Boosting Photocatalytic CO2 Reduction on CsPbBr3 Perovskite Nanocrystals by Immobilizing Metal Complexes

Article abstract of DOI:10.1021/acs.chemmater.9b04582

Converting CO₂ into chemical fuels with a photocatalyst and sunlight is an appealing approach to address climate deterioration and energy crisis. Metal complexes are superb candidates for CO₂ reduction due to their tunable catalytic sites with high activity. The coupling of metal complexes with organic photosensitizers is regarded as a common strategy for establishing photocatalytic systems for visible-light-driven CO₂ reduction. While most of the organic photosensitizers generally contain precious metals and are available through onerous synthetic routes, their large-scale application in the photocatalysis is limited. Halide perovskite nanocrystals (NCs) have been considered as one of the most promising light-harvesting materials to replace the organic photosensitizers due to their tunable light absorption range, low cost, abundant surface sites, and high molar extinction coefficient. Herein, we demonstrate a facile strategy to immobilize [Ni(terpy)₂]²⁺ (Ni(tpy)) on inorganic ligand-capped CsPbBr₃ NCs and to apply this hybrid as a catalyst for visible-light-driven CO₂ reduction. In this hybrid photocatalytic system, the Ni(tpy) can provide specific catalytic sites and serve as electron sinks to suppress electron-hole recombination in the CsPbBr₃ NCs. The CsPbBr₃-Ni(tpy) catalytic system achieves a high yield (1724 μmol/g) in the reduction of CO₂ to CO/CH₄, which is approximately 26-fold higher than that achieved with the pristine CsPbBr₃ NCs. This work has developed a method for enhancing the performance of photocatalytic CO₂ reduction by immobilizing metal complexes on perovskite NCs. The methodology we present here provides a new platform for utilizing halide perovskite NCs for photocatalytic applications.

Full text of DOI:10.1021/acs.chemmater.9b04582

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Article
Boosting Photocatalytic CO2 Reduction on CsPbBr3
Perovskite Nanocrystals by Immobilizing Metal Complexes
Zhoujie Chen, Yangguang Hu, Jin Wang, Qing Shen, Yaohong Zhang,
Chao Ding, Yu Bai, Guocan Jiang, Zhengquan Li, and Nikolai Gaponik
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b04582 • Publication Date (Web): 28 Jan 2020
Downloaded from pubs.acs.org on January 30, 2020
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Boosting Photocatalytic CO Reduction on
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CsPbBr3
Perovskite
Nanocrystals
by
Immobilizing Metal Complexes
†,#
‡,#
*,†
§
§
§
Zhoujie Chen, Yangguang Hu, Jin Wang, Qing Shen, Yaohong Zhang, Chao Ding, Yu
‡
ǁ
*,†
ǁ
Bai, Guocan Jiang, Zhengquan Li and Nikolai Gaponik
†Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Zhejiang
Normal University, Jinhua, Zhejiang 321004, P. R. China.
‡School of Chemistry and Materials Science, University of Science and Technology of China,
Hefei, Anhui 230026, P. R. China.
§Faculty of Informatics and Engineering, University of Electro-Communications, 1-5-1
Chofugaoka, Chofu, Tokyo 182-8585, Japan.
ǁPhysical Chemistry, TU Dresden, Bergstraße 66b, D-01062 Dresden, Germany
#
These authors contributed equally.
Corresponding Author
* E-mail: wangjin@zjnu.edu.cn.
*
E-mail: zqli@zjnu.edu.cn.
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ABSTRACT: Converting CO into chemical fuels with a photocatalyst and sunlight is an
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appealing approach to address climate deterioration and energy crisis. Metal complexes are
superb candidates for CO reduction due to their tunable catalytic sites with high activity. The
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coupling of metal complexes with organic photosensitizers is regarded as a common strategy for
establishing photocatalytic systems for visible-light-driven CO reduction. While most of the
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organic photosensitizers generally contain precious metals and are available through onerous
synthetic routes, their large-scale application in the photocatalysis is limited. Halide perovskite
nanocrystals (NCs), have been considered as one of the most promising light-harvesting
materials to replace the organic photosensitizers due to their tunable light absorption range, low
cost, abundant surface sites, high molar extinction coefficient. Herein, we demonstrate a facile
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strategy to immobilize [Ni(terpy) ] (Ni(tpy)) on inorganic ligand-capped CsPbBr NCs and to
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apply this hybrid as catalyst for visible light-driven CO reduction. In this hybrid photocatalytic
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system, the Ni(tpy) can provide specific catalytic sites and serve as electron sinks to suppress
electron-hole recombination in the CsPbBr NCs. The CsPbBr -Ni(tpy) catalytic system
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achieves a high yield (1724 μmol/g) in the reduction of CO to CO/CH , which is approximately
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6-fold higher than that achieved with the pristine CsPbBr NCs. This work has developed a
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method for enhancing the performance of photocatalytic CO reduction by immobilizing metal
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complexes on perovskite NCs. The methodology we present here provides a new platform for
utilizing halide perovskite NCs for photocatalytic applications.
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INTRODUCTION
Photocatalytic CO reduction offers a potential solution to simultaneously address the
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problems of the depletion of fossil fuels and global warming.(1,2) Although significant advances
have been reported,(3-8) the development of robust, selective, and inexpensive visible-light-driven
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photocatalysts for CO reduction remains crucial. In recent years, metal complexes have been
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widely used for photocatalysis.(9-12) These metal complexes often have multi- electron
conjugated structures, which contribute to their electron transfer and storage abilities.(13,14)
Moreover, the catalytic activity of metal complexes can be easily tailored by altering the central
metal ions or the surrounding ligands.(15) These attractive properties make them superb
candidates for multielectron-transfer catalysis in CO reduction. However, most metal
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complexes have limited visible-light response, which is an obstacle to their use in visible-light-
driven CO reduction. A common strategy for broadening its visible-light absorption range is to
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couple these metal complexes with organic photosensitizers, such as fac-tris(2-
phenylpyridine)iridium (Ir(ppy) ), tris(bipyridine)ruthenium(II) chloride (Ru(bpy) ), or organic
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dye.(16-18) These sensitizers, however, either contain precious metals or suffer the problem of
onerous synthetic routes, which limits their large-scale application.
Compared to organic photosensitizers, semiconductor nanocrystal (NCs) generally have a
higher molar extinction coefficient and exhibit a broader visible-light absorption range.(19)
Furthermore, these NCs have the advantages of convenient synthesis, tunable light absorption
range, low cost, and abundant surface sites.(2,7,20-22) Therefore, colloidal NCs are potentially ideal
candidates to replace organic photosensitizers. Among others, halide perovskite NCs have been
found to be great alternatives to well-established chalcogenide NCs.(23) In addition to the
advantages mentioned above, the perovskite NCs also have relatively low nonradiative
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recombination rates, tunable chemical composition and structure via post-synthetic
transformations.(24) Furthermore, perovskite NCs have also proven to be one of the most
promising materials for designing highly efficient photocatalytic systems.(25-40)
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Therefore, the coupling of perovskite NCs with metal complexes is expected to solve the
problems caused by the use of organic photosensitizers and form more efficient photocatalytic
systems. In this context, the visible-light absorption range of metal complexes can be expanded
by coupling with perovskite NCs; meanwhile these metal complexes are likely to provide
catalytic sites for perovskite NCs and capture photoexcited electrons from perovskite NCs for
selective photocatalytic reduction of CO2. Furthermore, coupling metal complexes with
perovskite NCs has the following advantages: 1) the abundant binding sites on perovskite NCs
can be readily modified for immobilizing metal complexes; 2) the immobilized metal complexes
can provide catalytic sites for perovskite NCs, and these specific catalytic sites are expected to
enhance the catalytic performance of the CO reduction; 3) the metal complexes can serve as the
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electron sinks which are able to accept multiple photoexcited electrons from the perovskite NCs
in a stepwise manner. This is particularly important for multielectron reductions of CO , which
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can proceed at relatively low potentials.(41) In other words, this architecture can combine the
advantages of homogeneous (e.g., metal complexes) and heterogeneous (e.g., perovskite NCs)
catalysis and potentially provides an efficient system for CO reduction.
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In this work, we present a novel hybrid photocatalyst system based on immobilizing
transition metal complexes onto modified CsPbBr perovskite NCs for visible-light-driven CO
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reduction. As a proof of concept, Ni(tpy) (nickel complex with 2,2’:6’,2’’-terpyridine,
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[
Ni(terpy) ] ) was selected as the metal complex. One of the reason for choosing Ni(tpy) is that
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it does not contain noble metal and it has polypyridyl rings for capturing and storing electrons.(42-
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43)
In this system (CsPbBr -Ni(tpy)), the catalytic efficiency of the CsPbBr NCs was found to be
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dramatically improved by the immobilizing of Ni(tpy). The photocatalytic activity of CsPbBr3-
Ni(tpy) for CO reduction is 26-fold higher than that of pristine CsPbBr NCs. Further
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investigations revealed the electron transfer kinetics between the CsPbBr NCs and Ni(tpy)
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through time-resolved fluorescence decay (TRPL) and transient absorption (TA) spectra. We
demonstrated that the coupling of metal complexes is a promising strategy to improve the
photocatalytic properties of perovskite NCs. This strategy also provides a platform for designing
precious-metal-free photocatalysts for CO reduction.
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EXPERIMENTAL SECTION
Chemicals. Cs CO (99.9%), octadecene (ODE, 90%), oleic acid (OA, 90%), PbBr (98%),
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ethyl acetate (EA, 99.8%, Anhydrous), acetonitrile (ACN, 99.8%, Anhydrous), ferrocene (98%)
were purchased from Sigma-Aldrich. Oleylamine (OAm, 80-90%) and 2,2′:6′,2″-terpyridine
(terpy, 97%) were purchased from Alfa Aesar. Ammonium hexafluorophosphate (NH PF ,
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5%), NiCl ·6H O (99.9%), hexane, ethanol, ether, acetone, toluene and tetrabutylammonium
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hexafluorophosphate (TBAPF , 98%) were purchased from Aladdin. All chemicals were used as
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received without any further purification. CO (>99 atom% C) was purchased from Sigma
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Aldrich.
Preparation of the Cs-OA Precursor. Cs CO (100 mg), OA (0.6 mL), and ODE (4 mL)
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were loaded into a 50 mL three-neck round-bottom flask. The reaction system vacuumed at 100
ºC for 1h, and then heated under N flow to 150 °C until all the Cs CO are dissolved in ODE.
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The temperature of the Cs-Oleate precursor was cooled down to 100 ºC and maintaining at this
temperature before injection.
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Synthesis of pristine CsPbBr NCs. The organic ligands (OL) capped CsPbBr NCs were
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prepared according to a literature procedure.(44) Briefly, ODE (5 mL) and PbBr (0.188 mmol)
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were loaded into 50 mL 3-neck round-bottom flask and dried under vacuum at 120 ºC for 1h.
Subsequently, dried OAm (0.5 mL) and dried OA (0.5 mL) were injected at 140 ºC under N2.
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After complete dissolution of a PbBr salt, the temperature was raised to 180 ºC and Cs-oleate
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solution (0.5 mL, 0.125 M) was quickly injected and, 5 s later, the reaction mixture was cooled
by the ice-water bath. The crude NCs solution was dispersed in 5 mL hexane, and the
undissolved particles were separated by centrifuging the solution at 6000 rpm for 5 min. The
supernatant was collected and the pristine CsPbBr particles were precipitated by adding EA
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(hexane: EA = 1:1, v:v), and the aggregation was separated by centrifuging the solution at 6000
rpm for another 5 min. After centrifugation, the supernatant was discarded and the particles
were dried for further characterizations and applications.
Preparation of PF modified CsPbBr NCs (PF -CsPbBr ). Ligand exchange was
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performed under dark condition by a modified literature procedure.(45) Briefly, the pristine
CsPbBr (5 mg) was re-dispersed in EA (5 mL, solution A). An ethanol solution of NH PF (40
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μL, 20 mM) was added into solution A and stirred in the dark for 30 min to form PF modified
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CsPbBr NCs. The stripped particles were centrifuged (5000 rpm, 5 min) and dried in air.
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Similar procedures were employed for the preparation of EA treated CsPbBr NCs except the
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absence of NH PF .
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Assembly of CsPbBr -Ni(tpy). In a typical process, the PF -CsPbBr NCs were re-
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dispersed in EA (5 mL, solution B). An acetonitrile solution of Ni(tpy) (40 μL, 6 mM) (detailed
chemical structure was shown in Figure S1) was added into solution B and stirred in the dark for
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0 min to form CsPbBr -Ni(tpy) composites. The CsPbBr -Ni(tpy) composites were separated
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by centrifuging the resulted solution at 5000 rpm for 5 min. After centrifugation, the supernatant
was discarded and the CsPbBr -Ni(tpy) catalysts were dried for further characterizations and
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applications.
RESULTS AND DISCUSSION
Figure 1. (a) Schematic of the formation of CsPbBr -Ni(tpy) photocatalysts. (b) Fourier
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transform infrared (FT-IR) spectra of the pristine CsPbBr (black), PF -CsPbBr (dark green) and
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CsPbBr -Ni(tpy) (dark red). (c) Variations in the zeta (ζ) potentials of the pristine CsPbBr , PF -
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CsPbBr and CsPbBr -Ni(tpy) in EA.
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High-quality colloidal CsPbBr NCs were synthesized using a typical hot injection method
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developed by Protesescu et al. (see experimental for details).(44) The pristine CsPbBr NCs are
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usually capped by long aliphatic chain-containing organic ligands such as OA and OAm.(46) The
presence of these bulky ligands forms a barrier on the surface of the NCs, thereby hindering the
assembly of CsPbBr -Ni(tpy). At the same time, these ligands also inhibit the adsorption of
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potential substrates (e.g., CO ) and reduce the catalytic activity of NCs. Therefore, in order to
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maximize the coupling of NCs to Ni(tpy) and/or to CO , it is necessary to remove or replace the
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OLs. To this end, we propose a novel strategy to assemble CsPbBr -Ni(tpy) catalysts. As
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depicted in Figure 1a, first, the EA can help to remove the native organic molecules capped on
the CsPbBr NCs (Figure S2), upon the ligand removal, the NCs display a more positively
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charged surface enabling a higher amount of PF to immobilize on the CsPbBr NCs via
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electrostatic interactions. Replacing native OL with PF can significantly reduce the steric
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hindrance of the NC surface. At the same time, the surface charge of the NCs may be altered by
-
the negatively charged PF . Subsequently, Ni(tpy) was introduced into the PF -CsPbBr NCs.
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Due to the strong electrostatic interaction between PF and Ni(tpy) on NCs, Ni(tpy) was
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immobilized on CsPbBr to form a CsPbBr -Ni(tpy) catalyst (details of the CsPbBr -Ni(tpy)
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assembly process are presented in the Experimental Section).
To monitor the assembly process, we examined the ligand species present on the NC
samples using FT-IR spectroscopy. As shown in Figure 1b, for the pristine CsPbBr , three
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typical bands for hydrocarbon species appear at 2852, 2925 and 1466 cm , indicating the
presence of long-chain organic ligands.(47) Additionally, four bands assigned to C=O, COO , N-
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H and N-H vibrations appear at 1710, 1394, 1633 and 3420 cm , respectively, which indicate the
(48)
coexistence of OAm and OA on the surface of the pristine CsPbBr NCs.
At the same time,
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revealing the desorption of OA and OAm from the NC surface. In addition, the appearance of
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the characteristic band of PF at 835 cm confirmed the adsorption of PF on the NC surface
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see Figure S3 for the characteristic bands of NH PF ). Compared with the PF -CsPbBr , the
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CsPbBr -Ni(tpy) shows new bands from 800 to 1600ꢀcm (Figure 1b) being characteristic for
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Ni(tpy)-containing species (see Figure S3 for the FTIR spectrum of Ni(tpy)). The FT-IR also
indicated that the Ni(tpy) were adsorbed on the surface of CsPbBr rather than physically mixed
3
with CsPbBr . Compared with the Ni(tpy) or the CsPbBr , there is no any shifts in the peaks for
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the CsPbBr -Ni(tpy). The FT-IR spectrum of CsPbBr -Ni(tpy) is a superposition of Ni(tpy) and
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CsPbBr NC signals revealing the Ni(tpy) are immobilized to the NC surface by electrostatic
3
interactions rather than by chemical bonding. In addition, with the adsorption of the
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-
[
Ni(terpy) ] , some of the free PF (from Ni(tpy)) will be further adsorbed onto the NC surface,
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-1
leading to a higher peak intensity at 835 cm in the CsPbBr -Ni(tpy).
3
To investigate the mechanism of the CsPbBr -Ni(tpy) assembly, we measured the zeta (ζ)
3
potentials of various NC samples. As is seen from Figure 1c, the pristine CsPbBr NCs carry a
3
-
positive charge on their surfaces (ζ potential of +5.8 mV). After ligand exchange with PF , the
6
surface of the CsPbBr became negatively charged facilitating further introduction of positively
3
charged Ni(tpy). These results demonstrated that the OL on the CsPbBr NCs could be replaced
3
-
by PF , allowing the immobilizing of Ni(tpy) on the CsPbBr NCs via electrostatic interactions.
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The X-ray diffraction (XRD) patterns (Figure S4) indicated that the NCs were highly crystalline,
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and their cubic perovskite crystal phase was well maintained during the formation of CsPbBr3-
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images of CsPbBr and CsPbBr -Ni(tpy), the insets show the high resolution TEM (HRTEM)
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images of the corresponding samples.
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shows that the pristine CsPbBr NCs and CsPbBr -Ni(tpy) display similar profiles in the visible
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spectral region with absorption maxima around 508 nm. In addition, the spectrum of CsPbBr3-
Ni(tpy) showed apparent characteristic absorption peaks below 400 nm, confirming the presence
of Ni(tpy) on the NC surface. It is also evidenced that the Ni(tpy) alone could not absorb visible
light, we can infer that under visible-light irradiation, Ni(tpy) acts as the catalyst and CsPbBr3
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plays the role of the photosensitizer in the CsPbBr -Ni(tpy) hybrid system. It should be noted
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that the absorption tail for CsPbBr -Ni(tpy) is extended to ~600 nm (Figure 2a). One of the
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possible reasons may be a stronger coupling between NCs caused by the ligand exchange.(49)
The assembly of CsPbBr -Ni(tpy) was further confirmed by the XPS spectra shown in Figure 2b.
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To the signals of the characteristic elements (Cs, Pb and Br), we observed two peaks from Ni 2p
in the CsPbBr -Ni(tpy). Furthermore, in the high resolution XPS shown in Figure S5, we
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observed the shift of binding energies in Cs, Pb, Br and N. These outcomes are principally due
to the strong chemical interactions at the interface between the CsPbBr and the tpy, leading to
3
the redistribution of charges in CsPbBr -Ni(tpy). The TEM images in Figure 2c-2d show that the
3
pristine CsPbBr NCs are highly crystalline nanocubic, and the particle size of the CsPbBr NCs
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remained constant (ca. 9.8 nm) during ligand exchange and catalyst immobilization. High-angle
annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-
dispersive X-ray spectroscopy (EDS) elemental mapping shown in Figure S6 further confirm that
the five representative elements (i.e., Cs, Pb, Br, Ni and F) are uniformly distributed throughout
-
the sample indicating the presence of PF and Ni(tpy) on the surface of CsPbBr -Ni(tpy) NCs.
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(
b) XPS spectra of CsPbBr : the valence band edge region. From these data, we could estimate
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that the valence band (VB) position of CsPbBr NCs was -5.98 V vs. vacuum. (c) Typical cyclic
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voltammograms of Ni(tpy) under argon or CO atmosphere (2 mM Ni(tpy), 0.1 M TBAPF in
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EA solution, 100 mV s ), a ferrocene/ferrocenium (Fc /Fc) redox couple was used as an internal
standard (Figure S7). (d) Schematic representation of the CsPbBr -Ni(tpy) photocatalyst system
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developed in this work: immobilizing of Ni(tpy) metal complexes onto a CsPbBr NC
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photosensitizer through electrostatic interactions.
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Since CsPbBr -Ni(tpy) is formed by immobilizing Ni(tpy) on CsPbBr NCs, to evaluate
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whether CsPbBr -Ni(tpy) can provide a sufficient driving force for CO reduction, we
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determined the band structure of CsPbBr and the reduction potential of the Ni(tpy) catalyst.
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According to the Tauc plot of (hν) vs hν from the UV-vis absorption spectra, the band gap (E )
g
of CsPbBr is approximately 2.39 eV (Figure 3a). The VB of CsPbBr was also measured by the
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valence band XPS (Figure 3b), which displayed an edge of the maximum energy at about 1.78
eV vs. Fermi level. Because the band gap of the CsPbBr as found from the Tauc plot is 2.39 eV,
3
we can conclude that the VB and conduction band (CB) positions of CsPbBr NCs are around -
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5.98 V and -3.59 V vs. vacuum, which is in agreement with the literature results.(25)
Subsequently, we conducted the CV of Ni(tpy) under Ar and CO atmospheres, respectively (as
2
shown in Figure 3c). Under an inert atmosphere, the CV curve display two reversible
electrochemical features, which can be assigned to ligand-based electrochemical processes. In
its own turn, under CO condition, the two electrochemical features lose reversibility and the
2
intensity of the corresponding cathodic peaks increased significantly. The enhanced current
indicates electrocatalytic activity of Ni(tpy) toward CO2,(43) of which the singly reduced Ni(tpy)
upon reaction with CO to give some Ni-related active species (e.g., Ni-CO and the Ni-CO
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complex).(22) The above results allowed us to suggest the schematics of the photocatalytic CO2
reduction process with the CsPbBr -Ni(tpy) photocatalysts. As illustrated in Figure 3d, under
3
visible-light illumination, CsPbBr will absorb light and generate photoexcited electrons on the
3
CB edge. Considering that the CsPbBr NCs has a more negative CB potential than the CdS
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NCs, (22,50) and the CdS NCs can provide an efficient driving force to enable CO reduction at
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Ni(tpy),(22) we can conclude that the photogenerated electrons at the CB edge of CsPbBr can be
3
swiftly injected into Ni(tpy) for CO reduction.
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Figure 4. Photocatalytic CO reduction using CsPbBr -based NCs. (a) Production of gases
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3
using various photocatalysts based on the pristine CsPbBr , the pristine Ni(tpy) and CsPbBr -
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Ni(tpy); error bars correspond to standard deviations based on three experiments. (b) Changes in
gases production as a function of the added Ni(tpy). (c) Cycling production of CH and CO
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using CsBrBr -Ni(tpy). (d) Mass spectra showing CH (m/z=17) and CO (m/z=29) produced
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over CsBrBr -Ni(tpy) in the photocatalytic reduction of 3CO2. Conditions unless stated
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otherwise: 100 mW cm , λ > 400 nm (see the light spectra in Figure S8), 4 h, 25 °C (5 mg
photocatalysts in 5 mL of EA solution (EA/water (49:1, v:v)).
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The photocatalytic activity of the CsPbBr -based catalysts was assessed in CO -saturated
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EA in the presence of water. As shown in Figure 4a, the pristine CsPbBr shows very low
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catalytic activity toward CO reduction, and the pristine Ni(tpy) has no photocatalytic activity
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due to its inability to absorb visible light. However, when we immobilized the Ni(tpy) onto the
CsPbBr , the photocatalytic properties of the composite were significantly improved. For the
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CsPbBr , the small PF ligands offer a nearly naked NC surface with low charge-transfer
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resistance upon stripping the OLs (detailed evidence is provided in Figure S9-S11), which allows
more photogenerated electrons to reach the surface of CsPbBr . At the same time, Ni(tpy) can
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trap photogenerated electrons from CsPbBr and provide efficient catalytic sites near the NC
3
surface for CO reduction. As a result, the photocatalytic activity of CsPbBr -Ni(tpy) was
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impressively high, achieving 1724 mol/g of CO/CH , which are almost 26-fold higher than that
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observed for the pristine CsPbBr . These findings strongly indicate that the immobilization of
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metal complexes is a powerful approach for improving the photocatalytic activity of CsPbBr3
NCs for CO reduction.
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Since the photocatalytic activity of CsPbBr was directly affected by Ni(tpy), we altered the
3
amount of Ni(tpy) to explore their effect on the photocatalytic activity in CO reduction.
2
Catalysts with different Ni(tpy) loadings can be easily obtained by changing the amount of
Ni(tpy) added during the immobilization. With the increase of Ni(tpy) content from 0.06 to 0.48
μmol, the catalytic properties of CsPbBr -Ni(tpy) gradually increase and then approach a plateau.
3
At the maximum (0.24 μmol Ni(tpy)), the reduction of CO to CO/CH achieved the highest
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value of 1724 μmol/g. We attributed this to the increased number of Ni(tpy) immobilized on the
surface of CsPbBr , as in general, more catalytic sites can offer a higher activity. The plateau of
3
the catalytic activity with the amount of Ni(tpy) increasing over 0.24 μmol may result from the
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redundant Ni(tpy), which will only weakly immobilize to the NC surface, and the excess of
Ni(tpy) would detach from the CsPbBr during the catalytic process (see the evidence provided
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in the supporting information, Figure S12). The increase of the Ni(tpy) concentration also
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promoted greater electron transfer from the CsPbBr to the Ni(tpy) electron sinks, which can be
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verified by the quenching of the NC luminescence (Figure S13). This behavior is consistent with
previous works conducted with other semiconductor-metal complex systems.(20) It suggests that
Ni(tpy) are mainly immobilized onto CsPbBr rather than physically colliding with CsPbBr in
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solution. Furthermore, the maximum electron consumption rate (Relectron = 2R(CO) + 8R(CH ))
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-1 -1
of CsPbBr /GO is 29.8 mol g h , and the R
of the CsPbBr -Ni(tpy) exhibit a higher value
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electron
-1 -1
of 1252 mol g h . The latter is an excellent result as compared to other perovskite-based
photocatalysts for CO reduction reported in the literature (Table S1). As to the CsPbBr /GO
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3
composite, although GO provides an additional transfer channel for photoexcited electrons, it
lacks, most probably, effective catalytic sites. At the same time, in the case of CsPbBr -Ni(tpy),
3
the Ni(tpy) can serve as the electron sink and can supply catalytic sites for a targeted reaction
providing in our case a higher photocatalytic activity for the CO reduction.
2
Figure 4c illustrates the time-dependent CO and CH generation rates using CsPbBr -
4
3
Ni(tpy) as a catalyst. There is no obvious decrease in the gas evolution rate in the first 4 h of the
reaction (one cycle). This stable photocatalytic activity reduces for only less than 15ꢀ% of its
initial value after repeating the above process for four cycles. We also performed some
characterizations of the CsPbBr -Ni(tpy) photocatalysts after 2 h photocatalytic reaction (Figure
3
S14-S16), the results indicated that the CsPbBr -Ni(tpy) photocatalysts could keep stable during
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photocatalysis. Control experiments (Figure S17) confirmed that CO is the feedstock for
2
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product generation, which was verified further by isotope labeling experiments using CO .
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When CO was used as the feedstock, two major signals with m/z values of 29 and 17 assigned
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to CO and CH were detected (Figure 4d). Therefore, the generated CO and CH can be
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attributed to the photocatalytic reduction of CO . In addition, for CsPbBr -Ni(tpy), the apparent
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quantum efficiency (AQE) for CO and CH production under 450 nm monochromatic light was
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.23% (Figure S18). The AQE of the system correlates well with the absorption spectra of
CsPbBr . The dependency indicates that CO and CH evolution was induced by photoabsorption
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due to the band-gap excitation of CsPbBr3.
Figure 5. (a) Photoluminescence (PL) quenching of CsPbBr and CsPbBr -Ni(tpy) ( =
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3
80 nm). (b) TRPL of CsPbBr and CsPbBr -Ni(tpy) (the scatter points were fitted using a
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biexponential decay) (ex = 380 nm, em = 515 nm). (c) The TA traces for CsPbBr and
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The CsPbBr becomes an efficient photosensitizer after immobilizing Ni(tpy) primarily
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because of the efficient electron transfer from CsPbBr to Ni(tpy). To clarify the key role of
3
Ni(tpy) in facilitating electron transfer, we analyzed the CsPbBr and CsPbBr -Ni(tpy) samples
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3
using both steady state and time-resolved PL analyses (measurement details are given in the SI).
As shown in Figure 5a, CsPbBr exhibits a strong PL peak centered at approximately 515 nm
3
upon excitation with 380 nm light. After immobilizing the Ni(tpy), the PL intensity of CsPbBr3
is quenched. This PL quenching behavior corresponds to electron transfer from CsPbBr to
3
Ni(tpy). We then used TRPL spectroscopy to probe the electron transfer dynamics, and applied
biexponential fitting to the TRPL curves. As shown in Figure 5b, the PL decay of CsPbBr3
becomes significantly faster in the presence of Ni(tpy), and the average lifetimes of CsPbBr3
were shortened from 47.3 ns to 5.3 ns after the immobilizing Ni(tpy). CsPbBr NCs often show
3
two lifetimes: a long PL lifetime (τ ) that may correlate to the band-edge electron-hole radiative
1
recombination, and a short PL lifetime (τ ) that can be attributed to exciton recombination with
2
the involvement of surface states or electron transfer.(51) According to the fitting results, pristine
CsPbBr possesses a long lifetime (τ ) of 52.5 ns and a short lifetime (τ ) of 9.4 ns with
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2
amplitudes of 57.0% and 43.0%, respectively. In contrast, CsPbBr -Ni(tpy) has two shorter
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lifetimes: a long lifetime (τ ) of 8.2 ns and a short lifetime (τ ) of 1.7 ns with amplitudes of
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0.5% and 79.5%, respectively. These results suggest that the PL decay of CsPbBr mainly
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occurs through band-edge electron-hole radiative recombination. For CsPbBr -Ni(tpy), the
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average lifetime becomes shorter due to a smaller contribution of the long-lived and a larger
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contribution of the short-lived components. Thus, the band-edge radiative recombination is
remarkably suppressed, and the photogenerated electrons in the CB of the CsPbBr are swiftly
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injected into the Ni(tpy). The PL decay results are also in line with the decrease in the PL
emission intensity. Therefore, these results corroborate that the immobilized Ni(tpy) catalysts
can effectively suppress charge recombination and promote charge separation in the CsPbBr3
photosensitizers.
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TA spectroscopy can provide insights into the charge carrier dynamics through the analysis
of the ultrafast excited-state decay processes. Compared with time-resolved PL, TA
spectroscopy has much better time resolution. Here, we use TA spectroscopy to elucidate the
relationship between the electron transfer kinetics and photocatalytic performance. The TA
spectra of CsPbBr and CsPbBr -Ni(tpy) samples were acquired with excitation by a 470 nm
3
3
pump pulse, and probed at 508 nm (see the SI for a detailed description). Figure 5c shows their
absorption change (ΔA) curves as a function of decay times. In the CsPbBr NCs, previous
3
researches have suggested that the exciton bleach feature in the ps-ns scale is a good reflection of
electron trapping/transfer.(52) After the attachment of Ni(tpy), the average lifetime (τ ) decreased
n
from 4337 ps (CsPbBr ) to 65.98 ps (CsPbBr -Ni(tpy)). The change of TA kinetic should be
3
3
mainly result from the electron transfer, considering that CO reduction happens on the Ni(tpy)
2
(53)
site. The corresponding rate constant (k ) of which can be obtained by eq 1
et
ket = 1/τ(CsPbBr
3
-Ni(tpy)) − 1/τ(CsPbBr
3
)
(1)
where τ(CsPbBr
3
-Ni(tpy)) is the lifetime of CsPbBr -Ni(tpy) and τ(CsPbBr corresponds to the lifetime of
3
)
3
1
0
–1
CsPbBr . On the basis of their average lifetimes, the calculated k is 1.49 × 10 s . These
3
et
results confirm that the Ni(tpy) complexes can serve as electron sinks, enabling rapid electron
transfer from the CsPbBr to the catalytic centers, the TA decay results are also in line with the
3
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decrease in the PL decay. Therefore, these results corroborate that the immobilized Ni(tpy)
catalysts can effectively suppress charge recombination and promote charge separation in the
CsPbBr photosensitizers. Overall, the above results suggest efficient charge separation in
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CsPbBr and the delivery of the electrons to the Ni(tpy) complexes for CO reduction. In
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addition, the photocurrent response suggests that their photocurrent is well reproducible upon
on/off cycling of the light source (Figure 5d). Compared with the CsPbBr , a 60% enhancement
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in the photocurrent response was observed for the CsPbBr -Ni(tpy) sample, indicating efficient
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charge separation in the CsPbBr -Ni(tpy), which is consistent with their PL and TA behaviors.
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Overall, the above results suggest an efficient charge separation in CsPbBr and the transfer of
3
the electrons to the Ni(tpy) complexes for CO reduction.
2
CONCLUSIONS
In summary, a CsPbBr -Ni(tpy) hybrid photocatalytic system was successfully fabricated
3
through a facile approach involving the immobilizing of Ni(tpy) complexes onto CsPbBr via
3
electrostatic interactions. Immobilizing the metal complexes onto CsPbBr is crucial for
3
promoting the catalytic activity of CsPbBr for CO reduction as it facilitates electron transfer
3
2
from CsPbBr to the Ni(tpy) catalytic centers. The resulting photocatalyst exhibits a high yield
3
(
1724 μmol/g) of CO and CH , which is approximately 26-fold higher than that of the pristine
4
CsPbBr . This work has developed a method for enhancing the performance of photocatalytic
3
CO reduction by immobilizing metal complexes on perovskite NCs. The methodology we
2
present here provides an important platform for exploring new types of catalysts based on
perovskite NC systems. Further optimization of our system aiming to improving the catalytic
performance can include the coupling of CsPbBr NCs with more efficient catalysts and/or
3
tuning the perovskite material compositions.
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ASSOCIATED CONTENT
Supporting Information. The Supporting Information is available free of charge on the ACS
Publications website at http://pubs.acs.org. See Supporting Information for material synthesis,
characterization methods, and control experiments.
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AUTHOR INFORMATION
Corresponding Authors
*
*
E-mail: wangjin@zjnu.edu.cn.
E-mail: zqli@zjnu.edu.cn.
ORCID
Jin Wang: 0000-0003-4740-4304
Zhengquan Li: 0000-0002-0084-5113
Nikolai Gaponik: 0000-0002-8827-2881
Author Contributions
#Z.C., and Y.H. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work was financially supported by National Natural Science Foundation of China
(
21701143, 21975223), Zhejiang Provincial Natural Science Foundation of China
LGG19B010002), and MEXT KAKENHI grant (Grands 26286013, 17H02736).
(
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