Photocatalytic water splitting into hydrogen by solar energy is a promising route to address increasing global energy consumption and environmental issues [1-4]. However, developing highly efficient and low-cost visible light-driven photocatalysts to meet practical needs remains a major challenge [5-10]. In the past decades, various visible light photocatalysts, such as CdS, graphitic carbon nitride (g-C3N4), and TaON, have widely been developed for H2 generation [11-16]. Among them, g-C3N4 has been extensively investigated toward hydrogen production under visible light owing to its unique inherent characteristics such as good visible light absorbance, strong reduction potential for H2 evolution, and excellent photochemical stability [17-20]. Consequently, numerous strategies have been developed to improve its photocatalytic activity such as coupling with other semiconductors [21-26], forming mesoporous or nanosheet structures [27-34], and loading with co-catalysts [35-41]. However, the photocatalytic activity of modified g-C3N4 remains inadequate under visible light irradiation owing to rapid recombination of photogenerated electron-hole pairs on its surface (Fig. 1(a)). Therefore, it is of high interest to develop new strategies to improve the hydrogen-evolution performance of g-C3N4.
Loading with noble metals, such as Pt, is regarded as one of the most ideal strategies to enhance photocatalytic hydrogen evolution over g-C3N4 owing to the improved charge separation and accelerated surface catalytic reactions [42, 43] that can be attained. However, the widespread application of this strategy is limited by the high cost and limited abundance of Pt metals. Noble metal Ag is an attractive alternative because of its considerably lower cost [44-46]. It has also been demonstrated that loading Ag nanoparticles on g-C3N4 photocatalysts can improve the photocatalytic H2-evolution performance. For example, Ge et al. [47] demonstrated that the H2-evolution rate of g-C3N4 could be enhanced by loading metallic Ag. The H2-evolution rate of the Ag-loaded photocatalyst was ~11.7 times higher than that of pure g-C3N4. Bai et al. [48] reported that the H2 evolution activity over core-shell Ag@C3N4 photocatalyst was 30 times higher than that over pure g-C3N4. Additionally, Chen et al. [49] reported the synthesis of a Ag quantum dot-modified g-C3N4 with improved hydrogen production rates. Although the above results clearly demonstrate the benefit of modifying g-C3N4 with Ag nanoparticles, Ag-modified g-C3N4 photocatalysts still show a considerably lower photocatalytic H2-evolution performance when compared with Pt-modified photocatalysts. A possible reason is that the metallic Ag surface is not an effective active site for H2 evolution owing to the weaker bond strength of Ag-H (37 kcal mol-1) when compared with that of Pt-H (60 kcal mol-1) [50] (Fig. 1(a)). To further improve the photocatalytic H2-evolutin activity of g-C3N4/Ag, it is highly desirable to modify the Ag surface to promote its interfacial hydrogen-evolution reactions. Recently, it was reported that the strong affinity between Ag and thiocyanate (SCN-) ions could enhance the interfacial electron transfer rate and facilitate the reduction of protons on Ag/TiO2 under UV light irradiation. Therefore, it is expected that SCN- ions adsorbed on a Ag surface can serve as effective interfacial active sites to improve the visible light photocatalytic performance of g-C3N4/Ag photocatalysts toward hydrogen production (Fig. 1(a)).
In the present study, SCN--modified g-C3N4/Ag photocatalysts were synthesized via a two-step process: photoinduced deposition of Ag on g-C3N4 surface and selective loading of SCN- on the Ag surface by an impregnation method. The g-C3N4/Ag-SCN photocatalysts exhibited improved photocatalytic H2-production performance compared with bare g-C3N4, g-C3N4/Ag, and g-C3N4/SCN. A possible photocatalytic mechanism was proposed to account for the improved photocatalytic performance. To our knowledge, this is the first report showing the improved photocatalytic H2 generation of g-C3N4/Ag by selective loading of SCN- ions. Considering the low cost and high efficiency achieved by SCN- ions, the latter show great potential for use in the design and preparation of high-performance silver-modified photocatalysts.
All chemicals were of analytical grade and supplied by Shanghai Chemical Reagent Ltd. (China) and used as received without further purification. Distilled water was used in all experiments.
The g-C3N4 photocatalyst was synthesized via a simple calcination-hydrothermal procedure using melamine powder as the precursor [51]. In a typical procedure, 10 g of melamine was calcined at 550 ℃ in a muffle furnace for 4 h in ambient atmosphere. After cooling to room temperature, the yellow product was collected and ground into powder. To increase its specific surface area, 0.5 g of the above sample was dispersed into 70 mL of distilled water with constant stirring for 1 h. The mixture was then transferred into a Teflon-lined stainless steel autoclave at 180 ℃ for 2 h. After natural cooling to room temperature, the precipitation was collected by filtration, washed with distilled water, and dried at 60 ℃ for overnight. Finally, the product was ground into powder and labeled as g-C3N4. The nitrogen adsorption-desorption analysis revealed that after hydrothermal treatment, the specific surface area of the resultant g-C3N4 sample was 47.2 m2 g-1, which was much higher than that of the bulk g-C3N4 sample (2.3 m2 g-1) prepared in the absence of a hydrothermal treatment [52].
Silver nanoparticles were loaded onto the g-C3N4 surface via a photodeposition method. Briefly, 0.05 g of the as-prepared g-C3N4 powder was uniformly dispersed into 10 mL of distilled water and stirred at room temperature for 10 min. Subsequently, 1.38 mL of AgNO3 solution (0.01 mol L-1) and 0.3 mL of lactic acid solution (10 vol%), as an electron donor, were injected into the above suspension. After visible light irradiation using four low-power LEDs (3 W, 420 nm, Shenzhen Lamplic Science Co. Ltd.) for 30 min, the resultant sample was filtered and washed with distilled water for several times to obtain the g-C3N4/Ag photocatalyst.
Both the preparation of the g-C3N4/Ag-SCN photocatalysts and their photocatalytic hydrogen-production experiments were conducted in a 100-mL three-neck Pyrex flask at ambient temperature and atmospheric pressure. In a typical experiment, 0.05 g of g-C3N4/Ag was dispersed into 80 mL of lactic acid solution (10 vol%) under continuous stirring for 10 min followed by injection of a known volume of potassium thiocyanate solution (0.1 mol L-1).
For the photocatalytic runs, prior to irradiation, the above suspension solution was bubbled with nitrogen for 30 min to remove dissolved oxygen. Four low-power LEDs (3 W, 420 nm, Shenzhen Lamplic Science Co. Ltd) were used as the light source to trigger the photocatalytic reaction. The LEDs were positioned radially 1 cm away from the reactor. During visible light irradiation, continuous stirring was applied to keep the photocatalyst particles in a suspension state. To measure the H2 amount, 0.4 mL of gas was intermittently sampled through a septum, and its composition was analyzed on a gas chromatograph (Shimadzu GC-2014C, Japan, with nitrogen as the carrier gas) equipped with a 5-Å molecular sieve column and a thermal conductivity detector. The final concentrations of SCN- in the suspension solution were adjusted to 0, 0.1, 0.2, 0.3, 0.5, 0.7, 1.0 mmol L-1. After the photocatalytic reaction, the precipitation was filtered, washed with deionized water several times, and finally dried at 60 ℃ for 12 h to obtain the g-C3N4/Ag-SCN(X mM) samples, where X represents the final concentration (mmol L-1) of the SCN- anions.
X-ray diffraction (XRD) measurements were performed on a Rigaku Ultima Ⅲ X-ray diffractometer (Japan) using Cu Kα radiation. The morphology of the materials prepared was analyzed via field-emission scanning electron microscopy (FESEM) on a JEOL JEM-7500F microscope (Japan). Further morphological and structural characterizations were performed via transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) using a JEOL JEM-2100F microscope. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a KRATOA XSAM800 XPS system with Mg Kα source. All of the binding energies were referenced to the C 1s peak at 284.8 eV for the surface adventitious carbon. UV-visible (UV-vis) absorption spectra were obtained using a UV-visible spectrophotometer (UV-2450, Shimadzu, Japan). For the UV-vis diffuse reflectance studies, BaSO4 was used as the reflectance standard.
Photoelectrochemical measurements were performed in a standard three-electrode system with platinum wire as the counter electrode, saturated Hg/Hg2Cl2 (in saturated KCl) as the reference electrode, and Na2SO4 aqueous solution (0.5 mol L-1) as the electrolyte. The results were recorded at an electrochemical workstation (CHI660E). The visible light irradiation was provided by one 3 W LED (420 nm light source with a 90 mW cm-2 power). A fluorine-doped tin oxide (FTO) conductor glass was used as the working electrode. To prepare the working electrode, typically, 10 mg of the sample powders was added to 1 mL of anhydrous ethanol and 1 mL of Nafion D-520 dispersion (5%, w/w, in water and 1-propanol, Alfa Aesar) and then evenly dispersed by ultrasound to obtain a suspension. The suspension was spread onto a 1 cm × 1 cm FTO glass with the sides protected by Scotch tape and then dried at 60 ℃ for 1 h. Last, linear sweep voltammetry (LSV) curves were measured at potentials ranging from 0.1 to -0.5 V and a scan rate of 10 mV s-1.
Fig. 1(b) illustrates the synthesis of the g-C3N4/Ag-SCN photocatalysts via a facile photodeposition process of metallic Ag on g-C3N4 followed by selective adsorption of SCN- ions on the Ag surface by an impregnation method. First, g-C3N4 was prepared via a simple calcination-hydrothermal procedure, as reported in our previous report. The resulting g-C3N4 powder was pale yellow. To prepare g-C3N4/Ag, g-C3N4 powder was uniformly dispersed into an AgNO3 aqueous solution with lactic acid solution as an electron donor. After visible light irradiation for 30 min, Ag+ ions were reduced on the g-C3N4 surface to form Ag0 nanoparticles. The resulting sample was dark brown, indicating successful deposition of Ag nanoparticles. To prepare g-C3N4/Ag-SCN photocatalyst, KSCN solution was injected into a g-C3N4/Ag suspension under nitrogen atmosphere. The SCN- ions selectively adsorbed on the Ag nanoparticle surface owing to the strong affinity between silver and sulfur [53], as shown in Fig. 1(b). Thus, owing to the adsorption of SCN- ions on the Ag surface, the prepared g-C3N4/Ag-SCN photocatalyst (Fig. 1(c)) was slightly less brown than the g-C3N4/Ag sample. The above results indicate the successful preparation of the g-C3N4/Ag and g-C3N4/Ag-SCN photocatalysts.
The phase structure and composition of the as-prepared g-C3N4/Ag-SCN, g-C3N4/Ag, and pure g-C3N4 photocatalysts were first analyzed by XRD (Fig. 2(a)). The XRD pattern of the pure g-C3N4 displayed two characteristic diffraction peaks, which can be well indexed to the hexagonal phase of g-C3N4 (JCPDS 87-1526) [54]. Specifically, the weak peak at 13.2° is typically oriented melons, which can be indexed to (100) facets and the strong peak at 27.6° represents the interplanar stacking of aromatic units, corresponding to (002) facets [55]. For the Ag-modified g-C3N4 photocatalysts, only weak diffraction peaks of metallic Ag could be detected in addition to the diffraction peaks of g-C3N4 owing to the small deposition amounts (~3 wt%) and high dispersion of the Ag species. To further observe the characteristic diffraction peak of metallic Ag, their slow-scan spectra are shown in Fig. 2(b). The g-C3N4/Ag and g-C3N4/Ag-SCN photocatalysts displayed comparable peak diffraction intensities owing to the similar amount of metallic Ag phase incorporated in the photocatalyst.
To further demonstrate the successful loading of Ag nanoparticles on the g-C3N4 surface and the selective adsorption of SCN- on the surface of Ag nanoparticles, the morphology and microstructures of the prepared samples were investigated by SEM and TEM, and the results are shown in Fig. 3. As observed, the g-C3N4 sample (Fig. 3(a)) was composed of many agglomerates of several micrometers in size. The g-C3N4/Ag, g-C3N4/Ag-SCN, and g-C3N4/SCN samples retained the same morphology as that of the g-C3N4 sample owing to the mild modification process used and minimal amounts of Ag and SCN ions deposited (Fig. 3(b)-(d)). According to the energy-dispersive X-ray spectroscopy (EDS) results (insets in Fig. 3(b) and (c)), the amount of metallic Ag incorporated was ~0.2 at% in the g-C3N4/Ag and g-C3N4/Ag-SCN photocatalysts. A new S signal could be observed in the EDS pattern of the g-C3N4/Ag-SCN(0.3 mM) sample (Fig. 3(c)), suggesting that the SCN- ions were loaded on the g-C3N4/Ag surface. To observe the morphology and phase structure of the Ag nanoparticles, TEM analysis was conducted. The TEM images of g-C3N4/Ag-SCN(0.3 mM) are shown in Fig. 3(e) and (f). As indicated by the red circles, the Ag nanoparticles (< 20 nm in size) were homogeneously dispersed on the g-C3N4 surface. The above results confirm the successful synthesis of the g-C3N4/Ag-SCN photocatalysts.
XPS was used to further confirm the chemical states of Ag and SCN- on the g-C3N4 surface. Fig. 4(a) shows the XPS survey spectra of the pure g-C3N4, g-C3N4/Ag-SCN, and g-C3N4/SCN samples. All samples displayed the main peaks of C and N elements from g-C3N4 [56]. Small XPS peaks of Ag and S elements could also be found in the g-C3N4/Ag-SCN samples. More detailed information of the elements in the samples could be obtained from the corresponding high-resolution XPS spectra in Fig. 4(b)-(e). The C 1s XPS spectra displayed two distinct peaks at about 284.8 and 288.2 eV, which can be attributed to the pollutant from the XPS equipment and the triazine rings (N-C=N), respectively (Fig. 4(b)). The N 1s XPS spectra (Fig. 4(c)) displayed three main peaks at about 398.8, 400.4, and 401.8 eV, which can be ascribed to the sp2-bonded nitrogen in N-containing aromatic rings (C-N=C), tertiary nitrogen (N-(C)3), and the amino functional groups (C-N-H) on the fringes of g-C3N4 [57, 58], respectively. The Ag 3d XPS spectra (Fig. 4(d)) displayed peaks with binding energies of 367.6 eV (Ag 3d5/2) and 373.9 eV (Ag 3d3/2), which can be ascribed to Ag0 [59]. The S 2p XPS spectra of g-C3N4/Ag-SCN and g-C3N4/SCN showed weak peaks at 161.4 eV (Fig. 4(e)), which can be attributed to adsorbed SCN-. According to the element component analysis based on the XPS results (Table 1), the calculated amounts of Ag and S in g-C3N4/SCN(0.3 mM) were ~0.45 and 0.22 at%, respectively. The XPS results further proved that Ag and SCN- were successfully incorporated on the g-C3N4 surface.
The light absorption properties of the pure g-C3N4, g-C3N4/Ag, and g-C3N4/Ag-SCN photocatalysts were characterized by UV-vis diffuse reflectance spectroscopy (Fig. 5). The g-C3N4 sample featured an absorption edge of ~460 nm, corresponding to a band gap of ~2.7 eV. After surface deposition of Ag nanoparticles, the resultant g-C3N4/Ag showed strong visible light absorption features in the range of 460-800 nm, which can be attributed to the localized surface plasmon resonance (LSPR) effect of metallic Ag nanoparticles. Modification of g-C3N4/Ag with SCN- to form g-C3N4/Ag-SCN resulted in a slight decrease in visible light absorption, and the corresponding sample was of a lighter brown (inset of Fig. 5). The above results suggest that the LSPR effect of the Ag nanoparticles can be slightly influenced by the addition of SCN-, further confirming the formation of a strong coupling interface between Ag and SCN-. The g-C3N4/SCN photocatalyst exhibited a comparable UV-vis spectrum to that of pure g-C3N4, indicating that the adsorption of SCN- alone on the g-C3N4 surface cannot cause the change observed in visible light absorption. The above results confirm the successful preparation of the g-C3N4/Ag-SCN photocatalysts.
The photocatalytic performance of the samples prepared was evaluated by monitoring the hydrogen evolution under visible light irradiation; the results are shown in Fig. 6(a). The pure g-C3N4 sample displayed a very low photocatalytic H2-evolution activity (~0.15 μmol h-1), as consistent with the previously reported results [60]. However, upon modification with Ag co-catalyst, the photocatalytic H2-evolution activity of the resultant g-C3N4/Ag sample showed a slight improvement (~0.71 μmol h-1) owing to effective separation of the photogenerated electrons from the g-C3N4 surface to the Ag nanoparticles. When KSCN selectively adsorbed onto the Ag surface, all resulting g-C3N4/Ag-SCN samples exhibited remarkably higher photocatalytic H2-evolution activities than g-C3N4/Ag. Particularly, g-C3N4/Ag-SCN(0.3 mM) displayed the highest photocatalytic activity (~3.89 μmol h-1), which is higher than that of pure g-C3N4 by a factor of 25.5. However, further increasing the amount of SCN- ions led to a slight decrease in the photocatalytic performance, as consistent with the performance of widely reported co-catalyst-modified photocatalysts [61]. Furthermore, g-C3N4/SCN(0.3 mM) displayed comparable photocatalytic H2-evolution activity (~0.15 μmol h-1) to that of pure g-C3N4, indicating that the SCN- ions alone on the g-C3N4 surface (without metallic Ag) have a negligible effect on the photocatalytic H2-evolution activity. To evaluate the H2-evolution stability of the g-C3N4/Ag-SCN photocatalysts, g-C3N4/Ag-SCN(0.3 mM) was subjected to repeated photocatalytic runs. The results are shown in Fig. 6(b). As observed, the g-C3N4/Ag-SCN photocatalyst could maintain a stable and effective photocatalytic performance under visible light irradiation during repeated tests. Hence, the g-C3N4/Ag-SCN photocatalyst can be regarded as a highly efficient visible light photocatalyst for H2 production.
Based on the results obtained, a mechanism for the photocatalytic H2-evolution over g-C3N4/Ag-SCN photocatalysts is proposed and schematically illustrated in Fig. 7(a). In the g-C3N4/Ag-SCN photocatalyst, the SCN- anions with strong electronegativity can be selectively adsorbed on the metallic Ag surface owing to its strong affinity for sulfur to form Ag-SCN- bond [53]. The sulfur atoms in the SCN- ions can effectively capture positive H+ ions from solution owing to their strong nucleophilicity [62], while the SCN- ions with a strong electronegativity can facilitate electron transfer from the Ag surface [45], subsequently promoting the direct reduction of protons to hydrogen. A more detail photocatalytic mechanism of g-C3N4/Ag-SCN photocatalyst is proposed in Fig. 7(b). After band gap excitation of bulk g-C3N4, the Ag nanoparticles first act as an electron sink to rapidly capture photogenerated electrons from the conduction band of g-C3N4 and then serve as an electron-transfer mediator to steadily transport photogenerated electrons to the adsorbed SCN- ions. The SCN- anions serve as interfacial active sites to effectively absorb protons from solution and then promote the subsequent interfacial H2-evolution reaction. Furthermore, the higher photocatalytic H2-evolution activity of g-C3N4/Ag-SCN when compared with that of the single-component-modified g-C3N4/Ag and g-C3N4/SCN photocatalysts indicates the synergistic effect of Ag as an electron-transfer mediator and SCN- as an interfacial catalytic active site. Therefore, it is apparent that the synergistic effect between Ag and SCN- accounts for the greatly improved photocatalytic H2-evolution performance of the g-C3N4/Ag-SCN photocatalyst.
To further understand the role of metallic Ag and SCN- co-catalysts in the photocatalytic H2-evolution reaction over g-C3N4/Ag-SCN photocatalyst, the photoelectrocatalytic activities of the various samples were investigated. Fig. 8 displays the polarization curves of the g-C3N4, g-C3N4/Ag, g-C3N4/Ag-SCN, and g-C3N4/SCN samples. As observed, the Ag loading onto g-C3N4 surface enhanced the current density, indicating that Ag functions as a co-catalyst to rapidly capture electrons from the g-C3N4 surface. Upon further selective adsorption of SCN- on the Ag surface, the prepared g-C3N4/Ag-SCN sample displayed a further improved current density, which can be attributed to the fact that the adsorbed SCN- can serve as an interfacial catalytic active site to effectively adsorb H+ ions from solution, thereby promoting the effective reduction of H+ to H2.
g-C3N4/Ag-SCN photocatalysts were synthesized via facile photodeposition of metallic Ag on g-C3N4 and subsequent selective adsorption of SCN- ions on the Ag surface by an impregnation method. The resulting g-C3N4/Ag-SCN photocatalysts showed a higher photocatalytic H2-evolution performance than g-C3N4, g-C3N4/Ag, and g-C3N4/SCN under visible light irradiation. In particular, g-C3N4/Ag-SCN(0.3 mM) achieved the highest photocatalytic activity (3.89 μmol h-1), which is higher than that of pure g-C3N4 by a factor of 25.5. The improved photocatalytic performance of the g-C3N4/Ag-SCN photocatalyst can be attributed to the synergistic effect of metallic Ag and SCN- ions, whereby the Ag nanoparticles function as an effective electron-transfer mediator for the steady capture and rapid transportation of photogenerated electrons, while the adsorbed SCN- serves as an interfacial active site to effectively absorb protons from solution and promote rapid interfacial H2-evolution reaction. Considering the present facile synthesis and its high efficacy, the present work may provide new insights into preparing high-performance photocatalytic materials.
2017,
Vol. 38 Issue (12): 1990-1998 
PDF
2017,
Vol. 38 Issue (12): 1990-1998 PDF