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Degradation Chemistry and Stabilization of Exfoliated Few-Layer Black Phosphorus in Water Taiming Zhang, Yangyang Wan, Huanyu Xie, Yang Mu, Pingwu Du, Dong Wang, Xiaojun Wu, Hengxing Ji, and Lijun Wan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02156 • Publication Date (Web): 25 Mar 2018 Downloaded from http://pubs.acs.org on March 25, 2018
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Degradation Chemistry and Stabilization of Exfoliated FewLayer Black Phosphorus in Water Taiming Zhang1‡, Yangyang Wan1‡, Huanyu Xie1, Yang Mu2, Pingwu Du1, Dong Wang3, Xiaojun Wu1,4*, Hengxing Ji1* and Lijun Wan1,3 1
Department of Materials Science and Engineering, CAS Key Laboratory of Materials for Energy Conversion, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), and CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230026, China.
2
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China.
3
Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences and Beijing National Laboratory for Molecular Sciences, Beijing 100190, China 4
Synergetic Innovation of Quantum Information & Quantum Technology, University of Science and Technology of China, Hefei, Anhui 230026, China KEYWORDS: black phosphorus, stability, degradation chemistry.
ABSTRACT: Exfoliated black phosphorus (BP), as a monolayer or few-layer material, has attracted tremendous attentions owing to its unique physical properties for applications ranging from optoelectronics to photocatalytic hydrogen production. Approaching intrinsic properties has been, however, challenged by chemical reactions and structure degradation of BP in ambient conditions. Surface passivation by capping agents has been proposed to extend the processing time window, yet contamination or structure damage rise challenges for BP applications. Here, we report experiments combined with first-principle calculations that address the degradation chemistry of BP. Our results show that BP reacts with oxygen in water even without light illumination. The reaction follows a pseudo first order parallel reaction kinet333ics, produces PO2 , PO3 , and PO4 with reaction rate constants of 0.019, 0.034, and 0.023 per day, respectively, and occurs preferentially from the P atoms locating at BP edges, which yields structural decay from the nanoflake edges in water. In addition, a negligible decay ratio (0.9 ± 0.3 mol%) and preserved photocatalytic activity of BP are observed after storing in deoxygenated water for 15 days without surface passivation under ambient light. Our results reveal the chemistry of BP degradation and provide a practical approach for exfoliation, delivery and application of BP.
INTRODUCTION Few-layer orthorhombic black phosphorous (BP) can be isolated by mechanical exfoliation owing to its anisotropic lamellar structure and weak interlayer interactions, and has a thickness-dependent bandgap that spans from 0.3 to 2 eV, 2 -1 -1 high hole mobility of 1,000 cm V s , and highly anisotropic 1 electrical, thermal, and optical properties. The unique structure and outstanding physical properties make BP attractive for high-performance electronic and optoelectronic devices
2,3
applications. The recent success in preparing few-layer BP suspension by liquid phase exfoliation, especially in water (a 4 safe and environmentally benign solvent), triggers the appli5,6 cations of BP for optothermal therapy, electrochemical 7,8 oxygen evolution reaction, and photocatalytic hydrogen 9 generation. These research progresses open exciting possibilities with few-layer BP in past literature, however, BP demonstrates fast oxidation and degradation in ambient 10 conditions, which is the major hurdle for BP in both processing and application. A deeper understanding of the degradation chemistry is essential towards stabilized BP and the establishment of promised applications. Some efforts have been devoted to explore the underneath mechanism of BP degradation. It is believed that illumination, oxygen, and water are simultaneously required for pho11,12 toactivated oxidation of BP in ambient environment. However, other work shows that deoxygenated water oxidizes BP at the defects and plane edges with the assistance of 13 illumination. In addition, some literatures imply that oxygen is the main reason and water does not play a primary 14,15 role. Therefore, the degradation mechanism of BP in ambient condition is still unclear. Nevertheless, based on the proposed degradation mechanisms, surface passivation procedures have been developed to protect the BP against water 16 and oxygen such as covalent functionalization, surface co17 10-12 ordination and coating. The accompanied contamination and damage from the capping agent rise challenges for BP application. Therefore, an efficient route to keep BP stable without capping agent is demanded. Here we discuss the results of a joint experimental and theoretical study on few-layer BP aqueous suspension with the goal of comprehensively addressing the degradation mechanism of BP. We analyzed the degradation products of 33BP in oxygenated water, and found that PO2 , PO3 , and 3PO4 ions are the major final products. By monitoring the concentrations of these BP oxidation products in water and
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using first-principle calculations, we showed that BP reacts with oxygen in oxygenated water and follows a pseudo first order parallel reaction kinetics with reaction rate constants -1 333of 0.019, 0.034, and 0.023 day for PO2 , PO3 , and PO4 , respectively. The BP structure decays, as a result of oxidation, preferentially from the edge-site P atoms as monitored by insitu atomic force microscopy (AFM). When stored in deoxygenated water with ambient illumination, BP, without any surface passivation, is chemically stable with a negligible decay (mole fraction of ≤ 1.0 mol%) and preserved photocatalytic activity for at least 15 days. EXPERIMENTAL SECTION Preparation of bulk BP and few-layer BP. Bulk BP was prepared through a low-pressure transport route on the basis 18,19 of literature. 500 mg of red phosphorus, 20 mg of Sn, and 10 mg of SnI4 were sealed in an evacuated ampoule bottle. The sealed ampoule was transferred to a tube furnace and heated at 930 K for 3 h. Then the temperature was dropped to 830 K in 15 h followed by a natural cooling process to yield dark bulk BP. After purification, the product was stored in glove box for further analysis. Few-layer BP was prepared by liquid exfoliation. The obtained bulk BP was ground and dispersed in deoxygenated DI water followed by shearmixing exfoliation (Motor-L5M-A, Silverson Machines Ltd., UK.) for 6 h at a shear speed of 10000 rpm. Then, the BP suspension was centrifuged at 6000 rpm for 30 min, and the supernatant was collected for further study. Characterizations. The structure of BP was characterized by XRD (D/max-TTR III with Cu Kα radiation of λ = 1.54178 Å operating at 40 kV and 200 mA). The absorbance was measured using a UV-vis absorption spectrometer (UV-3600, Shimadzu). Ultrasonic treatment was performed on the samples before UV-vis measurements to minimize the effect of flake-restacking on the measurement. SEM was performed on JSM-2100F, JEOL Ltd, and Raman spectra were acquired with Renishaw inVia with 532 nm laser excitation. In-situ AFM was performed with Demension Icon, Bruke. For this study, the samples were prepared by drop-casting few-layer BP suspension on SiO2/Si (300 nm) substrate and subsequent vacuum drying. The in-situ AFM images were acquired in oxygenated water at room temperature. TEM was performed on JEM-ARM200F at an accelerating voltage of 200 kV. AEC was performed on ICS-1100 (Thermo). An IonPac AS19 RFIC analytical column (4 × 250 mm) was used with an IonPac AG19 as Guard Column (4 × 50 mm). The column temperao ture was 30 C. The experiments were carriered with 30 mM KOH as eluent. The concentration values calculated from the UV-vis and AEC profiles are the average number of three samples. The deviations of the concentration values stands for the difference between samples. The photocatalytic H2 evolution measurement. The photocatalytic reaction was carried out in a pyrex reaction cell connected to a closed gas circulation and evacuation system (Labsolar–IIIAG, Perfectlight). The light source was a 300 W Xe lamp equipped with a UV cut-off filter (λ > 420 nm). Na2S and Na2SO3 was added to the few-layer BP aqueous suspension as the electron donor, and the mixed solution was processed with ultrasonic-processing until dissolved under N2 atmosphere. Subsequently, the mixed solution was transferred to the pyrex reation cell then the reactor was connected to system. Before irradiation, the system was vacuumed to
remove oxygen. Hydrogen gas from sample (constantly magnetic stirring) was measured by gas chromatography (GC7806, argon as a carrier gas) using a thermal conductivity detector (TCD). For each evaluation of hydrogen generation, 1.5 ml of the quantitative loop was injected into the GC and was quantified by a calibration plot to the internal CH4 standard. Computational Methods. All first-principles calculations were performed based on density functional theory imple20,21 mented in Vienna ab initio simulation package (VASP). Exchange-correlation functional of generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) 22 was used. To study the edge state of BP, various 1D phosphorene nanoribbons were constructed and relaxed until the -1 residual forces were below 0.03 eV Å with fine k-point samplings. To simulate a ribbon, a unit cell with periodic boundary condition was used. A vacuum space of at least 15 Å was included in the unit cell to minimize the interaction between the system and its replicas resulting from the periodic boundary condition. The plane-wave cutoff energy of 500 eV was used and the convergence threshold for self-consistent -5 iteration is 10 eV. The climbing-image nudged elastic band (CI-NEB) method was employed to search the minimum energy paths (MEP) and locate the transition states during the process of water molecule dissociated adsorption on the restructurable zigzag and diff-zigzag edge of BP, respective23 ly. RESULTS AND DISCUSSION Previous studies on BP degradation mechanisms at ambient conditions focus on the structure evolution of few-layer BP by using AFM and transmission electron microscopy (TEM), yet the BP oxidation chemistry is still ambiguous, for instance the reaction kinetics and final chemical product of the oxidation reaction, leading to debates on the key factors 3that responsible for the degradation of few-layer BP. POx ions, a mixture of P of different oxidation states, are suggested to be the final products of few-layer BP degradation in 5,10,11 3water. Identifying the chemical composition of POx and tracking their concentrations will provide a chemical vision in understanding the degradation chemistry. Anionexchange chromatography (AEC) separates anions based on their affinity to the ion exchanger, which has been used to separate and quantify anion species with concentration of < 1 24 ppm in water, providing a reliable method to study the oxidation products of BP in water. Plus, the intensity of the UV-vis absorption of a 2D materials suspension in liquid is sensitive to the solid concentration, which has been used to measure the mass concentration of BP in a variety of sol25,26 vents. Therefore, both the reactant (few-layer BP flakes) 3and oxidation product (POx ) could be studied at a meantime by UV-vis and AEC, respectively, to provide a comprehensive study on the degradation mechanism of BP. Few-layer BP was prepared by vapor phase transfer method followed by shear-mixing exfoliation in deoxygenated water, and the mass concentration of BP in water was measured by UV-vis spectroscopy (See EXPERIMENTAL SECTION, Figure S1 and Figure S2 for details). Raman spectrum (Figure 1a) and TEM analysis (Figure 1b) indicate that the BP structure was preserved after shear-mixing exfoliation in water. The BP suspension in deoxygenated water (inset of Figure 1c and Figure S1d) contains nanoflakes with average
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Figure 1. BP aqueous suspension. (a) Raman spectrum and TEM image of BP nanoflake. (b) TEM image and selected-area electron diffraction pattern showing the crystal structure of BP. (c) Size distribution of BP nanoflakes in water measured by dynamic light scattering; the inset shows a 10 ml vial containing BP in water. (d) AFM image and thickness distribution of BP nanoflakes used in this study. (e) AEC profiles and (f) UV-vis spectra of BP suspension in oxygenated water stored in air under ambient light. (g) The increase of oxidized P (∆[ox-P]) at different storage conditions. (h) Normalized UV-vis absorption intensities measured at wavelength of 460 nm for BP aqueous suspensions at different storage conditions.
lateral size of ∼300 nm (Figure 1c) and average thickness of ∼1.8 nm (Figure 1d) which were measured by dynamic light scattering and AFM, respectively. The concentration of the -1 prepared few-layer BP suspension was 864 ± 3 µmol L (27.9 -1 ± 0.1 mg L BP) as measured by UV-vis absorption spectroscopy (See Supporting Information for details). To study the degradation process of BP in water we tracked the changes of the BP suspensions at room temperature that were stored at four different conditions: i) kept in air (oxygenated water) under ambient light; ii) kept in air (oxygenated water) and dark; iii) protected by N2 (deoxygenated water) under ambient light; iv) protected by N2 (deoxygenated water) in dark. Three samples were studied at each storage condition to calculate the mean absolute deviation of the experimentally measured values. AEC analysis was performed on these samples for every 3 days, and the chromatography spectra of the BP suspension kept in air under ambient light (condition (i)) are presented in Figure 1e. In contrast to the flat line of DI water (Figure S3), Figure 1e shows three intensive peaks with retention times of 3.5, 5.7, and 11.7 minutes, which can be assigned to 333PO2 , PO3 , and PO4 , respectively (Figure S4). The intensities of the three peaks increased with storage time, indicating 3the generation of POx from the oxidation of BP (Figure 1e). A minor peak at a retention time of 4.6 minutes, of which the intensity changed with storage time irregularly, has been identified as Cl from water (Figure S3). The intensities of the peaks locating at retention times of 6.0, 6.9, and 13.6 minutes were almost unchanged for 15 days, which can be assigned to
the impurities introduced when preparing BP suspension. At the meantime, the suspensions used for chromatography study were subjected to UV-vis analysis to measure the con17,27 The decreased UV-vis absorpcentration of few-layer BP. tion with storage time (Figure 1f) indicates the degradation of 27 few-layer BP according to previous study, which is in ac3cordance with the increased concentrations of POx ions (Figure 1e). Therefore, the combined chromatography and UV-vis studies indicate the degradation of BP in oxygenated water when stored without light illumination. And we found 3three chemical species with retention times of 3.5 (PO2 ), 5.7 33(PO3 ), and 11.7 (PO4 ) minutes that can be assigned to the BP degradation products. The concentration of the oxidized P ([ox-P]) can be calcu333lated as a sum of concentrations of PO2 , PO3 , and PO4 333([ox-P] = [PO2 ] + [PO3 ] + [PO4 ]) from the AEC profiles. The [ox-P]0 of the freshly prepared BP suspension was 227 ± 5 -1 µmol L partly due to the oxidation of BP during the shearmixing and centrifugation process. To study the degradation rate of BP in water, we plotted the increase of oxidized P (∆[ox-P] = [ox-P]t - [ox-P]0) with storage time. The ∆[ox-P] was zero at the initial state, and increased to 123 ± 8 and 102 ± -1 9 µmol L after keeping in oxygenated water under ambient light and in dark for 15 days, respectively, corresponding to the mole fractions of degraded BP of 14.2 ± 0.9 mol% and 11.8 ± 1.0 mol% (Figure 1g). When the few-layer BP was kept in deoxygenated water for 15 days, the ∆[ox-P] increased to 8 ± 3
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Figure 2. The minimum energy pathways (MEP) for H2O and O2 dissociation over the surface and edges of black phosphorene are obtained by using DFT calculations. (a) to (d) display the MEP for the dissociation of H2O molecule on surface, two reconstructed zigzag types, and armchair type edges of BP. The calculated energy barriers range from 1.54 to 2.59 eV. (e) and (f) display the calculated MEP of O2 dissociation on the basal plane and reconstructed zigzag edge of BP with the energy barrier of 0.77 and 0.31 eV, respectively. The purple, red, and green balls represent phosphorous, oxygen, and hydrogen atoms, respectively. The inset maps are the regional configurations of initial state (IS), transition state (TS) and final state (FS) in these reactions, and the monolithic structures are shown in Figure S6. -1
(under ambient light) and 7 ± 6 µmol L (in dark), corresponding to the mole fractions of degraded BP of 0.9 ± 0.3 mol% and 0.8 ± 0.7 mol%, respectively. On the other hand, the concentration of few-layer BP ([BP]), which was measured from the UV-vis spectroscopy with respect to the storage time, was plotted in Figure 1h. The [BP] decreased by 25.7 ± 1.6 mol% (condition (i)), 21.5 ± 2.4 mol% (condition (ii)), 4.7 ± 0.7 mol% (condition (iii)), and 4.5 ± 0.5 mol% (condition (iv)) after keeping for 15 days. We suggest that the larger mole fraction of degraded BP measured by UV-vis spectroscopy is due to the restacking of the BP flakes, according to the increased average thickness of BP flakes after storing in deoxygenated water for 15 days that was measured by AFM (Figure S5). A similar behavior of aqueous suspension of hexagonal boron nitride (h-BN) was also observed (Figure S5), which should be able to assign to the restacking of h-BN flakes since h-BN is nondegradable in water. The above results indicate that few-layer BP is chemically stable in deoxygenated water both with and without
First-principles calculations are performed to understand the role of water and oxygen in BP degradation (See details in Supporting Information Figure S6–S8 and Table S1). The minimum energy pathways (MEP) for the dissociation of H2O and O2 molecules on the surface or edges of BP are calculated, as shown in Figure 2. It is shown that the dissociation of H2O molecule on the perfect surface of BP is endothermic, which requires to overcome an energy barrier of 1.98 eV (Figure 2a). Differently, the dissociation of O2 molecule on BP’s surface is exothermic, but needs to overcome an energy barrier of 0.77 eV. Similar results can be found on BP’s edges. Our calculation (Figure S7 and S8 and Table S1) have shown that the zigzag-type edge is energetically favorable than armchair-type, and reconstruction of edges usually 28,29 happened, consistent with previous work. Here, two reconstructed zigzag type edges (Figure S8) and one armchair type (Figure S7) edges are chosen as examples to simplify the calculations. As shown in Figure 2b to 2d, the dissociation of H2O on BP’s edges becomes exothermic, but the energy barriers are still high with the values of 2.59, 1.54, and 1.95 eV for the two reconstructed zigzag type and the armchair type edges of BP, respectively, implying that the BP’s reconstructed edges are still chemical stable to H2O. However, the energy barrier for O2 dissociation on BP’s reconstructed zigzag edges is only 0.31 eV (Figure 2f and Figure S8). Therefore, O2 species can react with BP at room temperature, and our theoretical calculation suggests that the oxidation and structure degradation of BP is most likely to occur at the edges of BP. To gain further insight, BP flakes were drop-casted on SiO2/Si substrate and immersed in oxygenated water. The morphology evolution of BP was measured by in-situ AFM in water. The AFM image shows a BP flake with lateral size of ∼500 nm (Figure 3a) and thickness of ∼2.5 nm (Figure 3b). After storing in oxygenated water for 3 days, the edge decayed while the flake thickness was unchanged. The decay of the BP from the flake edge is clearer from the AFM image and height profile acquired after the sample was stored in oxygenated water for 6 days. This phenomenon is different from the degradation process of BP observed in air by AFM where the generation and growth of small bumps (water beads) are seen at the BP surface by AFM, and the structure 11 degradation of BP starts from the top-site P atoms. We suggest that such difference between the BP degradation processes in water and in air is owing to the dispersion of water around the BP nanoflake. Water allows for the dissociation of oxidized P from the BP to accelerate the structure degrada12 tion. Water beads were found accumulated at the top surface of PB nanoflakes in air. Therefore, the BP nanoflakes in air decay preferentially at the top surface of the flake. The BP nanoflakes suspended in water were surrendered by water molecules and dissolved oxygen. Therefore, the degradation of BP in water starts preferentially from the nanoflake edges where the reaction energy barrier (Figure 2) is lower. Whereas, we show that the oxidization and structure degradation of BP preferentially from the flake edges when BP is dispersed in oxygenated water, agreeing with the lower oxidation
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PO = BP BP PO 1 − " ⑵ = + + #$ = BP BP PO 1 − " ⑶ = + + PO = BP BP PO 1 − " ⑷ = + +
BP = BP ⑸ The reaction rate constants were calculated by monoexponential fitting (Figure 4a), which yielded the values of kI, kIII, -1 and kV of 0.019, 0.034, and 0.023 day , respectively, and the -1 [BP]0 value of 174 µmol L .
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The mole fractions of PO2 , PO3 , and PO4 in water can be qualitatively explained with their disengagement energy (a) (b) Relative value of POx (%)
The oxidation kinetics of few-layer BP in oxygenated water was acquired by plotting and fitting the evolution curves of 333∆[PO2 ], ∆[PO3 ], and ∆[PO4 ] (Figure 4a). The kinetics 3indicated a monoexponential increase of POx (x = 2, 3, and 34, Figure 4a). On the other hand, the mole fractions of PO2 33(∼25 mol%), PO3 (∼45 mol%), and PO4 (∼30 mol%) in water were almost unchanged (Figure 4b), and the AEC analysis on the NaH2PO2, NaPO3, and NaH2PO4 aqueous solutions 333shown that interchange among PO2 , PO3 , and PO4 in oxygenated water is prohibited (Figure S9). Therefore, the oxidation states of P were determined when O2 was reacting with BP. According to Henry’s law, the concentration of -1 aqueous O2 in water is ∼260 µmol L in ambient conditions, and can be considered unchanged when reacting with BP because of the slow reaction rate. Plus, the above analysis shows that BP can be oxidized in dark. Hence, the degradation of BP in oxygenated water can be understood by a parallel pseudo-first-order reaction of BP and dissolved O2 (Figure 4a). The BP oxidation rate, r, can be written as:
-1
In this model, the deduced [BP]0 (174 µmol L ) is significantly lower than the initial concentration of BP measured -1 by UV-vis spectroscopy (864 ± 3 µmol L ). Such difference may be owing to the microscopic structure of BP in water. Generally, the parallel pseudo-first-order reaction model considers that the reactants are molecules dissolved in solvent. Hence, all of the reactants have a same opportunity to convert to products. However, BP is in the form of nanoflakes, and the reactive P atoms are locating at the edgesites in oxygenated water. Therefore, the deduced [BP]0, stands for the concentration of P atoms at the edge-sites of the nanoflakes, is qualitatively lower than the concentration of BP.
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Journal of the American Chemical Society barrier from BP’s edge. Here, the generation of H3POx (x=2, 3, or 4) groups from reconstructed zigzag type edge of BP are investigated as an example, which is the most stable in energy (See Supporting Information Table S1). The formation of H3POx on BP’s reconstructed edge is exothermic with large formation energy (see Supporting Information Figure S10). As shown in Figure 4c, the disengagement of H3POx from BP’s edge is exothermic with reaction enthalpies ranging from -0.8 to -1.2 eV, which overcomes an energy barrier ranging from 0.31 to 0.44 eV. Interestingly, the formation of H3PO3 has the lowest energy barrier, corresponding to the 3largest mole fraction of PO3 in solution. Different, the formation of H3PO2 has the highest energy barrier, consistent 3with the lowest mole fraction of PO2 in solution. The order of the disengagement energy barrier qualitatively determined 3the relative concentration of POx in solution. We found the degradation products and their mole fractions as well as the degradation rates of the BP nanoflakes is independent on the flake’s layer number, which is in accordance to the edge site preferred oxidation reaction of BP in water (Figure S11).
edges. The BP aqueous suspension can be stored under ambient light in sealed container after N2 bubbling for months without degradation for applications such as photocatalytic hydrogen production.
For the last part, we suggest that BP aqueous suspension, under ambient light, can be stored in sealed container after N2 bubbling (deoxygenated water) for months (Figure S12), or can sustain in oxygenated water for several days for applications. We compared the Raman spectra of the freshly prepared BP with that after keeping in deoxygenated water under ambient light for 15 days (Figure 5a). The changes in positions and intensities of the BP vibration modes were almost undetectable, indicating the preservation of the BP structure. We also analyzed the function stability of BP by testing the photocatalytic activity of BP for hydrogen production (Figure 5b). The freshly prepared BP aqueous suspension presented a -1 -1 hydrogen production rate of 138 µmol g h in a testing period of 540 minutes, which is in accordance to our previous 9 study. And the hydrogen production rates of BP after storing in deoxygenated water for 7 and 15 days were 138 and 125 -1 -1 µmol g h , respectively. The slight decay of the hydrogen production activity may be ascribed to the restacking of BP nanoflakes according to our UV-vis studies (Figure 1f and S5).
*
[email protected] (a)
freshly prepared 300 350 400 450-1 500 Wavenumber (cm )
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.” Additional structure characterization, extinction coefficient, standard curves, corresponding discussions of additional supporting experimental data and computational details. (PDF)
AUTHOR INFORMATION Corresponding Author *
[email protected] Author Contributions ‡
T.Z. and Y.W. contributed equally.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT We appreciate funding support from the National Natural Science Foundation of China (51672262, 21373197, 21573204, 21421063), the MOST (2016YFA0200602), Strategic Priority Research Program of CAS (XDB01020300), 100 Talents Program of the Chinese Academy of Sciences, National Program for Support of Top-notch Young Professional, and Fundamental Research Funds for the Central Universities (WK3430000003), iChEM, and by USTCSCC, SCCAS, Tianjin, and Shanghai Supercomputer Centers.
REFERENCES
(b) stored for 15 days
Hydrogen production (µmol)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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freshly prepared stored for 7 days stored for 15 days
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Figure 5. Stability of BP in deoxygenated water. (a) Raman spectra and photographs of freshly prepared BP nanoflakes in deoxygenated water and those stored under ambient light for 15 days. (b) Hydrogen production activities of BP nanoflakes in deoxygenated water.
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In conclusion, parallel pseudo-first-order reaction of BP with oxygen in water without light illumination were identi333fied. The reaction generates PO2 , PO3 , and PO4 with reac-1 tion rate constants of 0.019, 0.034, and 0.023 day , respectively, and occur preferentially from the P atoms locating at BP
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k I =0.019 day k I I I =0.034 day 1.0 BP v ac +H 3 PO 2 BP va c +H 3 PO 3 BP v ac +H 3 PO 4 0.5 0.0 -0.5 -1.0 ∆ E = 0.44 eV 0 . 34 eV 0.31 eV 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 Reaction coordinate
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