Photocatalysts such as heterostructured photocatalysts have shown promise for use in environmental remediation and water splitting [1, 2]. In heterostructured photocatalysts, which are produced by incorporating cocatalysts into narrow band gap semiconductors, the internal electric field that can build up at the interface can be used to rapidly break up electron/hole pairs, thereby enhancing the solar-to-fuel conversion efficiency [3]. Li et al. [4-6] reported that dual cocatalysts consisting of Pt and PdS on CdS achieved the highest quantum efficiency (93%) obtained to date in H₂ production by water splitting. Grätzel et al. [7] immobilized a molecular cocatalyst on Cu₂O to produce a photocathode for CO₂ reduction, effectively enhancing the solar-to-fuel conversion efficiency. Nanocrystalline semiconductors such as CdS have advantages such as controllable well-defined geometric structures, few body and surface defects, and large surface areas [8]. These have been widely studied in recent decades [9-11] but the precise synthesis of sophisticated CdS heterostructures (HCs) with controllable well-defined structures is still a challenge. CdS suffers from severe photo-erosion [12]. One strategy for tackling such problems is to deposit a protective layer of ultrathin carbon around CdS; this greatly improves the photocatalytic stability and activity [13]. Another strategy is to synthesize HCs such as PbSe-CdS [14], which can pull electrons/holes away from CdS; this protects CdS from photo-erosion and improves the photocatalytic activity. One-dimensional HCs with tipped materials, such as Au-tipped CdSe-seeded CdS (CdSe@CdS) nanorods (NRs) are of particular interest because the long axis of the rod naturally provides a direct path for charge transport [15]. The production of such structures involves synthesis of semiconductor nanoparticles and subsequent heterogeneous nucleation and growth via a mild deposition process.

In our previous studies, we investigated the deposition of symmetric tips such as a metal (Au), metal oxide (Fe₃O₄), or sulfide (PdS) onto CdS NRs, i.e., the materials deposited on both ends of the NRs are the same [16]. In this process, it is necessary to minimize homogeneous nucleation of the metal precursor and/or Ostwald ripening of the semiconductor particles during deposition. Au-CdS NRs and PdS-CdS NRs show good charge separation [17]. Here, we report the deposition of asymmetric tips, i.e., one tip is Au and the other tip is PdS, on CdSe@CdS NRs. We used the obtained nanostructures in water splitting for H₂ production. The H₂ production efficiency and photocatalytic stability of CdS were greatly enhanced by efficient charge separation.

Scheme 1 shows the procedure for PdS-CdSe@CdS-Au NR synthesis. The synthesis of CdSe-seeded CdS NRs was described in detail in an earlier publication [18]. Au-tipped CdSe@CdS NRs with a match-stick structure were obtained by controlling the concentration of the Au precursor. In a typical synthesis, a HAuCl₄ solution (20 mg/mL, 2 mL) and a dodecylamine toluene solution (28 mg/mL, 2 mL) were mixed. A CdSe@CdS NR stock solution (~50 μmol/L, 1 mL) was injected into the mixed solution and the reaction was performed for ca. 30 min at room temperature. PdS was then deposited on the other end of the Au-CdSe@CdS NRs by cation exchange. A mixture of CdSe@CdS NR toluene solution (1 mL), oleylamine (2 mL), and oleic acid (1 mL) was rapidly injected into a Pd(acac)₂ precursor solution at 180 ℃ and the reaction was continued for ca. 15 min under N₂. The product was transferred into an aqueous solution by ligand exchange with mercaptoundecanal acid and used for water splitting. An aqueous solution of the obtained photocatalyst (100 mL) was placed in a reactor under a 300 W Xe lamp. Na₂S (0.5 mol/L) and Na₂SO₃ (0.5 mol/L) were used as sacrificial reagents. The solution was thoroughly degassed and the amounts of H₂ evolved were determined by gas chromatography with Ar as the carrier gas.

Fig. 1 shows the structural characterization results for the as-synthesized CdSe@CdS NRs, Au-tipped CdSe@CdS NRs with a match-stick structure (Au-CdSe@CdS NRs), and asymmetrically tipped CdSe@CdS NRs (PdS-CdSe@CdS-Au NRs). The CdSe@CdS NRs are ca. 50 nm in length and 4.2 nm in diameter. The NR size is highly monodispersed. The transmission electron microscopy (TEM) image in Fig. 1(b) shows that round Au tips were deposited on one end of the CdSe@CdS NRs with a selectivity of ca. 85% by controlling the concentration of the Au precursor. The energy-filtered transmission electron microscopy (TEM) image in Fig. 1(c) confirms that the majority of the CdSe@CdS NRs were decorated with Au on only one tip. The average size of the Au tips is ca. 1.5 nm. Fig. 1(d), (e), and (f) show TEM and high-resolution TEM images, and the energy-dispersive X-ray spectrum (EDS) of PdS-CdSe@CdS-Au NRs. These show that one end of the CdSe@CdS NRs is tipped with PdS, and the other end is tipped with Au. The clearly observed lattice spacing of ca. 0.336 nm is attributed to the (002) plane of the CdS body of the NRs. The lattice spacing of ca. 0.24 nm on the tip is attributed to the (111) plane of crystalline Au. The other tip of the NR clearly differs from the CdS and Au parts; its shape and size differ from those of the Au tip. We performed control experiments (Fig. S1) in which PdS-CdSe@CdS-PdS NRs were synthesized. The PdS tips also lacked of crystalline lines. Metallic Pd would show crystalline lines [16], therefore we conclude that the other tip is PdS; this is in good agreement with the literature [19]. Furthermore, the EDS data in Fig. 1(f) confirm the presence of Pd and S, indicating formation of PdS. After PdS deposition, the length of the CdSe@CdS NRs decreased slightly because of partial cation exchange of CdS with Pd precursors. The selectivity for the PdS-CdSe@CdS-Au NRs was ca. 70%, as shown in Fig. S2. These results show that we successfully synthesized asymmetrically tipped CdSe@CdS NRs, with one Au tip and one PdS tip. Note that although bare CdSe@CdS NRs can be easily synthesized using solvothermal methods, the pyrolysis of metal-organic precursors at a high temperature gives highly crystalline CdSe@CdS NRs.

Fig. 2(a) shows the ultraviolet-visible (UV-Vis) absorption spectra of CdSe@CdS NRs, Au-CdSe@CdS NRs, and PdS-CdSe@CdS-Au NRs. The CdSe@CdS NRs give four distinct absorption bands; the peaks at λ @ 470, 430, and 390 nm correspond to the shell CdS, and the weak peak at λ @ 580 nm corresponds to the CdSe core. The deposition of Au on only one tip did not greatly change the peaks at λ = 430 and 380 nm because the volume of Au is small relative to that of the NR body. After deposition of the PdS tip to form asymmetric PdS-CdSe@CdS-Au NRs, the CdS absorption peaks became ambiguous, indicating strong electronic coupling between the CdS shell and its two tipped domains. Fig. 2(b) shows that the CdSe@CdS NRs gave a photoluminescence (PL) peak at l @ 597 nm, with a full width at half maximum of ca. 35 nm. After Au and PdS deposition, the PL intensity decreased to nearly zero because of strong fluorescence quenching by Au and PdS; this indicates that rapid charge separation occurred. Note that the PdS PL peak at λ = 650 nm was not observed because of the relatively small volume of PdS.

Fig. 3(a) shows the photocatalytic activities of the PdS-CdSe@CdS-Au NRs, Au-CdSe@CdS NRs, and CdSe@CdS NRs. For the CdSe@CdS NRs and Au-CdSe@CdS NRs, the amount of H₂ produced increased linearly with time. The amount of H₂ produced by each sample was ca. 50 mmol in 5 h, and Au-CdSe@CdS gave a slightly better performance than CdSe@CdS. For the PdS-CdSe@CdS-Au NRs, the amount of H₂ produced increased linearly in the first 3 h and then reached a plateau at ca. 1100 mmol at around 5 h. This is more than two orders of magnitude greater than the production achieved by CdSe@CdS and Au-CdSe@CdS. This proves that the asymmetrically tipped PdS-CdSe@CdS-Au NRs had significantly higher photocatalytic activity in water splitting. A plateau is reached because of NR photo-erosion. The TEM images in Fig. 3(b) and 3(c) show photo-erosion tests of the as-obtained PdS-CdSe@CdS-Au NRs and CdSe@CdS. CdSe@CdS was completely destroyed after photoirradiation for 1.5 h, but the PdS-CdSe@CdS-Au NRs retained their shapes and edges even after 4 h of photoirradiation. This clearly proves that the asymmetrically tipped PdS-CdSe@CdS-Au NRs had significantly improved photostability. In H₂ production over a longer period, PdS-CdSe@CdS-Au corroded after 5 h, as shown in Fig. S3, therefore use of the NRs over long periods of time would result in structural damage. In summary, incorporation into asymmetrically tipped PdS-CdSe@CdS-Au NRs significantly improved the photocatalytic activity and photostability of CdS. The performances of these NRs in the photodegradation of organic dyes (Fig. S4) showed a similar trend.

We performed control experiments to explore the synthetic mechanism, as shown in Fig. S1. High selectivity for the formation of PdS on the two ends of CdSe@CdS NRs can be achieved by increasing the temperature and the concentrations of the Pd and S precursors. According to previous reports, PdS formation mainly involves kinetically driven cation exchange [19]. We therefore hypothesized that cation exchange on one tip only is feasible. The kinetically controlled formation of CdSe@CdS NRs with Au on only one tip was reported earlier [18]. The reason for one-tipped NR formation is as follows. First, the chemical potential of a nanocrystal surface is inversely related to its radius (the Gibbs-Thomson effect). This suggests that secondary nucleation and growth should occur more rapidly on the tips than on the sides. Secondly, the atomic structure of cadmium chalcogenide NRs is not centrosymmetric; one tip is sulfur rich, resulting in preferential deposition of Au on that tip. We therefore tentatively propose that the mechanism of PdS-CdSe@CdS-Au NR formation is as follows. Au is deposited on the favored end, and this increases the chemical potential of that tip, preventing deposition of PdS on the same end. PdS is then deposited on the other end via partial cation exchange. On the basis of the H₂ production experimental results and literature reports, we propose the H₂ production mechanism shown in Fig. 4. The energy band gaps of CdS and PdS are 2.4 and 1.6 eV, respectively [6]. As discussed above, excited electrons from the CdS NRs are transferred to the Fermi level of Au and holes are transferred to PdS, because of band alignment. This greatly facilitates charge separation, and electron and hole transport to the opposite ends, greatly reducing recombination rates. Protons are therefore efficiently reduced on CdS NRs and Au tips, and this significantly increases the H₂ production efficiency. We suggest that the large surface-to-volume ratio of the NRs and ease of access to reactants because of the small NR diameter also play a role.

In conclusion, we synthesized asymmetrically tipped PdS-CdSe@CdS-Au NR HCs by a wet-chemical method. Their UV-Vis absorption spectra showed strong electronic coupling between both tips, i.e., PdS and Au, and the CdS body, resulting in partial disappearance of the CdS absorption peaks. PdS-CdSe@CdS-Au NRs gave a H₂ production rate of ca. 1100 mmol over 5 h. This is two orders of magnitude greater than the rate achieved using Au-CdSe@CdS NRs, with only one tip. PdS-CdSe@CdS-Au withstood 4 h of photoirradiation, compared with 1.5 h for CdSe@CdS NRs, indicating that the photostability of PdS-CdSe@CdS-Au NRs is better than that of CdS. The experimental results and control experiments indicated that the improved photocatalytic activity and stability are the result of efficient charge separation and rapid transport in asymmetrically tipped linear PdS-CdSe@CdS-Au HCs.