Introduction

Metal chalcogenides have been the hot topics in nanomaterials since decades ago, and these materials have been studied or applied in many fields such as catalysts (Seh et al. 2017), electronics and devices (Wang et al. 2012), bio-systems (Sapsford et al. 2013), and sensors (Comini et al. 2009). To function these metal chalcogenides, the first step is always about the synthesis. However, a specific method is often limited to the synthesis of one specific material, which raises the cost of production. Thus, even after many years of researches and explorations, experts are still looking forward to put up one similar, simple, and sixpenny method to synthesize a class of nanomaterials rather than just one (Zhou et al. 2018; Lin et al. 2017; He et al. 2018). One-pot synthesis is totally in accordance with such requirements (Tong et al. 2017), just like a bridge joining the start and the end, and nothing more. Among all the methods aimed at one-pot synthesis, hydrothermal method, in which the reaction is set in aqueous media at certain temperature, is one of the earliest-developed, simplest, and the most suitable way to synthesize nanomaterials in industry (Rabenau 1985; Feng and Xu 2001; Darr et al. 2017). Yet studied for a long time, hydrothermal method still lacks the universalities to synthesize a class of nanomaterials, especially metal chalcogenides that have been commonly researched during these years. One of the most important factors is to adopt one suitable and universal reducing reagent.

To face the problems referred to above, here we present the nearly general one-pot synthesis of metal (mainly referred to as the transition ones) selenides, sulfides and oxides through a surfactant-free hydrothermal method with oxalic acid (OA) as the reducing reagent at 220 °C for 24 h. One of the advantages of the one-pot synthesis is that we can easily prepare composites consisting of the components that can be synthesized solely under the same conditions. Thus, this work also attempts to prepare nanostructured composites by simply adding the required sources into one pot. 23 compounds have been synthesized through the same strategy, which is highly adoptable for further researches on the properties and applications of metal chalcogenides synthesized through this method, together with the synthesis of other nanostructured materials. What is more, shape-/composition-controlled synthesis of vanadium oxides, CdSe and nickel sulfides have also been realized by simply controlling the amount of OA, which is quite inspiring for the studies on the particle growth and the surfactant-free shape engineering of other nanomaterials.

Experimental section

Synthesis of all the nanomaterials

All of the nanomaterials referred to in this thesis were synthesized through nearly the same hydrothermal method. Typically, an adequate amount of oxalic acid (C2H2O4·2H2O, AR), stoichiometric metal precursors and chalcogen precursors (SeO2, AR; sublimed sulfur, AR) were dissolved in deionized water separately and then added to deionized water. Then the as-prepared solution was sealed in a Teflon-lined autoclave, kept at 220 °C for 24 h, after which the autoclave was cooled down in air. Finally, the precipitate was washed and centrifuged in ethanol and then deionized water several times, followed by drying in air at 60 °C for 6 h. Note that the amount of oxalic acid used in each synthesis is referred as times toward the stoichiometric amount required for the reduction of SeO2, S, or metal precursors to the desired output (of which the metal is reduced to be in lower valence).

Characterizations

High-resolution transmission electron microscopy (HRTEM) images were collected by using a JEOL JEM-2100F high-resolution transmission electron microscope operating at 300 kV, and the samples for HRTEM investigation were prepared by depositing a drop of a dilute dispersion in anhydrous ethanol onto a copper grid and leaving the solvent to evaporate at room temperature. X-ray diffraction (XRD) patterns were recorded by a Shimadzu XRD-6100 X-ray diffractometer with Cu Kα irradiation (λ = 0.15406 nm) operated at 40 kV and 30 mA in the 2θ scan range of 10°–80° (5°–80° for vanadium oxides) at a scan rate of 5° min−1. Scanning electron microscopy (SEM) images were obtained on a JSM-6610 scanning electron microscope operated at 20 kV. Field emission scanning electron microscopy (FESEM) images and energy-dispersive X-ray spectroscopy (EDS) maps were operated on a JSM-7800F field emission scanning electron microscope equipped with an energy-dispersive X-ray spectrometer operated at 10 kV.

Results and discussion

Nanomaterials of individual component

To the compounds of the sole component, lots of nanomaterials of specific shapes (shown in Fig. 1, S1-3) have been synthesized, such as 0D nanoparticles (such as MoSe2 and MoO2), 1D nanowires (such as VO2), 2D nanosheets (such as WSe2 and Cu2Se), and 3D hierarchical structures (such as CdS). Obviously more metal selenides than sulfides can be synthesized through this method, which probably means that OA is not so powerful to reduce sulfur to S2−. As this work emphasizes the role of OA as the reducing reagent, only sublime sulfur, rather than Na2S that is commonly used to prepare sulfides, is used to provide S atoms in all the synthesis. Well, even fewer oxides have been synthesized here, because it is well known that metal oxides can be easily synthesized by just heating the metal sources (such as metal halides and acetates) in water or air without the addition of any other reagents. Thus, the synthesis of metal oxides can hardly display the role of OA as the reducing reagent. Here, only metal sources in higher valence are adopted to prepare oxides. After many attempts to synthesize the desired metal chalcogenides, it is commonly found that the amount of OA has an optimal value of about three times as large as the stoichiometric one (as is put forward in Table S1). Less amount of OA may lead to the generation of other metal chalcogenides especially oxides, while a denser concentration of OA would also cause the existence of metal acetates. For those that can hardly form metal acetates, like Mo, the higher the concentration of OA, the purer would the final product be. On the other hand, the commonly optimal amount of OA is not adequate for the production of MoO2 nanoparticles from MoO3.

Fig. 1
figure 1

FESEM images of as-synthesized nanomaterials: (1) CdSe (scale bar: 2.5 µm), (2) NiSe (1 µm), (3) SnSe2 (200 nm), (4) ZnSe (1 µm), (5) Cu2Se (1 µm), (6) WSe2 (200 nm), (7) PbSe (1 µm), (8) Bi2Se3 (2 µm), (9) MoSe2 (100 nm), (10) CdS (5 µm), (11) NiS (10 µm), (12) ZnS (2 µm), (13) Cu2S (5 µm), (14) PbS (1 µm), (15) Bi2S3 (5 µm), (16) V3O7·H2O (1 µm), (17) MoO2 (200 nm), (18) MnO2 (300 nm)

Full size image

Nanostructured composites

Besides nanomaterials of sole components, composites, consisting of only two components and prepared using the same hydrothermal method, have also been synthesized, demonstrating the nearly universal applicability. For example, this work tried to synthesize three-composite compounds: PbS–ZnS (Fig. 2), Cu2S–ZnS (Fig. S5) and CdSe–PbSe (Fig. S6). With SEM, EDS, XRD and XPS characterizations, basically one-composite compound is found to be formed from the combustion of two components, where each one takes one specific shape similar to that of the material solely synthesized.

Fig. 2
figure 2

Basic characterizations of PbS–ZnS composites prepared through the one-pot hydrothermal process. a XRD pattern, in which the top pattern is based on the standard JCPDS card No. 89-2181 (ZnS), while the bottom is based on No. 05-0592 (PbS). b EDS mappings of as-prepared PbS–ZnS composite. Scale bar: 2 µm. c XPS spectra of the as-prepared sample

Full size image

Here, we mainly focus on PbS–ZnS composites, while the analysis of the other materials is given in the Supplementary Information file. The XRD pattern in Fig. 2a clearly verifies the existence of PbS and ZnS. Combined with the EDS characterizations displayed in Fig. 2b, PbS takes the shape of microcube and ZnS the microsphere. Also, the Pb atoms seem to diffuse in the ZnS crystals or at least on the surface. The XPS spectra in Fig. 2c further demonstrate the coordination of the atoms. The binding energy at 1045.0 eV corresponds to Zn 2p1/2, and the one at 1021.8 eV to Zn 2p3/2, which means that Zn in this material is in Zn2+ state (Li et al. 2010). Each of the Pb 4f peak was resolved into two bands, of which the 4f7/2 peak contains a main peak at at 137.5 eV and a shoulder peak at 138.8 eV, while the 4f5/2 peaks at 142.4 eV and 143.7 eV, respectively (Jang et al. 2010). The core-level S 2p XPS of PbS–ZnS was deconvoluted into four peaks, and the strong peaks at 160.7 eV and 161.7 eV correspond to S 2p1/2 and 2p3/2 of S2− with a satellite peak at 168.3 eV (Zhou et al. 2017). In addition, another strong peak at 162.6 eV can be assigned to a characteristic bimetallic metal–S bond (Sivanantham et al. 2016), which is in accordance with XRD data of the formation of two separate metal sulfides.

Shape-/composition-controlled synthesis

On vanadium oxides

Shape-controlled synthesis of nanomaterials is always one of the hot topics in the nanosize field, as the shape usually has great influence on the applications and even intrinsic properties of a specific material. OA, the most common dicarboxylic acid in air, has the smallest size among all the dicarboxylic acids. Its size, structure and saturation vapor pressure place it in the category of intermediate volatility species, making itself a good candidate for particle formation and growth studies (Bilde et al. 2015). The detailed synthesis is displayed in Table S2. As obviously depicted in Fig. 3 and S7, when the amount of OA is just one-fifth of that adequate to exactly reduce V5+ to V4+ (labeled as ×1/5, the same as followed), the product is VO2 nanoribbons (taking the B phase, JCPDS No. 81-2392). However, when the amount is slightly increased to ×1/3, great changes occur and the product becomes V3O7·H2O ultralong nanowires (JCPDS No. 85-2401). The VO × 1/3 sample is highly crystalline, but as the amount increases to ×3/5 the peaks in the XRD patterns become weak while keeping the same composition and morphology. Unexpectedly, the product turns back to be VO2 with the amount added up to ×1, and the corresponding shape also changes to microrods with a much shorter length. On continuously increasing the amount of OA, the composition of the output remains the same while the shape changes gradually from microrod to nanobelt (with a shorter length) and finally nanosheets. The distance between the crystal planes in the magnified HRTEM image of Fig. 3b (of which the data come from the sample VO × 7/5) is measured to be around 0.19 nm, which corresponds to the (600) crystal plane. This result demonstrates that the growth direction of as-prepared VO2 nanobelts prefers the [100] crystal direction (Strelcov et al. 2009; Guiton et al. 2005; Sohn et al. 2007). Combining the XRD patterns and FESEM images, the probable growth mechanism can be obtained. The minimal amount of OA, ×1/5, could probably be exactly adequate to reduce V2O5 to VO2 (Chirayil et al. 1998). While increasing the amount, the addition of CO2 (generated from OA) and coordinated H2O may enhance the trend that drives V to be in a more positive state, leading to the generation of V3O7·H2O (Chirayil et al. 1998). Notably, as the amount reaches ×3/5, the peaks in the XRD pattern become quite weak, which means the existence of V3O7·H2O is probably unstable. Thus when the amount is much larger, together with the decrease of the pH, this reaction system gets more ‘power’ to drive V atoms to a more negative state, leading to the generation of VO2 again. The OA molecules are proposed to prefer the attachment with the crystal planes parallel to the [100] direction, which is positive for the direct reduction of V5+ to V4+. Thus, a larger concentration of OA promotes growth perpendicular to the [100] direction and leads the final shape from the 1D nanobelt to 2D nanosheet (Mclaren et al. 2009).

Fig. 3
figure 3

a XRD patterns of as-prepared vanadium oxides with the amount of OA being ×1/5, ×1/3, ×3/5, ×1, ×7/5, and ×2 from the top to the bottom, despite that the two patterns are based on the standard JCPDS PDF cards on the tips (the top one corresponds to V3O7·H2O, while the bottom to VO2). b HRTEM images, SAED pattern and EDS mappings of VO × 7/5. Scale bars in the HRTEM image, the magnified one, SAED pattern, and EDS mappings: 200 nm, 2 nm, 5 nm−1, and 3 µm, respectively. c Enlarged FESEM images of vanadium oxides synthesized with different amounts of OA. Scale bar: 200 nm, 200 nm, 200 nm, 2 µm, 400 nm, and 400 nm, labeled from the left to the right

Full size image

On nickel sulfides

Similar phenomena are also found in the synthesis of nickel sulfides. As shown in Fig. 4a, b and S8a, the product contains pure NiS2 microcubes with smooth surface when the amount is ×1, while becoming composites consisting of NiS2 and NiS microcubes, but with cubic holes on the surface when the amount increased to ×2. Then, the output remains pure NiS after reaction with a larger amount of OA, which means that NiS2 is probably metastable (Roffey et al. 2016). As the amount increased to ×3 and ×4, the shapes undergo great changes and become ginger sugar-like structures and microcubes consist of elongated cuboids. Combining the works reported before and the specific phenomena, we propose that the growth of nickel sulfides synthesized here results from the oriented aggregation process (Yuwono et al. 2010). In the previous step, nickel sulfide nanocubes burn out in the synthesis of all the four samples. While the amount is ×1, the final product is NiS2 in which Ni has higher valence. Similar to vanadium oxides, when the amount of OA increases, the metal of the product changes to a lower valence. In the following steps, the nanocubes experience the oriented aggregation process. For the NS × 1 sample, the uniform nanocubes aggregate to form microcubes (Yang and Gao 2006). However, as the NS × 2 particles are a mixture of NiS2 and NiS, the oriented aggregation goes not so smooth as the coordination of atoms of NiS2 and NiS doesn’t quite fit, leading to the formation of microcubes with cubic holes. As the amount of OA reaches ×3, the growth mechanism goes in another way, such that the OA molecules probably partly play the role of surfactant with which the as-generated nanocubes prefer to orient, aggregated first as nanorods (Pan et al. 2014), followed by the oriented aggregation of these nanorods to form ginger sugar-like structures (Yuwono et al. 2010; Kim et al. 2001). But when the amount reaches ×4, the aggregation of as-formed nanorods becomes random, probably because the OA molecules play two roles in this step: on one hand, they promote the formation of nanorods aggregated from nanocubes and the microcubes form finally; on the other hand, the over-dosed OA molecules exaggerate the randomness of the reaction system, thus leading to the random aggregation of nanorods.

Fig. 4
figure 4

a XRD patterns of nickel sulfides ×1, ×2, ×3, and ×4 from the top to the bottom despite the two patterns being based on the standard JCPDS PDF cards on the tips (the top one corresponds to NiS2 No. 89-3058, while the bottom corresponds to NiS No. 12–0041). b SEM images of nickel sulfides prepared with different amounts of OA. Scale bar: 10 µm for all the four images. c EDS mappings of CdSe nanoparticles, of which the length of each particle displayed here is about 15.6 µm, 5.8 µm, and 10.3 µm, labeled, respectively, from the top to the bottom

Full size image

On cadmium selenide

The magic power of OA is also effective for CdSe, though without phase evolution. As shown in Fig. 4c and S8b, the shape of as-prepared CdSe nanoparticles changes from pagoda-like structure to dendrite structure and banana leaf-like structure through the hydrothermal process with an amount of OA ×2, ×3, and ×4, respectively. We propose that the growth of CdSe × 2 pagoda-like structures takes a secondary nucleation and growth, though in one pot (Zhang et al. 2006; Sounart et al. 2007). First of all, the hexagonal nanoplates generate and aggregate together to form the ‘dais’ (Gerdes et al. 2017). Then, the ‘first floor’ is built by the co-axial growth of several nanorods, and the upper ‘floors’ are built in sequence in the same way but with fewer and fewer nanorods (Rice et al. 2013). Finally, the number of co-axial nanorods is so small that they cannot allow the subsequent growth above them, leading to the generation of the sharp top. In addition, such process would never occur without the linkage of OA molecules between specific crystal planes of CdSe. For the CdSe dendrites (×2), the self-assembly growth prefers the direction along c ([002] crystal direction) with step-by-step nucleation and intergrowth on the already formed nanorods (Chen and Gao 2005; Kim et al. 2012). When the amount of OA increased to ×4, the branches of as-prepared nanocrystals become wide and thin, just like banana leaves, which is probably because the addition of OA molecules on certain crystal planes of CdSe prevents further growth perpendicular to these planes, thus promoting the growth along other directions.

Conclusions

In summary, this work develops a nearly general one-pot, surfactant-free hydrothermal method with oxalic acid as the reducing reagent to synthesize metal selenides, sulfides and oxides of sole components or composites. By simply changing the amount of oxalic acid, shape-/composition-controlled syntheses can be achieved toward vanadium oxides, nickel sulfides and CdSe. The method developed here is inspiring for the synthesis of other nanomaterials and the shape-/composition-controlled synthesis provides a simple strategy for the shape engineering of nanomaterials and enriches the knowledge about crystal growth. With or without modifications, the nanomaterials prepared here can be applied in many fields such as catalysts, supercapacitors, electronics, and biosystems, depending on the intrinsic nature of each material.