination. The photocatalytic CO2 reduction performances of CsPbBr3 NC film and CsPbBr3 NC/Pd NS composite films were subsequently investigated. The incident light was provided by a Xenon lamp, with the UV region being cut-off by a 420 nm-cut optical filter. The photocatalytic performances as function of the Pd NS amounts were plotted in the Figure 3a and summarized in the Table 2. It is apparent that the incorporation of the Pd NS can obviously enhance the electron consumption rate. Particularly, the optimized performance as high as 33.79 μmol/g h was achieved based on 600 μL Pd NS, corresponding to a 2.43-fold enhancement of pristine CsPbBr3 NC (9.86 μmol/g h). Moreover, this photocatalytic performance also exceeds the previous reported CsPbBr3 QD/GO photocatalyst and a series of traditional visible light absorption materials (Table S3). It can be noted in the time-course product generation plots that the greatly enhanced electron consumptions were mainly contributed by the increased generation rates of CH4 (Figure S11), which agreed well with other cases involved the Pd co-catalyst.37,

42-43

However, superfluous Pd amount was harm to the

photocatalytic performance, resulting from the little photocatalytic CO2 reduction response of Pd NS itself in this experiment (not shown), which ultimately competed in the light absorption with the active CsPbBr3 NC. Quantum efficiency (QE) tests were also conducted as a function of wavelength. As shown in Figure 3b, a significant enhancement on the QE in the range of the 380-600 nm can be figured out after incorporating the Pd NS. Since the Pd was detrimental to the light harvesting of CsPbBr3 NC (Figure S9), the enhancement of QE should root in the enhanced charge separation which had been confirmed in the PL and fs-TAS tests, or modified chemical reaction kinetics,

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according to the definition of the QE.44 To confirm whether there were side products coming from the surface organic matters, we conducted a controlling photocatalytic reaction in inert N 2 atmosphere. The results demonstrated that replacing CO2 by N2 largely restrained the photocatalytic responses (Table S4) to 2 % of those in CO2 atmosphere, confirming that the CO2 was the feedstock for the product generation. This result can be also found by the isotope labeling experiments. As shown in Figure S12, after the photocatalytic reaction within m/z=18) atmosphere, the signals of

13

CH4 (m/z=17) and

13

13

CO2 (m/z=45) and H2O (vapor,

CO (m/z=29) were clearly observed.

Moreover, the isotope experiment within CO2 (m/z=44) and H218O (vapor, m/z=20) further confirmed that the photo-holes would take part in the water oxidation during the photocatalytic reactions (Figure S13), according to the peak at m/z=36 (18O2). Additionally, a 3-recycled photocatalytic measurement was performed, in which the photocatalytic performance kept constant (Figure S14) demonstrating that the CsPbBr3 NC/Pd NS (600) composite was quite robust in the solid-vapor system. This deduction was further corroborated by the UV-vis spectra (Figure S15) and XPS plots (Figure S5) since the absorption behaviors and chemical states of the composed elements were consistent before and after photocatalytic reactions, respectively. Moreover, it is noteworthy that all the CsPbBr3 NC and CsPbBr3 NC/Pd NS samples were deposited onto the glass substrates for the photocatalytic reactions, which not only ensured the close and constant package of CsPbBr3 NC and Pd NS, but also made it more convenient to recycle when compared to that where particulate photocatalyst was suspended in solution.

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Figure 4. Photoelectrochemical tests in 0.05 M TBAPF6/dichloromethane electrolyte under AM 1.5G illumination: (a) Amperometric I-t curves; (b) photoelectrochemical impedance spectroscopy (PEIS) plots of CsPbBr3 NC and CsPbBr3 NC/Pd NS. The CsPbBr3 NC and CsPbBr3 NC/Pd NS samples were further fabricated as photoelectrodes for photoelectrochemical measurements. The amperometric I–t tests (Figure 4a) showed that both the CsPbBr3 NC and CsPbBr3 NC/Pd NS electrodes exhibited a fast and highly repeatable photocurrent response to light on-off cycles under a bias of -0.4 VAg/AgCl and simulated AM 1.5G solar irradiation. The highest photocurrent response was achieved on CsPbBr3 NC/Pd NS (600) sample (74 μA cm-2), which corresponds to a 2.4-fold increment compared to the pristine CsPbBr3 NC (31 μA cm-2). The order of the photocurrent density was consistent with the photocatalytic performances in the CO2 reduction experiments. Photoelectrochemical impedance spectroscopy (PEIS) was subsequently conducted, in which the semi-arcs at relative low-frequency range of the Nyquist plots gradually

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decreased with increasing the Pd amount (Figure 4b). Fitting by a simple R-C equivalent circuit model,45 the result demonstrated that the charge transfer resistance was lowered from 47894 Ω to 20240 Ω after introducing the Pd NS (Table S5), indicating more pronounced charge transfer at the close electronic contact by the Schottky junction and high conductivity of the metallic Pd NS.46-48 The PEC measurements again proved that the excess Pd NS may lower the photo-responses. Thus, the Pd amount should make a trade-off between the competitive light absorption and the charge transfer enhancement. To summarize, photocatalytic reduction of gaseous CO2 with H2O vapor was successfully implemented by perovskite CsPbBr3 nanocrystal. Such solid-vapor mode avoids the use of labile organic solvents and warrants the stability of CsPbBr3 as well. Furthermore, by loading the CsPbBr3 nanocrystals onto the two-dimensional Pd nanosheets, significant enhancements on both the photoelectron consumption rate (from 9.86 μmol/g h to 33.9 μmol/g h) in photocatalytic CO2 reduction and photocurrent density (from 31 μA cm-2 to 74 μA cm-2) in photoelectrochemical tests were attained by the composite materials. The quenched PL intensity by over 90 %, speeded PL decay and TAS decay, along with reduced electrochemical impedance revealed the performance promotions were stemmed from the accelerated charge separation & transfer by the construction of metal/semiconductor Schottky contact and the high catalytic property of Pd NS. We believe the solid-vapor mode can be extended in the upcoming studies by other halide perovskites or their related composites, and the rationally designed halide perovskite/metal concept could provide a feasible strategy for the purpose of high performance photocatalyst and other optoelectronic devices.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.xxxxxxxx: Experimental section; XRD patterns; AFM images; supplementary TEM images; XPS plots; UV-vis absorption spectra and photograph of the photocatalyst; TGA and FT-IR spectra; TAS spectra; Time courses plots of product evolution; GC-MS spectra; PEIS parameters. AUTHOR INFORMATION Corresponding Author: *Email: [email protected]

AUTHOR CONTRIBUTIONS †These authors contributed equally.

ACKNOWLEDGMENT The authors acknowledge the financial supports from the National Natural Science Foundation of China (21875288, 91433109), the GDUPS (2016), the Program of Guangzhou Science and Technology (201504010031), the Fundamental Research Funds for the Central Universities, and the NSF of Guangdong Province (S2013030013474). REFERENCES (1) Li, K.; Peng, B.; Peng, T., Recent Advances in Heterogeneous Photocatalytic CO2 Conversion to Solar Fuels. ACS Catal. 2016, 6, 7485-7527. (2) IPCC, Climate Change 2014-Impacts, Adaptation and Vulnerablity: Regional Aspects, Cambridge University Press, 2014. (3) Oschatz, M.; Antonietti, M., A search for selectivity to enable CO2 capture with porous adsorbents. Energy Environ. Sci. 2018, 11, 57-70. 14 ACS Paragon Plus Environment

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