Synthesis of Chestnut-Shell-Pigment-Modified Polyacrylamide Superabsorbent Optimized by Response Surface Methodology

Chestnut shell pigment was employed to modify a polyacrylamide superabsorbent. The synthesis was conducted through free-radical polymerization in aqueous solutions by using N, N’-methylene bisacrylamide and ammonium persulfate as a crosslinker and an initiator, respectively. This work focused on the optimization of the synthesis conditions by surface response methodology. The effects of the doses of chestnut shell pigment, crosslinker, and initiator on the water absorbency of the superabsorbent were studied. The optimized condition was found to be 14.47% of the acrylic acid, 0.16% of the chestnut shell pigment, 0.71% of the crosslinker, 0.32% of the initiator, 4.06 % of NaOH, and 80.28% of H2O in quality percentage, under which the water absorbency was 264.2g/g. Introduction Superabsorbents are a kind of hydrophilic polymer that is able to absorb and retain huge amounts of water or aqueous solutions even under pressure. They are widely used in several fields, such as soil improvement, coal dewatering, water-holding afforestation, water-saving irrigation, water-blocking tapes, hygienic products, and drug delivery systems [1]. Melanins are a class of macromolecular pigments polymerized from phenolic and/or indolic substances [2]. They are rich in hydroxyl, carboxyl and amino hydrophilic functional groups. These groups readily bind a variety of chemical substances such as heavy metals and water [3], and endow detoxication and water-holding capabilities to the pigments [4-5]. Therefore, modification of traditional superabsorbent with melanin maybe gives some special properties to the superabsorbent. However, to our knowledge, there is no report on modifying polyacrylamide superabsorbent with melanin. China is the biggest chestnut producer in the world, and chestnut plays very important roles in the solution of issues concerning agriculture, countryside and farmers in China, especially in hilly regions. Chestnut shell is a processing residue from the food industry and has not been utilized as yet. It contains about 15% of pigment, namely, chestnut shell pigment. We have intensively characterized the pigment on properties and structures. It was considered as allomelanin and found being rich in phenolic hydroxyl and carboxyl functional groups [6]. This study aimed to optimize the synthesis process of chestnut-shell-pigment-modified polyacrylamide superabsorbent by response surface methodology. Materials and Methods Materials Chestnut fruits (Castanea mollissima) were purchased from a market in Kunming, China, and peeled manually. The chestnut shell was crushed to pass 4-mm screens and used to extract the pigment following the procedure described in our previous work [7]. All the reagents used were of analytical-reagent grade. 2nd Annual International Conference on Advanced Material Engineering (AME 2016) © 2016. The authors Published by Atlantis Press 923 Superabsorbent Synthesis In a flask were successively introduced 7.108 g (equivalent to 0.1 mol) of acrylamide, a certain amount of 2% (w/w) chestnut shell pigment solution (dissolved in 0.1 mol/L NaOH), a certain amount of N,N'-methylene bisacrylamide as a crosslinker and 10 mL NaOH solution with a concentration of 5 mol/L. After the solid matter was sufficiently dissolved, pure water was added to give a total volume of 45 mL. The flask was preheated in a water bath at 40 °C with a magnetic stirrer (200 RPM) for 5 min, followed by addition of a certain amount of ammonium persulfate as an initiator. The reaction was going on to form a gel until the magnetic stirrer cannot rotate. The flask was removed from the water bath and cooled to ambient temperature before the gel was chopped. A solution of 50 mL 1 mol/L NaOH was added, and the chopped gel was hydrolyzed at 90 °C for 1 h. After cooling to the ambient temperature, the gel was rinsed with 100 mL pure water 3 times, air-blast dried at 100 °C for 12 h, crushed, and screened to collect 20 to 40 mesh particles for absorbency measurement. Water Absorbency and Nickel Sorption Measurement The water absorbency was measured using a tea bag method [8], and the nickel sorption capacity was determined according to reference [9]. Experimental Design and Data Analysis The doses of the pigment, crosslinker, and initiator for maximizing the water absorbency were optimized using a central composite design and response surface methodology. The chosen independent variables used in this study were coded according to Eq. (1): xi=(Xi − Xi0)/ΔX (1) where xi is the coded value of the independent factor, Xi the real value of the independent factor, Xi0 the real value of the independent factor at the central point, and ΔX the step change value. The behavior of the system is explained by a quadratic equation, which is used to predict the optimal water absorbency (Y), based on the coded values of the independent factors (Xi):

Introduction
Superabsorbents are a kind of hydrophilic polymer that is able to absorb and retain huge amounts of water or aqueous solutions even under pressure. They are widely used in several fields, such as soil improvement, coal dewatering, water-holding afforestation, water-saving irrigation, water-blocking tapes, hygienic products, and drug delivery systems [1]. Melanins are a class of macromolecular pigments polymerized from phenolic and/or indolic substances [2]. They are rich in hydroxyl, carboxyl and amino hydrophilic functional groups. These groups readily bind a variety of chemical substances such as heavy metals and water [3], and endow detoxication and water-holding capabilities to the pigments [4][5]. Therefore, modification of traditional superabsorbent with melanin maybe gives some special properties to the superabsorbent. However, to our knowledge, there is no report on modifying polyacrylamide superabsorbent with melanin.
China is the biggest chestnut producer in the world, and chestnut plays very important roles in the solution of issues concerning agriculture, countryside and farmers in China, especially in hilly regions. Chestnut shell is a processing residue from the food industry and has not been utilized as yet. It contains about 15% of pigment, namely, chestnut shell pigment. We have intensively characterized the pigment on properties and structures. It was considered as allomelanin and found being rich in phenolic hydroxyl and carboxyl functional groups [6]. This study aimed to optimize the synthesis process of chestnut-shell-pigment-modified polyacrylamide superabsorbent by response surface methodology.

Materials
Chestnut fruits (Castanea mollissima) were purchased from a market in Kunming, China, and peeled manually. The chestnut shell was crushed to pass 4-mm screens and used to extract the pigment following the procedure described in our previous work [7]. All the reagents used were of analytical-reagent grade.
2nd Annual International Conference on Advanced Material Engineering (AME 2016)

Superabsorbent Synthesis
In a flask were successively introduced 7.108 g (equivalent to 0.1 mol) of acrylamide, a certain amount of 2% (w/w) chestnut shell pigment solution (dissolved in 0.1 mol/L NaOH), a certain amount of N,N'-methylene bisacrylamide as a crosslinker and 10 mL NaOH solution with a concentration of 5 mol/L. After the solid matter was sufficiently dissolved, pure water was added to give a total volume of 45 mL. The flask was preheated in a water bath at 40 °C with a magnetic stirrer (200 RPM) for 5 min, followed by addition of a certain amount of ammonium persulfate as an initiator. The reaction was going on to form a gel until the magnetic stirrer cannot rotate. The flask was removed from the water bath and cooled to ambient temperature before the gel was chopped. A solution of 50 mL 1 mol/L NaOH was added, and the chopped gel was hydrolyzed at 90 °C for 1 h. After cooling to the ambient temperature, the gel was rinsed with 100 mL pure water 3 times, air-blast dried at 100 °C for 12 h, crushed, and screened to collect 20 to 40 mesh particles for absorbency measurement.

Water Absorbency and Nickel Sorption Measurement
The water absorbency was measured using a tea bag method [8], and the nickel sorption capacity was determined according to reference [9].

Experimental Design and Data Analysis
The doses of the pigment, crosslinker, and initiator for maximizing the water absorbency were optimized using a central composite design and response surface methodology. The chosen independent variables used in this study were coded according to Eq. (1): where x i is the coded value of the independent factor, X i the real value of the independent factor, X i0 the real value of the independent factor at the central point, and ΔX the step change value. The behavior of the system is explained by a quadratic equation, which is used to predict the optimal water absorbency (Y), based on the coded values of the independent factors (X i ): where β 0 is the intercept term, β i the linear effect, β ii the squared effect, and β ij the interaction effect.
The Design Expert 8.0.6 (Stat-Ease Inc., USA) software was used for experiment design and data analysis. A three-factorial (2 3 ) design with six replicates at the central points and six star-points was employed to the quadratic model. The distance of the star points from the central design equals to ±1.682. Three independent factors (doses of the chestnut shell pigment, crosslinker, and initiator corresponding to the actual factors X 1 , X 2 , and X 3 , respectively) were studied at five levels, and twenty experiments were carried out randomly (Table 1).

Result
Response surface methodology can be used to evaluate effects of multiple parameters on response variables. To minimize the experimental runs for optimizing the synthesis process of the superabsorbent, a typical three-factor central composite design was adopted. The design matrix and the response values obtained from the experimental works are listed in Table 2. The water absorbency was in a range from 196.87 to 261.50 g/g. The standard numbers from 15 to 20 at the center point were used to determine the experimental error. The quadratic model was selected according to the suggestion of the software. The experimental results in Table 2 were fitted to a full quadratic equation by multiple regression analysis using the software. The parameters for this equation and their significance levels are shown in Table 3. The X 2 2 was insignificant in the full equation and deleted to get a reduced model in order to improve the fitting accuracy. The coefficient of multiple determination, R 2 , was found to be 0.9438, which means that the model could explain 94.38 % of the total variations in the system. The relatively high value of R 2 suggests that the equation well represented the empirical relationship between the response and the independents under the given experimental domain. In order to check the adequacy of the reduced model to represent the system further, an analysis of variance (ANOVA) was performed and the results are given in Table 3. The model and the lack-of-fit terms were significant and insignificant, respectively, supporting the applicability of the reduced model. Among the linear, quadratic, and interaction forms of independent variables, X 1 , X 2 , X 3 , X 1 ×X 2 , X 2 ×X 3 , X 1 2 , and X 3 2 were significant. Eq. (3), describing the relationship between the coded variables and the water absorbency, was derived from the reduced model. The equation is shown as follows: Three-dimension response surface and two-dimension contour plots were used to analyze the dependence of the water absorbency on the independent variables and to investigate the interaction between the independent variables by using the Design-Expert software. Fig. 1 is the response surface and contour plots of the effects of the doses of chestnut shell pigment and crosslinker on the water absorbency. The interaction between the doses of the chestnut shell pigment and crosslinker can be obviously found in Fig. 1. At a low pigment dose (e.g. 2 mL), the water absorbency decreased with the increase of the crosslinker dose, whereas at a high pigment dose (e.g. 4 mL), the water absorbency increased with the increase of the crosslinker dose. Vice versa, at a low crosslinker dose (e.g. 0.25 g), the water absorbency decreased with the increase of the pigment dose, whereas at a higher dose of the crosslinker (e.g. 0.35 g), the water absorbency increased with the increase of the pigment dose. This can be explained as follows. At the low pigment dose, the reaction was relatively overdosed with the crosslinker, and further increasing the crosslinker enhanced the crosslinking degree and produced the three-dimensional reticulation with small pore size, which provided less space for water binding due to steric hindrance. At the high pigment dose, the crosslinker is not enough to form a favorable crosslinking degree for water binding, therefore, increasing the crosslinker made the performance better. The water absorbance behaviors depending upon the pigment dose at different crosslinker doses can also be explained by using this theory.  The Design-Expert software is a powerful tool for optimizing experimental conditions and was used to maximize the water absorbency. The optimal synthesis conditions were found to be the pigment solution (X 1 ) of 4.00 mL, the crosslinker (X 2 ) of 0.35 g, and the initiator (X 3 ) of 0.16 g. The components in the system based on the percentage by weight (w/w) were acrylamide of 14.47%, the chestnut shell pigment of 0.16%, the crosslinker of 0.71%, the initiator of 0.32%, NaOH of 4.06%, and H 2 O of 80.28 %. Under this condition, the calculated water absorbency was 261.3g/g, which is very close to the experimental one, 264.2 g/g, further indicating the feasibility of using the response surface methodology with central composite design to predict the water absorbency of the chestnut-shell-pigment modified polyacrylamide superabsorbent.

Discussion
Superabsorbents are widely used in many fields such as agriculture, industries, and health care. Sometimes a multiple-functional product is a better option. For example, the seedlings planted in the heavy-metal-contaminated soil in arid or semi-arid regions suffer from the stress of the drought and the heavy metals. A superabsorbent with higher heavy-metal binding capacity is more suitable for application under this condition because it can solve the two problems simultaneously. Wang et al. [10] prepared an unmodified polyacrylamide superabsorbent with a water absorbency of 220 g/g and a nickel adsorption capacity of 1.31 mmol/g. The chestnut-shell-pigment-modified polyacrylamide synthesized in this study gave an absorbency of 264.2 g/g and a nickel adsorption capacity of 1.56 mmol/g, which is more efficient to bind water and nickel. The abundant hydroxyl and carboxyl groups in the molecules of the chestnut shell pigment have strong abilities to bind water through hydrogen bonds and heavy metal through chelation and ion-exchange [7,9]. The chestnut shell pigment comes from a biomass and is renewable, easy-available and non-toxic to human beings and environments. So using such a material to modify traditional polyacrylamide superabsorbent has a good prospect for applications.


extfieldsofstudy: Chemistry Medicine

text:
Indacenodibenzothiophenes: synthesis, optoelectronic properties and materials applications of molecules with strong antiaromatic character

Exchanging fused benzenes for benzothiophenes results in pronounced antiaromaticity in the indacene core.

Introduction
First predicted by Hückel in 1931 1 and later proposed by Breslow in 1967, 2 antiaromaticity describes conjugated cyclic systems containing 4n p electrons that are destabilized in comparison to a suitable reference compound. 3 Many groups have sought to further expand this concept, 3a,c,d,4 and the criteria for aromatic and antiaromatic compounds have been succinctly dened by Breslow,3b Krygowski, 3e and Wiberg. 4i In addition to containing 4n p-electron cyclic conjugation, antiaromatic compounds have decreased delocalization, smaller HOMO-LUMO energy gaps, and increased bond length alternation in comparison to aromatic molecules. 3b,e,4i Antiaromatic compounds also exhibit paratropic 1 H NMR chemical shis and large, positive nucleus independent chemical shi (NICS) values. 3b,4b,d,5 Although experimental examples of antiaromatic compounds are quite rare, they nonetheless have attracted the attention of chemists due to theoretical interest and potential materials science applications. 4e,5b,6 Over the last 20 years, chemists have focused on the rational design of organic electronic materials. 7 The synthesis and rigorous optical, electronic, and solid-state characterization of small molecule organic semiconductors has enhanced enormously our understanding of the fundamental principles necessary to design materials for high performing organic light emitting diodes (OLEDs), organic eld effect transistors (OFETs), organic photovoltaics (OPVs), or other organic electronic devices. 7e-k,8 Highly conjugated polycyclic hydrocarbons (CPHs) are a fascinating class of compounds oen utilized in organic electronics due to their desirable photophysical and electronic properties. 7a-d,7h-k,9 The study of these CPHs has resulted not only in functional materials, but also provided insight into fundamental chemical principles, such as the nature of the carbon-carbon bond, singlet-biradical character, and aromaticity and antiaromaticity, knowledge that in turn can be applied to designing better performing materials. 7a-d,h-k,9a,e,f,10 Although acenes are widely used in organic electronics, their susceptibility to oxidative and photolytic degradation has led researchers to explore alternative acene-like topologies. 7j,9d,11 Studies on acene-like structures reveal that decreasing the aromaticity within a ring system or the inclusion of antiaromatic rings into materials could greatly improve their electrical conductivity and increase charge mobilities in the solid state. 4e,5b,6,12 Recently, indenouorenes, with their 6-5-6-5-6 fused ring systems and overall 4n p electrons, have received particular interest as indeno [1,2-b]uorene (1) derivatives show ambipolar charge transport in both single-crystal and thin-lm OFETs (Fig. 1). 13 Since 2010, our group 14 and others 10g,i,13b,15 have investigated compounds based on the indenouorene (IF) scaffold (IFs 1-7 Fig. 1). 13a,16 The inclusion of two carbonaceous ve-membered rings imparts an intrinsic ability to accept electrons reversibly, 16i which in turn has led to the examination of IFs and their derivatives as potential ambipolar and n-type materials. Recently, we explored thieno-fusion on the IF skeleton and published initial reports on the synthesis and characterization of indacenodithiophene and indacenodibenzothiophene (anti-IDT 3a and syn-IDT 4, anti-IDBT 6a and syn-IDBT 7a), 16l diindenothienoacenes (DI[n]T 5a), 16k and their selenophene analogues (IDS 3b and DIS 5b) 16f (Fig. 1). Given the low-lying LUMO levels and small electrochemical energy gaps of IDBTs 6a and 7a, we were keen to further expand the chemistry of these compounds. We also sought to explore more promising crystal morphologies for the IDBTs by varying the substituents at the apical carbon of the ve-membered ring. Herein we report a combined synthetic, computational, structural, and materials study of anti-IDBTs 6 and syn-IDBTs 7. We describe in detail the pronounced antiaromaticity of the s-indacene core, the preparation of IDBTs 6b-f and 7b-f (Scheme 1) along with the improved synthesis of their respective precursors (IDBT diones , anti-indacenodibenzothiophene (anti-IDBT) 6a, and syn-indacenodibenzothiophene (syn-IDBT) 7a.
10 and 11), and nally the respective optical, electrochemical, solid-state, and materials properties of the new compounds.
Determination of the NICS value offers a means to assess the aromaticity or antiaromaticity of a ring system. 4a-d Negative NICS values indicate a diatropic ring current (aromatic), while positive NICS values indicate a paratropic ring current (antiaromatic). Although NICS values are useful for determining local ring currents, they are unable to expound upon global and semi-global ring currents. Recently, Stanger developed the NICS-XY scan, which can be used to explore these types of ring currents (global, semi-global, local, diatropic or paratropic) in at p-conjugated systems. 17 The NICS-XY scans of s-indacene, [1,2-b]IF 1 0 , anti-IDBT 6 0 and syn-IDBT 7 0 are shown in Fig. 4, along with those of the isoelectronic hydrocarbon analogues of the IDBTs, namely unknown indacenodinaphthalenes (IDNs) 8 0 and 9 0 . The B3LYP/ 6-311+G* NICS-XY values were taken 1.7Å above the molecular plane and employed the s-only model to take only p contributions into consideration. 17,18 The NICS-XY scan of s-indacene shows two strong (25.7 ppm) paratropic ring currents on the ve-membered rings (B rings) and a slightly smaller (23.1 ppm) paratropic ring current over the center six-membered ring (A ring). This NICS-XY scan suggests that s-indacene contains a global paratropic ring current as well as local paratropic ring currents over each of the rings with the current over the outer ve-membered B rings being the strongest. In contrast, the NICS-XY scan of 1 0 reveals two diatropic ring currents (À9.2 ppm) that are clearly visible over the outer most benzene rings (C ring), while the central s-indacene motif is weakly paratropic (largest NICS value of 7.4 ppm). This result further conrms our assertion that [1,2-b]IF 1 0 is two aromatic benzene rings fused to a weakly paratropic para-xylylene core.
The pronounced paratropicity of the s-indacene core of IDBTs 6 0 and 7 0 is noticeably evident from their respective NICS-XY scans. anti-IDBT 6 0 is slightly less paratropic (NICS values of 21.5 ppm on B ring and 18.0 ppm on A ring) than s-indacene while showing an overall similar NICS-XY scan. The NICS-XY scan of the syn-IDBT 7 0 core is nearly indistinguishable from that of the parent s-indacene. syn-IDBT 7 0 possesses NICS-XY values of 24.9 ppm and 21.9 ppm for the B ring and A ring, respectively, compared with 25.7 and 23.1 ppm for s-indacene. Both 6 0 and 7 0 have strong, nearly equal diatropic ring currents (À14.3 and À13.3 ppm, respectively) in the outer-most benzene rings (D ring), while the thiophene ring (C ring) of both 6 0 and 7 0 is clearly non-aromatic with NICS values of 0.07 and 0.11 ppm, respectively. In contrast, the NICS values for the s-indacene core in isoelectronic anti-IDN 8 0 and syn-IDN 9 0 are roughly half of the IDBT values-B ring and A ring values are 14.0 and 10.5 ppm for 8 0 and 12.9 and 10.6 ppm for 9 0 , respectively, reecting the competition between the paratropic and diatropic ring currents. The diatropic ring currents in the C and D rings of 8 0 and 9 0 (8 0 : À8.7/À14.1, 9 0 : À9.4/À14.9 ppm, respectively), however, are analogous to the NICS values predicted for the C ring of 1 0 and the D ring of 6 0 and 7 0 , as might be expected for the purely hydrocarbon naphthalene unit.

ACID calculations
The anisotropy of induced current density (ACID) method 19 is used to visualize the ring currents 20 for s-indacene, 1 0 , and 6 0 -9 0 at the TPSSh/SVP level of theory (Fig. 5). 21 According to the ACID calculations, the parent s-indacene exhibits strong antiaromatic character (shows a counter-clockwise ring current) (Fig. 5a). A critical isosurface value (CIV) can be assigned to weak points in  a cyclic system of delocalized electrons. ‡ 22 A high CIV represents strong conjugation, and a low CIV reects weak delocalization at a critical point. In the case of s-indacene, the isosurface ruptures at a CIV of 0.111 (see ESI † for all CIV data). In contrast, the ACID plot for [1,2-b]IF 1 0 exhibits a diamagnetic ring current (aromatic) in the outer benzene rings that seem to disturb the paramagnetic ring current of the indacene core (Fig. 5b). The isosurface in the indacene core ruptures earlier (CIV ¼ 0.071) than in that of s-indacene, further conrming that [1,2-b]IF 1 0 is weakly paratropic.  [1,2-b]IF 1 0 (blue), anti-IDBT 6 0 (red), syn-IDBT 7 0 (green), anti-IDN 8 0 (purple) and syn-IDN 9 0 (orange). The ACID plot of anti-IDBT 6 0 exhibits diamagnetic ring currents in the outer benzene rings and a strong paratropic ring current in the indacene core (Fig. 5c). Similarly, the ACID plot of syn-IDBT 7 0 also exhibits diamagnetic ring currents in the outer benzene rings and a strong paratropic ring current in the indacene core (Fig. 5d). The isosurface of the s-indacene core for 6 0 ruptures at a CIV of 0.084 and at a CIV of 0.099 for syn-IDBT 7 0 . The trend of CIVs nicely matches the trend seen in the NICS-XY scans, namely, the antiaromaticity of the s-indacene core increases from anti-IDBT 6 0 to syn-IDBT 7 0 with 7 0 being nearly as antiaromatic as the parent s-indacene. For IDNs 8 0 and 9 0 ( Fig. 5e and f) the CIV are 0.076 and 0.074, respectively, falling in-between the values of 1 0 and 6 0 /7 0 , analogous to the NICS-XY scans (see ESI † for more details).
The combined 1 H NMR data, NICS-XY scans, and ACID results paint a fairly clear picture of the antiaromatic nature of IDBTs 6 0 and 7 0 . The NICS-XY and ACID calculations also reconrm the strong paratropicity of s-indacene while further solidifying our argument that this same structural unit within [1,2-b]IF 1 0 be regarded as weakly paratropic. All three data sets show increasing antiaromaticity of the tricyclic core from [1,2-b]IF 1 0 to IDNs 8 0 and 9 0 , then to anti-IDBT 6 0 and syn-IDBT 7 0 with the parent sindacene showing the highest degree of paratropicity. Direct benzannelation of the indacene core (as in 1 0 ) strongly reduces the antiaromaticity of the indacene unit because the diatropic (clockwise) ring current of benzene encounters the paratropic (counter-clockwise) current of the indacene at the bond of annelation (and the benzene obviously dominates the conicting currents). This conict is mitigated somewhat in naphtho-fused IDNs 8 0 and 9 0 ; however, the thiophene rings in 6 0 and 7 0 essentially isolate the conicting ring currents, leaving the paratropic s-indacene current almost intact. Given these results, we believe that IDBTs 6 0 and 7 0 are best described as substituted s-indacenes with the appended phenyl groups planarized by thioether linkages (Fig. 2, 4). Although not a signicant difference, calculations at the B3LYP/cc-pVDZ level of theory indicate that 6 0 is more stable than 7 0 by 1.29 kcal mol À1 .

Synthesis
Encouraged by the computational studies, we set out to prepare new derivatives of 6 and 7. The typical strategy for the synthesis of IFs and their related congeners is addition of a nucleophile to the corresponding dione followed by a SnCl 2 -mediated reductive dearomatization. Our initial report on the assembly of diones 10 and 11 utilized a Stille cross-coupling to construct key precursors 12 and 13, respectively. 16l We have improved the preparation of diesters 12 and 13 by employing a Suzuki-Miyaura cross-coupling (Scheme 1), allowing us to selectively synthesize diones 10 or 11 on gram scale from commercially available starting materials without the use of column chromatography. Concurrent work on a separate project suggested that performing the nal SnCl 2 -mediated reduction under rigorous anhydrous and anaerobic conditions could induce a signicant increase in product yield. As a prototypical example, the synthesis of compound 6f proceeds in 12% yield when using N 2 -sparged toluene as received, whereas performing the nal reduction under rigorous anaerobic and anhydrous conditions furnishes 6f in 59% yield. This strategy was extended to a series of diaryl-and diethynyl-substituted IDBTs possessing electron-withdrawing or donating groups (6a-f, 7a-f). § The reduction for electron-withdrawing arenes such as (6d, e and 7d, e) was sluggish and required the addition of a small amount of triuoroacetic acid for the reaction to proceed smoothly.
Optoelectronic properties Fig. 6 shows the electronic absorption spectra for IDBTs 6a-f and 7a-f. These data, along with the experimentally determined electrochemical data, are summarized in Table 1. anti-IDBTs 6a-f have a major absorption centered around 375 nm (3.31 eV) while syn-IDBTs 7a-f exhibit a major absorption from 330 to 300 nm (3.76 to 4.13 eV). As with the previously reported mesityl derivatives 6a and 7a, 6b-f and 7b-f show low energy absorptions ranging from 683 to 643 nm (1.82 to 1.93 eV). Timedependent density functional theory (TDDFT) calculations at the B3LYP/cc-pVDZ level of theory show that these S 0 / S 2 oneelectron excitations are predominantly HOMO / LUMO or HOMOÀ1 / LUMO transitions, depending on the nature of the substituent appended to the IDBT backbone (see ESI † for more details).
Interestingly, there are differences in the low energy absorptions of the anti-IDBTs compared to the syn-IDBTs. The anti-IDBTs (6b-f) have a l max ranging from 643-682 nm (39 nm [0.11 eV] range) whereas the syn-IDBTs (7b-f) have a l max ranging from 665 to 683 nm (18 nm [0.04 eV] range). This difference is reected in the onset of absorbance for the two isomers: the anti isomers have an onset of absorption ranging from 678 to 709 nm (31 nm [0.07 eV] range) while the syn isomers have an onset of absorption ranging from 712 to 722 nm (10 nm [0.03 eV] range). From the computational data, however, one cannot ascertain 'more variability' in one IDBT family versus the other. For the computed S 0 / S 1 excitation energies, the range for IDBTs 6a-f is 0.27 eV while that in IDBTs 7a-f is 0.25; a similar story unfolds for the adiabatic ionization potentials (see below), where the range for IDBTs 6a-f is 0.60 eV while that for IDBTs 7a-f is 0.58 eV. Moreover, the trends in the S 0 / S 1 transition energies and adiabatic ionization potentials are nearly identical for both families.
As observed with all other reported IFs, IDBTs 6a-f and 7a-f are non-emissive, a fact consistent with the lack of oscillator strength (f) determined for the S 0 / S 1 transitions in all TDDFT calculations on the IDBTs. 23 The S 0 / S 1 excitations in the IDBTs consist of a one-electron transition between orbitals of (nearly) the same symmetry (b g , based on the C 2h symmetry of 6 0 and 7 0 , see below), making the excitations orbitally forbidden.
As was seen in our study of mesityl-substituted [1,2-b]IF 1, both 6a and 7a are hypsochromically shied (ca. $40 nm) from 6b-f and 7b-f. This is easily explained by examination of the crystal structures of 7a vs. 7b. The mesityl groups of 6a and 7a are nearly orthogonal to the core of the molecule with a dihedral angle between the average planes of the aryl group and IDBT core of 75.0 and 59.6 , respectively, whereas 7b has dihedral angles of 33.2 and 29.3 . The large dihedral angle of the mesityl group to the IDBT core in 6a and 7a limits electronic communication between the orthogonal p systems, effectively limiting the delocalization of the p orbitals of 6a and 7a when compared to the other aryl substituents (see the discussion of the molecular orbitals below for more details).   Fig. 7 CV data of anti-IDBTs 6b, 6e, and 6f (left) and syn-IDBT 7b, 7e, and 7f (right).

Electrochemistry
All IDBTs undergo one reversible reduction in solution. A second reduction is observed for all of the IDBTs; however, its reversibility is dependent upon the aryl or ethynyl substituent. IDBTs 6a-f and 7a-f all display an oxidation wave ranging from irreversible to reversible depending on the aryl or ethynyl substituent. Cyclic voltammograms (CVs) of 6b, e, f and 7b, e, f are displayed in Fig. 7 (see the ESI † for CVs of 6a, c, d and 7a, c, d). All electrochemical data are compiled in Table 1. E 1 red values range from À0.80 to À0.16 V vs. SCE (6a and 7e, respectively) and E 1 ox values range from 0.92 to 1.36 V vs. SCE (6a and 6e, respectively). Electron-withdrawing groups shi E 1 red and E 1 ox to more positive values while electron-donating groups shi E 1 red and E 1 ox to more negative values. Once again, interesting electronic differences between the syn-and anti-isomers are observed. The CV determined E 1 red values for the syn-IDBTs are consistently more energetically stabilized when compared to their anti-IDBT counterparts, and the electrochemically determined E gap (E 1 ox À E 1 red ) is consistently smaller for the syn-IDBTs. The E 1 red values of the diaryl anti-IDBTs (6a-f) have an average E 1 red 0.15 eV higher than their syn-IDBT (7a-f) counterparts.{ As with the optical transitions, the computed ionization potentials (IP) and electron affinities (EA) at the B3LYP/cc-PVDZ level of theory follow the experimental trends quite well (see ESI †), with the syn-IDBTs generally possessing larger EAs (and more energetically stabilized LUMO energies) than the anti-IDBT counterparts with the same chemical functionalization.
Analysis of the frontier molecular orbitals provides insight behind the observed optoelectronic characteristics. Select frontier molecular orbitals for 6 0 and 7 0 , each with C 2h symmetry, are given in the ESI. † For each system, the splitting between the HOMOÀ1 (a u symmetry) and HOMO (b g symmetry) is rather small, 0.13 eV for 6 0 and 0.26 eV for 7 0 , with the HOMO energies for 6 0 and 7 0 nearly identical. The LUMO of 7 0 , however, is 0.13 eV more energetically stable than that of 6 0 (both LUMOs have b g symmetry), a result that already provides insight into the larger EAs/smaller reduction potentials observed for the syn-IDBT series. In both cases, the HOMO and LUMO mainly reside on the carbon framework, with the sulfur playing a larger role in the LUMO of 6 0 and the HOMO of 7 0 . Notably, the various chemical substituents, depending on the relative donor or acceptor strength (and conguration of the phenyl group), impart changes to the molecular orbitals. First, the energetic gap between the HOMO and HOMOÀ1 is reduced across the full molecular series with respect to the parent 6 0 and 7 0 species, with the donor substituents in 6b, 6c, and 7b leading to an energetic repositioning of the HOMO and HOMOÀ1, i.e., the HOMO is the a u symmetric orbital in these systems. It may be expected that the donor strength of the mesityl substituents in 6a and 7a could also lead to such an inversion; however, the lack of extension of the HOMO or HOMOÀ1 onto the phenyl rings, due to the orthogonal orientation of the mesityl units, prevents this from occurring and the frontier orbitals simply become energetically destabilized when compared to 6 0 and 7 0 through inductive effects. The reordering of the orbitals leads to the varying descriptions of the S 0 / S 1 and S 0 / S 2 described above. A combination of wave function delocalization on the phenyl rings and inductive effects due to the uorine atoms in 6d/7d and 6e/7e lead to large energetic stabilization of the frontier orbitals, and subsequently the larger IPs/smaller oxidation potentials and larger EAs/smaller reduction potentials of these systems versus the other phenyl substituents. The picture differs somewhat when comparing 6f and 7f: in both cases, the LUMO extends onto the ethynyl arm, energetically stabilizing the LUMO when compared to the parent anti-and syn-IDBT systems. In 6f, however, the HOMO and HOMOÀ1 switch their order and the a u parent orbital extends onto the ethynyl arm, destabilizing the HOMO, while this does not appear to be the case in 7f (though the energetic splitting between the HOMO and HOMOÀ1 is quite small, 0.02 eV). Overall, the molecular orbitals reveal the rather complex interplay between the base anti-and syn-IDBT structures and the nature of the chemical substituents that in turn determine the relative electrochemical and optical characteristics observed experimentally.

Solid-state structures
Single crystals of 6f and 7b suitable for X-ray diffraction (XRD) were obtained via the slow diffusion of CH 3 CN into CHCl 3 , while single crystals of 6d and 7d were grown via the slow evaporation of a concentrated solution in chlorobenzene at À40 C. The molecular structures of 6d, f and 7b, d are shown in Fig. 8. As expected, varying the substituents of the IDBT core greatly inuences the observed packing motif (Fig. 9). The solidstate morphologies of both 6d and 7d appear to be largely driven by arene/peruoroarene interactions ( Fig. 9a and b). anti-IDBT 6d packs in 1-D chains with a distance between average planes of the IDBT core of 3.48Å. The peruorophenyl groups of one 1-D chain are centered over the aromatic aryl group (D ring) of the neighboring chain. The distance between the center of the peruorophenyl ring and center of the D ring is 3.76Å, which is in good agreement with the center-to-center distances (3.4-3.8Å) observed for other 1 : 1 arene/peruoroarene crystal structures. 24 The dihedral angle between the plane of the IDBT core and the plane of the peruorophenyl ring of 6d is 54.9 , signicantly smaller than the dihedral angle between the average plane of the mesityl group and IDBT core of 6a (75.0 ). The molecular structure of syn-IDBT 7d contains three symmetrically independent molecules with two of these forming a 1-D chain. The distance between the average planes of the 1-D chain is 3.57Å. The third molecule bridges the 1-D chains with the center of the peruorophenyl rings of the bridging molecule 3.51-3.74Å from the center of the D ring for the molecules of the 1-D chain. The dihedral angle between the average plane of the peruorophenyl ring and the plane of the IDBT core ranges from 44.0-47.5 . This dihedral angle is signicantly smaller than the same angle observed in 6d and can be explained by a weak S/F interaction. 25 The distance between the sulfur of the thiophene ring (C ring) and the nearest uorine of the peruorophenyl ring ranges from 2.98-3.03Å, well below the S-F van der Waals radius of 3.27Å.
syn-IDBT 7b appears to pack on the cusp between a 1-D and 2-D structure (1-D chain of.B-A-B 0 .) with distances of 3.49 and 3.42Å between the average planes (B-A and A-B 0 , respectively) (Fig. 9c). The close C-S contacts between 1-D columns are 3.60 and 3.51Å, while the close C-C contacts are 3.55 and 3.45Å (B-A and A-B 0 , respectively). The centroids of the IDBT cores are Fig. 9 Solid state crystal packing diagrams of (a) anti-IDBT 6d, (b) syn-IDBT 7d, (c) syn-IDBT 7b, and (d) anti-IDBT 6f. Hydrogen atoms omitted for clarity. Ellipsoids drawn at 50% probability level. offset by 3.11 and 4.67Å (B-A and A-B 0 ). Interestingly, there appear to be multiple S-S interactions between the 1-D chains with S-S distances of 3.48, 3.65 and 3.68Å (S-S van der Walls radius is 3.60Å).
anti-IDBT 6f packs in 1-D slip-stacked columns with a distance between the average planes of the IDBT core of 3.36Å and short C-C contacts of 3.36Å (Fig. 9d). There is a large amount of molecular overlap in the packing of 6f with the center of the outer benzene ring shied only 1.15Å from the center of the central s-indacene ring. Given the large overlap and sub-van der Waals C-C contacts in 6f, we elected to focus on this compound for potential device fabrication.

Electronic band structure
As an initial evaluation, the intermolecular electronic couplings (t) and electronic band structure of 6f reveal the potential for hole and/or electron transport (the band structures for 6d and 7d are in the ESI †). Using the fragment orbital approach 26 at the B3LYP/cc-pVDZ level of theory, the HOMO:HOMO electronic coupling (for hole transport) of the p-stacked dimer is found to be 50 meV, while the LUMO:LUMO electronic coupling (for electron transport) is twice as large at 100 meV. These electronic couplings are on par with those of other high performing organic semiconductor materials. Examination of the electronic band structure along various crystallographic directions of the triclinic (TRI 1a ) lattice (Fig. 10), using the experimental unit cell and atomic coordinates, reveals that the valence band dispersion is 78 meV, while the conduction band dispersion is 383 meV; relaxation of the atomic coordinates leads to a decrease of the band gap, while the band dispersions remain unaffected (see the ESI † for further details). Note that the valence band maximum is at G, while the conduction band minimum is at X; therefore, the band gap is indirect, but only because the conduction band following G-X is slightly sloped downwards (the G-G band gap is marginally larger). Overall, the magnitudes of the electronic couplings (and band widths) suggest that 6f could possess reasonable hole and electron transport characteristics.

Organic eld effect transistors
Top-gate, bottom-contact OFETs of 6f were fabricated on SiO 2 substrates. Source and drain contacts were patterned by photolithography, with 5 nm of Ti and 40 nm Au deposited by ebeam evaporation. anti-IDBT 6f (1 wt% in room-temperature chlorobenzene) was then spin-coated onto this substrate. Undiluted Cytop uoropolymer was used as a gate dielectric and spin-coated over the organic semiconductor. Our earlier work has shown that these processing conditions yield a dielectric thickness of 1400 nm. 27 The OFETs were subsequently annealed at 110 C for one hour in a vacuum oven to cross-link the dielectric layer and le to cool overnight. A 60 nm layer of Au was thermally evaporated through a shadow mask aligned over the conduction channel, and this served as the gate electrode. The resulting OFETs were measured under ambient conditions, and an example for current-voltage characteristics is included in Fig. 11. We calculated the eld-effect mobility m from the saturation regime of device operation, at an applied drain-tosource voltage of V DS ¼ À40 V by standard procedures. 28 Fig. 11a shows the evolution of the drain current I D with the applied gate-to-source voltage V GS for this particular device of channel length L ¼ 20 mm and channel width W ¼ 35 mm. This device exhibits a hole mobility of m ¼ 0.44 cm 2 V À1 s À1 , and a threshold voltage of V th ¼ 1.33 V. Measurements over 10 devices yielded an average hole mobility of m avg ¼ 0.14 cm 2 V À1 s À1 AE 0.12 cm 2 V À1 s À1 . The low threshold voltage indicates a low density of trap states at the semiconductor-dielectric interface, which is to be expected for the transistors with Cytop gate dielectric. 27 Fig. 11b shows the transport characteristics for the same device. The transition from linear to saturation regime is clear for all investigated gate voltages. The non-linearity in the low-voltage region may originate from parasitic contact effects. 29 These contact effects could be minimized by using surface treatments on the contacts which could also increase the measured mobility of devices; therefore, the values quoted for charge carrier mobility should be seen as a lower bound. The electrical measurements show hole-only charge transport. The lack of a measured electron transport likely originates from the fact that Au contacts were used, and while the $5 eV work function of Au aligns well with the estimated solid-state IP of 6f, the energy barrier to inject electrons is too large (see Table 1). 30

Conclusions
In summary, our analysis of the NICS-XY scans and ACID plots of IDBTs 6 0 and 7 0 indicate a strong antiaromatic ring current in the central s-indacene core of these molecules, and that IDBTs 6 and 7 are best thought as phenyl-substituted s-indacene derivatives with thioether linkages planarizing the appended phenyl groups to the indacene core. We have improved the synthesis of dione precursors 10 and 11 to yield multigram quantities without the use of column chromatography and demonstrated that up to 500 mg (6f) of the nal IDBT can be produced in one batch. We have undertaken a detailed experimental and computational analysis of IDBT 6a-f and 7a-f and demonstrated that the optoelectronic, electrochemical characteristics and the solid-state morphology can be signicantly altered by the choice of aryl/ethynyl substitution. Analysis of the intermolecular electronic couplings and electronic band structure of 6f indicated that it could perform well as the active element in an OFET device. Fabrication of a device with 6f produced the highest hole mobilities recorded to date using a fully conjugated indenouorene derivative. Future work will focus on further exploration of the antiaromatic properties of IDBTs 6 and 7 as well as benzothiophene fusion as a means to control aromaticity/antiaromaticity in other IF related structures. dation under grant ECCS-1254757. C. R. thanks the University of Kentucky Vice President for Research for start-up funds, and Dr Gjergji Sini for fruitful discussions. K. U. was partly supported by the Osaka University Scholarship for Short-term Overseas Research Activities 2013. HRMS were obtained at the Mass Spectrometry Facilities and Services Core of the Environmental Health Sciences Center, Oregon State University, supported by grant #L30-CS00210, National Institute of Environmental Health Sciences, National Institutes of Health.
Notes and references ‡ It should be noted that NICS-XY values and ACID plot CIV values for antiaromatic compounds are exaggerated and should be treated with some caution. The results presented in this paper should not be seen as a denitive value of antiaromaticity, but rather as a method for comparison between similar systems. § In our hands, isolation of 7f proved to be highly problematic as we were unable to obtain more than 1-2 mg of moderately pure material. Further attempts at purication resulted in complete degradation of the material and multiple attempts were necessary to produce the data provided in this manuscript. { For a discussion on the differences between optically determined, electrochemically-determined and computationally determined HOMO/LUMO levels, energy gaps, and their terminology, please see: J.-L. Bredas, Mater. Horiz. 2014, 1, 17.


extfieldsofstudy:
Materials Science
Medicine

text:
Atomic scale observation of oxygen delivery during silver–oxygen nanoparticle catalysed oxidation of carbon nanotubes

To probe the nature of metal-catalysed processes and to design better metal-based catalysts, atomic scale understanding of catalytic processes is highly desirable. Here we use aberration-corrected environmental transmission electron microscopy to investigate the atomic scale processes of silver-based nanoparticles, which catalyse the oxidation of multi-wall carbon nanotubes. A direct semi-quantitative estimate of the oxidized carbon atoms by silver-based nanoparticles is achieved. A mechanism similar to the Mars–van Krevelen process is invoked to explain the catalytic oxidation process. Theoretical calculations, together with the experimental data, suggest that the oxygen molecules dissociate on the surface of silver nanoparticles and diffuse through the silver nanoparticles to reach the silver/carbon interfaces and subsequently oxidize the carbon. The lattice distortion caused by oxygen concentration gradient within the silver nanoparticles provides the direct evidence for oxygen diffusion. Such direct observation of atomic scale dynamics provides an important general methodology for investigations of catalytic processes.

H eterogeneous catalysis plays a major role in the production of energy and chemicals as well as in pollution control 1 . Metal and metal-based nanoparticles (NPs), which are frequently used as active components for catalytic reactions, have captured broad attention in the past decades, especially for their potential applications in solving environmental problems [2][3][4] . Development of highly efficient oxidation catalysts at low temperatures is crucial to dramatically reduce the emissions of particulates (mostly soot particles) into the environment 5 . Among many NP-based catalysts, Ag NPs hold many advantages due to their high reactivity and relatively low cost 6,7 . As demonstrated recently, Ag/Ag 2 O NPs possess lower activation energy [8][9][10][11] and are assumed to be ideal catalysts for oxidizing carbonaceous materials. An interesting phenomenon of silver NP channelling graphene has been revealed which shows that graphene edges can dictate the morphology of NPs 12 .
Carbon nanotube (CNT) 13 , a very important member of the family of carbonaceous materials, has been used as a model material for studying the oxidation processes of carbon-based materials [14][15][16] because of its excellent mechanical and electrical properties and wide range of potential applications [17][18][19] . Previous studies proposed an oxidation reaction model: the oxidation first occurs on the outside layer of a pristine CNT and the initiation temperature of the CNT oxidation depends on the wall thickness of the CNTs 20 . However, the atomic scale understanding of the intrinsic nature of active sites is still elusive. For example, how do the oxygen molecules dissociate and how do they transport to the CNTs? It is important to understand the relationships among the atomic structures of Ag NPs, the atomic scale dynamics and the catalytic oxidation processes in order to develop better supported metal catalysts for desired catalytic properties.
In this communication, an aberration-corrected environmental transmission electron microscopy (AC-ETEM) is used to directly record the processes of silver-based NPs catalysed oxidation of multi-wall CNTs (MW-CNTs). The number of oxidized carbon atoms is directly estimated with a semi-quantitative method and thus the CO x (including CO and CO 2 ) formation rate, which is equivalent to the turnover frequency (TOF), is obtained. Theoretical calculations,, together with the direct AC-ETEM observation, suggest that a mechanism similar to the Mars-van Krevelen process dominates the oxidation process. The contact interfaces between the Ag/AgO x NPs and the CNTs have the lowest oxygen concentration, while the exposed surfaces of the silver NPs possess the highest oxygen concentration. Such an oxygen gradient within the NPs provides the driving force to sustain the delivery of the oxygen atoms to the contact interfaces, between the Ag/AgO x NPs and the CNTs, where the carbon atoms can be oxidized. The continuous evolution of the NPs, the Ag/CNT interfaces, and the oxidation of the CNTs are monitored and tracked. To the best of our knowledge, this is the first atomic scale observation of the oxidation of carbon materials by metal NPs. On the basis of the experimental observation, we propose that the oxygen molecules dissociate at the particle surfaces and the dissociated oxygen atoms diffuse through the NPs to reach the Ag/CNT interfaces and subsequently oxidize the CNTs. It is expected that such an approach to study catalytic reactions is general and may be extended to investigations of other important catalytic reactions.

Results
Sample preparation. The CNTs, prepared via a chemical vapour deposition method, were suspended in ethyl alcohol and a drop of the solution was then transferred to a molybdenum TEM grid coated with a thin holey-carbon film. The FEI Titan G2 AC-ETEM with a monochromator was employed to carry out the oxidation experiments. The environmental chamber can allow gas pressures up to B20 mbar (ref. 21) (for more details, please visit http://www.fei.com). To acquire high-quality images, the TEM grid (without exposure to electron beam) was heated to 450°C in 2 mbar O 2 for 45 min inside the ETEM to remove the contaminants on the surfaces of the CNTs. Many rounds of experiments confirmed that such a treatment does not oxidize the multi-wall CNTs (Supplementary Fig. 1 and Supplementary Note 1) and the contaminants were removed from the surfaces of the CNTs. To minimize the electron beam effects on the CNTs, all the experiments were conducted at 80 kV (ref. 22). A Gatan Inconel heating holder was used to control the temperature of the sample treatment and the catalytic oxidation reaction. All the images were recorded by a Gatan UltraScan CCD camera.
The pre-treated and clean CNTs were then loaded with AgNO 3 aqueous solution (Supplementary Note 2). The dried TEM grid was then put into the microscope chamber and 2 mbar of H 2 was allowed into the microscope chamber to reduce the sample at 250°C for 2 h. During the reduction process, metallic Ag NPs were formed ( Supplementary Figs 2 and 3). Some of the Ag-containing NPs attached to the CNTs were only partially reduced (Supplementary Note 3). After this treatment, various sizes of silver NPs were finely dispersed onto the surfaces of the CNTs. During the sample synthesis and treatment processes the electron beam was always blanked. Our experiences with ETEM showed that the high-energy electrons inside an electron microscope ionize oxygen molecules to generate oxygen radicals, which can easily oxidize carbonaceous materials even at ambient temperatures. Because of this limitation, we could not continuously observe the dynamic processes of the catalytic oxidation reaction with the electron beam on. We did, however, observe the same region, with atomic scale resolution, repeatedly for many cycles of the oxidation reaction by pumping out the oxygen for observation after each cycle of the oxidation reaction. In a strict sense, our observations were not exactly in situ due to the limitation by the electron beam-induced ionization of oxygen molecules. In this paper, we will denote such an experiment as quasi-in situ, since we tracked the same Ag/CNT interface for many oxidation cycles. Previous studies suggest that the initial oxidation of CNTs by Ag NPs starts at B200°C, much lower than the initial oxidation temperature of CNTs without the use of Ag NPs as catalysts 3,5,[23][24][25] . Therefore, a reaction temperature of 250°C was chosen for the catalytic oxidation of our MW-CNTs.
Quasi-in situ oxidation experiments. The oxidation reaction was conducted inside the AC-ETEM at 250°C for several cycles to examine the atomic scale dynamic oxidation process. After each cycle of the oxidation reaction, the residual oxygen inside the microscope chamber was thoroughly purged out and the sample temperature was maintained at B250°C. After purging and evacuation, the chamber vacuum pressure was o2 Â 10 À 6 mbar and the electron beam was de-blanked to record images of the marked regions of interest. Figure 1a shows AC-ETEM images of CNTs after the sample treatment process revealing Ag NPs, with an average size of B5 nm in diameter, decorating the surfaces of the CNTs. This initial examination served as a baseline for further monitoring of the structural evolutions of the Ag/CNT system. Several Ag NPs on the CNTs were selected as regions of interest and were monitored after each reaction cycle. After appropriate images were taken, the electron beam was blanked and the sample was oxidized in 2 mbar O 2 at 250°C for 25 min. The regions of interest, before and after the oxidation reaction, were recorded at both low and high magnifications. Figure 1b shows the image, after 25 min oxidation reaction, of the same area as shown in . During the oxidation process, the Ag NPs attached to the external surfaces of the CNTs either cut through or drilled holes into the CNTs, which can be used to improve the solubility of the CNTs 26 .
Since the dynamic evolution of the oxidation reaction could not be directly observed because of the effects of electron beaminduced ionization of oxygen molecules (Supplementary Fig. 9 and Supplementary Note 8), a more elaborative set of experiments were conducted to extract the atomic scale information of the oxidation processes. Figure 2 shows a series of atomic resolution AC-ETEM images demonstrating clearly the changes of the atomic structure caused by the oxidation reaction at 250°C. By maintaining similar imaging conditions, the different phase-contrast AC-TEM images can be directly compared. Figure 2a shows a faceted Ag NP and the atomic layers of the MW-CNT before the oxidation reaction. However, after careful analysis of the lattice fringes of the 'Ag' NP in Fig. 2a, we concluded that it does not fit either the Ag metal or any existing phases of silver oxides (Supplementary Tables 1 and 2 and Supplementary Note 9), it is reasonable to assume that such ultrasmall size Ag NPs can be oxidized easily. In another word, small Ag NPs are difficult to be reduced during the sample preparation process. Electron energy loss spectroscopy (EELS) results also show the existence of oxygen as shown in Supplementary Fig. 10 (see Supplementary Note 10 for more details). So, we can label these NPs as an AgO x NP and the oxygen content within the NPs will be discussed later. The focus-dependence phase contrast of the AC-TEM images can be utilized to help locating the edges of a b After the first oxidation cycle (2 mbar O 2 at 250°C for the first 300 s), the outmost wall of the MW-CNT, which was in direct contact with the perimeter of the AgO x NP, was oxidized, resulting in desorption of the carbon atoms as CO and/or CO 2 molecules. At the end of the second oxidation cycle (Fig. 2c, 2 mbar O 2 at 250°C for the second 300 s), the AgO x NP had catalysed the oxidation of two walls of the MW-CNT. Figure 2d shows the same sample area after the third cycle of oxidation (2 mbar O 2 at 250°C for the third 300 s), clearly revealing the removal of four layers of the graphitic carbon and the advancement of the AgO x NP into the MW-CNT. After another 900 s oxidation reaction (2 mbar O 2 at 250°C for another 900 s), the AgO x NP almost sank entirely into the interior of the MW-CNT and all the seven layers of the MW-CNT which had been crossed by the AgO x NP were completely oxidized. Furthermore, by comparing Fig. 2a with Fig. 2e, a B30°rotation of the NP, along the direction indicated by the curved red arrow (Fig. 2a) had occurred due to the asymmetric shape of the NP and the fact that carbon oxidation occurs only if the carbon atoms are in contact with the Ag atoms, the forward advancement of the AgO x NP depends on the particle's shape and its dynamic interaction with the CNT. After the oxidation and the desorption of the carbon atoms, the side of the NP with larger contact area sank into the CNT faster than the side with smaller contact area, resulting in a torque force on the NP. Such a rotational precession of metal NP catalysts is of interest and may provide useful information on the nature of catalysed oxidation reactions. The tortuous track, created by AgO x due to the oxidation of the CNT layers further suggests that the shape of the AgO x NP controls its progression during the oxidation processes of the CNTs (Supplementary Fig. 12 and Supplementary Note 12).
To probe the nature of the oxidation mechanism, the structural change of the AgO x NP was carefully analysed after each oxidation cycle. Figures 3a-e shows the enlarged image taken from Fig. 2a-e. A set of 10 interplanar spacings (as marked by the blue and pink arrows in Fig. 3a-e), d, were carefully measured from the outside of the NP (that is, the side farthest away from the NP/CNT contact interface indicated by the blue double heads arrow), labelled as d o , and the inner side (that is, the side closest to the NP/CNT contact interface, indicated by the pink doublehead arrow), labelled as d I , are shown in Fig. 3f-j. The d o and d I are, respectively, 2.53 and 2.53 nm in Fig. 3f; 2.55 and 2.53 nm in Fig. 3g; 2.60 and 2.50 nm in Fig. 3h; 2.57 and 2.50 nm in Fig. 3i; and 2.56 and 2.50 nm in Fig. 3j. The largest relative increase in lattice spacing of B4% between d o and d I was demonstrated in Fig. 3h. The interplanar spacing of 0.35 nm of the CNTs was adapted as the internal calibration reference (see Supplementary  Fig. 11a). We did not use the value of 0.34 nm (ref. 20) as the standard since the size effect 27 may play a roll as well. Regardless of the absolute value of the lattice spacing of the CNTs, we just used it as an internal reference. Since we measured many spacings and repeated the measurement for each oxidation cycle, it was estimated that the error in determining the lattice spacing should be o0.7%, far smaller than the measured lattice spacing differences between d o and d I . The EELS analyses of the NPs suggest the existence of O within these particles. Furthermore, according to the first-principles calculations (Supplementary Fig. 13 and Supplementary Note 13), the lattice spacing of the Ag-based FCC lattice can be influenced by the content of O and a B10% lattice expansion can be expected for an oxygen content of B20%. The presence of different lattice values within a single NP, as measured experimentally, can be attributed to the different local oxygen concentration within the NP. This correlation is reasonable provided that O 2 molecules adsorb and dissociate on Ag which will be discussed later. By integrating all these experimental observations with theoretical calculations, we propose a working mechanism for the Ag NP catalysed oxidation of the CNTs. The oxygen atoms, originated from the dissociative adsorption of the gas phase oxygen molecules on the exposed surfaces of the Ag NP, diffuse into the Ag NP at moderate temperatures. The oxidation of the CNT carbon atoms requires the availability of the atomic oxygen at the NP/CNT contact interface, which is provided by the atomic oxygen within the Ag NP. During the oxidation process, the atomic oxygen at the NP/CNT contact interface is used to oxidize carbon and evaporate as CO x ; while on the exposed surfaces of the NP, a constant supply of oxygen molecules is provided which dissociate and diffuse into the NP. Therefore a dynamic equilibrium of the oxygen concentration within the NP is established: Higher oxygen content near the exposed surfaces of the NP and lower oxygen content at the Ag/CNT interface, resulting in a gradient of atomic oxygen within the NP. Therefore, during the catalytic oxidation of the CNTs, the AgO x NPs act as a media to convert gas phase molecular oxygen into atomic oxygen and then supply them to the NP/CNT contact interfaces to oxidize the CNTs. In addition to the example demonstrated in Fig. 3, other examples were shown in the Supplementary Fig. 14 and Supplementary Note 14.
The amount of the oxidized carbon atoms can be estimated from the AC-ETEM images provided that we use a reasonable model for the shape of the NPs. As shown in Fig. 2b, after the first oxidation cycle, the contact region between the NP and the outmost CNT wall was oxidized, the number of removed carbon atoms can be estimated from a simple model (Supplementary Fig. 15 and Supplementary Fig. 16, see more details in Supplementary Note 15 and Supplementary Note 16). If we assume that each layer of the CNT can be taken as a curved graphene layer with a planar density of B38.17 atoms per nm ( Supplementary Fig. 15) 3,28 , then the loss of the number of carbon atoms (due to formation of CO and/or CO 2 ) can be estimated as shown in Supplementary Table 3. The cumulative number of carbon atoms removed from the individual MW-CNT (Fig. 2) was quantitatively estimated to be 261, 275, 373 and 1,570 after the first oxidation cycle (300 s), the second cycle (total of 600 s), the third cycle (total of 900 s) and the fourth cycle (total of 1,800 s), respectively. The specific rates of the carbon oxidation by the individual Ag NP were estimated to be 0.87, 0.92, 1.24 and 1.74 carbon atoms per second, respectively (red line in Fig. 4). The increase of the specific rate with reaction time is a reflection of the increase in the Ag/CNT contact area when the Ag NP advanced during the oxidation process. If we assume that each surface Ag atom is catalytically active for the oxidation of the carbon then the TOF of the Ag active sites can be deduced (blue line in Fig. 4). Since the nature of active sites did not change after each oxidation cycle, the TOF should remain constant which was estimated to be B0.165 nm À 2 s À 1 (Ag) (dashed blue line in Fig. 4).

Discussion
With the help of first-principles simulations, more insights into such silver-based NP catalysed oxidation processes can be acquired. According to the density functional theory (DFT) calculations ( Supplementary Figs 17 and 18, and Supplementary Note 17), silver can readily reduce the energy barrier (B0.36 eV) in O 2 dissociation 29 . Nevertheless, O 2 dissociation is strongly endothermic on graphene or CNT 30 . It implies that the surfaces of silver NPs rather than the CNTs play the key role for O 2 dissociation at low temperatures, which explains why experimentally the carbon oxidation only occurred at the sites of CNTs that are in direct contact with Ag NPs. Furthermore, the DFT calculation shows that the oxidation of Ag surface is unstable due to the low thermal stability. The ab initio molecular dynamic simulation suggests that the O atom in Ag lattice could diffuse to the interfaces between Ag NPs and CNTs (Fig. 5a) resulting in oxidation of CNTs (Fig. 2). On the basis of previous studies, two catalytic mechanisms were proposed: Spillover mechanism 31 , suggesting that metal NPs activate O 2 molecules to produce active oxygen species which diffuse to the particle surface to oxidize the substrate 32 , and the analogy of the Marsvan Krevelen mechanism 33,34 , implying that oxidation occurred via the movement of oxygen species within the lattice of the metal NPs. The AC-ETEM results (Figs 2 and 3) suggest that Ag NPs dissociate the gas phase oxygen molecules and then served as an oxygen-ion channel to deliver active oxygen species to the CNTs during the oxidation process, similar to the Mars-van Krevelen mechanism 33,34 .
Electronic structure calculations suggested that the oxygen diffusion path lies midway between Ag atoms 35 , and the presence of oxygen within the Ag NP can weaken the Ag-Ag bonds, giving rise to a higher probability for the oxygen species jumping from site to site 36 even at low temperatures 37,38 . Figure 5b,c show the schematics illustrating the oxidation process of a silver oxide NP on the tip and the external surface of a CNT, respectively. As shown in Figure 5c, the O 2 is dissociated at the surface of NP and the NP acts as a dynamic reservoir for atomic O, providing active O atoms for CNT oxidation.
During the oxidation process, once the C atoms on the CNT were oxidized and CO x molecules (marked in Fig. 5b,c) formed and released, an oxygen concentration gradient (as marked by the yellow arrows in Fig. 5b,c) would form inside the NP to establish a dynamic equilibrium: High concentration of oxygen species near the exposed surfaces of the AgO x NPs and low oxygen concentration near the Ag/CNT interfaces. The oxygen species far from the contact area will supply this consumption by transiting oxygen species to the contact area, driven by the oxygen concentration gradient within the NPs. The continuous need of the atomic oxygen at the NP/CNT interfaces further demands the dissociative adsorption of the gas phase oxygen molecules (O 2 (ads) ) at the exposed surface of the NPs. The incorporation of the atomic oxygen increases the interplanar spacing of the silver NP 39 . When the oxygen gas supply was stopped, the oxygen concentration near the exposed surfaces of the Ag NPs should be significantly reduced. Since the AC-ETEM images were acquired without the presence of the oxygen, the balanced oxygen concentration near the Ag/CNT interfaces, which is not enough to oxidize the carbon atoms of the CNTs was trapped inside the NP which causes the lattice expansion as evidenced by the AC-ETEM images.
This result provides very useful and original understanding of how the catalysed oxidation of carbon-based materials occurs on an atomic scale. Together with first-principles simulations, we reveal that an oxygen concentration gradient dominated oxidation process, which suggests that the oxygen molecules dissociate on the Ag NPs and diffuse through the Ag NPs to reach the Ag/CNT interfaces and subsequently oxidize the carbon. Such direct observation of atomic scale dynamics provides an important general methodology not only for investigations of catalytic processes but also for improving advanced catalyst systems.

Methods
Quasi-in situ oxidation test. Before the oxidation test, the TEM grid with dispersed CNTs was heated in the AC-ETEM up to 450°C and then 99.9999% ultrapure oxygen was introduced with a pressure of 2 mbar for 45 min to remove the surface contaminants. After such treatment, the CNTs were examined to make sure their cleanness. The sample holder with the TEM grid was then taken out of the ETEM and a drop of AgNO 3 aqueous solution was loaded to the grid to deposit Ag-containing precursors to the CNTs and the support carbon films. The sample holder was then re-inserted back into the ETEM again and the whole system was reduced inside the microscope chamber with 2 mbar H 2 at 250°C for 2 h, resulting in the formation of finely dispersed metallic Ag or AgO x NPs. In situ oxidation reaction was performed with 2 mbar ultrapurity oxygen at 250°C. The electron beam was blanked during the oxidation process in order to eliminate the electron beam-induced ionization effects 20 . At the end of each oxidation cycle, the oxygen was purged out from the ETEM chamber while maintaining the sample temperature at 250°C. The final chamber vacuum pressure was better than 2 Â 10 À 6 mbar before the electron beam was de-blanked for examination. The same CNTs were located after each cycle of oxidation reactions. A total of four oxidation cycles were performed sequentially in order to extract information on the dynamic processes of the Ag catalysed oxidation of the CNTs. To minimize the electron beam effects on the CNTs, even without the presence of oxygen, all the experiments were conducted at 80 kV (ref. 22). The average dose used for imaging was B1250 e À Å À 2 s À 1 . We estimated that for each cycle of examination the total time for the CNTs exposed to the electron beam was about 80 s, resulting in a cumulative electron dose of 5 Â 10 5 e À Å À 2 for each cycle of examination. Repeated experiments confirmed that such operation conditions did not result in observable electron beam-induced damages of the Ag/CNT system. Simulation details. Our calculations are based on DFT and use of a plane-wave, pseudopotential formalism [40][41][42] . Exchange and correlation effects are included within the Perdew-Burke-Ernzerhof form of the generalized gradient approximation 43 . For modelling the O 2 dissociation process at the Ag or graphene surface, we used the supercell geometry separated by a vacuum gap equivalent to 15 Å. To obtain the energy barrier of each dissociation pathway, calculations are performed with a plane-wave cutoff energy of 550 eV, and the same k-points (4 Â 4) in the surface Brillouin zone was used for all structures. The total energy convergence criterion was 0.01 meV. All the atoms in the systems were fully relaxed until the largest residual force on any one was o0.02 eV Å À 1 . A Fermi surface smearing of 0.1 eV was used and the energy extrapolated to zero temperature. We included the spin-polarization energy in the calculation of the total energy of the free atoms and molecules. For the molecular dynamic simulation at infinite temperatures, the temperature was controlled by Noise-Hoove method and the time step was 2 fs.
Data availability. The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information files and from the corresponding author upon reasonable request.