# A Low-Temperature Hydrothermal Synthesis of Prussian Blue Nanocrystal and Its Application in H2O2 Detection

Authors: J. Yang; D. H. Li; L. L. Ji
Journal: Journal of Chemistry
 (2022)

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## Abstract

Uniform Prussian blue Fe3[Fe(CN)6]2 nanocrystals were synthesized by a direct dissociation and reduction of the single-Fe(III)-source precursor K3Fe(CN)6 under low-temperature hydrothermal conditions. UV-visible absorption spectrum, Fourier transform infrared spectrum, X-ray diffraction, field emission scanning electron microscope, X-ray photoelectron spectra, high-resolution transmission electron microscopy, and electrochemical testing were used to characterize and verify the synthesized Prussian blue nanocrystal product. The size of the synthesized product had a strong dependence on the acidity condition and the concentration of K3Fe(CN)6 solution. This result may facilitate not only the exploration of preparing Fe3[Fe(CN)6]2 nanocrystals for particular applications but also an in-depth explanation of the nature of the hydrothermal reaction. The Prussian blue nanocrystals were deposited onto an electrode support through lyotropic liquid crystalline templates to detect hydrogen peroxide (H2O2) by reduction reaction. Cyclic voltammograms showed that the Prussian blue modified electrode was of excellent electrocatalytic activity for H2O2. This electrode demonstrated a detection limit (1 × 10−7 M) and a linear range starting from the detection limit and extending over 6 orders of magnitude of H2O2 concentrations (1 × 10−7 to 1 × 10−1 M), which was of excellent performance in detecting H2O2.


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## Body

## 1. Introduction

Prussian blue (PB) as the first synthetic coordination compound is a mixed-valence iron (II) hexacyanoferrate (III) compound with a face-centered-cubic configuration, in which Fe3+ at the N-coordinated site is in high-spin state and Fe2+ at the C-coordinated site is in a low-spin state as shown in Figure 1 (inset) [1]. In view of its structural characteristics, it has obtained more and more attention as a kind of traditional material: a pigment in various dyes [2, 3], electrochemistry [4, 5], ion-exchange [6], and electrochemical applications [7], developing and utilizing the open zeolite-like structure of PB [8].Figure 1
XRD pattern of Prussian blue nanocrystal.In recent years, PB and PB analogues have fascinated renewed and growing interest in many new fields, for instance, molecular magnets [9–11], electrochemistry [4, 12], biosensing devices [13], and optics [14–16] due to their unique properties. Also, Kaye et al. have reported another new excellent property, for dehydrated PB analogues of M3[Co(CN)6]2 (M = Mn, Fe, Co, Ni, Cu, and Zn), which showed good performance in the field of hydrogen storage [17].The general synthesis method of PB and its analogues Mim+[M′(CN)6]n− is based on the double-source precipitation reaction of the Mm+ cations and the [M′(CN)6]n− anions in a neutral solution [18–21]. Our group synthesized Fe4[Fe(CN)6]3 nanocubes [22] and Co3[Co(CN)6]2 nanostructures [23] with morphologies of polyhedra, cubes and rods by using a single-source K4[Fe(CN)6] and K3[Co(CN)6], respectively. The Mn+ ions (Co3+ and Fe2+) dissociated from the single-source precursors were unstable and easy to form Mm+ (Co2+ and Fe3+). However, in this process, Fe3+ ions, which are also generated by the dissociation of the single-source precursor of [Fe(CN)6]3− ions but differed from these, are more steady in water; but Fe3+ ions are reduced, by the generated carbon monoxide (CO) from the precursor in acid solution, to generate Fe2+ ions, which combine the undissociated [Fe(CN)6]3− anions to produce PB Fe3[Fe(CN)6]2. PB is of an electrocatalytic effect on the reduction of hydrogen peroxide (H2O2) [24, 25]. It exhibited good analytical performance of activity, selectivity, and sensitivity [25]. In this text, Prussian blue synthesized by hydrothermal technology was investigated as a selective electrocatalyst for the reduction of H2O2. It exhibited advantageous analytical characteristics of low detection limit and wide linear range [24].


## 2. Experimental

All reagents that were purchased from Changchun Chemical Reagent Co. Ltd. were of analytical grade and used as received directly. Hydrogen peroxide (30% solution) was obtained from Sinopharm Chemical Reagent Co. Ltd. Brij-56 (polyoxyethylen (n = 10) cetyl ether) was purchased from J&K Chemical Ltd. Tween-60 (polyoxyethylen (n = 20) sorbitan monostearate) was purchased from Shanghai Yuanye Biotechnology Co. Ltd. AOT (2-ethylhexylsodium sulfoxinate) was purchased from Macklin. All solutions were made from deionized water.In a conventional hydrothermal synthesis [26], 0.6 mmol K3Fe(CN)6 was added to 60 mL HCl solution with a concentration of 0.06 mol/L. The K3Fe(CN)6 solution was well stirred and poured into a stainless Teflon-lined autoclave of 80 mL which was heated at 80°C for 12 h, and then the autoclave was cooled to 25°C naturally. The blue solid samples were collected by using filtration and washed repeatedly with water and anhydrous ethanol, respectively. Then, the product was dehydrated in the oven at 60°C for 12 h. Prussian blue prepared in the above way was used for X-ray diffraction, UV-vis absorption spectrum, Fourier transform infrared, field emission scanning electron microscope, and X-ray photoelectron spectra.Prussian blue prepared in this way was also made into an electrode. The PB-modified electrode was made by electrodeposition of Prussian blue through a hexagonal liquid crystal template and then was electrochemically activated as described in the literature [27].The field emission scanning electron microscope (FE-SEM) was operated on the EM-30Plus [28]. The UV-visible absorption spectrum was performed on a UNICO WFZ UV-2802PC/PCS spectrometer. The X-ray diffraction (XRD) measurement was carried out on a D2 PHASER diffractometer with a Cu Kα radiation (λ = 0.15405 nm) at a scanning rate of 5° per minute from 10° to 90°. High-resolution transmission electron microscopy (HRTEM) images were obtained using a JEM-2100UHR STEM/EDS. X-ray photoelectron spectra (XPS) tests were recorded by using a Thermo Scientific™ K-Alpha™ with an Mg Kα radiator. The laser diffraction particle size analysis of Prussian blue prepared was tested by the Nano-ZS90 laser particle sizer of Malvern company. All the electrochemical tests were conducted with a PARSTAT4000 and a conventional three-compartment electrochemical cell. Fourier transform infrared (FTIR) spectrum was measured using a Bruker IFS 125HR FTIR spectrometer. A Pt foil and a saturated calomel electrode (SCE) were the counter and the reference electrodes, respectively [29].


## 3. Results and Discussion

The powder X-ray diffraction was performed to verify the crystalline structure of the products. The XRD patterns of the products were shown in Figure1. The X-ray diffraction reflections were mostly indexed accordingly as could be seen in Figure 1. All the reflections could be easily referred to a pure face-centered cubic phase of [space group: Fm3¯m (225)] Fe3[Fe(CN)6]2 with lattice constant a = 10.19 Å, which was consistent with the standard values for the bulk cubic Fe3[Fe(CN)6]2 (JCPDS 52-1907).The UV-vis absorption spectrum exhibited that the synthesized products revealed strong broad-spectrum absorption in the range of 500 nm–1000 nm, which is shown in Figure2. The product showed an obvious peak at 699 nm. The broad band with λmax at 699 nm was in consistent with an intermetal charge-transfer band from FeII to FeIII, which reflected the formation of PB [3].Figure 2
UV-vis absorption spectrum of Prussian blue nanocrystal.Figure3 showed the FTIR spectrum of the product from 500 cm−1 to 3500 cm−1 at 25°C. The FTIR spectrum displayed an obvious peak at 2081 cm−1, which owed to the charge transfer and the stretching vibration band of C≡N of Fe2+-CN-Fe3+. In addition, the other obvious peak was observed at 519 cm−1, which showed that M-CN-M′ (M stands for metal) structure was formed [19]. Both the UV-vis and the FTIR spectra features were associated with the CN stretching vibration mode in Fe2+-CN-Fe3+ of PB.Figure 3
FTIR spectrum of Prussian blue nanocrystal.From the SEM results of Figure4, it could be seen that many highly dispersed samples with uniform sizes were obtained. The high-magnification FE-SEM image (Figure 4(b)) showed that there was a large number of cubic Fe3[Fe(CN)6]2 crystallites with an average diameter of 400 nm, and the edges of the nanocrystals had some fissures. Based on the molar weight of Fe, the yield was above 95%.Figure 4
FE-SEM images of the Prussian blue nanocrystal synthesized (a) at a lower magnification and (b) at a higher magnification.To further investigate the synthetic Prussian blue nanocrystal products, XPS spectra of N1s and Fe2p1/2 were recorded and shown in Figure 5. For N1s, the peak was located at 399.1 eV, which could be referred to N of cyanide. Similarly, for Fe2p1/2, the spectrum appeared at two peaks, at 724.99 eV and 721.01 eV, respectively. The one at 724.99 eV was attributed to Fe(III), and the other was attributed to Fe(II) [30, 31].Figure 5
XPS spectra of Prussian blue nanocrystal.As well known, due to its high value of stability constant (Ks = 1.0 × 1042), [Fe(CN)6]3− ions are very steady in neutral solution at 25°C, and almost no Fe3+ ions can be detected. However, under the hydrothermal condition of acidic solution, [Fe(CN)6]3− can first slowly and partially dissociate to generate Fe3+ ions and CO (reaction 1), which leads to uniform PB deposition [32]. Also, at the airtight of about 80°C, CO can partially dissolve in water and reduce Fe3+ to produce Fe2+ as revealed in reaction 2 (another reacts with the dissolved oxygen in the water), which can combine undissociated [Fe(CN)6]3− to form PB Fe3[Fe(CN)6]2 (the procedure is shown in reaction 3) [33]. The mechanism on synthesizing PB was illuminated.TheKsp of Fe3[Fe(CN)6]2 is very small (10−31), which could be the dynamic of the whole reaction 4, which could hinder Fe ions from transforming to FeOOH (or Fe2O3). Our group had synthesized single-crystal micropines of magnetic α-Fe2O3 by hydrothermal reaction of K3[Fe(CN)6] in a neutral solution at 140°C [34]. In this research, when the reactant temperature was increased to 140°C under the acid concentration of 0.06 M and 0.12 M, the as-synthesized products still consisted of Fe3[Fe(CN)6]2, which were stable in an acidic solution. Additional experiments had also been conducted to verify the reaction mechanism. When the acid concentration was decreased to 0.04 M, the products changed from Fe3[Fe(CN)6]2 to α-Fe2O3, which showed that enough CO generated in the process also had an important role in the synthesis of Fe3[Fe(CN)6]2.Large numbers of experiments indicated that the size distribution and size of the synthesized PB nanocrystal also had a strong dependence on the acidity and the concentration of K3Fe(CN)6. For example, as the acid concentration increased from 0.06 M to 0.12 M, the K3[Fe(CN)6] concentration remained unchanged at 0.01 M, then the mean diameter of PB changed between 400 nm (Figure 6(a)) and 500 nm (Figure 6(b)). The acid concentration must be higher than the value of 0.06 M; otherwise, almost no PB can be detected in the synthesized products. In addition, when the concentration of K3[Fe(CN)6] was changed to 0.05 M and 0.10 M while maintaining a constant acid concentration of 0.06 M, the sizes of Fe3[Fe(CN)6]2 nanocrystals increased to 500 nm (Figure 6(c)) and 800 nm (Figure 6(d)). Therefore, the size of PB nanocrystals could be adjusted by controlling the kinetic parameters of the reaction process, that was, acidity and the reactant concentration.Figure 6
TEM images of PB nanocrystals prepared at 80°C with different acidity and the concentration of K3[Fe(CN)6]: (a) 0.06 M H+, 0.01 M K3[Fe(CN)6]; (b) 0.12 M H+, 0.01 M K3[Fe(CN)6]; (c) 0.06 M H+, 0.05 M K3[Fe(CN)6]; (d) 0.06 M H+, 0.10 M K3[Fe(CN)6].Effects of different reaction conditions on PB particle size distribution were explained by laser diffraction particle size analysis, as shown in Figure7. For the samples synthesized by the above four methods, the particle size ranges were about 400 nm, 500 nm, 500 nm, and 800 nm, respectively, which results were consistent with TEM image results.Figure 7
Effects of different reaction conditions on PB particle size distribution. (a) 0.06 M H+, 0.01 M K3[Fe(CN)6]; (b) 0.12 M H+, 0.01 M K3[Fe(CN)6]; (c) 0.06 M H+, 0.05 M K3[Fe(CN)6]; (d) 0.06 M H+, 0.10 M K3[Fe(CN)6]).Cyclic voltammetry was used to examine the electrocatalytic performance of the PB-modified electrode. A pair of stable reduction and oxidation peaks were generated between 0.5 V and −0.4 V on the cyclic voltammograms as shown in Figure8. When 0.01 M H2O2 was added to the buffer solution, the reduction peak increased prominently, which indicated that the PB-modified electrode had excellent electrocatalytic activity on the electroreduction of H2O2.Figure 8
Cyclic voltammograms of the PB-modified electrode response to buffer solution before (dash line) and after (solid line) adding 0.01 M hydrogen peroxide.Analytical performances of PB-modified electrodes in H2O2 detection were studied as shown in Figure 9 which presented a typical calibration plot for H2O2 detection. The working electrode potential was 50 mV. The PB-modified electrode demonstrated a low detection limit (1 × 10−7 M) and a linear range starting exactly from the detection limit and stretching over 8 orders of magnitude of H2O2 concentrations (1 × 10−7−1 × 10−1 M). The significant deviation from the linearity was observed only at 0.1 M hydrogen peroxide content. Hence, Prussian blue is a superior electrocatalyst in H2O2 reduction.Figure 9
Calibration plot for H2O2 detection with the PB-modified electrode.The sensitivity and response time of the PB-modified electrode were studied by the chronoamperometry method. When 0.01 M H2O2 was added continuously, the amperometric response curve of the PB-modified electrode was shown in Figure 10. The response to H2O2 was rapid, the response time was less than 5 s, and the sensitivity was 4.6 μA/(mmol/L). Compared with other methods, the PB-modified electrode had the superiority of high sensitivity and wide linear range.Figure 10
Amperometric response curve of the PB-modified electrode for successive addition of 0.01 M H2O2.


## 4. Conclusions

In conclusion, we have successfully developed a unique synthesis technology for Prussian blue nanocrystals through a novel hydrothermal reduction of the single-source precursor K3Fe(CN)6 in acidity solution. Various characterizations verified that the synthesized products were Prussian blue nanocrystals. The size of the synthesized Prussian blue nanocrystals can be tuned by controlling the acidity condition and the reactant concentration. This result may facilitate not only the exploration of preparing Fe3[Fe(CN)6]2 nanocrystals for potential applications but also a deeper cognizance of the essential characteristics of hydrothermal reaction. Electrodeposition of the synthetic PB nanocrystals onto the electrode was carried out through liquid crystalline plating mixtures. The PB-modified electrode could effectively detect H2O2 by electrochemical reduction reaction. The detection limit was 1 × 10−7 M. The ability of Prussian blue to sensitively and selectively detect H2O2 by its reduction is of high importance for analytical chemistry.



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