Synthesis of metal chalcogenide semiconductor nanocrystals in controllable shape and size are of great significance for modern science and technology. Transition metal dichalcogenides MX₂ (M = Mn, Fe, Co, Ni, Cu, Zn; X = S, Se) with pyrite structures have been given much attention due to their novel electrical, magnetic, and optical properties.^[1–5] A great number of experimental and theoretical studies have been reported on MnX/MnX₂ during recent decades.^[6–14] Recently, many studies have intensely focused on DMSs due to their special physical properties and potential application such as blue/green light emitters.^[15] In particular, manganese selenides (MnSe₂, MnSe) possess strong sp-d band electron exchange interactions between electron/hole band states and 3d electron states of introduced Mn²⁺ ions; therefore they have a unique magnetic function and can be used to fabricate DMSs.^[16,17] Traditionally, molecular beam epitaxy (MBE),^[18] organometallic vapor phase epitaxy (OMVPE),^[19] direct reaction metal Mn with pure Se,^[20] hydrothermal,^{[6,8,21–29]} and solvothermal process^[9,30,31] were employed to synthesize manganese selenide. These methods usually need complex apparatus, severe reaction conditions, high temperature or toxic raw materials, which may restrict the massive preparation of manganese selenide. Thus, a new synthesis with tunable particle size, morphology, and structure was highly desirable in order to realize the final applications of these functionalized particles with good properties.

In this paper, we developed a controllable synthesis to make MnSe_x particles under hydrothermal conditions. This synthesis involves heating of reactants in a particular solvent under high-pressure with a system such as an autoclave. This method was a simple, convenient and controllable procedure, which provided an effective way to synthesize the selenide materials. Some factors such as pH value, reaction temperature and time were critical and had a great influence on the size, morphology, structure and phase purity of as-made particles. Moreover, the optical and magnetic properties of these samples were reported.

All the chemicals with analytical grade were purchased from Aladdin and used as received without further purification.In a typical experiment for the synthesis of MnSe/MnSe₂ particles, the mixture of 5-mL deionized water, 1.5-mmol manganese acetate tetrahydrate [Mn(ac)₂4H₂O], and 1.5-mmol acetic acid were kept stirring at room temperature. 10-mL freshly prepared NaHSe solution (0.15 M) and 2-mL 80-wt% hydrazine hydrate solution [N₂H₄·H₂O] were added to the manganese acetate solution, and then the mixture was adjusted to different pH values with 10-M NaOH solution or 10-M acetic acid. After being stirred for 10 min, the solution was transferred into a Teflon-lined stainless autoclave with 25-mL capacity. Then the sealed autoclave was heated to 180 °C and kept for 1 h to 24 h. After the reaction was completed, the autoclave was cooled down to room temperature on its own, the precipitate was separated by centrifugation and transferred into 10 mL of citric acid (1 M) solution to remove the by-product Mn(OH)₂ if necessary. Then the products were washed with distilled water and anhydrous ethanol several times, and dried under vacuum at 60°C for 4 h.

The crystal structure of dried particles was characterized on x-ray diffractometer (XRD) with Cu K_α radiation. The morphology of these particles was examined with JEOL-2100 transmission electron microscopy (TEM). A UV-visible spectroscopy was used to investigate the optical property of these nanoparticles at room temperature. The magnetic property was done on a superconducting quantum interference device (SQUID) from 300 K down to 5 K.

Based on the phase diagram, MnSe is stable at normal condition with a rock salt (NaCl) structure, in which the second nearest-neighbor Mn atoms are joined by Se atoms located on the octahedral sites (See Fig. 1(a)).^[17,32] The pyrite MnSe₂ is a face-centered cubic phase and resembles the NaCl structure. The Mn atoms replace the Na atoms and occupy all the octahedral sites which are formed by six nearest Se₂ groups. The dumbbell-shaped Se₂ groups replace the Cl atoms, being surrounded by six nearest Mn atoms as well (See Fig. 1(b)).

Fig. 1.

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	Fig. 1. The schematic illustration of the crystal structures of (a) MnSe and (b) MnSe2.

Figure 2(a) illustrates a series of XRD patterns of MnSe₂ and MnSe particles synthesized under different pH values for 12 hours. It is clear that there are two sets of diffraction patterns. When the pH value is less than 8, the diffraction peaks can be indexed as the pure pyrite MnSe₂ phase with lattice constant a = 5.350 Å. With pH values increasing, the main diffraction peaks are dominated with cubic MnSe phase. Once the pH value is greater than 9, a pure cubic MnSe phase with lattice constant c = 5.520 Å is developed. It is obvious that there is a phase transition from pyrite phase to cubic phase with pH value increase. In order to further investigate this phase transition in detail, a series of a small amount of solution was taken out from the autoclave after reacting for 1 h, 3 h, 6 h, 9 h, 12 h, and 24 h, respectively (see Table 1). The XRD patterns reveal that the pyrite MnSe₂ was dominant in the solution after 1-h reaction (See Fig. 2(b)). With the extension of reaction time, the diffraction peak of cubic MnSe appeared and gradually became a main phase in the reacting solution. Eventually, the phase transition from pyrite MnSe₂ to cubic MnSe was completed after 12-h reaction. Furthermore, such a phase transition will be much easier at high reaction temperature in an autoclave.^[33–35] However, when the pH values of the solutions are lower than 7, this phase transition cannot be fully completed, even with a long reaction time (48 h) or high reaction temperature (240 °C). The solution always contains a mixture of MnSe₂ and MnSe particles. Table 1 illustrates the effects of reaction time and pH values on the particle sizes, purity and the phase of the samples have been investigated.

Fig. 2.

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	Fig. 2. The XRD patterns of (a) as-grown MnSe and MnSe2 particles and (b) MnSex particles synthesized under different reaction time.

Table 1.

Table 1.

Table 1. The experimental conditions for preparation of MnSe/MnSe2 particles from Mn(ac)2 and Se powder (molar ratio = 1:1). . Sample No. Time/h T/°C pH values Size Phase 1 12 180 6.00 80 nm MnSe2 2 12 180 8.00 500 nm MnSe 3 12 180 10.00 1.5 μm MnSe 4 1 180 8.00 MnSe2 5 3 180 8.00 500 nm MnSe2/MnSe 6 6 180 8.00 MnSe2/MnSe 7 9 180 8.00 MnSe2/MnSe 8 24 180 8.00 MnSe 9 12 150 8.00 MnSe2 10 12 180 13 MnSe/Mn(OH)2 11 12 180 5 MnSe2

Table 1. The experimental conditions for preparation of MnSe/MnSe2 particles from Mn(ac)2 and Se powder (molar ratio = 1:1). .

Figure 3 shows the transmission electron microscopy (TEM) images of as-made MnSe₂ and MnSe particles synthesized at different pH values. It is clear that the pure MnSe₂ sample contains hexagon nanodiscs with an average diameter of about 80 nm (See Fig. 3(a), in the figure, the dark part is the side view of these nanodiscs). Although the pure MnSe sample also consists of hexagon discs, the size of these discs increases with pH values and is much larger than that of the pure MnSe₂ nanodiscs (See Fig. 3(b)). Once the initial pH value is higher than 8, the as-grown particles are cubic due to the loss of dangling bond difference, which is discussed later. Figure (d) is the statistics diagram of the as-prepared particles’ sizes. It is clear that the particle sizes increase from nanometer to micrometer with increasing pH values.

Fig. 3.

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	Fig. 3. (a)–(c) TEM images of as-grown MnSe and MnSe2 particles. (d) The statistics diagram of the as-prepared particles’ sizes as a function of pH values.

Figure 4 shows the SEM images of as-grown particles of Samples 1, 2, and 3. Compared to the previous reported method with NaSeO₃ as the selenium source,^[28] the particles synthesized by our method are evidently much smaller and more regular. What is more, this method is more environmentally friendly since the nontoxic Se powder was used to replace the toxic NaSeO₃.

Fig. 4.

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	Fig. 4. SEM images of as-grown MnSe2 and MnSe particles.

Generally, only one kind of material other than the multiple ones would be obtained under certain growth conditions. However, during our experiment process, materials with two kinds of components in various shapes are synthesized. It is of great value to understand the growth mechanism and phase transition mechanism during this process, which may help us to control synthesized transition metal chalcogenides in the future.

According to the equilibrium constant for Mn(OH)₂ formation given by Ref. [36]: Mn²⁺ + 2OH⁻ ↔ Mn(OH)₂, K_so = 10^−12.1, K_sp = 2.06 × 10⁻¹³, T = 298 K. Generally, the nucleation temperature is about 300 K. According to the equilibrium equation, Mn(OH)₂ starts to form once pH value is greater than 6.72. We may further speculate that if the pH value of the solution is higher than the theoretical value of 8.16, the concentration of Mn²⁺ will be much lower than 10⁻⁵ mol/L (due to the formation of Mn(OH)₂), which will prevent the formation of MnSe_x nuclei. Thus, Mn(OH)₂ is the main presentation of Mn source at room temperature. Increasing the temperature up to 180 °C, Mn(OH)₂ will dissolve according to the reaction equilibrium constant, which leads to slow growth of the existing nuclei but with no formation of new nuclei.^[37] As pH > 7, N₂H₄·H₂O serves as reductant and ensures the Se²⁻ in the solution. Furthermore acetic acid molecules slow down the growth considerably by coordinating to the surface of the nanocrystal forming a steric barrier for reactants.^[38,39] At this stage, acetic acid may selectively attach itself to a preferential crystallographic face, resulting in growth of particles along a particular plane, making them more orientated towards a particular shape. While the value of pH is lower than 8, more Mn²⁺ ion is presented instead of Mn(OH)₂, which leads to the formation of more fresh nuclei at low temperature. The size of particles with lower pH values is smaller than that with higher pH values. However, if further decreasing pH value (pH < 5), the existing Se²⁻ is highly reactive and can easily be oxidized by N₂H₄·H₂O to form Se particles. Furthermore, MnSe₂ particles will be obtained due to the reaction of MnSe and Se in this acidic solution.^{[8,24,26,29,40]} If the pH is above 12, the white flake of Mn(OH)₂ was observed in the as-prepared samples and confirmed by XRD patterns. This may suggest that the main product is Mn(OH)₂ instead of MnSe_x with extremely high pH value.

It has been established that the most commonly and energetically favored edge structures of MoS₂ are Mo zigzag (Mo-zz) terminations and S zigzag (S-zz) terminations,^[3] which alternatively constructs the six edges of a hexagonal-shaped MoS₂ nanosheet. Since MnSe_x have the same hexagonal crystal structures as MoS₂, in which Mn (Se) atoms replace the position of Mo (S) atoms, it should follow a similar principle. During the growth process of our experiment, the concentration of Mn atoms plays a significant role in the shape evolution. As the pH values are lower than 7, Se₂²⁻ atoms are the dominant as the source of Se, which leads to the formation of MnSe₂ and is further confirmed by the XRD patterns. For Mn-zz terminations, each Mn atom has two dangling bonds, while for Se-zz terminations, the dangling bonds become one. This difference may result in distinct chemical activity of those terminations, which may further lead to their diverse growth rates. As the pH values increase, the concentration of Mn atom decreases, meanwhile the concentration of Se atom increases. At the surface of nuclei of MnSe where Se atoms are sufficient, the main form is Se²⁻ instead of Se₂²⁻, and the difference between Mn-zz terminations and Se-zz terminations eventually disappear, which leads to the random growth rates and the cubic particles as the other works reported.^[6–14,41] The effect of the concentration of Mn atoms on the phase transition attests the assumption, too.

The optical properties of as-made MnSe particles strongly depend on the particle sizes.^[42] The precipitated as-made MnSe_x particles exhibit a different color from black to brown with pH value increase as illustrated in Fig. 5(a). From the reflection spectra (See Fig. 5(b)), it is clear that the wavelength of reflection peak between 495 nm and 590 nm gradually decreases with the increase of pH values. On the basis of the complementary color principle, the wavelength between 495 nm and 590 nm corresponds to yellow and green regions, which just matches the observed color alteration as seen in Fig. 5(a). The yellow–green color is believed to originate from MnSe aggregates.^[23,38] For samples synthesized with low pH values (pH < 7), the spectra intensity continuously increase with the wavelength above 500 nm.

Fig. 5.

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	Fig. 5. (a) Optical photograph and (b) the reflection spectra of as-grown MnSe2/MnSe particles synthesized with different pH values at 180 °C for 12 h.

It is well known that the bulk MnSe₂ and MnSe show antiferromagnetic ordering.^[43] However, the magnetic behavior of particles may be different from that of bulk materials.^[44,45] Figure 6 shows the room temperature and low temperature hysteresis loops of as-made MnSe_x particles. It is clear that the room temperature hysteresis loops exhibit some weak ferromagnetic behavior with low coercivity, remanence, and saturation magnetic moment. With temperature decreasing, both coercivity and saturation magnetic moment increase. However, the remanence remains at a low value even at 5 K. The ferromagnetic mechanism is not fully understood and needs to be investigated in detail.

Fig. 6.

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	Fig. 6. The magnetic hysteresis loops of as-made particles synthesized at 180 °C for 12 h measured at (a) 300 K and (b) 5 K. (Insets: zoom in of hysteresis loops around coercivity).

In summary, we have demonstrated a controlled synthesis method, which could produce a series of MnSe and MnSe₂nano/micro crystals conveniently on a large scale. The pH value has a strong influence on the morphology, size, and structure of synthesized particles. The phase transition mechanism of MnSe₂ to MnSe is discussed. The concentration of OH⁻ will affect the development of nuclei. Moreover, MnSe₂ and MnSe particles exhibit different optical properties. The reflection spectra of MnSe clearly show a peak with wavelength around 500 nm–600 nm. The magnetic measurement reveals that all samples exhibit a ferromagnetic behavior with low coercivity and remanence.