Synthesis of HfSe₂ and CuHfSe₂ crystalline materials using the chemical vapor transport method and their applications in supercapacitor energy storage devices

1 Introduction

With increasing concerns over the use of fossil fuels for energy generation and their environmental hazards, the scientific community is now emphasizing the utilization of alternative green energy sources and storage devices. Electrochemical capacitors, also known as supercapacitors, provide an alternative energy solution that can be used in various applications, such as renewable energy generation, electrical transportation, uninterruptible power supplies, and electrical machinery [1]. This is due to their outstanding advantages, including high specific capacitance, high power density, fast charging capability, long cycling life, large-scale production, low cost, and a wide operating voltage range [2,3,4,5,6].

So far, several materials have been tested as electrode materials for supercapacitor applications, including transition metal oxides, graphene, carbon nanotubes, MXene, and composites based on a combination of multiple materials [7,8,9,10]. There are also reports on metal–organic frameworks (MoFs) and even various waste material-based efficient energy storage devices [11,12,13]. In addition, layered transition metal dichalcogenide (TMDC) materials have emerged as an auspicious material class for electrochemical energy storage devices [14,15,16]. As electrode materials, TMDCs have demonstrated promising features in various aspects, such as excellent stability when used in a single-crystal form [6], increased capacitance with phase transition [17], compatibility for on-chip integration [18], and ability to form heterostructures with other layered materials [19]. In order to further enhance the energy storage capability of layered TMDC materials, different strategies have been employed, including phase or compositional modifications [20,21], increasing active surface areas [22], and doping [23].

For example, encompassing graphene and its composites with graphene oxide and graphite oxide using the liquid-phase exfoliation technique was investigated for supercapacitor energy storage devices [24]. The authors employed a simple sonication technique for different time durations to prepare graphene, GO, and GrO composites. They found that the sample prepared with a 48-h duration showed the best results with a capacitance of 534 F/g and 94% retention due to its high conductivity and more oxygen incorporation, which led to a more dominating redox reaction phenomenon. In a recent report by Tan and coworkers [25], a reduced graphene oxide (rGO)-supported copper/manganese-based MOF composite was fabricated by adopting a low-cost technique. They tested three electrode materials, namely Cu/MnO _x -rGO, Cu/MnS _x -rGO, and Cu/MnC _x -rGO, prepared by heating at 300°C for 3 h in air, with equal-weighted sulfur under argon and at 600°C in argon, respectively. They reported a specific capacitance of >700 F/g with 92% retention. Furthermore, Gokulsaswath et al. [26] synthesized zirconium disulfide (ZrS₂) quantum dots using the chemical bath deposition (CBD) method and characterized them using XRD, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) techniques for compositional and morphological analysis. Being a zero-dimensional material, ZrS₂ possesses a large surface area, which is an important parameter for electrochemical supercapacitors. The authors tested their energy storage properties using cyclic voltammetry, charge–discharge curves, and impedance spectroscopy in a standard three-electrode system. The synthesized ZrS₂ particles showed a specific capacitance of 101 F/g, determined using charge–discharge curves. Notably, in the aforementioned studies, the addition of carbon additives such as activated carbon and carbon black, typically comprising 10% (by mass) of the active electrode material, has become prevalent. To a certain extent, this practice obscures the true intrinsic behaviour of the materials under investigation. The addition of carbon-based additives increases conductivity, reduces voltage drop, and ultimately enhances performance significantly compared to the original value.

Among several layered TMDC materials, although hafnium diselenide (HfSe₂) has been explored for several applications, it has still not been considered for energy storage applications [27,28]. In this work, we have synthesized hafnium diselenide single crystals using the chemical vapor transport (CVT) method. The grown crystals were employed as an electrochemical supercapacitor electrode material, and clear insights into their intrinsic behavior were reported without using any carbon additives. HfSe₂ single crystals were further doped with copper, and comparative analysis was performed between Cu-doped (CuHfSe₂) and -undoped HfSe₂. The grown single crystals exhibited enhanced and stable performance with CuHfSe₂ under a three-electrode measurement setup. The Cu-doped electrode expedited electronic conductivity, which in turn improved the specific capacitance and stability, demonstrating potential for this newly tested 2D TMDC material in energy storage devices.

2 Experimental

2.1 Materials synthesis and electrode preparation

The reagents utilized in this study were purchased from Sinopharm Chemical Reagent and Sigma-Aldrich and were utilized without further purification steps. For CVT growth, stoichiometric amounts of hafnium–selenide and copper–hafnium–selenide powders were mixed with 20 mg iodine as a transport agent. In the next step, powder materials were put into the quartz ampoules with an inner diameter and length of 5 mm and 25 cm, respectively. After this, ampoules were evacuated to a vacuum of 10⁻³ Torr and tightly sealed. Finally, the sealed tubes were placed in a double temperature zone split-type furnace and heated at 850 and 950°C for a duration of 5 days. After 5 days, the furnace was cooled to room temperature, and the ampoule tubes containing HfSe₂ and CuHfSe₂ crystals (ranging in size from several microns to millimeters) were obtained. It is worth mentioning that erstwhile quartz ampoules were thoroughly cleaned with diluted HNO₃ and in succession washed with DI water and dried in an oven at 60°C. For the electrode preparation, bulk millimeter-sized single crystals of HfSe₂ and CuHfSe₂ were cleaned with IPA and then thoroughly ground to obtain a very fine powder. Then, 2 mg of this powder was added to a DI water, IPA, and Nafion combined solution in volumes of 750, 250, and 10 µl, respectively, and sonicated for several hours. Then, 5 µl of the prepared solution was placed on a working electrode (3 mm diameter) and used for measurements after drying in an oven at room temperature (Figure 1).

Figure 1

Schematic illustration of CVT growth setup using a double-zone furnace and the electrode preparation process.

SEM images were taken using a ZEISS Gemini Field Emission SEM, and the same was used to obtain elemental EDS spectra of the samples. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi X-ray photoelectron spectrometer. The crystal structure of the grown materials was analyzed with X’Pert Pro Super (Philips Co., Netherlands). For the SEM, EDS, and XPS measurements, fine powder-like samples were prepared by grinding the flakes, while for XRD characterization, one large-sized flake was selected and used after proper washing. All electrochemical measurements were conducted on a nickel foam utilizing finely ground powder of our sample materials. These measurements were performed using a Corrtest Instrument Potentiostat.

3 Results and discussion

X-ray diffraction patterns of HfSe₂ and CuHfSe₂ single crystals showed major peaks along the (001), (003), (004), and (005) planes, as shown in Figure 2(a). The XRD patterns obtained showed a prominent crystalline nature of the grown HfSe₂ single crystals, which retained the 2-H phase and matched well with the standard JCPDS card no. 37-1488. XRD spectra of CuHfSe₂ further revealed that the peak positions shifted to lower degrees compared to HfSe₂, which confirmed successful doping of Cu atoms into HfSe₂. These results are in accordance with the previously reported works of TMDC doping with transition metals [29,30]. The slight shift in peaks occurred due to a mismatch in the ionic crystal radii of copper and hafnium (Cu⁺¹ ∼ 91 pm, Hf⁺⁴ ∼ 85 pm) [31]. A new peak at around 25° also appeared in the Cu-doped HfSe₂ XRD spectrum. The appearance of this peak may be attributed to a minor presence of Cu₂Se in the sample; however, it did not alter or disrupt the original structure of the grown material, as evidenced by the presence of other sharp and intense diffraction peaks corresponding to CuHfSe₂.

Figure 2

(a) XRD patterns of HfSe₂ and CuHfSe₂ single crystals, (b) EDS of CuHfSe₂, and (c) and (d) SEM images of HfSe₂ and CuHfSe₂ single crystals.

The EDS was performed to study the elemental peak analysis of the CuHfSe₂ material, as presented in Figure 2(b). The presence of Cu peaks clearly manifests the existence of an adequate proportion of Cu in the synthesized material.

Scanning electron microscopy images of the HfSe₂ and CuHfSe₂ crystals are shown in Figure 2(c) and (d), respectively. The figures revealed that most of the grown crystals are in hexagonal shape, with sizes in several microns.

Figure 3 shows the chemical composition of CuHfSe₂ single crystals performed using XPS. Figure 3(a) depicts the Hf 4f spectra with two main binding energy peaks of 4f_7/2 and 4f_5/2 appearing around 16.1 and 18.2 eV, respectively. These two peaks correspond to HfSe_2, while a small peak observed at 19.2 eV confirmed the presence of HfO _x . Se 3d scan, shown in Figure 3(b), is composed of two foremost peaks, which are Se 3d_3/2 (∼54.47 eV) and Se 3d_5/2 (∼53.58 eV), attributed to Hf–Se bonding along with a small peak of the Se–Se bonding state in the grown sample (∼55.8 eV). Figure 3(c) illustrates the Cu 2p spectra with binding energy peaks for Cu 2p_3/2 and Cu 2p_1/2 appearing at around 932.5 and 952.4 eV, respectively. The occurrence of these characteristic peaks matches well with the previously reported work [28,32]. HfSe₂ is known to be more prone to oxidation compared to its other counterpart TMDCs [33]. The manifestation of the HfO _x phase in the XPS spectra may be due to the exposure of samples to air during characterization, but not during the growth (since the sample tubes were vacuumed and sealed for crystal growth).

Figure 3

XPS spectra for the (a) Hf 4f, (b) Se 3d, and (c) Cu 2p core levels.

To unveil the electrochemical performance of the prepared electrode materials, cyclic voltammetry (CV) was performed in the potential range from 0 to 0.5 V with scan rates ranging from 10 to 200 mV s⁻¹, as shown in Figure 4(a) and (b). The galvanostatic charge–discharge profiles of HfSe₂ and CuHfSe₂ are shown in Figure 4(c) and (d), respectively. The capacitance values of the prepared electrodes were calculated using CV profiles. First, the area under the CV curve was calculated by integrating the curve, whereas equation (1) was used to determine the capacitance values [8]:

where m is the mass of the electrodes, v is the scan rate, V _a − V _b is the potential window, and ∫I(V) dV is the integrating CV area. The highest calculated capacitance values were 44.7 and 151.83 F/g for HfSe₂ and CuHfSe₂ electrodes, respectively, at a scan rate of 10 mV s⁻¹. It can be observed that all CV curves show a semi-rectangular shape at all scanning rates, exemplifying the presence of electrochemical double-layer capacitor (EDLC) behavior. CuHfSe₂ electrodes exhibited significantly higher capacitance than undoped HfSe₂, primarily due to improved electrical conductivity. It is well known that conductivity, surface area, porosity, functional group attachment, and interlayer spacing are the main contributing factors for efficiency enhancement. The electrode materials prepared were tested for stability with 5,000 charge–discharge cycles. Figure 5(a) depicts the stability trend of the CuHfSe₂ electrode with more capacitance retention compared to the HfSe₂ electrode, as shown in Figure 5(b). The initial capacitance loss observed during the early charge–discharge cycles can be attributed to the stabilization period required for the system to reach electrochemical equilibrium. Similar behavior of initial capacitance loss has also been observed in previous reports [34,35]. After preliminary cycles, the CuHfSe₂ electrode showed very promising stability features with no further loss in the capacitance, while the HfSe₂ electrode experienced ultimate 50% capacitance loss at the end of 5,000 cycles. It is well established that the stability of any diatomic transition metal sulfide/selenide is related to the ideal bond strength or the bond dissociation energies of its constituent elements [36,37]. It has been reported that moduli (based on the force of stretching and strain between the bonds of the transition metals and chalcogen elements) of TMDCs increase on moving from lower-number groups to the higher-number groups [38]. A similar trend was observed in our synthesized TMDC materials, i.e., compared to the undoped HfSe₂, after doping with Cu, the formation of its bond with the host selenium elements by replacing the Hf atoms, the overall bond strength of CuHfSe₂ is increased, which is eventually reflected in the stability test. Moreover, for the broader interest of readers, a comparison between the present work and some previously published articles is presented in Table 1.

Figure 4

Cyclic voltammograms of (a) HfSe₂ and (b) CuHfSe₂. Charge–discharge profiles of (c) HfSe₂ and (d) CuHfSe₂ electrodes.

Figure 5

Capacitance retention for the (a) CuHfSe₂ and (b) HfSe₂ electrodes. (c) Electrochemical impedance spectroscopy and (d) specific capacitance versus scan rate curve.

After charging–discharging stability measurements, the morphological stability of the CuHfSe₂ electrode material was further confirmed by SEM, as shown in Figure S1. It is clear from the SEM image that there are no significant changes in the morphological structure of our grown material, which shows its stable behavior; however, due to the repeated charging and discharging cycles, a very small amount of nanosized lump formation can be seen on the material’s surface. This formation may be due to the agglomeration of sulfate ions or minor sulfonation on the material surface. Moreover, XRD diffraction was performed to examine any structural changes in the tested electrode material, as shown in Figure S2. It was revealed that most of the peaks in the CuHfSe₂ spectra reappeared after electrochemical measurements with no significant changes, thus confirming the stability of the electrode material. The slight peak broadening is attributed to the ion agglomeration at the material surface, which is also evident from the SEM image. To further ascertain the characteristics of the electrode materials synthesized, electrochemical impedance spectroscopy (EIS) was conducted in the applied frequency range of 0.01–1 MHz, as shown in the Nyquist plot in Figure 5(c). It is well known that the higher curve in the lower frequency range of the EIS spectra (Warburg impedance) reflects better supercapacitive behavior [39,40,41]. In this regard, the CuHfSe₂ electrode illustrated better conductivity due to the addition of Cu, as compared to the undoped HfSe₂ electrode, which demonstrates a lower Warburg impedance. The effect of Cu doping was further investigated by analyzing the interface (R _s) and charge transfer resistance (R _ct) values using the measured EIS spectra. From the experimental EIS data and its simulation fitting, it was observed that the Cu-doped HfSe₂ electrode showed better conductivity with interfacial and charge transfer resistance values of 0.72 and 3.97 Ω, compared to the undoped HfSe₂ electrode, whose values were 0.87 and 5.6 Ω, respectively. EIS simulation and best curve fitting circuit models were also constructed, as shown in the EIS inset images. Figure 5(d) shows the capacitance versus scan rate trend, which was obtained from the measured cyclic voltammograms of the electrodes. The graphs of both electrode materials show a decrease in the capacitance trend with increasing scan rates. The observed trend is obvious because the increasing scan rate charges are swept at faster rates, which provides fewer chances for them to interact with the active electrode material, resulting in a decrease in the electrode capacitance [42].

The ideal supercapacitor device oscillates between a capacitor in the low-frequency region and a resistor in the high-frequency region [43]. Therefore, in order to examine the capacitive behavior of the prepared electrodes, Bode plots, showing phase angle–frequency trends, are shown in Figure 6(a). It can be observed that in the low-frequency region, the Cu-doped HfSe₂ electrode shows a higher phase angle compared to HfSe_2, with the calculated magnitudes of around 54° and 51°, respectively, at 0.1 Hz. To further understand the intrinsic behavior of our electrode materials, the range of frequencies versus capacitance is plotted in Figure 6(b). The capacitance was calculated by using the following equation [6]:

where f is the frequency and Z″ is the imaginary impedance. It is clear from the figure that the Cu-doped electrode showed a higher magnitude of capacitance for a wide range of frequencies, contrary to the undoped electrode. These observations are in accordance with the above-mentioned CV and charge–discharge results.

Figure 6

(a) Phase angle versus frequency and (b) capacitance versus frequency plots.

The Ragone plot showing energy and power densities comparison for HfSe₂ and CuHfSe₂ electrodes is shown in Figure 7. The energy and power density values were calculated from the charge–discharge curves at current densities of 0.1–1 A/g using the following equations [44]:

and

where C and V are the specific capacitances measured from the charge/discharge curves and potential window, respectively, and ∆t is the discharge time of one cycle. It was found that at a current density of 0.1 A/g, the CuHfSe₂ electrode had a highest energy density value of 5.16 W h/kg with a power density of 25 W/kg; on the other hand, the HfSe₂ electrode exhibits energy and power density values of 0.340 W h/kg and 30 W/kg, respectively. Both electrodes exhibit a maximum power density value of 250 W/kg on reaching the current density value of 1 A/g, while the energy densities were 0.782 and 0.034 W h/kg for CuHfSe₂ and HfSe₂ electrodes, respectively.

Figure 7

Power and energy densities comparison for the HfSe₂ and CuHfSe₂ electrodes.

Among semiconductor materials, those with higher conductivity can provide a greater number of charge carriers to participate in electrochemical reactions, making them more valuable as electrode materials for electrochemical supercapacitor applications. The enhanced performance of our CuHfSe₂ electrode material is related to its more conductive nature due to the addition of Cu metal, resulting in a reduction in impedance according to electrochemical impedance spectra. To further confirm this point, the role and contribution of Cu orbitals to the material’s conduction and valence band electronic states were analyzed by calculating the band structure and PDOS, as shown in Figure 8. It can be seen in Figure 8(a) that HfSe₂ has a clear energy gap, which is a typical hallmark for semiconductor materials. The introduction of Cu atoms likely introduces additional states within the band gap, leading to a narrowing of the gap. As a result, the density of states of the valence band shifts and its upper edge significantly crosses the Fermi level. Figure 8(b) clearly reveals that after Cu doping, the band gap narrows and the valence band crosses the Fermi level. This implies that Cu-doped HfSe₂ behaves more like a semi-metal, where the Fermi level intersects the valence band, resulting in metallic conduction at the Fermi level, rather than semiconducting behavior, as typically observed in HfSe₂. The addition of Cu also increases the charge carrier density, and it also introduces additional energy levels within the band gap, effectively reducing the energy required for electrons to move from the valence band to the conduction band. These energy levels can hybridize with the existing ones, leading to a reduction in the band gap and a shift in the Fermi level. The hybridization of p-orbitals according to PDOS distributions in the valence bands is found to be a key contributor close to the Fermi level. Therefore, due to the crossing of the valence band and strong hybridization of p-orbitals near the Fermi level, it transformed toward a semimetal. The process of charge transfer became very swift in CuHfSe₂, which resulted in better electrochemical performance. Ultimately, the doping of Cu into HfSe₂ modifies its electronic structure. From these findings, it can be stated that the energy gap and charge carrier concentration close to the Fermi level play a vital role in determining the electronic conductivity of the grown material. Our density functional theory (DFT) calculation results agree with the experimental findings of electrochemical impedance spectroscopy and clearly verify that the enhanced energy storage response of the CuHfSe₂ material was due to its more conductive nature.

Figure 8

DFT-based PDOS calculations for the (a) HfSe₂ and (b) CuHfSe₂ materials.

4 Conclusion

In this work, we report for the first time CVT-synthesized crystalline hafnium diselenide (HfSe₂) and its use for supercapacitor energy storage devices. Its potential was further explored by doping it with Cu metal (CuHfSe₂), which resulted in its efficiency enhancement by threefold after doping. A specific capacitance value of 44.7 F/g was estimated for the HfSe₂ electrode, and a significant increase of 151.83 F/g was observed at a scan rate of 10 mV/s after Cu doping. This enhanced capacitance value for the doped sample is due to increased layer spacing and conductivity due to Cu doping. Due to these factors, the CuHfSe₂ electrode surpassed the HfSe₂ electrode in terms of capacitance and stability features, as confirmed by the experimental EIS analysis and DFT calculations. It is worth mentioning that the obtained capacitance values and other electrochemical analyses presented in the study reflect the pure intrinsic features of the materials, as no carbon additives were used for the electrode preparation, contrary to a common practice. The obtained values can be further enhanced manifold after adding carbon additives. Moreover, the grown CuHfSe₂ nanocrystals can be considered as a promising supercapacitor electrode material with good capacitance, stability, and energy density due to the unique microstructure of such 2D layered materials.