Selective Synthesis, Morphological Control, and Energy Applications

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In-situ Self-assembly Generated 3D Hierarchical Co3O4 Micro/Nanomaterial Series: Selective Synthesis, Morphological Control and Energy Applications Qingqing Miao, and Suojiang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14543 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

In-situ Self-assembly Generated 3D Hierarchical Co3O4 Micro/Nanomaterial

Series:

Selective

Synthesis,

Morphological Control and Energy Applications Qingqing Miao and Suojiang Zhang* Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, No. 1 Zhongguancun North Second Street, Haidian District, Beijing 100190, P.R. China Abstract A simple in-situ self-assembly selective synthetic strategy for one step controllable formation of various 3D hierarchical Co3O4 micro/nanomaterials with peculiar morphologies, uniform size and high quality is successfully developed. The morphological control and related impact factors are investigated and clarified in detail. The results further clarify the corresponding mechanisms on the reaction process, product generation, calcining process, as well as the formation of specific morphologies. Furthermore, the superior catalytic properties of these materials are confirmed by two typical Co-based energy applications on the decomposition of an important solid rocket propellant-ammonium perchlorate (AP) and dye-sensitized solar cells (DSSCs). The addition of Co3O4 materials for AP obviously decreases the decomposition temperatures for about 118-140 ℃ and increases the exothermic heat to a great extent. As the substituted counter electrodes of DSSCs, the 3D hierarchical Co3O4 materials exhibit the attractive photovoltaic performances. These findings provide a facile and effective way for designing new types of 3D hierarchical materials towards high catalytic activity for energy devices. Keywords: In-situ, 3D hierarchical Co3O4 micro/nanomaterials, superior catalytic properties, ammonium perchlorate, dye-sensitized solar cells

1. Introduction In recent years, increasing attention has been paid to inorganic materials due to their versatile characters and various applications in many fields. Among them, metal oxide materials have played an increasingly important part. In particular, as a typical p-type semiconductor, Co3O4 with the direct optical band gap at 1.48 and 2.19 eV,1 has attracted considerable interest. Owing to the unique physico-chemical properties such as high theoretical capacity,2 controllable size, adjustable shape, stable chemical properties, low cost, earth-abundance and environmentally friendliness, Co3O4 based-materials have been evaluated with the good application value in wide-ranging fields of supercapacitors,3,4 lithium-ion batteries,5,6 lithium-O2 batteries,7 gas sensors,8,9 catalysts,10,11-12,12 magnetic materials,13,14 and electrochromic

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devices.15 To date, considerable efforts have been devoted to developing different Co3O4 structures,16-26 including nanoparticles,26-27 nanowires,4, 18, 28-32 nanorods, 33-35 nanotubes,14, 36 nanosheets37-39 and nanospheres.11, 40-42 The various Co3O4 materials obtained by different methods described above undoubtedly showed outstanding performances. However, there is still no comprehensive understanding and unified cognition about the relationship between the microstructures and properties of Co3O4 materials. As we all know, the microstructure of materials (crystalline phase, morphology, size, surface area, etc.) is the key factor that affects the performance.43-47 Therefore, it’s very important to investigate the microstructures of the materials with the corresponding formation mechanisms. Various low-dimensional nanostructures of 0D nanoparticles, 1D nanowires and 2D nanosheets, have been developed with good application performance due to the advantages of low-dimensional nanomaterials, such as high surface area, short ion transport path. However, their serious self-aggregations prevent further applications. It’s a great challenge to avoid the disadvantages of low-dimensional nanomaterials and preserve the advantages simultaneously. 3D hierarchical micro/nanostructure, consisting of low-dimensional nanoscale building blocks, is regarded as a promising method to solve above problems. Furthermore, it’s worth noting that the materials with 3D hierarchical structures always exhibit additional features besides the conventional ones of the above-mentioned structures.4, 14, 28-31, 42, 48 To achieve the target 3D hierarchical micro/nanomaterials, the in-situ self-organization method exhibits various advantages compared with traditional synthetic methods, such as precipitation, mechanical milling, thermolysis and hydrothermal method. It’s effective to adjust and control the microstructures of the crystalline. The 3D hierarchical micro/nanomaterials with peculiar morphologies, uniform size, high quality and additional features can be achieved. Furthermore, the in-situ self-organization method avoids the complex procedures of transferring and fixing traditional materials to substrates for device applications. The problems of introduction of binder, unstable connection with substrate, unsatisfactory repeatability, unideal electron transfer and device performance can be resolved. Moreover, the internal resistance of the substrate in device applications will be reduced.49 Therefore, it is of great concern to achieve insitu self-organized 3D hierarchical Co3O4 micro/nanomaterials with different morphologies and definitize related structure formation mechanisms for high performance in specific applications. In this study, we report a simple, low cost and effective selective synthetic technique for one step controllable formation of Co3O4 with various 3D hierarchical micro/nanostructures by an in-situ self-assembly strategy. A series of 3D bionic hierarchical Co3O4 micro/nanomaterials with peculiar morphologies, uniform size and high quality were successfully achieved. The in-situ self-assembly strategy exhibits superior advantages over traditional synthetic method. The morphological control and related impact factors were systematically investigated. The results clarified the corresponding mechanisms on the reaction process, product generation, calcining process, as well as the formation of specific morphologies. Furthermore, the remarkable catalytic properties of these materials were confirmed by two typical Co-based applications on the decomposition of an important solid rocket propellant-ammonium perchlorate (AP) and dye-sensitized solar cells (DSSCs). The new and comprehensive insights into the in-situ self-assembly design, morphological control and formation mechanisms of 3D hierarchical Co3O4 micro/nanomaterials will be helpful for the development of synthetic technology of high-quality materials and promoting their applications in broader fields.

2. Experimental section


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2.1 Materials CoCl2·6H2O (99%), Co(SO4)2·4H2O (99%), Co(NO3)2·6H2O (98%), urea (CO(NH2)2, 99%), ammonium fluoride (NH4F, 98%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). I2 (99%), LiI (99%), guanidinium thiocyanate (GuSCN, 97%) and 4-tert-butylpyridine (TBP, 99%) were purchased from Sigma-Aldrich. 1-Methyl-3propylimidazolium iodide (PMII, 99%) and 1-ethyl-3-methylimidazolium tetrafluroborate (EMImBF4, 98%) were obtained from Linzhou Keneng Material Technology Co., Ltd. Titanium(II) dioxide (TiO2, P25) was purchased from Degussa. FTO glass (2.2 mm, square resistance 15 Ω/□) was obtained from OPV Tech. N719 dye was purchased from Solaronix. All chemicals were used as received without any further purification. 2.2 Preparation of Different Co(OH)F Precursors In a typical synthesis procedure, cobalt source (Co(NO3)2·6H2O, CoCl2·6H2O, Co(SO4)2·4H2O, 5 mmol), NH4F (10 mmol), and CO(NH2)2 (25 mmol) were dissolved in 50 mL of water under stirring. After 10 min, the obtained solution was transferred into Teflon-lined stainless steel autoclaves. Then, a piece of cleaned substrate (FTO glass, Al, Ti and Cu) was immersed into the reaction solutions with different orientations (horizontal upward, horizontal downward, oblique upward, oblique downward). Then, the autoclaves were sealed and maintained at 120℃ for 5 h. Thereafter, the reaction solutions were cooled down to room temperature spontaneously. The substrates were rinsed with distilled water several times in order to remove the residual reactant and impurity. The reaction 1#-5# were obtained from the cobalt source combined with NH4F and CO(NH2)2 in 50 ml H2O with the reaction time of 5 h. The cobalt sources for the reactions 1#-3# and 5# were Co(NO3)2·6H2O, and reaction 4# was CoCl2·6H2O. The corresponding molar ratio of reactants (cobalt source, NH4F and CO(NH2)2) for 1#-5# were 4:10:20, 4:20:20, 5:25:25, 5:10:25 and 5:10:25 for 1#-5# respectively. Co(SO4)2·4H2O was also used as the cobalt source with the molar ratio of 5:10:25.The reaction times were varied from 0-10 h. In the experiments on investigation of reaction concentration, the reaction 5# was selected as the target reference with the molar concentrations of all the reactants varying from 1/10, 1/5 to 1/2 of the original values. In the experiments on investigation of ionic liquid, 10 mmol EMImBF4 was added into the reaction solution for investigation on the ionic liquid-assisted reactions. The traditional synthetic method was conducted without any substrate. 2.3 Preparation of the 3D Hierarchical Co3O4 Materials Various 3D hierarchical Co3O4 materials were prepared by heating the corresponding Co(OH)F precursors at 400 °C in air for 4 h, followed by a natural cooling to room temperature to obtain the as-synthesized products. 2.4 AP Decomposition Application AP decomposition investigations are conducted by using the pure AP sample or the mixtures of AP with 2 wt. % Co3O4 materials (mass ratio: AP: Co3O4 = 2:98) and analyzed by the Thermal gravimetric analysis (TG) from 50℃ to 600℃ at a heating rate of 5 ℃/min in air flow and differential scanning calorimetry (DSC) from 100℃ to 500℃at a heating rate of 10 ℃/min under N2 flow. 2.5 DSSC Application



Preparation of TiO2 Photoanode. A 10 µm thick layer of 20 nm TiO2 (P25, Degussa, Germany) was attached to a FTO glass by screen printing technique. The obtained film was sintered at 500 °C for 30 min. After cooling to 80 °C, the TiO2 films were immersed in 5 x 10-4 M solution of N719 dye in ethanol for 20 h. Fabrication of Solar Cells. The dye-sensitized photoanode (area: 0.16 cm2) was assembled with a platinized counter electrode with the corresponding electrolyte injected into the cell. A redox (I-/I3-) electrolyte composed of 0.05 M I2, 0.1 M LiI, 0.6 M 1-propyl-3-methylimidazolium iodide PMII and additives of 0.1 M guanidinium thiocyanate (GuSCN) and 0.5 M 4-tertbutylpyridine (TBP) in acetonitrile. 2.6 Characterization SEM images were taken by a field-emission scanning electron microscope (SU8020, Hitachi, Japan). The measurement parameters of the accelerating voltage, the E beam current and the work distance was set as 5.0 kV, 10 µA and 4 mm respectively. TEM and HRTEM observations were recorded on a JEM 1200EX instrument operated at 200 kV (JEOL, Japan). Infrared absorption spectra were recorded on a Fourier transform infrared (FT-IR) spectrometer (Nicolet 380, USA). Laser Raman spectra were studied on a laser Raman spectrometer (LabRam HR800, Horiba Jobin Yvon, France). XRD patterns were measured on a Smartlab(9) X-ray powder diffractometer (Rigaku Corporation, Japan) with Cu Kα radiation (λ= 1.5418 Å) at scanning angles 2θ from 5 ℃ to 80 ℃at room temperature. XPS spectra were measured on an ESCALAB 250Xi XPS system (Thermo Fisher Scientific, Germany). Thermogravimetric studies were analyzed by using a TGA-50 system (Shimadzu, Japan) from 50 ℃ to 600 ℃ at a heating rate of 5 ℃/min in air flow and DSC measurements were carried out at DSC-60A (Shimadzu, Japan) at a heating rate of 5 ℃ min-1 in a temperature range from 100 ℃ to 500 ℃ at a heating rate of 10 ℃ /min under N2 flow. The Computerized Tomography measurements were conducted on a 3D X-ray Computerized Tomography (Xradia 410 Versa, Carl Zeiss, USA). Currentvoltage curves were conducted by a Keithley digital source meter (Keithley 2400, USA) under a solar simulator simulating the AM 1.5 spectrum (100 mW/cm2, Class AAA, Oriel, USA). The incident light intensity was calibrated with a standard silicon solar cell (Newport, USA).

3. Results and discussion 3.1 Morphological Control and Related Impact Factors 3.1.1 Reactants and Ratio. The developed cost-effective in-situ self-assembly controllable strategy for one step syntheses of Co3O4 with various 3D hierarchical structures was simply realized by homogeneously distributing Co(NO3)2·6H2O or CoCl2·6H2O, NH4F, CO(NH2)2 and solvent of water in the Teflon-lined stainless steel autoclaves with a piece of clean substrate facing upward. After the modified reaction process and further annealing, the various 3D bionic hierarchical Co3O4 materials under controlled conditions with peculiar morphologies, uniform size, high specific surface area and high quality could be successfully achieved. Fig. 1 shows the SEM images of the obtained 3D hierarchical Co3O4 materials with different reactants and concentration after a reaction time of 5 h. The bionic products from reaction 1#-3# and 5# were obtained from the cobalt source of Co(NO3)2·6H2O combined with NH4F, CO(NH2)2 in 50 ml H2O. The corresponding molar ratio of reactants were 4:10:20, 4:20:20, 5:25:25 and 5:10:25 for 1#-3# and 5# respectively. Reaction 4# used CoCl2·6H2O as the cobalt source and the molar ratio of reactants were the same with


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reaction 5#. As shown in Fig. 1b, the dandelion flower-like, zephyranthes grandiflora flower-like, sunflower-like, chrysanthemum ball-like and cotton rose flower-like structures were obtained for 1#-5# respectively, which are very similar to the corresponding plants of the nature in Fig. 1a. Comparing with reaction 1#, the product of 2# changes to a zephyranthes grandiflora flower-like structure from a dandelion flower-like appearance when increasing the molar concentration of NH4F from 10 to 20 M. While further enlarging the molar concentration of all the reactants, the morphology of 3# transforms to sunflower-like structure. For reaction 5#, the final product shows the cotton rose flowerlike structure as varying the molar concentration of Co(NO3)2·6H2O and CO(NH2)2 to 5 and 25 M respectively. Changing the cobalt source of Co(NO3)2·6H2O to CoCl2·6H2O or CoSO4·7H2O, the hierarchical structures (Fig. 1 and Fig. S1) exhibit the similar chrysanthemum ball-like features. The high-magnification SEM images in Fig. 1c-d indicate that the final 3D hierarchical structured products are composed of 1D nanostructures such as nanochains, nanorods or nanowires with a porous structure and rough granular surfaces constituted by the successive nanoparticles (Fig. S2), which further constitute the different morphological types of petals. It is interesting that the petals with 1D nanostructures share a common centre and extend radically in the outward direction. The diameters of the 1D nanostructures are about 50-60 nm, 50-65 nm, 65-95 nm, 40-60 nm and 60-90 nm for 1#-5# respectively. It can be seen from Fig. 1e that the large-scale and uniform 3D hierarchical Co3O4 materials are obtained via our in-situ self-assembly controllable strategy. The hole diameters of the 3D materials range from 10-15 µm for 1#, 4# and 5#, 6-10 µm for 2#, and 7-10 µm for 3#. The typical 3D morphology of 5# under the X-ray computerized tomography is shown in Fig. S3.

Fig. 1 SEM images of the in-situ synthesized 3D hierarchical structured Co3O4 materials 1#-5# grown on FTO glasses with different reactants and concentration after a reaction time of 5 h. a) Pictures of plants in the nature. b-e) SEM images of Co3O4 materials at various magnifications.

Fig. 2a-b shows the XRD results of the synthesized 3D hierarchical structured Co3O4 materials 1#-5#. The peaks in the XRD patterns can be indexed to cubic phase Co3O4 (JCPDS Card No. 43-1003). No other characteristic peaks of impurities are found, revealing the high purity of the obtained Co3O4 materials. Furthermore, the results above and the comparison XRD patterns in Fig. S3 confirm the cubic phase Co3O4 materials derived from different cobalt sources of



Co(NO3)2·6H2O, CoCl2·5H2O and CoSO4·7H2O. The XRD spectra of the corresponding Co3O4 materials 1#-5# with reaction time of 5 h without substrate are exhibited in Fig. S5, showing the same cubic phase. The strong absorption characteristic peaks in Fig. 2c at about 660 cm-1 and 560 cm-1 can be assigned to the ν Co-O. As the Raman spectra shown in Fig. 2d and Fig. S6, there are five Raman active modes for the Co3O4 materials. The characteristic band at around 195 cm-1 is attributed to the F32g symmetry. The bands at 476 cm-1 and 516 cm-1 are assigned to the Eg and the F 2 2g symmetry, whereas the peaks at 610 cm-1 and 680 cm-1 are assigned to the F 1 2g and A 1g symmetry.50

Fig. 2 Comparison of the XRD spectra of a) the in-situ synthesized Co3O4 materials 5# and b)1#-5# with reaction time of 5 h. c) IR spectra and d) Raman spectra of the in-situ synthesized Co3O4 materials 1#-5# with reaction time of 5 h.

Fig. 3 XPS spectra of the in-situ synthesized 3D hierarchical Co3O4 materials 1#-5#. a,d,g,j,m) Co3O4. b,e,h,k,n,) Co 2p. c,f,i,l,o) O 1s.


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For further determining the chemical compositions of the synthesized Co3O4 materials 1#-5#, XPS are conducted with the results shown in Fig. 3. Fig. 3a,d,g,j,m exhibit the whole XPS spectra of Co3O4 materials 1#-5# with the corresponding characteristic peaks assigned in the figures. The survey scan spectra denote the presence of Co and O elements in the obtained Co3O4 materials. In Fig. 3b,e,h,k,n, the peaks with binding energies of about 780.5 eV and 795.6 eV and spin energy separation of 15 eV are assigned to the Co 2p3/2 and Co 2p1/2 characteristic spin-orbit peaks of the Co3O4 phase.51 The existence of Co3O4 can be further confirmed by the characteristic O 1s peaks at 530.0 eV in Fig. 3c,f,i,l,o, which are designated to the oxygen species in the Co3O4 phase. The O 1s peaks at 531.3 eV is corresponds to the oxygen species in hydroxyl.33 The above results provide the further confirmation on the chemical compositions and structural features of the obtained Co3O4 materials.

Fig.4 a) TEM images and b)-c) HRTEM images of the in-situ synthesized Co3O4 materials 2#. The inset is the corresponding SAED pattern. d) SAED pattern of Co3O4 materials 2#.

Fig.5 a) TEM images and b)-c) HRTEM images of the in-situ synthesized Co3O4 materials 5#. The inset is the corresponding SAED pattern. d) SAED pattern of Co3O4 materials 5#.



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The detailed structural and morphological information about the 3D hierarchical Co3O4 materials are investigated by TEM and HRTEM. Fig. 4 and Fig. 5 exhibit the typical TEM images of the in-situ 3D hierarchical Co3O4 materials 2# and 5#, respectively. The structural features of the obtained Co3O4 materials, as mentioned above, the porous and rough granular surfaces constituted by successive nanoparticles, can be further supported by the high magnification TEM figures in Fig. 4a,b and Fig. 5a. The ultrasonic method results in some broken parts from the taken materials. The individual nanocrystals are about 14-20 nm and 20-30 nm for Co3O4 2# and 5#, respectively. The HRTEM images in Fig. 4c and Fig. 5b,c are taken from the single nanoparticle. The lattice fringes spacing of 0.28 nm shown in Fig. 4c is corresponding to the (220) crystal plane of the cubic Co3O4. The result is in good agreement with the XRD pattern of Co3O4 (JCPDS Card No. 43-1003) in Fig. 2. Fig. 5b indicates lattice fringes spacings of 0.24 nm and 0.28 nm, which correspond to the (311) and (220) crystal planes of the cubic Co3O4 respectively. The lattice fringes spacing of 0.28 nm in Fig. 5c is corresponding to the (220) crystal plane of the cubic Co3O4 (JCPDS Card No. 43-1003). The inset figures in Fig. 4c and Fig. 5c indicate the high crystallinity of the obtained Co3O4 materials. Selected area electron diffraction (SAED) rings in Fig. 4d and Fig. 5d can be indexed to the cubic Co3O4 crystal structure. These results further confirm the structural features of the final 3D hierarchical Co3O4 products, which are composed of different morphological types of petals constituted by successive crystalline nanoparticles. Fig. S7 and Fig. S8 indicate the typical UV-Vis spectra of the Co3O4 materials 1#-5#. All the 3D hierarchical Co3O4 products show two characteristic absorption bands between 200-550 nm and 550-800 nm with the central values at ca. 390 nm and 690 nm, respectively. Based on the previous papers related the UV-Vis spectra of Co3O4, there are two optical band gap energies of Eopt1 and Eopt2 with the corresponding values of about 1.5 eV and 2.0 eV, respectively. 52-56 Ⅲ

Ⅱ

The valence band of Co3O4 shows a strong O 2p character. Co results in a subband inside the energy gap, while Co 3d orbits contributes mainly to the conduction band. Eopt1 represents a band edge of the ligand field absorption band at about Ⅱ

Ⅲ

Ⅱ

Ⅱ

550-800 nm, indicating the charge transfer process of O to Co . Eopt2 denates the charge transfer process of O to Co , corresponding to the absorption band between 200-550 nm. 3.1.2 Reaction Time. In order to understand the influence of reaction time on the morphology of the 3D hierarchical structured Co3O4 materials, the reaction times are adjusted from 0 to 10 h. Fig. 6 shows the representative morphology images of Co3O4 materials 1#-5# grown on FTO glasses with the reaction time of 0.5 h, 5 h and 10 h. Different from the typical bare FTO glass, all the five reactions obtain the clear granulated morphology at 0.5 h. After 5 h, the different bionic morphologies of the 3D hierarchical structured Co3O4 materials are obtained, just as shown in Fig. 1. Prolonging the reaction time to 10 h, 1#, 4#, 5# exhibit the similar but bigger flower-like morphologies compared with their counterparts at 5 h. It’s interesting that the final products of 3# changed to the multilevel hexagonal morphology, which is similar to that of 2#. This result could be tentatively attributed to the continuous orientated growth of nanoneedles on the sunflower-like structure of 3#. All the 3D hierarchical structured products at 10 h keep the porous structures and rough granular surfaces, which are composed of successive nanoparticles. The series of XRD patterns in Fig. 7 exhibit the insitu synthesized Co3O4 materials 1#-5# grown on FTO glasses with the reaction time from 0.5 h-10 h. When the reaction time is 0.5 h and 1 h, the characteristic peaks are assigned to the FTO glasses (Fig. S9) or the mix of cubic Co3O4 (JCPDS Card No. 43-1003) and FTO due to the thin Co3O4 films grown on the substrate. The slight right shifts of the XRD peaks for 0.5 h and 1 h in Fig. 7 a-e could be attributed to the in-situ measurement model by using all the Co3O4 samples grown on the FTO glasses for 0.5 h and 1 h. The slightly higher position of the samples with FTO glasses on the


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specimen holder should be responsible for the minor right shifted XRD peaks of 0.5 h and 1 h. When the reaction time is extended to 2 h or longer, the characteristic peaks are in good agreement with the cubic phase Co3O4 (JCPDS Card No. 43-1003). The results also certify the good crystallization of the Co3O4 materials at reaction time of 5 h. In the reaction process, no other characteristic peaks of impurities are found, confirming the high purity of Co3O4 products by the developed in-situ self-assembly controllable synthetic method.

Fig.6 SEM images of the in-situ synthesized Co3O4 materials 1#-5# with different reaction time.



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Fig.7 XRD of the in-situ synthesized Co3O4 materials 1#-5# grown on FTO glasses. a) 1# with reaction time of 0.5 h-10 h; b) 2# with reaction time of 0.5 h-10 h; c) 3# with reaction time of 0.5 h-10 h; d) 4# with reaction time of 0.5 h-10 h; e) 5# with reaction time of 0.5 h-10 h; f) comparison of 1#-5# with reaction time of 5 h. All the samples at 0.5 h and 1 h are measured with the FTO glasses due to the thin thickness of Co3O4 films, while the materials at 2-10 h are conducted using the nanomaterials only.

3.1.3 Reaction Concentration. Fig. 8 presents SEM images of the 3D hierarchical structured Co3O4 materials grown on FTO glasses with different concentrations based on reaction 5#. Changing the molar concentrations of all the reactants to 1/10, 1/5 and 1/2 of the original values, the morphologies of the reaction products gradually transformed from the nanowaires grown on the surface of FTO glasses (Fig. 8a and 8e) to orchid leaves-like structures (Fig. 8b and 8f) and fireball-like appearance (Fig. 8c and 8g). The diameters of nanowaires in Fig. 8a and 8e range from about 50-80 nm (the high-magnification SEM image is shown in Fig. S10a). In Fig. 8b and 8f, the 15-19 µm diametered orchid leaves-like 3D structures are assembled with petal-shaped structures, which are further composed of 1D nanowaires with the diameter of 60-100 nm (Fig. S10b and the insert of Fig. S10e). As for the hollow fireball-like appearance (Fig. 8c, 8g, the inset of Fig. S10f), the diameter is in the range of 8-12 µm. Interestingly, the flexible and curling nanowires with the diameter of 10-20 nm in Fig. S10c are further constituted by the successive nanoparticles, thus forming a porous structure and rough granular surface, just look like the “flames” on the fireball-like structure. Fig. S10d-f display the uniform feature of the large-scale in-situ prepared 3D hierarchical Co3O4 materials. Furthermore, the feature of radiating outward of the architecture remains from Fig. 8b and 8d.

Fig.8 SEM images of the 3D hierarchical structured Co3O4 materials grown on FTO glasses with different concentrations based on reaction 5#. a) All the concentrations of reactants is 1/10 of 5#; b) The concentrations of reactants decrease to 1/5 of 5#; c) The concentrations of reactants change to 1/2 of 5#; d) The original concentrations (molar concentration: Co(NO3)2·6H2O : NH4F : CO(NH2)2 = 5 M : 10 M : 25 M); e)-h) SEM images at different magnifications for Co3O4 materials with different reactant concentrations.


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3.1.4 Substrate and Orientation. The developed typical in-situ synthetic strategy of 3D bionic hierarchical Co3O4 materials was found to be easily extended to the other substrates, such as Al, Ti and Cu except for FTO. Fig. 9 shows the corresponding SEM images of the 3D hierarchical Co3O4 materials grown on different substrates. Fig. 9b displays the hexagonal structured Co3O4 materials grown on Al foil, which are assembled with two triangles with the side length of 12-16 µm. The obtained materials based on Cu foil and Ti sheet in Fig. 9c-9d show the similar structures to the cotton rose flower-like product grown on FTO glasses. However, the morphology on Cu foil exhibits a more sparse appearance and Ti sheet shows a more dense shape compared with the FTO glasses. The different morphologies on several substrates can be attributed to the different surface features which affect the morphology evolution.

Fig.9 SEM images of the 3D hierarchical structured Co3O4 materials grown on a) FTO glasses; b) Al substrate; c) Ti substrate and d) Cu substrate with the high-magnification SEM images as the insert figures.

The different orientations of the substrate were also investigated. As shown in Fig. 10, the dense and well ordered Co3O4 nanoneedle arrays grow uniformly on the FTO glasses. The morphologies of as-prepared Co3O4 materials appear as the different kinds of “grasses” on a lawn (Fig. 10b,c,e,f,h,i). It’s notable that each grass blade is composed of several nanoneedles, which are further constituted by the successive nanoparticles (Fig. S11a). The high-magnification SEM images in Fig. 10a reveal that the nanoneedles are about 70-90 nm in diameter when the FTO surface is oblique upward during the in-situ self-assembly synthetic process. While the Co3O4 nanoneedle arrays grow on the oblique downward FTO glass (Fig. 10d), the diameters of the nanoneedles rang from 40 to 60 nm with the thickness of about 2.5 µm (Fig. S11b,c). The diameters of the nanoneedles rang from 90 to 120 nm was obtained by the Co3O4 nanoneedle arrays grow on the horizontal downward FTO glass (Fig. 10g). It’s worth nothing that the SEM results shown in Fig. S12 confirm the large scale production of these ordered Co3O4 nanoneedle arrays with the extremely uniform surfaces.

Fig.10 SEM images of the in-situ synthesized 3D hierarchical Co3O4 materials on FTO glasses with different orientations. The morphology at different magnifications a-c) grown on the oblique upward FTO glass; d-f) on the oblique downward FTO glass; g-i) on the horizontal downward FTO glass.



3.1.5 Ionic Liquid-assisted Reactions. In order to expand the understanding of this in-situ self-assembly controllable synthetic method, the ionic liquid (IL)-assisted reactions were also investigated. The model IL of 1-ethyl-3methylimidazolium tetrafluroborate (EMImBF4) was added into the reaction system. Fig. S13 displays the hierarchical disc-shaped appearance of the obtained products with the diameter of 8-10 µm. The disc-shaped structure with rough granular surface is further assembled with successive nanoparticles and the nanoneedles in diameter of 60-80 nm on the side, as shown in the insert figures in Fig. S13a-c. 3.1.6 Comparison with Traditional Synthetic Method. The developed in-situ self-assembly one-step synthetic method for Co3O4 materials in this work is not only simple and low cost, but also easy to control the peculiar morphologies to obtain various 3D bionic hierarchical Co3O4 materials. The materials possess the features of uniform size and high quality. Under the same reaction conditions, the traditional synthetic methods were also conducted. As shown in Fig. S14, the SEM images exhibit the disordered, irregular and agglomerate morphology of the obtained Co3O4 materials synthesized by the in-situ method which do not grow on the substrate and the traditional method without any substrate under the same reaction conditions. The results give the further proofs on the advantages of the in-situ synthetic strategy over the traditional methods for Co3O4 materials. Moreover, the hierarchical and porous structured Co3O4 materials grown on different substrates could resolve the problems of traditional nanoparticles when fabricating devices, such as the complex process to transfer or fix the materials on substrates, unstable combination between them, as well as unideal electron transfer and repeatability. The features above make the various high-quality Co3O4 materials by in-situ strategy useful in the fields of catalysis, lithium ion batteries, chemical sensing, supercapacitors, electrochromism, energy conversion, etc. 3.2 Morphology Evolution and Formation Mechanism 3.2.1 Morphology Evolution. To clarify the actual morphology evolution process of the developed Co3O4 materials, a series of time-dependent investigations were conducted. The synthetic morphology evolution process of 1#-5# with varied reaction times are investigated in detail. Fig. 11a-f displays the representative SEM images taken from the different stages in morphology evolution processes of the in-situ synthesized 3D hierarchical Co3O4 materials 1#-5#. At the initial stage of 1#, as shown in Fig. 11a, the thin and unevenly distributed Co3O4 grown on the rugged surface of FTO glasses (the SEM image of bare FTO glass is shown in Fig. 6. Then the uniform polyporous Co3O4 film is formed and followed by the slightly thicker honeycomb-like film (Fig. 11b). Later, the nanoneedles grow on the side of four-sided cylinder (Fig. 11c) and then 2D disk-like structure with nanoneedles on the ring is formed (Fig. 11d). As the aggregation of 2D disk-like structures in different directions (Fig. 11e), the embryonic form of the dandelion flower-like 3D hierarchical structures are fabricated (Fig. 11f-h). When the reaction time is 5h, the bionic dandelion flower-like 3D hierarchical structured Co3O4 are achieved (Fig. 11i), just as mentioned in Fig. 1. When the reaction time is further prolonged to 10 h, the Co3O4 material turned bigger (Fig. 11j), but still with the similar morphologies compared with their counterparts at 5 h. 4# and 5# display the similar morphology evolution processes. However, after the honeycomblike film stage, the grass lawn-like structures of 5# is formed instead of the four-sided cylinders with nanoneedles. As for 2#, the relatively compact layers are first constructed and then 2D structured hexagonal nanoplanes consititued by numberous of tiny four-sided cylinders are formed. Subsequently, the nanoneedles grow on the two sides of the plane with certain orientations. As the orientated growth of the nanoneedles, the zephyranthes grandiflora flower-like structure


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are final fabricated. Here, the formation of hexagonal nanoplanes with nanoneedles may be ascribed to the minimization of the surface energy and Ostwald ripening.57 Similarly, with the same reactant molar ratio but different molar concentration, a similar process is observed for 3# with the nanoneedles mainly growing on the ring of the disk planes. Finally, the sunflower-like structures are obtained. All the 3D bionic hierarchical Co3O4 materials with different reaction times developed by the in-situ self-assembly synthetic method exhibit the high quality and high purity, just as the timedependent XRD results shown in Fig. 7.

Fig.11 a-j) Morphology evolution processes along with reaction time of the in-situ synthesized 3D hierarchical structured Co3O4 materials 1#-5#.

3.2.2 Formation Mechanism. The typical precursors of the in-situ synthesized Co3O4 materials before the calcination process could be regarded as Co(OH)F (JCPDS Card No. 50-0827), according to the XRD patterns shown in Fig. 12a and the 3D XRD spectra in Fig. 12b. Furthermore, the IR spectra of the precursors for Co3O4 materials are shown in Fig. 12c, which displays the distinct different result from the final Co3O4 products (the IR spectra of the bare FTO glass is shown in Fig. S15). The strong sharp absorption band at about 3500 cm-1 can be assigned to the characteristic peaks of OH. The broad absorption band at about 3200-3500 cm-1 and the weak absorption band at about



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1600 cm-1 are distributed to the characteristic peaks of stretching and bending vibrations of O-H from the absorbed H2O. The strong characteristic peaks of Co-O in Fig. 2c at about 660 cm-1 and 560 cm-1 disappear in all the spectra of the precursors. Correspondingly, the new characteristic peaks near 1000 cm-1 and 750 cm-1are found for bending vibrations of Co-OH and Co-F. The EDS results in Fig. 12d and Fig. 12e provide the direct proof for the constitution of Co, O and F in the precursors.

Fig.12 a) XRD patterns, b) 3D XRD spectra, c) IR spectra, d) EDS spectrum, e) EDS mapping of the Co(OH)F precursors for the in-situ synthesized Co3O4 materials 1#-5#.

Fig.13 Morphological changes of the Co3O4 products a-c) 1# and d-f) 2#: a) morphology of 1# before the calcination. b-c) morphology of 1# after the calcination process. d) morphology of 2# before the calcination. e-f) morphology of 2# after the calcination process.

The formation of the precursors Co(OH)F can be described with the following formulas:57 H NCONH 3H O → 2NH CO 2OH (1) NH F → NH F Co OH F → CoOHF

(2) (3)

The obtained precursors of Co(OH)F are rinsed and further calcinated in the air. Under the action of O2, Co(OH)F precursors are finally oxided into Co3O4 products. It’s noteworthy that the reactions in the calcination process result in the obvious morphology changes as shown in Fig. 13. After the calcination, the precursors of Co(OH)F with smooth surfaces (Fig. 13a and Fig. 13d) develop into the final 3D hierarchical structured Co3O4 products with porous structures and rough granular surfaces, which are actually constituted by the successive nanoparticles (Fig. 13b-c, Fig. 13e-f and


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Fig. S2). The results above clarified the corresponding mechanisms of the morphology evolution, reaction process, product formation, calcining process for the formation of specific morphologies. 3.3 Catalytic Properties 3.3.1 Catalytic Application for Decomposition of Ammonium Perchlorate. To investigate the potential catalytic activity of the developed Co3O4 materials, their effects on the decomposition of ammonium perchlorate (AP) are conducted, which is known widely used in high-energy fuels such as rocket propellant in rocket launch, explosive, firework, oxidizer, analytical reagent, bleaching agent, blowing agent, as well as medicine industry. It’s a remarkable fact that the thermal decomposition properties of AP directly influence the combustion behavior of the rocket propellants. The lower pyrolysis temperature and more exothermic heat facilitate the application in rocket propellants.58-61 The developed 3D hierarchical structured Co3O4 products 1#-5# by the in-situ self-assembly controllable synthetic method are applied for the decomposition of AP. Fig. 14a displays the TG curves for neat AP and the mixtures of AP with 2 wt % Co3O4 materials 1#-5#. There are two weight loss steps for neat AP and only one weight loss steps for the mixtures of AP with Co3O4 catalyzers. The initial thermal decomposition of pure AP takes place at about 283 ℃ with the final thermal decomposition at near 443℃. The initial thermal decomposition of AP with Co3O4 catalyzers 1#-5# are at about 235℃, 233℃, 234℃, 232℃ and 232℃. While, the corresponding final thermal decomposition temperatures are about 306℃, 308℃, 315℃, 296℃ and 301 ℃, respectively. The above results are atractive when compared with the other reported values of 353℃ for urchin-like Co3O4, 326 ℃ for stacked Co3O4, 325℃ for flower-like Co3O4 and 319℃ for nanosheet Co3O4, respectively.59-60 The Co3O4 material with disordered and agglomerate morphology by the traditional synthetic method achieve the final thermal decomposition temperature of about 316℃ (Fig. S16). Except for 3#, all of the Co3O4 catalyzers 1#, 2#, 4#, 5# exhibit excellent catalytic properties compared with the non-in-situ catalyzer. 3# shows a slightly higher final thermal decomposition temperature than the value of non-in-situ catalyzer. The result indicates that the final thermal decomposition temperatures of AP are remarkably decreased by about 128-147℃ for the obtained Co3O4 materials 1#-5#.

Fig.14 a) TG curves for AP and AP with 2 wt % Co3O4 materials 1#-5# (mass ratio: AP: Co3O4 = 2:98). DSC curves for thermal decomposition of b) AP and c-g) AP with 2 wt % Co3O4 materials 1#-5# respectively.

Furthermore, the DSC measurements are also conducted to clarify the exact decomposition process of AP and AP with Co3O4 catalyzers. As shown in Fig. 14b, the thermal decomposition of AP contains three steps: the endothermic



phase transition from orthorhombic to cubic at 246℃, the low-temperature decomposition (LTD) at about 314℃ and the high-temperature decomposition (HTD) at about 438 ℃ .62 The addition of Co3O4 catalyzers 1#-5# makes the decomposition steps of AP blend into one process and obviously decreases the decomposition temperatures and increases the exothermic heat at the same time (Fig. 14c-h). The mixtures of AP with 2 wt. % Co3O4 materials 1#-5# decreases the decomposition temperatures from 438℃ into 302℃, 306℃, 320℃, 295℃and 300℃respectively when compared with that of pure AP. 3# decreases the decomposition temperature of AP by about 118℃. While the corresponding values for 1#, 2#, 4#, 5# are 136℃,132℃,143℃,138℃,respectively. Although 3# exhibits the lowest decreasement on AP decomposition temperature, it achieves the highest exothermic heat of 1228 J/g. It’s also noted that the exothermic heat are increased into 1197 J/g, 994 J/g, 933 J/g and 1123 J/g for AP with Co3O4 catalyzers 1#, 2#, 4#, 5#. Compared with the value of AP, the exothermic heat of AP with Co3O4 catalyzers 1#-5# are improved. 1# and 5# exhibits both of the high decreasement on AP decomposition temperature and high exothermic heat. The results above definitely certify the significant effects on the decomposition of AP and the good catalytic properties of the Co3O4 materials 1#-5# by the developed in-situ self-assembly controllable synthetic method. 3.3.2 Catalytic Application for Dye-sensitized solar cells. As one of the low cost and environmental friendly alternatives, DSSCs have attracted significant attention since the breakthrough in 1991 by M. Grätzel and co-workers.63 A typical DSSCs consists a semiconductor oxide photoanode (usually TiO2, ZnO), a sensitizer, electrolyte and a counter electrode (CE, usually Pt). The theoretical analysis on the band diagram and electron transfer mechanism of DSSC is exhibited in Fig. S17. The electron transfer mechanism can be simply described as follows: under the action of sunlight, the sensitizer is photoexcited (Dye*), and the excited electrons are injected into the conduction band of TiO2 and then collected by the FTO substrate and transferred to the counter electrode through the external load. The reduction reactions between the excited sensitizer and the redox couple (iodide) in the electrolyte could realize the regeneration of the sensitizer. Subsequently, the iodide is regenerated by the reduction of triiodide from the counter electrode. The energy level difference between the Femi level of TiO2 and redox couple is corresponding to the value of open-circuit photovoltage (VOC). Counter electrode is a key factor that affects the photovoltaic performance of DSSCs. At present, the high efficiency of over 12% has been achieved based on a Pt counter electrode.64 However, the disadvantages of traditional Pt counter electrodes, such as high cost and rare, are considered as the critical factors limiting the costreduction of outdoor use of DSSCs. Therefore, considerable efforts have been devoted to investigating inexpensive and abundant alternative catalytic materials for counter electrodes.65-71

Fig.15 The I-V curves of the DSSCs based on the Pt and Co3O4 counter electrodes.


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The developed 3D hierarchical structured Co3O4 materials 1#-5# are applied as the counter electrodes of DSSCs. Fig. 15 exhibits the I-V curves of DSSCs based on the Pt and Co3O4 1#-5# counter electrodes. The corresponding parameters are summarized in Table S1. It’s clear that the device based the developed 3D hierarchical structured Co3O4 material 5# achieved a conversion efficiency of 5.68 %. The VOC, short-circuit photocurrent density (JSC) and fill factor (FF) of Co3O4 counter electrode are 0.68 V, 12.24 mA cm-2 and 0.69, respectively. Co3O4 1# and 4# counter electrodes achieve the similar conversion efficiencies of 5.68 % and 5.69 % to the value of 5# due to the similar morphologies and sizes. Traditional Pt counter electrode exhibits the conversion efficiency of 8.83 % with the corresponding VOC, JSC and FF values of 0.74 V, 17.19 mA cm-2 and 0.69, respectively. A slight lower conversion efficiencies of 5.55 % and 5.52 % with minor decreased JSC of 11.72 mA cm-2 and 11.55 mA cm-2 are obtained by 2# and 3# counter electrodes. The above results based on the developed 3D hierarchical structured Co3O4 materials are attractive when compared with the other Co3O4 materials.72-73 The results indicate the good catalytic activity of the developed 3D hierarchical Co3O4 materials as the counter electrodes of DSSC.

Conclusions A series of 3D bionic hierarchical Co3O4 micro/nanomaterials with peculiar morphologies, uniform size, and high quality were successfully achieved by a simple, low cost and controllable one step in-situ self-assembly synthetic technique. The hierarchical materials obtained by the in-situ synthetic strategy exhibits outstanding advantages compared with that achieved by the traditional synthetic method. The morphological control and related impact factors such as reactants, mole ratio, reaction time, reaction concentration, substrate, orientation, ionic liquid-assisted reactions were investigated and claried in detail. The related mechanisms on the reaction process, product generation, calcining process and morphology evolution are systematically and clearly clarified. Furthermore, the excellent catalytic properties of these materials are confirmed for both the decomposition of AP and the counter electrode of DSSCs. The addition of Co3O4 materials for decomposition of AP obviously decreases the decomposition temperatures for about 118-143 ℃ and increases the exothermic heat to a great extent at the same time. Furthermore, the 3D hierarchical Co3O4 materials exhibit the attractive photovoltaic performances as the substituted counter electrodes of DSSCs. Moreover, the obtained various 3D hierarchical structured Co3O4 materials could also be expected to have promising applications in catalysis, lithium ion batteries, chemical sensing, supercapacitors, etc. The new and comprehensive insights into the in-situ selfassembly design, morphological control and formation mechanisms of 3D hierarchical structured Co3O4 micro/nanomaterials will be helpful for the development of synthetic technology of high-quality materials and promote their applications in broader fields. The findings presented in this study will provide a facile and effective way for designing and synthesis of new types of 3D hierarchical materials towards high catalytic activity and energy devices.

Associated content *S Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1039/x0xx00000x.



SEM images of Co3O4 materials using CoSO4·7H2O as the cobalt source; High-magnification SEM images of Co3O4; 3D X-ray computerized tomography morphology of Co3O4; XRD spectra of Co3O4 materials using Co(NO3)2·6H2O and CoSO4·7H2O as the cobalt sources; XRD spectra of the Co3O4 materials without substrate. Raman spectra of the in-situ synthesized Co3O4 materials 1#-5# with reaction time of 5 h; UV-Vis spectra of Co3O4; SEM images of Co3O4 materials with different concentrations and different orientation; Cross section SEM images of Co3O4 on the oblique downward FTO glass; SEM images of Co3O4 based on ionic liquid-assisted reactions; SEM images of the Co3O4 materials by different synthesized methods; XRD and IR spectra of the bare FTO glass; TG curves for AP and AP with Co3O4 catalyzer by the non-in-situ method; Theoretical analysis on band diagram and electron transfer mechanism of DSSC; Photovoltaic performance of the DSSCs based on Pt and Co3O4 counter electrodes.

Author Information † Assoc. Prof. Q. Miao, Prof. S. Zhang Corresponding Author *E-mail: [email protected]. Fax: +86-10-82627080 ORCID Qingqing Miao: 0000-0002-0871-9090 Suojiang Zhang: 0000-0002-9397-954X Notes

The authors declare no competing financial interest.

Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No. 51203161, 21576264), the State Key Program of National Natural Science Foundation of China (Grant No. 21436010), and the Major Research plan of the National Natural Science Foundation of China (Grant No. 91434203). We thank for the financial support from the Key Laboratory of Green Process and Engineering, Chinese Academy of Sciences. We also thank Hui Wu for the assistance with computerized tomography measurements, who are from the analysis and test center of the Institute of Process Engineering, Chinese Academy of Sciences.

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