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Keywords:

  • carbon nitride;
  • hydrogen evolution reaction;
  • nanoparticles;
  • tungsten carbide;
  • water splitting

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Supporting Information

Tungsten carbide exhibits platinum-like behavior, which makes it an interesting potential substitute for noble metals in catalytic applications. Tungsten carbide nanocrystals (≈5 nm) are directly synthesized through the reaction of tungsten precursors with mesoporous graphitic C3N4 (mpg-C3N4) as the reactive template in a flow of inert gas at high temperatures. Systematic experiments that vary the precursor compositions and temperatures used in the synthesis selectively generate different compositions and structures for the final nanocarbide (W2C or WC) products. Electrochemical measurements demonstrate that the WC phase with a high surface area exhibits both high activity and stability in hydrogen evolution over a wide pH range. The WC sample also shows excellent hydrogen oxidation activity, whereas its activity in oxygen reduction is poor. These tungsten carbides are successful cocatalysts for overall water splitting and give H2 and O2 in a stoichiometric ratio from H2O decomposition when supported on a Na-doped SrTiO3 photocatalyst. Herein, we present tungsten carbide (on a small scale) as a promising and durable catalyst substitute for platinum and other scarce noble-metal catalysts in catalytic reaction systems used for renewable energy generation.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Supporting Information

Production of clean energy from renewable resources remains one of the greatest challenges for society. Hydrogen is considered a promising energy carrier because of its high energy density. The low-cost, efficient production of hydrogen in a sustainable manner remains a scientific and technological challenge.1 One of the most economical ways to harness hydrogen is from the photocatalytic overall water splitting (OWS) reaction by using solar energy, which does not leave a carbon footprint.2 This process requires a semiconductor with suitable band positions and surface active sites (cocatalysts) to enhance electrochemical redox reactions for water splitting.3 The efficiency of photocatalysts has yet to be improved for commercial applications.

During the overall splitting of water, the hydrogen evolution reaction (HER) should take place efficiently; this requires a catalyst that minimizes the activation energy of hydride formation on the surface as well as the formation of a molecule of diatomic hydrogen.4, 5 It is well known that platinum-group metals (Pt, Rh, Pd, etc.) are excellent HER electrocatalysts that possess appropriate bonding energy with adsorbed hydrogen; thus providing a low overpotential for the HER.6 To achieve the OWS, the surfaces of the catalysts should also be insensitive to the back-reaction of H2 and O2 producing H2O, that is, the oxygen reduction reaction (ORR). One successful method to achieve OWS is to coat metal particles with a Cr layer, thereby forming Cr/metal core/shell particles on the semiconductor.7 A study with a model electrode revealed that the Cr layer functioned to selectively permeate proton and hydrogen molecules, but not oxygen molecules.8 Once the metal particles were coated with the Cr layer, the OWS activity correlated well with HER electrocatalysis when using n-type GaN:ZnO as the photocatalyst.2 Although there are other factors to take into account, such as the semiconductor–cocatalyst interface junction (e.g., Schottky barrier),2, 9 the development of suitable cocatalysts for OWS is still necessary. It also remains challenging to develop highly active HER cocatalysts that have a low cost and utilize abundant materials.10

There are many approaches to develop efficient alternatives to noble-metal catalysts, such as bimetallic systems,11 core/shell structures,12, 13 and different transition-metal compounds.14 Among them, some sulfides and carbides, such as molybdenum disulfide10 and tungsten carbide,15 have been reported to exhibit interesting HER performance. Tungsten carbide was first reported to perform with platinum-like behavior in the catalysis of hydrogenolysis by Boudart’s group in the 1970s.16 Later, tungsten carbide was reported to show activity not only for the HER in acidic and neutral conditions,17 but also for the hydrogen oxidation reaction (HOR); thus confirming the potential of this material as a platinum substitute for electrocatalytic applications in polymer electrolyte membrane fuel cells and direct methanol fuel cells.18

Catalyst particles in the nanometer regime have many active sites on their surfaces, as well as distinct geometric effects with deeply unsaturated coordination sites and quantum confinement effects, which can provide excellent catalytic performance.19 However, to date, most catalytic studies with tungsten carbide reported the formation of relatively large particles with a low surface area.2022 If novel active materials, such as carbides, can be synthesized on a small scale (typically smaller than 5 nm) by developing new synthetic routes, the tailored materials could potentially be used as substitute noble metals in HER/HOR catalysis.

Some methods in the literature can be used to obtain nanosized carbide materials, such as high-temperature treatment of microporous carbon materials with metal precursors23 and chemical vapor deposition techniques to prepare carbon-encapsulated metal nanoparticles.24 A novel synthetic route utilizes organic compounds, such as urea,20 as carbon sources and also utilizes transition-metal precursors. In particular, Antonietti’s group developed a simple method to control Mo and W carbide and nitride phases, depending on the molar ratio of the metal precursor to urea.20 In their study, the variation in the urea-to-metal ratio led to phase variation from carbides to nitrides. In contrast, the controlled synthesis of early transition-metal nitride nanoparticles has also been reported based on the confinement of ordered pores present in the mesoporous graphitic carbon nitride (mpg-C3N4) used as the reactive template.2527 The ordered pores reflect the size of the original SiO2 nanoparticles as a template for mpg-C3N4 synthesis and strictly determine the size of the final nitride particles.26, 28

In this study, attempts to synthesize tungsten nanoparticles by using mpg-C3N4 as the reactive template led to the formation of tungsten carbide nanoparticles rather than their nitride counterparts. It was possible to control the final product phase based on the precursor ratio (metal to mpg-C3N4) and temperature applied during the synthesis. The electrocatalytic activity of these tungsten carbide nanoparticles was studied in the HER, HOR, and ORR. The experimental results demonstrated that the WC nanoparticles showed excellent performance in the HER but low activity for the ORR, which suggested that the WC may have functioned as a selective cocatalyst for the HER during OWS without introducing the back-reaction. Thus, the possibility of using nanosized tungsten carbide as a cocatalyst for photocatalytic OWS was explored. The results suggest that these nanoparticles may be candidate cocatalysts in the OWS.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Supporting Information

Synthesis and decomposition of mpg-C3N4

Figure S1 in the Supporting Information shows the isotherm acquired from the sorption experiment performed on the mpg-C3N4 template (inset) as well as the average pore size distribution calculated in the same experiment. The graph in the inset shows a hysteresis loop in the plot and reflects a type IV isotherm associated with the capillary condensation process in the structures of the mesopores, according to IUPAC classification. The specific BET surface area obtained was 248 m2 g−1, which was consistent with the formation of mesopores specifically designed by using colloidal silica as a hard template.25, 26 From the results in Figure S1 in the Supporting Information, a high value of the average pore volume (0.51 cm3 g−1) was obtained based on a Barret–Joyer–Halenda (BJH) plot, and the average desorption pore size was 64 Å with a narrow distribution, which accurately reflected the size of the original silica template used in the synthesis. Table S1 in the Supporting Information presents the textural properties discussed above and the elemental analysis obtained for the mpg-C3N4 reactive template used in this investigation. Elemental analysis gave a lower carbon to nitrogen ratio (≈0.63) when compared with the stoichiometry of C3N4 (0.75), which suggested a nitrogen-rich carbon nitride sample, possibly because the surface of the sample was mainly C-NH2, 2C-NH, and hydrogen-bonded OH groups or absorbed water molecules, as corroborated by spectroscopy studies and discussed below.

A FTIR spectrum of the mpg-C3N4 template is presented in Figure S2 in the Supporting Information; the FTIR spectrum of melamine is also displayed for comparison. The formation of the mpg-C3N4 structure was confirmed by the existence of a graphite-like sp2-bonding state and adsorption peak at 810 cm−1, which corresponded to the breathing mode of triazine (out-of-plane ring-bending vibration mode).29, 30 Characteristic stretching vibration signals of the tri-s-triazine heterocyclic rings are present in the range of 1200–1600 cm−1 with peak maxima at 1233, 1405, and 1565 cm−1.31, 32 The bands observed at 1317 and 1610 cm−1 are related to the C(sp2)[BOND]N and C(sp2)[DOUBLE BOND]N stretching modes, respectively, in a graphitic-type structure.32, 33 The weak signal at 2175 cm−1 can be assigned to the cyano group stretching band, which suggests that the condensation of the tri-s-triazine network is incomplete.34 The broad peaks observed from 2900 to 3400 cm−1 are generally related to the stretching and deformation modes of the residual N[BOND]H components and their intermolecular hydrogen bonding. It is possible to compare this region with the melamine spectrum above 2900 cm−1, where the signals at 3125, 3325, 3415, and 3466 cm−1 are attributed to N[BOND]H stretching vibration modes of the amino groups.35 Residual hydrogen atoms on the edges of the polymeric network bind through C[BOND]NH2 and 2C[BOND]NH bonds and are energetically stable.29 The absorbance of the OH band overlaps in this range and can be attributed to hydrogen-bonded OH groups and absorbed water molecules in the sample.36 Furthermore, there is no trace of the absorption bands for vSi-O-Si at 1000–1200 cm−1,37 which indicates successful removal of silica used for the synthesis of mpg-C3N4.

When mpg-C3N4 was exposed to a high temperature in a flow of inert gas (>900 K), it was completely decomposed into gaseous products, as confirmed by the complete loss of weight measured by thermogravimetric analysis (TGA). CAUTION! High-temperature decomposition of C3N4 causes the formation of hydrogen cyanide and cyanogen gases. During decomposition, a thin, yellow solid film at the cooled outlet of the reactor was recovered as a byproduct from the reaction. The FTIR spectrum of the film is shown in Figure S2 in the Supporting Information. The spectrum shows characteristics almost identical to those of the carbon nitride template, but with essential differences. The recovered byproduct exhibits the same characteristic vibrations for a CN heterocyclic ring and the dominant ring breathing located at 1200–1600 and 810 cm−1,2932 respectively. Similarly, a reflecting broad band above 2900 cm−1 can be attributed to N[BOND]H stretching and deformation modes.35 The absence of absorption bands at 1610 and 1317 cm−1 is an indication of the interrupted non-graphitic-like structure.32, 33 The signal at 2175 cm−1 assigned to a C[TRIPLE BOND]N or N[DOUBLE BOND]C[DOUBLE BOND]N stretching band is also a good indicator of the truncated nature of the network.34 However, the major absorption and broader peaks in the 1100–1250 cm−1 region may reflect new C[BOND]N stretching vibrations associated with a cross-linked structure. The disordered nature of the decomposition product makes it difficult to give unambiguous IR peak assignments.30

Figure 1 A shows the detection of the main gaseous products found when using MS during the temperature-programmed decomposition of the mpg-C3N4 template under a flow of argon. The products that remained gaseous after cooling to room temperature were observed between 705 and 1100 K, as reported previously.30, 32 At 940 K, clear peak maxima of the main signals were detected at 27, 28, and 52 amu; these were assigned to hydrogen cyanide (HCN, MW=27.02 g mol−1), nitrogen (N2, MW=28 g mol−1), and cyanogen (C2N2, MW=52.03 g mol−1), respectively. These results provide evidence for the formation of cyanogen (52 amu) as the main gaseous product formed during mpg-C3N4 decomposition. The recovered polymeric yellow film at the outlet of the reactor indicates the formation of heavier C3Nx species as a result of fragmentation of the tri-s-triazine heterocyclic rings in the carbon nitride material.32 Other nitrogen-containing gases and oxygenated hydrocarbons were not observed during the heating process. No remaining materials were recovered after the decomposition experiments, which clearly suggests that complete volatilization occurred during the decomposition of mpg-C3N4. From the findings of this study, we can derive Equation (1) for the decomposition of mpg-C3N4 at high temperatures and under inert conditions:(1)

  • equation image(1)
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Figure 1. Mass signals obtained A) during the temperature-programmed decomposition of mpg-C3N4 and B) during tungsten carbide nanoparticle synthesis.

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Hydrogen and excess nitrogen may arise from the surface C[BOND]NH2 and 2C[BOND]NH groups, as confirmed by FTIR spectroscopy. Moreover, the stoichiometry of the carbon nitride template produced was calculated from elemental analysis results.

Tungsten carbide synthesis using an mpg-C3N4 template

A reactive hard-templating method using the nanometer-sized confinement encountered in mpg-C3N4 has been reported as favorable for the synthesis of metal nitride nanoparticles. TiN,25 VN,38 GaN,38 Ta3N5,26, 39 and ternary Al–Ga–N and Ti–V–N27 have been successfully obtained. The decomposition reaction of the confining matrix serves as a nitrogen source for the metal source found in the pores of the carbon nitride.28 To the best of our knowledge, carbide materials have not been reported with carbon nitride as the reactive template.

The pores of the reactive template were impregnated with a chloride dialkoxide solution of pentavalent tungsten. A blue solution was prepared from [WCl6] and ethanol, as described in Equations (2) and (3):40(2), (3)

  • equation image(2)
  • equation image(3)

Figure 1 B shows the mass spectra obtained during the attempted synthesis of tungsten carbide materials from the tungsten source immersed in mpg-C3N4 (1:1, WCl6 to mpg-C3N4 weight ratio). The main gaseous products indicated by the 27, 28, and 52 amu signals were assigned as HCN, N2, and C2N2, respectively, as previously observed in the experiments for mpg-C3N4 decomposition. However, the peak maxima were shifted to lower temperatures at about 845 K with the tungsten source when compared with those without tungsten species (at 920 K), as shown in Figure 1 A. This reduction of temperature clearly indicates reactions of mpg-C3N4 with the tungsten precursor [WCl3(OC2H5)2]. Two peaks were observed in the nitrogen signal; the first one corresponded to the reaction of the mpg-C3N4 template with the tungsten precursor and the second one (≈705 K; Figure 1 B) corresponded to the decomposition of intact mpg-C3N4, as indicated by the signal shown in Figure 1 A. A solid white powder recovered at the outlet of the reactor was identified as ammonium chloride (NH4Cl) by X-ray diffraction (XRD) measurements. The crystals were most likely deposited at the outlet walls of the quartz tube as a result of the reaction between HCl and NH3 and originated from the tungsten precursor solution and carbon nitride decomposition, respectively.

A series of tungsten products was synthesized from the reaction with the mpg-C3N4 template under various conditions. First, Figure 2 A shows XRD patterns of the samples synthesized by varying the starting precursor weight ratio ([WCl6]/C3N4 ratio) while keeping the temperature constant at 1223 K. Other variables studied included the volume of ethanol and heating rate, but no significant effects were observed in the product structure as a result of these changes. Alternatively, prominent effects on the product structure were observed with variations in the weight ratio of [WCl6] to mpg-C3N4. A 1:1 ratio gave hexagonal δ-WC with a P6m2 space group (ICSD-77566), as identified in the XRD pattern (1:1 in Figure 2 A), with primary diffraction peaks detected at 2θ=31.5 (d=2.858, WC [0 0 1]), 35.7 (d=2.511, WC [1 0 0]), and 48.2° (d=1.887, WC [1 0 1]). A broad peak was observed between 64.1 and 65.4°, which corresponded to d=1.450 (WC [1 1 0]) and 1.426 (WC [0 0 2]), respectively. In the literature, the δ-WC phase is commonly referred to as α-WC, and this designation is used hereafter.41 As the amount of available carbon increased in the synthesis, a mixture of α-WC and α-W2C phases was obtained when using a 1:2 [WCl6]/C3N4 ratio and only broader peaks assigned to the α-W2C phase were observed at [WCl6]/C3N4 ratios of 1:4 and 1:8 (Figure 2 A). This α-W2C phase has a hexagonal structure with a P3m1 space group (ICSD-77568), with characteristic diffraction peaks observed at 2θ=37.9 (d=2.368, W2C [0 0 2]), 61.8 (d=1.498, W2C [1 1 0]), and 74.9° (d=1.266, W2C [1 1 2]). Finally, when the amount of carbon nitride template was reduced in the synthesis (2:1), a mixture of tungsten carbide phases and tungsten metal was identified. Unstrained cubic W0 (ICSD-43667) with primary diffraction peaks at 2θ=40.2 (d=2.239, W0 [1 1 0]), 58.2 (d=1.583, W0 [2 0 0]), and 73.1° (d=1.292, W0 [2 1 1]) was detected in the diffractogram in conjunction with the characteristic diffractions of the α-WC and α-W2C phases.

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Figure 2. The effects of the tungsten to carbon nitride weight ratio and synthesis temperature on the XRD patterns of the tungsten-based products. A) Variation of the [WCl6] to mpg-C3N4 weight ratio at 1223 K. B) Variation of the temperatures at a 1:1 weight ratio. C) Variation of temperatures at a weight ratio of 1:2. The asterisk (*) indicates the formation of tungsten oxide species (WO2, W18O49, and WO3).

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Figure 2 B shows XRD patterns of the samples synthesized at different temperatures (1073–1373 K), while keeping the ratio of [WCl6]/C3N4 at 1:1. At relatively low temperatures (<1223 K), broad peaks characteristic of hexagonal α-W2C were observed and indicated the formation of small particles, as expected from utilizing the confinement of the mpg-C3N4 pores. Hexagonal α-WC was formed at temperatures above 1223 K, whereas α-W2C was detected only slightly at both 1223 and 1323 K, as the minor phase in the product, and was almost undetectable at 1373 K. It is interesting to note that there was a clear phase transition at 1273 K where the primary diffraction peaks assignable to cubic W0 appeared together with the peaks for the α-WC phase. Some diffraction peaks assigned to WO2 and WO3 are also highlighted with an asterisk (*) in Figure 2; these peaks must have originated from the oxidation of tungsten metal during the passivation process at the end of the synthetic procedure. It was considered that metallic tungsten formed as a result of the eutectoid decomposition of α-W2C, in agreement with the W[BOND]C phase diagram, where metallic tungsten and α-WC could coexist below 1500 K.41 The peaks for α-WC became sharper at higher temperatures, which clearly indicated an increase in particle size because particle aggregation increases at high temperatures. The Scherrer equation was used to approximate the size of the crystallites in the samples42 in a range from 56 to 152 Å in the temperature range of 1223 to 1373 K, as listed in Table 1. This finding is consistent with a gradual growth of particle size with increasing temperatures. The phase assignments of the products obtained with XRD are also listed in Table 1. The results described above can lead to conceptual Equations (4)–(7):(5), (6), (7)

  • equation image(4)
  • equation image(5)
  • equation image(6)
  • equation image(7)
Table 1. Phase assignment from the XRD diffractogram and particle size evaluation from the Scherrer equation of selected samples produced at different temperatures with a constant precursor weight ratio of 1:1.
T [K]Phase from XRDFWHM at 2θ [º][a]Scherrer size [Å]Surface area [m2 g−1][b]
  1. [a] Full width at half maxima (FWHM) evaluation from the line broadening at the specified diffraction peak. [b] Nonporous spherical particles with homogeneous size distribution were assumed for calculations.

1073W2C61.823151
1173W2C61.829120
1223WC W2C48.25668
1273WC W2C W048.26360
1323WC48.27054
1373WC48.215225

Equation (4) should include extra amine/imide groups from C3N4, which form additional ammonia. Equation (5) indicates the disproportionation of α-W2C to form metallic W and α-WC. Metallic tungsten is not stable and it reacts with the remaining carbon or with oxygen (passivation).

The role of the W and mpg-C3N4 precursor ratio during the synthesis of tungsten carbide nanoparticles was explored in experiments in which the mass of mpg-C3N4 was doubled with respect to the amount of the tungsten precursor at different temperatures. It was observed that α-W2C initially formed at temperatures above 1073 K did not transform into α-WC even at 1273 K (Figure 2 C). For all temperatures ranging from 1073 to 1273 K, broad peaks were observed, mainly from α-W2C characteristic diffraction patterns. This result indicates that excess C3N4 (thus leading to excess carbon) suppresses the disproportionation of α-W2C to metallic W and WC [Eq. (5)]. Hereafter, the samples are denoted based on the major phases identified by XRD followed by the temperature used in the syntheses (e.g., W2C-1173, WC-1223, WC-1373).

The X-ray photoelectron spectroscopy (XPS) results of W 4f for the synthesized carbide materials obtained at different temperatures are shown in Figure 3. WO3 (99.9 %, Aldrich) was used as a reference material for the direct comparison of hexavalent oxidation states and gave spectra with binding energies for W 4f7/2 and W 4f5/2 of 35.8 and 38.0 eV, respectively, which was consistent with the values reported in the literature.43 All of the tungsten carbide samples showed W 4f signals above (31.3±0.1) eV, which corresponded to the metallic state of tungsten.21 For α-WC samples synthesized in the range of 1223–1373 K, the W 4f spectra show peaks at binding energies assigned to WC at 32.2 (W 4f7/2) and 34.4 eV (W 4f5/2), which closely agrees with reported values.15, 43, 44 These α-WC samples showed signals emitted from the partially oxidized surface of the materials because of the O2 passivation treatment and unavoidable exposure of the nanoparticles to the ambient conditions. The intensity of the W 4f signals assigned to the WO3 species (35.8 and 38.0 eV) decreased with an increase in temperature, whereas the carbidic W 4f signal intensity (32.2 and 34.4 eV) increased with increasing temperature, which corresponds to the concrete formation of WC, as observed in the XRD patterns (Figure 2 B). The α-W2C produced at lower temperatures (<1223 K) reflected a further reduced state of W with the peak location shifted 0.1 eV lower when compared with the α-WC samples.15 These XPS spectra did not show surface oxidation with any appreciable peaks at 38.0 or 35.8 eV (Figure 3). When α-WC samples were treated under substantial Ar ion sputtering, the intensity of the oxidative phase was diminished, which suggests the removal of consecutive layers of the oxide species without an effect on the W 4f carbidic signals (Figure S3 in the Supporting Information).

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Figure 3. W 4f XPS spectra of tungsten-based products obtained at different temperatures. The spectrum of WO3 is included as a reference for comparison. The metallic tungsten characteristic signal is indicated with an arrow at 31.3 eV.

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Table 2 shows the results (in mass percentages) of elemental analyses (C, H, N) of the synthesized W samples to confirm the formation of the carbide rather than the nitride form of the transition metal. The nitrogen content for all samples (0.1–0.8 %) was well below the possible stoichiometric values for tungsten nitride (7.07 and 3.66 % for WN and W2N, respectively). As listed in Table 2, the carbon content in the samples obtained below 1223 K (6.12 %) exceeded the stoichiometric value (3.16 %), as assigned from XRD (α-W2C). The samples produced at higher temperatures with an assigned stoichiometry of α-WC exhibited closer agreement with the theoretical carbon content. In the WC-1273 sample, the carbon content was 1.9 %, which was well under the stoichiometric value and supports earlier observations from XRD and XPS where a phase transition was observed at 1273 K and metallic tungsten led to the oxidation of the carbide phase. All of the samples produced at higher temperatures (>1223 K) and identified by XRD as α-WC showed similar carbon contents, which is a good indication of the reproducibility of the procedure and is in reasonable agreement with the results observed by XRD and shown in Figure 2 B.

Table 2. Elemental analyses and calculated BET surface areas of tungsten carbide products obtained at varying temperatures, while maintaining a precursor weight ratio of 1:1.
Sample[a]WeightElement [%]BET surface area
 ratio[b]CNH[m2 g−1]
  1. [a] The sample ID indicates the assigned phase from the XRD diffractogram and the synthesis temperature in K at which the product was obtained. [b] Precursor weight ratio ([WCl6]/mpg-C3N4).

W2C-10731:213.30.80.2145
W2C-11731:218.20.70.2141
W2C-1223A1:822.31.00.6
W2C-1223B1:418.80.80.6
W2C-1223C1:212.80.60.3125
WC-12231:18.30.60.1104
WC-12731:11.90.10.160
WC-13231:18.80.40.185
WC-13731:18.30.40.180

To further characterize the tungsten content in the resulting tungsten carbide samples, thermogravimetric analysis coupled with differential scanning calorimetry (TGA/DSC) was utilized to simultaneously monitor the heat flow in the experiment in addition to the weight change. For a WC-1373 sample under a flow of air (Figure S4 in the Supporting Information), the mass of the sample increased throughout the heating process until it was 16.5 % higher than that of the original product. The final theoretical stoichiometric mass gain of 18 % (for α-WC) is represented in Figure S6 in the Supporting Information by a dashed line. The 1.5 % difference correlates to the findings observed in elemental analysis where excess carbon was present in the sample. There are two subtle shoulders in the weight signal during the weight increment process, which is also reflected in the heat flow signal as two exothermic peaks with maxima at 713 and 800 K. The first peak can be attributed to the partial oxidation of the material towards possible intermediate mixed-valence species (i.e., WO2, W2O3, and/or W18O49) with the release of COx.45 The main peak represents the full oxidation of the sample towards WO3, as observed by the color change from black to bright yellow at the end of the experiment. The tungsten mass content in the original sample could be calculated from this experiment to be approximately 92.5 %, which is in close agreement with the stoichiometric value of 94 % for WC and further confirms the results obtained from elemental analysis and XRD.

Nitrogen sorption experiments were performed on the samples to obtain the BET surface area. Table 2 compiles the values of the surface areas of the materials. The surface area of the products decreased with increasing temperature. Higher surface areas were attained for W2C products compared with the WC products. The α-WC sample with the highest surface area (≈104 m2 g−1) was obtained at 1223 K with the precursor weight ratio maintained at 1:1. However, it is worth noting that the measured BET surface area for the tungsten carbide nanomaterials may be affected by the presence of excess carbon, especially at low temperatures where the amount of carbon is higher (>12 %, Table 2). As evaluated from the XRD diffractogram by using the Scherrer equation, the nanocrystallite size was 6–15 nm at higher temperatures (1223–1373 K) and about 2–3 nm at lower temperatures (1073–1173 K; Table 1). By using these sizes and assuming nonporous spherical particles with a homogeneous size distribution, the surface area of the W2C or WC particles can be approximated, as shown in Table 1. The surface areas obtained were slightly lower than the measured BET surface areas and the difference between these calculations may have originated from the presence of carbon and the error generated from the assumption of sphericity.

Figure 4 shows the results obtained from TEM investigations on W2C-1073 (A, B), WC-1223 (C, D) and WC-1373 (E, F). Low- and high-magnification micrographs are shown for representative samples obtained at different temperatures. As observed in Figure 4 A, C, and E, the particle size increases with increasing temperature, which is in agreement with the results observed from XRD and calculated from the Scherrer equation. The W2C-1073 sample exhibited a uniform particle size distribution (≈5 nm) that accurately reflected the original pore size of the mpg-C3N4 template, as observed in Figure 4 A. The HRTEM image of the WC-1223 sample showed a greater extent of aggregation and sintering, with nanoparticle sizes up to 10 nm (Figure 4 C). Finally, the samples obtained at an even higher temperature (WC-1373) showed sizes up to 25 nm. The HRTEM images on the right (Figure 4 B, D, and F) allowed us to observe the crystalline nature of the tungsten carbide nanoparticles produced in detail. It was possible to observe the remaining carbon present in the samples, as indicated by elemental analysis and TGA. Although it was not possible to quantify the amount of carbon present, energy-dispersive X-ray (EDX) spectroscopy was performed to confirm the chemical composition of the crystalline nanoparticles. A line profile of EDX spectroscopy was performed to study the local environment for a single nanoparticle observed in scanning transmission microscope (STEM) mode; the result shows that the nanocrystal is composed of tungsten and carbon; moreover, carbon was also observed in the surroundings of the nanoparticles (Figure S5 in the Supporting Information).

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Figure 4. High-resolution transmission electron microscopy (HRTEM) images of W2C-1073 (A, B), WC-1223 (C, D), and WC-1373 (E, F). The [WCl6] to mpg-C3N4 weight ratio used for the synthesis was 1:2 (A, B) and 1:1 (C–F).

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In summary, we have shown that by tuning the synthesis conditions, we are able to obtain materials with the desired tungsten carbide phase and a high surface area. This synthetic method led to the formation of extra carbon in addition to the tungsten phase, which plays an important role in inhibiting transformation of the α-W2C phase to the α-WC phase (Figure 2 B). As shown in the MS experiments, the tungsten precursor [WCl3(OC2H5)2] reacted with mpg-C3N4 inside the nanoconfinement of the mesoporous template to control the particle size of the “seeds” at relatively low temperatures (800–1000 K; Figure 1 B). When the synthetic conditions were optimized to obtain α-WC at higher temperatures (>1200 K), only α-WC was present in the final product, as confirmed by XRD, XPS, TGA, and elemental analysis. The high-surface-area tungsten carbide obtained could be utilized as a catalytically active site and, in this study, we investigated the electrochemical activity in water redox reactions.

Tungsten carbide tungsten carbide as an electrocatalyst

The electrochemical stability of WC under acidic conditions at room temperature was evaluated for the WC-1223 sample. A cyclic voltammetry (CV) experiment with a wide potential window (0.05–1.05 V vs. a reversible hydrogen electrode (RHE)) was performed to evaluate the stability of tungsten carbide materials. Tungsten carbide showed a major oxidation peak starting at 0.6 V versus RHE, which was consistent with similar observations found in the literature (Figure S6 in the Supporting Information).46 The oxidation peaks completely vanished after several cycles, which indicates the irreversible nature of the WC oxidation process. Great care must be taken to retain the original nature of the carbide structure to be able to correctly evaluate and compare the electrocatalytic performance of the materials.

The WC-1223 sample was tested for HOR activity in acidic solution and the results were comparable to those obtained for 40 % Pt/CB. The polarization curve of the WC-1223 material is presented in Figure 5 A. The HOR was evaluated by using a rotating disk electrode (RDE) with the rotation speed, ω, varying from 400 to 2500 rpm. For WC-1223, the HOR current was primarily controlled by a region of mixed kinetic/diffusion control at overpotentials up to 0.15 V versus RHE. Diffusion-limiting currents were prominent above 0.2 V versus RHE for curves obtained under rotational speeds below 900 rpm. It is worth noting that a wider overpotential region (0–0.3 V vs. RHE) can be used for kinetic analysis in the 2500 rpm polarization curve (Figure 5 A) where the measured current is primary kinetically controlled. Platinum polarization curves showed that diffusion-limited currents were rapidly obtained at 0.06 V versus RHE (Figure S7 in the Supporting Information). The results indicated that the HOR anodic currents were kinetically controlled in a very narrow potential range (0–0.05 V vs. RHE), in agreement with the literature.47 The linearity and intercept to zero observed in the inset of Figure 5 B proved that at sufficient overpotentials (i.e., 150 mV vs. RHE) the current is mainly diffusion-limited and proportional to the square root of ω.48 This total mass-transfer-limited condition is described by the Levich equation in the form shown in Equation (8):

  • equation image(8)
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Figure 5. A) HOR polarization curves on WC-1223 at varying rotation rates obtained by using a RDE. B) HOR Koutecký–Levich plots for WC-1223 obtained at varying overpotentials (mV vs. RHE). The inset shows the Koutecký–Levich plot for 40 % Pt/CB obtained at 150 mV versus RHE. C) Tafel plots using mass-transport-corrected HOR currents of the WC-1223 sample and 40 % Pt/CB (H2-saturated 0.5 M H2SO4, 50 mV s−1, 298 K).

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in which iL is the Levich current, B is the Levich constant, n is the number of electrons transferred in the half reaction, F is the Faraday constant, A is the surface area, D is the diffusion coefficient (298 K, 3.7×10−5 cm2 s−1, estimated from the product of H2 diffusivity at infinite dilution and the ratio of the dynamic viscosities of the electrolyte and pure water),47 ω is the angular rotational speed, υ is the kinematic viscosity (298 K, 1.2×10−2 cm2 s−1), and C0 is the H2 saturation concentration (298 K, 7.14×10−4M).49 From the 40 % Pt/CB experimental data obtained at 0.15 V versus RHE (shown in the inset of Figure 5 B), the least-squares regression fit returned a slope of BC0=6.032×10−3 mA rpm−0.5. At sufficient overpotential and at a specific rotational speed it is possible to estimate the surface area of the catalyst from Equation (8). Thus, the platinum surface area calculated from the Equation (8) was 0.0304 cm2. When a kinetic limitation is involved in the electron transfer reaction, as in the case for tungsten carbide (Figure 5 A), the currents at the RDE are described by the Koutecký–Levich equation [Eq. (9)]:

  • equation image(9)

in which iK represents the current in the absence of any mass-transport effects and, hence, the current is determined only by kinetic limitations. From Equation (9), the plot of the inverse of the current as a function of the inverse of the square root of the rotational speed will produce a straight line with an intercept corresponding directly to the kinetic current iK−1 and a slope of (BC0)−1. Figure 5 B depicts the Koutecký–Levich plots for tungsten carbide (WC-1223) at different overpotentials (mV vs. RHE). There is a noticeable contribution of kinetic limitation over a wide range of overpotentials (0.025–0.2 V vs. RHE). Non-zero intercepts shown in Figure 5 indicate that the rate of the electron-transport process was the limiting factor. However, at sufficient overpotentials (0.4 V vs. RHE), we can consider the kinetic component negligible and it is possible to assume complete diffusion-limiting conditions. From the least-squares regression lines obtained from Figure 5 B, the average slope observed from 0.075 to 0.4 V versus RHE was BC0=3.08×10−3 mA rpm−0.5. Similarly to the platinum calculations, the estimated tungsten carbide surface area was calculated to be 0.015 cm2. The mass-transport-corrected kinetic current was normalized by using the calculated surface area approximations from the Equation (9). Hence, diffusion-corrected Tafel plots of the polarization curves obtained from 40 % Pt/CB (2500 rpm) and WC-1223 (1600 rpm and 2500 rpm) are presented in Figure 5 C. The kinetic current was independent of the rotation rate above 1600 rpm, as observed in Figure 5 C, which ensures the accuracy of iK determination for WC-1223 in this rotation range. From the Tafel analysis (η=a+b log j) in Figure 5 C, the exchange current density (j0) can be extrapolated from the Tafel curve linear region to an intercept log j0 and represents the electrochemical reaction rate at equilibrium.48, 50 This kinetic parameter can be described as any system’s capacity to facilitate a net current (reaction rate) without incurring significant energy losses from activation.48 The Tafel slope (β) can be empirically calculated from the polarization curve between specific overpotentials (η), which provides insight into the possible mechanism of the reaction (see below). In the literature,6, 51, 52 three elementary reaction steps depict the mechanism for hydrogen oxidation/evolution on solid-state catalytic sites:

  • 1.
    The dissociative adsorption of hydrogen molecules on the catalyst surface, known as the Tafel reaction or recombination reaction for hydrogen evolution [Eq. (10)].(10)
    • equation image(10)
  • 2.
    The direct reaction of hydrogen molecules through the Heyrovsky reaction, also known as the ion-plus-atom reaction [Eq. (11)].(11)
    • equation image(11)
  • 3.
    The ionization of the H atom to produce one electron and one hydronium ion in solution, which results in an empty active site in the reaction, known as the Volmer reaction [charge-transfer reaction; Eq. (12)].(12)
    • equation image(12)

In the elementary steps mentioned above, Hads represents atomic hydrogen chemisorbed to a metal surface (i.e., platinum) and the symbol * denotes a free adsorption site. From Equations (10) to (12), it is possible to theoretically calculate the expected Tafel slope from surface coverage assumptions (see the Supporting Information). For the HER, the first common step is represented by the Volmer equation or discharge reaction [Eq. (12)]. Then there are two possibilities, either electrochemical desorption [Eq. (11)] or a recombination step [Eq. (10)]. The reversible reactions (HOR) follow a Tafel–Volmer or Heyrovsky–Volmer mechanism. As mentioned previously, platinum is one of the most studied catalysts for the HOR and HER in the literature and exchange current densities are difficult to obtain because of the fast kinetics and lack of accuracy during correction for mass-transport limitations in most cases. However, several groups have found that the Pt(110) surface is the most active for both the HOR and HER because it exhibits Tafel slopes of approximately 30 mV dec−1, which correspond to a Tafel–Volmer mechanism [Eqs. (10) and (12)].47, 50, 53 The recombination reaction or Tafel step is the rate-determining step (RDS), as reflected at low overpotentials with a measured Tafel slope of 28 mV dec−1 for the HOR in Figure 5 C (40 % Pt/CB), which is also in agreement with the findings for well-characterized polycrystalline Pt47, 54 and the theoretical value of 30 mV dec−1 obtained from Equation (10) [Eq. (13)].(13)

  • equation image(13)

in which R is the ideal gas constant, T is the absolute temperature, and F is the Faraday constant. When the electrochemical desorption step [Eq. (11)] is slow then Equation (14) is valid:(14)

  • equation image(14)

in which α is the transfer coefficient. Finally, if Equation (12) is the RDS or the rates of Equations  (11) and (12) are comparable, then a high slope value is obtained from Equation (15):

  • equation image(15)

The HOR Tafel plot obtained for tungsten carbide seems to have two apparent Tafel slopes of 43 (0.1–0.4 V vs. RHE) and 150 mV dec−1 (0.4–0.7 V vs. RHE; Figure 5 C). There is no clear linear region in the kinetic HOR polarization curve, which possibly indicates a potential-dependent reactive intermediate coverage.49 Moreover, polarization of the HOR cannot be simplified into a single linear Tafel equation and the kinetic parameters in the Tafel region are difficult to accurately determine. Nevertheless, on the basis of the Tafel slopes obtained, the WC-1223 sample exhibited a Heyrovsky–Volmer HOR mechanism with the ion–atom reaction as the RDS [Eqs. (11) and (12)]. At higher potentials, the rates of the electrochemical desorption [Eq. (11)] and the discharge reaction [Eq. (12)] may become comparable, which makes it difficult to define the RDS. However, the calculated slopes (>120 mV dec−1) indicate that at sufficiently high overpotentials the discharge reaction [Eq. (12)] may act as the RDS.

The catalysts synthesized at different temperatures were also examined for activity in the HER in 0.5 M H2SO4 at 298 K by using a RDE to achieve steady-state current conditions without diffusion effects on the measurements. Figure 6 A gives the voltammograms of tungsten carbide synthesized at different temperatures along with voltammograms of Pt and GCE. The acquired currents were tentatively normalized by the surface area calculated from the Levich analysis performed earlier for the HOR on WC-1223. The electrochemically active surface area of the platinum catalyst was calculated to be similar to the geometric area of the electrode (0.07 cm2). Cathodic currents attributed to the HER were observed for all carbide samples with an onset potential of approximately −0.1 V versus RHE. The WC-1223 sample exhibited the highest HER current among all of the samples with a hydrogen evolution overpotential below 100 mV versus RHE and it thus outperformed the rest of the samples. Figure 6 B shows Tafel plots for the HERs of the W2C-1073 and WC-1223 samples. Pt was an excellent catalyst for the HER, since it produced an exchange current density of j0≈1 mA cm−2. Among the synthesized tungsten-based nanoparticles, WC-1223 had the highest activity with j0=0.35 mA cm−2, followed by the W2C-1073 sample with an exchange current density of j0=0.28 mA cm−2, which is consistent with the polarization curves exhibited in Figure 6 A. However, it should be noted that for tungsten carbide materials, the exchange current density was normalized by the estimated surface area from Koutecký–Levich experiments rather than the electrochemically active surface area as for the platinum electrode. The WC-1223 sample exhibited a Tafel slope of 84 mV dec−1 (0.1 to 0.15 V vs. RHE), whereas the W2C-1073 sample exhibited a slope of 102 mV dec−1 (0.13 to 0.19 V vs. RHE). The results indicate a Volmer–Heyrovsky HER mechanism catalyzed by the tungsten carbide samples (WC-1223) with a rate-determining electrochemical desorption step from the ion to the atom reaction, as shown in Equations (10)–(12).51, 52, 55 When the tungsten carbide phase is different (W2C-1073), the HER mechanism changes to the extent that the discharge step or the Volmer reaction becomes rate limiting. The high values for the Tafel slope most likely resulted from variation in the surface coverage of the adsorbed hydrogen. Thus, it is possible that the rates of the discharge reaction and the electrochemical desorption are comparable, which then results in intermediate values of Tafel slopes between 40 and 120 mV dec−1. As seen from the results in Table 3, a steeper Tafel slope was obtained for the samples where WC was the major phase present, which indicates a clear difference in the reaction mechanism pathways for different carbide phases (W2C and WC). The higher slopes obtained in the HOR correlate with the values obtained for the HER, which indicates that the reversible hydrogen reaction may proceed by the same mechanism with a similar RDS. The higher current density observed for the WC-1223 sample (Figure 6 A) could be attributed to the monocarbide phase of the sample (as resolved by elemental analysis, XRD, TGA, and XPS) in combination with the high surface area of the material (Table 2). α-WC exhibits better hydrogen evolution performance than α-W2C under acidic conditions even though α-W2C has a higher surface area. This trend is explained by a volcano plot of exchange current as a function of the hydrogen binding energy of the material, developed by Nørskov and co-workers,6 and is consistent with recent DFT and experimental studies on transition-metal carbides.15 Therefore, the Pt-like electronic structure allowed α-WC to show higher activity than any other sample in this study, even though the α-W2C samples exhibited a higher surface area. However, as the synthesis temperature increased, the HER current decreased along with the decrease in the surface area of the samples because of the reduction in the number active sites available for hydrogen evolution.

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Figure 6. A) HER voltammograms of tungsten carbide samples synthesized at different temperatures (W2C-1073, W2C-1173, WC-1223, and WC-1273) along with voltammograms of a Pt electrode and a glassy carbon electrode (GCE). B) HER Tafel plots for W2C-1073, WC-1223, and Pt electrode. C) The stability test of the WC-1223 sample under a wide range of pH conditions. The first and 800th polarization cycles for the WC-1223 sample are shown in the inset (0.5 M H2SO4, in Ar, 50 mV s−1, 298 K).

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Table 3. HER evaluation at −0.3 V versus RHE and Tafel parameters calculated from kinetic measurements in an Ar-purged 0.5 M H2SO4 solution for selected samples at room temperature with a 50 mV s−1 scan rate.
SampleCurrent density[a] [mA cm−2]η [mV]log (io) [mA cm−2]β [mV dec−1]
  1. [a] At η=0.3 V.

W2C-107319130–1900.28102
WC-122338.5100–1500.3584
Pt97.910–30134

The stability test of the material with the best performance for the HER is shown in Figure 6 C. The first and the 800th polarization cycles are shown in the inset of Figure 6 C and are not significantly different; hence, the WC-1223 carbide sample demonstrated negligible current loss after 800 charging–discharging cycles between −0.3 and 0.1 V versus RHE in a 0.5 M H2SO4 solution at 298 K. This finding clearly indicates the high stability of the carbide material under acidic conditions. The activity in the HER was also evaluated under neutral and alkaline conditions and compared with that of the platinum catalyst. Figure 6 C shows the polarization curves obtained under different pH conditions and the results were compared with those obtained using 40 % Pt/CB (Table S2 in the Supporting Information). The onset potential for nonacidic conditions was very similar at −0.15 V versus RHE, although the current was lower than that under acidic conditions. As presented in the Supporting Information, the trend for WC-1223 was similar to that of 40 % Pt/CB; the activity decreased to 20 % of the original performance at neutral pH and then recovered to approximately 30 % of the activity observed under acidic conditions at pH 13. This result relates the electronic structure of tungsten carbide, which is similar to that of platinum, with the HER behavior of platinum, as previously discussed.1517, 21, 46

Figure 7 shows the experiments performed for the ORR on W2C-1173, WC-1223, and WC-1373 samples, and a platinum electrode. In this case, the recorded currents for the materials were normalized using the geometrical surface area of the electrode (jgeom). Special care must be taken when evaluating the ORR on tungsten carbides to prevent oxidation of the catalyst. As previously discussed, tungsten carbide oxidation was observed at positive potentials higher than 0.6 V versus RHE (Figure S6 in the Supporting Information). Therefore, the polarization experiments were performed only for positive potentials below 0.5 V versus RHE to ensure that the catalytic nature of the material was unaltered. As shown in Figure 7, the ORR activity for all of the tungsten-based samples was low and cathodic currents attributed to the ORR were observed at potentials as low as 0.3 V versus RHE for the best tungsten carbide sample. The WC-1223 sample showed the highest activity towards the ORR among the other samples tested. As expected, the platinum catalyst exhibited excellent performance towards the ORR with an onset potential of 0.9 V versus RHE, in contrast to tungsten carbide with 0.3 and 0.1 V versus RHE for WC-1223 and W2C-1073, respectively. The reduced ORR activity observed for tungsten carbide materials was viewed as an advantage for an overall water-splitting application, as further discussed below.

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Figure 7. Polarization curves in saturated O2 obtained for W2C-1173, WC1223, WC-1373, the Pt electrode, and GCE (0.5 M Na2SO4, pH 3.6, 50 mV s−1, 298 K).

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Photocatalytic overall water splitting

As described previously, Pt-group metal surfaces catalyze proton reduction, but are also highly active in inducing the back-reaction from H2 and O2 to water (ORR).56 In the literature, the Ni/NiO57, 58 core/shell structure and RuO259 have been extensively utilized as successful cocatalysts for OWS. Recently, a chromium shell was studied as a possible selective membrane to prevent O2 permeation to the metal surfaces.7 Apparently, the core/shell structure of the chromium-based layer protects the metallic nanoparticle surfaces to suppress the ORR (back-reaction), while maintaining hydrogen evolution (i.e., proton reduction).8 Moreover, photocatalytic OWS in a particulate-type system has been achieved only on metal/metal oxide surfaces with a core/shell structure. Therefore, from the results obtained from the HER and ORR experiments where WC materials catalyzed hydrogen evolution but revealed poor performance for the ORR, the samples were considered to be potential cocatalysts for the water-splitting reaction.

OWS was attempted under varying loading conditions on a Na-doped SrTiO3 (STO) photocatalyst, and the results are shown in Figure 8 A. This photocatalyst was active for OWS under UV irradiation (photon distribution in Figure S8 in the Supporting Information) when impregnated with Rh and Cr species, giving about 300 μmol h−1 H2 and about 150 μmol h−1 O2, as discussed elsewhere.56 The photocatalyst with 2.75 wt % WC/STO:Na showed the highest activity with the production of stoichiometric amounts of hydrogen and oxygen in the experiment. HRTEM images are presented in Figure 8 B and show a representative tungsten carbide particle (WC-1373) supported on the WC/STO:Na photocatalyst. When the loading of tungsten carbide on the semiconductor photocatalyst STO:Na reached 10 wt %, the hydrogen production rate decreased and oxygen gas was not observed as a product, like in the case of unmodified STO:Na. The lack of oxygen production was indicative of the oxidation of impurities or surface hydroxyl groups. OWS was achieved at low oxygen and hydrogen partial pressures in the WC/STO:Na system. Tungsten carbide showed reasonably high electrochemical hydrogen evolution activity, while exhibiting poor oxygen reduction performance, which may explain the system’s ability to catalyze OWS. WC was used as a cocatalyst for photocatalytic water splitting and functioned as a hydrogen evolution site, while preventing the back-reaction in a dual-role system. The similarity of the electronic structure of tungsten carbide to that of platinum may be reflected in its ability to improve charge separation and serve as a hydrogen-evolution active site. It is also possible, as demonstrated by XPS, that tungsten carbide undergoes oxidation on its surface to prevent back-reactions, while reflecting the low activity observed in the OWS experiments. Although further optimization of the synthetic and reaction conditions is needed, WC shows promise in the search for non-noble-metal cocatalysts for OWS. Furthermore, these findings show that tungsten carbide can be used for water splitting without a core/shell structure, which opens up new possibilities in the design of cocatalysts for photocatalytic OWS.

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Figure 8. A) OWS experiments performed on a recirculating reactor unit by using the STO:Na photocatalyst with varying loadings of the WC-1373 sample (300 W Xe lamp containing UV light, 100 mL milli-Q H2O, 50 mg photocatalyst). B) HRTEM micrographs of a) 1.25 wt % WC-1373/STO:Na; b) 5 wt % WC-1373/STO:Na.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Supporting Information

We successfully synthesized tungsten carbide nanoparticles with a size of approximately 5 to 15 nm from the confinement of the pores of a mpg-C3N4 template. Our results showed the formation of carbide nanoparticles, rather than nitride nanoparticles, from the carbon nitride template and indicated the role of mpg-C3N4 as a carbon source. The phase transformation of tungsten carbide was clarified by XRD studies. Two phases (α-WC and α-W2C) could be obtained from this synthetic procedure by optimizing the precursor weight ratio and synthesis temperature. We showed that was possible to control the desired phase in the product and to obtain carbide particles in the nanometer range. The WC phase could only be obtained at temperatures above 1223 K when the precursor weight ratio was maintained at 1:1. W2C was always produced below this temperature, regardless of the precursor ratio. The synthesis of the carbon nitride template was evaluated by means of MS and the main gaseous products produced during the synthesis of WC were identified as cyanogen gas, hydrogen cyanide, and nitrogen gas. High-surface-area WC (≈104 m2 g−1) samples were obtained and the carbidic phase was confirmed by elemental analysis, XRD, XPS, and TGA. Electrochemical studies showed that the WC-1223 sample exhibited the highest and most stable HER activity in an acidic medium among the samples prepared in this study. The same sample also demonstrated the ability to catalyze HOR and HER with good catalytic kinetics. Interestingly, tungsten carbide showed poor catalytic activity in the ORR, which makes it a potential cocatalyst candidate for photocatalytic water splitting. OWS was achieved under UV irradiation by loading the WC-1373 sample onto a Na-doped STO photocatalyst. Stoichiometric amounts of hydrogen and oxygen were observed with different loading conditions; 2.75 wt % loading gave the highest overall water-splitting rate. Although the performance of our sample is not yet comparable to that of platinum, this study introduces the great potential of tungsten carbide as a non-noble-metal alternative for use as a cathode catalyst in water electrolysis, as an anode catalyst in polymer electrolyte membrane fuel cells, and as a dual-role cocatalyst in OWS.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Supporting Information

Mesoporous graphitic carbon nitride synthesis: The reactive template mpg-C3N4 was synthesized as previously reported.26 Briefly, carbon nitride was synthesized from cyanamide (CA, 99 % Aldrich) and an aqueous colloidal silica suspension composed of silica nanoparticles (LUDOX SM-30, 30 wt %, Aldrich; ≈6 nm average diameters). The weight ratio of SiO2 and CA was maintained at 1:1 for all the samples. Usually, CA was dissolved in the required volume of colloidal silica and the suspension was slowly heated to 353 K until a solid homogeneous product was obtained. The solids were placed in a closed alumina crucible and heated under static air at 823 K for 4 h with a heating rate of 2.3 K min−1. A 4 M ammonium bifluoride (NH4HF2, ≥98.5 % Fluka) aqueous solution was utilized to remove silica from the final product, followed by a washing procedure with H2O (18.2 MΩ cm−1) and ethanol (≥99.8 % Aldrich).

Tungsten carbide nanoparticle synthesis: In a typical synthesis, [WCl6] (0.5 g; ≥99.9 % Aldrich) was dissolved in ethanol (0.56 mL). The metal precursor reacted vigorously with ethanol to produce hydrochloric acid and form a stable metal–orthoester precursor. Sonication of the mixture was used to aid the rapid solubilization of the metal precursor. Then, a drop-impregnation-type process was performed on 0.5 g of the mpg-C3N4 template to keep the weight ratio of the precursor compounds at 1:1. The final composite precursor was placed in an alumina boat crucible for heating under a flow of N2 (100 mL min−1). The temperature was varied from 1073 to 1373 K. Other synthetic variables investigated included heating rate, concentration of precursor solution, pore size of the carbon nitride templates, and carbon-to-metal precursor ratio. CAUTION! High-temperature decomposition of C3N4 causes the formation of hydrogen cyanide and cyanogen gases.

Sodium-doped strontium titanate synthesis: The UV-responsive photocatalyst STO:Na was synthesized by using the flux-assisted method, as reported in a recent review.56 The precursors used in the synthesis were strontium carbonate (SrCO3, 99.9 % Aldrich) and titanium oxide anatase (TiO2, 99.9 % Aldrich) in a 1:1 molar ratio. Strontium chloride hexahydrate (SrCl26 H2O, 99.9 % Aldrich) molten salt was used in excess as the reaction medium and sodium carbonate (Na2CO3, 99 % Aldrich) was introduced to induce sodium doping into the final semiconductor crystal. In a typical synthesis, 50 mmol of SrCl26H2O, 5 mmol of SrCO3, 5 mmol of TiO2, and 0.125 mmol of Na2CO3 were placed in that order into a mortar. The reagents were ground for 20 min until a very fine powder was obtained. The dry mixture was treated under atmospheric conditions at high temperature inside a platinum crucible. The temperature was changed at a rate of 6.25 K min−1 and the final temperature of 1423 K was maintained for 10 h by using a muffle furnace. Later, the product was washed with deionized water (500 mL) and filtered through a 0.45 μm membrane. The collected powder was dried at 363 K for 2 h under vacuum conditions.

Characterization of the templates and catalysts: Samples were characterized by elemental analysis, N2 sorption experiments, MS, FTIR spectroscopy, XRD, XPS, TGA, and TEM. MS was used to determine the decomposition products and probable reaction pathways by using an OMNI Star (GSD 320 O1) portable mass spectrometer with a tungsten filament from Pfeiffer Vacuum. The MS experiments were performed by using a tubular furnace under a flow of Ar (100 mL min−1) with a heating rate of 3.75 K min−1 from room temperature to 1123 K. Quartz wool was placed to stop any solids from blocking the capillary inlet of the mass spectrometer. Identical experimental conditions for the synthesis of tungsten carbide nanoparticles were used for the MS investigations, as described above. Adsorption spectra were recorded by using a PerkinElmer Spectrum 100 FTIR spectrometer. Elemental analysis was performed in a Flash 2000 Thermo Scientific CHNS/O analyzer. The BET surface areas, BJH pore sizes, and pore volumes were approximated at 77 K in a Micrometrics ASAP 2420 surface area and porosity analyzer using N2 as the probe molecule. A Bruker D8 Advance diffractometer (DMAX 2500) operating with a Cu energy source at 40 kV and 40 mA was used to crystallographically characterize the materials. The XPS spectra were calibrated against the carbon 1s photoelectron signal at 285 eV on an Amicus/ESCA 3400 from Kratos Analytical instrument operating with dual Mg/Al anodes with an energy source at 12 kV and 10 mA. TGA was performed in a Mettler-Toledo TGA/DSC1 Star system under air (100 mL min−1). TEM was used to characterize the morphology and the particle size distribution of the synthesized products. Microscopy characterizations were performed on a Titan ST transmission electron microscope from FEI operated at 300 kV. The samples were prepared by suspending them in ethanol and dispersing them by sonication. A drop of the solution was poured onto a copper-grid-supported carbon film. Finally, the grid was dried in air prior to observation.

Electrochemical measurements: The HER, HOR, and ORR were evaluated by electrochemical methods by using a RDE. A GCE with a geometric surface area of 0.071 cm2 was loaded with 1 mg cm−2 tungsten carbide nanoparticles. Typically, 9 mg of tungsten carbide sample was dispersed in 445 μL of ethanol with 15 min of ultrasonication. Then, 3.5 μL of the suspension was drop-coated into the GCE and dried in open air. The electrode was later treated under static air conditions at 473 K for 1 h A Pt/CB (≈40 %, FC-12, Ishifuku) electrode was used in the experiments for evaluation purposes. The electrolyte used for the HER and HOR electrochemical investigations was a 0.5 M H2SO4 solution, and a 0.5 M Na2SO4 solution was used for the ORR experiments. The pH was adjusted using H2SO4 or NaOH to the required pH value for each experiment. The measurements were performed by using a 16-channel research-grade potentiostat system (VMP3) from BioLogic Science Instruments in a conventional three-electrode single electrochemical cell. The working electrode was mounted on a rotator (RRDE-3A, BAS). An Ag/AgCl electrode and platinum wire were used as the reference electrode and counter electrode, respectively. The Ag/AgCl reference electrode was calibrated against the RHE potential and all potentials were expressed against the RHE value. Prior to all measurements, the electrochemical cell was bubbled for 30 min with Ar, H2, or O2 for HER, HOR, and ORR measurements, respectively. CV experiments were conducted with a 50 mV s−1 scan rate between −0.4 and 0.1 V versus RHE for hydrogen evolution, 0.0 to 0.3 V versus RHE for hydrogen oxidation under different rotational speeds, and −0.1 and 0.6 V versus RHE for ORRs. A constant rotational speed of 1600 rpm was used for all HER and ORR experiments.

Photocatalytic experiments: Tungsten carbide materials were tested as cocatalysts for the OWS. The obtained products were loaded on STO:Na by using an incipient wetness impregnation procedure.56 The loading amount and heating temperature under oxidizing or reducing conditions were explored. In a typical experiment, the required amount of tungsten carbide nanopowder was suspended in 50 mL of ethanol and sonicated for 15 min to obtain a stable dispersion. Later, an appropriate amount of STO:Na was added and the solution was heated to 363 K under vigorous stirring until all of the solvent was completely evaporated. The recovered product was dried under vacuum for 2 h at 363 K. Two different heat treatment procedures were employed, namely, in an oxidative environment (static air conditions) and a reducing atmosphere (5 % H2 in argon flow, 100 mL min−1). The temperature used for the heat treatment was varied from 473 to 623 K. OWS experiments were performed by using a recirculating reactor unit. The accumulated gaseous products were analyzed by using a Shimadzu GC with a Molecular Sieve 13X column. Photocatalytic OWS was performed under UV light irradiation by using a 300 W Xe lamp with a cold mirror (260–500 nm). Experiments for OWS were performed with 100 mL of H2O (pH 7, 18.2 MΩ cm−1) and 50 mg of photocatalyst.

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Supporting Information

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