Predictable Particle Engineering: Programming the Energy Level

Article Cite This: J. Am. Chem. Soc. 2018, 140, 7629−7636

pubs.acs.org/JACS

Predictable Particle Engineering: Programming the Energy Level, Carrier Generation, and Conductivity of Core−Shell Particles Conghui Yuan,*,†,‡ Tong Wu,† Jie Mao,† Ting Chen,† Yuntong Li,† Min Li,† Yiting Xu,†,‡ Birong Zeng,†,‡ Weiang Luo,†,‡ Lingke Yu,§ Gaofeng Zheng,§ and Lizong Dai*,†,‡ †

College of Materials, ‡Fujian Provincial Key Laboratory of Fire Retardant Materials, and §Department of Instrumental and Electrical Engineering, Xiamen University, Xiamen 361005, China

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S Supporting Information *

ABSTRACT: Core−shell structures are of particular interest in the development of advanced composite materials as they can efficiently bring different components together at nanoscale. The advantage of this structure greatly relies on the crucial design of both core and shell, thus achieving an intercomponent synergistic effect. In this report, we show that decorating semiconductor nanocrystals with a boronate polymer shell can easily achieve programmable core−shell interactions. Taking ZnO and anatase TiO2 nanocrystals as inner core examples, the effective core−shell interactions can narrow the band gap of semiconductor nanocrystals, change the HOMO and LUMO levels of boronate polymer shell, and significantly improve the carrier density of core−shell particles. The hole mobility of core−shell particles can be improved by almost 9 orders of magnitude in comparison with net boronate polymer, while the conductivity of core−shell particles is at most 30-fold of nanocrystals. The particle engineering strategy is based on two driving forces: catechol-surface binding and B−N dative bonding and having a high ability to control and predict the shell thickness. Also, this approach is applicable to various inorganic nanoparticles with different components, sizes, and shapes.

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INTRODUCTION Decoration of inorganic nanoparticles with polymeric shells has become a widely used strategy to fabricate advanced composite materials possessing potential applications in various fields including catalysis,1−3 energy,4−6 biology,7−10 and sensing.11−14 The advantage of this strategy is highly dependent on the synergistic effect between organic and inorganic components.15,16 Nevertheless, most of the reported synergies between inorganic core and polymeric shell are relatively simple or unidirectional. Toward different cores, the possible functions of polymeric shells are normally limited to stabilizer,17−19 protector,20−22 guest collector,23,24 or stimulusacceptor.25,26 It is still a great challenge to achieve bidirectional synergistic effects between the core and shell, therefore improving the intrinsic properties of both counterparts. On the other hand, the interaction between core and shell, as well as the matter and energy exchange between inner core and outer environment, is related to the shell thickness. Thus, a facile and versatile approach to precisely control the shell thickness and uniformity is desirable. Herein, we report unexpectedly bidirectional core−shell synergistic effects in hybrid particles composed of a boronate polymer (BP) shell and a nanocrystal core. Taking ZnO and anatase TiO2 (denoted as anTiO2) nanocrystals as examples for the inner cores, we show that the interactions between BP shell and nanocrystal core can (i) narrow the band gap of ZnO and anTiO2nanocrystals, (ii) change the HOMO and LUMO levels of BP, and (iii) dramatically increase the carrier density of © 2018 American Chemical Society

core−shell particles. Benefiting from the core−shell synergies, the conductivity of the core−shell particles is greatly improved compared with net BP, ZnO, and anTiO2. A supramolecular approach is developed to fabricate inorganic@BP (including nanocrystal@BP) core−shell particles. Condensation reaction between a catechol monomer and a boronic monomer with multiple functionalities on the surface of inorganic nanoparticles affords BP cross-linked networks, which generates two types of supramolecular interactions: first, a strong interaction between BP and nanocrystal contributed by the high binding affinity of catechol moiety to inorganic surface;27−29 second, a relatively weaker B−N dative bonding among BP chains, which is capable of organizing polymers into well-defined assemblies as demonstrated in our previous work.30−32 With this elegantly designed supramolecular approach, we are able to precisely control and predict the BP shell thickness on the surface of inorganic nanoparticles with various components, sizes, and shapes.

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RESULTS AND DISCUSSION Shell Thickness Control and Prediction. Three-armed catechol monomer (TBC) and boronic monomer (TBB) incorporated with imine moieties (Figures 1a, S1, and S2) were utilized as building blocks for BP.33 The procedure for the fabrication of core−shell particles is simple; condensation Received: March 21, 2018 Published: May 24, 2018 7629

DOI: 10.1021/jacs.8b03010 J. Am. Chem. Soc. 2018, 140, 7629−7636

Article

Journal of the American Chemical Society

Figure 1. Control and prediction of BP shell thickness: (a) schematic illustration for the coating process of BP on various inorganic nanoparticles; (b) TEM images of amTiO2@BPs derived from 0.02, 0.1, 0.15, 0.35, and 0.7 mg/mL of monomers, where ST represents the shell thickness; (c) darkfield TEM image and EDX mapping of a typical amTiO2@BP particle; (d) DLS diameters of amTiO2@BPs prepared by using different monomer concentrations; (e) SAXS profiles of amTiO2@BPs derived from different concentrations of monomers; (f) theoretical prediction curve for the BP shell thickness calculated from eq 1 (black solid curve) and the actual shell thickness variation versus monomer concentration (star-like discrete points indicate shell thickness calculated by DLS results; color spherical discrete points indicate shell thickness calculated from TEM images). Insets of (f) are the magnified TEM images of (b).

with the increase of monomer concentration from 0.02 to 0.85 mg/mL (Figure 1d and Table S1). To further verify the shell thickness control from a perspective of statistical analysis, smallangle X-ray scattering (SAXS) experiments were performed. amTiO2@BPs derived from 0.06, 0.1, and 0.15 mg/mL of monomers show broad peaks at q = 1.12, 0.78, and 0.54 1/nm in the SAXS profiles (Figure 1e), corresponding to domain spacing of ∼5.6, 8.0, and 11.6 nm, respectively. All the domain spacing values tested by SAXS are equal to the shell thicknesses measured from TEM images or DLS. We then anticipated establishing the functional relationship between the shell thickness and the monomer concentration. Toward monodispersed spherical amTiO2, the BP shell thickness ST can be expressed by the following equation:

reaction between TBC and TBB in an ethanol solution with the presence of target core particles leads to the formation of BP shell (Figure 1a). We first tested the possibility of precise control over the BP shell thickness. Monodispersed spherical amorphous TiO2 (denoted as amTiO2, diameter of ∼120 nm) nanoparticles were adopted as inner cores to facilitate the measurement and calculation of the shell thickness. TEM images (Figure 1b and insets of Figure 1f) of the resultant particles display a darker contrast inner core and a lighter contrast outer shell, providing the first indication for the formation of amTiO2@BP core−shell structure. Dark-field scanning TEM image shown in Figure 1c confirms the component difference between inner core and outer shell. Energy-dispersive X-ray spectroscopy (EDX) mapping of a typical particle (Figure 1c) indicates the coexistence of elements Ti, C, N, and B, especially that the size of Ti map is relatively smaller than the maps of C, N, and B. Statistical calculation from the TEM images shows that amTiO2@BPs derived from 0.02, 0.1, 0.15, 0.35, and 0.7 mg/mL of monomers possess shell thicknesses of ∼1.6, 7.8, 11.4, 25.4, and 43.5 nm, respectively. Dynamic light scattering (DLS) results give the same evolution trend, as the shell thickness gradually increases

ST =

3

where γ =

γCm + r 3 − r 3Vs ; 4πρN

(1)

Cm represents the total mass concentration of

TBB and TBC; r, Vs, ρ, and N represent the radius of inner core, the volume of the reaction solution, the density of BP, and the particle number of the cores, respectively (detailed formula derivation process is described in the Supporting Information). 7630


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Figure 2. Bidirectional core−shell interactions. TEM images of ZnO@BP (a) and anTiO2@BP (b) core−shell particles with different BP shell thicknesses. Plots of the transformed Kubelka−Munk function versus the energy of light: ZnO@BPs (c) and anTiO2@BPs (d). (e) Band gaps of ZnO@BPs and anTiO2@BPs. CV curves of ZnO@BPs (f) and anTiO2@BPs (g). HOMO/LUMO levels of ZnO@BPs and anTiO2@BPs (h). Schematic illustration for the core−shell interactions (i).

Since the values of N and ρ are difficult to test, the value of γ is unknown. Fortunately, it is possible to determine γ by choosing the already prepared core−shell particles with the given ST, inner core r, and monomer concentration Cm. We have determined γ for amTiO2 and plotted the theoretical profile of the shell thickness ST versus monomer concentration Cm (Figure 1f). The shell thicknesses calculated from TEM images and analyzed by DLS are also displayed in Figure 1f. The variation trend of the experimental shell thickness versus monomer concentration coincides well with the theoretical profile. With this theoretical predication, our approach demonstrates a high applicability in precise control over the shell thickness. For example, the theoretical profile predicts that the BP shell thicknesses would be 29.1 and 40.2 nm when the monomer concentrations are 0.4 and 0.6 mg/mL, respectively. Experimental results indicated that core−shell particles with ∼28.8 and 39.8 nm of BP shell thicknesses were fabricated with the above two monomer concentrations (Figure S6). Programmable Core−Shell Synergistic Effects. Utilizing ZnO and anTiO2 as inner cores, ZnO@BPs and anTiO2@ BPs were easily generated through the same procedure. Because of the high controllability and predictability of this approach, we were able to achieve the same BP shell thickness on the surface of both ZnO and anTiO2. As shown in parts a and b of Figure 2, ZnO@BPs and anTiO2@BPs with ∼3.8, 14.6, and 26.2 nm thicknesses of BP shells were selectively prepared. The shape of nanocrystals has no impact on the coating behavior, as every particle is covered with a uniform BP shell. Dark field TEM and EDS mapping results confirmed the formation of core−shell structure and the coexistence of C, N, B elements on the surface of ZnO and anTiO2 (Figure S7). ZnO and anTiO2 are semiconductors that have been widely used in solar cells,34−36 sensing,37,38 and catalysis.39−42 Much

effort has been focused on narrowing their band gap to optimize the photoelectric property.43−46 Herein, we demonstrate the first core−shell synergistic effect by showing that BP can significantly narrow the band gap of both ZnO and anTiO2. Under our experimental conditions, net ZnO and anTiO2 nanocrystals possess band gaps of 3.2 and 2.9 eV, respectively. With the increasing shell thickness of BP, the band gaps of [email protected], [email protected], and [email protected] decrease from 3.1, 3.0 to 2.9 eV (Figure 2c,e). This evolution trend is more evident for anTiO2@BP particles; [email protected], anTiO2@ BP14.6, and [email protected] possess band gaps of 2.5, 2.0, and 1.5 eV, respectively (Figure 2d,e). As a control experiment, we prepared ZnO/BP and anTiO2/BP mixtures with equal compositions to the corresponding core−shell particles, and their band gaps were found to be almost the same values with that of net ZnO or anTiO2 (Figure S8). The second synergistic effect between core and shell is that the semiconductor cores can change the HOMO and LUMO levels of BP shell. The cyclic voltammetry (CV) curves of BP itself display a stable and reversible oxidation and reduction process (Figure S9). BP has HOMO and LUMO levels of −5.08 and −2.95 eV, respectively. In the case of ZnO@BPs, with the decreasing of shell thickness from 26.2, 14.6 to 3.8 nm, the HOMO level of BP lifts up from −4.94, −4.91 to −4.88 eV, while the corresponding LUMO level lowers from −3.02, −3.15 to −3.32 eV (Figure 2f,h). anTiO2@BPs exhibit the same evolution trend, as the HOMO/LUMO levels of anTiO2@ BP26.2, [email protected], and [email protected] are −4.95/−3.09, −4.92/−3.14, −4.89/−3.24 eV, respectively (Figure 2g,h). Since a thinner shell can shorten the average distance between BP and nanocrystal, these results indicate that the influence of the inner core on the HOMO/LUMO levels of BP is distance dependent. We attribute the narrowed band gap of semi7631


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Figure 3. Carrier generation and electrical conductivity. ESR spectra of ZnO@BPs (a) and anTiO2@BPs (b) with various shell thicknesses; (c) normalized peak-to-peak height of the ESR signals versus shell thickness; (d) structure of the hole-only device; (e) experimental (symbols) and theoretical (lines) current-density versus voltage plots for ZnO@BPs with different layer thicknesses. Plots of current-density versus voltage of ZnO@BPs (f) and anTiO2@BPs (g) with different shell thicknesses (layer thickness 1.00 ± 0.04 μm). (h) Conductivity of the microscaled devices. (i) and inset of (h) are the structure and optical microphotograph of the microdevice.

an indication of the occurrence of charge separation of the polymer chains. For ZnO nanocrystals, an extremely weak signal at g = 2.052 implies the low electron concentration in the conduction band (Figure S12). Weak spin resonance signals can also be observed for anTiO2 nanocrystals; the signal of O2•− appears at g = 2.058, while the signals at g = 1.935 and 1.988 are attributed to Ti3+ radial (Figure S12). After the coating of BP, the as formed ZnO@BPs and anTiO2@BPs show dramatically enhanced signals in their ESR spectra (Figure 3a,b and Figure S13), implying the high concentration of carriers. A single and enhanced organic radical signal at g = 2.001 was detected for ZnO@BPs. anTiO2@BP particles displayed no signal of organic radical but showed high intensity signal of Ti3+ radical at g = 1.935 and 1.988. According to the attribution of the signals, the signal intensity of organic radical was normalized to the mass content of BP, while that of Ti3+ radical was normalized to the mass content of anTiO2. As shown in Figure 3c, the normalized organic radical signal intensities of ZnO@BPs are at least 100-fold of BP and decrease with the increasing of BP shell thickness. However, this is not the case for anTiO2@BPs, as a thicker BP shell leads to higher normalized signal intensity of Ti3+ radial. The normalized signal intensity of Ti3+ radicals was found to be at least 50-fold of anTiO2 (possible reasons for these results are explained in Scheme S1). In the ESR spectra at 77K (Figure S14a), ZnO@BPs show 55-fold signal intensity of that measured at room temperature, while anTiO2@BPs not only display enhanced signal intensity but also exhibit richer spin resonance species (Figure S14b). The dramatically improved carrier density implies that the core−shell particles may be of high electrical conductivity. Hole-only devices with a layer of core−shell particles sandwiched between an ITO electrode as hole injector and an evaporated Al contact as top electrode (Figure 3d) were first adopted to evaluate the conductivity of the core−shell particles. Since the diameters of the core−shell particles are hundreds of nanometers, the particle layers employed in our devices are relatively thick. Figure 3e displays the J−V characteristics of three ITO/ZnO@BPs/Al devices with layer thicknesses of

conductor nanocrystals and the change in the HOMO/LUMO levels of BP to the core−shell bidirectional interactions (Figure 2i). Migration of metal ions into the BP shell driven by metal− catechol interaction induces the defect formation in the semiconductor nanocrystals, therefore resulting in narrowed band gaps. On the other hand, incorporation of metal ions in the boronate polymer networks is responsible for the changed HOMO/LUMO levels of BP shell. The compositions of ZnO@BPs and anTiO2@BPs were characterized by X-ray photoelectron spectroscopy (XPS). Through the curve-fitting of high resolution XPS signals (Figure S10), peaks at 1022.5 (Zn 2p3/2, Zn−O−C), 531.8 (O 1s, Zn−O−C), 457.9 (Ti 2p3/2, Ti3+), and 529.4 eV (O 1s, Ti−O−C) indicate the generation of catechol−metal ion complexes. In this regard, the formation of core−shell structure can cause the composition changes of both semiconductors and BP. Control experiments were performed to further verify the effect of metal ions on the HOMO/LUMO levels of BP. For example, with the presence of metal compound (ZnCl2 or tetrabutyl orthotitanate), the as prepared BP−metal ion composites exhibit improved HOMO level and lowered LUMO level (Figure S11). We also coated BP on the surface of SiO2 nanospheres to afford SiO2@BPs and tested their CV curves (Figure S11). The calculated HOMO/ LUMO levels of SiO2@BPs with shell thicknesses of 2.4 and 10.5 nm are almost the same as that of BP. Thus, the evolution of HOMO/LUMO levels of BP is due to the composition change instead of the nanoscaled shell thickness. In comparison with the approach of complicated molecular design for tuning the HOMO/LUMO levels of organic semiconductors,47−49 our findings provide a much easier route. The ability to generate stable carriers such as electrons, holes, and radical species is particularly important for the conductivity of both organic and inorganic semiconductors. In this regard, ZnO@BPs and anTiO2@BPs display the third core−shell synergistic effect through the high ability to generate stable carriers. The feature of carriers in the core−shell particles was monitored by electron spin resonance (ESR) spectroscopy at room temperature. BP itself displays a typical polymeric radical cation signal at g = 2.001, as well as a spin resonance at g = 1.922 which is derived from radical anions (Figure 3a), giving 7632


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Figure 4. High adaptability of the coating approach: (a) TEM images of MSN@BPs, Au@BPs, Fe2O3@BPs, Fe3O4@BPs, ZnOrod@BPs, and Cu2O@BPs; (b) magnified TEM images of (a); (c) EDX line scanning analyses of typical core−shell particles.

8.86, 5.04, and 1.04 μm. The log J−log V plots of these devices reveal a quadratic dependence between current-density J and voltage V. We thus fitted the plots with a simple space-charge limited current (SCLC) model:50,51 J=

9 V2 ε0εrμp 3 8 L

To further compare the electrical conductivity between core−shell particles and semiconductor nanocrystals, the particles were processed into microdevices with particle layer (50 μm × 5 μm × 2 μm) gaped between two Ag electrodes (Figure 3i and inset of Figure 3h), using a nanojet printing technology. All the devices exhibit a simple ohmic characteristic as their I−V curves have a linear relationship between current and bias voltage (Figure 3h). The evolution of electrical conductivity of both ZnO@BPs and anTiO2@BPs with different BP shell thicknesses is similar to that of μp. With the presence of BP shell, the electrical conductivities of ZnO@BPs and anTiO2@BPs are improved at most 30- and 10-fold in comparison with ZnO and anTiO2 nanocrystals, respectively (Table S2). The enhanced electrical conductivity of the core− shell particles is contributed not only by the high carrier density but also the improved interparticle contact. Adaptability of the Particle Engineering Strategy. Motivated by the efficient synergy between BP shell and semiconductor nanocrystals, we expected to extend our coating approach to various inorganic nanoparticles. Starting from SiO2 nanospheres, we have further verified the high controllability and predictability of this coating approach (Figures S15, S16, and S17). Decoration of a BP shell on the surfaces of mesoporous SiO2 nanospheres (MSN), Au nanoparticles, Fe2O3 nanospindles, Fe3O4 nanospheres, ZnO nanorods, and Cu2O nanocubes has been easily achieved. The TEM images of all the resultant particles shown in Figure 4a,b have clear core− shell structures, and the composition difference between the core and shell is verified by EDS line scanning analyses (Figure 4c). The shell thicknesses of the as formed MSN@BPs, Au@ BPs, Fe2O3@BPs, Fe3O4@BPs, ZnOrod@BPs, Cu2O@BPs calculated from the statistical analyses of the TEM images are ∼16.1, 25.5, 12.4, 18.4, 11.8, and 17.2 nm, respectively, which agree well with the results measured by EDX line scanning analyses. All these core−shell particles could be completely isolated from the ethanol solution through a simple centrifugal process, and the yields were higher than 96 wt %. Therefore, our supramolecular approach has the ability to organize BP on the surface of inorganic nanoparticles with various compositions, sizes, and shapes.

(2)

where ε0εr is the permittivity of the core−shell particles, μp represents the hole mobility, and L is the thickness of the particle layer. We have found that εr is dependent on the thickness of BP shell. BP itself has a εr value of 3.1, while ZnO@BP 3.8 , ZnO@BP 14.6 , ZnO@BP 26.2 , an TiO 2 @BP 3.8 , [email protected], and [email protected] have εr values of 2.4, 2.9, 3.0, 41.8, 28.4, and 12.6, respectively. By use of μp = 2.04 × 10−2 cm2/(V s), the log J−log V plots of the ITO/ZnO@BPs/ Al devices can be well described by eq 2, especially at low voltage. The slope of the plots increases rapidly at voltage higher than 10 V, resulting in a deviation from the prediction plots. Probably, this is because the hole density p and mobility μp are related to the intensity of electric field.52,53 Figure 3f shows the J−V curves of BP and ZnO@BPs with different shell thicknesses. BP itself also exhibits a SCLC characteristic but with an extremely low hole mobility μp = 6.19 × 10−11 cm2/(V s). The conductivity of ZnO@BPs is enhanced by almost 9 orders of magnitude, in comparison with net BP (Figure 3f and Table S2). ZnO@BPs with different shell thicknesses exhibit the same SCLC behavior but possess different hole mobilities. The μp values of [email protected], ZnO@ BP14.6, and [email protected] are 2.04 × 10−2, 3.16 × 10−3, and 1.98 × 10−4 cm2/(V s). The J−V curve characteristics of anTiO2@ BPs (Figure 3g) are similar to that of ZnO@BPs but have a much lower hole mobilities. Moreover, thicker BP shell leads to a higher μp of anTiO2@BPs, as [email protected], [email protected], and [email protected] have μp values of 6.81 × 10−6, 1.35 × 10−6, and 4.32 × 10−7 cm2/(V s). The μp evolution trend of core− shell particles accords well with that of the radical concentration measured by ESR. Therefore, the ability to generate carriers mainly determinates the hole mobility of the core−shell particles. 7633


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and energy exchange between the inner core and the outer environment.

The condensation reaction between catechol monomer and boronic monomer for the formation of BP was characterized by Fourier transform infrared (FT-IR) spectroscopy (Figure S18) and XPS (Figure S10). TBC monomer has a high binding affinity to the surface of inorganic particles. As shown in the ζ potential evolution results, with the presence of TBC, the surface charge of the inorganic particles (such as amTiO2, ZnO, and anTiO2) changed from positive to negative evidently (Figure S19). Probably, this interaction has a high ability to restrict the condensation polymerization between TBC and TBB on the surface of inorganic particles. Moreover, the role of catechol functionality for the high efficiency coating approach was clarified by the observed surface recognition behavior of BP (Figure S20). 11B NMR spectra of boronic monomer and BP were also acquired to confirm the formation of B−N dative bond between boronate and imine moieties in the polymer chains (Figure S20). The thermostability of BP, ZnO@BPs, and anTiO2@BPs was characterized by thermogravimetric analysis (TGA) in N2 atmosphere. As displayed in Figure S21, BP begins to lose weight at temperature ∼125 °C, which can be attributed to the further condensation reaction between residual boronic and catechol moieties. The decomposition of the BP occurs at temperature ∼400 °C, implying the high thermostability of the polymer networks. No advance decomposition was observed for BP and semiconductor nanocrystals after the formation of core−shell particles. The adaptability of this approach was also tested by varying the building blocks of BP. Condensation polymerization of trifunctional catechol monomer (TAC) and boronic monomer (TAB) derived from the Schiff base formation reaction between tris(4-aminophenyl)amine and 3,4-dihydroxybenzaldehyde or 4-formylphenylboronic acid (Figures S3 and S4) also has the ability to form a uniform BP shell on the surface of inorganic nanoparticles (Figure S22a). Interestingly, when dual-functional catechol monomer (DAC) and boronic monomer (DAB) (Figure S5) were applied in the approach, an extremely thick BP shell could be created on the inorganic nanoparticle surface (Figure S22b).

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Materials. Building blocks for BP including TBC, TBB, TAC, TAB, DAC, and DAB were synthesized according to our previous work32,33 with slight optimization. Synthetic and characterization details were also described in the Supporting Information. Fe3O4 nanospheres (200−300 nm) and anTiO2 (∼100 nm) and ZnO nanoparticles (∼200 nm) were supplied by Aladdin. The preparation methods of other inorganic nanoparticles were described in the Supporting Information. Tetrabutylammonium tetrafluoroborate and ferrocene (Fc) were used as received from J&K Chemical. Characterization. TEM images were obtained from a JEM2100 instrument at an acceleration voltage of 200 kV. EDX mapping was performed on a FEI Talos F200S instrument. SEM images and EDX line scanning analysis were taken using a HiTaChi SU-70 SEM instrument. DLS measurements were performed on a Malvern Nanozetasizer. SAXS measurements were carried out on Anton PaarSAXSess mc2 using Cu Kα (λ = 1.541 84 Å, 40 kV, 50 mA) X-ray sources at room temperature. ESR measurements were performed on an electron spin resonance spectrometer (Bruker EMX-10/12), with sample weights fixed at 0.1 g. UV/vis diffuse reflectance spectra were acquired from a UV spectrophotometry (UV-3600). Cyclic voltammetry (CV) experiments were carried out on an electrochemical workstation (CHI 760E), employing a three-electrode cell with a scan rate of 0.1 V/s. Glassy carbon electrode, Ag wire, and Pt wire were used as working electrode, reference electrode, and counter electrode, respectively. Tetrabutylammonium tetrafluoroborate (0.1 M in dichloromethane) was supporting electrolyte, and Fc/Fc+ was external standard. The measurement system was argon purged for 30 min before testing. The hole mobility and conductivity of the devices were tested by using a computer controlled Keithley 2400 SourceMeter. For the measurement of hole mobility, particles were spin-coated from a ethanol solution under the protection of argon on a ITO glass (hole injector contact, 20 mm × 20 mm × 1 mm) as anode. An aluminum top contact with 100 nm thickness was used as cathode through magnetron sputtering method (JC500-3D magnetron sputtering instrument). To measure the conductivity of the particles, nanojet printing method (OPTOMEC AJ300 aerosol jet printer) was used to fabricate particle layer (50 μm × 5 μm × 2 μm) between two Ag contacts. Coating of BP on the Surface of Various Inorganic Nanoparticles. Inorganic nanoparticles such as amTiO2, SiO2, MSN, Au, Fe3O4, ZnO, anTiO2 nanoparticles, Fe2O3 nanospindles, Cu2O nanocubes, and ZnO nanorods were treated with extraction and washed with ethanol at least three times to eliminate the stabilizers, surfactants, or other small molecules attached on the particle surface. All these inorganic nanoparticles were redispersed in ethanol to form dispersions with a concentration of 1.0 mg/mL. To these solutions, catechol monomers such as TBC, TAC, and DAC in ethanol solutions were added. These mixture solutions were treated with sonication to ensure the dispersion of the inorganic nanoparticles and the binding of catechol monomer on the particle surface. Since the as-used inorganic nanoparticles are almost naked, they can easily aggregate in ethanol solutions. However, with the presence of catechol monomer, the dispersion capability of the inorganic nanoparticles in ethanol can be significantly improved. Then, ethanol solutions of boronic monomers including TBB, TAB, and DAB with an equal mole ratio to the catechol monomers were added. Within 10 min stirring, core−shell particles can be isolated from the reaction solutions through a simple centrifugation. After drying in vacuum, the yields of all core−shell particles were calculated to be higher than 96 wt %.

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CONCLUSION In summary, we have shown that a simple approach with two supramolecular driving forces including catechol-surface binding and B−N dative bonding can organize boronate polymers on the surface of inorganic nanoparticles with different components, sizes, and shapes. Through the adjustment of the monomer concentration, the shell thickness can be precisely controlled and predicted ranging from ∼1.0 to 50 nm. Unexpectedly synergistic effects between core and shell provide a blueprint for the design of advanced composite materials. By utilization of ZnO@BP and anTiO2@BP core− shell nanoparticles as examples, the core−shell synergies lead to the band gap narrowing of inner cores, HOMO/LUMO evolution of BP shell, and dramatic improvement of carrier density. Our strategy may be applicable in improving the photocatalytic, photoelectric, conductive, and sensing properties of both organic and inorganic semiconductors. Indeed, we have already found that the hole mobility of the core−shell particles is almost 9 orders of magnitude of BP itself. From the perspective of methodology, this approach may be of particular interest in the design of core−shell structured organic/inorganic hybrid particles. The precise control and prediction over the shell thickness make it possible to adjust the core−shell synergistic effect as well as to investigate the matter

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b03010. 7634


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Details of monomer synthesis, inorganic nanoparticles preparation, theoretical prediction, calculation and characterization (NMR, TEM, EDX, UV−vis, FTIR, ESR, XPS, DLS, TGA and CV), Figures S1−S22, Tables S1 and S2, and Scheme S1 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*C.Y.: [email protected] *L.D.: [email protected] ORCID

Conghui Yuan: 0000-0003-0126-7236 Birong Zeng: 0000-0003-1582-4929 Lizong Dai: 0000-0001-8049-4263 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51773172, 51673161); Scientific and Technological Innovation Platform of Fujian Province (Grant 2014H2006); Natural Science Foundation of Fujian Province (Grants 2015J01220, 2016J01257). The authors thank Yanping Zheng, Guorong Chen, Dingliang Tang, Xinyu Liu, and Wei Zheng for the measurement assistance.

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Journal of the American Chemical Society (53) Pasveer, W. F.; Cottaar, J.; Tanase, C.; Coehoorn, R.; Bobbert, P. A.; Blom, P. W. M.; de Leeuw, D. M.; Michels, M. A. J. Phys. Rev. Lett. 2005, 94, 206601.

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