Metal–Organic Layers for Electrocatalysis and Photocatalysis

Abstract

Metal–organic layers (MOLs) are two-dimensional analogues of metal–organic frameworks (MOFs) with a high aspect ratio and thickness down to a monolayer. Active sites on MOLs are more accessible than those on MOFs thanks to the two-dimensional feature of MOLs, which allows easier chemical modification around the catalytic center. MOLs can also be assembled with other functional materials through surface anchoring sites that can facilitate charge/energy transport through the hybrid material. MOLs are thus quite suitable for interfacial catalysis like electrocatalysis and photocatalysis. In this outlook, we focus on representative progress of constructing unique interfacial sites on MOLs with designer paths for charge separation and energy transfer, as well as cooperative cavities for superior substrate adsorption and activation. We also discuss challenges and potentials in the future development of MOL catalysts and catalysts beyond MOLs.

Short abstract

In this outlook, we focus on representative progress of constructing unique interfacial sites on metal−organic layers with designer paths for charge separation and energy transfer, as well as cooperative cavities for superior substrate adsorption and activation.

Two-dimensional (2D) materials have attracted a lot of attention since the development of graphene.¹ Metal–organic layers (MOLs) are 2D versions of metal–organic frameworks (MOFs).² They are also known as coordination nanosheets (CONASHs), metal–organic framework nanosheets (MONs), and two-dimensional supramolecular polymers (2DSPs) in the literature.³⁻⁵ The MOLs discussed in this review are all ultrathin nanosheets of a monolayer or a few layers (less than five layers) in thickness with an aspect ratio over 100. The MOLs inherit features of both MOFs and 2D materials. For example, like other 2D materials, MOLs have active sites completely exposed on the external surface and thus highly accessible, as compared to active sites in the internal channels of MOFs. The MOLs were also much more mechanically flexible than the MOFs and easier to be dispersed in solvents for a range of applications. On the other hand, like MOFs, MOLs also have a regular network structure from alternately connected metal nodes (secondary building units = SBUs) and organic bridging ligands that can be rationally designed, leading to structural diversity and tunability. MOLs have emerged as a promising material for applications in separation,⁶⁻¹⁵ sensing,¹⁶⁻²⁵ biomedicine,²⁶⁻³¹ and catalysis.^5,32−44

MOLs can be prepared via two categories of strategies: top-down exfoliation and bottom-up assembly (Figure 1). The top-down method produces MOLs by selectively breaking interlayer bonding in bulk MOF crystals through external forces such as sonication exfoliation,^9,45−54 solvent-induced exfoliation,^55,56 freeze–thaw exfoliation,¹³ intercalation, or chemical exfoliation.^12,57−60 These top-down methods are easy to scale up but difficult to obtain high quality materials of uniform thickness in many cases. These MOLs are quite suitable for scalable heterogeneous photocatalysis. The bottom-up assembly resembles MOF synthesis that uses a coordination reaction between the metal SBU and bridging ligands to construct networks but constrains its growth in the interlayer direction using either capping agents as modulators,^{27,44,61−65} surfactant-assisted synthesis,⁶⁶⁻⁷² or interfacial preparation.^5,73−80 MOLs produced by the bottom-up method can have regular morphology. Interfacial synthesis usually has issues of scale-up but may still be suitable for electrocatalysis as long as MOLs prepared on the interface can be transferred to electrode surface. Other methods based on modulator/surfactant need to consider the surface modification of prepared MOLs by these molecules, which may present mass transport barrier but may also provide a special microenvironment to tune the catalytical cavity. MOLs prepared by different methods have different unique properties that can be utilized in electrocatalysis and photocatalysis,⁸¹⁻⁸⁴ highlighting the importance of target-oriented synthetic design.

Figure 1.

(Top) Synthetic strategy of MOLs including top-down exfoliation and bottom-up assembly; (bottom) different techniques for structural characterization of MOLs. PXRD: Reprinted with permission from ref (73). Copyright 2010, Springer Nature. STEM: Reprinted with permission from ref (44). Copyright 2016, John Wiley and Sons. SEM: Reprinted with permission from ref (25). Copyright 2020 John Wiley and Sons. STM: Reprinted with permission from ref (87). Copyright 2016, American Chemical Society. AFM: Reprinted with permission from ref (5). Copyright 2015, John Wiley and Sons. EXAFS: Reprinted with permission from ref (62). Copyright 2017, American Chemical Society.

The MOL structure can be characterized by a range of different methods (Figure 1), including electron microscopy (EM), scanning probe microscopy (SPM), powder X-ray diffraction (PXRD), X-ray absorption fine structure spectroscopy (XAFS), and atomic pair distribution function method (PDF). High-resolution EM methods can visualize the network of the SBUs, while SPM methods are quite suitable for obtaining surface properties of nanosheets with a high resolution. Cryoelectron microscopy, which can slow down the electron beam damage of the ultrathin MOLs may help greatly to obtain atomic resolution images of these materials. Ultrathin nanosheets show distinct PXRD patterns that are different from 3D crystals, but the PXRD method usually cannot differentiate between the monolayer and nanosheets less than five layers in thickness. XAFS and PDF can be useful to differentiate between possible SBUs and/or to verify structures of active sites, but their interpretations must be evaluated with caution as usually a range of different structural models can fit in the same spectra.

As previous reviews of MOLs have provided detailed information on their synthesis,^2,82,85,86 this outlook will focus on the progress of MOLs for electrocatalysis and photocatalysis, highlighting the structural basis for tuning their catalytic performance. We will then discuss the current challenges and emerging opportunities in this field.

Constructing Unique Active Sites on MOLs

One major goal of MOL and MOF catalysis is to construct unique active sites that are unreachable from solution chemistry.

MOL and MOF catalysts bridge traditional homogeneous and heterogeneous catalysts but are also different from the two (Figure 2).

Figure 2.

(Top) Illustration of constructing MOLs and building catalytic centers via ligand functionalization and SBU modification; (bottom) unique features of MOLs bridging homogeneous and heterogeneous catalysts.

First, MOLs and MOFs have cavities that can interact with “transition state” species or “intermediates” in catalysis. This microenvironment mimics enzymes in which secondary interactions preconcentrate substrates and facilitate selective conversion. Secondary interaction between the substrate and a MOF cavity is also rich but still challenging to control. Surface modification of the MOL provides a new way to introduce the cooperative entities. As compared to modifying a MOF cavity, MOL surface modification does not have a space restriction and is thus more flexible.

Two examples from our group show the potential for MOL surface modification in photocatalysis and electrocatalysis (Figure 3). We prepared a MOL with a terpyri dine (TPY) site on the ligand: [Hf₆(μ₃-O)₄(μ₃-OH)₄(HCO₂)₆(TPY)₂].³⁶ Postsynthetic functionalization of the TPY by ferrous salt gives a Fe center, which catalyzes photocatalytic aerobic oxidation of tetrahydrofuran (THF) to 2-hydroxy tetrahydrofuran (2-OH-THF, 43%) and butyrolactone (BTL, 57%). The MOL can then be surface-modified by attaching monocarboxylates with hydrophilic/hydrophobic tails on the SBUs, which tune the selectivity toward 2-OH-THF versus BTL. Hydrophilic modification with gluconate (GA) finally leads to 100% selectivity toward the more desired BTL product. This fine-tuning of product selectivity via the changing microenvironment does not require redesign of the active center and can be rationally implemented.

Figure 3.

Surface modification of TPY-MOLs for photocatalysis and electrocatalysis; (right) functionalization of TPY-MOLs with Fe^II and GA for the selective oxidation of tetrahydrofuran; (left) modifying the SBUs of TPY-MOL with CoPP to form a cooperative site for CO₂ activation.

Using the same MOL with TPY site, we integrated cobalt-protoporphyrin (CoPP) onto the MOL by surface modification as the carboxylate group on CoPP can attach to the SBU (Figure 3). The surface-attached Co center cannot coordinate to TPY due to spatial isolation, but the two moieties can form a cooperative site that stabilizes a [TPYH⁺-^–O₂C-CoPP] intermediate in electrochemical CO₂ activation and reduction, leading to a high current density (1314 mA/mg Co, TOF = 0.4 s^–1) for CO production.³⁵ This construction of cooperative sites from two moieties is challenging for homogeneous catalysts. MOLs thus provide an interesting supramolecular platform for building multifunctional sites.

Second, MOLs and MOFs can stabilize active metal centers with low coordination numbers or low numbers of strong coordination.^36,42,44 In the Fe-TPY mentioned above, the Fe center on the MOL coordinates to the terpyridine moiety, while the other three coordination sites on Fe were occupied by labile solvent or counterions. Such a [Fe^II(TPY)]²⁺ moiety is not stable in the homogeneous system and tends to react with each other to form a [Fe^II(TPY)₂]²⁺ complex and free Fe²⁺ ions. This stabilization effect by MOL is manifested in the turnover number (TON) of the aerobic oxidation of THF, which was 820 ± 30 for the Fe-functionalized MOL and was only 45 ± 2 for the [Fe^II(TPY)]²⁺ complex in solution.³⁶

Third, the active sites on MOLs and MOFs have relatively regular and clear structures. Other heterogeneous catalysts like supported metal on metal oxide usually have multiple local structures capable of catalytical conversion. Even homogeneous catalysts in solution can possibly form dimers to complicate their catalytic behavior. Site isolation on MOLs and MOFs stabilizes the single-metal site, making it possible to build catalysts of uniform sites. The structure certainty of organic binding sites as well as the availability of a crystal structure from X-ray diffraction adds having confidence in the structure models of catalytical centers in MOFs and MOLs.

However, this notion of a “clear structure” of MOL and MOF cannot be overinterpreted. Single crystal structure represents average positions of atoms in MOF, while catalysis happens locally on a specific site. For example, defects are often important in MOF catalysis but can hardly be captured by X-ray diffraction of MOFs. In addition, strong peaks on PXRD patterns may only reflect long-range order, while the local structure is more important for catalysis. As a result, partial loss of sharp PXRD peaks of MOFs may not necessarily corresponds to destroying the structure of the local active site; on the other hand, retention of the PXRD pattern does not ensure a stable active site. Moreover, species captured by spectroscopic methods on the MOFs or MOLs were usually the stable species that may not necessarily turnover in catalytic cycle.

Besides, surface modifications of the MOLs were more challenging to characterize than structures in MOFs, as the single-crystal X-ray diffraction method can no longer be applied. Techniques developed for surface science such as SPM, sum-frequency generation (SFG) spectroscopy, and nano-IR can be useful to reveal the surface structure of MOLs, and aberration-corrected TEM has the potential to directly give the atomic image of the active center. These active sites on the MOLs were still much more manageable and tunable than those of many traditional heterogeneous catalysts.

Fourth, compared to active sites in MOFs, these sites on MOLs are more accessible as they were exposed on the external surface. This accessibility is critical for both electrocatalysis and photocatalysis. In electrocatalysis, these active sites can be in direct contact with the electrode, largely removing the electron conductivity requirement of the MOLs. In photocatalysis, as the excited state has a limited lifetime, and a more accessible excited state can greatly facilitate its utilization before decaying and being wasted. The accessibility of the active site also allows the approach of large organic substrates that can be challenging to accommodate in MOF channels. This key advantage of accessibility will be further exemplified in following sections.

Fifth, mechanically flexible and surface functionalized MOLs are suitable for assembling with a variety of other materials to obtain composite catalysts. The active sites can thus reside on the interface for interfacial catalysis. This is especially true for MOLs assembled on electrodes for electrocatalysis and on heterojunctions for efficient charge separation for photocatalysis.^34,88,89

Last but not least, the MOLs were still heterogeneous catalysts so that the active sites on MOLs can be easily isolated from the reaction mixture to shut down the reaction. The MOL catalysts can be recovered and reused, and more importantly they can be removed from the product by filtration to simplify workup of some organic transformations and avoid product contamination by heavy metals of the catalyst.⁴³

Facilitating Electron Transfer of MOLs for Electrocatalysis

Although MOFs were widely investigated as electrocatalysts, in principle, their activities would be hindered by poor electrical conductivity of most MOFs. Chemical diversity of conductive MOFs is still limited.⁹⁰ MOLs have monolayer thickness and can be assembled with a conductive electrode so that the active centers can be in direct contact with the electrode surface. Conductivity of the MOL skeleton is thus not necessary. The mesh structure of the MOL still allows counterions and substrates to contact the electrode and the active sites.

There were MOFs constructed from redox active linkers that can conduct electrons or holes through subsequent redox reactions of the linkers in the framework, a so-called “hopping” mechanism. However, active sites on these MOFs can still be different from active sites on the MOLs in direct contact with the electrode. The latter can reside in the inner Helmholtz layer of the electrical double layer on the electrode and thus feel an electrical field in the catalysis, while the former only accepts conducted oxidation/reduction equivalents to react with the substrate in a regular redox reaction.

MOLs and MOFs have been widely investigated in a variety of electrocatalysis studies such as the hydrogen evolution reaction (HER),^{5,88,89,91−95} oxygen evolution reaction (OER),^{32,54,57,87,96−99} overall water splitting,^33,100 oxygen reduction reaction (ORR),^101−104 carbon dioxide reduction (CO₂RR),^35,105−107 and electrocatalytic transformation of organic molecules.^34,108 Some reported MOLs show comparable or even better performance than benchmark noble metal-based catalysts. There have been several reviews on the electrocatalytic performance of MOLs.^82−84,109 Here we focus on how electron transfer is realized in these catalysts.

MOLs can be assembled with conductive materials. For example, we developed a method to grow MOLs around multiwalled carbon nanotubes (CNTs) with carboxylate on the surface (Figure 4a).³⁴ The resultant composite of conductive CNTs coated with the catalytic MOLs were then loaded onto a macroporous carbon electrode. We attached TEMPO-based active centers onto these MOLs for electrochemical oxidation of primary alcohols and oxidative dehydrogenation of cyclic aromatic amines. Compared to homogeneous solution of TEMPO as electrocatalysts, this catalyst assembly on the electrode provides much higher faradaic efficiency for these organic conversions as the wasted TEMPO shuttling between the working and counter electrode was prohibited for the solid catalyst.

Figure 4.

MOLs assembled on conductive substrates for electrocatalysis. (a) MOL-TEMPO on conductive multiwalled carbon nanotubes (CNTs) for electrochemical oxidation of alcohols. Reprinted with permission from ref (34). Copyright 2018, American Chemical Society. (b) Ni_0.8Fe_0.2(C₁₂H₆O₄)(H₂O)₄ grown on nickel foam for efficient electrocatalytic water splitting. Reprinted with permission from ref (33). Copyright 2017, Springer Nature.

Besides electrosynthesis of organic molecules, the MOL assembly is also beneficial for electrocatalytic activation of small molecules, which usually involves a gas component either as a reactant or a product. For example, ORR and CO₂RR have oxygen and CO₂ in the gas phase, respectively, while HER and OER generate H₂ and O₂. A gas–liquid–solid triple-phase interface is critical in these types of catalysis. Control of the microstructure of the catalyst composite and tuning of hydrophilicity and hydrophobicity can have a huge effect on the overall catalytic performance. A 2D PcCu-O₈–Co composed of copper phthalocyanine and Co²⁺¹⁰⁴ was loaded on CNTs to exhibit excellent electrocatalytic ORR activity (E_1/2 = 0.83 V vs RHE, n = 3.93, and j_L = 5.3 mA·cm^–2) in alkaline media. The mesoporous nest built from the CNTs certainly helped with the activity. It is also worth noting that this catalyst has a clear Co–O₄ center as a catalytic site, as revealed by in situ Raman spectroscopy and density functional theory (DFT) calculations. Similarly, a bimetal NiFe-based MOF nanosheet assembled on nickel foam with hierarchical porosity demonstrates superior electrocatalytic performance toward OER and overall water splitting (Figure 4b).³³

It is also beneficial to use conductive MOL in electrocatalysis. The π-conjugated metal bis(dithiolene) moieties can construct conductive MOLs that are electrocatalytically active.^110,111 Monolayer THTNi (THT = 1,2,5,6,9,10-triphenylenehexathiol) synthesized by the Langmuir–Blodgett method is an active HER catalyst with an overpotential of 333 mV at 10 mA·cm^–2.⁵ The NiS₄ center on THTNi can function as a HER active site in a wide pH range. Further tuning electronic structures by ligand and metal substitution produced other active centers (MS_xN_4-x) with the HER activity following the order of CoS₂N₂ > CoN₄ > CoS₄ > NiS₄,⁸⁹ resulting from different H* adsorption and H₂ desorption energies on these centers.

Some of the MOLs were not stable under electrocatalytic condition and can thus serve as a precursor to prepare the active ultrathin material.^57,112−114 For example, the NiFe-based SURMOF was found to form NiFe-based oxy-hydroxide nanosheets while retaining coordinated ligands BDC-X (BDC = 1,4-benzenedicarboxylic acid, X = NH₂, H, OCH₃, and Br) as the active phase denoted as 1-X. These oxyhydroxides showed superior activity with a low overpotential of 220 mV at 10 mA·cm^–2.¹¹⁴In situ electrochemical transformation method was developed to prepare ultrathin metal oxide or hydroxide nanosheets with excellent electrocatalytic activity. A Fe doped 2D-Co-NS (Fe:2D-Co-NS@Ni) was prepared and used for OER,⁵⁷ exhibiting an ultralow overpotential of 211 mV @ 10 mA·cm^–2. A bismuthene (Biene) of atomic thickness obtained from Bi-MOLs exhibited CO₂RR activity with ∼100% FE for formate.¹¹³

Charge Separation on MOLs for Photocatalysis

MOLs have several key features that are beneficial for photocatalysis. First, as compared to MOFs, MOLs have fewer issues of light scattering, thanks to its ultrathin morphology and dispersibility in solvents.¹¹⁵ Second, the accessibility of active centers on MOLs is critical in photocatalysis as the excited state has a limited lifetime (ns−μs for singlet excited state and μs-ms for triplet excited state) during which the contact with substrate must happen to escape futile energy dissipation.^38,39 Third, efficient energy transfer is critical for a range of photocatalysis, which will be detailed in the next section. Fourth, surface functionalization of a photoactive MOL can bring together two required components for photocatalysis: a chromophore for light harvesting and a catalytic center for chemical conversion.^37,40,41,116 We focus on designing these synergistic entities on MOLs to facilitate electron transfer between the two in this section.^{36−41,60,65,115−123}

A MOL from Lin’s group can serve as a typical example of hierarchically integrating photosensitizers ([Ru-(bpy)₃]²⁺) and active centers (M^I(bpy)(CO)₃X (M = Re, Mn)) in one skeleton for efficient photocatalysis (CO₂RR) (Figure 5).³⁷ The MOL (Hf₁₂-Ru) contains a Hf₁₂(μ₃-O)₈(μ₃-OH)₈(μ₂-OH)₆(TFA)₆ SBU (TFA = trifluoroacetate) and [Ru-(bpy)₃]²⁺-based linear ligand L-Ru. All the capping TFA groups on the SBU can be replaced by M^I(bpy)(CO)₃X moieties with carboxylate linkage on the bpy. The resultant multifunctional MOLs Hf₁₂-Ru-M (M= Re or Mn) exhibit high activity for photocatalytic CO₂ reduction to CO with a TON of 8613 which is about 70 times higher than that of a homogeneous control. The proximity of photosensitizers and catalytic centers in the MOL network facilitates electron transfer that initiates the photocatalysis.

Figure 5.

Schematic showing the synthesis of Hf₁₂-Ru-M (M = Re or Mn) via monocarboxylic acid exchange of Hf₁₂-Ru and sunlight-driven CO₂ reduction. Reprinted with permission from ref (37). Copyright 2018, American Chemical Society.

Energy Transfer on MOLs for Multi-Electron Injection in Photocatalysis

Energy transfer and light harvesting are important for photocatalysis involving multiple electron injections.^124−128 This is due to the diffusive feature of sunlight energy. A typical visible photon density of sunlight is ∼1 photon/nm²/ms, which translates into a maximum photoelectron generation rate of ∼1 ms^–1 on a single molecular photosensitizer. However, HER requires two electrons; OER is a four-electron process; and CO₂RR toward a methane product is an eight-electron process. Unfavorable side reactions may happen during the intervals between the arrived photons when the system was waiting for additional electron/hole injection, diminishing the photocatalytic efficiency. Green plants thus use connected chromophore arrays to form an antenna that captures sunlight energy in a much larger area and efficiently transfers the energy to a single reaction center for catalytic conversion.

MOFs and MOLs are arrays of light-absorbing organic ligands and can mimic light-harvesting systems in nature. Molecular chromophores as the ligands are separated by metal connecting nodes to avoid photoinduced charge transfer and aggregation induced quenching, just as the isolated chlorophyll in the chloroplast. The interchromophore coupling in the weak coupling region is similar to that in nature. Furthermore, the 2D MOLs mimic the 2D network of chlorophyll in the thylakoid membrane, which can efficiently transfer energy to reaction centers attached to the 2D film to drive energy gaining reactions. We performed careful studies on energy transfer and exciton extraction on MOLs and found that, although energy migration on the MOL is less efficient than that in the 3D MOF due to fewer numbers of connected paths in the 2D, the 2D excitons were easier to extract due to their accessibility on the surface (Figure 6a).⁶² Energy transfer at a reasonable rate still happens on the 2D MOL. We attached two photocatalytic centers Re^I(bpy)(CO)₃X and [Ir^III(bpy)Cp*OH]⁺ on a 2D MOL (Zr-TCBPE-MOL, TCBPE = tetrabenzoatetraphenylethylene) that can only transfer energy to the active centers (Figure 6b).¹²⁹ The light-harvesting effect of the MOL enhanced catalytic activity by 6 times for the Re center to produce CO and 110 times for the Ir center to produce HCO₂H as compared to homogeneous controls without energy transfer. We must note that 1e^– transfer photocatalysis that involves radical reaction, in principle, does not benefit from this energy transfer scheme.

Figure 6.

(a) MOLs and MOFs for studying energy transfer and exciton extraction; exciton occupation probabilities at t = 100 ps after injecting one exciton at the center at t = 0 ps. Reprinted with permission ref (62). Copyright 2017, American Chemical Society. (b) Multifunctionalized Zr-TCBPE-MOLs and the energy transfer process followed by the catalytic conversion of CO₂. Reprinted with permission from ref (129). Copyright 2019 The Royal Society of Chemistry.

Outlook

The 2D morphology of MOLs together with regular and tunable structures is promising for electrocatalysis and photocatalysis. Though progress has been made during the past several years, there are still great challenges to overcome. Preparation of uniform MOLs in a reasonable scale is still difficult. The number of existing MOLs is too limited to meet the needs of designing various catalysts. Up to now, compared with more than 88 000 kinds of MOFs, the number of MOLs is no more than one percent of that of MOFs, and many of these MOLs were based on a similar skeleton structure.¹³⁰ It is thus urgent to expand the diversity of these materials. This research demand can be combined with research of robotic combination chemistry and machine learning, which has been proven to be useful in expediting development of related materials.¹³¹

Stability of MOLs is another important aspect to consider. MOLs may undergo structural evolution under catalytic conditions. On the one hand, not only are more in situ spectroscopy and imaging studies needed to reveal the true active sites, but the durability of the catalyst also needs to be further verified through characterization of recovered sample, cycling experiments, and prolonged catalytic reaction. MOLs constructed from M–S and M–N coordination bonds are showing much improved chemical stability than those constructed from M–O bonds. Moreover, covalent organic frameworks (COFs) have seen rapid progress in the past few years, which can be much more stable than MOFs and MOLs. Development of monolayer of COFs is on the way, which can potentially change the landscape of 2D materials.

The high cost of MOLs/MOFs is often regarded as the Achille’s heel for catalysis application. Practical use of a material is determined by the balance between its cost and the benefit it can bring. Targeting catalytic products of higher value can certainly improve the chance of developing a practical MOL catalyst. Even for commodity chemical synthesis, if the turnover frequency of the catalyst is extremely high, the operation can be insensitive to the cost of the catalysts, as already demonstrated by the success of noble metal catalysts in the Monsanto process to produce acetic acid. In situations where activity and/or selectivity are critical, catalysts of high costs can also have merits as exemplified by the expensive three-way catalysts for exhaust treatment on cars. We believe the cost of MOL should NOT be its final obstacle. The real challenge is to develop high performance MOL catalysts to meet the needs of catalytic applications.

Besides practical use, MOLs are also a useful platform for the academic study of catalysis. Designing model catalysts to gain mechanistic insight into complex reactions may lead to some discovery that is transferrable to more practical catalyst systems.

These challenges are also opportunities. MOLs as electrocatalysts and photocatalysts are becoming an exciting research field. We hope the progress of MOLs, combining knowledge of molecular catalysts and heterogeneous catalysts. can bring something new to the field.

The work is supported by funding from the Ministry of Science and Technology of China (2016YFA0200702), the National Natural Science Foundation of China (No. 21671162).

The authors declare no competing financial interest.