Synthesis of {111} Facet-Exposed MgO with Surface Oxygen

Subscriber access provided by University of Newcastle, Australia

Article

Synthesis of {111} Facet-Exposed MgO with Surface Oxygen Vacancies for ROS Generation in the Dark Ying-juan Hao, Bing Liu, Li-gang Tian, Fa-tang Li, Jie Ren, Shao-jia Liu, Ying Liu, Jun Zhao, and Xiaojing Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16856 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

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

ACS Applied Materials & Interfaces

High-energy {111} facet exposed MgO with abundant oxygen vacancies is constructed, which exhibits an excellent bactericidal property for Escherichia coli in the dark because of the generation of 1O2 and •O2species through a termed chemisorption-activation path. 279x172mm (300 x 300 DPI)

ACS Paragon Plus Environment


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

Page 2 of 24

Synthesis of {111} Facet-Exposed MgO with Surface Oxygen Vacancies for ROS Generation in the Dark Ying-juan Hao, † Bing Liu, ‡ Li-gang Tian, † Fa-tang Li,*, † Jie Ren, † Shao-jia Liu, † Ying Liu, † Jun Zhao† and Xiao-jing Wang† †

College of Science, Hebei University of Science and Technology, Shijiazhuang 050018, China

‡

School of Chemical and Material Engineering, Jiangnan University, Wuxi, 214122, China

KEYWORDS: MgO; reactive oxygen species; surface oxygen vacancy; chemisorption; bacteriocide


Page 3 of 24

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


ABSTRACT

Seeking a simple and moderate route to generate reactive oxygen species (ROS) for antibiosis is of great interest and challenge. This work demonstrates that molecule transition and electron rearrangement processes can directly occur only through chemisorption interaction between the adsorbed O2 and highenergy {111} facet-exposed MgO with abundant surface oxygen vacancies (SOVs), hence producing singlet oxygen and superoxide anion radicals without light irradiation. These ROS were confirmed by electron paramagnetic resonance, in-situ Raman and scavenger experiments, respectively. Furthermore, the heat plays a crucial role for electron transfer process to accelerate the formation of ·O2-, which is verified by temperature kinetic experiments of nitro blue tetrazolium reduction in the dark. Therefore, the presence of oxygen vacancy can be considered as an intensification of the activation process. The designed MgO is one-step acquired via constructing reduction atmosphere during combustion reaction process, which has a similar ability as noble metal Pd to activate molecular oxygen and can be used as an effective bacteriocide in the dark.



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

Page 4 of 24

INTRODUCTION Reactive oxygen species (ROS) mainly including singlet oxygen (1O2), superoxide anion radicals (·O2-) and hydroxyl radicals (·OH) can cause the damage of cell membrane and the final death of cells.1-4 Although some recent works about bactericidal antibiotics have reported that robust antibacterial activities can realize in the absence of the generation of ROS,5,6 many researchers have devoted to seeking various strategies to produce ROS on various materials to kill unexpected bacteria aiming at higher efficiency. Early in 1967,7,8 the opinion that an effective collision of oxygen-oxygen could induce the molecule transitions in liquid oxygen has been observed. However, the mostly accepted formation mechanism of 1O2 is an energy transfer path from various photosensitizers capable of adsorbing an appropriate wavelength light and utilizing the energy to excite O2.9-13 Recently, Xiong et al. have reported that chemisorption of O2 on Pd {100} facet can directly enable the spin-flip activation of 1O2,14 whereas the generation of 1O2 was not found on the surface of Pd nanocubes.15Although density functional theories (DFT) have predicted that the singlet state of adsorbed O2 can be formed on reduced SnO2,16 ZnO17and copper cation exchanged chabazite,18 no significant experimental advances are achieved on the surface of non-noble metal oxide for the 1O2 generation to date. Furthermore, the direct adsorption and activation behaviors of molecular O2 on surface oxygen vacancies (SOVs) have been confirmed by scanning tunneling microscopy (STM) technique.19,20 It can be inferred that an increase in SOVs would be beneficial for the generation of 1O2 concerning the adsorption theory. Among many pioneering works, the formation of SOVs is usually via common postprocessing procedures, e.g. heat treatment21 and electron beam excitation,22 therefore these SOVs generally exist on the most thermodynamically stable crystal facet. It is known that nanocrystals with low-energy facet usually have lower catalytic activity than that with high-energy one.23-25 For example, oxygen-defect anatase TiO224 or BiOBr25 with dominant {001} facet could improve their photoactivities. Hence, how to construct SOVs in high-energy facet of nanocrystals to further improve their activities is a challenging and invaluable topic because of its unstable characteristic. ACS Paragon Plus Environment

Page 5 of 24

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


In this work, in-situ formation of SOVs on high-energy facet is realized by a simple and feasible method by constructing a reduction atmosphere conducive to the escape of lattice oxygen during the crystal formation process. The designed MgO with SOVs in {111} facet is acquired by solution combustion method, which has a similar ability as noble metal Pd14 or Ag26 to activate molecular oxygen, directly producing 1O2 and ·O2- in the dark. EXPERIMENT SECTION Synthesis of MgO. All chemicals except for commercial MgO appeared in this work were purchased from Aladdin (Shanghai, China) and used as received without further purification. MgO was synthesized by the high temperature solution combustion method. In detail, 0.010 mol of magnesium nitrate heptahydrate ((Mg(NO3)2·6H2O)) and 0.020 mol of urea (CN2H4O) were mixed and then the mixture was continually heated on an electric furnace until the complete combustion. Finally, the obtained product was MgO. The commercial MgO (chemistry pure) was received from Damao chemical reagent factory (Tianjin, China) as reference for comparison. Characterization. X-ray diffraction (XRD) pattern was performed on Rigaku D/MAX 2500 X-ray diffractometer using Cu-Kα radiation. The special surface area and porous structure were obtained at 77 K on Micromeritics Tristar 3020 adsorption instrument. The pore-size distribution was analyzed by Barret-Joyner-Halenda (BJH) method. Transmission electron microscopy (TEM) images were received on a JEOL JEM-2100 electron microscopy. The spherical aberration-corrected scanning transmission electron microscopy technique (STEM) images were accomplished on a JEOL JEM-ARM200F with an acceleration voltage of 200 KV. The determination of ·O2- radicals at room temperature was revealed on a Bruker EMX-8/2.7 X-band electron paramagnetic resonance (EPR) spectroscopy with the X-band at 9.86 GHz under vacuum conditions in the dark. 2,2-Diphenyl-1-picrylhydracil radical (DPPH) was used for g-value (g=2.0028) calibration. The formation of singlet oxygen was also observed by EPR characterization with 2, 2, 6, 6-tetramethylpiperdine (TEMP) as a quencher under dark conductions.1 The UV-Vis diffuse reflectance spectra (DRS) of the as-obtained samples were measured on Thermo ACS Paragon Plus Environment


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

Page 6 of 24

Scientific Evolution 220 UV-Vis spectrophotometer with a flashing xenon lamp as light source between 200 and 800 nm at room temperature. The photoluminescence (PL) spectra were confirmed on a Hitachi F-4600 fluorescence spectrophotometer altering excitation wavelength from 250 to 290 nm. The Fourier transform infrared (FTIR) spectra were carried out on a Shimadzu IR-Prestige 21 with potassium bromide as background sample under air atmosphere at room temperature in the range of 400-4000 cm-1. The X-ray photoelectron spectroscopy (XPS) spectra were obtained on a PHI 1600 ESCA XPS system with a calibration of C 1s peak at 284.6 eV. The peaks of Mg 1s and O 1s were deconvoluted by Lorentzian-Gaussian method, respectively. Magnetic properties of the as-obtained MgO and commercial MgO were measured on a Lakeshore155 vibrating-sample magnetometer (VSM) at room temperature. In-situ Raman spectra were carried out on a Renishaw UV RM1000 with a confocal microprobe Raman system under the same conditions (oxygen flow, 532 nm laser line with a power of 100%, a dwell time of 60 s and a scan number of 6). The samples without pretreatment were inserted in a homemade in-situ Raman sample cell and heated to different temperature in a flow of pure oxygen (35 mL/min) with a heating time 10 oC/min. During the heating process, the samples were not irradiated by 532 nm laser line. Finally, the Raman signals were directly recorded after 30 min holding time for each temperature. Raman spectra under nitrogen and oxygen flows were acquired to determine the adsorbed oxygen on the samples at room temperature. Oxygen temperature-programmed desorption (O2-TPD) experiments were achieved on a PCA-2200 chemical adsorption apparatus (BJBUILDER, Beijing). Before the analysis, the samples (0.1 g, 60-80 mesh) were purged by vacuum desorption at 300 oC for 30 min to remove the impurities on the surface. The samples were cooled to 50 oC and switched to oxygen flow (25 mL/min) for 60 min. Subsequently, the samples were swept with helium gas for 100 min. The temperature was increased from 50 oC to 600 o

C at a heating rate 10 oC /min under helium flow (30 mL/min). The dark current-voltage curves were measured on a CHI660E electrochemical station (Chenghua,

Shanghai) using conventional three-electrode quartz cells. The supporting electrolyte was a phosphate ACS Paragon Plus Environment

Page 7 of 24

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


buffer solution (pH=8) containing disodium hydrogen phosphate and sodium dihydrogen phosphate solution. Theoretic Calculation. The calculations were based on density functional theory (DFT) using projector augmented wave (PAW) methods, as implemented in the Vienna ab initial simulation package (VASP).27 The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was used for describing the exchange-correlation interactions. The valence electron configurations were set as Mg-3s2 and O-2s22p4, respectively. The cutoff energy was 400 eV. Herein, Oxygen vacancies calculations were performed in a 9-atom-thick MgO {100} slab and a 10-atom-thick MgO {111} slab. The supercells consisted of 80 Mg and 79 O for MgO{111} facet while 81 Mg and 80 O for MgO {100} facet. A 2×2×1 k-points Monkhorst-Pack k-mesh was employed for the irreducible Brillouin-zone sampling. The shapes of the slab and the volume remained unchanged when optimizing structure. Optimized structures were obtained by minimizing the forces on each ion until they were less than 0.05 eV/Å according to references [28-30]. The formation energies of oxygen vacancies in MgO were calculated with a energy convergence limit of 10-4 eV. The formation energy of oxygen vacancy was expressed as follows:31 Evac = Ecell-vac + 1/2EO2 - Ecell where Evac, Ecell-vac and Ecell were the formation energy of oxygen vacancy, the total energy of the optimized supercell with and without an oxygen vacancy, respectively. EO2 was the energy of free oxygen molecule (about -9.85 eV). Determination of superoxide anion radical in the dark. Because nitro blue tetrazolium (NBT) with a maximum absorbance at 259 nm could easily react with superoxide anion radicals,32 these radicals were determined by monitoring the reduction of 2.0×105 mol/L NBT in the dark on a Thermo Scientific Evolution 220 spectrophotometer.



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

Page 8 of 24

Antibacterial experiments in the dark. Considering that the negative charged Gram-negative Escherichia coli (E. coli) could grow under alkaline conditions,33 thus we selected it to assess the antibacterial activity of the samples. The E. coli material in this work was received from enzyme engineering laboratory, College of bioscience and bioengineering, Hebei University of Science and Technology. For antibacterial experiments, various amount MgO (0~0.075 mol/L) was homogeneously dispersed in 100 mL of solution containing E. coli cells at a cell density of about 107 colony-forming units (cfu)/mL. After incubation at 37 oC overnight, the bacteria concentration was determined by measuring the optical density at 600 nm on 723 UV-Vis spectrophotometer.34 In order to obtain plate photographs for colony units for E. coli, 1 mL of MgO suspension with various concentrations and 0.15 mL of E. coli solution were pipetted into a plate, and then appropriate amount of agar solution (about 50 oC) was added into the plate, shaking until the complete solidification. Blank glass was used as a control experiment. The whole process was carried out in the dark. Decolorization experiments in the dark. All the decolorization experiments of methyl violet (MV) in this work were instantaneously carried out in the dark. In a typical evaluation process, 0.10 g of catalyst was added to 100 mL MV (20 mg/L) aqueous solution. At given time intervals, 5 mL of the suspension was sampled and rapidly centrifuged (10000 r/min, 1 min) to remove the sample. The concentration was determined by spectroscopic analysis on the abovementioned spectrophotometer. The MV decolorization rate was reported as C/Co, where Co and C were the concentrations of 20 mg/L and after degradation, respectively. Meanwhile, the effects of nitrogen, oxygen and 1,4-diazabicyclo[2,2,2] octane (DABCO, 9.2 mmol/L, singlet oxygen scavenger) on the decolorization performances were also investigated to further confirm the decolorization mechanism. The oxidation product of MV dye was off-line analyzed on a Shimadzu LC-MS2010EV system, which was carried out in the negative ionization mode.


Page 9 of 24

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


During the control experiments, the effect of pH values caused by MgO itself on the decolorization performance was eliminated by adding hydrochloric acid because of its acid-base characteristic of MV, ensuring that the pH values of the solution remained the same before and after the degradation. RESULTS AND DISCUSSION Seen from Figure S1, no diffraction peaks arising from impurities are observed, indicating a high purity of the as-obtained sample. It is noted that a slight increase intensity of MgO {111} facet from the sample is also found in comparison with that from standard diffraction of MgO (JCPDS card No. 00045-0946), suggesting a high possibility of exposed {111} facet by this method. Figures S1b-1d reveal the single-crystalline nature of the as-obtained sample with an interplanar lattice fringe of 0.243 nm corresponding to the {111} facet of cubic MgO.

Figure 1.(a) Aberration-corrected annular-bright-field image of the as-obtained MgO, and line profiles of Mg (b) and O (c) atoms along the vertical direction with regarding to the dotted lines of (a), respectively, showing the atom intensity versus position. Similar to the work of Xu et al.,35 the aberration-corrected annular-bright-field (ABF) transmission electron microscopy technique is employed to identify the presence of oxygen vacancies in singlecrystalline MgO {111} facet. Figure 1 shows a high-magnification ABF STEM image of the as-obtained sample viewed along the [112] axis. Seen from Figure 1a, the dark spots are Mg sites while the bright ones are O sites (magnified at top left inset). It is noted that the contrast differences reveal the existence ACS Paragon Plus Environment


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

Page 10 of 24

of surface defects in MgO {111} facet. To further determine the origin of these defects, the line profiles showing the image intensity versus position in image (a) along the vertical direction about white and black dotted lines are displayed in Figure1b and Figure 1c, respectively. One can see that the concentration of O atoms undergoes a significant change while that of Mg atoms remains unchanged along the direction, showing the presence of SOVs in MgO {111} facet.35 Therefore, SOVs in a singlecrystalline MgO {111} facet was obtained for the first time via simple solution combustion process in this work. It can be explained that when the combustion atmosphere was in reduction caused by the excess fuels, the oxygen atoms on the MgO {111} facet would release, hence leaving SOVs.36 This process can be expressed using following equation.37 OO = VO•• + 1/ 2O2 (g) + 2e/

(1)

Herein, the formation of designed MgO with SOVs is put forward as follows: Mg ( NO3 ) 2 ⋅ 6 H 2O( s) + 2CO( NH 2 )2 (s) + 0.5O2 ( g ) ∆  → MgO( s) + 2CO2 ( g ) + 3N 2 ( g ) + 10H 2O( g )

(2)


Page 11 of 24

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


Figure 2. Oxygen vacancies determination of the as-obtained MgO by different characterizations: (a) UV-Vis DRS, (b) (αhν)2 versus hν curves, (c) PL spectra with 250, 270 and 290 nm as excitation wavelengths, (d) XPS survey scan spectrum, and high-resolution XPS of (e) O 1s and (f) Mg 1s. The optical measurements (DRS and PL) can offer some direct proofs of surface states on the sample surface. Figure 2a shows that the as-obtained MgO has an obvious optical absorption between 200-350 nm. It is noted that there appears a small absorption peak of color centers in the sample, as shown in the inset of Figure 2a. The energy band gap (Eg) can be estimated according to the expression of Tauc plots, (αhν)2=K(hν−Eg),38 where α and hν represents absorption coefficient and photon energy, respectively, K is a constant. The (αhν)2 vs. hν curve is shown in Figure 2b. The extrapolation of straight line to (αhν)2=0 axis give two band gaps of 3.8 and 4.7 eV, which are involved with color centers or lowACS Paragon Plus Environment


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

Page 12 of 24

coordinated sites.39,40 When the excitation wavelength is respectively 250 nm, 270 or 290 nm, a characteristic broad PL emission peak at about 375 nm is originated from low coordination Mg2+ species from the sample40 or single trapped electrons (Fs+ centers).41,42 Obviously, this phenomenon is well agreement with the DRS result. However, no changes about the peak positions at 452 and 470 nm are observed altering the excitation wavelengths from 250 to 290 nm. It indicates that there occur the fixed surface states related with SOVs, which are also supported by the following XPS results (Figure 2d-2f). Seen from Figure 2d, there are no impurity exists on the sample surface, which agrees well with Figure S2. The Lorentzian-Gaussian bands of O 1s peak at 529.5 and 531.8 eV (Figure 2e) are assigned to the lattice oxygen in MgO43 and oxygen vacancies44 while the two bands at 1303.8 and 1305.0 eV (Figure 2f) to Mg 1s in MgO45 and the low state Mg ions involved with the neighbor trapped electron in Fs+ centers, respectively.

Figure 3. Tilted side views of 2×2×1 supercell of MgO {100} and {111} facets with one oxygen vacancy, respectively. On basis of the above analysis, when the partial pressure of oxygen is insufficient, the lattice oxygen atoms can easily escape, accompanying with the formation of SOVs. We employed DFT calculations using PAW method to verify the possibility of the formation of SOVs in MgO {111} facet (Figure 3). For comparison, the formation energy of oxygen vacancies in MgO {100} facet mostly reported46,47 was also calculated in this work. The theoretical results show that the formation energies in {111} and {100} facets are respectively -4.89 eV and 6.18 eV, indicating the easier formation in the former facet. Notably, ACS Paragon Plus Environment

Page 13 of 24

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


we also found that the reconstruction of two Mg atoms from sub-layer could improve the stability of polar MgO {111} facet during the formation process of oxygen vacancy.48

Figure 4. (a) In-situ Raman spectra of the as-obtained MgO under O2 and N2 flows at room temperature; (b) EPR pattern of the generation of 1O2 with TEMP as a quencher in the dark. The arrows display the changes of EPR signal with prolonging the reaction time from 0 to 140 min; (c) room-temperature magnetization curves; (d) dark current-voltage curves; (e) solid EPR signal image; (f) Dark reduction of NBT by ·O2- within 5 h at different temperature. In-situ Raman spectra of the as-obtained MgO and commercial MgO under oxygen flow at different temperature are indicated in Figure 4a and Figure S3, respectively. Because bulk MgO has no Raman spectrum due to its inversion symmetry,49 the obtained Raman signal is only associated with the surface information. It is seen that some Raman bands at 701,1042, 1307 and 1552 cm-1 are detected on the asACS Paragon Plus Environment


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

Page 14 of 24

obtained sample surface (see Figure 4a) while 893, 990, 1087and 1552 cm-1 appear on the surface of commercial MgO (see Figure S3a).The Raman band at 1087 cm-1 is assigned to surface phonon modes50 and the others are mainly related with surface oxygen species.51 Herein, the band at 1552 cm-1 is attributed to O-O stretching mode of molecular O2 while the bands at 1042 and 1307 cm-1 are ascribed to ·O2- and adsorbed O2, respectively.51 It is notable that almost no Raman signal of ·O2- is found on the surface of commercial MgO. Compared with commercial MgO, no changes about these positions and intensities of surface oxygen species are found on the surface of the as-obtained sample even at higher temperature of 500 oC. It suggests that there appears a strong chemisorption of oxygen over the sample surface, which is verified by oxygen temperature-programmed desorption (O2-TPD) (Figure S4). It has been reported that a molecular collision (or perturbation) induce an electronic transition for the generation of 1O2species in theory.7,8 Considering the chemisortpion of O2 involving molecular collisions and electron rearrangements, it can be considered to induce a direct generation of 1O2 species. Figure 4b and Figure S5 provide some important EPR evidences for 1O2 generation on the surface of the as-obtained sample with TEMP as a quencher in the dark.1,52 With prolonging the reaction time, the intensity of EPR signals gradually increase, suggesting that 1O2 species indeed generate on the sample surface. The kinetics behavior of 1O2 species versus reaction time is indicated in Figure S5. When the reaction time exceeds 100 min, the intensity of EPR signals begins to decrease. This phenomenon could be caused by quenching effects resulted from intermolecular collisions.7 Similar to the work of Liu et al.,1 the characteristic EPR signals of 1O2 species are also observed in the absence of the as-obtained sample, which is mainly related with the presence of an impurity from commercial TEMP.53 However, no EPR signal of 1O2 species are detected on the surface of commercial MgO in the dark, as displayed in Figure S6. Therefore, we infer that the presence of SOVs would strengthen the activation ability for molecular oxygen. Figure 4c indicates that the room temperature paramagnetism spin-disorder is characteristic of the presence of abundant Fs+ centers of MgO,54,55 which agrees well with PL result. Compared with the as-obtained MgO, the diamagnetism of commercial MgO (inset in Figure 4c) is attributed to lack of Fs+ centers on the sample surface. Because the dark current-voltage curves can be ACS Paragon Plus Environment

Page 15 of 24

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


used to confirm an intermolecular electron transfer reaction, we have carried out dark reduction current experiments, as shown in Figure 4d. Obviously, the dark reduction current density of the as-obtained MgO is larger than that of commercial MgO under argon or oxygen atmosphere. Figure 4e indicates some EPR signals with g values at 1.999, 2.019 and 2.038, which are assigned to ·O2- species involved with Fs+ centers.4,54,55 These chemisorbed ·O2- species over the as-obtained sample were also verified by temperature kinetic reduction experiments of NBT in the dark (Figure 4f and Figure S7a), which was reported to be easily reacted with ·O2- species by Bleiski et al..32 However, the similar phenomenon did not occur due to lack of ·O2- species when commercial MgO was employed, as indicated in Figure S7b. This is in accordance with the results of Raman, magnetization and dark reduction current experiments.

Figure 5. (a-d) Plate photographs for colony forming units of E. coli as a function of MgO concentration (mol/L) in the dark: (a) 0; (b) 0.0125; (c) 0.0250; (d) 0.0375; (e) antibacterial activities of various samples. For a proof-of-concept application, the as-obtained MgO is used as antibacterial (Figure 5 and Figures S8-9) and wastewater treatment materials (Figures S10-13) to understand the oxidization activities of ·O2- and 1O2 species. Figure 5a-d shows that the as-obtained sample has a robust antibacterial efficiency towards negative charged Gram-negative E. coli. To eliminate the effect of surface basicity (or hydration) of MgO33 itself and highlight lethal antibacterial performances of the asobtained sample, we further investigated the antibacterial efficiencies of commercial Mg(OH)2 and ACS Paragon Plus Environment


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

Page 16 of 24

commercial MgO. Seen from Figure 5e, their antibacterial efficiencies towards E. coli achieved about 8% and 22% under the same conditions, respectively, which is agreement with the works of Pan33 and Leung56, confirming their antibacterial ability without the attendance of ROS. Therefore, the improvement in antibacterial efficiency of the as-obtained MgO is related with ROS generated on the sample surface in comparison with commercial MgO based on the above discussions. Figures S8-9 showed the morphology changes of E. coli after introducing the two materials without energy irradiation. In addition, the as-obtained sample was also applicable to pollutant elimination in the dark, as displayed in Figures S10-12. Table S1 and Figure S13 further show that this decolorization process is a chemical oxidation reaction rather than physical adsorption.

CONCLUSIONS In summary, high-energy MgO {111} facet with surface oxygen vacancies has been synthesized by solution combustion method. The as-obtained MgO shows an excellent molecular oxygen activation ability, directly producing reactive oxygen species that include superoxide anion radicals and singlet oxygen without light irradiation. These reactive oxygen species are confirmed by in-situ Raman, EPR and scavenger results. SEM results indicate that E. coli could be killed within 2 h in the dark when the concentration of the as-obtained sample was 0.05 mol/L. On basis of these results, the antibacterial mechanism of the as-obtained MgO in dark is attributed to the production of reactive oxygen species on the sample surface. This work not only offers a new understanding for the generation mechanism of ROS species, but also will broaden the application of alkaline earth metal oxide..

ASSOCIATED CONTENT


Page 17 of 24

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


The Supporting Information is available free of charge via the Internaet at http://pubs.acs.org.: theoretical basis of oxygen vacancies, XRD and in-situ Raman patterns, EPR and O2-TPD results, SEM images of E. coli morphology changes, pollutant elimination efficiencies in the dark and HPLC results of products. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (21376061), Natural Science Foundation of Hebei Province (B2015208124, B2015208010), Top Young Talents Program in University of Hebei Province (BJ2017049), Scientific Research Foundation for High-Level Talent in University of Hebei Province (GCC2014057), and Foundation of Hebei University of Science and Technology (2014PT97).

REFERENCES (1) Liu, K.; Liu, Y. L.; Yao, Y. X.; Yuan, H. X.; Wang, S.; Wang, Z. Q.; Zhang, X. Supramolecular Photosensitizers with Enhanced Antibacterial Efficiency. Angew. Chem., Int. Ed.2013, 52, 8285–8289. (2) Xing, C. F.; Xu, Q. L.; Tang, H. W.; Liu, L. B.; Wang, S. Conjugated Polymer/Porphyrin Complexes for Efficient Energy Transfer and Improving Light-activated Antibacterial Activity. J. Am. Chem. Soc. 2009, 131, 13117–13124.



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

Page 18 of 24

(3) Wu, J. M.; Kao, W. T. Heterojunction Nanowires of AgxZn1-xO-ZnO Photocatalytic and Antibacterial Activities under Visible-Light and Dark Conditions. J. Phys. Chem. C 2015, 119, 1433– 1441. (4) Prasanna, V. L.; Vijayaraghavan, R. Insight into the Mechanism of Antibacterial Activity of ZnO: Surface Defects Mediated Reactive Oxygen Species even in the dark. Langmuir 2015, 31, 9155–9162. (5) Keren, I.; Wu, Y. X.; Inocencio, J.; Mulcahy, L. R.; Lewis, K. Killing by Bactericidal Antibiotics Does Not Depend on Reactive Oxygen Species. Science 2013, 339, 1213–1216. (6) Liu Y. Y.; Imlay, J. A. Cell Death from Antibiotics Without the Involvement of Reactive Oxygen Species. Science 2013, 339, 1210–1213. (7) Pettschnick, R. P. H.; Hoytink, G. J. Collision Induced Electronic Transitions of Oxygen. Chem. Phys. Lett. 1967, 1, 145–148. (8) Kearns, D. R. Physical and Chemical Properties of Singlet Molecular Oxygen. Chem. Rev. 1971, 71, 395–427. (9) DeRosa, M. C.; Crutchley, R. J. Photosensitized Singlet Oxygen and Its Applications. Coordin. Chem. Rev. 2002, 233–234, 351–371. (10) Ge, J. C.; Lan, M. H.; Zhou, B. J.; Liu, W. M.; Guo, L.; Wang, H.; Jia, Q. Y.; Niu, G. L.; Huang, X.; Zhou, H. Y.; Meng, X. M.; Wang, P. F.; Lee, C. S.; Zhang, W. J.; Han, X. D. A Graphene Quantum Dot Photodynamic Therapy Agent with High Singlet Oxygen Generation. Nat. Commun. 2014, 5, 4596. (11) Kawasaki, H.; Kumar, S.; Li, G.; Zeng, C. J.; Kauffman, D. R.; Yoshimoto, J.; Iwasaki, Y.; Jin R.H. Generation of Singlet Oxygen by Photoexcited Au25(SR)18 Clusters. Chem. Mater. 2014, 26, 2777–2788.


Page 19 of 24

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


(12) Vankayala, R.; Sagadevan, A.; Vijayaraghavan, P.; Kuo, C. L.; Hwang, K. U. Metal Nanoparticles Sensitize the Formation of Singlet Oxygen. Angew. Chem., Int. Ed. 2011, 50, 10640–10644. (13) Kovalev, D.; Fujii, M. Silicon Nanocrystals: Photosensitizers for Oxygen Molecules. Adv. Mater. 2005, 17, 2531–2544. (14) Long, R.; Mao, K. K.; Ye, X. D.; Yan, W. S.; Huang, Y. B.; Wang, J. Y.; Fu,Y.; Wang, X. S.; Wu, X. J.; Xie, Y.; Xiong, Y. J. Surface Facet of Palladium Nanocrystals: A Key Parameter to the Activation of Molecular Oxygen for Organic Catalysis and Cancer Treatment. J. Am. Chem. Soc. 2013, 135, 3200– 3207. (15) Dahal, E.; Curtiss, J.; Subedi, D.; Chen, G.; Houston, J. P.; Smirnov, S. Evaluation of the Catalytic Activity and Cytotoxicity of Palladium Nanocubes: The Role of Oxygen. ACS Appl. Mater. Interfaces 2015, 7, 9364–9371. (16) Oviedo, J.; Gillan, M. J. First-Principles Study of the Interaction of Oxygen with the SnO2 (110) Surface. Surf. Sci. 2001, 490, 221–236. (17) Zav'yalova, L. M.; Myasnikov, I. A.; Zav'yalov, S. A. Chemisorption of Singlet Oxygen on the Zinc Oxide Surface. Khim. Fiz. 1987, 6, 473–476. (18) Pérez-Badell, Y.; Solans-Monfort, X.; Sodupe, M.; Montero, L. A. A DFT Periodic Study on the Interaction between O2 and Cation Exchanged Chabazite MCHA (M =H+, Na+ or Cu+): Effects in the Triplet-Singlet Energy Gap. Phys. Chem. Chem. Phys. 2010, 12, 442–452. (19) Tan, S. J.; Ji, Y. F.; Zhao, Y.; Zhao, A. D.; Wang, B.; Yang, J. L.; Hou, J. G. Molecular Oxygen Adsorption Behaviors on the Rutile TiO2(110)-1×1 Surface: An in Situ Study with Low-Temperature Scanning Tunneling Microscopy. J. Am. Chem. Soc. 2011, 133, 2002–2009. (20) SetvÍn, M.; Aschauer, U.; Scheiber, P.; Li, Y. F.; Hou, W. Y.; Schmid, M.; Selloni, A.; Diebold, U. Reaction of O2 with Subsurface Oxygen Vacancies on TiO2 Anatase (101). Science 2013, 341, 988–990. ACS Paragon Plus Environment


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

Page 20 of 24

(21) Wang, J. P.; Wang, Z. Y.; Huang, B. B.; Ma, Y. D.; Liu, Y. Y.; Qin, X. Y.; Zhang, X. Y.; Dai, Y. Oxygen Vacancy Induced Band-Gap Narrowing and Enhanced Visible Light Photocatalytic Activity of ZnO. ACS Appl. Mater. Interfaces 2012, 4, 4024–4030. (22) Scheiber, P.; Fidler, M.; Dulub, O.; Schmid, M.; Diebold, U.; Hou, W. Y.; Aschauer, U.; Selloni, A. (Sub)Surface Mobility of Oxygen Vacancies at the TiO2 Anatase (101) Surface. Phys. Rev. Lett. 2012, 109, 136103. (23) Zhou, Z. Y.; Tian, N.; Li, J. T.; Broadwell, I.; Sun, S. G. Nanomaterials of High Surface Energy with Exceptional Properties in Catalysis and Energy Storage. Chem. Soc. Rev. 2011, 40, 4167–4185. (24) Liu, G.; Yang, H. G.; Wang, X. W.; Cheng, L. N.; Lu, H. F.; Wang, L. Z.; Lu, G. Q.; Cheng, H. M. Enhanced Photoactivity of Oxygen-Deficient Anatase TiO2Sheets with Dominant {001} Facets. J. Phys. Chem. C 2009, 113, 21784–21788. (25) Li, H.; Shang, J.; Ai, Z. H.; Zhang, L. Z. Efficient Visible Light Nitrogen Fixation with BiOBr Nanosheets of Oxygen Vacancies on the Exposed {001} Facets. J. Am. Chem. Soc. 2015, 137, 6393– 6399. (26) Gonchar, A.; Risse, T.; Freund, H. J.;Giordano, L.; Valentin, C. D.; Pacchioni, G. Activation of Oxygen on MgO: ·O2- Radical Ion Formation on Thin, Metal-Supported MgO (001) Films. Angew. Chem., Int. Ed. 2011, 50, 2635–2638. (27) Iyikanat, F.; Sahin, H.; Senger, R. T.; Peeters, F. M. Vacancy Formation and Oxidation Characteristics of Single Layer TiS3. J. Phys. Chem. C 2015, 119, 10709–10715. (28) Duan, Z. Y.; Henkelman, G. CO Oxidation at the Au/TiO2 Boundary: The Role of the Au/Ti5c Site. ACS Catal. 2015, 5, 1589–1595.


Page 21 of 24

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


(29) Song , W. Y.; Liu, J.; Zheng, H. L.; Ma, S. C.; Wei, Y. C.; Duan, A. J.; Jiang, G. Y.; Zhao, Z.; Hensen, E. J. M. A Mechanistic DFT Study of Low Temperature SCR of NO with NH3 on MnCe1-xO2 (111), Catal. Sci. Technol. 2016, 6, 2120–2128. (30) Liu, B.; Zhao, Z.; Henkelman, G.; Song, W. Y. Computational Design of a CeO2-Supported PdBased Bimetallic Nanorod for CO Oxidation. J. Phys. Chem. C 2016, 120, 5557–5564. (31) Agarwal, V.; Metiu, H. Energy of Oxygen-Vacancy Formation on Oxide Surfaces: Role of the Spatial Distribution. J. Phys. Chem. C 2016, 120, 2320–2323. (32) Bleiski, B. H. J.; Shlue, G. G.; Bajuk, S. Reduction of Nitro Blue Tetrazolium by CO2- and O2Radicals. J. Phys. Chem. 1980, 84, 830–833. (33) Pan, X. H.; Wang, Y. H.; Chen, Z.; Pan, D. M.; Cheng, Y. J.; Liu, Z. J.; Lin, Z.; Guan, X. Investigation of Antibacterial Activity and Related Mechanism of a Series of Nano-Mg(OH)2. ACS Appl. Mater. Interfaces 2013, 5, 1137–1142. (34) Ivask, A.; ElBadawy, A.; Kaweeteerawat, C.; Boren, D.; Fischer, H.; Ji, Z. X.; Chang, C. H.; Liu, R.; Tolaymat, T.; Telesca, D.; Zink, J. I.; Cohen, Y.; Holden, P. A.; Godwin, H. A. Toxicity Mechanisms in Escherichia Coli Vary for Silver Nanoparticles and Differ from Ionic Silver. ACS Nano 2014, 8, 374–386. (35) Xu, Z. T.; Jin, K. J.; Gu, L.; Jin, Y. L.; Ge, C.; Wang, C.; Guo, H. Z.; Lu, H. B.; Zhao, R. Q.; Yang, G. Z. Evidence for a Crucial Role Played by Oxygen Vacancies in LaMnO3 Resistive Switching Memories.Small 2012, 8, 1279–1284. (36) Li, F. T.; Ran, J. R.; Jaroniec, M.; Qiao, S. Z. Solution Combustion Synthesis of Metal Oxide Nanomaterials for Energy Storage and Conversion. Nanoscale 2015, 7, 17590–17610. (37) Wu, Q. P.; Krol, R. Selective Photoreduction of Nitric Oxide to Nitrogen by Nanostructured TiO2 Photocatalysts: Role of Oxygen Vacancies and Iron Dopant. J. Am. Chem. Soc. 2012, 134, 9369–9375. ACS Paragon Plus Environment


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

Page 22 of 24

(38) J. Tauc (Ed.), Amorphous and Liquid Semiconductors, Plenum Press, New York, 1974. (39) Sterrer, M.; Berger, T.; Diwald, O.; Knözinger, E. Energy Transfer on the MgO Surface, Monitored by UV-Induced H2 Chemisorption. J. Am. Chem. Soc. 2003, 125, 195–199. (40) Chizallet, C.; Costentin, G.; Lauron-Pernot, H.; Krafft, J. M.; Che, M.; Delbecq, F.; Sautet, P. Assignment of Photoluminescence Spectra of MgO Powders: TD-DFT Cluster Calculations Combined to Experiments. Part I: Structure Effects on Dehydroxylated Surfaces. J. Phys. Chem. C 2008, 112, 16629–16637. (41) Rosenblatt, G. H.; Rowe, M. W.; Williams, G. P. J.; Williams, R. T.; Chen, Y. Luminescence of F and F+ Centers in Magnesium Oxide. Phys. Rev. B 1989, 39, 10309–10318. (42) Rinke, P.; Schleife, A.; Kioupakis, E.; Janotti, A.; Rödl, C.; Bechstedt, F.; Walle, C. G. V. FirstPrinciples Optical Spectra for F Centers in MgO. Phys. Rev. Lett. 2012, 108, 126404. (43) Cabailh, G.; Lazzari, R.; Cruguel, H.; Jupille, J.; Savio, L.; Smerieri, M.; Orzelli, A.; Vattuone, L.; Rocca, M. Stoichiometry-Dependent Chemical Activity of Supported MgO(100) Films. J. Phys. Chem. A 2011, 115, 7161–7168. (44) Mülller, K.; Torres, D.; Park, J. B.; Liu, P.; Stacchiola, D.; Starr, D. E. NiO-MgO and CoO-MgO Thin-Film Solid Oxide Solutions on a Mo (100) Support: Formation, Reduction, and Influence of the Support. J. Phys. Chem. C 2013, 117, 280–287. (45) Zhang, W. B.; Tang, B. Y. Stability of MgO (111) Polar Surface: Effect of the Environment. J. Phys. Chem. C 2008, 112, 3327–3333. (46) Tobita, M.; Ho, S. Electronic Structure Calculation of MgO (001) Surface with Aggregated Oxygen Vacancies. J. Chem. Theory Comput. 2008, 4, 1057–1064.


Page 23 of 24

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


(47) Cabailh, G.; Lazzari, R.; Cruguel, H. Jupille, J.; Savio, L.; Smerieri, M.; Orzelli, A.; Vattuone, L.; Rocca, M. Stoichiometry-Dependent Chemical Activity of Supported MgO (100) Films. J. Phys. Chem. A 2011, 115, 7161–7168. (48) Wolf, D. Reconstruction of NaCl Surfaces from a Dipolar Solution to the Madelung Problem. Phys. Rev. Lett. 1992, 68, 3315–3318. (49) Böckelmann, H. K.; Schlecht, R. G. Raman Scattering from Microcrystals of MgO. Phys. Review. B 1974, 10, 5225–5231. (50) Ishikawa, K. First-order Raman Scattering in MgO Microcrystals. J. Appl. Phys. 1985, 57, 973– 975. (51) Long, R. Q.; Wan, H. L. In Situ Confocal Microprobe Raman Spectroscopy Study of CeO2/BaF2Catalyst for the Oxidative Coupling of Methane. J. Chem. Soc. Faraday Trans. 1997, 93, 355–358. (52) Das, K. C.; Das, C. K. Thioredoxin, a Singlet Oxygen Quencher and Hydroxyl Radical Scavenger: Redox Independent Functions. Biochem. Biophys. Res. Commun. 2000, 277, 443–447. (53) Lion, Y.; Delmelle, M.; Vorst.A. V. D. New Method of Detecting Singlet Oxygen Production. Nature. 1976, 263, 442–443. (54) Diwald, O.; KNözinger, E. Intermolecular Electron Transfer on the Surface of MgO Nanoparticles. J. Phys. Chem. B 2002, 106, 3495–3502. (55) Pacchioni, G.; Ferrari, A. M.; Giamello, E. Cluster Models of O2-Adsorption on Regular and Defect Sites and Fs Centers of the MgO (100) Surface. Chem. Phys. Lett. 1996, 255, 58–64. (56) Leung,Y. H.; Ng, A. M. C.; Xu, X. Y.; Shen, Z. Y.; Gethings, L. A.; Wong, M. T.; Chan, C. M. N.; Guo, M. Y.; Ng, Y. H.; Djurišić, A. B.; Lee, P. K. H.; Chan, W. K.; Yu, L. H.; Phillips, D. L.; Ma, A. ACS Paragon Plus Environment


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

Page 24 of 24

P. Y.; Leung, F. C. C.; Mechanisms of Antibacterial Activity of MgO: Non-ROS Mediated Toxicity of MgO Nanoparticles Towards Escherichia Coli. Small 2014, 10, 1171–1183.


Synthesis of {111} Facet-Exposed MgO with Surface Oxygen

Recommend Documents