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* ABSTRACT
* MAIN
* CONCLUSION
* EXPERIMENTAL DETAILS
* SUPPLEMENTARY MATERIAL
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ELECTRONIC AND CATALYTIC ENGINEERING IN TWO-DIMENSIONAL VDW METAL–ORGANIC
FRAMEWORKS THROUGH ALLOYING
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Free Submitted: 23 March 2021 Accepted: 02 August 2021 Published Online: 18
August 2021
* ELECTRONIC AND CATALYTIC ENGINEERING IN TWO-DIMENSIONAL VDW METAL–ORGANIC
FRAMEWORKS THROUGH ALLOYING
*
Applied Physics Reviews 8, 031411 (2021); https://doi.org/10.1063/5.0051219
Yuxia Shen, Bohan Shan, Christopher Muhich, Srishti Gupta, Han Li, Patrick Hays,
Ying Qin, Shiljashree Vijay, Joseph Winarta, Bin Mu, and Sefaattin Tongaya)
more...View Affiliations
* School for Engineering of Matter Transport and Energy, Arizona State
University, Tempe, Arizona 85287, USA
* a)Author to whom correspondence should be addressed: sefaattin.tongay@asu.edu
View Contributors
* Yuxia Shen
* Bohan Shan
* Christopher Muhich
* Srishti Gupta
* Han Li
* Patrick Hays
* Ying Qin
* Shiljashree Vijay
* Joseph Winarta
* Bin Mu
* Sefaattin Tongay
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ABSTRACT
Bimetallic metal-organic framework (MOFs) alloys, in which heterogeneous metal
clusters are incorporated into their backbone, are capable of highly selective
separations and catalysis. Due to limitations in our fundamental understanding
of their alloying, however, established methods result in phase-separated or
amorphous two-dimensional (2D) MOFs or lack precise control over alloy ratios.
Here, our results demonstrate 2D MOF alloys where metal cation ratios (M1 and
M2) in M1xM21-xBDC (M1 or M2= Zn, Cu, Ni, Co, Fe, Mn) can be engineered on
demand by controlling the metal salt dissociation constants. Resulting MOF
alloys exhibit a highly 2D nature with excellent crystallinity and minute
control over metal cation ratios. Our experimental and theoretical results show
that their electronic bandgaps and photoexcited carrier lifetimes can be
engineered by metal cation alloying. Interestingly, 2D alloyed MOFs enable
high-efficiency photo-catalytic water reduction performance in Co/Ni MOF alloys
owing to the spatially separated metal clusters in 2D MOF alloys.
MAIN
Section:
ChooseTop of pageABSTRACTMAIN <<CONCLUSIONEXPERIMENTAL DETAILSSUPPLEMENTARY
MATERIALAUTHORS' CONTRIBUTIONSChoose
Metal–organic frameworks (MOFs) fabricated from a wide array of metal cations
offer unique properties and functionalities.1–31. S. Yang, X. Lin, W. Lewis, M.
Suyetin, E. Bichoutskaia, J. E. Parker, C. C. Tang, D. R. Allan, P. J.
Rizkallah, P. Hubberstey, N. R. Champness, K. M. Thomas, A. J. Blake, and M.
Schröder, Nat. Mater. 11, 710 (2012). https://doi.org/10.1038/nmat33432. J. A.
Mason, J. Oktawiec, M. K. Taylor, M. R. Hudson, J. Rodriguez, J. E. Bachman, M.
I. Gonzalez, A. Cervellino, A. Guagliardi, C. M. Brown, P. L. Llewellyn, N.
Masciocchi, and J. R. Long, Nature 527, 357 (2015).
https://doi.org/10.1038/nature157323. L. J. Wang, H. Deng, H. Furukawa, F.
Gándara, K. E. Cordova, D. Peri, and O. M. Yaghi, Inorg. Chem. 53(12), 5881–5883
(2014). https://doi.org/10.1021/ic500434a For example, transition metals
possessing unfilled d-orbitals generate coordinatively unsaturated or open metal
sites, which are particularly selective in recognizing small molecules44. H.
Furukawa, K. E. Cordova, M. O'Keeffe, and O. M. Yaghi, Science 341(6149),
1230444 (2013). https://doi.org/10.1126/science.1230444 and facilitating carrier
transfer in heterogeneous catalysis.55. S. Zhao, Y. Wang, J. Dong, C.-T. He, H.
Yin, P. An, K. Zhao, X. Zhang, C. Gao, L. Zhang, J. Lv, J. Wang, J. Zhang, A. M.
Khattak, N. A. Khan, Z. Wei, J. Zhang, S. Liu, H. Zhao, and Z. Tang, Nat. Energy
1, 16184 (2016). https://doi.org/10.1038/nenergy.2016.184 Especially when these
three-dimensional (3D) traditional MOFs are synthesized in two-dimensional (2D)
form, they offer added functionalities and sensitivity owing to their much
enhanced surface-to-volume ratios.6–86. Q. Jiang, P. Xiong, J. Liu, Z. Xie, Q.
Wang, X.-Q. Yang, E. Hu, Y. Cao, J. Sun, Y. Xu, and L. Chen, Angew. Chem. Int.
Ed. 59(13), 5273–5277 (2020). https://doi.org/10.1002/anie.2019143957. C. Li, X.
Hu, W. Tong, W. Yan, X. Lou, M. Shen, and B. Hu, ACS Appl. Mater. Interfaces
9(35), 29829–29838 (2017). https://doi.org/10.1021/acsami.7b093638. M. Zhao, Q.
Lu, Q. Ma, and H. Zhang, Small Methods 1(1–2), 1600030 (2017).
https://doi.org/10.1002/smtd.201600030 In these 2D MOFs, metal clusters and
organic linkers are constructed by coordination bonding only in the 2D
landscape, while the adjacent layers are bonded via hydrogen bonds or weak van
der Waals (vdW) force. Considering the vital role played by the judicious choice
of metal cations, MOFs with two different metal types (bimetallic MOFs) enable
remarkable improvement in properties analogous to traditional alloying, as long
as phase separation can be prevented and alloy ratios can be engineered on
demand.
Previously, metal cation alloying in bulk MOFs has only been demonstrated a few
times wherein the ratio of the metal cations (composition) was not engineered
but set at a given value. By incorporating secondary metal cation, it was shown
it is possible to enhance nitrogen sorption (bimetallic HKUST-1)99. M. A.
Gotthardt, R. Schoch, S. Wolf, M. Bauer, and W. Kleist, Dalton Trans. 44(5),
2052–2056 (2015). https://doi.org/10.1039/C4DT02491E or improve the performance
of Li-O2 batteries in bimetallic Mn/Co in MOF-74).1010. S. H. Kim, Y. J. Lee, D.
H. Kim, and Y. J. Lee, ACS Appl. Mater. Interfaces 10(1), 660–667 (2018).
https://doi.org/10.1021/acsami.7b15499 Similar alloying approaches can, in
principle, open a myriad of possibilities, but there exist great challenges to
fabricate 2D MOF alloys, since the steric configuration and coordination numbers
of metal cations ultimately determine if the 2D structure can be retained within
a stable phase and elemental composition.
Here, we present a systematic approach to synthesize 2D MOF bimetallic alloys
with any combination of 3d transition metal cations from half-filled Mn (3d5) to
fully occupied Zn (3d10) (Fig. 1) with a controllable amount of alloy ratios.
The two different metal ions, “M1” and “M2,” are linked with benzene-1,
4-dicarboxylic acid (BDC); therefore, the synthesized 2D MOF alloys are denoted
as “M1xM21-xBDC” (M1 or M2= Zn, Cu, Ni, Co, Fe, Mn; x value ranging from 0 to 1)
[Fig. 1(a)]. The microscopy88. M. Zhao, Q. Lu, Q. Ma, and H. Zhang, Small
Methods 1(1–2), 1600030 (2017). https://doi.org/10.1002/smtd.201600030 and
spectroscopy1111. Y. Shen, B. Shan, H. Cai, Y. Qin, A. Agarwal, D. B. Trivedi,
B. Chen, L. Liu, H. Zhuang, B. Mu, and S. Tongay, Adv. Mater. 30(52), 1802497
(2018). https://doi.org/10.1002/adma.201802497 studies (Figs. S1 and S2) reveal
these synthesized 2D M1xM21-xBDC layers are highly crystalline and retain their
2D nature at desired alloying ratios. Systematic studies are presented to
explain the growth mechanism and dynamics. Additionally, we examine the
photocatalytic behavior of our 2D MOF alloys, which revealed that
high-efficiency photocatalytic water splitting can be attained. Overall, these
studies introduce the world's first 2D MOFs alloys involving different metal
cations and establish how their physical properties change to achieve
high-efficiency catalytic devices.
FIG. 1. Schematic structure of 2D vdW MOF alloys (M1xM21-xBDC) and involved
metal types in this article. (a) The basic building units in M1xM21-xBDC; (b)
involved metal types in partial periodic elemental table; (c) top view of
obtained M1xM21-xBDC with randomly arranged M1 and M2 clusters.
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FIG. 2. The featured competitive growth in 2D MOF alloys. (a) The relative
competition between M1 and M2, M1 or M2= Ni, Co, Cu, and Zn. Calculated odds
ratio of M1/M2 in alloys over M1/M2 in precursors are displayed as one diagonal
line and curves with denoted values. Besides, the colored competitive spectrum
based on log odds ratio values indicates the superiority degree between M1/M2;
(b) typical AFM height profile of Ni0.52Co0.48BDC few layers demonstrate the vdW
nature; (c) versatility of the proposed synthesis strategy to produce 2D MOF
alloys by substituting M1/M2 = Zn/Cu, Zn/Ni, Cu/Ni, and Cu/Co. SEM and EDS
mapping images suggest their lamellar morphology and uniform composition.
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Competitive growth in 2D MOF alloys
As mentioned in Main, the synthesis of uniform 2D MOF alloys is not achievable
by arbitrary mixing metal ions. Through our systematic investigation of a
M1xM21-xBDC prototype, we observe a unique competitive growth phenomenon that
reveals the formation of uniform 2D MOF crystals with adjustable alloy ratio at
large scales.
Before we discuss our successful route to generate fully alloyed 2D MOFs using
Co-Ni as the testbed material system, we describe our initial attempts to
synthesize the MOF alloys using a conventional approach. The existing approach
uses BDC, pyridine, and metal salts with identical anions, i.e., Ni and Co
chloride (NiCl2/CoCl2= 1/1), as precursors. In our studies, we have dissolved
NiCl2 and CoCl2 in DMF to obtain a uniform metal cation solution. After adding
pyridine into this metal solution, it was mixed with ligand solution dropwise.
Typical products from similar growth runs have resulted in a mixture of NiBDC
(green) and Co0.21Ni0.79BDC alloy (purple) crystals as shown in Fig. S3(a),
while the unreacted Co or Ni cations remained in the solution without
participating in the growth process. Careful EDS mapping of Ni-Kα and Co-Kα has
also shown the presence of Co0.21Ni0.79BDC along with NiBDC [Fig. S3(b)]. Even
though the NiCl2 and CoCl2 precursors were used at a 1:1 ratio, the presence of
Ni-rich Co0.21Ni0.79BDC alloy and NiBDC implies that the reaction constant ([M+
cation]/[M-O coordination species]) for NiCl2 is far greater than that of CoCl2.
More specifically, Ni ions are more reactive with the BDC ligand than Co during
the NixCo1-xBDC formation, and only a fraction of Co cations in the solution
coordinate into the alloyed structure. These trends can be observed across the
entire Co/(Co+Ni) precursor range used in our growth process, as depicted by
solid red dots in Fig. 2(a). Here it is noteworthy to mention that only
precursor solutions with over 80% Co cations produced a CoNiBDC alloy structure,
and even then the final Co content of the MOF was only 52%. Precursor salt
mixtures below 80% CoCl2 always produced separated mixtures consisting of NiBDC
and low Co content alloys.
FIG. 3. Optical properties of M1xM21-xBDC alloys. (a) UV-Vis absorbance
spectroscopy spectra of CoxNi1-xBDC (x = 0, 0.34, 0.52, 0.66, 0.78, and 1)
crystals; (b) DFT calculated absorbance spectra of NiBDC, Co0.5Ni0.5BDC, and
CoBDC; (c) and (d) extracted optical bandgap values of CoxNi1-xBDC and 2D MOF
alloys with other metal pairs; (e) projected density of states (DOS) of
Co0.5Ni0.5BDC.33. L. J. Wang, H. Deng, H. Furukawa, F. Gándara, K. E. Cordova,
D. Peri, and O. M. Yaghi, Inorg. Chem. 53(12), 5881–5883 (2014).
https://doi.org/10.1021/ic500434a
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The growth dynamics of 2D MOF alloys is characterized by the different reaction
constants between the two metal cations. Here we use odds ratio
OR = [M1/M2(Alloy)]/[M1/M2(Precursor) as an indicator to illustrate the relative
reactivity of M1 over M2 cation. Systematic results involving various metal
pairs are present in Fig. 2(a). The data in the orange region indicates M1 has a
lower reaction constant than that of M2, while the purple region is the
opposite. In the case that M1 and M2 have comparable reactivities, their OR
values fall along the solid diagonal line.
We find that competing growth occurs in other metal pairs as well. Starting from
their chloride precursors (MCl2, M= Zn, Cu, Co, and Ni), the relative
competitions between these metal cations are demonstrated in Fig. 2(a). Starting
with a 1:1 ratio of bimetal precursors, the relative reactivity of Ni, Co, and
Cu are all higher than that of Zn. We presume that electronegativity is a key
parameter; however, other effects, such as d-orbital configuration and
coordination number, should also be considered. Our typical SEM and EDS mapping
results [Fig. 2(c)] demonstrate the classic 2D morphology and uniform alloying
of these alloys, including Ni0.78Co0.22BDC, Ni0.92Zn0.08BDC, Cu0.50Co0.50BDC,
and Cu0.89Zn0.11BDC.
How can one achieve better control over alloy ratios using a molar mixture of
metal cation precursors and prevent the formation of NiBDC or CoBDC? Since NiCl2
precursors are more reactive than CoCl2, we have substituted the Ni chloride
precursor with the less soluble Ni acetylacetonate salt (acac = CH3-O-O-CH3).
This, in principle, provides fewer reactive Ni precursor ions at any given time,
enabling similar Co and Ni coordination rates with the BDC molecules. This
should result in a MOF alloy where the metal ratios are dictated by the initial
precursor quantities. These results are shown by yellow dots in Fig. 2(a). It
can be clearly seen that the resulting Co/(Co+Ni) alloy ratios closely follow
the ratio of Co to Ni precursor used, thereby enabling us to synthesize uniform
NixCo1-xBDC alloys with continuous alloy ratios (x = 0.10, 0.20, 0.30, 0.41,
0.57, 0.71, 0.80). It is noteworthy that even when the fraction of the Co
precursor is low, using Ni(acac)2, with its small dissociation constant, leads
to predictable alloy ratios. However, we note that when the CoCl2 precursor
fraction is high (80% and above), the alloy does deviate from linearity while
still remaining capable of achieving a high Co concentration of 2D MOF alloys.
Overall, the combination of CoCl2 and Ni(acac)2, which have similar
disassociation constants, produces a single phase, meaning only 2D MOF alloys,
without NiBDC or CoBDC segments. Additionally, the contribution of pyridine on
the growth of crystalline 2D CoNiBDC alloys is indispensable. A detailed
discussion about the roles of pyridine is involved in supplementary material and
Fig. S4.
Tunable band gaps and electron transfer between hetero clusters
Similar to alloying in traditional or 2D material systems,12–1412. K. Wu, M.
Blei, B. Chen, L. Liu, H. Cai, C. Brayfield, D. Wright, H. Zhuang, and S.
Tongay, Adv. Mater. 32(17), 2000018 (2020).
https://doi.org/10.1002/adma.20200001813. B. Huang, M. Yoon, B. G. Sumpter,
S.-H. Wei, and F. Liu, Phys. Rev. Lett. 115(12), 126806 (2015).
https://doi.org/10.1103/PhysRevLett.115.12680614. J. Kang, S. Tongay, J. B. Li,
and J. Q. Wu, J. Appl. Phys. 113(14), 143703 (2013).
https://doi.org/10.1063/1.4799126 alloying 2D MOFs enables one to engineer the
physical properties of the synthesized systems such as the bandgap. Here, we
have performed UV-Vis absorption spectroscopy measurements [Fig. 3(a)] on
CoxNi1-xBDC (x = 0, 0.12, 0.31, 0.72, and 1) and performed comprehensive
theoretical studies to understand how increasing Co concentration influences
their behavior [Fig. 3(b) and Fig. S5]. Parent NiBDC (x = 0) exhibited two main
absorption peaks located at 1.84 eV and 3.05 eV, while CoBDC (x = 1) has two
predominant transitions around 2.27 eV and 3.83 eV. We find that as the Co
concentration (x) increases, the absorbance spectra of CoxNi1-xBDC gradually
shifts from NiBDC to CoBDC like features, instead of a simple addition of two
pure MOFs. This implies that the concentrations of Co and Ni cations and their
interaction are playing key roles in the electronic band structure of
CoxNi1-xBDC. The interaction of Co and Ni cations through BDC linker can also be
evidenced by our x-ray photoelectron spectroscopy (XPS) measurements (Fig. S6).
For example, in Co0.52Ni0.48BDC, Ni 2p3/2 peak shifted from 854.7 eV (NiBDC) to
856.1 eV (alloy) compared to NiBDC, while the binding energy of Co 2p3/2 varied
in alloy by 500 meV compared to CoBDC.
Furthermore, the Tauc plots of CoxNi1-xBDC (x= 0, 0.12, 0.31, 0.72, and 1) have
been analyzed to investigate their bandgap evolution, as shown in Fig. 3(c) and
Fig. S7. NiBDC (x = 0) exhibits an optical bandgap at 3.75 eV, and increasing Co
concentration (x > 0) leads to a continuous reduction in its bandgap decreasing
by 450 ∼550 meV until it reaches 3.11 eV for CoBDC or x = 1. It can be seen from
Eg vs Co fraction (x) curves that the correlation is not linear, meaning
increasing x does not result in a linear change in the bandgap. Instead, a small
amount of Co incorporation into NiBDC host matrix causes large changes (small x
limit). The behavior is referred to as to negative band bowing and implies that
the band structure is mainly dictated by contributions from the Co orbitals,
which will be discussed in the section Theoretical understanding of alloying in
2D MOFs. Here, we also show other achieved bandgap parameters for other 2D MOF
alloys by designing metal pairs for the first time, as shown in Fig. 3(d).
Mn0.89Zn0.11BDC has the smallest bandgap (3.42 eV), while Ni0.47Zn0.53BDC holds
the largest bang gap (4.01 eV) among all these types of alloys. These results
confirmed the key role of metal types in the determination of MOF bandgap, which
was only theoretically predicted 3,15,163. L. J. Wang, H. Deng, H. Furukawa, F.
Gándara, K. E. Cordova, D. Peri, and O. M. Yaghi, Inorg. Chem. 53(12), 5881–5883
(2014). https://doi.org/10.1021/ic500434a15. J. Castells-Gil, N. M. Padial, N.
Almora-Barrios, J. Albero, A. R. Ruiz-Salvador, J. González-Platas, H. García,
and C. Martí-Gastaldo, Angew. Chem. Int. Ed. 57(28), 8453–8457 (2018).
https://doi.org/10.1002/anie.20180208916. M. A. Syzgantseva, C. P. Ireland, F.
M. Ebrahim, B. Smit, and O. A. Syzgantseva, J. Am. Chem. Soc. 141(15), 6271–6278
(2019). https://doi.org/10.1021/jacs.8b13667 before.
Theoretical understanding of alloying in 2D MOFs
To understand the electronic behavior of the 2D MOF systems, we performed
density functional theory (DFT) analysis of the single metal and 50/50% Co/Ni
(Co0.5Ni0.5BDC) alloyed materials. All calculations were completed in
VASP17,1817. G. Kresse and J. Furthmüller, Phys. Rev. B 54(16), 11169 (1996).
https://doi.org/10.1103/PhysRevB.54.1116918. G. Kresse and J. Furthmüller,
Comput. Mater. Sci. 6(1), 15–50 (1996).
https://doi.org/10.1016/0927-0256(96)00008-0 using the HSE061919. J. Heyd, G. E.
Scuseria, and M. Ernzerhof, J. Chem. Phys. 124(21), 219906 (2006).
https://doi.org/10.1063/1.2204597 density functional, with 10% exact exchange.
The wavefunction was constructed by a summation of plane waves with energies up
to 1000 eV, and the Brillion Zone was sampled on a 4 × 4 × 1 Γ-point centered
Monkhorst-Pack grid. The alloy was modeled by a supercell consisting of
1 × 2 × 1 primitive cells with two Ni or two Co atoms paired. We attempted to
construct the alloy with mixed metal centers, but this was unstable. The
wavefunction of NiCoBDC was sampled on a 4 × 2 × 1 k-point grid.
The NiBDC and CoBDC band edges are composed of metal d states, with additional
mixing of the organic states below and above the valence and conduction band
edges, respectively, as shown in Fig. S5. Additionally, Co and Ni d states form
a mid-gap state 1.28 and 1.25 eV above the valence band maximum. These states
give rise to a band edge-to-band edge gap of 2.83 eV and 3.04 eV and a
mid-gap-state-to-band edge gap of 1.37 and 1.87 eV for Co and Ni, respectively.
While these band gaps are substantially lower than the experimentally measured
bandgap, these are not the photoactive states. The calculated absorption spectra
[Fig. 3(b)], calculated using a frequency-dependent complex dielectric function,
shows adsorption maxima at 3.74 and 4.30 eV for Co and Ni, respectively.
Applying Tauc plot analysis of the calculated adsorption spectrum results in an
“apparent” bandgap of 3.25 and 4.04 eV, which matches well with the experimental
values of 3.1 and 3.75 eV.
The band edge-to-band edge absorption between 1.5 and 2.25 eV is very weak for
NiBDC, as is seen by the lack of adsorption in the calculated spectra at the
true bandgap, but more significant for CoBDC. We attribute the poor adsorption
in Ni to a limited coupling between the states near the band edges because of
their similar Ni d-states e2g symmetries. Although the metal transition in CoBDC
(Cod→ Cod) is insufficient as well, the slight mixing of Cod with chelating N
and C states from the capping agent (noted as “Cod→ Cap” near the orange
spectrum) leads to an increment in the absorption.
The major absorption peaks are located at ∼2.98 and 4.30 eV for NiBDC and 3.75
and 4.76 for CoBDC. Both result from electron excitation from the metal d state
to the benzene π states of the linker molecules and the N and C states of the
capping agents. Upon excitation, electrons are expected to quickly relax from
the benzene states through the continuum of states to the metal d-states at the
conduction band edge. The poor coupling between the band edge states suggests
that electrons and holes at those states should have long lifetimes, and thus be
excellent for driving chemical reactions.
The Co0.5Ni0.5BDC alloy retains many of the distinctive electronic
characteristics of both NiBDC and CoBDC, rather than a blending of behavior. As
seen in the density of states (DOS) plot and the adsorption spectra, the peaks
associated with Ni and Co centers from the pure phases are retained though
slightly shifted, which parallels the experimental findings. We attribute this
to the fact that each metal dimer is composed of either Co or Ni and the metal
centers are relatively far from each other. The slight changes to the adsorption
energies from the un-alloyed material are attributed to altered lattice
parameters and small redistribution of electron densities around each metal
center due to the electronic communication. We do note that the major adsorption
peak in NiCoBDC is a combination of metal d–benzene π transitions and Co to Ni
transition. The latter is unique to the mixed Co0.5Ni0.5BDC alloy because of the
presence of two metal centers.
The Co0.5Ni0.5BDC alloy is expected to demonstrate a unique density of states
[Fig. 3(e)] from its parent materials because of the alignment of its bands. The
highest occupied band is mostly Co in character, while the lowest unoccupied
band is Ni in character. This results in the lowest energy significant
excitation arising from Co d-states to benzene π-states and Co to Ni transition,
though we expect the former to be more strongly coupled. The lower energy of the
Ni conduction band states as compared to the Co states results in the excited
electron relaxing to and localizing on Ni centers. Similarly, the photogenerated
hole localizes on the Co centers, from where it originated, leaving the electron
and the whole physically separated. This further increases quasi-particle
lifetime as compared to the unalloyed MOF by physically separating the excited
electrons and holes, thus decreasing the transition dipole moment, and the
recombination probability.
Photocatalytic water splitting
The presence of two different, spatially separated metal clusters and the
calculation results suggests that photo-excited polarons are rapidly split into
electrons and holes. This behavior hints at the possibility of long
photo-excited carrier lifetimes and potentially high catalytic performance. We
have carried out systematic photocatalytic water splitting studies on several
samples from Co0.52Ni0.48BDC and compared our results to those from parent CoBDC
and NiBDC sheets to elucidate the role of two metal cation alloys and their
catalytic performance. In a typical measurement, the working electrode made of
MOF catalyst film was subject to photons at select energies from 325 nm to
625 nm. These MOF catalyst films were deposited onto the conductive side of ITO
glass [Fig. 4(a)], and the thickness of the catalyst film is determined by SEM
images [Fig. 4(c)]. Next, the photo-generated electrons pass through an external
circuit and are recorded by a potentiostat setup [Fig. 4(b)]. Finally, the
photocatalysis performance is evaluated by the photocurrent density as well as
the incident photon to current conversion efficiency (IPCE).
FIG. 4. The photocatalytic performance of Co0.52Ni0.48BDC, CoBDC, and NiBDC. (a)
Schematic illustration of the deposition of MOF film; (b) photo of
photocatalytic measurement setup; (c) SEM morphology of working electrode with
measured thickness; (d) photon-generated current density after white light
irradiation for 30 s; (e) incident photon to current conversion efficiency
(IPCE) as a function of incident photon wavelength.
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The comparison of photocurrent densities of Co0.52Ni0.48BDC, CoBDC, and NiBDC is
presented in Fig. 4(d). Under the white light irradiation, the photon-generated
current density with a maximum value in Co0.52Ni0.48BDC is 1.94 μA/cm2. CoBDC
and NiBDC only hold 1.0 and 0.17 μA/cm2, respectively. Similarly, the improved
photocatalytic behavior of Co0.52Ni0.48BDC is verified in the comparison of IPCE
plots [Fig. 4(e)]. The maximum value of IPCE (IPCE max) of the alloy is 9.3%
with the incident energy of 3.45 eV. And the IPCE value retains higher than 5%
within the incident light wavelength of 450 nm. This is more than twice the IPCE
max of CoBDC and NiBDC, 4.1% and 3.0%, respectively. Here, we note that the
photo-generated current of the Co/Ni-BDC sample decays slightly faster with time
than that of CoBDC, as shown in Fig. 4(d). We attribute this to slightly reduced
crystalline quality of CoNiBDC samples compared with CoBDC arising from
increased defects/crystallinity mediated by Ni incorporation (with faster
reaction rates).
The enhanced photon to current conversion in the bimetallic alloys is attributed
to the electron–hole separation caused by the presence of two dissimilar metal
centers, which localizes the holes and electrons on two distinct and physically
separated sites. As described above, the DFT electronic structure shows that the
photoexcited electrons and hole of Co0.52Ni0.48BDC localize on the physically
separated Co/Ni metal centers [Fig. 3(e)], and that these states are only weakly
coupled, in contrast to the single metal MOFs, which show stronger coupling
[Fig. 3(b)]; thus, the charge carriers are expected to separate and remain
separated. The theoretical charge carrier separation of Co0.52Ni0.48BDC
overcomes the well-known photocatalytic limitation of electron–hole pair
recombination.20–2220. Q. Li, B. Guo, J. Yu, J. Ran, B. Zhang, H. Yan, and J. R.
Gong, J. Am. Chem. Soc. 133(28), 10878–10884 (2011).
https://doi.org/10.1021/ja202545421. W. Wang, X. Xu, W. Zhou, and Z. Shao, Adv.
Sci. 4(4), 1600371 (2017). https://doi.org/10.1002/advs.20160037122. C. Chu, Q.
Zhu, Z. Pan, S. Gupta, D. Huang, Y. Du, S. Weon, Y. Wu, C. Muhich, E. Stavitski,
K. Domen, and J.-H. Kim, Proc. Natl. Acad. Sci. USA 117(12), 6376–6382 (2020).
https://doi.org/10.1073/pnas.1913403117
In order to confirm our conclusion of photocatalytic activity, we have conducted
electrochemical impedance spectroscopy (EIS). The EIS Nyquist plot represents a
measure of the rate of the reaction on the photocatalyst, and, thus, a smaller
radius reflects an effective separation of charge carriers.23–2523. S. Li, K.
Ji, M. Zhang, C. He, J. Wang, and Z. Li, Nanoscale 12(17), 9533–9540 (2020).
https://doi.org/10.1039/D0NR01696A24. W. Zhan, L. Sun, and X. Han, Nano-Micro
Lett. 11(1), 1 (2019). https://doi.org/10.1007/s40820-018-0235-z25. A. Murali,
P. K. Sarswat, and H. Y. Sohn, Mater. Today Chem. 11, 60–68 (2019).
https://doi.org/10.1016/j.mtchem.2018.10.007 The EIS Nyquist plots of NiBDC,
CoBDC, and Co0.52Ni0.48BDC and the equivalent circuit are shown in Fig. S9.
Among these three catalysts, bimetallic CoNiBDC displays the smallest
semicircle, which indicates the highest photocatalytic activity.2626. J. Ângelo,
P. Magalhães, L. Andrade, and A. Mendes, Appl. Surf. Sci. 387, 183–189 (2016).
https://doi.org/10.1016/j.apsusc.2016.06.101 Based on the equivalent circuit,
such high photocatalytic performance is ascribed to the lower charge transfer
resistance and space charge capacitance, which allows efficient charge
separation of electron/hole pairs. By contrast, 2D MOFs with mono-type metal
cluster have more difficulty reaching the semiconductor/electrolyte interface
where the photocatalytic reaction takes place, and more electrons have the
possibility of recombining before reacting.
This conclusion is further supported by the electrochemical impedance
spectroscopy (EIS) results [Fig. 4(d)], in which HP-CdS exhibits the smallest
radius, revealing the lowest charge-transfer resistance.
CONCLUSION
Section:
ChooseTop of pageABSTRACTMAINCONCLUSION <<EXPERIMENTAL DETAILSSUPPLEMENTARY
MATERIALAUTHORS' CONTRIBUTIONSChoose
We have demonstrated a scalable synthesis of 2D vdW MOF alloys with tunable
metal type and ratio. The presented methodology is universal for obtaining 2D
M1M2BDC (M1 or M2= Zn, Cu, Ni, Co, Fe, and Mn) layers with vdW nature and great
crystallinity. Overall results highlight the importance of metal cation
precursor selection to ensure similar reaction rates to achieve 2D MOF alloys
with well-engineered alloy composition and avoid the formation of binary phases.
Using experimental and theoretical simulations, for the first time, our results
show that alloying in 2D MOFs allows for engineering their physical behavior and
offer insight into the mechanism behind the bandgap engineering. Enhanced
catalytic performance on MOF alloy Co0.52Ni0.48BDC, in comparison to CoBDC and
NiBDC, suggests the extended electron/hole lifetime by spatial separation in
alloys. Overall, our findings are anticipated to open new avenues of fine
control over the synthesis of 2D MOF alloys and to offer fundamental insights
for the potential photocatalytic applications of bimetallic 2D MOF alloys.
EXPERIMENTAL DETAILS
Section:
ChooseTop of pageABSTRACTMAINCONCLUSIONEXPERIMENTAL DETAILS <<SUPPLEMENTARY
MATERIALAUTHORS' CONTRIBUTIONSChoose
Growth procedures
Zn(NO3)2 6H2O, Cu(NO3)2 3H2O, NiCl2 6H2O, Ni(acac)2, CoCl2 6H2O, MnCl2 4H2O,
FeCl3, benzene-1,4-dicarboxylic acid (H2BDC), pyridine, and acetone were
purchased from Sigma-Aldrich without further purification. N,
N-Dimethylformamide (DMF) was purchased from Fisher Scientific.
CoCl2 6H2O and NiCl2 6H2O were mixed at a certain molar ratio with a total
1.33 mmol. The mixed metal salts and pyridine (0.18 ml) were dissolved in 10 ml
DMF to form the first precursor solution. In the second container, 2 mmol H2BDC
was dissolved in 5 ml DMF. Both solutions were then bath-sonicated for 2 min.
The first precursor solution was added into the second container dropwise. The
mixture solution was transferred into a hydrothermal reactor, which is sealed in
an autoclave with a Teflon interior. The reaction was carried out in an isotherm
oven at 120 °C for 24 h. After the reaction stops, crystals were scooped out and
cleaned by washing using fresh DMF and acetone for three times, respectively. At
last, collected samples were dried in vacuum oven at 100 °C for 10 h.
Other MOF alloys were synthesized via the similar procedure, except for
replacing NiCl2 6H2O and CoCl2 6H2O with other metal precursors accordingly.
Characterization
The imaging of 2D MOF alloys was performed using a Philips XL30 environmental
scanning electron microscope (SEM) at a voltage of 10 kV with the assistance of
Au sputter coating. The Raman spectroscopy measurements were taken using a
Renishaw InVia Raman microscope under 100 objective lens with 488 nm laser at
37.5 μW as the excitation source. AFM topography was conducted in the contact
mode, in which the scan size was 256 × 256, and the scanning speed was set to
1 Hz. Data were analyzed by Gwyddion software. The powder XRD of samples was
collected using a PANalytical Aeris powder x-ray diffractometer with Cu Kα
radiation (λ = 1.542 Å) with a scan step of 0.02°. Optical properties were
measured on a UV-Vis-Lambda 950 spectrometer with an incident photon wavelength
from 200 to 800 nm. M1xM21-xBDC samples were collected between two pieces of
clean sapphire to form a sandwich configuration. The whole part was stabilized
with adhesive tape around the outside of the sapphires. The optical properties
of blank sapphires were pre-measured as the baseline.
Theoretical simulations
All quantum mechanical calculations were conducted using periodic boundy
condition as implemented in the Vienna Ab initio Simulation Package
(VASP)17,1817. G. Kresse and J. Furthmüller, Phys. Rev. B 54(16), 11169 (1996).
https://doi.org/10.1103/PhysRevB.54.1116918. G. Kresse and J. Furthmüller,
Comput. Mater. Sci. 6(1), 15–50 (1996).
https://doi.org/10.1016/0927-0256(96)00008-0 using the HSE061919. J. Heyd, G. E.
Scuseria, and M. Ernzerhof, J. Chem. Phys. 124(21), 219906 (2006).
https://doi.org/10.1063/1.2204597 density functional, with 10% exact exchange.
The wavefunction was constructed by a summation of planewaves with energies up
to 1000 eV, and the Brillion Zone was sampled on a 4 × 4 × 1 Γ-point centered
Monkhorst-Pack grid. The wavefunction of NiCoBDC was sampled on a
4 × 2 × 1 k-point grid. Projector augmented wavefunctions (PAW) were used to
reduce the computational cost. All structures were fully relaxed.
Frequency-dependent complex dielectric function was calculated as outlined by M.
Fox.2727. M. Fox, “ Optical Properties of Solids,” American Association of
Physics Teachers ( Oxford University Press, 2002).
SUPPLEMENTARY MATERIAL
Section:
ChooseTop of pageABSTRACTMAINCONCLUSIONEXPERIMENTAL DETAILSSUPPLEMENTARY
MATERIAL <<AUTHORS' CONTRIBUTIONSChoose
See the supplementary material for more information that supports the findings
of this study, including morphology and structure of 2D MOF alloys, phase
separation, roles of capping agent, DFT calculation, optical properties, and
electrochemical impedance spectroscopy.
AUTHORS' CONTRIBUTIONS
Section:
ChooseTop of pageABSTRACTMAINCONCLUSIONEXPERIMENTAL DETAILSSUPPLEMENTARY
MATERIALAUTHORS' CONTRIBUTIONS <<Choose
Y.S. and B.S. conducted all the synthesis of MOF crystals and structural
analyses. H.L., Y.Q., S.V., and J.W. performed optical characterization and EDS
elemental mapping. Y.S. conducted the rest of the measurements. C.M. and S.G
provided DFT calculation and its analyses. Y.S., C.M., P.H., B.M., and S.T.
wrote the manuscript. S.T. supervised all the research. All authors reviewed the
manuscript prior to its submission.
ACKNOWLEDGMENTS
S.T. acknowledges support from Grant No. DOE-SC0020653, NSF DMR Grant No.
1552220, DMR Grant Nos. 1904716 and 1955889, and NSF CMMI Grant Nos. 1825594 and
1933214. S.T. also acknowledges support from Applied Materials Inc. C.L.M. and
S.G. acknowledge support from the National Science Foundation Nanosystems
Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT;
Grant No. ERC-1449500). Calculations were conducted in part on the Extreme
Science and Engineering Discovery Environment (XSEDE), supported by NSF (Grant
No. ACI-1548562), through the Bridges high-performance computer at the
Pittsburgh Supercomputing Center (allocation ECD190001). S.T also acknowledges
NSF ECCS 2052527 and DMR 2111812 for spectroscopy and electrical
characterizations.
DATA AVAILABILITY
The data that support the findings of this study are available within the
article and its supplementary material.
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