Activating Co nanoparticles on graphitic carbon nitride by tuning the Schottky barrier via P doping for the efficient dehydrogenation of ammonia-borane

Abstract

Herein, a highly active Mott–Schottky nanocatalyst with tunable Schottky barrier was constructed by embedding Co nanoparticles (NPs) into phosphorus-doped graphitic carbon nitrides (P_xCN) for the enhanced dehydrogenation of ammonia-borane (AB). Analyses show that P doping can efficiently decrease the work function of g-C₃N₄ and elevate the conduction band edge potential; thus, P doping offers a simple approach to tune the Schottky barrier of Co/P_xCN by varying the doping amount of P atoms. Upon rationally tuning the Schottky barrier from 0.60 to 0.78 eV, the obtained photocatalyst with the molar composition Co/P_3.59CN showed a remarkable TOF (total turnover frequency) value of 67.09 mol H₂ per mol metal per min for the dehydrogenation of AB under visible light at 298 K, which, to the best of our knowledge, is one of the highest values reported to date for non-noble metal-based monometallic catalysts and is even comparable to those of some noble metal catalysts. This study opens a new prospect for the rational design and preparation of metal/g-C₃N₄ nanocatalysts with tunable Schottky barriers and provides a simple way to elevate the catalytic activity of these nanocatalysts.

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Introduction

In recent years, the rapidly increasing energy demands have necessitated the development of alternative energy sources that are clean and renewable.¹ Hydrogen has been considered as an ideal candidate for clean energy production because it possesses a higher gravimetric energy density than petroleum (120 kJ g⁻¹ for hydrogen vs. 44 kJ g⁻¹ for petroleum) and generates water as the only by-product.^2,3 The key point of the so-called “hydrogen-based economy” is to find a safe and efficient hydrogen storage medium.^4,5 Various complex hydrides, such as borane-nitrogen (BN) and ammonia-borane (NH₃BH₃, AB) compounds,^6,7 have been investigated for hydrogen storage. Ammonia-borane has attracted significant attention due to its relatively high hydrogen content (19.6 wt%) and a low molecular weight (30.7 g mol⁻¹).^2,8,9 AB can release three equivalents of hydrogen quickly when approprite catalysts are used under mild conditions. Thus, efficient catalysts for the dehydrogenation of AB are highly desired.

However, to date, the traditional catalysts used for the hydrolytic dehydrogenation of AB are mostly based on noble metals such as Pt, Au, and Pd. Although they can exhibit excellent catalytic activity, their high cost limits their practical applications in large scale.^10–13 Thus, the development of catalysts based on non-noble metals with high efficiency for the dehydrogenation of AB is highly required. Previously, a series of studies have been reported on non-noble metal, including Cu, Co, and Ni, catalysts;^3,14–22 among them, Co-based catalysts have recently attracted substantial attention because of their low cost and relatively high catalytic activity.^23–25

The current studies are mainly focused on the catalysts based on ultrafine metal nanoparticles (NPs), which contribute to the catalytic activity because their high surface areas can provide abundant active sites. However, the agglomeration of Co NPs due to their high surface energy may reduce the catalytic activity of the corresponding catalyst.²⁶ An efficient strategy to inhibit the agglomeration of these NPs is the dispersion of Co NPs on a high-surface-area support.⁶ More importantly, Mott–Schottky rectification occurred when a proper semiconductor was used as a support, which could efficiently enhance the electron density of Co NPs. Due to the difference between the work functions of the Co NPs and semiconductor support (exemplified using an n-type semiconductor here), the electrons migrate from the semiconductor support to the Co NPs; this makes the Co NPs more “noble” until the Fermi level on both sides of the interface reaches an equilibrium. This Mott–Schottky rectification between the Co NPs and the semiconductor support mainly depends on the band structure of the semiconductor.²⁷ Thus, a potential barrier, which is called Schottky barrier, is generated at the interface of the semiconductor support and the Co NPs.^28,29 As the Schottky barrier can restrict the backward transfer of electrons, it plays a key role in enhancing the separation efficiency of the photo-generated electron/hole pairs. Therefore, the electron density of the Co NPs is enriched and the catalytic activities are promoted.

Among the various semiconductors available for embedding the metal NPs, graphitic carbon nitride (g-C₃N₄) has been considered as an ideal candidate due to its suitable band structure, where the Fermi level of most of the metals is located between the conduction band and valence band of g-C₃N₄.^27,30–33 Although several heterogeneous catalysts constructed by non-noble metal NPs and g-C₃N₄ have been investigated for the dehydrogenation of AB,^3,34–36 their catalytic activities are still not sufficient.^14,23 In principle, rationally tuning the band structure of g-C₃N₄ could be an effective approach to increase the Schottky barrier of Mott–Schottky heterojunctions, causing enrichment of the electron density of metal NPs. Doping g-C₃N₄ with heteroatoms, such as B, P, S, I, and F, has been proved to be an efficient way to modify the electron structure of g-C₃N₄.^37–41 However, only few studies have been reported on the relationship between the heterogeneous atoms doped into g-C₃N₄ and Schottky barrier, especially on the impact of heteroatom doping on the catalytic activities.

Herein, based on that consideration, we constructed a Mott–Schottky interface between the Co NPs and phosphorus-doped graphitic carbon nitrides (P_xCN) for the efficient dehydrogenation of AB under visible light, and the Schottky barrier of Co/P_xCN was rationally tuned by adjusting the doping content of the P atoms. In our study, we found that P doping can decrease the work function, narrow the bandgap, and elevate the conduction band edge potential of g-C₃N₄; thus, P doping offers an efficient method to tune the Schottky barrier of Co/P_xCN. The Schottky barrier of Co/P_xCN can be enlarged from 0.60 eV to 0.78 eV by varying the doping amount of P atoms. The obtained photocatalyst with the molar composition Co/P_3.59CN shows a remarkable TOF (total turnover frequency) value of 67.09 mol H₂ per mol metal per min for the dehydrogenation of AB under visible light at 298 K. This work provides new insights into the control of the catalytic activity of the catalysts based on metal nanoparticles embedded into a semiconductor support. By rationally tuning the Schottky barrier of metal/g-C₃N₄ nanocatalysts, the catalytic activity of heterojunctions was optimized using a small doping amount of atoms.

Experimental

Materials

Melamine (C₃H₆N₆, Sinopharm Chemical Reagent Co. Ltd, >99%), sodium borohydride (NaBH₄, Sinopharm Chemical Reagent Co. Ltd, 96%), ethylene glycol (C₂H₆O₂, Sinopharm Chemical Reagent Co. Ltd, >99%), 1-hydroxyethylidene-1,1-diphosphonic acid (C₂H₈O₇P₂, HEDP, Meryer Chemical Technology Co. Ltd, >99%), cobalt chloride hexahydrate (CoCl₂·6H₂O, Tansoole, ≥99%), and ammonia-borane (NH₃BH₃, AB, Macklin, 97%) were used as received without further purification. Deionized water (DI) was utilized in all the experimental processes.

Synthetic procedures

Synthesis of P_xCN. Phosphorus-doped graphitic carbon nitride was obtained according to the method previously reported in the literature.⁴² Melamine (4.8 g) and HEDP in different amounts (0.0, 0.2, 0.4, 0.6, and 0.8 g) were dissolved in a 64 ml mixed solution of DI water and ethylene glycol (with a volume ratio of 15 : 1) at 80 °C. After removing the solution, the mixed powder was dried overnight at 60 °C under vacuum conditions; then, the resultant mixture was calcined at 550 °C for 4 hours in air. Finally, P-doped g-C₃N₄ was obtained after cooling the product down to room temperature. The results of Inductively coupled plasma emission spectroscopy (ICP) analyses indicate that the percentages of P atoms in P_xCN are 0.0%, 2.16%, 3.59%, 5.26%, and 8.49% (Table S1 in the ESI†). In order to express the doping amount of P atoms, the resulting samples were labelled as P_xCN, where x (x = 0, 2.16, 3.59, 5.26, and 8.49) refers to the percentage of the P atoms. Energy dispersive X-ray spectroscopy elemental mapping images suggest that the P atoms are well dispersed in g-C₃N₄ (Fig. S1 and S2†).

Synthesis of Co/P_xCN. Typically, 9 mg of the as-prepared P_xCN and 0.017 mmol of CoCl₂·6H₂O were dispersed in 5 mL DI water under magnetic stirring for 24 h. Then, 2 mL of a freshly prepared NaBH₄ aqueous solution (containing 0.017 mmol NaBH₄) was added to the as-prepared solution. After centrifugation, the catalyst was washed with DI water and dried in vacuum at 60 °C overnight. The as-obtained black solid samples were used for further characterizations.

Characterization

Powder X-ray diffraction (XRD) measurements were carried out by Rigaku D/max-2600PC. Fourier transform infrared spectroscopy (FTIR) was conducted using Thermo Fisher Nicolet 6700. X-ray photoelectron spectroscopy (XPS) spectra were obtained via Kratos AXIS Ultra DLD. UV-visible absorption spectra were acquired using PerkinElmer Lambda 950. The energy dispersive X-ray spectroscopy (EDS) analysis and elemental mapping were performed using Hitachi S-4800. Inductively coupled plasma emission spectroscopy (ICP) analyses were conducted via Varian ICP 710.

Electrochemical measurements

The electrochemical measurements were carried out in a conventional three-electrode cell using the CHI 760E workstation as an electrochemical analyzer. A Pt wire and a Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively. Catalyst powder in a certain amount was dispersed in distilled water containing ethanol and Nafion solution. Subsequently, blending and sonication were conducted to obtain a homogeneous slurry. The working electrode was obtained by dropwise adding the slurry on a glassy carbon electrode. Herein, 0.1 M KOH aqueous solution without any additive was used as an electrolyte. Mott–Schottky plots were obtained at the frequencies of 500, 750, and 1000 Hz in the bias potential range from 1.0 to −1.0 V.

Catalytic activity measurements

The catalytic activity of the hybrid catalysts for the dehydrogenation of AB was measured using a classic water-filled gas burette. Typically, 9 mg of substrate (P_xCN), 0.017 mmol of CoCl₂·6H₂O, and 3 ml of DI water were filled in a two-necked jacketed round-bottom flask under vigorous stirring. After this, 0.85 mmol AB and 0.017 mmol NaBH₄ dissolved in 2 ml DI water were quickly added to the flask (the molar ratio of AB and Co is n_AB : n_Co = 50 : 1, where n means the mole number). The molar ratio of AB/NaBH₄ is 50 : 1; thus, the hydrogen gas generated by NaBH₄ can be ignored in this system. The reactor was irradiated by a 300 W Xe lamp with a 420 nm cutoff filter. The volume of the generated hydrogen gas was quantified by the gas burette. To get the apparent activation energy (E_a) of the hydrolysis of AB catalyzed by the nanocatalysts, the catalytic performance was investigated at different temperatures (288, 293, 298, and 303 K), which was thermostated by a water bath. The total turnover frequency (TOF) value was calculated by the following equation:
where n_AB and n_metal refer to the mole number of NH₃BH₃ and Co, respectively, and t refers to the reaction time (min).

Results and discussion

Direct evidences for the formation of P-doped g-C₃N₄ (P_xCN) were obtained by XRD and FTIR (Fig. 1). The XRD patterns of all samples exhibit two distinct diffraction peaks at 13.0° (100) and 27.3° (002) (Fig. 1a), which could be ascribed to the basic crystal structure of g-C₃N₄ (JCPDS card no. 87-1526).³⁹ The strong peak at 27.3° corresponds to the (002) interlayer stacking, whereas the peak at 13.0° is ascribed to the (100) in-plane ordering of tri-s-triazine units. The intensity of both peaks decreased with an increase in the amount of P atoms; this indicates that the doping of P atoms increased the disturbance of the graphitic-like structure and significantly decreased the crystallinity of g-C₃N₄.⁴⁰ Especially, the (002) peak of P_5.26CN and P_8.49CN exhibits an obvious left shift when compared with that of P₀CN, suggesting that the doping of P atoms loosened the interlayer stacking system and slightly increased the distance between layers.^43,44 The molecular structure of P_xCN was investigated by the FTIR spectra. As can be observed from Fig. 1b, the characteristic IR spectra of P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN are similar to that of P₀CN. The broad peaks between 3000 and 3500 cm⁻¹ are attributed to N–H stretching, whereas the peaks in the 1200–1600 cm⁻¹ region correspond to the typical stretching modes of CN heterocycles, and the sharp peak at around 803 cm⁻¹ is related to triazine units.^45,46 A new peak was observed at 950 cm⁻¹ after P doping. It is presumed that this peak originates from P–N stretching, suggesting that the P atoms have successfully squeezed into the triazine units of g-C₃N₄ and probably substitute the C atoms to form P–N bonding (Fig. 1c).^42,47

Fig. 1 (a) XRD patterns and (b) FTIR spectra of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN. (c) Schematic of the structure of P_xCN.

To explore the modification of the electronic structure of g-C₃N₄ caused by the doping of P atoms, X-ray photoelectron spectroscopy (XPS) was performed. As shown in Fig. 2a, the XPS survey spectrum of P₀CN exhibits three peaks, which are ascribed to C 1s, N 1s, and O 1s. The C 1s spectrum exhibits two remarkable peaks centered at 284.8 eV and 288.2 eV, which could be assigned to graphitic carbon (C–C) and sp²-hybridized carbon in the N-containing triazine ring (N–C=N), respectively. The N 1s spectra can be deconvoluted into three peaks located at 398.6 eV, 399.8 eV, and 401.0 eV. These peaks originated from the sp²-hybridized nitrogen in the triazine rings (C–N=C), tertiary nitrogen N–(C)₃ groups, and amino functions carrying hydrogen (C–N–H). The XPS survey spectrum of P_3.59CN is similar to that of P₀CN, except for the weak signals of the P atoms. The P 2p peak of P_3.59CN centered at 133.4 eV is ascribed to the P–N bonding³⁸ (P–C bonding is ∼1–2 eV lower³⁷), which is in good agreement with the results of the FTIR spectra. From these XPS results, we can again confirm that the P atoms had squeezed into the skeleton and probably replaced the C atoms in g-C₃N₄. Moreover, it was found that the peaks of N–C=N and C–N=C shifted to lower energies by 0.20 and 0.14 eV after the introduction of the P atoms, respectively; this indicates that the P atoms acted as good electron donors for g-C₃N₄. That is, the addition of the P atoms could significantly enhance the electron density of the g-C₃N₄ networks.

Fig. 2 (a) XPS survey spectra of P₀CN and P_3.59CN. (b–d) The C 1s, N 1s, and P 2p deconvoluted XPS spectra of P₀CN and P_3.59CN, respectively.

The UV-visible absorption spectra, Mott–Schottky plots, and XPS valence band spectra of P_xCN were obtained to further investigate the change in the band structure caused by the modification of the electron structure of g-C₃N₄. As shown in Fig. 3a, the introduction of the P atoms significantly enhanced the absorption of visible light for g-C₃N₄, resulting in an ambiguous color change from pale yellow for P₀CN to black for P_8.59CN (Fig. S3†). Moreover, the band absorption edge of P_xCN displays an obvious red shift when compared with that of P₀CN. With an increase in the doping content of P atoms, this shift in the band absorption edge increased. Thus, the bandgap gradually decreased from 2.79 eV for P₀CN to 2.57 eV, 2.27 eV, 2.24 eV, and 2.22 eV for P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN, respectively (Fig. 3b). Therefore, we can conclude that the bandgap can be well-tuned by simply controlling the doping content of the P atoms. The flat-band potentials of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN obtained from the Mott–Schottky plots are −0.73, −0.82, −0.94, −0.97, and −1.01 V vs. Ag/AgCl, respectively (Fig. 4a–e). The flat-band potential against Ag/AgCl can be converted to that against SHE (standard hydrogen electrode) by the following equation:⁴¹

Fig. 3 (a) UV-Visible absorption spectra and (b) band gap energy spectra of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN.

Fig. 4 Mott–Schottky plots of (a) P₀CN, (b) P_2.16CN, (c) P_3.59CN, (d) P_5.26CN, and (e) P_8.49CN. (f) XPS valence band spectra of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN. (g) Band structures of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN (E_C: conduction band, E_F: Fermi level, and E_V: valence band).

Therefore, the flat-band potentials of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN were calculated to be 0.235, 0.145, 0.025, −0.005, and −0.045 V vs. SHE, respectively. According to the reference standard for which 0 V vs. SHE is equal to −4.44 V vs. E_vac (vacuum level), the Fermi level (E_F) can be calculated by the following equation:⁴⁸

E_F = −V_fb (vs. SHE) × e − 4.44 eV

Accordingly, the E_F values of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN were calculated to be −4.67, −4.58, −4.46, −4.43, and −4.39 eV, respectively. Hence, it can be observed that the work functions of P-doped g-C₃N₄ efficiently decreased owing to the enrichment of the electron density of g-C₃N₄ networks caused by the doped P atoms. The energetic gap between the valence band edge potential and Fermi level was determined by the XPS valence band spectra,⁴⁸ which was 2.52, 2.29, 2.03, 2.09, and 2.19 eV for P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN, respectively (Fig. 4f). Thus, the valence band edge potentials of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN were calculated to be −7.19, −6.87, −6.49, −6.52, and −6.58 eV, respectively. In addition to the bandgap energies of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN deduced from the UV-visible absorption spectra (Fig. 3b), the conduction band edge potentials of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN were calculated to be −4.40, −4.30, −4.22, −4.28, and −4.36 eV, respectively. In our study, it was found that the decrease in the bandgap slowed down when the doping content of the P atoms was higher than 3.59%, and the elevation of Fermi level also followed this trend when the P loading amount was above 3.59% (Fig. 4g and Table 1). However, the energetic gap between the Fermi level and conduction band edge potential decreased from 0.27 eV for P₀CN to 0.28 eV, 0.21 eV, 0.11 eV, and 0.03 eV for P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN, respectively; this is in good agreement with the previous theory.⁴⁹ Therefore, the P_xCN catalysts acquire highest conduction band edge potential when the doping content of P atoms is 3.59%. When Mott–Schottky heterojunctions were constructed using Co NPs, this elevated conduction band edge potential enlarged the Schottky barrier of Co/P_xCN. As a result, we can conclude that the conduction band edge potential, the bandgap, and the work function of P_xCN can be well-tuned by simply adjusting the doping amount of the P atoms.

Table 1 The percentages of P atoms and band structures of P₀CN, P_2.16CN, P_3.59CN, P_5.26CN, and P_8.49CN

Samples	P atom percentage (%)	Bandgap (eV)	Fermi level^a (eV)	Conduction band^a (eV)	Valence band^a (eV)
a The potential is against the vacuum level.
P0CN	0	2.79	−4.67	−4.40	−7.19
P2.16CN	2.16	2.57	−4.58	−4.30	−6.87
P3.59CN	3.59	2.27	−4.46	−4.22	−6.49
P5.26CN	5.26	2.24	−4.43	−4.28	−6.52
P8.49CN	8.49	2.22	−4.39	−4.36	−6.58

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In order to couple the Co NPs with P_xCN, cobalt chloride hexahydrate was in situ reduced by NaBH₄ using P_xCN as a substrate. As shown in the TEM images (Fig. S4†), the Co NPs are well dispersed on the surface of P_3.59CN. The high-resolution TEM (HRTEM) images show that the interlayer spacing in the particle is 0.204 nm, which matches well with the (111) lattice plane of metallic Co.^50,51 The formation of metallic Co NPs was again confirmed by the XRD pattern of Co/P_3.59CN (ICP analysis indicates that the actual loading amount of Co was 9.52 wt%). A weak and broad peak was observed at 44.2°, which was ascribed to the metallic Co NPs (Fig. S5†). The chemical state of the metallic Co NPs was revealed by the XPS spectra of Co/P_3.59CN (Fig. S6†). The Co 2p spectra were deconvoluted into four peaks. The peaks located at 780.8 eV, 782.8 eV, 786.1 eV, and 777.6 eV are ascribed to Co²⁺, Co³⁺, shakeup satellites, and metallic Co, respectively.⁵⁰ Oxidized Co are clearly existed in the sample. However, after the etching treatment, it was found that the intensity of the peak of metallic Co dramatically enhanced; this suggested that metallic Co exists inside the oxidation layer. This implies that an oxidation layer was formed on the surface of the Co NPs, which was beneficial to the stability of Co/P_3.59CN in air.⁵²

The Mott–Schottky plots of Co/P₀CN, Co/P_3.59CN, and Co/P_8.49CN were obtained to explore charge rectification and the changes in the Schottky barrier. As shown in Fig. 5, the flat-band potentials of Co/P₀CN, Co/P_3.59CN, and Co/P_8.49CN were calculated to be −0.68, −0.76, and −0.79 V vs. Ag/AgCl, respectively. When these values were compared with those of P₀CN, P_3.59CN, and P_8.49CN (−0.73, −0.94, and −1.01 V vs. Ag/AgCl, respectively), it was found that the flat-band potentials of P₀CN, P_3.59CN, and P_8.49CN exhibited an obvious positive shift after the loading of the metallic Co NPs. This means that the electrons migrate from the P_xCN supports to the surface of the Co NPs, leading to the enrichment of the electron density of Co NPs.⁵³ Hence, an interfacial electric field, which is beneficial for the separation of electron/hole pairs at the interface of the Co NPs and P_xCN supports, must have generated at the interface of the Co NPs and P_xCN, and an upward band bending of the conduction band and valence band must have occurred.⁵⁴ As the Schottky barrier is related to the work function of the Co NPs (5 eV)^55,56 and the conduction band edge potential of P_xCN, the Schottky barriers of Co/P₀CN, Co/P_2.16CN, Co/P_3.59CN, Co/P_5.26CN, and Co/P_8.49CN were calculated to be 0.60, 0.70, 0.78, 0.72, and 0.64 eV, respectively, indicating that the Schottky barrier of Co/P_xCN was enlarged by the doping of P atoms. Thus, we can conclude that the Schottky barrier of Co/P_xCN can be well-tuned by simply varying the doping amount of the P atoms (Fig. 6). As the Schottky barrier can restrict the backward transfer of electrons, the enlarged Schottky barrier undoubtedly improves the separation efficiency of the photo-generated electron/hole pairs. Thus, more electrons can transfer from the P_xCN supports to the Co NPs, leading to the enrichment of the electron density of Co NPs. Finally, the catalytic performance of the Co NPs for the dehydrogenation of AB was remarkably elevated.

Fig. 5 Mott–Schottky plots of (a) Co/P₀CN, (b) Co/P_3.59CN, and (c) Co/P_8.49CN.

Fig. 6 Band structures and Schottky barriers of Co/P₀CN, Co/P_2.16CN, Co/P_3.59CN, Co/P_5.26CN, and Co/P_8.49CN (E_C: conduction band, E_F: Fermi level, and E_V: valence band).

The catalytic activities of Co/P₀CN, Co/P_2.16CN, Co/P_3.59CN, Co/P_5.26CN, and Co/P_8.49CN for the dehydrogenation of AB were investigated under visible-light irradiation. To eliminate the thermal influence caused by the irradiation, the temperature was maintained at 298 K by a water bath during the reaction. As can be observed in Fig. 7, the catalytic activity was remarkably promoted after the doping of P atoms. Obviously, Co/P_3.59CN possesses the highest catalytic performance in the hydrolysis of AB. Its TOF value is up to 67.09 mol mol⁻¹ min⁻¹ under visible-light irradiation at 298 K, which is one of the highest values achieved to date for non-noble metal-based monometallic catalysts without NaOH treatment and is even comparable to those of some noble metal catalysts (Table 2). It can be observed that the catalytic activities of the nanocatalysts have a positive proportional relationship with the Schottky barrier (Fig. 7b). Therefore, we can conclude that the enhanced photocatalytic activity of Co/P_3.59CN may be due to the enlarged Schottky barrier. As discussed above, the introduction of P atoms enriched the electron density of the g-C₃N₄ networks. Thus, the conduction band edge potential, the bandgap, and the work function of P_xCN could be well-tuned by adjusting the doping amount of the P atoms. When the doping amount of the P atoms reached 3.59%, the maximum Schottky barrier of 0.78 eV was acquired for Co/P_3.59CN. As a result, more electrons were injected into the surface of the Co NPs, which made the Co NPs more “noble”. Consequently, the catalytic performance of Co/P_3.59CN in the hydrolysis of AB was significantly enhanced (Fig. 8).

Fig. 7 (a) Hydrogen evolution curves for the AB hydrolysis catalyzed by Co/P₀CN, Co/P_2.16CN, Co/P_3.59CN, Co/P_5.26CN, and Co/P_8.49CN under visible-light irradiation ([AB] = 170 mM, 5 mL, n_Co/n_AB = 0.02, and T = 298 K). (b) TOF values of Co/P₀CN, Co/P_2.16CN, Co/P_3.59CN, Co/P_5.26CN, and Co/P_8.49CN for the AB hydrolysis and the corresponding Schottky barriers.

Fig. 8 Schematic of the dehydrogenation of AB catalyzed by Co/P_3.59CN under visible-light irradiation.

Table 2 TOF values and activation energy values for the hydrolysis of AB catalyzed by different monometallic-based catalysts

Catalysts	TOF ((H2)mol per (Cat-M)mol per min)	Activation energy, Ea (kJ mol^−1)	Ref.
a The reaction was promoted by the addition of NaOH. b The reaction was tested under visible-light irradiation.
Co/P3.59CN	67.09^b(48.52)	34.33	This work
Co/g-C3N4	55.6^b	—	23
40 wt% Pt/C	55.56	—	57
Co/CTF-1	42.3	42.7	58
Ni@3D-(N)GFs	41.7	—	59
Ni2P	40.4	44.6	15
CuNPs@SCF	40.0	—	60
Co/PEI-GO	39.9	28.2	61
Ni/ZIF-8	35.3(85.7^a)	42.7(28^a)	62
Ni@MCS-30	30.7	—	16
Pd/CN-CS	27.7	35.3	34
CoP	24.9(72.2^a)	46.7	63
Ni/CNT	23.53	—	64
Ni/CN	18.7^b	36	3
Co/graphene	13.8	32.75	65
Ni@MCM-41	9.3	—	16
Ni NPs/C	8.8	28	66
Co@C–N@SiO2-800	8.4	36.1	67

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Visible light can be used to promote the catalytic performance of Co/P_xCN in the AB hydrolysis. As shown in Fig. 9a, the evolution rate of hydrogen under the visible-light irradiation is substantially faster than that without irradiation in the AB hydrolysis catalyzed by Co/P_3.59CN. This is because visible-light irradiation can excite the P-modified carbon nitride support and inject more electrons into the Co NPs.

Fig. 9 (a) Hydrogen evolution curves for the AB hydrolysis catalyzed by Co/P_3.59CN (10 wt%) with and without visible-light irradiation ([AB] = 170 mM, 5 mL, n_Co/n_AB = 0.02, and T = 298 K). (b) Hydrogen evolution curves for the AB hydrolysis catalyzed by Co/P_3.59CN with different Co loading amounts in the range from 3 wt% to 40 wt% under visible-light irradiation and the corresponding (c) reaction rates and (d) TOF values ([AB] = 170 mM, 5 mL, and T = 298 K).

The catalytic activity of Co/P_3.59CN with different loading amounts of Co ranging from 3 wt% to 40 wt% was evaluated to investigate the effect of Co loading on the catalytic activity of Co/P_3.59CN. As can be observed from Fig. 9b, the catalytic activities gradually increased with an increase in the loading amount of Co up to 30 wt%. However, a further increase in the Co loading amount to 40 wt% led to poor catalytic activity (Fig. 9c). This is most likely because limited sites are available for building an effective metal–semiconductor contact on the surface of P_3.59CN.²⁷ Although Co/P_3.59CN acquired the best catalytic activity at the Co loading amount of 30 wt%, the optimum catalytic performance with the highest TOF was achieved at the Co loading amount of 10 wt% (Fig. 9d).

The catalytic performance of Co/P_3.59CN at different temperatures (288–303 K) was evaluated to study the reaction kinetics of AB hydrolysis. As shown in Fig. 10a, the decomposition time for Co/P_3.59CN at 288 K is 199 s, whereas the decomposition time at 303 K is 96 s. This means that the hydrogen generation rate accelerated with an increase in temperature. This result indicates the vital role of temperature in the dehydrogenation of AB. The apparent activation energy (E_a) of the hydrolysis of AB was obtained by the following Arrhenius equation:⁵⁸

Fig. 10 (a) Hydrogen evolution curves and (b) Arrhenius plots for AB hydrolysis catalyzed by Co/P_3.59CN at different temperatures in the range of 288–303 K ([AB] = 170 mM, 5 mL, and n_Co/n_AB = 0.02).

The Arrhenius plots of ln k vs. the inverse absolute temperature (1/T) are shown in Fig. 10b. From the equation, the E_a value for Co/P_3.59CN was calculated to be 34.33 kJ mol⁻¹, which is relatively low as compared to those of the other non-noble metal-based monometallic catalysts for AB hydrolysis (Table 2).

Since the stability and reusability of the nanocatalysts are also important factors for the practical application of these nanocatalysts, the recycling test of Co/P_3.59CN was conducted under visible-light irradiation at 298 K. As shown in Fig. 11, the catalysts exhibited high catalytic activity even after 8 runs, and the volume of the hydrogen generated by AB remained unchanged; this suggested good stability and recyclability of Co/P_3.59CN. The TEM images and XRD patterns of Co/P_3.59CN exhibit no obvious change after eight cycles (Fig. S7†), again demonstrating the high stability of Co/P_3.59CN.

Fig. 11 Hydrogen evolution curves for AB hydrolysis catalyzed by Co/P_3.59CN during an eight-cycle reusability test ([AB] = 170 mM, 5 mL, n_Co/n_AB = 0.02, and T = 298 K).

Conclusions

In conclusion, herein, we developed a simple method to tune the Schottky barrier of Co/P_xCN by varying the doping amount of P atoms for the efficient dehydrogenation of AB. By simply adjusting the doping amount of P atoms from 0 to 3.59%, the Schottky barrier could be controlled from 0.60 eV to 0.78 eV because the doped P atoms act as good electron donors for g-C₃N₄, leading to the enrichment of the electron density of g-C₃N₄ networks. Upon further increasing the doping content of P atoms, the energetic gap between the Fermi level and conduction band edge potential was significantly decreased, resulting in a decrease in the Schottky barrier of Co/P_xCN. The maximum TOF value for Co/P_3.59CN in the dehydrogenation of AB is up to 67.09 mol mol⁻¹ min⁻¹ under visible-light irradiation at 298 K. To the best of our knowledge, this is one of the highest values achieved to date for non-noble metal-based monometallic catalysts without NaOH treatment and is even comparable to those of some noble metal catalysts. This study provides an easy and feasible method to tune the Schottky barrier of metal/g-C₃N₄-type Mott–Schottky nanocatalysts and provides us a new vision to facilitate their catalytic activity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of Shanghai (No. 17ZR1420000), the Shanghai talent development funding for the project (No. 2017044), the International Science & Technology Cooperation Program of Shanghai (No. 18520723300), the Shanghai Science and Technology Commission (19XD1431800), and the China Postdoctoral Science Foundation (No. 2018M642021).

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Footnote

† Electronic supplementary information (ESI) available: TEM, XPS and EDS elemental mapping images. See DOI: 10.1039/d0qi00659a

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