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Article

Synthetic Mechanism Studies of Iron Selenides: An Emerging Class of Materials for Electrocatalysis

by
Bo Hou
1,*,
David Benito-Alifonso
2,
Richard F. Webster
3,
David Cherns
4,
M. Carmen Galan
2 and
David J. Fermín
2,*
1
School of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA, UK
2
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
3
Electron Microscopy Unit, University of New South Wales, Sydney, NSW 2052, Australia
4
H H Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
*
Authors to whom correspondence should be addressed.
Catalysts 2021, 11(6), 681; https://doi.org/10.3390/catal11060681
Submission received: 12 April 2021 / Revised: 19 May 2021 / Accepted: 26 May 2021 / Published: 27 May 2021
(This article belongs to the Special Issue Emerging Nanocatalysts for Efficient Oxygen Reduction, Oxygen Evolution and Hydrogen Evolution)
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Versions Notes

Abstract

:
Solution-processed iron selenide nanocrystals (NCs) have recently attracted considerable attention in electrocatalysis water splitting. Nevertheless, a primary challenge in current iron-based NCs chemical synthesis is controlling phase purities between each chalcogen (monochalcogenide, dichalcogenides, and oxides), which requires a comprehensive understanding of the reaction mechanisms at the early stages of nucleation. Herein, we investigate the fundamental steps in transforming molecular organoiron and organoselenium precursors to iron selenides NCs with the view of developing universal synthesis protocols for phase pure metal selenium and metal oxides NCs. The main intermediate species and volatile by-products are identified by high-resolution electron microscopy and Nuclear Magnetic Resonance (NMR) spectroscopy (1H, 13C, and 31P). Experimental evidence suggests that the phase determining factor is the coordinating reactivity difference between olefins (1-octadecene, oleylamine), tributylphosphine and trioctylphosphine associated with their corresponding Se bond cleavage. This work proposes organoselenium interconversion reaction mechanisms during iron selenides synthesis, offering a universal synthetic strategy for other electrocatalytically or photocatalytically active layered metal selenides materials.

1. Introduction

Since the discovery of superconductivity from iron compounds [1,2], iron-based semiconductors, particularly iron selenides, have attracted great interests from physicists and chemists [3,4,5,6,7,8,9,10]. Considerable research on iron selenides has been carried out not only to aim at high-temperature superconductivity but also various other novel applications [4,11]. For instance, among the complex phase diagram, FeSe and FeSe2 have been demonstrated as photo-absorbers for photovoltaics [12], anode materials for sodium-ion batteries [8,10], cathode materials for lithium-air batteries [5], and non-linear optical materials for bioimaging [9,13]. Recently, excellent electrocatalysis performance in oxygen evolution reaction (OER) [6,14], hydrogen evolution reaction (HER) [15,16,17], and overall water splitting [7,11] have also been discovered and demonstrated in FeSe2 and FeSe. Indeed, iron selenides present strong potential and unique electrical and optical properties for electrocatalysis, and their performance is strongly related to the stoichiometric ratio between Fe and Se and their phase purity [6,8,11,16,17,18].
Iron selenides could crystallize in the form of FeSe (α-FeSe and β-FeSe), Fe3Se4, Fe7Se8 and FeSe2, depending on the composition of the binary phase. The phase diagram of iron selenides is similar to the NiAs system [19]. The structure of iron selenides may be hexagonal, NiAs-like structures [20] with a composition of Fe7Se8 (H-phase) or as a monoclinic structure of the same phase with composition Fe3Se4 (M-phase). FeSe crystallizes in tetragonal (PDF = 85-0735) or hexagonal structure (PDF = 75-0608), while FeSe2 crystallizes in cubic (PDF = 48-1881) or orthorhombic marcasite-type structures (PDF = 21-0432) [21]. Regarding the bandgap of FeSe, large discrepancies can be found in the literature ranging from 0.14 eV to 3.00 eV [22,23,24]. FeSe2 has been reported as a p-type material with a bandgap of around 1 eV [25].
FeSe and FeSe2 have been fabricated using several high-temperature approaches such as metal-organic chemical vapor deposition [26], mechano-synthesis [27], vapor selenization [21,25,28,29], mechanical alloying [3,30,31], reactive sputtering, and spray pyrolysis [32,33,34]. However, the products were extremely unstable at high temperature, giving rise to phase changes in the deposited film [18]. Therefore, synthetic routes at low temperatures have been developed, including chemical bath deposition [24], electrodeposition [35], and organic metallic chemical synthesis [36,37,38,39]. Conventional ‘hot-injection’ method is commonly adopted in synthesizing metal chalcogenide NCs [40,41,42] such as CdSe [43,44,45,46,47,48,49,50], ZnSe [51,52,53,54] CuInZn(SSe)3 [55,56,57], CuZnSn(SSe)4 [58,59], PbS [55,60,61,62,63], PbSe [64,65,66], HgSe [67,68] or HgTe [69,70]. Until now, organic metallic reactions via ‘hot-injection’ and corresponding thermolysis approaches attracted the most interest for synthesizing iron selenides due to their better phase and crystal structural controllability than the other low-temperature synthesis methods [9,13,71,72,73,74]; however, a detailed understanding of its chemical reaction mechanisms was rarely reported.
In the present work, we investigate the fundamental steps in transforming molecular organoiron and organoselenium precursors to iron selenides NCs during the organometallic reaction to develop universal synthetic protocols for preparing phase pure FeSe and FeSe2 NCs. Four common organic reaction media have been chosen, and the main intermediate species and volatile by-products were identified by Transmission Electron Microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM) and NMR spectroscopy (1H, 13C, and 31P). Phosphine Lewis bases such as tributylphosphine (TBP) or trioctylphosphine (TOP) were chosen due to their strong coordination to the chalcogenide component. In addition, long alkane chain olefin compounds such as 1-octadecene (1-ODE) and oleylamine (OLA) were also selected since both reagents are the most popular phosphine-free solvents for metal chalcogenide nanocrystal synthesis. Experimental evidence suggests that the phase determining factor is the coordinating reactivity difference between olefins (1-ODE and OLA), TBP, and TOP associated with their selective Fe-Se bond cleavage.

2. Results and Discussion

2.1. Chemical Reaction Mechanisms in the ‘Hot-Injection’ Approach

A double cold trap apparatus was employed to extract the volatile reaction intermediates. Se precursor was prepared by dissolving Se in TBP. The iron precursor was formed by iron (II) acetate (Fe(AC)2) reacting with stearic acid (SA) in 1-ODE. As shown in Figure S1 (Supplementary Information), the reaction product generated from the first reaction of iron acetate with SA and 1-ODE is monitored. The volatiles generated from the reaction were trapped in the double ‘cold trap’, which was equipped with a Dewar with liquid nitrogen. The formation of the iron precursor was examined by taking aliquots during the heating stage prior to the injection of Se precursors.
Figure 1a,b shows 1H and 13C NMR analysis of the product (Sample I) collected by the ‘double trap’, which was generated after loading iron acetate, SA and 1-ODE into the reaction vessel and heating at 60 °C and 130 °C for half an hour, respectively. In Figure 1a, the formation of acetic acid is revealed by the strong intermolecular hydrogen bonding in carboxylic acid dimers, which leads to very low-field broad signal δ = 9.74 (sample I) and 11.00 (commercial, acetic acid, ≥99.7%, Sigma-Aldrich) in the range of 9–15 ppm. The up-field proton signal is similar between Sample I and commercial acetic acid, such as the singlet at around 2 ppm is also characteristic (CH3) of acetic acid. Further evidence is provided by the 13C NMR spectra, as shown in Figure 1b, in which identical carbon shifts are observed for Sample I and acetic acid standard sample.
Figure 1c shows the 13C NMR analysis of iron-containing species collected from the iron precursor. Compared to the 13C NMR spectra of standard SA (analytical grade, Fisher Scientific) and acetic acid, the reaction solution shows negligible acetic acid content. On the other hand, iron stearate (FeSt2) can be resolved with identical chemical shifts to stearic acid except for the absence of carbonyl’s chemical shift (13C, δ = 180 ppm) ascribed to the screening effect upon bonding with iron.
However, after injection of Se precursor (Se in TBP), no color change occurs even when the heating time is extended to 30 min. Furthermore, as shown in Figure 1d, there are no characteristic or prominent lattice reflections from the XRD analysis of the crude solution, especially at large angles. From the result achieved so far, the only conclusion is the formation of amorphous metal or organic iron complex rather than highly crystalline nanoparticles.
Based on the NMR and XRD analysis, a plausible reaction pathway is proposed in Scheme 1. Fe(AC)2 reacts with SA in the presence of 1-ODE via a ligand exchange reaction, generating FeSt2 as iron precursors and volatile acetic acid. Due to the strong bonding between Se and phosphorus, the Se-TBP bond is not readily cleaved under these conditions after the injection of Se precursors. Consequently, no crystalline products are formed in the end.
The second investigation is carried out by adjusting Se precursors’ reactivity from replacing TBP by OLA. Since the coordination between OLA and Se is weaker than that of phosphines [45,55], we anticipate a more labile Se-OLA bond cleavage, thus accelerating the reaction of Se with FeSt2 and resulting in the formation of the desired iron selenide NCs.
Figure 2a shows the XRD of as-prepared nanocrystals obtained in the presence of OLA. In contrast to the TBP approach, highly crystalline orthorhombic FeSe2 is achieved, with each XRD feature assigned according to the standard PDF card index (PDF = 21-0432 and 74-0247).
Figure 2b,c displayed the TEM analysis results, which exhibits the expected layer type morphology] The HRTEM in Figure 2d resolved the reflection from the (110) plane, with a lattice spacing of 3.8 ± 0.2 Å, which is close to the value for single crystal orthorhombic iron diselenide.
Based on the XRD, TEM and HRTEM analysis results, a plausible reaction pathway of the second approach is proposed, as shown in Scheme 2. The mechanism for the FeSt2 formation is the same as Scheme 1. However, the reaction between selenium and OLA will generate organoselenium complexes, as reported in our previous works [45,59] These less reactive organoselenium complexes react with FeSt2 at elevated temperature and facilitate the formation of FeSe2 NCs.
The third investigation involves the replacement of 1-ODE by OLA in the iron precursor. In that case, both iron and Se precursor were prepared in the presence of OLA. After injection of the Se precursor, strongly magnetic particles are formed, as revealed by the strong adsorbing on the magnetic stirring bars. TEM analysis revealed spherical shape nanoparticles. Figure 3b shows the XRD analysis of the final product from the third approach, with no evidence of iron selenides. High purity Fe3O4 nanocrystals are found as confirmed by the well-aligned XRD reflections compared with standard face-centered cubic Fe3O4 PDF card (PDF = 75-0033). The crystal domain size is calculated to be 10.9 ± 4.3 nm from the Scherrer equation. HRTEM analysis resolved the reflection from the (220) plane in Figure 3c with a lattice spacing of 3.0 ± 0.2 Å confirmed the cubic structure. Furthermore, the (111) plane (Figure 3d) with a lattice spacing of 5.0 ± 0.2 Å also resolved, which are close to the bulk Fe3O4 structure value.
Based on XRD, TEM, and HRTEM analysis of the data, a plausible reaction pathway for the third approach is proposed in Scheme 3. The critical step in iron precursor formation is the anion exchange process driven by the depletion of acetic acid and the construction of FeSt2. As shown in Scheme 3, when 1-ODE is replaced by OLA, the formation of FeSt2 is prohibited by a condensation reaction between OLA and SA instead of the anion exchange reaction between SA and acetic acid. Therefore, when elevating the temperature after the injection of Se precursors, iron acetate will first undergo a thermolysis reaction generating Fe3O4 NCs rather than reacting with Se species.
Furthermore, as discussed earlier in Scheme 2 and Scheme 3, the formation of FeSt2 and Se-C organoselenium species is crucial to achieving high purity FeSe2 without the formation of Fe3O4. Therefore, an excess amount of SA, vacuum drying and suitable selenium precursors are the critical factors in the FeSe2 NCs synthesis through the ‘hot-injection’ approach. It should be noted that the organic metallic chemical synthesis of iron nanoparticles, iron metal or iron-nonmetal alloy is also a rather complex reaction process [59]; however, similar approaches to our current work could be carried out in the future to elucidate the underlying synthetic mechanisms.

2.2. Selenium Extraction for the Synthesis of FeSe Nanoparticles

The synthesis of FeSe was carried out through the extraction of selenium from FeSe2 by TBP, analogous to the previous report [74]. However, no FeSe nanocrystals could be found in the final solution after the chemical extraction process (Figure S2). The XRD analysis in Figure S2 of the crude solution shows no prominent reflections from FeSe NCs, especially at large angles. A plausible chemical pathway can be proposed, as shown in Scheme S1. It can be concluded that after reacting with TBP, all the FeSe2 changed to amorphous metal or organic iron complex rather than highly crystalline NCs.
An alternative approach was carried out by replacing TBP with TOP. During the reaction between FeSe2 and TOP, 1 mL of the reaction solution was syringed out under Ar protection, and J. Young NMR tube was employed for 31P NMR analysis (denoted as Sample II) since TOP is air sensitive. Aliquots were also taken out for monitoring crystal structure evolution by XRD analysis. Figure 4 shows the typical 31P NMR analysis result. Taking TOP standard as a reference, a new singlet with a chemical shift of δ = 36.28 ppm is appeared after the phase transformation. The signal is indexed to P = Se correlation which is consistent with the literature value [75].
The as-prepared FeSe nanoparticles show flake-like features, as shown in Figure 5a of the TEM analysis. The HRTEM image in Figure 5b resolves the lattice fringes of the (001) plane, with a lattice spacing of 5.9 ± 0.2 Å, consistent with the bulk FeSe tetragonal crystal value (PDF = 85-0735). Figure 5c shows the XRD analysis of the aliquots during the extraction reaction. Well-defined reflections can be revealed in Figure 5c, which displays series of reflection evolutions during phase transformation from orthorhombic FeSe2 to a tetragonal FeSe crystal structure.
Based on the analysis of reaction intermediates extracted from TEM and XRD, a plausible reaction mechanism was proposed, as shown in Scheme 4. FeSe2 reacted with TOP and diffuse half the equivalents of Se generating FeSe and TOPSe. It should be noted that the delicate detail step reaction mechanism remains unclear; however, a conclusive claim can be made that the amount of TOP added into the solution and the reaction duration are crucial factors. For instance, we found an excess amount of TOP or extending the reaction time will result in the formation of amorphous materials instead of FeSe NCs.

2.3. Optical Properties Analysis

Reflectance is the fraction of light reflected from a surface as a function of wavelength. When properly measured, reflectance provides information about the optical properties of materials, since the light that is not reflected is either absorbed due to its chemical composition, or is scattered and/or transmitted, depending on mechanical structure.
Generally, the surfaces of substances reflect light differently depending on their texture. Smooth surfaces such as mirrors reflect the beam in exactly the same direction as the incident beam. On the other hand, rough surfaces substance exhibit diffuse reflection, whereby the rays of the incident beam are scattered in all directions. Normally, a typical surface is in neither smooth or completely rough, and therefore has both specular and diffuse components. Therefore, using an integrating sphere which collects all of the beams reflected by the material surface, one can estimate the optical properties of the substance. To extract the band gap from reflectance measurements, the functions proposed by Tau [76] and Kubelka-Munk were employed (see derivative process below).
h ν α 1 / n = A h ν − E g a p
F R ∞ = 1 − R ∞ 2 2 R ∞ = k s = A c s
h ν ∗ F R ∞   2 = A h ν − E g a p
where: h: Plank’s constant, ν: frequency of vibration, α: absorption coefficient,   E g a p : band gap, A: proportional constant. The value of n is denoted according to the nature of the materials: for direct allowed transition, n = ½; for direct forbidden transition, n = 3/2; for indirect allowed transition, n = 2; for indirect forbidden transition, n = 3. For both FeSe and FeSe2, direct allowed transitions have previously been reported. Therefore, in this case, the value of n in Equation (1) is 2.
Figure 6a,e shows the multiple total reflection measurements on different areas of the glass slides covered by FeSe2 and FeSe, respectively. Since it is difficult to ensure the homogeneous coverage of the slides by drop-casting, the position of the slide was directed toward the light source together with multiple measurements to minimize errors. By transforming the reflectance experimental data (Figure 6a,e) following Equations (1)–(3), the optical band gap of FeSe2 (Figure 6b–d) and FeSe (Figure 6f–h) were predicted from the intercepts of the tangent line of the curves to the abscissa.
High absorption and low reflectance can be observed from Figure 6a, indicating that the bandgap of FeSe2 is less than 1.2 eV. Based on the Tau and Kubelka-Munk equations, [76] the optical band gap of FeSe2 is estimated to be 0.67 eV. For FeSe NCs, the reflectance measurement shows a change in the reflectance at a wavelength of around 700 nm for all three measurements. By performing the same transformation, the optical band gap of FeSe is estimated to be 1.73 eV.
It should be noted that in Figure 6a,e, the reflectance increases swiftly when the incident light has a wavelength below 400 nm. This could be attributed to a scattering effect. In the Rayleigh approximation as depicted in Equation (4):
I = I 0 ( 1 + cos 2 θ 2 R 2 ) 2 π λ 4 n 2 − 1 n 2 + 2 2 ( d 2 ) 6
where I0 is the light intensity before the interaction with the particle, R is the distance between the particle and the observer, θ is the scattering angle, n is the refractive index of the particle, and d is the diameter of the particle. For instance, taking some common semiconductors as examples, and setting the distance between the particle and observer at 0.2 m, the size of the particle as 100 nm, I0 equal to unity, and taking and the refractive index of CdSe, [77] CdTe, [77] CdS, [77] ZnSe, [77] ZnO, [78] TiO2, [79] PbSe [77] from literature values. The evolution of I as a function of λ and refractive index n is shown in Figure 7. From the plot, an increase in scattering is observed as the wavelength of incident light decreases. Further explanation should be given about the index of refraction, since the index of refraction of materials is also a function of wavelength. The values taken from the literature are measured under different wavelength conditions. However, in the current study, it is assumed that the refraction index of the selected materials remains constant during the light scattering process, and focusing only on the variation of light scattering at the surface of the semiconductor as a function of incident light wavelength. The term scattering refers to a general physical process, such as when the ray form of light, sound or even moving particles are forced to change their trajectory by one or more localized non-uniformities in the medium which they are in. Reflection can be simply understood as being a special type of scattering process, whereby, the line normal to the surface equally divides the angle between incident light and reflected light, in accordance with the so-called law of reflection. Therefore, as noted before, a light scattering will induce the enhancement of reflection within the integrating sphere reflection measurement.

3. Materials and Methods

3.1. ’Hot-Injection’ Approach

The synthesis of FeSe2 was carried out under standard oxygen/water-free conditions using a Schlenk line. Two types of selenium precursors are prepared: selenium in oleylamine (OLA) and selenium in tributylphosphine (TBP). Se in oleylamine: 0.1342 g (1.7 mmol) Se (99.99%, Sigma-Aldrich, Dorset, UK ) and 10 mL oleylamine (technical grade, 70%, Sigma-Aldrich, Dorset, UK) were loaded into a two-neck flask under Ar and vacuum drying at 60 °C and 130 °C for 30 min, respectively. Then, Se was allowed to dissolve completely by heating at 200 °C under Ar overnight. Se in TBP: a Se stock solution (1 M) was prepared by dissolving 0.79 g of selenium in 10 mL of TBP, yielding a colorless solution.
Two types of iron precursors were prepared, which are Fe(AC)2 in 1-octadecene (1-ODE) and Fe(AC)2 in OLA. In 1-ODE: 0.34786 g (2 mmol) Fe(Ac)2 (95%, Sigma-Aldrich, Dorset, UK), 1.4224 g (5 mmol) stearic acid (SA, analytical grade, Fisher Scientific, Loughborough, UK), and 5.0 mL 1-ODE (technical grade, 90%, Sigma-Aldrich, Dorset, UK) were loaded into a 50 mL two neck flask under Ar. After loading, the whole system is vacuum dry at 60 °C and 130 °C for half an hour, respectively. In OLA: a similar precursor solution was prepared but replacing 1-ODE with OLA.
The Se precursor (transparent colorless for Se in TBP or transparent yellowish solution for Se in OLA) was injected into the iron precursor (prepared in 1-ODE or OLA) under Ar at 130 °C.
After injection, the temperature increased to 200 °C under vigorous stirring. The reaction temperature was held at 200 °C for 20 min before cooling to room temperature. Toluene (7 mL) was injected into the final solution, and the solution stored under Ar. The purification of as-prepared FeSe2 involves precipitation by adding isopropyl alcohol and centrifugation at 8000 rpm for 10 min. To remove the excess of organic stabilizer, centrifugation was carried out three times.

3.2. TBP and TOP Extraction Se from Iron Diselenide

The procedure is similar to the method reported recently8] Approximate 30 mg of purified FeSe2 was loaded into a two-neck flask under Ar. 10 mL TOP or TBP was added, and the mixture was dried in a vacuum at 60 °C for 30 min. The temperature increased to 180 °C and held at this temperature for 50 min with vigorous stirring. The mixture was allowed to be cooling down to room temperature. The NCs were purified by precipitation with hexane and methanol mixture and centrifugation (3 times). The final purified FeSe NCs were re-dispersed in toluene and stored under Ar.

3.3. NMR, XRD and TEM Methodologies and Instrumentation

Reactions requiring anhydrous conditions were performed under an atmosphere of either anhydrous nitrogen or argon. All glassware was flame-dried prior to use, and glass syringes and needles were placed in an oven (150 °C) for at least 2 h and allowed to cool in desiccators under an atmosphere of anhydrous nitrogen. 1H NMR and 13C NMR spectra were measured in a 400 MHz Varian INOVA 400 instrument (Varian, Crawley, UK). Chemical shifts are quoted in parts per million (ppm) and referenced to SiMe4 (1H NMR 0 ppm) and CDCl3 (13C NMR 77.16 ppm). The 13C NMR sequence parameters were set up as follows: 45° pulse of 7.70 μs, τ = 1.0 s, and acquisition time of 1.28 s. The 31P NMR spectra were measured in a Jeol ECP (Eclipse) 300 instrument (JEOL, Herts, UK). The crystal structure of the products was investigated by powder XRD (BRUKER D8, Cu Kα radiation λ = 1.54 Å, Durham, UK), JEOL 2011 200 kV Hi Resolution TEM (HRTEM, JEOL, Herts, UK) fitted with an EDX Oxford Instruments ISIS 300 system (Oxford Instruments plc, Abingdon, UK).

3.4. Optical Properties Analysis

Typically, FeSe2 or FeSe nanocrystals which dispersed in toluene were drop-casting onto a quartz slide. Then, the reflectance of the slides is analyzed at a 90-degree incident bean angle position on a Perkin Elmer Lambda 35 spectrometer (PerkinElmer, Beaconsfield, UK).

4. Conclusions

In summary, FeSe2, FeSe and Fe3O4 NCs were successfully produced through a well-controlled synthetic strategy. The underlying reaction mechanism was, for the first time, monitored by in situ NMR analysis of intermediates generated during the synthesis of FeSe2 from Fe(AC)2. Plausible reaction mechanisms are proposed based on the detected intermediate species. Upon understanding the chemical reaction process, iron oxide NCs were synthesized by thermolysis reaction. It was found that Fe3O4 is the most stable iron oxide phase during the thermolysis of iron acetate, rather than Fe2O3 or FeO. By ‘chemical etching’ of FeSe2, tetragonal crystal structure FeSe nanoparticles were synthesized successfully. As determined by 31P NMR, TOP was found to play a role in the lattice diffusion of Se from FeSe2. The formation of FeSe was found subsequently with the cleavage of the Fe-Se bond and the formation of the Se-P bond. A plausible reaction mechanism of this phase transformation process was also proposed. The optical properties of as-prepared FeSe and FeSe2 were investigated by integrating sphere total reflection measurements. It was found that the optical band gap of FeSe2 is around 0.67 eV, and FeSe is 1.73 eV. Given the complex nature of the phase diagram of iron chalcogenides, the fundamental understanding of phase pure synthesis of FeSe and FeSe2 NCs are of paramount importance for their photocatalysis and electrocatalysis applications. This work provides comprehensive studies on the organoselenium interconversion reaction mechanisms during iron selenides synthesis, which offer a universal chemical synthetic pathway for preparing phase pure electrocatalytically or photocatalytically active layered metal selenides materials.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11060681/s1, Figure S1: A ‘double cold trap’ set up for the collection of the volatile reaction intermediate species, Scheme S1: Plausible reaction mechanism for the synthesis of FeSe from FeSe2 and TBP, Figure S2: The powder XRD pattern of the final product generating from the preparation of FeSe, which employs TBP as an extraction reagent.

Author Contributions

Conceptualization, B.H. and D.J.F.; methodology, B.H., D.B.-A., R.F.W.; validation, D.C., M.C.G. and D.J.F.; formal analysis, B.H., D.B.-A., R.F.W.; investigation, B.H., D.B.-A., R.F.W.; resources, D.C., M.C.G. and D.J.F.; data curation, D.C., M.C.G. and D.J.F.; writing—original draft preparation, B.H. and D.J.F.; writing—review and editing, D.B.-A., R.F.W., D.C. and M.C.G.; visualization, B.H., D.B.-A., R.F.W.; supervision, D.C., M.C.G. and D.J.F.; project administration, D.J.F.; funding acquisition, M.C.G., and D.J.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research is supported by the Engineering and Physical Sciences Research Council (EP/G031088/1), SUPERGEN Consortium on Excitonic Solar Cells (D.J.F. and B.H.) and EP/J002542/1 (M.C.G. and D.B.-A.) and ERC COG:648239 (M.C.G.).

Acknowledgments

The authors are grateful to Jonathan Jones and Simon R Hall for their valuable supports and comments on this research.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Kamihara, Y.; Watanabe, T.; Hirano, M.; Hosono, H. Iron-Based Layered Superconductor La[O1−xFx]FeAs (x = 0.05−0.12) with Tc = 26 K. J. Am. Chem. Soc. 2008, 130, 3296–3297. [Google Scholar] [CrossRef] [PubMed]
  2. Yi, W.; Sun, L.; Ren, Z.; Lu, W.; Dong, X.; Zhang, H.-j.; Dai, X.; Fang, Z.; Li, Z.; Che, G.; et al. Pressure effect on superconductivity of iron-based arsenic-oxide ReFeAsO0.85 (Re = Sm and Nd). EPL Europhys. Lett. 2008, 83, 57002. [Google Scholar] [CrossRef] [Green Version]
  3. Hsu, F.-C.; Luo, J.-Y.; Yeh, K.-W.; Chen, T.-K.; Huang, T.-W.; Wu, P.M.; Lee, Y.-C.; Huang, Y.-L.; Chu, Y.-Y.; Yan, D.-C.; et al. Superconductivity in the PbO-type structure α-FeSe. Proc. Natl. Acad. Sci. USA 2008, 105, 14262–14264. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  4. Nitsche, F.; Goltz, T.; Klauss, H.H.; Isaeva, A.; Müller, U.; Schnelle, W.; Simon, P.; Doert, T.; Ruck, M. Room-Temperature Synthesis, Hydrothermal Recrystallization, and Properties of Metastable Stoichiometric FeSe. Inorg. Chem. 2012, 51, 7370–7376. [Google Scholar] [CrossRef] [PubMed]
  5. Yoo, H.; Lee, G.-H.; Kim, D.-W. FeSe hollow spheroids as electrocatalysts for high-rate Li–O2 battery cathodes. J. Alloy. Compd. 2021, 856, 158269. [Google Scholar] [CrossRef]
  6. Li, W.; Niu, Y.; Wu, X.; Wu, F.; Li, T.; Hu, W. Heterostructured CoSe2/FeSe2 Nanoparticles with Abundant Vacancies and Strong Electronic Coupling Supported on Carbon Nanorods for Oxygen Evolution Electrocatalysis. ACS Sustain. Chem. Eng. 2020, 8, 4658–4666. [Google Scholar] [CrossRef]
  7. Chanda, D.; Tufa, R.A.; Birdja, Y.Y.; Basu, S.; Liu, S. Hydrothermally/electrochemically decorated FeSe on Ni-foam electrode: An efficient bifunctional electrocatalysts for overall water splitting in an alkaline medium. Int. J. Hydrogen Energy 2020, 45, 27182–27192. [Google Scholar] [CrossRef]
  8. Lv, C.; Liu, H.; Li, D.; Chen, S.; Zhang, H.; She, X.; Guo, X.; Yang, D. Ultrafine FeSe nanoparticles embedded into 3D carbon nanofiber aerogels with FeSe/Carbon interface for efficient and long-life sodium storage. Carbon 2019, 143, 106–115. [Google Scholar] [CrossRef]
  9. Kwon, J.; Jun, S.W.; Choi, S.I.; Mao, X.; Kim, J.; Koh, E.K.; Kim, Y.-H.; Kim, S.-K.; Hwang, D.Y.; Kim, C.-S.; et al. FeSe quantum dots for in vivo multiphoton biomedical imaging. Sci. Adv. 2019, 5, eaay0044. [Google Scholar] [CrossRef] [Green Version]
  10. Choi, J.H.; Park, S.-K.; Kang, Y.C. A Salt-Templated Strategy toward Hollow Iron Selenides-Graphitic Carbon Composite Microspheres with Interconnected Multicavities as High-Performance Anode Materials for Sodium-Ion Batteries. Small 2019, 15, 1803043. [Google Scholar] [CrossRef]
  11. Panda, C.; Menezes, P.W.; Walter, C.; Yao, S.; Miehlich, M.E.; Gutkin, V.; Meyer, K.; Driess, M. From a Molecular 2Fe-2Se Precursor to a Highly Efficient Iron Diselenide Electrocatalyst for Overall Water Splitting. Angew. Chem. Int. Ed. 2017, 56, 10506–10510. [Google Scholar] [CrossRef]
  12. Qin, Z.; Yang, B.C.; Zhang, S.R.; Li, K.; Yan, S.Q.; Lv, X.W.; Li, Z.J. Flower-like pyrite FeSe2 nanoparticles with enhanced optical properties by hot-injection. Vacuum 2015, 111, 157–159. [Google Scholar] [CrossRef]
  13. Mao, X.; Kim, J.-G.; Han, J.; Jung, H.S.; Lee, S.G.; Kotov, N.A.; Lee, J. Phase-Pure FeSex (x = 1, 2) Nanoparticles with One- and Two-Photon Luminescence. J. Am. Chem. Soc. 2014, 136, 7189–7192. [Google Scholar] [CrossRef]
  14. Guo, K.; Zou, Z.; Du, J.; Zhao, Y.; Zhou, B.; Xu, C. Coupling FeSe2 with CoSe: An effective strategy to create stable and efficient electrocatalysts for water oxidation. Chem. Commun. 2018, 54, 11140–11143. [Google Scholar] [CrossRef]
  15. Theerthagiri, J.; Sudha, R.; Premnath, K.; Arunachalam, P.; Madhavan, J.; Al-Mayouf, A.M. Growth of iron diselenide nanorods on graphene oxide nanosheets as advanced electrocatalyst for hydrogen evolution reaction. Int. J. Hydrogen Energy 2017, 42, 13020–13030. [Google Scholar] [CrossRef]
  16. Gao, R.; Zhang, H.; Yan, D. Iron diselenide nanoplatelets: Stable and efficient water-electrolysis catalysts. Nano Energy 2017, 31, 90–95. [Google Scholar] [CrossRef]
  17. Xu, X.; Song, F.; Hu, X. A nickel iron diselenide-derived efficient oxygen-evolution catalyst. Nat. Commun. 2016, 7, 12324. [Google Scholar] [CrossRef]
  18. Yuan, B.; Luan, W.; Tu, S.-t. One-step synthesis of cubic FeS2 and flower-like FeSe2 particles by a solvothermal reduction process. Dalton Trans. 2012, 41, 772–776. [Google Scholar] [CrossRef]
  19. Schuster, W.; Mikler, H.; Komarek, K. Transition metal-chalcogen systems, VII.: The iron-selenium phase diagram. Mon. Für Chem. 1979, 110, 1153–1170. [Google Scholar] [CrossRef]
  20. Terzieff, P.; Komarek, K. The paramagnetic properties of iron selenides with NiAs-type structure. Mon. Chem. 1978, 109, 651–659. [Google Scholar] [CrossRef]
  21. Hamdadou, N.; Bernède, J.C.; Khelil, A. Preparation of iron selenide films by selenization technique. J. Cryst. Growth 2002, 241, 313–319. [Google Scholar] [CrossRef]
  22. Feng, Q.J.; Shen, D.Z.; Zhang, J.Y.; Li, B.S.; Li, B.H.; Lu, Y.M.; Fan, X.W.; Liang, H.W. Ferromagnetic FeSe: Structural, electrical, and magnetic properties. Appl. Phys. Lett. 2006, 88, 012505. [Google Scholar] [CrossRef] [Green Version]
  23. Luo, H.; Furdyna, J.K. The II-VI semiconductor blue-green laser: Challenges and solution. Semicond. Sci. Technol. 1995, 10, 1041. [Google Scholar] [CrossRef]
  24. Ubale, A.U.; Sakhare, Y.S.; Bhute, M.V.; Belkhedkar, M.R.; Singh, A. Size-dependent structural, electrical and optical properties of nanostructured iron selenide thin films deposited by Chemical Bath Deposition Method. Solid State Sci. 2013, 16, 134–142. [Google Scholar] [CrossRef]
  25. Ouertani, B.; Ouerfelli, J.; Saadoun, M.; Zribi, M.; Rabha, M.B.; Bessaïs, B.; Ezzaouia, H. Optical and structural properties of FeSe2 thin films obtained by selenization of sprayed amorphous iron oxide films. Thin Solid Film. 2006, 511–512, 457–462. [Google Scholar] [CrossRef]
  26. Liu, K.W.; Zhang, J.Y.; Shen, D.Z.; Shan, C.X.; Li, B.H.; Lu, Y.M.; Fan, X.W. Electronic and magnetic properties of FeSe thin film prepared on GaAs (001) substrate by metal-organic chemical vapor deposition. Appl. Phys. Lett. 2007, 90, 262503. [Google Scholar] [CrossRef] [Green Version]
  27. Campos, C.E.M.; de Lima, J.C.; Grandi, T.A.; Machado, K.D.; Drago, V.; Pizani, P.S. XRD, DSC, MS and RS studies of Fe75Se25 iron selenide prepared by mechano-synthesis. J. Magn. Magn. Mater. 2004, 270, 89–98. [Google Scholar] [CrossRef]
  28. Hamdadou, N.; Khelil, A.; Morsli, M.; Bernàde, J.C. Iron diselenide thin films synthesized by soft selenization of iron films. Vacuum 2005, 77, 151–156. [Google Scholar] [CrossRef]
  29. Ouertani, B.; Ouerfelli, J.; Saadoun, M.; Bessaïs, B.; Ezzaouia, H.; Bernède, J.C. Transformation of amorphous iron oxide thin films predeposited by spray pyrolysis into a single FeSe2-phase by selenisation. Sol. Energy Mater. Sol. Cells 2005, 87, 501–511. [Google Scholar] [CrossRef]
  30. Campos, C.E.M.; de Lima, J.C.; Grandi, T.A.; Machado, K.D.; Pizani, P.S. Structural studies of iron selenides prepared by mechanical alloying. Solid State Commun. 2002, 123, 179–184. [Google Scholar] [CrossRef]
  31. Chang, C.-C.; Wang, C.-H.; Wen, M.-H.; Wu, Y.-R.; Hsieh, Y.-T.; Wu, M.-K. Superconductivity in PbO-type tetragonal FeSe nanoparticles. Solid State Commun. 2012, 152, 649–652. [Google Scholar] [CrossRef]
  32. Wadia, C.; Alivisatos, A.P.; Kammen, D.M. Materials Availability Expands the Opportunity for Large-Scale Photovoltaics Deployment. Environ. Sci. Technol. 2009, 43, 2072–2077. [Google Scholar] [CrossRef]
  33. Luther, G.W. Pyrite synthesis via polysulfide compounds. Geochim. Cosmochim. Acta 1991, 55, 2839–2849. [Google Scholar] [CrossRef]
  34. Ennaoui, A.; Fiechter, S.; Pettenkofer, C.; Alonso-Vante, N.; Büker, K.; Bronold, M.; Höpfner, C.; Tributsch, H. Iron disulfide for solar energy conversion. Sol. Energy Mater. Sol. Cells 1993, 29, 289–370. [Google Scholar] [CrossRef]
  35. Kwon, H.; Thanikaikarasan, S.; Mahalingam, T.; Park, K.; Sanjeeviraja, C.; Kim, Y. Characterization of electrosynthesized iron diselenide thin films. J. Mater. Sci. Mater. Electron. 2008, 19, 1086–1091. [Google Scholar] [CrossRef]
  36. Akhtar, M.; Akhtar, J.; Malik, M.A.; Tuna, F.; Helliwell, M.; O’Brien, P. Deposition of iron selenide nanocrystals and thin films from tris(N,N-diethyl-N′-naphthoylselenoureato)iron(iii). J. Mater. Chem. 2012, 22, 14970–14975. [Google Scholar] [CrossRef]
  37. Shi, W.; Zhang, X.; Che, G.; Fan, W.; Liu, C. Controlled hydrothermal synthesis and magnetic properties of three-dimensional FeSe2 rod clusters and microspheres. Chem. Eng. J. 2013, 215–216, 508–516. [Google Scholar] [CrossRef]
  38. Sines, I.T.; Schaak, R.E. Phase-Selective Chemical Extraction of Selenium and Sulfur from Nanoscale Metal Chalcogenides: A General Strategy for Synthesis, Purification, and Phase Targeting. J. Am. Chem. Soc. 2010, 133, 1294–1297. [Google Scholar] [CrossRef]
  39. Chen, L.; Yang, X.; Fu, X.; Wang, C.; Liang, C.; Wu, M. Facile Solvothermal Synthesis of Uniform Iron Selenide Nanoplates. Eur. J. Inorg. Chem. 2011, 2011, 2098–2102. [Google Scholar] [CrossRef]
  40. Owen, J.; Brus, L. Chemical Synthesis and Luminescence Applications of Colloidal Semiconductor Quantum Dots. J. Am. Chem. Soc. 2017, 139, 10939–10943. [Google Scholar] [CrossRef]
  41. García-Rodríguez, R.; Hendricks, M.P.; Cossairt, B.M.; Liu, H.; Owen, J.S. Conversion Reactions of Cadmium Chalcogenide Nanocrystal Precursors. Chem. Mater. 2013, 25, 1233–1249. [Google Scholar] [CrossRef]
  42. Sowers, K.L.; Swartz, B.; Krauss, T.D. Chemical Mechanisms of Semiconductor Nanocrystal Synthesis. Chem. Mater. 2013, 25, 1351–1362. [Google Scholar] [CrossRef]
  43. Benito-Alifonso, D.; Tremel, S.; Hou, B.; Lockyear, H.; Mantell, J.; Fermín, D.J.; Verkade, P.; Berry, M.; Galan, M.C. Lactose as a “Trojan horse” for Quantum Dot Cell Transport. Angew. Chem. Int. Ed. 2013, 53, 810–814. [Google Scholar] [CrossRef] [Green Version]
  44. Hou, B. Colloidal Quantum Dots: The Artificial Building Blocks for New-Generation Photo-Electronics and Photochemistry. Isr. J. Chem. 2019, 59, 637–638. [Google Scholar] [CrossRef] [Green Version]
  45. Hou, B.; Benito-Alifonso, D.; Webster, R.; Cherns, D.; Galan, M.C.; Fermín, D.J. Rapid phosphine-free synthesis of CdSe quantum dots: Promoting the generation of Se precursors using a radical initiator. J. Mater. Chem. A 2014, 2, 6879–6886. [Google Scholar] [CrossRef] [Green Version]
  46. Hou, B.; Parker, D.; Kissling, G.P.; Jones, J.A.; Cherns, D.; Fermín, D.J. Structure and Band Edge Energy of Highly Luminescent CdSe1−xTex Alloyed Quantum Dots. J. Phys. Chem. C 2013, 117, 6814–6820. [Google Scholar] [CrossRef]
  47. Sayevich, V.; Robinson, Z.L.; Kim, Y.; Kozlov, O.V.; Jung, H.; Nakotte, T.; Park, Y.-S.; Klimov, V.I. Highly versatile near-infrared emitters based on an atomically defined HgS interlayer embedded into a CdSe/CdS quantum dot. Nat. Nanotechnol. 2021. [Google Scholar] [CrossRef]
  48. Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X. Solution-processed, high-performance light-emitting diodes based on quantum dots. Nature 2014, 515, 96–99. [Google Scholar] [CrossRef]
  49. Shen, H.; Gao, Q.; Zhang, Y.; Lin, Y.; Lin, Q.; Li, Z.; Chen, L.; Zeng, Z.; Li, X.; Jia, Y.; et al. Visible quantum dot light-emitting diodes with simultaneous high brightness and efficiency. Nat. Photonics 2019, 13, 192–197. [Google Scholar] [CrossRef]
  50. Zhao, Q.; Gouget, G.; Guo, J.; Yang, S.; Zhao, T.; Straus, D.B.; Qian, C.; Oh, N.; Wang, H.; Murray, C.B.; et al. Enhanced Carrier Transport in Strongly Coupled, Epitaxially Fused CdSe Nanocrystal Solids. Nano Lett. 2021, 21, 3318–3324. [Google Scholar] [CrossRef]
  51. Banski, M.; Afzaal, M.; Malik, M.A.; Podhorodecki, A.; Misiewicz, J.; O’Brien, P. Special Role for Zinc Stearate and Octadecene in the Synthesis of Luminescent ZnSe Nanocrystals. Chem. Mater. 2015, 27, 3797–3800. [Google Scholar] [CrossRef]
  52. Hou, B.; Li, Y.; Liu, Y.; Yuan, B.; Jia, M.; Jiang, F. A simple way of shape-controlled synthesis of ZnSe nanocrystals: Nanodots, nanoflowers, and nanotubes. CrystEngComm 2009, 11, 1789–1792. [Google Scholar] [CrossRef]
  53. Kim, T.; Kim, K.-H.; Kim, S.; Choi, S.-M.; Jang, H.; Seo, H.-K.; Lee, H.; Chung, D.-Y.; Jang, E. Efficient and stable blue quantum dot light-emitting diode. Nature 2020, 586, 385–389. [Google Scholar] [CrossRef] [PubMed]
  54. Han, C.-Y.; Lee, S.-H.; Song, S.-W.; Yoon, S.-Y.; Jo, J.-H.; Jo, D.-Y.; Kim, H.-M.; Lee, B.-J.; Kim, H.-S.; Yang, H. More Than 9% Efficient ZnSeTe Quantum Dot-Based Blue Electroluminescent Devices. ACS Energy Lett. 2020, 5, 1568–1576. [Google Scholar] [CrossRef]
  55. Hou, B.; Sohn, M.; Lee, Y.-W.; Zhang, J.; Sohn, J.I.; Kim, H.; Cha, S.; Kim, J.M. Chemically encoded self-organized quantum chain supracrystals with exceptional charge and ion transport properties. Nano Energy 2019, 62, 764–771. [Google Scholar] [CrossRef]
  56. Song, H.; Lin, Y.; Zhang, Z.; Rao, H.; Wang, W.; Fang, Y.; Pan, Z.; Zhong, X. Improving the Efficiency of Quantum Dot Sensitized Solar Cells beyond 15% via Secondary Deposition. J. Am. Chem. Soc. 2021, 143, 4790–4800. [Google Scholar] [CrossRef] [PubMed]
  57. Song, H.; Lin, Y.; Zhou, M.; Rao, H.; Pan, Z.; Zhong, X. Zn-Cu-In-S-Se Quinary “Green” Alloyed Quantum-Dot-Sensitized Solar Cells with a Certified Efficiency of 14.4%. Angew. Chem. Int. Ed. 2021, 60, 6137–6144. [Google Scholar] [CrossRef]
  58. Kattan, N.; Hou, B.; Fermín, D.J.; Cherns, D. Crystal structure and defects visualization of Cu2ZnSnS4 nanoparticles employing transmission electron microscopy and electron diffraction. Appl. Mater. Today 2015, 1, 52–59. [Google Scholar] [CrossRef] [Green Version]
  59. Hou, B.; Benito-Alifonso, D.; Kattan, N.; Cherns, D.; Galan, M.C.; Fermín, D.J. Initial Stages in the Formation of Cu2ZnSn(S,Se)4 Nanoparticles. Chem. A Eur. J. 2013, 19, 15847–15851. [Google Scholar] [CrossRef]
  60. Hou, B.; Cho, Y.; Kim, B.S.; Hong, J.; Park, J.B.; Ahn, S.J.; Sohn, J.I.; Cha, S.; Kim, J.M. Highly Monodispersed PbS Quantum Dots for Outstanding Cascaded-Junction Solar Cells. ACS Energy Lett. 2016, 1, 834–839. [Google Scholar] [CrossRef]
  61. Hou, B.; Cho, Y.; Kim, B.-S.; Ahn, D.; Lee, S.; Park, J.B.; Lee, Y.-W.; Hong, J.; Im, H.; Morris, S.M.; et al. Red green blue emissive lead sulfide quantum dots: Heterogeneous synthesis and applications. J. Mater. Chem. C 2017, 5, 3692–3698. [Google Scholar] [CrossRef] [Green Version]
  62. Hou, B.; Kim, B.-S.; Lee, H.K.H.; Cho, Y.; Giraud, P.; Liu, M.; Zhang, J.; Davies, M.L.; Durrant, J.R.; Tsoi, W.C.; et al. Multiphoton Absorption Stimulated Metal Chalcogenide Quantum Dot Solar Cells under Ambient and Concentrated Irradiance. Adv. Funct. Mater. 2020, 30, 2004563. [Google Scholar] [CrossRef]
  63. Hou, B.; Jung, S.-H.; Zhang, J.; Hong, Y.; Kim, B.-S.; Sohn, J.I.; Lee, E.K.; Choi, B.L.; Whang, D.; Cha, S.; et al. Growth of quantum dot coated core-shell anisotropic nanowires for improved thermal and electronic transport. Appl. Phys. Lett. 2019, 114, 243104. [Google Scholar] [CrossRef]
  64. Campos, M.P.; Hendricks, M.P.; Beecher, A.N.; Walravens, W.; Swain, R.A.; Cleveland, G.T.; Hens, Z.; Sfeir, M.Y.; Owen, J.S. A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions near the Homogeneous Limit. J. Am. Chem. Soc. 2017, 139, 2296–2305. [Google Scholar] [CrossRef]
  65. Shrestha, A.; Batmunkh, M.; Tricoli, A.; Qiao, S.Z.; Dai, S. Near-Infrared Active Lead Chalcogenide Quantum Dots: Preparation, Post-Synthesis Ligand Exchange, and Applications in Solar Cells. Angew. Chem. Int. Ed. 2019, 58, 5202–5224. [Google Scholar] [CrossRef]
  66. Lin, Q.; Yun, H.J.; Liu, W.; Song, H.-J.; Makarov, N.S.; Isaienko, O.; Nakotte, T.; Chen, G.; Luo, H.; Klimov, V.I.; et al. Phase-Transfer Ligand Exchange of Lead Chalcogenide Quantum Dots for Direct Deposition of Thick, Highly Conductive Films. J. Am. Chem. Soc. 2017, 139, 6644–6653. [Google Scholar] [CrossRef]
  67. Livache, C.; Martinez, B.; Goubet, N.; Gréboval, C.; Qu, J.; Chu, A.; Royer, S.; Ithurria, S.; Silly, M.G.; Dubertret, B.; et al. A colloidal quantum dot infrared photodetector and its use for intraband detection. Nat. Commun. 2019, 10, 2125. [Google Scholar] [CrossRef] [Green Version]
  68. Tang, X.; Wu, G.f.; Lai, K.W.C. Plasmon resonance enhanced colloidal HgSe quantum dot filterless narrowband photodetectors for mid-wave infrared. J. Mater. Chem. C 2017, 5, 362–369. [Google Scholar] [CrossRef]
  69. Qu, J.; Rastogi, P.; Gréboval, C.; Lagarde, D.; Chu, A.; Dabard, C.; Khalili, A.; Cruguel, H.; Robert, C.; Xu, X.Z.; et al. Electroluminescence from HgTe Nanocrystals and Its Use for Active Imaging. Nano Lett. 2020, 20, 6185–6190. [Google Scholar] [CrossRef]
  70. Ma, Y.; Zhang, Y.; Yu, W.W. Near infrared emitting quantum dots: Synthesis, luminescence properties and applications. J. Mater. Chem. C 2019, 7, 13662–13679. [Google Scholar] [CrossRef]
  71. Yarema, M.; Yarema, O.; Lin, W.M.M.; Volk, S.; Yazdani, N.; Bozyigit, D.; Wood, V. Upscaling Colloidal Nanocrystal Hot-Injection Syntheses via Reactor Underpressure. Chem. Mater. 2017, 29, 796–803. [Google Scholar] [CrossRef] [Green Version]
  72. Li, D.; Pan, D.; Liu, W.; Li, X.; Chen, M.; Li, S.; Li, Y.; Tan, J.; Sun, D.; Wang, Z.; et al. Controllable Phase Transition for Layered β-FeSe Superconductor Synthesized by Solution Chemistry. Chem. Mater. 2017, 29, 842–848. [Google Scholar] [CrossRef]
  73. Greenfield, J.T.; Kamali, S.; Lee, K.; Kovnir, K. A Solution for Solution-Produced β-FeSe: Elucidating and Overcoming Factors that Prevent Superconductivity. Chem. Mater. 2015, 27, 588–596. [Google Scholar] [CrossRef]
  74. Oyler, K.D.; Ke, X.; Sines, I.T.; Schiffer, P.; Schaak, R.E. Chemical Synthesis of Two-Dimensional Iron Chalcogenide Nanosheets: FeSe, FeTe, Fe(Se,Te), and FeTe2. Chem. Mater. 2009, 21, 3655–3661. [Google Scholar] [CrossRef]
  75. Liu, H.; Owen, J.S.; Alivisatos, A.P. Mechanistic Study of Precursor Evolution in Colloidal Group II−VI Semiconductor Nanocrystal Synthesis. J. Am. Chem. Soc. 2006, 129, 305–312. [Google Scholar] [CrossRef]
  76. Tauc, J. Optical properties and electronic structure of amorphous Ge and Si. Mater. Res. Bull. 1968, 3, 37–46. [Google Scholar] [CrossRef]
  77. Bass, M.; DeCusatis, C.; Enoch, J.; Lakshminarayanan, V.; Li, G.; MacDonald, C.; Maha- jan, V.; Van Stryland, E. Optical Properties of Materials, Nonlinear Optics, Quantum Optics. In Handbook of Optics, 3rd ed.; Mcgraw-Hill: New York, NY, USA, 2009; Volume IV. [Google Scholar]
  78. Bass, M. Handbook of Optics, 2nd ed.; Mcgraw-Hill: New York, NY, USA, 1994; Volume II. [Google Scholar]
  79. Devore, J.R. Refractive Indices of Rutile and Sphalerite. J. Opt. Soc. Am. 1951, 41, 416–417. [Google Scholar] [CrossRef]
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Figure 1. 1H (a) and 13C NMR (b) analysis of volatile species Sample I collected from ‘cold trap’. 13C NMR analysis of the reaction intermediate, which was taken during Fe(Ac)2, reacted with SA in the presence of 1-ODE (c). X-ray Powder Diffraction (XRD) diagram of final product generated through hot injection of TBP-Se into iron precursor prepared by Fe(AC)2 reacted with SA in the present of 1-ODE (d).
Figure 1. 1H (a) and 13C NMR (b) analysis of volatile species Sample I collected from ‘cold trap’. 13C NMR analysis of the reaction intermediate, which was taken during Fe(Ac)2, reacted with SA in the presence of 1-ODE (c). X-ray Powder Diffraction (XRD) diagram of final product generated through hot injection of TBP-Se into iron precursor prepared by Fe(AC)2 reacted with SA in the present of 1-ODE (d).
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Scheme 1. Plausible reaction pathway of the synthesis of FeSe2 by ‘hot-injection’ method employing Fe(AC)2 and SA as an iron precursor, and TBP for solubilization of Se.
Scheme 1. Plausible reaction pathway of the synthesis of FeSe2 by ‘hot-injection’ method employing Fe(AC)2 and SA as an iron precursor, and TBP for solubilization of Se.
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Figure 2. (a) Powder XRD pattern of as-prepared FeSe2 nanocrystals through employing FeSt2 as an iron precursor and OLA for solubilization of Se. The vertical lines on the bottom correspond to orthorhombic marcasite-type structures of FeSe2 (PDF = 21-0432 and 74-0247). (b,c) TEM analysis of as-prepared FeSe2, the scale bar is 50 nm. (d) HRTEM analysis of as-prepared FeSe2 resolved the reflection from the (110) plane, with a lattice spacing of 3.8 ± 0.2 Å, the scale bar is 2 nm.
Figure 2. (a) Powder XRD pattern of as-prepared FeSe2 nanocrystals through employing FeSt2 as an iron precursor and OLA for solubilization of Se. The vertical lines on the bottom correspond to orthorhombic marcasite-type structures of FeSe2 (PDF = 21-0432 and 74-0247). (b,c) TEM analysis of as-prepared FeSe2, the scale bar is 50 nm. (d) HRTEM analysis of as-prepared FeSe2 resolved the reflection from the (110) plane, with a lattice spacing of 3.8 ± 0.2 Å, the scale bar is 2 nm.
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Scheme 2. Plausible reaction pathway for the second approach for the synthesis of FeSe2, employing FeSt2 as an iron precursor and OLA for solubilization of Se.
Scheme 2. Plausible reaction pathway for the second approach for the synthesis of FeSe2, employing FeSt2 as an iron precursor and OLA for solubilization of Se.
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Figure 3. TEM (a), XRD (b) and HRTEM (c,d) analysis of as-prepared Fe3O4 nanoparticles. (a) TEM analysis of Fe3O4 nanoparticles with scale bar equal to 20 nm. (b) Powder XRD pattern of as-prepared Fe3O4 NCs by Scheme 3, the vertical line on the bottom corresponds to face centered cubic Fe3O4 (PDF = 75-0033). (c,d) HRTEM of Fe3O4 resolving the lattice spacing of the (220) plane is 3.0 ± 0.2 Å and (111) plane is 5.0 ± 0.2 Å. The scale bar in HRTEM images is all equal to 2 nm.
Figure 3. TEM (a), XRD (b) and HRTEM (c,d) analysis of as-prepared Fe3O4 nanoparticles. (a) TEM analysis of Fe3O4 nanoparticles with scale bar equal to 20 nm. (b) Powder XRD pattern of as-prepared Fe3O4 NCs by Scheme 3, the vertical line on the bottom corresponds to face centered cubic Fe3O4 (PDF = 75-0033). (c,d) HRTEM of Fe3O4 resolving the lattice spacing of the (220) plane is 3.0 ± 0.2 Å and (111) plane is 5.0 ± 0.2 Å. The scale bar in HRTEM images is all equal to 2 nm.
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Scheme 3. Plausible reaction scheme for the synthesis of Fe3O4 from Fe(Ac)2 and OLA. It should be noted that OLA shows on the arrow refer to the solvent for preparing the Se precursor.
Scheme 3. Plausible reaction scheme for the synthesis of Fe3O4 from Fe(Ac)2 and OLA. It should be noted that OLA shows on the arrow refer to the solvent for preparing the Se precursor.
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Figure 4. 31P NMR analysis of reaction intermediates during the reaction of FeSe2 with TOP. The top spectrum corresponds to standard TOP. The bottom spectrum corresponds to Sample II, taken from the crude reaction mixture by J. Young NMR tube.
Figure 4. 31P NMR analysis of reaction intermediates during the reaction of FeSe2 with TOP. The top spectrum corresponds to standard TOP. The bottom spectrum corresponds to Sample II, taken from the crude reaction mixture by J. Young NMR tube.
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Figure 5. TEM (a) and HRTEM (b) analysis of as-prepared FeSe NCs, the lattice spacing is 5.9 ± 0.2 Å, and the scale bar is 20 nm and 2 nm, respectively. (c) A series of XRD patterns derived from preparing FeSe nanocrystals through extracting Se from FeSe2. The vertical line on the bottom corresponds to tetragonal FeSe (PDF = 85-0735) and orthorhombic marcasite-type structures of FeSe2 (PDF = 21-0432), respectively.
Figure 5. TEM (a) and HRTEM (b) analysis of as-prepared FeSe NCs, the lattice spacing is 5.9 ± 0.2 Å, and the scale bar is 20 nm and 2 nm, respectively. (c) A series of XRD patterns derived from preparing FeSe nanocrystals through extracting Se from FeSe2. The vertical line on the bottom corresponds to tetragonal FeSe (PDF = 85-0735) and orthorhombic marcasite-type structures of FeSe2 (PDF = 21-0432), respectively.
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Scheme 4. Plausible reaction scheme for the synthesis of FeSe from FeSe2 and TOP.
Scheme 4. Plausible reaction scheme for the synthesis of FeSe from FeSe2 and TOP.
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Figure 6. Optical band gap estimation of FeSe2 (a–d) and FeSe (e–h) from the reflectance measurement, different colour solid line attributed to the same sample in different measurement. The bandgap values of FeSe2 (b–d) and FeSe (f–h) are collected from the intercepts on the abscissa.
Figure 6. Optical band gap estimation of FeSe2 (a–d) and FeSe (e–h) from the reflectance measurement, different colour solid line attributed to the same sample in different measurement. The bandgap values of FeSe2 (b–d) and FeSe (f–h) are collected from the intercepts on the abscissa.
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Figure 7. Rayleigh scattering of semiconductors as a function of wavelength, as determined by employing Equation (4). The arrow indicates the effect of increasing of refractive index.
Figure 7. Rayleigh scattering of semiconductors as a function of wavelength, as determined by employing Equation (4). The arrow indicates the effect of increasing of refractive index.
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Hou, B.; Benito-Alifonso, D.; Webster, R.F.; Cherns, D.; Galan, M.C.; Fermín, D.J. Synthetic Mechanism Studies of Iron Selenides: An Emerging Class of Materials for Electrocatalysis. Catalysts 2021, 11, 681. https://doi.org/10.3390/catal11060681

AMA Style

Hou B, Benito-Alifonso D, Webster RF, Cherns D, Galan MC, Fermín DJ. Synthetic Mechanism Studies of Iron Selenides: An Emerging Class of Materials for Electrocatalysis. Catalysts. 2021; 11(6):681. https://doi.org/10.3390/catal11060681

Chicago/Turabian Style

Hou, Bo, David Benito-Alifonso, Richard F. Webster, David Cherns, M. Carmen Galan, and David J. Fermín. 2021. "Synthetic Mechanism Studies of Iron Selenides: An Emerging Class of Materials for Electrocatalysis" Catalysts 11, no. 6: 681. https://doi.org/10.3390/catal11060681

APA Style

Hou, B., Benito-Alifonso, D., Webster, R. F., Cherns, D., Galan, M. C., & Fermín, D. J. (2021). Synthetic Mechanism Studies of Iron Selenides: An Emerging Class of Materials for Electrocatalysis. Catalysts, 11(6), 681. https://doi.org/10.3390/catal11060681

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