Boron Radicals Identified as the Source of the Unexpected Catalysis

Jan 14, 2019 - Atomically thin boron nitride (BN) nanosheets were generally considered to be chemically inert until the recent discovery of the surpri...
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Boron Radicals Identified as the Source of the Unexpected Catalysis by Boron Nitride Nanosheets Zhen Liu, Jingquan Liu, Srikanth Mateti, Chunmei Zhang, Yingxin Zhang, Lifen Chen, Jianmei Wang, Hongbin Wang, Egan H. Doeven, Paul S Francis, Colin J. Barrow, Aijun Du, Ying Chen, and Wenrong Yang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06978 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Boron Radicals Identified as the Source of the Unexpected Catalysis by Boron Nitride Nanosheets Zhen Liu, † Jingquan Liu, # Srikanth Mateti, $ Chunmei Zhang,

&

Yingxin Zhang, § Lifen Chen, †

Jianmei Wang, † Hongbin Wang, † Egan H. Doeven, † Paul S. Francis, † Colin J. Barrow, † Aijun Du, & Ying Chen,*, $ and Wenrong Yang*,† †

School of Life and Environmental Sciences, Deakin University, Geelong, VIC 3216, Australia

#

College of Materials Science and Engineering, Qingdao University, Qingdao, 266071, China

$

Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia

&

School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Brisbane, QLD 4001, Australia §

School of Material and Chemical Engineering, Ningbo University of Technology, Ningbo,

315000, China *Correspondence

and

requests

for

materials

should

be

addressed

to

W.Y.

([email protected]) or Y.C. ([email protected]) ABSTRACT: Atomically thin boron nitride (BN) nanosheets were generally considered to be chemically inert until the recent discovery of the surprising catalysis. However, the origin of this unusual catalytic activity remains unclear. We have observed the free boron radicals at the edges and defective sites of boron nitride nanosheets, and demonstrated with both experimental and theoretical approaches that the boron radicals in the nanosheets can catalyze the chromogenic

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reaction of 3,5,3′,5′-tetramethylbenzidine, and serve as a source of reactive radicals for the coreactant electrogenerated chemiluminescence of tris(2,2′-bipyridine)ruthenium(2+). These findings suggest BN nanosheets as a type of non-metal catalysts. KEYWORDS: boron nitride nanosheet • free radical • catalysis • electrochemiluminescence •spin trapping Atomically thin boron nitride nanosheets (BNNSs) are an important two-dimensional (2D) nanomaterial with superior mechanical and thermal conducting properties.1-4 However, investigation of the chemical reactivity of BNNSs has been limited by their chemical stability and poor solubility in water.5-8 Boron nitride (BN) nanomaterials have been deemed to be chemically inert, but recent reports indicated that they are able to be functionalized9-12 and can catalyze some reactions.13-16 For example, BN nanotubes were functionalized by polymer chains via surface initiated atom transfer radical polymerization.13 BN nanoplatelets were used as solid radical initiators for the aerobic oxidation of thiophenol to diphenyldisulfide.14 Porous BN was found to be catalytically active in acetylene hydrochlorination due to the ability of polarizing and activating acetylene at its armchair edges.15 More recently, selective oxidative dehydrogenation of propane to propene has been reported, which showed active sites on hexagonal BN (h-BN) as a catalyst.17 In this reaction, the high selectivity was attributed to the stabilization of the propyl radicals by the nitroxyl radical site. 2D BN edges avoided the creation of a highly reactive propyl radical, 18 and the over-oxidation of the adsorbed species was prohibited.19 However, the precise origin of the catalysis in BN nanomaterials remains unclear.20 Despite these promising reports, there is a lack of understanding of the catalytic activity of BNNSs. Herein, we performed a series of experiments and carried out a number of crucial controls to detect the radicals on BNNSs using electronic spin resonance (ESR) spectroscopy.21 BNNSs were 2 ACS Paragon Plus Environment

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found to be able to quench the free radicals of 2,2-diphenyl-1-picrylhydrazyl (DPPH, Scheme 1), and the radical concentrations of BNNSs were enhanced by narrowing the lateral dimensions of BNNSs. Density functional theory (DFT) calculations predicted that unsaturated boron atoms at the edges or defects of BNNSs might be the site of the free radicals. We then evaluated the catalytic activity of the radicals of BNNSs with the chromogenic reaction of 3,5,3′,5′-tetramethylbenzidine (TMB, Scheme 1).

Scheme 1. A radical scavenging method for the detection of boron radicals on BNNSs, and the catalytic activity of BNNSs for the oxidation of TMB. RESULTS The radicals on the as-prepared BN nanomaterials were investigated using ESR spectroscopy. ESR is a useful technique to study the unpaired electrons of materials.22 Every electron has a magnetic moment and spin quantum number, with magnetic components.23 The magnetic moment aligns itself into two energy levels under an external magnetic field. An unpaired electron moves between the two energy levels by either absorbing or emitting a photon of energy to meet the resonance condition.24 In an ESR measurement, a series of paramagnetic centers (e.g., free radicals) is treated 3 ACS Paragon Plus Environment

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with microwaves at a fixed frequency. By increasing the magnetic field, the gap between the two energy states is widened until it matches the energy of the microwaves.25 Because more electrons exist in the lower state, according to the Maxwell–Boltzmann distribution, there is a net absorption of energy, which is detected and converted into a spectrum.26 The line width of ESR spectra indicates the dynamics of atoms. Furthermore, the amplitude of the ESR signal is proportional to the number of the unpaired electrons present in the sample, enabling quantification of the radicals.27,28 Together with spin trapping and radical scavenging activity, different free radicals can be accurately probed by ESR spectroscopy. It is generally hypothesized that the radicals were boron or nitrogen radicals at the edges or defects of BN due to the cleavage of B-N bonds.29-31 We first examined the bulk h-BN, BNNSs terminated by hydroxyl groups (BNNS-OH), BNNSs terminated by amino groups (BNNS-NH2) and BN nanoparticles terminated by hydroxyl groups (BNNP-OH) in powders (Figure S4). In all the spectra, we only observed a weak ESR peak for BNNP-OH. The defective spherical structure and small dimensions could be the reason for the detection of radicals on BNNP-OH. There was a very low ratio from edge to surface on BNNSs, thus the cleavage of B-N bonds, which happened mostly at the edges, was greatly inhibited. On the other hand, the ESR spectra of BNNSs dispersed in solutions were also examined using a radical scavenging method which detected the decrease in ESR peaks of DPPH due to the formation of covalent bonds between radicals. The covalent binding between nitrogen radicals of DPPH and boron radicals of BNNSs was first investigated by DFT.32 Single nitrogen vacancy (VN) was created by removing single N atom from a 10 × 10 h-BN monolayer as shown in Figure 1a. The magnetic moment was calculated to be 1 µB during optimization due to the unpaired electrons in B atoms, which was in good agreement with a previous study.33 DPPH is a free radical with magnetic moment of 1 µB. However, the

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DPPH/VN system became non-magnetic because a chemical bond was formed between N from DPPH and the B atom (position 1) around VN (Figure 1b). The adsorption energy for DPPH/VN system was calculated to be -3.32 eV based on Eq. 1 (Supporting Information), indicating a strong interaction between DPPH molecules and the B-atoms on the 2D VN nanosheets.34 For BN nanoribbon with a bare B-atom as shown in Figure 1c, the magnetic moment was also calculated to be 1 µB. After the adsorption of DPPH radical, N atom from the DPPH molecule could form a chemical bond with B atoms at the edges (Figure 1d), leading to a nonmagnetic system. The binding energy was calculated to be as high as -3.64 eV.

Figure 1. DFT study of the covalent bonding of DPPH and BNNS. (a) Optimized geometry for a 10 × 10 supercell h-BN sheet with one VN, leading to B-terminated nanohole; (b) DPPH adsorbed on the 2D VN h-BN nanosheet; (c) Optimized geometry for hydrogen passivated BN edge with one unsaturated B-atom; (d) DPPH adsorbed B-terminated BN edge. Brown, blue, red, white, and orange atoms represent C, N, O, H, and B atoms, respectively. The functional groups of BNs were characterized by FTIR (Figure S1) and energy-dispersive Xray spectroscopy (Table S1). The morphology of prepared BN nanomaterials was displayed in 5 ACS Paragon Plus Environment

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Figure 2 and Figure S2. By sonication and solvothermal reaction, the lateral dimensions of BN were narrowed from microns to nanometers (Figure 2a-c and Figure S3). The ESR spectrum of DPPH in Figure 2d exhibited five sets of symmetric peaks between 3430 and 3500 gauss. BNNSOH was a weak quencher of DPPH radicals while BNNS-OH after sonication in water (BNNSOH(s)) strongly deactivated the DPPH. BN quantum dots (BNQD-1) thoroughly scavenged DPPH peaks (bottom curve), indicating a high concentration of radicals. First, exfoliating BNNS-OH into monolayers significantly improved the surface area for the reaction with DPPH. Second, cleavage of B-N bonds generated more structural defects to accommodate the free boron radicals.

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Figure 2. Morphologies and ESR spectra of BN nanomaterials. (a) SEM image of BNNS-OH; AFM images of (b) BNNS-OH(s) and (c) BNQD-1; (d) ESR spectra of DPPH (0.5 mM) by adding different BN nanomaterials (0.1 mg mL-1). Similarly, the increase in radical concentrations by narrowing BN dimensions was observed with the other BN nanomaterials in both water and DMF (Figure 3). Radicals on BNNS-OH (red in Figure 3a) and BNNS-NH2 (red in Figure 3b) were barely detected, which was consistent with the results obtained with powder samples. With a similar morphology, BNNS-OH(s) (green in Figure 3a) showed a similar capacity to scavenge DPPH radicals with BNNP-OH(s) (green in Figure 3c). The radical concentration of BNNS-NH2(s) (green in Figure 3b) was a little weaker than the other two due to its relative larger lateral dimension. Radicals of BNQDs were measured by mixing DPPH in ethanol and BNQDs in DMF, and were found to be more concentrated than others (blue curves). To exclude the influence of solvent, both DPPH and BNs were dispersed in DMF, and similar results were achieved (Figure 3d-f). To be monolayer or few-layer, the surface area of BNNS is determined by controlling lateral dimension and creating porous structure during bond cleavage. Therefore, we think the cleavage of B-N bonds is the driving force for the formation of active radicals. As a result, we concluded that the radicals on BNNSs are significantly improved when narrowing lateral dimensions of BNNSs, while the functional groups contribute to the dispersity but rarely affect the radical numbers. Table 1 summarized the concentrations of free radicals on different BNs. Taking BNNP-OH in water as an example, the intensity difference between the two central peaks was 8.1 which can be read from Figure 3c. So the remaining and scavenging radical concentrations of DPPH were calculated to be 0.27 mM and 0.23 mM (Figure S5). Since the applied concentration of BNNP-OH was 0.1 mg mL-1, the radical concentrations on BNNP-OH was calculated to be 2.3 mmol g-1. The concentrations of radicals on the other BNs

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were calculated by the same method. To make it less complicated, henceforward, we selected BNNS-OH(s) as a model of exfoliated BNNSs in the following discussion because the morphology and radical concentrations of BNNSs were very similar. BNQD-1 was selected as a model of BNQDs.

Figure 3. ESR spectra of DPPH (0.5 mM) by adding BN nanomaterials (0.1 mg mL-1) with different dimensions in water (a-c) or DMF (d-f).

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Table 1. Summary of the calculated results of radical concentration on BNs. BN category

Radical concentrations (mmol g-1) In water/ethanol

In DMF

BNNS-OH

0.23

1.43

BNNS-OH(s)

3.67

3.01

BNQD-1

> 5.0

> 5.0

BNNS-NH2

1.16

1.52

BNNS-NH2(s)

3.40

3.64

BNQD-1

> 5.0

> 5.0

BNNP-OH

2.30

3.31

BNNP-OH(s)

3.39

4.03

BNQD-3

> 5.0

> 5.0

To confirm that the boron radicals are the origin of the active sites on BNNSs, we then utilized UV-Vis spectroscopy to study the covalent binding between DPPH and BNs. Due to the strong absorption band at 520 nm, DPPH showed a deep violet color. This color turned to pale yellow when the radicals were neutralized (Figure 4a), which enabled visual monitoring of the reaction.21 In the UV-Vis spectra (Figure 4b), the peak (520 nm) of DPPH blue shifted and weakened after BNNS-OH(s) binding (green curve), and a much stronger peak shift was observed by BNQD-1 binding (blue curve), indicating the covalent interactions between BNs and DPPH. Fourier transform infrared spectroscopy (FTIR) also gave important information (Figure 4c). N-O bending (880 cm-1) and C-H stretching (2853 and 2924 cm-1) vibrations of DPPH were not affected by BNNS-OH(s), indicating that DPPH did not react with BNNS-OH(s) via its conjugated structure 9 ACS Paragon Plus Environment

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or nitro groups. And B-N stretching peak (1377 cm-1, black curve) experienced a redshift to 1405 cm-1 due to a covalent bond generated on BN skeleton. Based on bond ionicity considerations, protons were supposed to deactivate N radicals as soon as their formation in aqueous solution.35,36 Previous reports indicated oxygen functionality, defect and dimensional structure of carbonaceous materials are responsible for their active sites.37,38 We have concluded the decisive effect of lateral dimensions above. To rule out the potential effect on O and N atoms, we carried out experiments to determine whether radical concentrations of BNNSs were affected by their edge functional groups (Figure 3). The results show there is little difference of boron radical concentrations in both OH and NH2 terminated BNNSs. Based on these observations, we propose the functional groups (OH and NH2) are not dehydrogenated and not able to produce active radical sites in BNNSs. Therefore, we conclude that there are no O or N radicals on the surface of BNNSs. These findings confirm that the boron radicals are the origin of the active sites on BNNSs.39

Figure 4. Investigation of the covalent bonding between DPPH and BN nanomaterials. Photograph (a) and characterizations of the covalent bonds between DPPH and BNs by UV-Vis absorption (b) and FTIR (c).

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Having active radicals on their surface, BNNSs could be suitable to catalyze reactions in which highly aggressive chemical species are formed as intermediates. Based on this hypothesis, we tested the activity of BNNS-OH(s) to promote catalytic decomposition of H2O2 to generate hydroxyl radicals (•OH), which is commonly catalyzed by enzymes or metal ions.40,41 Thereafter, the chromogenic reaction of TMB by •OH resulted in a color change from colorless to blue by converting amino groups to imino groups. Here we showed that BNNS-OH(s) promoted the oxidation of TMB due to the decomposition of H2O2. Without BNNS-OH(s), TMB reacted slowly with H2O2 and resulted in a slight change of the color (Figure 5a). With both BNNS-OH(s) and H2O2, there was a marked color change and an increase in the absorption at 650 nm. The color of the solution unexpectedly became yellow, which was due to the formation of a stable state of TMB with a carbonyl group on each benzene ring.40 Figure 5b presents the proposed mechanism of catalysis using BNNS-OH(s) for TMB oxidation. The boron radicals on BNNS-OH(s) converted H2O2 to •OH, resulting in the oxidation of TMB.42,43 The decomposition of H2O2, which was catalyzed by BNNS-OH(s), was investigated ESR spectra using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a •OH trapping agent (Figure 5c). As shown in the ESR spectra, the addition of BNNS-OH(s) into DMPO+H2O2 markedly improved the 1:2:2:1 peaks which were very similar to those of DMPO-OH by the classical Fenton reaction. This indicated that BNNS-OH(s) was highly efficient in promoting H2O2 decomposition due to the presence of boron radicals. Figure 5d shows the kinetic study of TMB oxidation, in which the increase in absorption at 650 nm was measured over time. In comparison, the catalytic activities of bulk h-BN, BNNS-OH, BNNP-OH and BNNSNH2 were studied by the same method. It was found these powders made minimal contributions to the catalysis of TMB oxidization (Figure S6). This was consistent with the ESR measurements, which indicated there were no radicals in these samples, and thus they were not catalytically active.

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Furthermore, this reaction was also conducted at the absence of H2O2 (Figure S7). BNNS-OH(s) were able to oxidize TMB, which showed a blue color and a peak at 650 nm in the UV-Vis spectrum. We concluded that some hydroxyl radicals were released from BNNS-OH(s) due to instability of B-OH bonds. A small amount of iron may be an impurity in the prepared BN nanomaterials, but the influence of catalytic activity can be ignored.44-46 Grant et al.17 reported the catalytic activity of h-BN for the oxidative dehydrogenation of propane to propene. The authors proposed that an oxygen-terminated armchair edge of BN (>B– O–O–N