Ordered CoIII‐MOF@CoII‐MOF Heterojunction for Highly Efficient Photocatalytic Syngas Production

The design of advanced metal–organic framework (MOF) catalysts for solar‐driven conversion of CO2 into syngas (CO/H2 mixture) is beneficial. Herein, the design of a joint MOF heterostructure consisting of orderly assembled CoII‐ and CoIII‐based Prussian blue analogs (PBAs) driven by their spontaneous lattice match in the growth process is reported. As‐prepared H/CoIII‐PBA@CoII‐PBA cage is a mesocrystal and exhibits superior photocatalytic syngas production activity (VCO up to 50.56 mmol g−1 h−1, CO/H2 = 1:1), which is among the best state‐of‐the‐art heterogeneous photocatalysts in the literature. Theoretical calculations and experimental results confirm that CoIII‐PBA exerts a stronger affinity for CO2 molecules than CoII‐PBA, thus serving as the active site. The built‐in electric field in the CoIII‐PBA@CoII‐PBA heterojunction can direct the fast transport of photogenerated electrons from CoII‐PBA to the active CoIII‐PBA. In the present case, the engineering of electronics outweighs morphological engineering to enhance the catalytic properties of CoIII‐MOF@CoII‐MOF for CO2‐to‐syngas conversion.

DOI: 10.1002/smsc.202200085 The design of advanced metal-organic framework (MOF) catalysts for solardriven conversion of CO 2 into syngas (CO/H 2 mixture) is beneficial. Herein, the design of a joint MOF heterostructure consisting of orderly assembled Co II -and Co III -based Prussian blue analogs (PBAs) driven by their spontaneous lattice match in the growth process is reported. As-prepared H/Co III -PBA@Co II -PBA cage is a mesocrystal and exhibits superior photocatalytic syngas production activity (V CO up to 50.56 mmol g À1 h À1 , CO/H 2 = 1:1), which is among the best state-of-the-art heterogeneous photocatalysts in the literature. Theoretical calculations and experimental results confirm that Co III -PBA exerts a stronger affinity for CO 2 molecules than Co II -PBA, thus serving as the active site. The builtin electric field in the Co III -PBA@Co II -PBA heterojunction can direct the fast transport of photogenerated electrons from Co II -PBA to the active Co III -PBA. In the present case, the engineering of electronics outweighs morphological engineering to enhance the catalytic properties of Co III -MOF@Co II -MOF for CO 2to-syngas conversion.
In addition, studies have reported the morphological engineering of MOFs, for example, building cage-like structures, whereas they mostly suggested that the large cavity of the cage may favor the catalytic performance of MOFs. [20a,22] In contrast, the electronic engineering of MOFs has not been adequately explored.
In this work, we prepared ordered Co III -MOF@Co II -MOF heterojunctions with intimate contact, in which the two Co-MOFs (with a Prussian blue analog [PBA] as an example) have distinct valence Co states but spontaneously assembled through oriented attachment, forming a Co III -PBA/Co II -PBA mesocrystal. Specifically, Co III -PBA forms a cage to encapsulate Co II -PBA, forming either a cubic block or a smaller cage. The mechanism behind the excellent performance of Co III -PBA@Co II -PBA in syngas generation (V CO = 50 mmol g À1 h À1 , CO/H 2 = 1:1) was explored by experimental characterization along with density functional theory (DFT) calculations.

Establishment of Different Co III -PBA@Co II -PBA Structures
Different Co-PBA structures were obtained using the same procedure by varying the stoichiometry of the starting materials, that is, Co(CH 3 CO 2 ) 2 and K 3 Fe(CN) 6 . When the content of Co(CH 3 CO 2 ) 2 was low concerning K 3 Fe(CN) 6 , a singlecomponent PBA that can be identified as K 2 Co[Fe(CN) 6 ] (JCPDS No.75-0038) from X-ray diffraction (XRD) was obtained ( Figure 1a and Figure S1a, Supporting Information), hereafter referred to as Co II -PBA. The Fourier transform infrared (FT-IR) spectrum ( Figure 1b and Figure S1b, Supporting Information) of Co II -PBA displays a CN vibration peak at %2083 cm À1, which can be ascribed to Co II -NC-Fe II . [23] Increasing the Co 2þ content in the starting materials gives rise to a new set of XRD peaks, whose profile is analogous to that of Co II -PBA but whose peak positions shift to higher angles, indicating a contraction of cell parameters (Figure 1a and S1a, Supporting Information). The FT-IR spectra revealed a new CN vibration peak at %2120 cm À1 which can be related to Co III -NC-Fe II (Figure 1b and S1b, Supporting Information). The results of elemental analysis and energy-dispersive spectrometry (EDS, Figure S2, Supporting Information) showed a reduced K content when compared to pristine Co II -PBA (K 2 Co[Fe(CN) 6 ]). Therefore, the new phase can be deemed KCo[Fe(CN) 6 ] (hereafter referred to as Co III -PBA), and the obtained product is a heterostructure comprising Co II -PBA and Co III -PBA.
In the scanning electron microscopy (SEM) ( Figure S3a, Supporting Information) and transmission electron microscopy (TEM) images (Figure 1c), pure Co II -PBA appeared as solid cubes with a uniform particle size of approximately %400 nm ( Figure S3b,c, Supporting Information). Each Co II -PBA cube was a single crystal with a spot-like selected-area electron diffraction (SAED) pattern ( Figure S3d, Supporting Information), and the K, Fe, and Co elements were distributed homogeneously in the C and N matrices ( Figure S4, Supporting Information). The morphology of the Co III -PBA@Co II -PBA heterostructure depended on the Co 2þ dosage in the starting materials during material synthesis. The SEM and TEM images (Figure 1d and S5, Supporting Information) demonstrate that when the Co/Fe ratio of the starting materials was 1.50, additional Co III -PBA was decorated on the edge of the previously described solid Co II -PBA cube, yielding a product hereafter referred to as S/Co III -PBA@Co II -PBA. The Co II -PBA cube was enclosed in a Co III -PBA cage, whose columns were %80 nm thick. When the Co/Fe ratio of the starting materials was further increased to 2.20, the obtained heterostructure no longer had an enclosed Co II -PBA cube but consisted of a Co II -PBA cage inside a larger Co III -PBA cage (Figure 1e and S6, Supporting Information). However, the particle size of the heterostructure (hereafter referred to as H/Co III -PBA@Co II -PBA) was unchanged, as each Co II -PBA cage was still %400 nm in length. The columns of the Co II -PBA and Co III -PBA cages were %30 and %80 nm thick, respectively ( Figure S7, Supporting Information). The outer Co III -PBA cage was in intimate contact with the inner Co II -PBA cage, as can be seen from the high-resolution TEM (HRTEM) image ( Figure S8, Supporting Information). Elemental mapping (Figure 1f and S9, Supporting Information) showed a uniform distribution of K, Fe, Co, C, and N throughout the heterostructure for both S/Co III -PBA@Co II -PBA and H/Co III -PBA@Co II -PBA.
The Co/Fe ratio can be determined from EDS to be 1.39 for Co II -PBA, 1.22 for S/Co III -PBA@Co II -PBA, and 1.59 for H/Co III -PBA@Co II -PBA ( Figure S10, Supporting Information). The X-ray photoelectron spectroscopy (XPS) images of Co II -PBA, H/Co III -PBA@Co II -PBA, and S/Co III -PBA@Co II -PBA show similar Fe 2p 3/2 spectra, with peaks at 708.7 eV ascribed to the presence of Fe 2þ . [24] In the Co 2p 3/2 spectra, Co 2þ appears at 782.1 eV, and Co 3þ appears at 785.1 eV ( Figure 2a). The XPS peak of Co 3þ intensified significantly upon introducing Co III -PBA ( Figure 2b).
Therefore, the formation of Co III -PBA@Co II -PBA can be rationalized as follows: as the redox potential of Co 2þ /Co 3þ (À1.83 eV vs normal hydrogen electrode (NHE)) is lower than that of [Fe 3þ (CN) 6 ] 3À /[Fe 2þ (CN) 6 ] 4À (0.36 eV vs NHE) ( Figure S11, Supporting Information), the oxidation of Co 2þ by [Fe 3þ (CN) 6 ] 3À will give Co 3þ and [Fe 2þ (CN) 6 ] 4À . The reaction between Co 2þ and [Fe 2þ (CN) 6 ] 4À afforded Co II -PBA (Co II -NC-Fe II ). When the starting materials had a low Co/Fe ratio of 0.20, the relatively small amount of Fe 2þ more readily joined Co II to give Co II -NC-Fe II as the precipitate, probably because of the relatively low solubility of Co II -PBA. In the PBA structure, electron redistribution can occur between the two metal sites, with cyanide (ÀCN) groups as the bridge. [25] When the Co/Fe ratio in the starting materials increased to 1.50, Co III -PBA (Co III -NC-Fe II ) was formed when [Fe 2þ (CN) 6 ] 4À reacted with Co 3þ . The precipitation of Co III -PBA (Co III -NC-Fe II ) becomes feasible, followed by the formation of Co II -NC-Fe II , spontaneously creating a heterostructure. This reveals that MOFs can regulate their growth behavior to form intriguing and complex cages or solid heterostructures of assembled architectures.

Oriented Assembly of Co III -PBA on Co II -PBA
Co II -PBA and Co III -PBA have well-resolved lattice fringes in the HRTEM image (Figure 2c,d, and S12, Supporting Information), d (200) = 0.499 and 0.509 nm for Co III -PBA and Co II -PBA, respectively. The observed d spacings indexed to the diffraction of the (200) facets of PBA agree well with the results calculated from XRD, that is, d (200) = 0.498 and 0.509 nm for Co III -PBA and Co II -PBA, respectively. There is an evident oriented attachment of two distinct building blocks (difference in Co valences), as 1) the (200) facets of Co III -PBA are parallel to the (200) facets of Co II -PBA, and 2) on each edge of the cage-in-cage architecture, the outer Co III -PBA always shares the same orientation as the inner Co II -PBA (Figure 2e and S13, Supporting Information). As a result, both S/Co III -PBA@Co II -PBA and H/Co III -PBA@Co II -PBA can be identified as mesocrystals, which can diffract electrons of the Co II -PBA single crystal. In addition, all the samples displayed the typical <002>, <022>, and <020> zone axes of cubic PBA (Figure 2f and S14, Supporting Information). www.advancedsciencenews.com www.small-science-journal.com The Brunauer-Emmett-Teller (BET)-specific surface area of Co II -PBA (1.4 cm 2 g À1 ) is much lower than that of S/Co III -PBA@Co II -PBA (156.6 cm 2 g À1 ) and H/Co III -PBA@Co II -PBA (162.4 cm 2 g À1 ) ( Figure 3a and Table S1, Supporting Information). There are abundant microspores (%0.7-1.3 nm in size) in both S/Co III -PBA@Co II -PBA and H/Co III -PBA@Co II -PBA ( Figure S15, Supporting Information). The cavity of the hollow H/Co III -PBA@Co II -PBA cage was at the submicrometer scale.
Thermogravimetric (TG) analysis verified that Co II -PBA accommodated much less H 2 O than H/Co III -PBA@Co II -PBA ( Figure 3b). Specifically, the dehydration of adsorbed or crystallized water at <200°C amounts to 8.6 and 22.4 wt% weight loss for Co II -PBA and H/Co III -PBA@Co II -PBA, respectively. After the dehydration process, Co II -PBA and H/Co III -PBA@Co II -PBA had the same decomposition temperature profiles that matched the literature values for the PBA structures. [26] The CO 2 adsorption capacity was measured using CO 2 temperature-programmed desorption (CO 2 -TPD), and the CO 2 -TPD peak at <400°C was taken as the release of adsorbed CO 2 because the PBA structure collapsed at >400°C. Figure 3c shows that Co II -PBA has a much weaker adsorption capability for CO 2 than H/Co III -PBA@Co II -PBA. Figure 3d and Table S1, Supporting Information, show that the CO 2 adsorption capacity of the PBA is positively correlated with the Co III -NC-Fe II (Co III -PBA), ranking in the order of H/Co III -PBA@Co II -PBA (29.1 cm 3 The binding and activation of CO 2 on the surfaces of Co II -PBA (Co II -NC-Fe II ) and Co III -PBA (Co III -NC-Fe II ) were modeled by DFT calculations. While CO 2 can be chemically bound to the surfaces of both Co II -PBA and Co III -PBA, the CO 2 adsorption energy (ΔE ad-CO2 ) is harmful to both Co II -PBA and Co III -PBA; Co III -PBA seems to bind CO 2 more strongly than Co II -PBA. Figure 3e shows that the bond lengths of CO 2 are 1.42 and 1.62 Å on Co II -PBA but are elongated to 1.50 and 1.64 Å on Co III -PBA as a result of stronger chemical interaction. Furthermore, Figure 3f shows that ΔE ad-CO2 is À2.57 eV for Co III -PBA (Co III -NC-Fe II ) but À2.04 eV for Co II -PBA (Co II -NC-Fe II ). Hence, the DFT results are consistent with the experimental findings and further demonstrate that Co III -PBA (Co III -NC-Fe II ) enhances the adsorption and activation of CO 2 .

Syngas Production
The performance of photocatalytic syngas production was then compared among Co II -PBA, S/Co III -PBA@Co II -PBA, and H/Co III -PBA@Co II -PBA. To this end, the bandgaps of Co II PBA, S/Co III PBA@Co II PBA, and H/Co III PBA@Co II PBA ( Figure S16 and Table S2, Supporting Information) were first investigated, estimated to be 2.40, 2.00, and 2.30 eV, respectively, according to the Tauc plot. The position of conduction band minimum (E CBM ) of Co II PBA, S/Co III PBA@Co II PBA, and H/Co III PBA@Co II PBA is also estimated to be À0.87, À0.62, and À0.77 eV, respectively, based on the Mott-Schottky plots ( Figure S17 Figure S18, Supporting Information) [27] and the photogenerated electrons in the lowest unoccupied molecular orbital (LUMO) of Ru can be sent to the conduction band of PBA for the reduction reaction ( Figure S19, Supporting Information). The photocatalytic reactions were then performed under normal photocatalytic conditions. Here, [Ru(bpy) 3 ]Cl 2 ·6H 2 O (bpy = 2,2 0 -bipyridine) is used as the photosensitizer and triethanolamine (TEOA) as the electron donor. Figure 4a shows that the gas yields were low when Co II -PBA was used (V CO = 11.18 mmol g À1 h À1 , V H2 = 8.59 mmol g À1 h À1 ) but improved dramatically for H/Co III -PBA@Co II -PBA (V CO = 50.56 mmol g À1 h À1 , V H2 = 41.63 mmol g À1 h À1 ). This enhanced performance presumably arises from Co III -NC-Fe II (Co III -PBA), which provides more reactive sites to adsorb CO 2 and accommodate H 2 O. Because H/Co III -PBA@Co II -PBA and S/Co III -PBA@Co II -PBA cages have identical chemical components with very similar phases and differ only in morphology, they serve as ideal pairs for examining the role of the cage and the active metal sites in the catalytic reaction. It does not seem to be critical to catalyst performance, as S/Co III -PBA@Co II -PBA gives even higher gas yields of V CO = 51.2 mmol g À1 h À1 and V H2 = 45.8 mmol g À1 h À1 ( Figure S20a, Supporting Information). For both heterostructures, the presence of Co III -PBA significantly improved charge transfer and allowed the redox reaction to proceed efficiently. [28] The 1 H-NMR measurement ruled out the formation of liquid products, such as CH 3 OH, HCOOH, and HCHO ( Figure S20b, Supporting Information), and the isotopic 13 C-labelled experiment confirms that 13 CO was obtained from the reduction of 13 CO 2 ( Figure S20c, Supporting Information). Figure 4b shows that no gaseous products were formed without light, TEOA, or [Ru(bpy) 3 ]Cl 2 ·6H 2 O. The gas yield was also negligible without the input of the PBA catalyst, and only H 2 was obtained when the reaction was conducted under an Ar atmosphere. Notably, the tendency of CO generation matches well with the absorption spectrum of the Ru photosensitizer ( Figure S20d, Supporting Information). Hence, the solar-driven CRR and HER over PBA were driven by the excitation of [Ru(bpy) 3 ]Cl 2 ·6H 2 O, whose electrons were sent to the active site on PBA for subsequent reduction reactions. [29] Photocatalysis using H/Co III -PBA@Co II -PBA generated syngas with a CO/H 2 ratio of approximately 1:1 ( Figure S21, Supporting Information), which can be used for hydroformylation, one of the most common industrial reactions for the production of important chemical commodities. [30] When assessed based on the combined gas yield (mmol g À1 h À1 ), H/Co III -PBA@Co II -PBA outperforms many state-of-the-art heterogeneous photocatalysts reported in the literature (Figure 4c nd Table S2, Supporting Information), such as Fe-SAs/ N-C (V CO = 4.50, V H2 = 4.95), [31] Co 3 O 4 -NS (V CO = 23.00, V H2 = 16.12), [32] Co-ZIF-9 (V CO = 8.36, V H2 = 5.98), [9] etc. H/Co III -PBA@Co II -PBA maintained good catalytic activity over four repeated cycles (Figure 4d), and the hollow architecture remained intact in the retrieved catalysts, as confirmed by www.advancedsciencenews.com www.small-science-journal.com XRD, FT-IR, XPS, and SEM ( Figure S22, Supporting Information). Thus, the H/Co III -PBA@Co II -PBA cage is a robust catalyst for syngas production. The catalytically active sites in the heterostructures were further investigated. To this end, control samples, including FeFe PB (without Co) and CoCo PBA (without Fe), were prepared. As shown in Figure S23, Supporting Information, the FeFe PB composed of the Fe element only gave negligible CO production (V CO = 2.51 mmol g À1 h À1 ), while the Co-bearing CoCo PBA and FeCo PBA (i.e., H/Co III -PBA@Co II -PBA) gave considerably high CO production efficiency. This indicates that Co sites act as CRR active sites in the heterostructure, while the synergy of Fe and Co in the PBA structure promotes CRR performance. In situ FT-IR spectroscopy characterization further confirmed the adsorption and activation of CO 2 on the catalysts (Figure 4e). The result under the light irradiation is as follows: the *CO 2 À species at 1522 and 1685 cm À1 , monodentate carbonate groups (m-CO 3 2À ) at 1459, 1509, and 1560 cm À1 , bidentate carbonate (b-CO 3 2À ) at 1336, 1359, and 1490 cm À1 , and carbonate group (HCO 3 2À ) at 1401, 1432, and 1469 cm À1 were detected. It indicates the adsorption and activation of CO 2 on the catalysts to generate the critical intermediates of CO 2 reduction. [33] Furthermore, the IR peaks at 1637 and 1538 cm À1 intensified, demonstrating the generation of COOH*, a key intermediate in reducing CO 2 to CO. [34] In addition, the bridged CO* absorption peak at 1700-1800 cm À1 was also detected, suggesting CO product generation. Hence, a probable reduction pathway involving "CO 2 ! *CO 2 ! *COOH ! *CO ! CO" is proposed for the presenting system (Figure 4f ). [35]

Directed Transport of Photogenerated Electrons
The enhanced charge transfer in the heterostructures can be observed from the steady-state photoluminescence (PL) spectra, time-resolved PL (TRPL) spectra, and photocurrent and electrochemical impedance spectroscopy (EIS) measurements. [36] TRPL spectra in Figure 5a confirm the enhanced charge transfer efficiency in the heterostructures [37] because H/Co III -PBA@Co II -PBA/Ru (306.8 ns) and S/Co III -PBA@Co II -PBA/Ru (346.9 ns) displayed shorter average lifetimes than Co II -PBA/Ru (351.7 ns). Furthermore, the recombination of light-excited charge carriers was examined using PL spectroscopy. As shown in Figure 5b, about the photosensitizer [Ru(bpy) 3 ] 2þ with a characteristic emission peak at approximately 607 nm, [38] the heterostructures can promote charge transfer, leading to a decrease in the PL intensity in the presence of heterostructures, ranking in the order of H/Co III -PBA@Co II -PBA/Ru < S/Co III -PBA@Co II -PBA/Ru < Co II -PBA/Ru < Ru. The trends of the EIS spectra ( Figure 5c) and photocurrent curves (Figure 5d) show that H/Co III -PBA@Co II -PBA was the best among the samples for driving charge transfer.
We also performed theoretical calculations to assess the charge-transfer pathways in the heterostructures. Figure 6a,b shows that the work function is 5.69 and 5.46 eV for the (001) Figure 5. a) TRPL spectra, b) PL spectra, c) EIS measurement, and d) photocurrent of Co II -PBA, S/Co III -PBA@Co II -PBA, and H/Co III -PBA@Co II -PBA.
www.advancedsciencenews.com www.small-science-journal.com plane of Co II -PBA and Co III -PBA, respectively. Therefore, upon formation of the Co III -PBA/Co II -PBA heterojunction, electrons redistribute between Co III -PBA and Co II -PBA to establish a built-in electric field. With this internal electric field, the electrons generated upon light irradiation experience electrostatic attraction and moved from Co II -PBA to Co III -PBA. Both Co II -PBA and Co III -PBA contain active sites that can catalyze the HER and CRR; however, when Co III -PBA is present, it is Co III -PBA, rather than Co II -PBA, which is the ultimate destination of the photogenerated electron before the electron is delivered from the catalyst to the reactant molecule (CO 2 and H 2 O). Furthermore, the Pt photodeposition experiment and transient photovoltage (TPV) spectra can validate the transfer of photogenerated electrons from Co II -PBA to Co III -PBA. 1) The Pt photodeposition experiment (Figure 6c) shows that the Pt particles prefer to deposit on the Co III -PBA domain because of the accumulation of photogenerated electrons on Co III -PBA to reduce H 2 PtCl 6 into Pt. [33] 2) In the TPV spectra (Figure 6d), both the pure Co II -PBA and H/Co III -PBA@Co II -PBA exhibited a negative signal, indicating that the photogenerated electrons can migrate to the surface under light irradiation. [39] About pure Co II -PBA, H/Co III -PBA@Co II -PBA has an increased TPV intensity (Figure 6e and S24, Supporting Information), suggesting the transfer of photogenerated electrons from Co II -PBA to Co III -PBA in the heterostructure.

Significance of Findings
We outline the findings as follows.

Effective Catalyst Design
It is feasible to construct an effective Co II -PBA/Co III -PBA by engineering the oxidation state of the metal sites (i.e., Co in this work) of the two PBAs, wherein the change in the Co valence state helps establish the heterojunction and boosts the charge transfer, thus tuning the geometric and electronic state of the active metal sites to promote the redox reaction. The lattice matching between Co II -PBA and Co III -PBA, for which a similar phase structure and lattice match is critical (Figure 1i), enables the spontaneously ordered alignment to create a robust catalyst with strong interactions between its components, as is evident from the high structural stability and facile electron transfer of the heterostructure. [40] In this case, the electronic properties of the heterostructure outweigh the morphology while determining the catalyst activity. The reason includes the performance of the hollow cage-in-cage H/Co III -MOF@Co II -MOF is similar to the compact solid S/Co III -MOF@Co II -MOF. Both heterojunctions are similar in size and the thickness of their inner core, and the transport of photogenerated charges thus does not differ substantially from a purely geometrical standpoint. Nevertheless, we anticipate that the cage-in-cage MOF@MOF architecture will demonstrate potential applications in other areas that exploit their versatile and highly tunable compositions and structures. [41] 2. 5

.2. Delicate MOF@MOF Construction
Epitaxial growth is currently the standard means to create MOF@MOFs, and it is difficult to match the lattice parameters

Conclusion
The interface engineering of Co II /Co III in heterostructured MOF mesocrystals (Co III -PBA@Co II -PBA) was achieved. The oriented assembly of Co III -PBA nanoparticles occurred around the cubic prisms of Co II -PBA, with Co III -PBA forming a cage structure that enclosed Co II -PBA. DFT calculations and experimental results confirmed that: 1) Co III -PBA has a stronger affinity for CO 2 than Co II -PBA and 2) the photogenerated electrons can be quickly transferred from Co II -PBA to Co III -PBA through the built-in electric field in the heterojunction of Co III -PBA@Co II -PBA. In the photocatalytic CO 2 -to-syngas process, the electronics of the Co site in Co III -PBA@Co II -PBA seem to be more critical than morphology, as S/Co III -PBA@Co II -PBA and H/Co III -PBA@Co II -PBA have similar photocatalytic performances, while both strongly outperform many recently reported photocatalysts for solar-driven syngas production. The excellent syngas production can be attributed to the directional transfer of high-energy electrons to the more reactive metal centers in the Co III -PBA.

Experimental Section
Materials: Chemicals in experiments were of analytical grade, with potassium hexacyanoferrate(III) (K 3 Fe(CN) 6  Synthesis of Co-PBA: Solution A is prepared by dissolving cobalt(II) acetate tetrahydrate and trisodium citrate dihydrate (0.3 g) in DI water (40 mL). Potassium hexacyanoferrate(III) (0.2 g) was dissolved in DI water (60 mL) to form solution B. Dropwise addition of solution B to solution A was accomplished under magnetic stirring, and the mixture was stirred for another 24 h at ambient temperature. The precipitate was collected by centrifugation, washed with DI water and ethanol, and dried under a vacuum. The obtained product depended on the dosage of cobalt(II) acetate tetrahydrate in the starting materials, about 0.03, 0.23, and 0.33 g for Co II -PBA, S/Co III -PBA@Co II -PBA, and H/Co III -PBA@Co II -PBA, respectively.

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