Cu1.94S synthesis steps areprovided in the method section. All of the reagents were purchased commercially and used without further purification. The sulfur source was 1-dodecanethiol and the copper source was Cu(CH3COO)2or Cu3(BTC)2 sol prepared followingreferences in literature 22.PXRD (Fig. 1a) and FTIR spectra (Fig. 1b) were used to analyze the powder material phase and to investigate the presence of organic cappingagentson particle surfaces, respectively. It is observable that bothcatalysts prepared by either Cu(CH3COO)2 (upper) or Cu3(BTC)2 sol (below) display similar PXRD peaks. All of the diffraction peaks can be indexed as djurleite Cu1.94S (PDF#23‒0959). As for FTIR spectra, there are stretching vibration absorption peaks indicating Cu‒O (726 cm‒1), C-H (2857 and 2927 cm‒1) caused by 1-dodecanethiol, and C = O present in carboxyl groups (1467 and 1645 cm‒1) from CH3COO‒ (upper) or BTC3‒ (below).
Cu1.94S morphologies were characterized using TEM. Cu(CH3COO)2 and Cu3(BTC)2 prepared Cu1.94S aggregate as randomly distributed nanoparticles with diameters of 6.1 ± 0.9 nm (Fig. 2a-c, denoted as RA) and Cu1.94S ordered superstructure with diameters of 5.1 ± 0.4 nm (Fig. 2d-i, denoted as SS), respectively. The inset FFT pattern in Fig. 2e shows high stacking order, six-fold axes, and confirms a HCP superstructure(Figure S1)10. Ordering also occurs in SS over hundreds of nanometers (Fig. 2g). Small angle XRD of SS shows equidistance’s of 7.98 nm, which is coincidental with the periodicity found in ordered structure (Fig. 2h) images. Single particle HRTEM images of SS (Fig. 2i) shows good crystallinity and the lattice fringe can be indexed to the (804) plane, which corresponds to the strongest XRD diffraction peak found at a 2θ of 37.62° (Fig. 1a).
Photocatalytic hydrogen evolution experiments were conducted to characterize catalyst performance. Tris(2,2’-bipyridine)ruthenium(ii) complex [Ru(bpy)3]Cl2, was added as visible-light harvesting agent. An exact description of the catalyst system (denoted as [Ru(bpy)3]Cl2, Ru-RA, or Ru-SS) is provided in the experimental section. UV-vis spectra (Fig. 3a) show that orange [Ru(bpy)3]Cl2, Ru-RA, and Ru-SS absorb visible light intensely, while black copper sulfides exhibit weak absorption. This result also shows that [Ru(bpy)3]Cl2 is the strongest photosensitizer. Photoluminescence (PL) spectra (Fig. 3b) were used to investigate photo-induced electron and hole recombination. Obvious fluorescence quenching occurred in Ru-SS, which indicates effective inhibition of recombination.
Capacitance measurements were conducted to obtain a Mott-Schottky plot (Figure S2), from which one can see that SS is a p-type semiconductor with a flat band potential of 1.68 V and a top valence band of 1.78 V. Solid UV–vis absorption spectra (Figure S3) shows that the bottom conduction band (CB) of SS is ‒0.22 V, which is lower than E(H2O/H2). This CB is more positive than the HOMO of [Ru(bpy)3]Cl2 (‒0.88 V)[23] and permits electron transfer from [Ru(bpy)3]Cl2 to SS (Fig.3c). Theadaptive band structure enable Ru-SS to photo-catalyze hydrogen evolution.
Photocatalytic H2 evolution reactions (HER) were also carried out under visible light irradiation as a model reaction. H2 evolution rates found in Ru-SS were 2482.00 µmol g‒1 h‒1 and about five times greater than that of Ru-RA (520.29 µmol g‒1 h‒1). The Cu1.94S-SS system in this paper shows high H2 evolution rates. As shown in Table 1, this rate was even found to be higher than those of some other metal sulfide nanocomposites27 − 33.
Table 1
Comparison of Ru-SS with nano-copper sulfides for photocatalysis of H2 evolution reactions
Catalyst
|
H2 evolution rate
µmol g− 1 h− 1
|
Reference
|
Ru-SS
|
2482.00
|
this work
|
Ru-RA
|
520.29
|
this work
|
SS
|
20.46
|
this work
|
[Ru(bpy)3]Cl2
|
0
|
this work
|
Cu1.94S
|
34.038
|
24
|
Cu1.94S
|
20
|
25
|
Cu1.94S
|
11
|
26
|
Photoelectrochemistry, photophysics, and electrical performance were further studied to elucidate this markedly improved catalysis. Conditions for photoelectrochemical measurement are described in the experimental section. Both Ru-RA and Ru-SS show fast photocurrent response, as shown in Fig. 4a. Ru-SS exhibits higher photocurrent density, which indicates faster charge separation. Time-resolved fluorescence decay spectra (Fig. 4b) show that the average fluorescence lifetimes for [Ru(bpy)3]Cl2, Ru-RA, and Ru-SS are 148.95, 155.22, and 162.88 ns, respectively. Ru-SS shows the longest lifetime, which indicates the best electron-hole pair separation or lowest electron-hole recombination rates. Nyquist plot were used to measure the electrical conductivity of RA and SS. Smaller Nyquist arc radii seen in SS (Figure S4) show higher electrical conductivity, which suggests faster interparticle charge transfer. Therefore, SS related samples provide relatively fast photo response, low electron-hole recombination rates, and fast interparticle charge transfer. These properties collectively enable significantly improved photocatalytic performance.
Fine structure investigation is one of the challenges inhibiting deep understanding and rational design of catalysts. RA and SS exhibit distinguished catalytic activity but show only stacking pattern differences according to TEM. Thus X-ray Absorption Fine Structure (XAFS) was used for further characterization and results are shown in Fig. 5 and Table S1. Figure 5c shows EXAFS peaks of curves in R space that are characterized as Cu‒S bonds. According to local structure fitting (Table S1), the Cu‒S average coordination number (CN) is 1.5 and 3.1 for SS Cu1.94S and RA Cu1.94S, respectively. This is much lower than that of bulk Cu1.94S, as shown in its lattice structure (Figure S5), but is reasonable for nanomaterials with large quantities of unsaturated surface coordinate ions. Here the unsaturated coordinate ions are copper. Closed packed structures in SS can provide extra stability for abundant dangling bonds upon the CN difference of SS and RA. Cu‒O bonds are present in incomplete Cu2+‒BTC3‒ (SS) or Cu2+‒CH3COO‒(RA), which is deduced from IR spectra (Fig. 1b) and confirmed by EXAFS calculation. The lower Cu‒O CN in SS (1.2) than that in RA (2.6) indicates that the amount of BTC3‒ (SS) is less than that of CH3COO‒ (RA) on the nanoparticle surfaces. Relative anion amounts were also identified using zeta potentials, which are − 10.07 mV and ‒18.35 mV for SS and RA, respectively. Less negative zeta potentials present in SS indicate less negatively charged carboxylate and thus more unsaturated copper.
Normalized Cu K-edge X-ray absorption near edge structures (XANES) of SS and RA are shown in Fig. 5b. The lower absorption edge of SS shows that the oxidation number of copper in SS is lower than that in RA. This may be due to a less abundance of Cu2+ from incompletely broken Cu2+‒BTC3‒ than that from incompletely broken Cu2+‒CH3COO‒.
More surface dangling bonds, lower Cu‒O CN, and lower oxidation number indicate that SS has less BTC3‒ on nanoparticle surfaces. This is potentially due to close-packed ordered arrangements. In addition, the steric hindrance of BTC3− is greater than that of CH3COO‒. When considering molecular structures, it can be deduced that the close packed ordered superstructure, as well as residue BTC3‒, block efficient transport of large [Ru(bpy)3]Cl2 (1.329 nm⋅1.329 nm⋅1.329 nm) and triethanolamine (TEOA, 1.150 nm⋅1.150 nm⋅1.070 nm) (Figure S6) outwards but allow small H2O to enter the interior of SS.
This process and key factors for remarkably enhanced catalytic performance found in SS photocatalyst is summarized in Scheme 1. First, incident visible light induces metal-to-ligand charge transfer and generates electron-hole pairs in [Ru(bpy)3]Cl2. Afterwards, electrons move to Cu1.94S due to their adaptive band structure. Then, 1-dodecanethiol and carboxylate on particle surfaces, along with the close packed structure, block large [Ru(bpy)3]Cl2 and TEOA transport outwards but allow small H2O to enter the interior of Cu1.94S aggregates. Electrons are used to reduce H2O to H2, while holes are sacrificed by TEOA around [Ru(bpy)3]Cl2. The catalytic region is divided into reduction zones (interior) and oxidation zones (outside) for SS. In comparison with RA, SS prepared by MOFs exhibit ordered superstructure and show relatively higher electron conductivity caused by uniform electromagnetic fields and interparticle interactions. SS has also more surface dangling bonds and offers more catalytic active sites. Therefore, catalysts containing SS provide more efficient separation of electrons and holes. Close packed SS with more steric hindrance provided by BTC3‒ also block [Ru(bpy)3]Cl2 and TEOA more efficiently.
In summary, we constructed Cu1.94S nanocrystal ordered superstructure and clearly show that the materials have beneficial properties distinct from single and randomly aggregated counterparts. The close-packed ordered nanoparticles were simply prepared by careful selecting MOFs as metal sources that simultaneously endow materials with more catalytic active sites and more efficient redox. Therefore, as prepared model Cu1.94S exhibits greatly improved HER catalytic performance of 2482.00 µmol g‒1 h‒1 and provides guidelines to inform future catalyst design.