Multidimensional Morphology Control for PS-b-P 4 VP Templated Mesoporous Iron (III) Oxide Thin Films

demonstrate tremendous potential in multiple applications. In the present work, the synthesis of morphology-controlled α -Fe 2 O 3 thin films is realized with polystyrene-block-poly(4-vinyl pyridine) (PS-b-P 4 VP) diblock copolymer assisted sol-gel chemistry. The solvent category (DMF and 1,4-dioxane) and polymer-to-FeCl 3 ratio used for the solution preparation are systematically varied to tune the morphology of the thin films. For both solvents, DMF and 1,4-dioxane, nanocluster structures are obtained with low PS-b-P 4 VP concentration. When the concentration of PS-b-P 4 VP reaches the critical micelle concentration, spherical and wormlike porous structures are specifically formed in the DMF and 1,4-dioxane solvent system, respectively. Further increasing the polymer-to-FeCl 3 ratios leads to the enlargement of the spherical pore sizes in the DMF system, whereas the center-to-center distances of the wormlike structures in the 1,4-dioxane system decrease. Moreover, DMF/1,4-dioxane solvent mixtures with different volume ratios are applied for the sol–gel solution preparation to gain more insight into how the solvent selectivity affects the thin film morphology. By adjusting the preferential affinity of the solvent mixture to the polymer blocks, a spherical to wormlike pore shape transition is observed with a critical Δ χ value of around 0.77. and XPS characterization results the hematite phase all The morphology control at the


Introduction
Hematite (α-Fe 2 O 3 ), as the most common iron oxide polymorph, is characterized by its abundance, nontoxicity, good thermodynamic, and chemical stability as well as large bandgap (around 2.1 eV). [1][2][3][4] Compared with the bulk form, for a proper dispersion in the block copolymer is required. For example, Barandiaran and Kortaberria [1] modified Fe 2 O 3 maghemite NPs with polystyrene (PS) brushes and prepared hybrid polystyrene-block-poly(4-vinyl pyridine) (PS-b-P 4 VP)/ Fe 2 O 3 nanocomposites with tailored magnetic properties. The morphology change of the neat and composite thin films was systematically investigated with respect to a dioxane vapor annealing time. Abul Kashem et al. [31] prepared a series of polystyrene-block-polyisoprene/maghemite NPs (PS-b-PI/Fe 2 O 3 ) composite thin films by grafting PS chains on to maghemite NP surface and studied the structural evolution of the thin film structures with respect to the NP concentration. Yao et al. [32] controlled the alignment of polystyrene coated maghemite NPs within a block copolymer matrix by employing different external magnetic fields. The nano-and microstructure and the magnetic behavior of the hybrid films were investigated as a function of the NP concentration. Emrick et al. [33] modified CdSe NPs with trioctylphosphine oxide (TOPO) and polyethylene glycol polymer, which allowed for the specific location of the NPs in the PS and P 2 VP domains of the block polymer PS-b-P 2 VP. However, this method involves sophisticated NP preparation and modification processes and serious NP agglomeration can be provoked through a high loading of the polymer matrix with NPs. In the case of the in situ reduction approach, the metal precursors are specifically cooperated in one domain of the block copolymer and the metal oxide NPs can be obtained by removing the organic species with pyrolysis or oxygen plasma treatment. Accordingly, the complex surface modification of the NPs is circumvented. For example, Hardy et al. [5] synthesized highly ordered iron oxide NPs (α-Fe 2 O 3 ) through well-defined ferrocene-containing triblock copolymers by UV/ozonolysis and thermal pyrolysis. Sohn et al. [2] realized the concurrent self-assembly of gold and iron oxide NPs by adding dodecanethiol-protected gold NPs and FeCl 3 to the toluene solution of PS-b-P 4 VP micelles. To the best of our knowledge, the polymer concentration and the solvent effect for block copolymer templated α-Fe 2 O 3 thin film synthesis has been rarely investigated. Related research in our group demonstrated the prominent influence of the PS-b-P 4 VP concentration on the ZnO morphology and the solvent effect during the polystyrene-block-polyethylene oxide (PS-b-PEO) templated TiO 2 and SnO 2 synthesis. [26,34,35] The formation of thermodynamically stable block polymer micelles is mainly governed by the following three contributions: The stretching degree of the core-forming polymer chains, the interfacial tension between the micelle core and the surrounding solvent, and the repulsive interactions among corona-forming chains. [25] The first two contributions can be affected by factors including copolymer composition and concentration, poor solvent content in the solution, and the preferential affinity of the common solvent. [36] The third contribution mainly refers to block polymers, which contain easily ionized chains during micellization. In this situation, the electrostatic repulsions among the polymer chains turn out to be the major component affecting the micellar structures. Accordingly, the morphology control for this kind of block polymer can be realized via the addition of acids, bases, or salts, such as HCl, NaOH, or NaCl, etc. [37] In the present work, the PS-b-P 4 VP diblock copolymer templated α-Fe 2 O 3 thin film synthesis is systematically investigated in terms of the solvent category and PS-b-P 4 VP concentration. Two kinds of solvents with different preferential affinities to the polymer blocks (DMF and 1,4-dioxane) and three diblock copolymer (PS-b-P 4 VP) to precursor (FeCl 3 ) weight ratios (ω PS-b-P4VP :ω FeCl3 ) are used for the solution preparation. Moreover, a series of DMF/1,4-dioxane mixtures with different compositions are confected for tuning the preferential affinity of the solvent for tracking the continuous evolution of the nanostructures. Due to keeping the FeCl 3 concentration in each sol-gel stock solution identical, the influence of the ionization of the P 4 VP chains on the thin film morphology is ignored in the following analysis. The phase composition of the thin films is determined by X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The surface and inner film morphologies are probed with scanning electron microscopy (SEM) and grazing-incidence small-angle X-ray scattering (GISAXS), respectively. Based on the multidimensional characterization results, the underlying mechanism for governing the structural evolution of the thin films is revealed.

Sample Preparation
The preparation routine of PS-b-P 4 VP templated α-Fe 2 O 3 thin films is schematically shown in Figure 1a. For the sol-gel stock solution preparation, 2 mL solvent (DMF or 1,4-dioxane) was equally divided into two portions for dissolving FeCl 3 powder and PS-b-P 4 VP polymer, respectively. 20 mg FeCl 3 powder was weighed in a N 2 glove box to get rid of the effect of moisture in the air. After being taken out from the glove box, the FeCl 3 powder was dissolved with 1 mL solvent (DMF or 1,4-dioxane). Meanwhile, PS-b-P 4 VP polymer solutions with different concentrations were prepared by dissolving 10 mg, 20 mg, or 40 mg PS-b-P 4 VP into 1 mL of the same solvent. The prepared FeCl 3 and PS-b-P 4 VP solutions were stirred for 1 h for complete dissolution. Afterwards, the FeCl 3 and polymer solutions were mixed and followed by further stirring for one hour to incorporate the inorganic FeCl 3 part into the micellar structures of the PS-b-P 4 VP. Three ω PS-b-P4VP :ω FeCl3 ratios with fixed FeCl 3 concentrations, 1:2, 1:1, and 2:1, were used for investigating the polymer concentration effect in each solvent system. The component phase diagram of the different sol-gel solutions is shown in Figure S1 (Supporting Information). Figure S2 (Supporting Information) refers to the photograph of the prepared FeCl 3 and PS-b-P 4 VP/FeCl 3 sol-gel solutions. Via complexation between iron ions and the applied organic solvents (as sketched www.advmatinterfaces.de in Figure 1b,c), the black-brown FeCl 3 powders were completely dissolved and clear solutions were obtained. In addition, no precipitates were observed after mixing the FeCl 3 and PS-b-P 4 VP solutions. For the sol-gel solutions prepared with DMF/1,4-dioxane solvent mixture, 40 mg PS-b-P 4 VP and 20 mg FeCl 3 (ω PS-b-P4VP :ω FeCl3 = 2:1) were first dissolved with 1 mL DMF/1,4-dioxane solvent mixture, respectively. Then, the sol-gel solution was prepared with the same routine as the pure solvent counterpart. For the thin film deposition, the prepared solgel solutions were spin-coated on pre-cleaned silicon substrates with a Delta 6 RC TT spin coater (SÜSS Micro-Tec Lithography GmbH) at 2000 rpm for 60 s. To remove the polymer template, the spin-coated thin films were calcined at 500 °C for 2 h with a heating rate of 1 °C min -1 under ambient atmosphere. Due to the preferential affinity of the solvent to the polymer blocks, various micellar structures formed in the sol-gel stock solution. Moreover, the vinyl pyridine side group on the P 4 VP chain has one free electron pair that is not involved in bonding and therefore can bind or react with Fe 3+ ion as a good ligand. [38] Accordingly, the Fe 3+ species can be confined in the P 4 VP domains through complexation (Figure 1d). By calcining the spin-coated thin films in air, the polymer template PS-b-P 4 VP is removed and mesoporous α-Fe 2 O 3 films formed simultaneously.

Thin Film Characterization
The crystallinity of the α-Fe 2 O 3 thin films was studied with a D8 ADVANCE X-ray diffractometer using an X-ray wavelength of 1.54 Å. Raman spectra were acquired with an "inVia Reflex Raman System," which consisted of a research-grade optical microscope (Leica DM2700M, Magnification 5×, 20×, 50×) and a high-performance Raman spectrometer (Renishaw). For both, the XRD and Raman characterization, drop-casting was used for the film preparation to increase the measurement signal. X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALAB 250Xi spectrometer (monochromatized Al Kα radiation, h ν = 1486.6 eV) at ambient conditions. The surface morphology of the α-Fe 2 O 3 thin films was probed with a high-resolution field emission scanning electron microscope (SEM, Zeiss Gemini NVision 40) at a working distance of 3.5 mm and an acceleration voltage of 5 kV. Grazingincidence small-angle X-ray scattering (GISAXS) measurements for all samples were performed at the P03 beamline of PETRA III storage ring. [39] The wavelength and incidence angle used for the measurement was 0.96 Å and 0.35°, respectively. The sample to detector distance was 4340 mm and a Pilatus 300K (Dectris Ltd.) detector with a pixel size of 172 μm was used for recording the scattering signal. A python program called Directly Programmable Data Analysis Kit (DPDAK) was used for calibration and data analysis. [40] [41] The XRD characterization indicates that the crystallinity of the thin films is almost unaffected by the utilized solvent or ω PS-b-P4VP :ω FeCl3 ratios. Besides the XRD characterization, the phase composition of the calcined thin films is further confirmed by the Raman spectra. The vibrational modes of hematite at the first Brillouin zone center can be represented by

Phase Composition
The A 1u and A 2g modes in the equation are acoustic and optically silent. The remaining symmetrical (g) modes are active in Raman, and the asymmetrical (u) ones are active in infrared (IR) spectra. Figure 2b shows the Raman spectra of the calcined thin films prepared with different solvents and ω PS-b-P4VP :ω FeCl3 ratios. The peak located at 221 cm -1 corresponds to A 1g mode and the remaining two peaks at 288 and 405 cm -1 are attributed to the E g modes. [10,42] The appearance of these three Additionally, Figure 3a,b shows the core-level Fe 2p XPS spectra for the calcined thin films prepared with ω PS-b-P4VP :ω FeCl3 of 2:1 in DMF and 1,4-dioxane solvent. The major Fe2p 1/2 and Fe2p 3/2 peaks of the two samples are observed centered at 710.8 eV and 724.4 eV with a spin energy separation of 13.6 eV. The two weak peaks that appear at 718.9 and 732.4 eV correspond to the shakeup satellites of Fe2p 3/2 and Fe2p 1/2 . All peaks are typical characteristics of α-Fe 2 O 3 . [3,43,44] Apart from the Fe 2p peaks, prominent O 1s and C 1s peaks are also clearly visible in the full-range XPS survey spectra of both thin films, which can be ascribed to the organic functional group residues of the polymeric template after calcination ( Figure 3c).   :ω FeCl3 ratio used for the thin film preparation is labeled above the respective XRD or Raman data.

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corresponding ω PS-b-P4VP :ω FeCl3 values used for the thin film preparation are 1:2, 1:1, and 2:1 for Figure 4a-c, respectively. The thin film prepared with ω PS-b-P4VP :ω FeCl3 of 1:2 is mainly composed of small nanoclusters (Figure 4a). When increasing the ω PS-b-P4VP :ω FeCl3 ratio to 1:1, a nanopattern consisting of spherical pore structures is formed. The porous structure of the thin film is directly associated with the template effect of the micelles in the solution. Due to the preferential affinity of the DMF solvent to the P 4 VP chains, spherical micelles consisting of PS cores and P 4 VP/Fe 3+ coronas are prone to be formed, no matter in the DMF solution or the final dry film. The structural transition from nanoclusters to the patterned nanostructures can be related to the micellization of the PS-b-P 4 VP template at the critical concentration, which is proportional to the concentration of the block copolymer. However, the formed micellar structures are labile at the early stage of the micellization due to the highly swollen condition of the core-forming chains by the solvent, which provides a strong probability for micelle fusion. This fusion might account for the existence of the secondary large pore structures seen in Figure 4b. [45] By further increasing the ω PS-b-P4VP :ω FeCl3 ratio to 2:1, a more uniform distribution of the pore structures and a slight increase of the pore size are observed within the thin film (Figure 4c). The influence of copolymer concentration on the micellar morphology can be tentatively explained by the equation from a theory for small-molecule surfactant micelles. [25] N agg is the average number of polymer chains in an aggregate, C represents the copolymer concentration and CMC is the copolymer concentration at the critical water content. As seen in Equation (2), N agg is proportional to the copolymer concentration C. For spherical micelles, the increase of the N agg value yields larger core dimensions and an increase in the stretching degree of the core-forming blocks. This process is energetically favorable because the formation of a larger core size decreases the aggregate number and the total interfacial area within the system. Cross-section images of the α-Fe 2 O 3 thin films are  Figure S4 (Supporting Information).

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shown in Figure S3 (Supporting Information). It is observed that all the thin films are composed of a mesoporous α-Fe 2 O 3 monolayer. Figure 4d shows the corresponding power spectral density (PSD) functions extracted from the SEM images. The enhanced ordering of the nanostructures can be manifested in the FFT patterns by the appearance of a sharp peak in the PSD functions. The sharp peaks of the PSD functions show the q values of the most dominant lateral feature sizes in the SEM images. [46] Specifically, the peak values of the thin films prepared with ω PS-b-P4VP :ω FeCl3 ratios of 1:2, 1:1, and 2:1 are determined to be 0.31, 0.34, and 0.28 nm -1 , which correspond to feature sizes of 20.3, 18.5, and 22.4 nm, respectively. The system correlation length is a measure for the lateral ordering of structures within the thin film, which is inversely proportional to the full width at half maximum (FWHM) of the firstorder peak of the PSD functions. [47] The calculated correlation lengths for the nanostructures shown from Figure 4a With the SEM measurements, the local structure on the film surface is probed. However, in practical applications, the large area homogeneity and the inner structure of the thin film are also of great importance. Grazing-incidence smallangle X-ray scattering (GISAXS) ideally complements microscopic imaging techniques as it readily provides statistical structure information at both, the surface and the bulk of the thin film. [48] Figure 5a-c shows the 2D GISAXS data of the α-Fe 2 O 3 thin films prepared with DMF solvent. The shape of the Bragg peaks indicates the presence of well-ordered nanostructures in the horizontal direction of the thin films. [49] To extract the lateral structure information within the thin films, horizontal line cuts are performed at the material characteristic Yoneda peak as indicated by the white arrow in Figure 5a. Figure 5d shows the horizontal line cuts of the 2D GISAXS data. The additional broad peak in the low q y region for the thin film prepared with ω PS-b-P4VP :ω FeCl3 of 1:1 can be assigned to the existence of the fused large pore structures within the film. Moreover, the shift of the most pronounced scattering peaks in Figure 5d is in good accordance with the behavior of the PSD functions, which qualitatively reflects that the prepared thin films possess the same structural evolution trend within the local and large probed area. For quantitatively extracting the in-plane structure information of the α-Fe 2 O 3 thin films, the horizontal line cuts in Figure 5d are fitted within the framework of the distorted-wave Born approximation (DWBA) using a local monodisperse approximation (LMA). [50] In the modeling, Gaussian distributed form factors and structure factors are assumed. [51] The form factors denote the shape and size of the scattering objects, which is assumed to be standing cylinders, and the structure factors refer to the center-to-center distances between the scattering objects in a 1D paracrystal arrangement, respectively. For more details, see Supporting Information. Error bars indicate the scope of the goodness of fit. The fitted radii and distances are shown in Figure 5e,f. The most conspicuous scattering peak in the high q region is fitted to extract the second structure information. When the ω PS-b-P4VP :ω FeCl3 ratio increase from 1:2 to 2:1, the center-to-center distances of the second structure are determined to be (20.0 ± 1.4), (17 ± 3), and (22 ± 2) nm. The fitted center-to-center distances are in good agreement with the feature sizes calculated from the main peak of the PSD functions, which reflects a good consistency of the main feature sizes in the local and large probed areas of the thin film. In addition, the corresponding radii of the second structure are fitted to be (5.6 ± 0.3), (4.5 ± 0.4), and (5.0 ± 0.4) nm. Based on the fitted center-to-center distances and radii, the pore sizes formed by the nanostructures can be calculated with the approach [34,52] Pore size center to center distance 2 structure radius Considering that the thin film prepared with a ω PS-b-P4VP :ω FeCl3 ratio of 1:2 is mainly composed of discrete nanoclusters and no distinct pore structures can be observed, the pore size calculation is only performed for the other two samples. Accordingly, the pore sizes calculated from the second center-to-center distances and radii are (8 ± 4) and (12 ± 3) nm for the thin films prepared with ω PS-b-P4VP :ω FeCl3 of 1:1 and 2:1. In addition, the weak scattering intensity in the higher q y region for all line cuts is fitted with a smaller first center-to-center distance and radius. The first center-to-center distances for the thin films prepared with ω PS-b-P4VP :ω FeCl3 ratios of 1:2, 1:1, and 2:1 are determined to be (10.0 ± 1.5), (8.6 ± 0.9), and (11.0 ± 1.2) nm, and the corresponding radii are fitted to be (2.6 ± 0.5), (2.3 ± 0.4), and (2.6 ± 0.5) nm. According to Equation 3, the first pore sizes are calculated to be (4.0 ± 1.7) and (6 ± 2) nm for the thin films prepared with ω PS-b-P4VP :ω FeCl3 of 1:1 and 2:1. Moreover, the additional broad peak in the low q y region for all thin films is fitted with a third distance and radius. Specifically, the center-to-center distance of the thin films prepared with ω PS-b-P4VP :ω FeCl3 ratios of 1:2, 1:1, and 2:1 is determined to be (210 ± 25), (90 ± 19), and (120 ± 18) nm, and the corresponding radius is determined to be (6.5 ± 0.5), (5.0 ± 0.5), and (5.5 ± 0.5) nm, respectively. The pore sizes generated by the third structure are calculated to be (80 ± 20) and (109 ± 19) nm for the thin films prepared with ω PS-b-P4VP :ω FeCl3 of 1:1 and 2:1. The small and large pore sizes calculated from the first and third structure factors can be ascribed to the locally collapsed or fused large pore structures within the thin film. Notably, all the modeled feature sizes in the GISAXS data analysis can be identified with surface features in the SEM images ( Figure 4). The good consistency between the GISAXS fit results and the SEM topography demonstrates that the nanostructures observed within the micron size range extend over a larger area on the thin film. :ω FeCl3 ratio is 1:2, discrete nanoclusters accompanied with large aggregates are observed. By increasing the ω PS-b-P4VP :ω FeCl3 ratio to 1:1 and 2:1, uniformly distributed worm-like structures with enhanced order, as verified by the FFT patterns, are formed. Figure 6d shows the corresponding PSD functions calculated from the SEM images. Due to the polydispersity of the nanostructures, no distinct peak is observed in the PSD function of the thin film prepared with a ω PS-b-P4VP :ω FeCl3 ratio of 1:2. The sharp peaks shown in the other www.advmatinterfaces.de two PSD functions can be used for determining the feature sizes and order. The peak centers of the thin films prepared with ω PS-b-P4VP :ω FeCl3 ratios of 1:1 and 2:1 are determined to be 0.23 and 0.24 nm -1 , which corresponds to feature sizes of 27.3 and 26.2 nm, respectively. The FWHM of the thin films prepared with ω PS-b-P4VP :ω FeCl3 ratios of 1:1 and 2:1 are determined to be 0.11 and 0.13 nm -1 , which corresponds to correlation lengths of 9.1 and 7.7 nm, respectively. The fitted peak position and calculated correlation lengths indicate that both, the feature sizes and order of the thin films are decreased by increasing the ω PS-b-P4VP :ω FeCl3 ratios from 1:1 to 2:1. The drastic morphology variation of the thin films with respect to the solvent category can be tentatively explained by a preferential affinity of the solvent to the polymer blocks. Based on the characteristic parameters of the different polymers and solvents, the polymer-solvent interaction parameters (χ P−S ) is calculated via  www.advmatinterfaces.de and 23.0 MPa 1/2 , respectively. [53] Table 2 are the corresponding χ P−S values calculated by Equation (4). According to the Flory-Huggins theory, the poly mer and solvent are completely miscible when χ P-S is less than 0.5. The preferential affinity of the solvent to a certain block can be calculated by the difference between the χ PS-S and The calculated Δχ for the PS-b-P 4 VP/DMF and PS-b-P 4 VP/1,4-dioxane polymer-solvent pairs are 1.1 and -0.09. The high Δχ value of PS-b-P 4 VP/DMF reflects the inferior solubility of the PS blocks in DMF, which leads to the significantly inhibited stretching of the PS blocks and thus promotes the formation of spherical PS micellar cores. However, the slightly negative Δχ value of PS-b-P 4 VP/1,4-dioxane polymer-solvent pair indicates that the 1,4-dioxane has a very slight preferential affinity to the PS block. Accordingly, the appearance of the aggregates in Figure 6a and the wormlike structures in Figure 6b,c can be ascribed to the weak selectivity of 1,4-dioxane and an enhanced stretching of the PS blocks. In addition, the inter-domain distance of the wormlike  :ω FeCl3 ratio from 1:2 to 2:1. The first, second, and third nanostructures used for the GISAXS data modeling are marked with white circles in (a-c). For clarity, the zoom-in view of the nanostructure is presented in Figure S5 (Supporting Information).

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structure decreases by increasing the ω PS-b-P4VP :ω FeCl3 ratio from 1:1 to 2:1, which is against the trend shown in the DMF solvent system. The decreased inter-domain distance induced by the increase of the ω PS-b-P4VP :ω FeCl3 ratio can be assigned to the unique shape of the micellar structure. It is observed that the thin films prepared with ω PS-b-P4VP :ω FeCl3 of 1:1 and 2:1 are mainly composed of interconnected wormlike structures, which is prone to deform into more elongated wormlike micelles for accommodating more polymer chains and maintaining low surface energy in the monolayer geometry. Figure 7a-c shows the 2D GISAXS data of the α-Fe 2 O 3 thin films prepared with 1,4-dioxane. Compared with the thin film prepared with a ω PS-b-P4VP :ω FeCl3 ratio of 1:2, the clearly visible Bragg rod in the lateral direction of the other two thin films can be attributed to an enhanced in-plane order of the nanostructures. Figure 6d shows the corresponding line cuts made from the 2D GISAXS data. The line cuts clearly show the sharpening of the Bragg peak by increasing ω PS-b-P4VP :ω FeCl3 ratio from 1:2 to 1:1. In addition, the Bragg peak shifts towards larger q y values and broadens by further increasing the ω PS-b-P4VP :ω FeCl3 ratio from 1:1 to 2:1. The shift and width of the scattering peaks in Table 2. Polymer-solvent interaction parameters (χ P-S ) of different polymer-solvent pairs.  Figure 6d, which demonstrates that the feature size variation trend is consistent in the local and large area of the thin film.

Thin Film Morphology Evolution with respect to the Solvent Category and ω PS-b-P4VP :ω FeCl3 Ratios
From the discussion above, the good consistency between the SEM topography and the structures extracted in the GISAXS data analysis demonstrates that a large area homogeneity is achieved for all α-Fe 2 O 3 thin films. In addition, it is clearly demonstrated that the α-Fe 2 O 3 thin films prepared with a ω PS-b-P4VP :ω FeCl3 ratio of 1:2 irrespective of the solvent, in DMF and 1,4-dioxane possess similar nanocluster structures. The main structural differences caused by the solvent category consist in the distinct spherical and worm-like nanostructures formed at higher ω PS-b-P4VP :ω FeCl3 ratios. Accordingly, a schematic illustration for the micellar structure evolution, which is directly related to the α-Fe 2 O 3 nanostructures, is given with respect to the different solvent category and ω PS-b-P4VP :ω FeCl3 ratios in Figure 8. By increasing ω PS-b-P4VP :ω FeCl3 ratio from 1:1 to 2:1, a completely different structure evolution tendency is observed for the spherical and wormlike structures. Specifically, the expansion of the spherical micelles with the increase of the ω PS-b-P4VP :ω FeCl3 ratio can be explained by the theory for small molecule surfactant micelles. The average number of polymer chains in an aggregate (N agg ) is proportional to copolymer concentration, and the increased N agg of the spherical micelles contributes to the enhanced stretching of the core-forming blocks and therefore the expansion of the micelles. However, for the interconnected wormlike structures acquired with the 1,4-dioxane solvent, the increase of the ω PS-b-P4VP :ω FeCl3 ratio promotes the formation of more elongated wormlike micelles for accommodating more polymer chains in the monolayer geometry.

Relationship between Thin Film Morphology and Δχ Value of the Cosolvent
The solubility parameter of a solvent mixture can be tuned by changing its composition. [54] For tracking the successive morphology evolution with respect to the solubility parameter of a solvent, we study DMF/1,4-dioxane solvent mixtures with different compositions at a fixed ω PS-b-P4VP :ω FeCl3 ratio of 2:1 for the sol-gel solution preparation. The acquired thin film morphologies are shown in Figure 9. The morphology of the thin films remains almost constant when increasing the volume fraction of 1,4-dioxane from 0 to 20 vol% (Figure 9a-c). The inflection from spherical to worm-like nanostructure is triggered, when the volume fraction of 1,4-dioxane reaches 30 vol% ( Figure 9d). As shown in Figure 9d, the wormlike structure appears in a form of micron-sized islands among the spherical pore structures. The formation of this unique hybrid structure is likely due to the incomplete micellar structural transformation in the sol-gel For unraveling the relationship between the thin film morphologies and the preferential affinity of the cosolvent, a phase diagram of the α-Fe 2 O 3 thin film morphologies is plotted as a function of the volume fractions of 1,4-dioxane and the Δχ values of different polymer-solvent pairs (Figure 10). The specific polymer-solvent interaction parameters (χ P-S ) used for the Δχ calculation are listed in Table S1 (Supporting Information).
According to the coordinate system, the thin film morphology is primarily templated by the spherical micelle structure in the region of 0.88 ≤ Δχ ≤ 1.09. The spherical-to-wormlike micellar transformation occurs at the Δχ value of 0.77. The further decrease of the Δχ value from 0.65 to -0.09 yields the neat wormlike nanostructures.
In order to investigate the influence of the cosolvent compositions on the feature sizes and order of the α-Fe 2 O 3 thin films, PSD functions of the SEM images from Figure 9 are calculated. As shown in Figure 11a, a significantly enhanced intensity in the low q region is observed for the thin film prepared with 30 vol% 1,4-dioxane. This can be ascribed to the appearance of the micron-sized islands within the thin film. Figure 11b,c refer to the corresponding peak and FWHM values extracted from the PSD functions. A progressive shift of the peak position towards smaller q values is observed by increasing the volume fraction of 1,4-dioxane from 0 to 90 vol%, which indicates the continuous expansion of the feature sizes of the pore structures. However, a slight shift of the peak position toward larger q values is observed by increasing the 1,4-dioxane volume fraction from 90 to 100 vol%. This shift is likely caused by the viscosity difference between the DMF/1,4-dioxane co-solvent and pure 1,4-dioxane. [55] In addition, the extracted FWHM values (Figure 11c) indicate that the order of the nanostructure is decreased with the increase of the volume fraction of 1,4-dioxane. The highest FWHM value of the thin film prepared with 30 vol% 1,4-dioxane can be ascribed to the mixture of the micron-sized island structures, which greatly diminishes the ordering of the thin film.

Conclusion
Mesoporous α-Fe 2 O 3 thin films with different morphologies are synthesized by using FeCl 3 as precursor and the amphiphilic diblock copolymer PS-b-P 4 VP as a template. The XRD, Raman,

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and XPS characterization results confirm the hematite phase in all films. The morphology control at the molecular level for the thin films is realized by changing the following two factors: the solvent category and the ω PS-b-P4VP :ω FeCl3 ratio. The SEM and GISAXS characterization results demonstrate the structural consistency on the local and large areas of all thin films. When the ω PS-b-P4VP :ω FeCl3 ratio used for sol-gel solution preparation is 1:2, nanocluster structures are observed in both DMF and 1,4-dioxane solvent. This nanostructure can be assigned to the weak templating effect of the low PS-b-P 4 VP concentration. By increasing the ω PS-b-P4VP :ω FeCl3 ratio to 1:1 , the hierarchical spherical pore structure is formed in the DMF solvent system, whereas the wormlike structure is formed in the 1,4-dioxane system, which signifies the initiation of the phase separation of the PS-b-P 4 VP template in the sol-gel solution. The hierarchical structure in the DMF case can be assigned to the instability of the micellar structure, which leads to a partial fusion of the smaller spherical micelles. In addition, the distinctly different morphologies present in the DMF and 1,4-dioxane system can be attributed to the different preferential affinity of the solvent. Further increasing the ω PS-b-P4VP :ω FeCl3 ratio to 2:1 leads to the expansion of the spherical pore structure, whereas the wormlike pore structure shrinks. The expansion of the spherical pore structure can be explained by the increased average number of polymer chains in an aggregate (N agg ). However, the unique interconnected wormlike structure tends to deform into a more elongated wormlike structure for accommodating more polymer chains in the monolayer geometry. Moreover, DMF/1,4-dioxane solvent mixture with different component ratios are prepared for exploring the influence of the selectivity of the solvent on the nanostructure of the thin film. It is demonstrated that the spherical-to-wormlike porous structural transformation occurs at the critical Δχ value of 0.77 and the structural transformation leads to the expansion of the feature sizes of the nanostructures. The slight shrink of the feature size by increasing the volume fraction of 1,4-dioxane from 90 to 100 vol% is likely due to the viscosity difference between the DMF/1,4-dioxane cosolvent and the pure 1,4-dioxane. Except for the hybrid structure of the spherical and wormlike structure, the ordering of the nanostructures is gradually decreased with the increase of the 1,4-dioxane component in the cosolvent. Given by the advantages of large area homogeneity and multiple morphologies, the α-Fe 2 O 3 thin films prepared in the present work show enormous application prospects in the fields of photovoltaics and sensors.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.