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Article

Enhanced Alkaline Hydrogen Evolution on Gd1.0/Ndx (x = 0.5, 1.0, 3.0, and 6.0%)-Doped TiO2 Bimetallic Electrocatalysts

by
Mohammed Alsawat
1,*,
Naif Ahmed Alshehri
2,
Abdallah A. Shaltout
3,
Sameh I. Ahmed
4,
Hanan M. O. Al-Malki
1,
Manash R. Das
5,6,
Rabah Boukherroub
7,
Mohammed A. Amin
1,* and
Mohamed M. Ibrahim
1,*
1
Department of Chemistry, College of Science, Taif University, Taif 21944, Saudi Arabia
2
Physics Department, College of Science, Al-Baha University, Aqiq, Al-Baha 65431, Saudi Arabia
3
Spectroscopy Department, Physics Research Institute, National Research Centre, El Behooth St., Dokki, Cairo 12622, Egypt
4
Department of Physics, College of Science, Taif University, Taif 21944, Saudi Arabia
5
Advanced Materials Group, Materials Sciences and Technology Division, CSIR-North East Institute of Science and Technology, Jorhat 785006, India
6
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
7
Centre National de la Recherche Scientifique, Université Lille, Université Polytechnique Hauts-de-France, UMR 8520, IEMN, F-59000 Lille, France
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(8), 1192; https://doi.org/10.3390/catal13081192
Submission received: 8 May 2023 / Revised: 5 July 2023 / Accepted: 8 July 2023 / Published: 8 August 2023
(This article belongs to the Special Issue Exclusive Papers of the Editorial Board Members and Topical Advisory Panel Members of Catalysts in Section Electrocatalysis)
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Abstract

:
The work reports a facile synthesis of high thermally stable nanocrystalline anatase TiO2 nanoparticles (NPs) doped with different atomic concentrations (0.5, 1.0, 3.0, and 6.0%) of Gd3+ and Nd3+ ions by a template-free and one-step solvothermal process, using titanium(IV) butoxide as a titanium precursor and dimethyl sulfoxide (DMSO) as a solvent. The structure and morphology of the Gd3+, Nd3+, and 0.5%Gd3+-0.5%Nd3+/doped TiO2 NPs have been characterized by using various analytical techniques. The Gd3+/ and Nd3+/TiO2 molar ratios were found to have a pronounced impact on the crystalline structure, size, and morphology of TiO2 NPs. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies revealed the proper substitution of Ti4+ by Gd3+ and Nd3+ ions in the TiO2 host lattice. The as-prepared Gdx/TiO2, Ndx/TiO2, and Gd1.0/Ndx/TiO2 bimetallic NPs, x = 0.5, 1.0, 3.0, and 6%, have been investigated as electrocatalysts for hydrogen evolution reaction (HER) in 1.0 M KOH solution using a variety of electrochemical techniques. At any doping percentage, the Gd1.0/Ndx/TiO2 bimetallic NPs showed higher HER catalytic performance than their corresponding counterparts, i.e., Gdx/TiO2 and Ndx/TiO2. Upon increasing the Nd content from 0.5 to 6.0%, the HER catalytic performance of the Gd1.0/Ndx/TiO2 bimetallic NPs was generally enhanced. Among the studied materials, the bimetallic Gd1.0/Nd6.0/TiO2 NPs emerged as the most promising catalyst with an onset potential of −22 mV vs. RHE, a Tafel slope of 109 mV dec−1, and an exchange current density of 0.72 mA cm−2. Such HER electrochemical kinetic parameters are close to those recorded by the commercial Pt/C (onset potential: −15 mV, Tafel slope: 106 mV dec−1, and exchange current density: 0.80 mA cm−2), and also comparable with those measured by the most active electrocatalysts reported in the literature. The synergistic interaction of Gd and Nd is thought to be the major cause of the bimetallic catalyst’s activity.

1. Introduction

The ever-growing demand for clean, renewable, environmentally friendly, and cost-effective energy sources has resulted from meeting the needs of modern society, which is expanding and evolving quickly. Utilizing hydrogen (H2) as a possible energy source with the possibility of replacing fossil fuels has proven to be a particularly alluring strategy in this context, partly because of its high combustion heat (287 kJ/mol) and the release of green byproduct water [1]. A significant portion of H2 is produced from fossil fuels using a steam reforming method that also produces CO2, which is thought to be a potential cause of environmental global warming [2].
One of the most advantageous processes for producing very pure H2 gas is alkaline water electrolysis. The effectiveness of alkaline water electrolysis is, however, constrained by the sluggish kinetics of the hydrogen evolution process (HER) in alkaline media [3,4,5]. Effective electrocatalysts are suggested for use as cathode materials in water electrolysis cells because they produce significant amounts of H2 at low overpotentials by accelerating the kinetics of the HER. Electrode materials based on platinum (Pt) are the most reliable and effective HER electrocatalysts. The excessive cost of these components, however, raises the price of water electrolyzers. As a result, one of the primary goals for the efficient production of H2 on a large scale is the deployment of innovative, affordable, and effective HER electrocatalysts [6,7].
For the purpose of achieving good electrochemical performances, researchers have designed nanostructures of various Pt group metals (Ru, Rh, Ir, and Pd) with high surface-to-volume ratios [8]. Nitrogen-doped reduced graphene oxide (rGO)-based Pt-TiO2 nanostructures [9], monolayer Pd and Au supported on Mo2C [10], monolayer Pt supported on bulk tungsten carbide (WC) [11], etc., were employed as HER electrocatalysts, demonstrating outstanding activity.
Extensive research has been conducted on alternative non-noble (Fe, Co, Ni, Mo), MoS2 coupled with a perovskite oxide, and metal-free (carbon-based) electrocatalysts in an effort to avoid using precious noble metals [12,13,14,15,16,17]. Transition metal chalcogenides [18], carbides [19], metal alloys [20], and complexes [21] are examples of other non-noble electrocatalysts. Bimetallic NPs, which are composed of two elements, have drawn a lot of attention for their efficiency as HER electrocatalysts as well as for their potential use in a variety of energy storage applications [22,23,24,25]. They have greater catalytic characteristics compared to their monometallic counterparts, which is primarily ascribed to their improved and tunable chemical, physical, and cooperative interactions [25,26]. Au-Pd [27,28] and Au-Ni [29] bimetallic electrocatalysts exhibited high catalytic performance for the HER among bimetallic alloys. Excellent HER activity may be found in alloy nanostructures made of Cu-Pt [30], Au-Pt [31], and other metals. Core-shell nanostructures can also take the place of Pt in a number of HER reactions. Examples of outstanding performances include Au@Pt [32], Cu@Pd/Ti [33], Au@Pd [34], Au@Pd [35], and Au@CdS core-shell nanostructures [36].
In this work, a facile synthesis of Gd3+ and Nd3+-doped TiO2 NPs at different atomic concentrations (0.5, 1.0, 3.0, and 6.0%) of Gd3+ and Nd3+ ions, as well as the mixed 0.5%Gd3+-0.5%Nd3+-doped TiO2 NPs, using titanium(IV) butoxide as a titanium precursor and dimethyl sulfoxide (DMSO) as a solvent, is presented. The lack of usage of amphiphilic surfactants, capping agents, or block copolymers, in contrast to the majority of earlier preparation techniques, can significantly reduce production cost of the synthesized catalysts. Our process is highly straightforward, economical, and easily scalable, as it required only three basic ingredients. The reducing agent and stabilizing layer in this approach were both performed by the DMSO solvent, which is believed to adhere to the surface of the NPs, preventing their agglomeration.
The structure and morphology of the obtained doped TiO2 NPs, namely, Gdx/TiO2, Ndx/TiO2, and Gd1.0/Ndx/TiO2 bimetallic NPs, x = 0.5, 1.0, 3.0, and 6%, were characterized by advanced surfaces analysis techniques. Using linear sweep voltammetry, the newly synthesized electrode materials were assessed for the first time as HER electrocatalysts in 1.0 M KOH aqueous solution. Chronoamperometry and repetitive cyclic polarization measurements were used to evaluate the stability of the best performing electrocatalyst.

2. Results and Discussion

2.1. Characterization of Gdx/TiO2, Ndx/TiO2, and Gd0.5/Nd0.5/TiO2 NPs

The formation of Gdx/TiO2, Ndx/TiO2, and Gd0.5/Nd0.5/TiO2 NPs was characterized by Fourier-Transform Infrared Spectroscopy (FT-IR) and simultaneous Thermogravimetric Analysis-Differential Thermal Analysis (TGA-DTA) (Supporting information, Figures S1 and S2).
Energy dispersive X-ray fluorescence (EDXRF) was used to quantify the elemental contents of the Gdx/TiO2, Ndx/TiO2, and Gd0.5/Nd0.5/TiO2 NPs. Before carrying out the EDXRF measurements, a series of Gdx/TiO2, Ndx/TiO2, and Gd0.5/Nd0.5/TiO2 NPs were homogenously distributed inside the powder sample holder of 25 mm diameter. Mylar foil (4.5 µm-thick, Chemplex Industries, Inc., Palm City, FL, USA) was installed inside the sample holder. Furthermore, fine powders of Gdx/TiO2, Ndx/TiO2 and Gd0.5/Nd0.5/TiO2 NPs were also prepared and measured under the same conditions. Based on the direct excitation from the X-ray tube, only the characteristic radiation of Nd, Gd, and Ti was detected, Figure 1, Figure 2 and Figure 3. As illustrated in Figure 1, Figure 2 and Figure 3, the characteristic Kα and Kβ lines of Ti were free from spectral interference and were detected at photon energies of 4.509 and 4.932 keV, respectively. Additional sum peaks of Ti were recognized at photon energies of 9.019 and 9.452 keV. Using the EDXRF spectra of the current nanocomposites, standard-less quantitative elemental analysis based on the fundamental parameter approach was performed. The proposed method is based on the conversion of the measured intensities (primary, secondary, and ternary) of each element of interest to concentrations in wt.%. Standard-less software “UniQuant” delivered from the manufacturer was utilized for this purpose. The current method has many advantages, such as how fast it is, its simplicity, complete absence of the standard materials, and automatic matrix correction. Figure 1 and Figure 2 depict the X-ray fluorescence spectra of the Gdx/TiO2 and Ndx/TiO2 series at different concentrations of Gd and Nd ranging from 0.5 to 6 wt.%. The characteristic L lines of Gd were detected, and all of them are free from spectral interferences. The detected Gd-L lines are Lα1, Lβ1, Lβ2, Lγ1, Lγ2, and Lι at photon energies of 6.058, 6.714, 7.103, 7.786, 8.084, and 5.36 keV, respectively, Figure 1 and Figure 3. The characteristic radiations of Nd-Lα1, Nd-Lβ1, Nd-Lβ2 were recognized at 5.231, 5.722, and 6.09 keV, respectively, Figure 2 and Figure 3. In the case of the Gd0.5/Nd0.5/TiO2 NPs, there was a spectral interference between Nd-Lβ2 at 6.09 keV and Gd-Lα1, which was located at 6.058 keV, Figure 3. Additional spectral interferences between Nd-Lγ1 and Gd-Lβ1 were observed at 6.602 and 6.714 keV, respectively. Although there was a remarkable spectral interference between L lines of Nd and Gd, the Nd-Lα1 and Nd-Lβ1 at, respectively, 5.231 and 5.722 keV were free from interference and can be used. In addition, the Gd-Lβ2 at 7.103 keV and Gd-Lγ1 at 7.786 keV were also free from spectral interference. As shown in Figure 1 and Figure 2, the characteristic L lines of Gd and Nd increase as the concentration of Gd and Nd increases. The quantitative elemental analysis results of the Gdx/TiO2 and Ndx/TiO2 NPs are presented in Table 1. The obtained concentrations (wt.%) of both Gdx/TiO2 and Ndx/TiO2 NPs are almost in good agreement with the initial prepared ratios. Table 2 summarizes the quantitative elemental analysis results of Gd0.5/TiO2, Nd0.5/TiO2, and Gd0.5/Nd0.5/TiO2 NPs. In the case of Gd0.5/Nd0.5/TiO2 NPs, the L lines of Gd and Nd are close to Ti-K lines and below 10 keV. The obtained results are in accordance with expected ratios.
X-ray diffraction (XRD) was employed to check the Gd3+- and Nd3+-doped TiO2 phases and their crystallinity. Both Gdx/TiO2 and Ndx/TiO2 (x = 0.5, 1.0, 3.0, and 6.0 wt.%) samples only comprised diffraction peaks of the tetragonal anatase phase (JCPDS # 01-084-1286) [37]. No peaks for any oxide byproducts were observed for all the samples, most probably due to the XRD detection limit. This reflects the substitution of both Gd and Nd (up to 6%) in the TiO2 anatase lattice forming a homogeneous solid solution. The structural information for all the refined phases was obtained by Rietveld refinements [38,39]. The anatase phase with the tetragonal lattice was refined in the space group I 4 1 / a m d [40] and dominated the composition of all TiO2 samples (Table 1). Figure 4a–c shows the calculated and observed diffraction patterns from Rietveld refinement for three selected samples where Rwp (%) = 4 to 5.
Figure 4d–f displays the tetragonal lattice parameters and the cell volume of the anatase phase as a function of Gd content. According to Vegard’s law [41], the cell parameter a increased, which was attributed to the replacement of Ti with larger Gd atoms inside the anatase lattice. On the other hand, the cell parameter c decreased upon increasing the doping content and could be understood as a relaxation of the lattice due to the expansion of the a cell parameter. In turn, the volume of the anatase tetragonal cell was dominated by the enlargement of the a parameter and featured a slight increase with increasing the Gd and Nd content. In addition, the crystallite size was about 20 nm in the case of Gd doping, while doping with Nd reduced the crystallite size to about 10 nm, Table 3.

2.2. Composition and Chemical State Analysis

The composition and chemical state of pure TiO2, Gdx/TiO2, Ndx/TiO2, and Gd0.5/Nd0.5/TiO2 NPs were characterized by electron spectroscopy for chemical analysis (ESCA) set up using a monoenergetic Al-Kα (1486.6 eV). Figure 5 exhibits the survey spectrum of the pure TiO2 nanoparticles as well as the core level spectra of the Ti 2p and O 1s. The core levels plot of Ti and O were recognized not only in the pure TiO2, but also in the doped TiO2, Figure 5, Figure 6 and Figure 7. The peak of the C 1s at 284.6 eV as well as the KVV Auger line at 1233 eV originate from the adsorption of carbon on the surface due to contamination. The C 1s peak was used as a reference to correct the charge shift in the doped TiO2 [42]. Two principle peaks of Ti 2p at the binding energies of 458.8 and 464.4 eV were allocated to Ti 2p3/2 and Ti 2p1/2, respectively. Besides, the Ti Auger lines were assigned to LM23M23 and L3M23M45 at 1103 and 1073 eV, respectively. The spin-orbit splitting of 5.7 eV between the two peaks of Ti 2p confirms the existence of titanium dioxide [43]. Therefore, the oxidation state of titanium is mostly +4, and this is consistent with the literature data.
The shifts of the Ti 2p to high binding energy with Gd and Nd could be attributed to either new species of the TiO2 or the inelastic scattering of Gd and Nd atoms within the anatase crystal structure [44].
The peak of O 1s was estimated at 530.1 eV at the low content of doping (0.5%). At the high doping level (6.0%) of Gd and Nd, two peaks of O 1s were observed at 529.8 and 530.7 eV, respectively, Figure 6 and Figure 7. The different oxygen species could be the main reason for the shoulder at higher binding energy. Additional KLL Auger lines could be recognized at around 1000 eV. The two peaks of O 1s in the doped TiO2 were attributed to the crystal lattice oxygen at 529.8 eV and hydroxyl oxygen (Ti-OH) for pure TiO2 at 530.7 eV, Figure 6 and Figure 7 [45]. However, the different oxygen species could be the main reason for the shoulder at higher binding energy. As the concentrations of Gd and Nd increase from 0.5% to 6% in the anatase phase, remarkable shifts of the O 1s peak was observed, which suggests the presence of Gd2O3 and Nd2O3 in the anatase crystal [46,47]. Additionally, the remarkable shifts of the O 1s peak could be attributed to the surface relaxation effects [48].
In the case of Gd-doped TiO2, two weak peaks of Gd were identified at 142 and 152 eV ascribed to 4p3/2, 4d5/2, respectively, Figure 6. Due to the low detector efficiency at high Z elements, the statistical distribution of the Gd 4d peaks was poor, and the Gd 3d peaks could not be detected at a low concentration of Gd (0.5%). The binding energy of Gd 4d5/2 at 142.4 eV indicates the trivalent oxidation state of gadolinium, mostly in the form of Gd2O3, which is in agreement with reference [49].
The spin orbit splitting between the two peaks of Gd 4d equals 5.6 eV, and it also agrees with the literature [49]. Based on the obtained spectra of Gdx/TiO2 NPs, the characteristic peaks of Gd were successfully evidenced in the TiO2, Figure 6. The valence state of Ndx/TiO2 NPs was also demonstrated. The complete doping behavior was also confirmed in the case of Nd x/TiO2 NPs. The two characteristic peaks of Nd 3d3/2 and Nd 3d5/2 were located, respectively, at 995.8 and 975.9 eV, Figure 7. The values of the binding energy of Nd 3d agree with those reported by Wang et al. [50]. The spin orbit splitting between the two peaks of Nd 3d equals 20 eV, Figure 7.
As seen from Figure 5, Figure 6, Figure 7 and Figure 8, the intensity of O 1s peak decreased after doping with Gd or Nd compared with the pure TiO2, which confirms the successful doping in TiO2. In the case of Gdx/TiO2 and Ndx/TiO2 NPs, the binding energies (BE) of the two peaks of Ti 2p1/2 and 2p3/2 at 464.8 and 458.9 eV, respectively, are lower than those of pure TiO2. Besides, the BE of the two peaks of Ti in Gdx/TiO2 are also lower than those of Ndx/TiO2 NPs, Figure 5, Figure 6, Figure 7 and Figure 8. The decrease in Ti 2p BE might be attributed to the Ti4+ and O2- local environment change by the introduction of Gd or Nd atoms.
As shown in Figure 6 and Figure 7, these shifts in the BE of Ti 2p and O 1s peaks could be ascribed to the formation of Ti –Gd and Ti–Nd bonds on the grain boundaries of the crystallites, reducing the Ti4+ BE. Figure 6 and Figure 7 revealed an increase of Nd 3d and Gd 4d peak intensities upon increasing the doping level from 0.5 to 6.0 wt.%. At a concentration of 0.5 wt.% of Nd and Gd, the O 1s and Ti 2p intensities had a remarkable decrease. The spin orbit splitting of Nd 3d and Gd 4d remained the same as the previous cases.
Table 4, Table 5, Table 6 and Table 7 summarize the measured binding energies, full widths at half maximum (FWHM), peak areas, and atomic concentrations for pure TiO2, Gdx/TiO2 and Ndx/TiO2 NPs, and Gd0.5/Nd0.5/TiO2 NPs. The non-stoichiometric atomic ratios of O:Ti of TiO2 could be ascribed to two main reasons. The first one is the quantitative surface analysis capability of the XPS is limited to a few nanometers (1–10 nm). Secondly, due to the chemical treatments of TiO2, Gdx/TiO2 and Ndx/TiO2 NPs, and Gd0.5/Nd0.5/TiO2 NPs [51], the atomic ratios of O:Ti of TiO2 could be relatively non-stoichiometric, especially at the surface of the NPs. As illustrated in Table 4, Table 5 and Table 6, the O:Ti atomic ratios in Ndx/TiO2 samples are higher than those of Gdx/TiO2. These results support the reported fact that Nd3+ ions are more electropositive than Gd3+ ions. Therefore, the Nd doping may create a more oxygen-rich nano-phase structure.
SEM imaging and EDX were used jointly to elucidate the morphology and elemental composition of the Gdx/TiO2 and Ndx/TiO2 NPs. Figure 9 shows the SEM photographs of the typical Gdx/TiO2 and Ndx/TiO2 samples. From the images, the Gdx/TiO2 and Ndx/TiO2 existed essentially in the form of spherical particles and presented porous structures similar to those of TiO2. The morphological study revealed that for both TiO2 and Gdx/TiO2 samples, the surface looked almost the same with slightly whitish portion, indicating the deposition of Gd. Based on the SEM results, the Ti Kα-fluorescence signals of the pure TiO2 and Gdx/TiO2 samples were also obtained by EDX analysis (Figure 9). Table 8 gives semi-qualitative information about the elemental and atomic percentages in the TiO2 and Gdx/TiO2 samples.

High Resolution Transmission Electron Microscopy (HR-TEM) Analysis

The morphology of the samples and the corresponding chemical composition were further determined by, respectively, HR-TEM and selected area electron diffraction (SAED) patterns. Figure 10 depicts typical TEM and HR-TEM images (insets SAED patterns) of Nd0.5/TiO2, Gd0.5/TiO2, and Gd0.5/Nd0.5/TiO2 NPs. As evidenced in the TEM images, the majority of the TiO2 nanoparticles consist mainly of quasi-spherical and cubic particles. The HR-TEM images revealed a characteristic lattice spacing of 0.352 nm for the TiO2 anatase (101) plane. The average size of both Gd0.5/TiO2 and Nd0.5/TiO2 slightly changed with the increase of the RE3+ content; for example, the average particle size range was 10–12 nm.
To estimate the optical band gap, UV-vis diffuse reflectance spectra were measured to analyze the red-shifts in the absorption regions. The Kubelka–Munk equation αhν = A(hν −Eg)2, where α, h, ν, and Eg and A are the absorption coefficient, Plank constant, light frequency, band gap, and the proportionality constant, respectively, was used for band gap determination, Figure 11 and Figure 12. For pure TiO2, a band gap energy of 3.18 eV was determined, which is in accordance with that of other reports [52,53,54], while for RE-doped TiO2, the band gap energy decreased due to the red-shift of absorbance (see Figure 11 and Figure 12 and Table 9), suggesting that gadolinium and neodymium improved the visible light absorbance of TiO2.

2.3. Electrocatalytic Activity Studies for the Hydrogen Evolution Reaction (HER)

2.3.1. Cathodic Polarization Measurements

Figure 13a presents the cathodic polarization plots of our synthesized catalysts, namely, Gdx/TiO2 and Ndx/TiO2 with various RE doping percentages (0.5, 1, 3, and 6%). The cathodic polarization curves of the Gd1.0/TiO2 NPs with varying Nd content, Gd1.0/Ndx/TiO2 (x = 0.5, 1, 3, and 6%) were also recorded. Measurements were conducted in 1.0 M KOH solution in comparison with bare GCE and TiO2/GCE. The polarization curves in Figure 13a also comprised the cathodic response of a commercial Pt/C catalyst as a reference point. The corresponding Tafel plots are exhibited in Figure 13b, and the fitting Tafel parameters are depicted in Table 10.
As a well-known eminent HER electrocatalyst, the Pt/C catalyst achieved amongst the investigated catalysts the lowest HER’s onset potential, EHER~−15 mV vs. RHE, with the steepest reduction (catalytic) currents. In contrast, the bare GC electrode displayed inferior catalytic activity, clear from its humble catalytic current generated at a larger EHER (−720 mV vs. RHE).
On the other hand, there was a substantial improvement in the HER catalytic activity in the reductive sweep curves of the synthesized Gdx/TiO2, Ndx/TiO2, and Gd1.0/Ndx/TiO2 nanocomposites. This enhanced HER catalytic activity occurred to different extents depending on the type of studied catalyst, RE doping percentage and the bimetallic nanocomposite Gd1.0/Ndx/TiO2 composition.
It follows, from Figure 13a, that the HER catalytic activity of Ndx/TiO2 and Gdx/TiO2 catalysts enhanced with RE doping percentage. In addition, at any RE doping percentage, the Ndx/TiO2 electrocatalyst exhibited higher HER activity than Gdx/TiO2. This was evident from their EHER values recorded in Table 10. For instance, at a doping percentage of 3.0%, an EHER value of −103 mV vs. RHE was recorded for Nd3.0/TiO2, which is 57 mV more anodic (active direction) than that of the Gd3.0/TiO2 catalyst (160 mV vs. RHE). The lower EHER values of the Ndx/TiO2 catalysts led to higher exchange current density values, jo. For example, a jo value of 15.9 × 10−2 mA cm−2 was recorded for the Nd3.0/TiO2 catalyst. This Nd3.0-TiO2 catalyst’s jo value is ~14.2 times greater than that measured for the Gd3.0/TiO2 catalyst (1.12 × 10−2 mA cm−2). The higher HER catalytic activity of Ndx/TiO2 catalysts was also testified from their lower overpotentials required to generate a current density of 10 mA cm−2, η10. For example, the Nd3.0/TiO2 catalyst required an η10 of 244 mV to deliver a current density of 10 mA cm−2, which is 188 mV anodic to that attained by Gd3.0/TiO2 (432 mV).
These results highlight the high HER catalytic efficiency of the Ndx/TiO2 catalyst. The high Ndx/TiO2 electrocatalyst’s HER activity compared to that of the Gdx/TiO2 electrocatalysts can be chiefly attributed to, as evidenced from XRD studies (revisit Table 3), the former’s smaller crystallite size (~10 nm) than that of the latter (~20 nm). The Ndx/TiO2 electrocatalyst’s smaller crystallite size is translated into higher electrochemical active surface area (EASA), as estimated from cyclic voltammetry measurements performed at various potential scan rates (Figure S3, Supporting Information).
The HER catalytic performance of the investigated Gd1.0/Ndx/TiO2 bimetallic NPs (x = 0.5, 1, 3, and 6%) is positioned far beyond that of their corresponding individuals at any studied RE doping percentage, namely, Gdx/TiO2 and Ndx/TiO2 (x = 0.5, 1, 3, and 6%). In addition, the Gd1.0/Ndx/TiO2 bimetallic NPs’ catalytic activity for the HER was enhanced with increasing Nd content, approaching that of the commercial Pt/C electrocatalyst at a composition of Gd1.0/Nd6.0/TiO2. This was evident from the EHER and η10 values recorded for the tested Gd1.0/Ndx/TiO2 bimetallic NPs, Table 10, that shift towards a more anodic direction with an increase in the Nd content. Thus, high cathodic currents, and hence large amounts of H2, could be generated at low overpotentials, denoting efficacious catalytic performance for the HER.
Increasing the Nd doping percentage in the Gd1.0/Ndx/TiO2 bimetallic NPs from x = 0.5 up to 6.0% has also led to higher jo values, which represents another line of evidence for improved catalytic performance for the HER. The kinetics of the HER became therefore faster if both Gd and Nd NPs were combined as bimetallic, Gd/Nd/TiO2, rather than individually loaded on TiO2, i.e., Gd/TiO2 and Nd/TiO2. Another supported bimetallic NPs electrocatalysts, such as Au–Pd [27,28], Au–Ni [29], Cu–Pt [30], and porous Cu–Ti [55] exhibited analogous findings for the HER.
Bimetallic catalysts exhibit eminent catalytic characteristics, which are not observed in their individual monometallic counterparts, through cooperative interactions (synergistic effects) [27,28,29,30,55]. Such catalytic characteristics comprise increased electrocatalytic activity, improved chemical/physical stability, a greater surface area, and increased catalyst selectivity. On these grounds, the HER catalytic activity of our synthesized TiO2-supported bimetallic Gd/Nd catalyst is possibly a result of the synergistic effect between Gd and Nd, abundant catalytically active sites, and an increasingly accessible electrochemical surface area. The increased accessible electrochemical surface area of the investigated bimetallic catalysts, namely, Gd1.0/Nd0.5/TiO2, Gd1.0/Nd1.0/TiO2, Gd1.0/Nd3.0/TiO2, and Gd1.0/Nd6.0/TiO2 catalysts compared with their individual monometallic counterparts (Gd/TiO2 and Nd/TiO2) was evidenced from cyclic voltammetry measurements (Figure S3, Supporting Information).
As revealed from Table 10, the Gd1.0/Nd6.0/TiO2 bimetallic nanocomposite, the best catalyst here, exhibited EHER, η10, and jo values of −22 mV vs. RHE, −109 mV dec−1, and 0.72 mA cm−2, respectively. These HER electrochemical kinetic parameter values are very close to those measured for the commercial Pt/C (−15 mV vs. RHE, −106 mV dec−1, and 0.8 mA cm−2). These findings reflect the outstanding HER catalytic performance of Gd1.0/Nd6.0/TiO2 electrocatalyst that surpassed many effective electrocatalysts and are comparable with the most efficient ones reported in the literature (Table S1, Supporting Information).
An additional substantial electrochemical parameter, the Tafel slope, was also employed to assess and compare the HER catalytic efficiency of the investigated catalysts. In order to identify the major HER mechanism over the studied catalysts under such alkaline conditions, the measured Tafel slope values (Table 10) are contrasted with the standard values depicted in Equations (1)–(3). Such equations constitute the HER path in alkaline electrolytes on a particular catalyst [56]. The first water dissociation step (Volmer step, Equation (1)) is unavoidably the subject of the HER process under alkaline conditions, as few protons are present in alkaline electrolytes [56]. Following the Volmer step, either the two adsorbed hydrogen atoms are combined on the catalyst surface, forming a molecule H2 (Tafel step, Equation (2)), or a hydrated proton is directly bonded to the adsorbed hydrogen atom that requires the transfer of the electron from the catalyst surface (Heyrovsky step, Equation (3)).
H 2 O + e − → H ads + HO −       ( Volmer Step ) b = 2.34 R T α F ≅ 120   m V / d e c
H ads + H ads → H 2       ( Tafel Step ) b = 2.34 R T ( 1 + α ) F ≅ 30   m V / d e c
H 2 O + e − + H ads → H 2 + HO −       ( Heyrovsky ) b = 2.34 R T 2 F ≅ 40   m V / d e c
Table 10 reports a Tafel slope value of 110 mV dec−1 for the commercial Pt/C catalyst, which is in accordance with that recorded in the literature [57]; a good proof of the accuracy of the electrochemical measurements is utilized here. The Tafel lines of all tested Gdx/TiO2 and Ndx/TiO2 catalysts are parallel to each other with Tafel slopes of around 140 mV dec−1. With Tafel slopes ranging from 108 to 113 mV dec−1, the three studied Gd1.0/Ndx/TiO2 bimetallic catalysts’ Tafel lines are parallel to that of the Pt/C catalyst (110 mV dec−1). The obvious decrease in the Tafel slope value from about 140 mV dec−1 for Gdx/TiO2 and Ndx/TiO2 catalysts to about 110 mV dec−1 for the four tested Gd1.0/Ndx/TiO2 bimetallic catalysts adds another line of evidence for the enhanced HER kinetics on Gd1.0/Ndx/TiO2 bimetallic catalyst surfaces. The explanation for this is that lower Tafel slopes usually indicate an abundance of active sites on the catalyst surface [58]. This result suggests an alkaline HER mechanism on the surface of the three studied bimetallic electrocatalysts that is similar to that taking place on the commercial Pt/C electrocatalyst. The Volmer step as the rate limiting step for the HER is part of that mechanism [57].
The catalysts’ electrochemical active surface area (EASA) is another important metric used to compare their catalytic activity [59]. The inaccurate estimation of the specific capacitance of the composites, however, made it extremely difficult to quantify EASA for binary and ternary catalysts [60]. As a result of this, an alternative technique for assessing the catalytic activity of electrocatalysts is based on their electrochemical double-layer capacitance (Cdl), which has a direct link with EASA [61]. The values of Cdl (Table 11) were calculated in this investigation using cyclic voltammetry (CV) measurements performed at various potential sweep rates, as mentioned in Section S1 (Supporting Information).
The results in Table 11 clearly showed that the Cdl values rose as the tested NPs’ doping % in TiO2 increased, with Nd NPs being more efficient than Gd NPs at every doping percentage examined. Depending on the amount of Nd in the Gd1.0/Ndx/TiO2, the Cdl values further increased when Nd was co-doped with Gd. The obtained results demonstrated the catalytic influence of the synergistic interaction between Gd and Nd, as well as the abundance of catalytically active sites and a rising amount of accessible electrochemical surface area [27,28,29,30].
The surface sites that can be exploited for adsorption and desorption processes are more accessible and catalytically active in catalysts with higher Cdl values [62].
The value of EASA was calculated from Cdl using Equation (4) [62]:
EASA = Cdl/Cs
where Cs is the specific capacitance for an electrode with 1.0 cm2 of flat, uniform surface area; it is typically between 20 and 40 mF cm−2. Table 11 summarizes the EASA values calculated for the materials under investigation using a flat electrode with an average value of 30 mF cm−2.
At any measured RE doping %, the Gd1.0/Ndx/TiO2 bimetallic NPs clearly achieved EASA values higher than those of their equivalent individuals, thus confirming the cooperative interactions (synergistic effects) between Gd and Nd in catalyzing the HER [27,28,29,30]. The highly active surface area of the Gd1.0/Ndx/TiO2 catalysts (1307, 1580, 1853, and 2097 cm2 for x = 0.5, 1.0, 3.0, and 6.0%, respectively) might have contributed to the appreciable rise in their Cdl values (39.2, 47.4, 55.6, and 62.9 mF cm−2 for x = 0.5, 1.0, 3.0, and 6.0%, respectively).
Cyclic voltammetry data, Figure S3, and Equation (5) [63], were employed to estimate the number of active sites n for the investigated materials.
n = Q/2F
where F is the Faraday constant (96,485 C mol−1) and 2 denotes the stoichiometric number of electrons that the HER of the electrode consumes. The studied Gd1.0/Ndx/TiO2 bimetallic NPs (x = 0.5, 1.0, 3.0, and 6%) clearly displayed n values higher than those of their equivalent individual counterparts at any investigated RE doping percentage, namely, Gdx/TiO2 and Ndx/TiO2 (x = 0.5, 1.0, 3.0, and 6%). With increasing Nd content in the Gd1.0/Ndx/TiO2 bimetallic NPs, the value of n increased approaching that computed for the commercial Pt/C electrocatalyst (n = 30.94 × 10−8 mol−1) at a composition of Gd(1.0)-Nd (6.0), n = 29.43 × 10−8 mol−1. These results confirmed the higher HER kinetics when the Gd and Nd NPs were combined into a hybrid bimetallic NP, Gd1.0/Ndx/TiO2.

2.3.2. Faradaic Efficiency Calculations for the HER

The investigated catalysts’ HER Faradaic efficiency (%) values were also calculated in order to further assess and compare their electrocatalytic activity. A controlled galvanostatic electrolysis was conducted to measure the amount of H2 gas evolved (Vm, in mol) per hour using gas chromatography (GC), as reported in Section S4 (CGE), Equation (6).
Vm = mol gas (GC)
The value of Vc, the predicted amount of the released gas based on the charge transferred, is then computed using Equation (7) [64], assuming 100% Faradaic efficiency during the employed CGE:
Vc = Q(CGE)/nF
where F is the Faraday constant (96,485 C), Q(CGE) is a representation of the charge transferred through the WE during the CGE operation, and n (2H+ + 2e = H2, n = 2) is a mathematical representation of the number of electrons exchanged during the HER. The value of ε is derived by dividing Vm by Vc. The measured electrocatalyst’s Faradaic efficiency (ε%) is then calculated by multiplying the ratio (Vm/Vc) quotient by 100, Equation (8) [64].
Faradaic efficiency (ε%) = [Fn (mol gas(GC))100]/Q(CGE)
The calculated and measured quantities of H2 evolved for the examined electrocatalysts during the first hour of CGE are summarized in Table 12, which revealed that the tested Gd1.0/Ndx/TiO2 bimetallic NPs, x = 1.0 and 6%, exhibited ε% values that are much higher than those of their equivalent individuals at any studied RE doping percentage, Gdx/TiO2 and Ndx/TiO2 (x = 1.0 and 6%). Additionally, it was also noticed that as Nd content was increased in Gd1.0/Ndx/TiO2 from 1.0 to 6.0%, the HER’s ε% value also enhanced from 92.9 to 98.7%, thus approaching that of the commercial Pt/C electrocatalyst (99.5%). These results provided another piece of evidence for the enhanced HER kinetics when both Gd and Nd dopants were brought together on TiO2, forming the Gd1.0/Ndx/TiO2 bimetallic NPs rather than being loaded separately, i.e., Gd1.0/TiO2 and Ndx/TiO2.

2.4. Best Catalyst’s Long-Term Stability Tests

Excellent electrocatalysts must meet a number of criteria, one of which is long-term stability. To assess the stability of the best catalyst for the HER, two main electrochemical approaches were used. They comprise 72 h of controlled potential electrolysis (chronoamperometry) measurements as well as continuous (repetitive) cyclic polarization (CP) up to 10,000 cycles, Figure 14.
It follows from the CP measurements, Figure 14, that the catalyst’s polarization curve maintained a high degree of similarity with just minor current losses after 10,000 cycles, thusreflecting good stability in the catalytic activity. CP findings were validated by electrolysis results at a static overpotential (inset of Figure 14); the current remained essentially constant during the run.

3. Experimental

3.1. Synthesis of Gd3+- or Nd3+- as Well as 0.5%Gd3+-0.5%Nd3+-Doped TiO2 NPs

Nanocrystalline TiO2 nanoparticles (NPs) doped with different atomic concentrations (0.5, 1.0, 3.0, and 6.0%) of Gd3+ and Nd3+ ions were prepared adopting our previous method for the synthesis of pure TiO2 nanoparticles [51]. The obtained materials were referred to as x% Gd or x% Nd-doped TiO2, where %x is the percent content of Gd3+ or Nd3+ ions in the sample. Mixed 0.5%Gd3+-0.5%Nd3+-doped TiO2 NPs were also obtained by the same method using a mass ratio of 0.5% of both Gd3+ and Nd3+ ions.

3.2. Electrocatalytic Activity Measurements

Electrochemical Setup

Electrochemical characterizations were carried out using a standard double-jacketed three-electrode electrochemical cell. A graphite rod (99.999% pure, Sigma-Aldrich, Darmstadt, Germany) and mercury/mercury oxide, Hg/HgO, NaOH(0.1 M), served as the cell’s auxiliary and reference electrodes, respectively. The working electrode was a 3 mm glassy carbon (GC) loaded with catalyst (WE). Section S2 of the Supporting Information file gives the full description of the WE preparation for electrochemical experiments.
To evaluate the performance and stability of the synthesized electrocatalysts toward the HER, various electrochemical techniques were used, as reported in Section S3 (Supporting Information).
The electrochemically active surface area (EASA) of the catalysts was estimated using cyclic voltammetry (CV) tests carried out at various potential scan rates (ν: 20–120 mV s−1) covering the potential range (0.32–0.42 V vs. RHE), which only permits the capacitive current to flow. The catalyst Cdl can be determined by plotting the difference in current density between anodic and cathodic scans (ΔJ = Janodic – Jcathodic) against the slope of the ΔJ vs. ν plot at 0.37 V vs. RHE.

4. Conclusions

In this work, a facile and efficient one-pot method for the synthesis of RE3+ (Gd3+ or Nd3+)-doped TiO2 NPs with different atomic concentrations (ca. 0.5–6.0%), namely, Gdx/TiO2, Ndx/TiO2, and Gd1.0/Ndx/TiO2 bimetallic NPs, x = 0.5, 1.0, 3.0, and 6%, as efficient cathode materials for H2 production, was developed. The structure and morphology of the obtained materials were characterized by using various techniques, which indicated that the prepared RE-doped TiO2 NPs were pure-phase and uniformly dispersed. The crystallite size was about 20 nm in the case of Gd3+-doping, while doping with Nd3+ decreased the crystallite size to about 10 nm. Linear cathodic polarization measurements were used to examine the as-prepared NPs as active electrocatalysts for effective hydrogen generation in alkaline solution (1.0 M KOH). The studied TiO2-doped bimetallic NPs, namely, Gd1.0/Ndx/TiO2, x = 0.5, 1.0, 3.0, and 6%, showed higher HER catalytic performance than their corresponding individual counterparts at any tested RE doping percentage, namely, Gdx/TiO2 and Ndx/TiO2. The Gd1.0/Ndx/TiO2 HER catalytic performance was enhanced with increasing Nd content from 0.5 up to 6.0%. The Gd1.0/Ndx/TiO2 maximum HER catalytic activity was attained at x = 6% with HER electrochemical kinetic parameters (onset potential: −22 mV, Tafel slope: 109 mV dec−1, and exchange current density: 0.72 mA cm−2), approaching the performance of the commercial Pt/C electrocatalyst (onset potential: −15 mV, Tafel slope: 106 mV dec−1, and exchange current density: 0.80 mA cm−2). The synergistic interaction of Gd and Nd is thought to be the major cause of the bimetallic catalyst’s activity. The simplicity and originality of the procedure adopted here to synthesize such hybrid NPs, together with their high HER catalytic activity in the dark, are the fundamental features of this work.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13081192/s1, Figure S1. FT-IR spectra of DMSO, Gd3+.nBuO, and Gd-doped TiO2 NPs before (red) and after (blue) annealing process; Figure S2. TGA-DTA curves of Gdx-doped TiO2 NPs recorded from room temperature to 1000 °C before (a) and after (b) calcination; Figure S3. Cyclic voltammograms recorded for studied catalysts at various potential scan rates (20–100 mV s−1) measured in a non-Faradaic region of the voltammograms. Measurements were conducted in deaerated KOH solution (1.0 M) at room temperature; Figure S4. Double-layer capacitance measurements for determining the electrochemically-active surface area of Gdx/TiO2 and Ndx/TiO2 (x = 0.5, 1.0, 3.0, and 6.0%) catalysts; Figure S5. Double-layer capacitance measurements for determining the electrochemically-active surface area of Gd1.0/Ndx/TiO2 (x = 0.5, 1.0, 3.0, and 6.0%) catalysts; Table S1: Comparison of HER catalytic activity of our best performing electrocatalysts with the highly efficient ones reported in the literature in alkaline solutions [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87].

Author Contributions

Conceptualization, M.A., N.A.A. and H.M.O.A.-M.; methodology, S.I.A., M.R.D. and A.A.S.; software (XRD, XRF, and XPS) and formal analysis, H.M.O.A.-M.; investigation, M.A. and N.A.A.; resources, M.A.A. and M.M.I.; data curation, M.A., R.B., M.A.A. and M.M.I.; writing—original draft preparation, A.A.S., M.A.A. and M.M.I.; writing—review and editing, M.A.A. and M.M.I.; visualization, M.A.A. and M.M.I.; supervision, M.A., M.A.A. and M.M.I.; project administration, M.A.; funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

The researchers would like to acknowledge Deanship of Scientific Research, Taif University for funding the work.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The EDXRF spectra of the Gdx/TiO2 nanocomposites.
Figure 1. The EDXRF spectra of the Gdx/TiO2 nanocomposites.
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Figure 2. The EDXRF spectra of the Ndx/TiO2 nanocomposites.
Figure 2. The EDXRF spectra of the Ndx/TiO2 nanocomposites.
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Catalysts 13 01192 g003
Figure 3. The EDXRF spectra of the Gd0.5/TiO2, Nd0.5/TiO2, and Gd0.5/Nd0.5/TiO2 nanocomposites.
Figure 3. The EDXRF spectra of the Gd0.5/TiO2, Nd0.5/TiO2, and Gd0.5/Nd0.5/TiO2 nanocomposites.
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Catalysts 13 01192 g004
Figure 4. XRD patterns of (a) Gdx/TiO2 NPs and (b) Ndx/TiO2 NPs, (c) calculated (red) and recorded (black) diffraction patterns of Gd0.5/TiO2, Nd0.5/TiO2 and Gd0.5/Nd0.5/TiO2, (d,e) lattice parameters as a function of Gd and Nd content, (f) cell volume as a function of Gd and Nd content.
Figure 4. XRD patterns of (a) Gdx/TiO2 NPs and (b) Ndx/TiO2 NPs, (c) calculated (red) and recorded (black) diffraction patterns of Gd0.5/TiO2, Nd0.5/TiO2 and Gd0.5/Nd0.5/TiO2, (d,e) lattice parameters as a function of Gd and Nd content, (f) cell volume as a function of Gd and Nd content.
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Catalysts 13 01192 g005
Figure 5. X-ray photoelectron spectroscopy analysis of the pure TiO2 nanoparticles including (A) a survey and core level spectra of (B) Ti 2p and (C) O 1s.
Figure 5. X-ray photoelectron spectroscopy analysis of the pure TiO2 nanoparticles including (A) a survey and core level spectra of (B) Ti 2p and (C) O 1s.
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Catalysts 13 01192 g006
Figure 6. X-ray photoelectron spectroscopy of the Gd/TiO2 nanoparticles at 0.5% and 6.0% of Gd including (A) a survey and core level spectra of (B) Ti 2p, (C) O 1s, and (D) Gd 4d.
Figure 6. X-ray photoelectron spectroscopy of the Gd/TiO2 nanoparticles at 0.5% and 6.0% of Gd including (A) a survey and core level spectra of (B) Ti 2p, (C) O 1s, and (D) Gd 4d.
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Catalysts 13 01192 g007
Figure 7. X-ray photoelectron spectroscopy of the Nd-doped TiO2 nanoparticles at 0.5 and 6.0 wt.% of Nd including (A) a survey and core spectra of (B) Ti 2p, (C) O 1s, and (D) Nd 3d.
Figure 7. X-ray photoelectron spectroscopy of the Nd-doped TiO2 nanoparticles at 0.5 and 6.0 wt.% of Nd including (A) a survey and core spectra of (B) Ti 2p, (C) O 1s, and (D) Nd 3d.
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Catalysts 13 01192 g008
Figure 8. XPS core level spectra of (A) O 1s, (B) Ti 2p, (C) Gd 4d, and (D) Nd 3d of the Gd0.5/Nd0.5/TiO2 NPs.
Figure 8. XPS core level spectra of (A) O 1s, (B) Ti 2p, (C) Gd 4d, and (D) Nd 3d of the Gd0.5/Nd0.5/TiO2 NPs.
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Catalysts 13 01192 g009
Figure 9. SEM/EDX examinations of Gd0.5/TiO2 (A,B) and Nd0.5/TiO2 NPs (C,D).
Figure 9. SEM/EDX examinations of Gd0.5/TiO2 (A,B) and Nd0.5/TiO2 NPs (C,D).
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Catalysts 13 01192 g010
Figure 10. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images (inset SAED patterns) of Gd0.5/TiO2, Nd0.5/TiO2, and Gd0.5/Nd0.5/TiO2 (average particle size 12 ± 0.50 nm). (A,B) Gd0.5/TiO2, (C,D) Nd0.5/TiO2, and (E,F) Gd0.5/Nd0.5/TiO2.
Figure 10. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images (inset SAED patterns) of Gd0.5/TiO2, Nd0.5/TiO2, and Gd0.5/Nd0.5/TiO2 (average particle size 12 ± 0.50 nm). (A,B) Gd0.5/TiO2, (C,D) Nd0.5/TiO2, and (E,F) Gd0.5/Nd0.5/TiO2.
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Figure 11. Kubelka–Munk function plots of Gdx/TiO2 NPs for different Gd atomic concentrations.
Figure 11. Kubelka–Munk function plots of Gdx/TiO2 NPs for different Gd atomic concentrations.
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Catalysts 13 01192 g012
Figure 12. Kubelka–Munk function plots of Ndx/TiO2 NPs for different Nd atomic concentrations.
Figure 12. Kubelka–Munk function plots of Ndx/TiO2 NPs for different Nd atomic concentrations.
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Catalysts 13 01192 g013
Figure 13. Cathodic polarization measurements for (a) the HER and (b) the corresponding Tafel plots recorded for the investigated catalysts. Measurements were carried out in 1.0 M KOH solution at a scan rate of 5 mV s−1 at room temperature. (1) bare GCE; (2) TiO2/GCE; (3) Gd0.5/TiO2/GCE; (4) Gd1.0/TiO2/GCE; (5) Gd3.0/TiO2/GCE; (6) Gd6.0/TiO2/GCE; (7) Nd0.5/TiO2/GCE; (8) Nd1.0/TiO2/GCE; (9) Nd3.0/TiO2/GCE; (10) Nd6.0/TiO2/GCE; (11) Gd1.0/Nd0.5/TiO2/GCE; (12) Gd1.0/Nd1.0/TiO2/GCE; (13) Gd1.0/Nd3.0/TiO2/GCE; (14) Gd1.0/Nd6.0/TiO2/GCE; (15) Pt/C.
Figure 13. Cathodic polarization measurements for (a) the HER and (b) the corresponding Tafel plots recorded for the investigated catalysts. Measurements were carried out in 1.0 M KOH solution at a scan rate of 5 mV s−1 at room temperature. (1) bare GCE; (2) TiO2/GCE; (3) Gd0.5/TiO2/GCE; (4) Gd1.0/TiO2/GCE; (5) Gd3.0/TiO2/GCE; (6) Gd6.0/TiO2/GCE; (7) Nd0.5/TiO2/GCE; (8) Nd1.0/TiO2/GCE; (9) Nd3.0/TiO2/GCE; (10) Nd6.0/TiO2/GCE; (11) Gd1.0/Nd0.5/TiO2/GCE; (12) Gd1.0/Nd1.0/TiO2/GCE; (13) Gd1.0/Nd3.0/TiO2/GCE; (14) Gd1.0/Nd6.0/TiO2/GCE; (15) Pt/C.
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Catalysts 13 01192 g014
Figure 14. Long-term stability tests recorded for the best performing electrocatalyst (Gd1.0/Nd6.0/TiO2) in 1.0 M KOH solution at room temperature for the HER. LSV measurements were conducted at a scan rate of 50 mV s−1. Insets are chronoamperometry measurements (j vs. t) performed on the catalyst at a constant applied potential of −0.32 V vs. RHE.
Figure 14. Long-term stability tests recorded for the best performing electrocatalyst (Gd1.0/Nd6.0/TiO2) in 1.0 M KOH solution at room temperature for the HER. LSV measurements were conducted at a scan rate of 50 mV s−1. Insets are chronoamperometry measurements (j vs. t) performed on the catalyst at a constant applied potential of −0.32 V vs. RHE.
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Table 1. Elemental quantitative analysis in wt.% of the Gdxx/TiO2 and Ndxx/TiO2 composites using Energy dispersive X-ray fluorescence (EDXRF) analysis.
Table 1. Elemental quantitative analysis in wt.% of the Gdxx/TiO2 and Ndxx/TiO2 composites using Energy dispersive X-ray fluorescence (EDXRF) analysis.
Gdx/TiO2 Composites Ndx/TiO2 Composites
SampleGd, %TiO2, %SampleNd, %TiO2, %
Gd0.50/TiO21.80 ± 0.0697.92 ± 0.07Nd0.5/TiO21.13 ± 0.0598.67 ± 0.06
Gd1.0/TiO22.20 ± 0.0797.46 ± 0.08Nd1.0/TiO22.16 ± 0.0797.45 ± 0.08
Gd3.0/TiO22.39 ± 0.0797.24 ± 0.08Nd3.0/TiO27.71 ± 0.1290.95 ± 0.14
Gd6.0/TiO23.07 ± 0.0896.45 ± 0.09Nd6.0/TiO210.89 ± 0.2587.19 ± 0.17
Table 2. Elemental quantitative analysis in wt.% of the Gd0.5/TiO2, Nd0.5/TiO2 and Gd0.5/Nd0.5/TiO2 composites using EDXRF.
Table 2. Elemental quantitative analysis in wt.% of the Gd0.5/TiO2, Nd0.5/TiO2 and Gd0.5/Nd0.5/TiO2 composites using EDXRF.
SampleGd0.5/TiO2Nd0.5/TiO2Gd0.5/Nd0.5/TiO2
Gd2.01 ± 0.07-2.11 ± 0.07
Nd-1.13 ± 0.051.04 ± 0.05
TiO297.85 ± 0.0798.67 ± 0.0696.35 ± 0.09
Table 3. The cell parameters and crystallite size of Gdx/TiO2, Ndx/TiO2, and Gd0.5/Nd0.5/TiO2 NPs as calculated from the Rietveld refinements.
Table 3. The cell parameters and crystallite size of Gdx/TiO2, Ndx/TiO2, and Gd0.5/Nd0.5/TiO2 NPs as calculated from the Rietveld refinements.
xGdx/TiO2Ndx/TiO2
a (A)c (A)V (A3)D (nm)a (A)c (A)V (A3)D (nm)
0.53.7872 (14)9.499 (4)136.24 (16)21 (2)3.7834 (12)9.454 (4)135.33 (14)12 (1)
1.03.7885 (15)9.497 (5)136.31 (17)19 (2)3.7863 (15)9.447 (5)135.43 (17)11 (1)
3.03.7895 (14)9.497 (4)136.39 (16)18 (1)3.7928 (16)9.425 (5)135.58 (18)10 (2)
6.03.7899 (16)9.496 (5)136.40 (18)18 (1)3.7964 (14)9.405 (4)135.56 (16)10 (2)
0.5%Gd-0.5%Nd/TiO2Pure TiO2
3.7915 (18)9.452 (3)135.87 (17)12 (2)3.7854 (14)9.4842 (5)135.9 (17)17 (3)
Table 4. The measured binding energies, full widths at half maximum (FWHM), peak areas, and atomic concentrations for the pure TiO2 NPs.
Table 4. The measured binding energies, full widths at half maximum (FWHM), peak areas, and atomic concentrations for the pure TiO2 NPs.
PeakBinding Energy (eV)FWHM (eV)Peak Area, kcps (eV)Atomic Conc. (at.%)
O 1s530.777.493398.373.4
Ti2p1/2458.294.742892.526.5
Ti2p3/2463.98
Table 5. The measured binding energies, full widths at half maxima (FWHM), peak areas, and atomic concentrations for the Gd/TiO2 NPs.
Table 5. The measured binding energies, full widths at half maxima (FWHM), peak areas, and atomic concentrations for the Gd/TiO2 NPs.
PeakBinding Energy (eV)FWHM (eV)Peak Area, kcps (eV)Atomic Conc. (at.%)
Gd0.5/TiO2
O 1s530.773.6991.573.43
Ti 2p1/2458.293.31973.626.48
Ti 2p3/2463.98
Gd 4d3/21420.271.30.09
Gd 4d5/2152
Gd6.0/TiO2
O 1s530.773.711047.571.73
Ti 2p1/2458.293.231049.428.11
Ti 2p3/2463.98
Gd 4d3/21420.01.30.16
Gd 4d5/2152
Table 6. The measured binding energies, full widths at half maxima (FWHM), peak areas, and atomic concentrations for the Nd-doped TiO2 nanoparticles.
Table 6. The measured binding energies, full widths at half maxima (FWHM), peak areas, and atomic concentrations for the Nd-doped TiO2 nanoparticles.
PeakBinding Energy (eV)FWHM (eV)Peak Area, kcps (eV)Atomic Conc. (at.%)
Nd (0.5%)/TiO2
O 1s530.773.361139.175.12
Ti 2p1/2458.291.931108.724.71
Ti 2p3/2463.98
Nd 3d3/2995.800.7740.17
Nd 3d5/2975.9
Nd (6.0%)/TiO2
O 1s530.773.71266.574.59
Ti 2p1/2458.293.371175.925.18
Ti 2p3/2463.98
Nd 3d3/2975.90.011.780.23
Nd 3d5/2995.8
Table 7. The measured binding energies, full widths at half maxima (FWHM), peak areas, and atomic concentrations for the Gdx/TiO2 and Ndx/TiO2 NPs.
Table 7. The measured binding energies, full widths at half maxima (FWHM), peak areas, and atomic concentrations for the Gdx/TiO2 and Ndx/TiO2 NPs.
Peak Binding Energy, eVFWHM, eVPeak Area, kcpseVAtomic Concentration, (at.%)
O 1s538.53.83961.477.55
Ti 2p1/2458.292.89657.921.82
Ti 2p3/2463.98
Gd 4d3/21420.190.700.29
Gd 4d5/2152
Nd 3d3/2975.90.643.80.34
Nd 3d5/2995.8
Table 8. Catalyst composition using EDX analysis of TiO2, Gd/TiO2, and Nd/TiO2.
Table 8. Catalyst composition using EDX analysis of TiO2, Gd/TiO2, and Nd/TiO2.
ElementCompound
TiO2 (Atom%)Gdx/TiO2 (Atom%)Ndx/TiO2 (Atom%)
O73.0470.8172.36
Ti26.9629.0527.46
Gd-0.150-
Nd--0.185
Table 9. Band gaps of Gdx/TiO2 and Ndx/TiO2 NPs with different atomic concentrations.
Table 9. Band gaps of Gdx/TiO2 and Ndx/TiO2 NPs with different atomic concentrations.
0.5%1.0%3.0%6.0%
Gdx/TiO23.063.042.972.93
Ndx/TiO23.072.952.912.83
Table 10. Mean value (standard deviation) of the electrochemical HER kinetic parameters on the surfaces of our synthesized catalysts, Gdx/TiO2, Ndx/TiO2, and Gd1.0/Ndx/TiO2 loaded on a GCE. Measurements were conducted at room temperature in deaerated KOH solution (1.0 M) in comparison with bare GCE, TiO2/GCE, and Pt/C.
Table 10. Mean value (standard deviation) of the electrochemical HER kinetic parameters on the surfaces of our synthesized catalysts, Gdx/TiO2, Ndx/TiO2, and Gd1.0/Ndx/TiO2 loaded on a GCE. Measurements were conducted at room temperature in deaerated KOH solution (1.0 M) in comparison with bare GCE, TiO2/GCE, and Pt/C.
Tested CathodeOnset Potential
(EHER, mV vs. RHE)		Tafel Slope
(βc, mV dec−1)		Exchange Current Density
(jo, mA cm−2)		Overpotential
at j = 10 mA cm−2
(η10, mV)
bare GCE−720 (9.2)−165 (2.6)2.75 (0.05) × 10−5----
TiO2/GCE−215 (3.6)−152 (2.2)1.45 (0.03) × 10−3565 (7.6)
Gd0.5/TiO2/GCE−186 (3.2)−142 (1.7)2.51 (0.04) × 10−3511 (6.2)
Gd1.0/TiO2/GCE−175 (2.9)−143 (1.8)5.4 (0.15) × 10−3466 (5.1)
Gd3.0/TiO2/GCE−160 (2.8)−141 (2.1)1.12 (0.3) × 10−2432 (4.7)
Gd6.0/TiO2/GCE−145 (2.9)−142 (1.8)2.51 (0.04) × 10−2365 (4.2)
Nd0.5/TiO2/GCE−130 (2.2)−140 (1.5)5.62 (0.06) × 10−2332 (3.8)
Nd1.0/TiO2/GCE−118 (2.4)−139 (1.5)10.5 (0.3) × 10−2277 (3.5)
Nd3.0/TiO2/GCE−103 (1.5)−141 (1.6)15.9 (0.42) × 10−2244 (3.2)
Nd6.0/TiO2/GCE−85 (1.4)140 (1.5)43.6 (0.6) × 10−2 225 (2.2)
Gd1.0/Nd0.5/TiO2/GCE−72 (1.6)113 (1.4)35.5 (0.4) × 10−2 177 (1.9)
Gd1.0/Nd1.0/TiO2/GCE−60 (1.1)112 (1.4)44.7 (0.6) × 10−2 161 (1.8)
Gd1.0/Nd3.0/TiO2/GCE−38 (0.8)110 (1.8)50.2 (0.7) × 10−2 142 (1.7)
Gd1.0/Nd6.0/TiO2/GCE−22 (0.3)109 (1.5)72 (1.1) × 10−2 115 (1.8)
Pt/C−15 (0.2)−106 (1.2)80 (0.9) × 10−2106 (1.5)
Table 11. Estimated values for the examined electrocatalysts’ double-layer capacitance (Cdl), electrochemical active surface area (EASA), net voltammetry charge (Q), and number of active sites (n) based on CV measurements, Figure S3 (Supporting Information).
Table 11. Estimated values for the examined electrocatalysts’ double-layer capacitance (Cdl), electrochemical active surface area (EASA), net voltammetry charge (Q), and number of active sites (n) based on CV measurements, Figure S3 (Supporting Information).
Tested CathodeCdl/
µF cm−2		EASA/cm2Q × 103/Cn × 108/mol
TiO2 NPs alone4.08136.03.21.66
Gd0.5/TiO28.26275.37.33.78
Gd1.0/TiO221.2706.711.66.01
Gd3.0/TiO228.8960.025.213.06
Gd6.0/TiO236.41213.334.517.88
Nd0.5/TiO221.8726.710.85.60
Nd1.0/TiO229.9996.715.47.98
Nd3.0/TiO238.61286.731.716.43
Nd6.0/TiO246.91563.341.221.35
Gd1.0/Nd0.5/TiO239.21306.722.911.87
Gd1.0/Nd1.0/TiO247.41580.043.522.54
Gd1.0/Nd3.0/TiO255.61853.348.625.19
Gd1.0/Nd6.0/TiO262.92096.756.829.43
Pt/C65.42180.059.730.94
Table 12. Mean value (standard deviation) of VH2 (measured and calculated) obtained after 1 h of a controlled galvanostatic electrolysis (CGE) *, together with the Faradaic efficiency values, FE (%), for the studied catalysts.
Table 12. Mean value (standard deviation) of VH2 (measured and calculated) obtained after 1 h of a controlled galvanostatic electrolysis (CGE) *, together with the Faradaic efficiency values, FE (%), for the studied catalysts.
Tested CatalystH2 Measured by GC
(H2/μmol h−1)		Calculated H2 Based on the Charge Passed during ElectrolysisFE (%)
Charge Passed/CH2/μmol h−1
TiO2 NPs alone6.9 (0.12)2.4 (0.05)12.4 (0.2)55.4 (0.8)
Gd1.0/TiO211.8 (0.15)3.1 (0.055)15.9 (0.31)74.2 (1.1)
Gd6.0/TiO215.6 (0.3)3.6 (0.06)18.7 (0.38)83.6 (1.3)
Nd1.0/TiO214.3 (0.26)3.4 (0.052)17.8 (0.35)80.5 (1.2)
Nd6.0/TiO220.1 (0.35)4.3 (0.07)22.4 (0.4)89.8 (1.5)
Gd1.0/Nd1.0/TiO222.9 (0.4)4.8 (0.09)24.6 (0.42)92.9 (1.4)
Gd1.0/Nd6.0/TiO231.4 (0.55)6.1 (0.12)31.8 (0.5)98.7 (1.6)
Pt/C32.9 (0.3)6.4 (0.1)33.1 (0.36)99.5 (1.4)
* CGE: the catalyst is held at a current density of −10 mA cm−2 for 1 h in 1.0 M KOH solution at 25 °C.
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Alsawat, M.; Alshehri, N.A.; Shaltout, A.A.; Ahmed, S.I.; Al-Malki, H.M.O.; Das, M.R.; Boukherroub, R.; Amin, M.A.; Ibrahim, M.M. Enhanced Alkaline Hydrogen Evolution on Gd1.0/Ndx (x = 0.5, 1.0, 3.0, and 6.0%)-Doped TiO2 Bimetallic Electrocatalysts. Catalysts 2023, 13, 1192. https://doi.org/10.3390/catal13081192

AMA Style

Alsawat M, Alshehri NA, Shaltout AA, Ahmed SI, Al-Malki HMO, Das MR, Boukherroub R, Amin MA, Ibrahim MM. Enhanced Alkaline Hydrogen Evolution on Gd1.0/Ndx (x = 0.5, 1.0, 3.0, and 6.0%)-Doped TiO2 Bimetallic Electrocatalysts. Catalysts. 2023; 13(8):1192. https://doi.org/10.3390/catal13081192

Chicago/Turabian Style

Alsawat, Mohammed, Naif Ahmed Alshehri, Abdallah A. Shaltout, Sameh I. Ahmed, Hanan M. O. Al-Malki, Manash R. Das, Rabah Boukherroub, Mohammed A. Amin, and Mohamed M. Ibrahim. 2023. "Enhanced Alkaline Hydrogen Evolution on Gd1.0/Ndx (x = 0.5, 1.0, 3.0, and 6.0%)-Doped TiO2 Bimetallic Electrocatalysts" Catalysts 13, no. 8: 1192. https://doi.org/10.3390/catal13081192

APA Style

Alsawat, M., Alshehri, N. A., Shaltout, A. A., Ahmed, S. I., Al-Malki, H. M. O., Das, M. R., Boukherroub, R., Amin, M. A., & Ibrahim, M. M. (2023). Enhanced Alkaline Hydrogen Evolution on Gd1.0/Ndx (x = 0.5, 1.0, 3.0, and 6.0%)-Doped TiO2 Bimetallic Electrocatalysts. Catalysts, 13(8), 1192. https://doi.org/10.3390/catal13081192

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