Cadmium-free CuInS2/ZnS quantum dots as efficient and robust photosensitizers in combination with a molecular catalyst for visible light-driven H2 production in water

M. Sandroni ab, R. Gueret a, K. D. Wegner b, P. Reiss b, J. Fortage *a, D. Aldakov *b and M.-N. Collomb *a
aUniv. Grenoble Alpes, CNRS, DCM, 38000 Grenoble, France. E-mail: jerome.fortage@univ-grenoble-alpes.fr; marie-noelle.collomb@univ-grenoble-alpes.fr
bUniv. Grenoble Alpes, CEA, CNRS, INAC-SyMMES, 38000 Grenoble, France. E-mail: dmitry.aldakov@cea.fr

Received 12th January 2018 , Accepted 26th February 2018

First published on 10th April 2018


We demonstrate that cadmium-free core–shell CuInS2/ZnS quantum dots (QDs) are very efficient and robust visible-light absorbing photosensitizers for photocatalytic hydrogen production in a fully aqueous solution when associated with a molecular catalyst, a cobalt tetraazamacrocyclic complex. In the presence of ascorbate as a sacrificial electron donor, this new hybrid system exhibits a remarkable activity for hydrogen production under visible light irradiation at pH 5.0 with up to 7700 and 1010 turnover numbers versus catalyst and QDs, respectively, and with an initial turnover frequency per QD of 1.59 mmolH2 gQD−1 h−1. These are the best performances reported so far with cadmium-free QDs in combination with a molecular catalyst, highlighting the great potential of ternary chalcopyrite nanocrystals as efficient and robust materials for solar fuel production.



Broader context

The conversion of solar energy into fuels is a convenient way to store the intermittent and oddly distributed energy of photons in the form of chemical potential, as performed in photosynthetic systems. Artificial photosynthetic systems generally associate a photosensitizer, in charge of efficient light harvesting, with a reduction catalyst that converts photogenerated charges into chemical bonds. Photosensitizers are typically based on rare and expensive molecular ruthenium or iridium transition metal complexes, even though they exhibit insufficient stability during photocatalysis to achieve high performances. A promising alternative, recently explored for hydrogen production, is to replace these molecular photosensitizers by semiconductor nanocrystals (quantum dots) and to employ them in combination with noble metal-free H2-evolving catalysts. Quantum dots display indeed higher photostability and a large absorption in the visible domain and their optoelectronic properties are easily tuned by varying their size and composition. However, most of these “hybrid” systems use toxic and heavy metal-based quantum dots such as CdS, CdSe and CdTe, limiting their practical utilization. We demonstrate here that cadmium-free core–shell CuInS2/ZnS quantum dots are very efficient visible-light-absorbing photosensitizers for photocatalytic hydrogen production, in association with a molecular cobalt catalyst. This study highlights the great potential of ternary chalcopyrite nanocrystals as efficient and robust materials for solar fuel production. Our results also emphasize the importance of developing hybrid systems, combining the benefits of semiconductor nanoparticles, such as strong light absorption and high stability, to the efficiency of molecular H2-evolving catalysts.

Introduction

Hydrogen (H2) is considered as the future energy carrier and this sparked great scientific and economical interest for its green production by solar water splitting.1–4 A classical strategy for H2 production is based on a three-component molecular system operating in a homogeneous solution, combining a photosensitizer (PS) as a light collector, a proton reduction catalyst (Cat), and a sacrificial electron donor (SD) which supplies the system with electrons.5–8 In most of the studies, molecular PSs based on rare metal coordination complexes such as [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) or its derivatives9,10 were used, even though they exhibit insufficient stability during photocatalysis to achieve high performances.11,12 A promising alternative recently explored is to replace these molecular PSs by semiconductor nanocrystals (quantum dots, QDs) for the design of “hybrid photocatalytic systems” for H2 evolution.13,14 Metal chalcogenide QDs are characterized by unique photophysical properties, which make them superior to molecular dyes. They possess higher photostability and a broad absorption spectrum with high extinction coefficients, and additionally, their optical and electronic properties can be tuned by the variation of their size composition.14–16 Examples of hybrid photocatalytic systems for H2 production combining QDs with a molecular complex as a catalyst in a homogeneous solution started to emerge in 2011.17–33 There have also been some studies concerning the use of simple inorganic salts that could play the role of catalysts, when coordinated to capped ligands of QDs.30,34–39 Although the photocatalytic performance and the stability of these hybrid systems in water are considerably higher in comparison to “all-molecular” systems, they use toxic and heavy metal-based QDs such as CdS, CdSe and CdTe, which are not compatible with green chemistry and sustainable development.

Very recently, cadmium-free QDs40 and carbon dots (CDs)41–44 were employed in combination with noble metal-free H2-evolving molecular catalysts. However, these hybrid systems remain less efficient compared to those using Cd-based QDs. Indeed, the group of Chen40 reported a relatively high number of catalytic cycles (turnover number, TON) and turnover frequency (TOF) per QD using (AgIn)xZn2(1−x)S2 QDs with [Co(bpy)3]Cl2 as a catalyst and ascorbic acid (H2A) as an SD (TONQD ≈ 1200 and TOFQD = 276 mmolH2 gQD−1 h−1); however, the TON per catalyst remains quite low (TONCat < 10). Additionally, this photocatalytic system operates in organic solvents, which is not compatible with future water splitting applications. Other examples of Cd-free hybrid systems reported by the group of Reisner41–44 based on water-soluble carboxylate-capped CDs in association with a nickel phosphine catalyst (NiP) operate in a fully aqueous solution. However, although the low cost and appealing photostability of such metal-free CDs make them undoubtedly very interesting materials,44,45 their weakly negative reduction potential related to a low energetic conduction band makes them compatible only with catalysts exhibiting low overpotentials such as NiP. In addition, the restriction of their absorption to the UV region critically limits their application for solar fuel production. Thus, in the presence of ethylenediamine tetraacetic acid (EDTA) as an SD,41 TONCat and TOFQD values obtained for the CD–NiP hybrid system were only 64 and 41 μmolH2 gQD−1 h−1, respectively. The substitution of EDTA by a mixture of tris(carboxyethyl)phosphine (TCEP)/sodium ascorbate (NaHA) significantly improves the stability of the catalyst and thus the TONCat (up to 1094), but the TOFQD remains low (52.8 μmolH2 gQD−1 h−1).42 Finally, the visible-light absorption properties of CDs were significantly improved through graphitisation and N-doping,43 allowing decreasing the CDs concentration and thus increasing the TOFQD (7.95 mmolH2 gQD−1 h−1) with EDTA; however the TONCat is still low (around 20).43

Therefore, despite this recent progress, there is a strong need to explore new families of cadmium-free QDs to access more efficient hybrid systems for solar fuel production. In this work, we have investigated for the first time the use of core–shell CuInS2/ZnS QDs (CIS/ZnS)46,47 as visible-light-absorbing photosensitizers for photocatalytic H2 generation in combination with a molecular H2-evolving catalyst. CIS and CIS/ZnS are among the most studied ternary chalcopyrite-type QDs and have attracted great interest due to their applications in biolabeling, bioimaging and photovoltaic cells.47,48 With a band gap of 1.55 eV for the bulk, a broad size-tunable absorption spectrum in the UV-visible range combined with a high absorption coefficient (>105 cm−1) and long-living excitons (>100 ns), CIS QDs are very well-suited for visible light-driven photocatalysis.47 Additionally, CIS and CIS/ZnS core–shell QDs display low toxicity,49 and can be directly synthesized in water using adequate capping agents, which are compatible with sustainable H2 production. Potential use of CIS and CIS/ZnS as visible-light responsive photocatalysts for solar H2 generation and organic dye degradation has been recently studied.47,50,51 However, CIS alone in the presence of a sacrificial electron donor shows poor photocatalytic activity for solar H2 generation owing to fast charge carrier recombination. The photocatalytic activity can be improved using CIS/ZnS core–shell QDs52–54 or by incorporation of cocatalysts based on noble (Pt, Ru, Pd or Au)54–57 or non-noble metal nanoparticles (ZnO, MoS2).58–60 A very recent study has also shown that nonstoichiometric In-rich CuxInyS QDs exhibit better activity for photocatalytic hydrogen production than the stoichiometric ones (Cu/In = 1[thin space (1/6-em)]:[thin space (1/6-em)]1).61 The great potential of CIS/ZnS QDs to activate a molecular catalyst for reduction reactions was also demonstrated very recently by the group of Weiss for the photocatalytic reduction of CO2 to CO using iron porphyrins as catalysts.62

Herein, we couple water-soluble glutathione (GSH) ligand capped CIS/ZnS QDs with one of the most efficient H2-evolving molecular catalysts in water, the cobalt tetraazamacrocyclic complex [CoIII(CR14)Cl2]Cl (Cat1) (CR14 = 2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) (Scheme 1).26,63–66


image file: c8ee00120k-s1.tif
Scheme 1 Hybrid photocatalytic system composed of CIS/ZnS–GSH core–shell QDs (GSH = glutathione), the [CoIII(CR14)Cl2]Cl catalyst (Cat1) and the sacrificial electron donor sodium ascorbate, and of the [CoII(CR15)(H2O)2]Cl2 catalyst (Cat2).

With NaHA as an SD, this new hybrid photocatalytic system was found to be very active towards H2 evolution and robust in a fully aqueous solution at pH 5.0, and thus represents the first example of an efficient Cd-free hybrid system for H2 production, exhibiting both high TONCat and TONQD and high TOFQD. Importantly, the high performances of the CIS/ZnS–Cat1–NaHA system are coupled with very good stability of CIS/ZnS QDs under photocatalytic conditions, which can be reused at will several times without loss of activity.

Results and discussion

Water-soluble luminescent CIS/ZnS–GSH nanocrystals.

Synthesis. Water-soluble CIS/ZnS QDs were synthesized in aqueous solution using microwave heating, according to a previously reported procedure.67 This type of synthesis has the advantage of providing luminescent particles directly soluble in water, without an additional water transfer step which is likely to perturb the surface and introduce defect states affecting photoluminescence and charge transfer reactions. Glutathione (GSH) was used as a water-solubilizing ligand for the CIS/ZnS QDs and as a reductant to convert Cu2+ to Cu+ during the synthesis. A special double purification protocol allowed obtaining orange-emitting QD samples with a narrow size distribution and an emission maximum of 638 ± 2 nm (see the Experimental section and Fig. S1, ESI).
Structural characterization. The size of the nanocrystals was determined by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). TEM images of the samples evidenced nanocrystals with an average diameter of 2.2 ± 0.3 nm (Fig. 1a). The X-ray diffractogram exhibited very broad peaks, coherent with small particles in the cubic phase of CIS with a lattice parameter of 5.45 Å (Fig. 1b and Fig. S2, ESI). The absence of the diffraction peak at 2Θ = 17.9° excluded the tetragonal chalcopyrite phase characteristic for bulk CIS. The peak width in the powder XRD experiments also gave an indication of the crystallite size, which was calculated to be around 2 nm using the Scherrer equation, in good agreement with the size measured by TEM. EDX measurements confirmed an off-stoichiometric copper-deficient composition of Cu0.3InZn1.90S3.40, which is in line with the synthetic protocol, although a slight variation was observed in the Cu[thin space (1/6-em)]:[thin space (1/6-em)]In ratio between the precursors ([Cu]/[In] = 0.25) and the purified nanocrystals ([Cu]/[In] = 0.30) (Fig. S3, ESI). The amount of organic surface ligands was estimated by thermogravimetric analysis (TGA) under Ar (Fig. S4, ESI). Ligand desorption occurring after a plateau at 100 °C represented around 15% of the total sample mass.
image file: c8ee00120k-f1.tif
Fig. 1 (a) TEM image of CIS/ZnS–GSH nanocrystals; and (b) powder X-ray diffractograms of CIS/ZnS–GSH nanocrystals before (red) and after 95 hours (blue) of photocatalysis.
Optical characterization. The nanocrystals exhibit a broad absorption in water covering the whole visible range up to 650 nm, devoid of a clear excitonic peak (Fig. 2). Two features can be distinguished in the absorption spectrum: a shoulder at lower wavelengths (350–460 nm), and a low energy tail (460–650 nm) which is probably due to electronic transitions involving intra-gap states. Using the size of the inorganic core determined by TEM and XRD and the mass of the surface ligands from TGA, we estimated the extinction coefficient of the CIS/ZnS QDs to be 22[thin space (1/6-em)]600 M−1 cm−1 at 400 nm (see the ESI for the details of the calculation). Unlike the core CIS nanocrystals displaying no luminescence, the CIS/ZnS–GSH core–shell QDs where the surface is passivated by a ZnS shell are characterized by an orange emission with a photoluminescence (PL) maximum at 638 nm and a quantum yield of around 2%. The emission peak is very broad (FWHM ≈ 120 nm, 0.36 eV) (Fig. 2) and most probably involves electronic states located in the band gap of QDs.46,68 The copper-poor stoichiometry, together with the presence of a wide band-gap ZnS shell, plays an important role in shifting the luminescence to lower wavelengths with respect to stoichiometric CuInS2 nanocrystals.48,68 The excitation spectrum suggests that the emission is mostly due to the excitation of the low energy tail (Fig. 2). It shows a well-defined peak between 500 and 600 nm (Fig. 2), corresponding to the low-energy feature in the absorption spectrum. Time-resolved PL measurements revealed a multiexponential PL decay, which can be attributed to the presence of several radiative decay pathways (Fig. S5, ESI). A three-exponential fit can be used to fit the decay, leading to decay times of 13 ns (6%) and 100 ns (38%), and a longer component of 371 ns, accounting for the majority of the signal (56%) in good agreement with the literature data (see Table S1, ESI).48 Such long-lived emission is highly beneficial for electron transfer to the catalyst and from the SD.
image file: c8ee00120k-f2.tif
Fig. 2 Normalized absorption (blue line) and PL emission spectra (red line, λexc = 450 nm) and excitation spectrum (dotted line, recorded at λem = 638 nm) of CIS/ZnS–GSH nanocrystals, recorded in water at room temperature in a 1 cm quartz cuvette.
Photocatalytic hydrogen production. Photocatalysis experiments were performed under visible light irradiation in a deaerated aqueous solution containing CIS/ZnS–GSH nanocrystals, Cat1, and the ascorbic acid (H2A)/sodium ascorbate (NaHA) buffer (total concentration of 0.5 M) providing electrons to the system and maintaining a constant pH value during photocatalysis. NaHA was selected since it is an excellent SD in acidic water with a low oxidation potential (0.30 V vs. SCE),7,69 which can ensure an efficient electron transfer to the QDs. The H2 production was quantified in real time by gas chromatography. The photocatalytic performances in terms of TONCat, TONQD, TOFQD and nH2 of all systems studied are summarized in Table S2 (ESI). The performances of the system were first investigated at three different pH values of 4.5, 5.0 and 5.5, using 106 ± 4 μM CIS/ZnS–GSH, 10 μM Cat1 and 0.5 M H2A/NaHA (Fig. 3). The catalysis proved to be very efficient with immediate H2 production at all pHs used. An optimum photocatalytic activity was obtained at pH 5.0 with high TONCat and TONQD values of 5900 and 1110, respectively, corresponding to 295 μmol of H2 (7.2 mL) after 96 h of irradiation, and to a high TOFQD value of 1.59 mmolH2 gQD−1 h−1. At pH 4.5, the TOFQD value is slightly higher (1.80 mmolH2 gQD−1 h−1), but the photocatalytic system is less stable, resulting in slightly lower TONCat and TONQD values. Conversely, the initial TOFQD decreases significantly at a higher pH of 5.5 (1.06 mmolH2 gQD−1 h−1), but the photocatalytic system appears to be more stable over time, finally providing TON values similar to those obtained at pH 4.5 (Fig. 3). Indeed, while under more acidic conditions the formation of the hydride species is promoted by protonation of the reduced Co(I) form of Cat1 ([CoI(CR14)(X)n]+), thus improving the H2-evolving catalytic activity;64,65 the stability of the catalyst decreases.7,64
image file: c8ee00120k-f3.tif
Fig. 3 Photocatalytic hydrogen production (TONCat, nH2) as a function of time from a deaerated aqueous solution (5 mL) of NaHA/H2A (0.5 M) at different pHs under visible light irradiation (400–700 nm) in the presence of CIS/ZnS–GSH QDs (106 ± 4 μM) and Cat1 (10 μM).

The remarkable catalytic activity in terms of TONsCat, TONsQD and TOFsQD at all pH values studied makes this system the most active hybrid photocatalytic system reported so far, associating cadmium-free QDs with a molecular H2-evolving catalyst.40,44 Moreover, such high TONCat values with this catalyst have never been achieved in association with CdTe QDs26 or with the molecular [Ru(bpy)3]2+ (Ru) complex as photosensitizers. Indeed, the maximum TONCat value reached in water was 680 TONsCat at pH 4.1 with CdTe QDs (59 μM), Cat1 (75 μM) and HA/H2A (0.1 M),26 and 1010 at pH 4.0 with Ru (1 mM) and HA/H2A (1.1 M) and Cat1 (100 μM).64

Control experiments without Cat1 at pH 5.0 produced only 13.3 μmol of H2 after 70 h of irradiation (vs. 280 μmol of H2 with Cat1), highlighting the key role of the cobalt molecular catalyst and its association with CIS/ZnS–GSH QDs in the high performance of this photocatalytic system (Fig. S6, ESI). Another control experiment using Co(NO3)2 salt results also in weak H2 production (6.5 μmol) close to that of a blank experiment without catalyst, indicating that cobalt salts, which may potentially come from the Cat1 decomposition, are not responsible for H2 evolution (Fig. S6, ESI).

The excellent photocatalytic performance of the CIS/ZnS–GSH–Cat1–NaHA system is in part due to the exceptional stability of QDs, which show an activity for H2 production beyond 90 h of irradiation (Fig. 3). After 50 h, however, the H2 production strongly slowed down, indicating the degradation of at least one component. To identify the main cause of deactivation of the photocatalytic system, we studied the robustness of the QDs under photocatalytic conditions after several consecutive runs. The CIS/ZnS–GSH QDs were isolated from the reaction mixture by centrifugation after 21 h of irradiation, washed with water, and then redispersed in a fresh solution containing 10 μM Cat1 and 0.5 M NaHA/H2A. This solution was further irradiated for 21 h. After two consecutive separations and redispersions of the same batch of QDs, the photocatalytic activity was maintained (513 ± 22 TONQD) (Fig. 4 and Fig. S7, ESI). It was therefore possible to recycle the CIS/ZnS–GSH nanocrystals several times without significant loss of activity, which demonstrates their high stability. The intrinsic stability of this material was further confirmed by powder XRD analysis of the QDs isolated after 95 h of irradiation. Indeed, the same XRD pattern was obtained as that of the freshly prepared sample, in accordance with the full retention of the crystal structure of the QDs after photocatalysis (Fig. 1b).


image file: c8ee00120k-f4.tif
Fig. 4 Photocatalytic hydrogen production (TONCat, nH2) as a function of time from a deaerated aqueous solution (5 mL) of NaHA (0.36 M) and H2A (0.14 M) at pH 5.0 under visible light irradiation (400–700 nm) in the presence of CIS/ZnS–GSH QDs (110 μM) and Cat1 (10 μM) (1st run), and after centrifugation and redispersion of the QDs into a fresh solution of Cat1 (10 μM) and NaHA (0.36 M)/H2A (0.14 M) buffer (2nd and 3rd runs).

We thus attribute the loss of the photocatalytic activity to the degradation of the molecular catalyst that can also be coupled with the accumulation in solution of one of the oxidized forms of NaHA, dehydroascorbic acid (DHA).7 DHA is known to be a good electron acceptor capable of preventing any electron transfer to the catalyst by trapping the photo-generated electrons from the QDs and thus short-circuiting the catalysis.70–72

The H2-evolving performance of the photocatalytic system was further assessed at lower catalyst concentration while maintaining the concentration of QDs at 113 ± 7 μM and H2A/NaHA at 0.5 M (Fig. S8, ESI). When the concentration of Cat1 was decreased from 10 to 5 μM, TONCat values significantly increased, yielding numbers up to 7700. However, a further decrease in catalyst concentration to 1 μM did not lead to a further increase of the TONCat values taking into account the H2 produced by CIS/ZnS–GSH–NaHA solutions without any catalyst. This dependence of H2 production on the catalyst concentration does not follow the general trend observed for “fully” molecular photocatalytic systems using cobalt catalysts in water,66,70–76i.e. a considerable increase of TONCat values with increasing PS/Cat ratio by decreasing the catalyst concentration (≤1 μM). The decrease in the photocatalytic activities of the hybrid system at very low catalyst concentration (≈1 μM) is likely due to the reduced interaction between the QDs and the molecular catalyst, preventing an efficient electron transfer between the two components.

To evaluate the reducing power of CIS/ZnS–GSH QDs regarding the catalyst activation, we attempted to assess their redox properties by electrochemical studies. However, it is known that the observation of QDs’ redox processes by electrochemical techniques and those of CuInS2 in particular is challenging. So in our case cyclic voltammetry measurements of CIS/ZnS–GSH QDs did not give reliable results, as the oxidation and reduction peaks could not be clearly identified. The highest value for the conduction band of CIS/ZnS–GSH QDs has been reported to be −3.6 eV,48,77 which corresponds to −1.12 V vs. SCE. We hypothesize that in our case of small-sized, copper-deficient CIS/ZnS–GSH QDs, the conduction band exhibits a similar energy level. The driving force (ΔG°) for the reduction of Cat1 (E1/2(CoII/I) = −0.85 V vs. SCE) is thus largely exergonic (−0.27 eV), allowing efficient catalyst reduction. In order to evaluate whether the CIS/ZnS–GSH QDs were also able to reduce a catalyst presenting a lower reduction potential, we substituted Cat1 by the complex [CoII(CR15)(H2O)2]Cl2 (Cat2) having a larger pentaazamacrocyclic ligand, CR15 (CR15 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene) (Scheme 1). This complex presents an H2-evolving catalytic activity similar to Cat1 when coupled to [Ru(bpy)3]Cl2 and H2A/NaHA in water, but its reduction is cathodically shifted (E1/2(CoII/I) = −1.03 V vs. SCE).78 No significant H2 evolution was observed when Cat2 (10 μM) was tested with CIS/ZnS–GSH QDs (116 μM) and H2A/NaHA (0.5 M) compared to H2 produced by the solution without catalyst (Fig. S6 and Table S2, ESI). This indicates that the driving force is not sufficient to promote the reduction of Cat2G° of −0.09 eV), which corroborates the assumed conduction band level at around −3.6 eV.

Finally, we also performed comparative studies using the molecular complex [Ru(bpy)3]Cl2 (Ru) and Cd-containing QDs as reference photosensitizers under the same experimental conditions (Fig. 5). 3.5 nm CdSe QDs capped with oleylamine, trioctylphosphine and stearate ligands were synthesized in organic solvent (1-octadecene) and then transferred to the aqueous phase by ligand exchange with GSH to yield CdSe–GSH nanocrystals (see the Experimental section and Fig. S9, ESI). Experiments were first performed with the three PSs at a similar concentration of 68 μM in a buffered H2A/NaHA solution (0.5 M), using a Cat1 concentration of 10 μM (Fig. 5a). Under these conditions, the production of H2 is much more efficient with CIS/ZnS–GSH QDs. Indeed, TONCat and TONPS values of 3200 and 940, respectively, were measured for CIS/ZnS–GSH compared to 560 and 170 for CdSe–GSH, as well as 640 and 190 for Ru, respectively. The initial TOF is higher with Ru, fully consistent with the larger driving force for the catalyst reduction by the reduced state of this photosensitizer, [RuII(bpy)2(bpy˙)]+ (Ru) (E1/2(RuII/−) = −1.34 V vs. SCE), as compared to CIS/ZnS–GSH QDs.79,80 However, the markedly higher TON values obtained with CIS/ZnS–GSH QDs can be directly correlated with the much higher stability of the whole photocatalytic system under prolonged irradiation. Indeed, with CIS/ZnS–GSH QDs, H2 is produced for more than 90 h compared to only 5 h with Ru. The main cause of the deactivation of the photocatalytic system with Ru is the well-known poor stability of this molecular PS in its reduced state, Ru, which easily undergoes bipyridine substitution in acidic water.73,74 In contrast, the stability of the photocatalytic system with CdSe–GSH QDs proved to be excellent, exceeding 50 h, fully consistent with the higher stability of the inorganic nanocrystals with respect to the molecular ruthenium complex. However, the efficiency for H2 production does not reach that of CIS/ZnS–GSH QDs because the rate of hydrogen production is significantly lower, resulting in much lower TON values. Nevertheless, since the absorption properties of Ru in the visible range (λmax = 452 nm; ε = 14[thin space (1/6-em)]600 M−1 cm−1)79,80 and CdSe–GSH QDs (λmax = 570 nm; ε = 164[thin space (1/6-em)]000 M−1 cm−1) differ significantly from those of CIS/ZnS–GSH QDs, the direct comparison of their respective activity using the same concentration of PS is not ideal. We thus conducted further experiments using the previously reported optimal concentrations for both PSs, i.e. about 500 μM for Ru,26,64,65,75 and 5 μM CdSe–GSH QDs at pH 4.5,22,29,34,81 while maintaining the Cat1 concentration at 10 μM.


image file: c8ee00120k-f5.tif
Fig. 5 Photocatalytic hydrogen production (TONCat, nH2) as a function of time from a deaerated aqueous solution (5 mL) of NaHA/H2A (0.5 M) under visible light irradiation (400–700 nm) in the presence of Cat1 (10 μM) and different photosensitizers CIS/ZnS–GSH QDs, CdSe–GSH QDs, and [Ru(bpy)3]Cl2 at 68 μM (pH 5.0) (a), and at various concentrations (pH 4.5) (b).

In the case of Ru, increasing the PS concentration from 68 μM to 500 μM significantly increases the TONCat (up to 1500), but concomitantly decreases the TONPS to 65, as usually observed for such fully molecular photocatalytic systems (Fig. 5b). However, under these conditions, the stability of the photocatalytic system was still limited to 5 h.82 The decomposition of Ru was also confirmed by the absorption spectrum of the photocatalytic solution at the end of irradiation with new visible bands at 350 and 465 nm assigned to [Ru(bpy)2(HA)]+ species (Fig. S10, ESI).74

Regarding CdSe–GSH QDs, optical absorption in the visible range of these nanocrystals is considerably higher than that of CIS/ZnS–GSH: the integration of the visible region (400–700 nm) of the UV-vis absorption spectrum of 5 μM CdSe–GSH QDs is similar to that of 110 μM CIS/ZnS–GSH QDs (Fig. S11, ESI). Decreasing the concentration of CdSe–GSH from 68 to 5 μM while maintaining the concentration of Cat1 at 10 μM has a beneficial effect on the photocatalytic activity (Fig. 5b), since the amount of H2 produced increases. Thus, the TON with respect to the catalyst increases from 560 to 790 corresponding to a large increase of the TON versus CdSe–GSH from 170 to 3150. Nevertheless, the TONCat remains significantly lower than that reached with CIS/ZnS QDs (790 versus 4580, respectively).

Taken all together, these results demonstrate that CIS/ZnS–GSH QDs are a very promising substitute for the expensive and toxic metal-based benchmark photosensitizers for light-driven hydrogen production.

Conclusions

We demonstrated that cadmium-free CuInS2/ZnS core–shell nanocrystals are efficient photosensitizers for visible-light driven hydrogen production in association with a molecular catalyst. Water-soluble glutathione-capped CuInS2/ZnS QDs were directly prepared in water by a microwave-activated synthesis. Remarkable photocatalytic performances in terms of efficiency and stability were achieved under visible light irradiation using these nanocrystals with an efficient H2-evolving catalyst based on a first-row metal complex, the tetraazamacrocyclic cobalt complex Cat1, and ascorbate used as a sacrificial electron donor in water at pH 5.0. This combination represents the most efficient hybrid photocatalytic system reported so far for H2 production in terms of TON values with respect to catalyst and QDs (TONCat and TONPS) and of TOFPs using cadmium-free QDs in association with a molecular catalyst. Such high TONCat values (up to 7700) have never been obtained for this cobalt macrocyclic catalyst in a three-component photocatalytic system, neither in association with the [Ru(bpy)3]2+ complex (TONCat of 1500) nor with CdSe–GSH QDs (TONCat of 800) as photosensitizers. In the case of the molecular ruthenium complex, this is due to its poor stability as a PS, limiting the production of H2 at 5 h, while H2 production with CIS/ZnS–GSH QDs lasts for more than 90 h. These results also demonstrate the high stability of the tetraazamacrocyclic catalyst Cat1, as the latter can achieve very high TONCat values and is active for several hours when it is associated with a stable PS. In conclusion, our study highlights the importance of developing hybrid systems, combining the benefits of semiconductor nanocrystals, such as their high stability in water even at slightly acidic pH, to the efficiency of molecular H2-evolving catalysts. In this respect, ternary chalcopyrite nanocrystals have great potential as efficient and robust materials for solar fuel production to act as a relevant alternative to expensive and toxic metal-based photosensitizers.

Experimental

Materials and methods

Reactants. L-Glutathione reduced, Cu(NO3)2 dihydrate, InCl3, Zn(OAc)2, Na2S nonahydrate, CdO, Se powder, stearic acid, oleylamine, 1-octadecene, trioctylphosphine and mercaptopropionic acid were purchased from Sigma Aldrich or Fisher Scientific and used as received. Centrifugal filters Amicon Ultra-15 (MWCO 30 kDa) were purchased from Sigma-Aldrich, and Sephadex NAP-25 columns from GE Healthcare.
UV-visible absorption spectroscopy. Absorption spectra were measured on a Hewlett Packard 8452A Diode Array spectrophotometer using 1 cm quartz cuvettes (for nanocrystal characterization) or on a Varian Cary 300 spectrophotometer (for photocatalysis) using 1 cm (spectra of the catalyst mother solution) or 1 mm quartz cuvettes (spectra of the photocatalytic mixture before and after the experiment).
Photoluminescence spectroscopy. Excitation and emission spectra were recorded on a Fluorolog-3 spectrofluorometer from Horiba Scientific, equipped with an iHR320 spectrometer with a 1200 g mm−1 or 600 g mm−1 grating. The detector was a Hamamatsu R928 photomultiplier tube. Lifetime measurements were carried out using a NanoLED pulsed source from Horiba Scientific (emission wavelength 455 nm, pulse duration 1.3 ns), and the R928 photomultiplier tube. This value was determined in aqueous solution using [Ru(bpy)3]Cl2 as the reference (QY = 4%).83
Thermogravimetric analysis. Thermogravimetric analysis was performed on the dry particles using a Setaram TG 92-12 apparatus under an argon atmosphere. The particles were introduced in a platinum crucible which was heated from 30 to 650 °C at 10 °C min−1. An intermediate step of 10 minutes at 100 °C was applied in order to remove residual water before starting the ligand desorption.
Scanning electron microscopy/energy dispersive X-ray analysis. EDX spectra were recorded on a Zeiss Ultrascan 55 scanning electron microscope (working distance 7.1 mm, electron acceleration voltage 20 kV), equipped with a Bruker Quantax EDS detector. The nanocrystals were drop-cast on a silicon substrate from a concentrated aqueous solution and allowed to dry, in order to obtain thick layers.
Transmission electron microscopy. Transmission electron micrographs (TEM) were measured on a JEOL 3010 microscope operated at 300 kV. Samples in an aqueous solution were drop-cast on copper grids with 400 meshes and an ultrathin C film on a Holey Carbon support film.
X-Ray diffraction. X-Ray diffraction was carried out on dry nanocrystal samples, deposited on a disoriented Si substrate. Data were recorded on a Philips X’PERT diffractometer using Cu Kα radiation (λ = 1.54178 Å) and a linear X’Celerator detector.
Elemental analysis, 1H NMR and mass spectrometry. Elemental analyses were carried out with a C, H, N analyzer (SCA, CNRS). A 1H NMR spectrum was recorded on a Bruker Avance III 400 MHz spectrometer using a standard Bruker pulse sequence. Chemical shifts for the 1H NMR spectrum are referenced relative to residual protium in deuterated water (D2O δ = 4.79 ppm). ESI-MS spectra were registered on a Bruker Esquire 3000 Plus and Amazon speed ion trap spectrometer equipped with an electrospray ion source. The samples were solubilized in an ethanol/water mixture and analyzed in positive ionization mode by direct perfusion in the ESI-MS interface (ESI capillary voltage = 2 kV, sampling cone voltage = 40 V). Elemental analysis, 1H NMR and mass spectrometry were performed within the ICMG Chemistry NanoBio Platform, Grenoble.

Synthesis of CuInS2/ZnS–GSH nanocrystals

CIS/ZnS–GSH nanocrystal synthesis was adapted from a literature procedure.67 In a 3-neck round-bottomed flask, glutathione (307 mg, 1.0 mmol) and Cu(NO3)2·3H2O (12 mg, 0.05 mmol) were dissolved in 20 mL of degassed water. InCl3 (44 mg, 0.2 mmol) was added, together with 30 mL of degassed water, producing the formation of a milky suspension. The mixture was basified with 1 M NaOH solution, under Ar bubbling, until it reached pH 9–10 and became transparent. The flask was connected to a water condenser, and 2 mL of aqueous Na2S·9H2O (0.2 M) was quickly injected before heating. The synthesis was carried out in a Milestone StartSYNTH Microwave Synthesis Labstation, with contactless infrared temperature control.

Temperature program:

(1) r.t.−100 °C, t = 1 min, Pmax = 600 W

(2) 100 °C, t = 6 min, Pmax = 600 W

(3) air cooling, t = 20 min

Upon heating, the yellow solution turned dark orange. After cooling below 40 °C, aqueous Na2S·9H2O (0.2 M, 2.0 mL) and Zn(OAc)2·2H2O (0.2 M, 2.0 mL) were injected and the solution was heated again with microwave activation (same heating program). After cooling to room temperature the solution was purified to remove unreacted materials and side products. For this, the solution was first concentrated to 2–2.5 mL by centrifugal filtration (Amicon Ultra-15 filter) followed by size exclusion chromatography on a Sephadex column (NAP-25). This double purification also contributed to narrowing the size distribution by discarding the smallest and the largest fractions (Fig. S1, ESI). The resulting red fraction was collected and water was added to reach a volume of 5 mL. The samples were stored at 4 °C. The synthesis was repeated several times and displayed high reproducibility; particles with an emission maximum of 650 ± 12 nm were obtained.

For this study, the synthesis has been repeated four times in a row starting from the same stock solutions, in order to minimize experimental variations, yielding highly reproducible results in terms of luminescence properties (λem = 638 ± 2 nm) and composition.

Synthesis of CdSe–GSH nanocrystals

The synthesis of CdSe nanocrystals was adapted from a literature procedure.86 CdO (0.104 g, 0.8 mmol), stearic acid (5.692 g, 20 mmol) and oleylamine (OlAm, 6.6 mL, 20 mmol) were added to 40 mL of 1-octadecene (ODE) in a three-neck round-bottomed flask in a glovebox. The sealed flask was then connected to a Schlenk line, and the solution was degassed under vacuum for 30 minutes. After this time, the violet/brownish suspension was put under argon and heated to 250 °C. During the heating step, at around 180 °C, an almost colourless solution was obtained. Meanwhile, 10 mL of 0.4 M TOP–Se solution (4 mmol of Se powder in trioctylphosphine) were dissolved in 16.6 mL of ODE. The solution was quickly injected in the hot CdO solution, resulting in a temperature decrease from 250 to 205 °C. The solution turned yellow, orange and dark red as the nanocrystals growth proceeded, and the reaction was stopped 18 minutes after injection by removing the heating mantle. When the temperature reached 80 °C, 5 mL of MeOH and 5 mL of CHCl3 were added, and the mixture was poured into 200 mL of acetone. The precipitate, containing the CdSe QDs and excess stearic acid, was collected by centrifugation (6000 rpm, 5 minutes). Stearic acid was removed by centrifugation after addition of hexane (which only dissolves the QDs). After concentration of the hexane solution, a second purification was performed by precipitation with 4 mL of MeOH and 100 mL of acetone. The nanocrystals were redispersed in CHCl3.

The CdSe QDs were transferred to the aqueous phase using GSH. CdSe QDs dispersed in CHCl3 (10−4 M, 5 mL) were added to 5 mL of GSH solution (1 M in water, around 5000-fold excess, pH adjusted to 9). The biphasic mixture was degassed by argon bubbling and stirred vigorously until the QD transfer was complete, as indicated by a red aqueous phase and colourless organic phase. The nanocrystals dispersed in the aqueous phase were first concentrated by centrifugal filtration (30 kDa size exclusion, 5000 rpm, 8 minutes) and then further purified with Sephadex (NAP-25 column, elution with water). The red fraction was collected and the nanocrystals were stored in an aqueous solution at 4 °C, showing high colloidal stability over several months.

Synthesis of the catalysts

The [CoIII(CR14)Cl2]Cl complex (Cat1) was synthesized according to literature procedures.64,84 The compound was characterized by ESI-MS and 1H NMR (Fig. S12–S14, ESI). ESI-MS m/z for C15H22Cl3CoN4 (M): 387.05 [M − Cl]+, 352.08 [M − 2Cl]+, 315.10 [M − 3Cl]+, 158.54 [M − 3Cl]2+. 1H NMR (400 MHz, D2O) δ = 8.76 (q, J = 7.6 Hz, 1H), 8.60 (t, J = 8.4 Hz, 2H), 4.10 (m, 2H), 3.70 (m, 2H), 3.31 (t, J = 12.0 Hz, 1H), 3.13–3.01 (m, 4H), 2.97 (s, 6H), 2.50 (m, 2H), 2.19–2.01 (m, 2H).

The [Co(CR15)(H2O)2]Cl2 complex (Cat2) was synthesized according to a procedure adapted from the literature.85 2,6-Diacetylpyridine (686 mg, 4.2 mmol) was dissolved in ethanol (8.4 mL) at 40 °C, under stirring. To this solution was then added a solution of CoCl2·6H2O (1 g, 4.2 mmol) in distilled water (4.2 mL), leading to a pink solution which was warmed at 50 °C. After few minutes, triethylenetetraamine (0.63 mL, 4.2 mmol) was added and the color of the solution turned reddish-brown and cloudy. Acetic acid (0.17 mL) was then added to ensure the complete dissolution of the triethylenetetraamine. The resulting clear solution was stirred at 75 °C under argon for 1 night and then cooled to 0 °C. After 15 minutes, orange-red microcrystals were formed which were filtered off and washed with diethyl ether (931 mg). The filtrate was then concentrated under reduced pressure and cooled again to 0 °C to obtain a second fraction of the orange powder (145 mg). Overall yield 52%. Anal. calcd for C15H27Cl2CoN5O2·3H2O (492.12 g mol−1): C, 36.52; H, 6.74; N, 14.20. Found: C, 36.42; H, 6.90; N, 14.24. ESI-MS m/z for C15H27Cl2CoN5O2 (M): 367.0 [M − Cl − 2H2O]+, 166.2 [M − 2Cl − 2H2O]2+ (Fig. S15 and S16, ESI).

The purity of Cat1 and Cat2 was also verified by cyclic voltammetry and UV-vis spectroscopy.

Photocatalysis experiments

A homemade glass tube with a diameter of 2 cm fused with a round bottom flask was used as the photocatalysis reactor. The total volume of the tube was 169.5 mL (head space volume = 164.5 mL), while for the experiment using CdSe a glass tube of 28.5 mL (head space volume of 23.5 mL) was used. Continuous irradiation was performed under stirring at 298 K with a xenon lamp (150 W, Hamamatsu L8253, type LC8-03) equipped with a 400–700 nm large band filter, which was placed 4 cm from the sample.

In a typical experiment, the catalyst and the QDs were diluted to an appropriate concentration of 5 mL in a volumetric flask. Ascorbic acid and sodium ascorbate (Table S3, pKa = 4.1, ESI) were introduced at the bottom of the photocatalysis reactor together with the stirring bar (volume < 0.1 mL). Then, the solution containing PS and Cat was added, and the reactor was immediately closed with a rubber septum, covered with aluminum foil and degassed by argon bubbling for about 45 minutes. Since the ascorbic acid/ascorbate concentration was relatively high (0.5 M), no other buffer was added. After checking the quality of the degassing by gas chromatography (GC, injection of 100 μL of sample), irradiation was started. The amount of hydrogen evolved was quantified by GC analysis of the headspace gas of the glass tube. The GC analysis was performed with a Perkin Elmer AutoSystem XL Gas Chromatograph equipped with a 5 Å molecular sieve column (oven temperature = 303 K) and a thermal conductivity detector (TCD), which uses argon as the carrier gas. GC/TCD calibration was carried out using two samples of the reference gas (1% and 5% H2 in N2).

Catalyst and sacrificial electron donor addition to the washed particles. After 21 hours of irradiation, the reactor was opened and the mixture containing the quantum dots was transferred to a centrifugation tube. The quantum dots were isolated by centrifugation (15 minutes, 4000 rpm), and the supernatant was discarded. Water was added and centrifugation was repeated. The quantum dots were then diluted in 5 mL of a 10 μM aqueous solution of Cat1. Finally, the QD–Cat mixture was transferred to the reactor containing NaHA (0.36 M)/H2A (0.14 M) buffer and the stirring bar, and degassed for 45 minutes before starting the irradiation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors wish to thank the LABEX ARCANE (project QDPhotoCat ANR-11-LABX-0003-01) for financial support, including M. S.'s post-doctoral fellowship. This work was also supported by ICMG FR 2067 and the COST CM1202 program (PERSPECT H2O). K. D. W. acknowledges the LABEX SERENADE (project SAQUADO, ANR-11-LABX-0064) for his post-doctoral fellowship.

Notes and references

  1. N. Armaroli and V. Balzani, ChemSusChem, 2011, 4, 21–36 CrossRef CAS PubMed.
  2. A. J. Esswein and D. G. Nocera, Chem. Rev., 2007, 107, 4022–4047 CrossRef CAS PubMed.
  3. S. Ye, C. Ding, R. Chen, F. Fan, P. Fu, H. Yin, X. Wang, Z. Wang, P. Du and C. Li, J. Am. Chem. Soc., 2018, 140, 3250–3256 CrossRef CAS PubMed.
  4. G. Liu, S. Ye, P. Yan, F. Xiong, P. Fu, Z. Wang, Z. Chen, J. Shi and C. Li, Energy Environ. Sci., 2016, 9, 1327–1334 CAS.
  5. S. Berardi, S. Drouet, L. Francas, C. Gimbert-Surinach, M. Guttentag, C. Richmond, T. Stoll and A. Llobet, Chem. Soc. Rev., 2014, 43, 7501–7519 RSC.
  6. N. Queyriaux, R. T. Jane, J. Massin, V. Artero and M. Chavarot-Kerlidou, Coord. Chem. Rev., 2015, 304, 3–19 CrossRef PubMed.
  7. T. Stoll, C. E. Castillo, M. Kayanuma, M. Sandroni, C. Daniel, F. Odobel, J. Fortage and M.-N. Collomb, Coord. Chem. Rev., 2015, 304–305, 20–37 CrossRef CAS.
  8. W. T. Eckenhoff and R. Eisenberg, Dalton Trans., 2012, 41, 13004–13021 RSC.
  9. W. T. Eckenhoff, Coord. Chem. Rev. DOI:10.1016/j.ccr.2017.11.002.
  10. Y.-J. Yuan, Z.-T. Yu, D.-Q. Chen and Z.-G. Zou, Chem. Soc. Rev., 2017, 46, 603–631 RSC.
  11. R. S. Khnayzer, C. E. McCusker, B. S. Olaiya and F. N. Castellano, J. Am. Chem. Soc., 2013, 135, 14068–14070 CrossRef CAS PubMed.
  12. T. Stoll, M. Gennari, J. Fortage, C. E. Castillo, M. Rebarz, M. Sliwa, O. Poizat, F. Odobel, A. Deronzier and M.-N. Collomb, Angew. Chem., Int. Ed., 2014, 53, 1654–1658 CrossRef CAS PubMed.
  13. Z. Han and R. Eisenberg, Acc. Chem. Res., 2014, 47, 2537–2544 CrossRef CAS PubMed.
  14. M. Wang, K. Han, S. Zhang and L. Sun, Coord. Chem. Rev., 2015, 287, 1–14 CrossRef CAS.
  15. L.-Z. Wu, B. Chen, Z.-J. Li and C.-H. Tung, Acc. Chem. Res., 2014, 47, 2177–2185 CrossRef CAS PubMed.
  16. L. Clarizia, D. Russo, I. Di Somma, R. Andreozzi and R. Marotta, Energies, 2017, 10, 1624 CrossRef.
  17. F. Wang, W.-G. Wang, X.-J. Wang, H.-Y. Wang, C.-H. Tung and L.-Z. Wu, Angew. Chem., Int. Ed., 2011, 50, 3193–3197 CrossRef CAS PubMed.
  18. F. Wen, J. Yang, X. Zong, B. Ma, D. Wang and C. Li, J. Catal., 2011, 281, 318–324 CrossRef CAS.
  19. F. Wen, X. Wang, L. Huang, G. Ma, J. Yang and C. Li, ChemSusChem, 2012, 5, 849–853 CrossRef CAS PubMed.
  20. J. Huang, K. L. Mulfort, P. Du and L. X. Chen, J. Am. Chem. Soc., 2012, 134, 16472–16475 CrossRef CAS PubMed.
  21. F. Wang, W.-J. Liang, J.-X. Jian, C.-B. Li, B. Chen, C.-H. Tung and L.-Z. Wu, Angew. Chem., Int. Ed., 2013, 52, 8134–8138 CrossRef CAS PubMed.
  22. C.-B. Li, Z.-J. Li, S. Yu, G.-X. Wang, F. Wang, Q.-Y. Meng, B. Chen, K. Feng, C.-H. Tung and L.-Z. Wu, Energy Environ. Sci., 2013, 6, 2597–2602 CAS.
  23. A. Das, Z. Han, M. G. Haghighi and R. Eisenberg, Proc. Natl. Acad. Sci. U. S. A., 2013, 110, 16716–16723 CrossRef CAS PubMed.
  24. J.-X. Jian, Q. Liu, Z.-J. Li, F. Wang, X.-B. Li, C.-B. Li, B. Liu, Q.-Y. Meng, B. Chen, K. Feng, C.-H. Tung and L.-Z. Wu, Nat. Commun., 2013, 4, 2695 Search PubMed.
  25. X.-W. Song, H.-M. Wen, C.-B. Ma, M.-Q. Hu, H. Chen, H.-H. Cui and C.-N. Chen, Appl. Organomet. Chem., 2014, 28, 267–273 CrossRef CAS.
  26. C. Gimbert-Suriñach, J. Albero, T. Stoll, J. Fortage, M.-N. Collomb, A. Deronzier, E. Palomares and A. Llobet, J. Am. Chem. Soc., 2014, 136, 7655–7661 CrossRef PubMed.
  27. K. Han, M. Wang, S. Zhang, S. Wu, Y. Yang and L. Sun, Chem. Commun., 2015, 51, 7008–7011 RSC.
  28. W.-J. Liang, F. Wang, M. Wen, J.-X. Jian, X.-Z. Wang, B. Chen, C.-H. Tung and L.-Z. Wu, Chem. – Eur. J., 2015, 21, 3187–3192 CrossRef CAS PubMed.
  29. A. Das, Z. Han, W. W. Brennessel, P. L. Holland and R. Eisenberg, ACS Catal., 2015, 5, 1397–1406 CrossRef CAS.
  30. C. M. Chang, K. L. Orchard, B. C. M. Martindale and E. Reisner, J. Mater. Chem. A, 2016, 4, 2856–2862 CAS.
  31. J. Yang, D. Wang, H. Han and C. Li, Acc. Chem. Res., 2013, 46, 1900–1909 CrossRef CAS PubMed.
  32. H. Chen, Z. Sun, S. Ye, D. Lu and P. Du, J. Mater. Chem. A, 2015, 3, 15729–15737 CAS.
  33. S. Ye, R. Chen, Y. Xu, F. Fan, P. Du, F. Zhang, X. Zong, T. Chen, Y. Qi, P. Chen, Z. Chen and C. Li, J. Catal., 2016, 338, 168–173 CrossRef CAS.
  34. Z. J. Han, F. Qiu, R. Eisenberg, P. L. Holland and T. D. Krauss, Science, 2012, 338, 1321–1324 CrossRef CAS PubMed.
  35. Z. J. Li, X. B. Li, J. J. Wang, S. Yu, C. B. Li, C. H. Tung and L. Z. Wu, Energy Environ. Sci., 2013, 6, 465–469 CAS.
  36. Z. J. Li, J. J. Wang, X. B. Li, X. B. Fan, Q. Y. Meng, K. Feng, B. Chen, C. H. Tung and L. Z. Wu, Adv. Mater., 2013, 25, 6613–6618 CrossRef CAS PubMed.
  37. J. J. Wang, Z. J. Li, X. B. Li, X. B. Fan, Q. Y. Meng, S. Yu, C. B. Li, J. X. Li, C. H. Tung and L. Z. Wu, ChemSusChem, 2014, 7, 1468–1475 CrossRef CAS PubMed.
  38. Y. Peng, L. Shang, Y. T. Cao, G. I. N. Waterhouse, C. Zhou, T. Bian, L. Z. Wu, C. H. Tung and T. R. Zhang, Chem. Commun., 2015, 51, 12556–12559 RSC.
  39. F. Qiu, Z. Han, J. J. Peterson, M. Y. Odoi, K. L. Sowers and T. D. Krauss, Nano Lett., 2016, 16, 5347–5352 CrossRef CAS PubMed.
  40. Y.-J. Yuan, D.-Q. Chen, M. Xiong, J.-S. Zhong, Z.-Y. Wan, Y. Zhou, S. Liu, Z.-T. Yu, L.-X. Yang and Z.-G. Zou, Appl. Catal., B, 2017, 204, 58–66 CrossRef CAS.
  41. B. C. M. Martindale, G. A. M. Hutton, C. A. Caputo and E. Reisner, J. Am. Chem. Soc., 2015, 137, 6018–6025 CrossRef CAS PubMed.
  42. B. C. M. Martindale, E. Joliat, C. Bachmann, R. Alberto and E. Reisner, Angew. Chem., Int. Ed., 2016, 55, 9402–9406 CrossRef CAS PubMed.
  43. B. C. M. Martindale, G. A. M. Hutton, C. A. Caputo, S. Prantl, R. Godin, J. R. Durrant and E. Reisner, Angew. Chem., Int. Ed., 2017, 56, 6459–6463 CrossRef CAS PubMed.
  44. G. A. M. Hutton, B. C. M. Martindale and E. Reisner, Chem. Soc. Rev., 2017, 46, 6111–6123 RSC.
  45. G. A. M. Hutton, B. Reuillard, B. C. M. Martindale, C. A. Caputo, C. W. J. Lockwood, J. N. Butt and E. Reisner, J. Am. Chem. Soc., 2016, 138, 16722–16730 CrossRef CAS PubMed.
  46. D. Aldakov, A. Lefrancois and P. Reiss, J. Mater. Chem. C, 2013, 1, 3756–3776 RSC.
  47. M. D. Regulacio and M.-Y. Han, Acc. Chem. Res., 2016, 49, 511–519 CrossRef CAS PubMed.
  48. M. Sandroni, K. D. Wegner, D. Aldakov and P. Reiss, ACS Energy Lett., 2017, 2, 1076–1088 CrossRef.
  49. P. Reiss, M. Carrière, C. Lincheneau, L. Vaure and S. Tamang, Chem. Rev., 2016, 116, 10731–10819 CrossRef CAS PubMed.
  50. C. Ye, M. D. Regulacio, S. H. Lim, Q.-H. Xu and M.-Y. Han, Chem. – Eur. J., 2012, 18, 11258–11263 CrossRef CAS PubMed.
  51. K. Zhang and L. Guo, Catal. Sci. Technol., 2013, 3, 1672–1690 CAS.
  52. M. Xu, J. Zai, Y. Yuan and X. Qian, J. Mater. Chem., 2012, 22, 23929–23934 RSC.
  53. Y. Zhou, W. Hu, J. Ludwig and J. Huang, J. Phys. Chem. C, 2017, 121, 19031–19035 CAS.
  54. C. Ye, M. D. Regulacio, S. H. Lim, S. Li, Q.-H. Xu and M.-Y. Han, Chem. – Eur. J., 2015, 21, 9514–9519 CrossRef CAS PubMed.
  55. I. Tsuji, H. Kato, H. Kobayashi and A. Kudo, J. Phys. Chem. B, 2005, 109, 7323–7329 CrossRef CAS PubMed.
  56. L. Zheng, Y. Xu, Y. Song, C. Wu, M. Zhang and Y. Xie, Inorg. Chem., 2009, 48, 4003–4009 CrossRef CAS PubMed.
  57. T.-L. Li, C.-D. Cai, T.-F. Yeh and H. Teng, J. Alloys Compd., 2013, 550, 326–330 CrossRef CAS.
  58. F. Shen, W. Que, Y. He, Y. Yuan, X. Yin and G. Wang, ACS Appl. Mater. Interfaces, 2012, 4, 4087–4092 CAS.
  59. Y.-J. Yuan, D.-Q. Chen, Y.-W. Huang, Z.-T. Yu, J.-S. Zhong, T.-T. Chen, W.-G. Tu, Z.-J. Guan, D.-P. Cao and Z.-G. Zou, ChemSusChem, 2016, 9, 1003–1009 CrossRef CAS PubMed.
  60. Y.-J. Yuan, D. Chen, J. Zhong, L.-X. Yang, J. Wang, M.-J. Liu, W.-G. Tu, Z.-T. Yu and Z.-G. Zou, J. Mater. Chem. A, 2017, 5, 15771–15779 CAS.
  61. X.-B. Fan, S. Yu, F. Zhan, Z.-J. Li, Y.-J. Gao, X.-B. Li, L.-P. Zhang, Y. Tao, C.-H. Tung and L.-Z. Wu, ChemSusChem, 2017, 10, 4833–4338 CrossRef CAS PubMed.
  62. S. Lian, M. S. Kodaimati, D. S. Dolzhnikov, R. Calzada and E. A. Weiss, J. Am. Chem. Soc., 2017, 139, 8931–8938 CrossRef CAS PubMed.
  63. C. F. Leung, Y. Z. Chen, H. Q. Yu, S. M. Yiu, C. C. Ko and T. C. Lau, Int. J. Hydrogen Energy, 2011, 36, 11640–11645 CrossRef CAS.
  64. S. Varma, C. E. Castillo, T. Stoll, J. Fortage, A. G. Blackman, F. Molton, A. Deronzier and M.-N. Collomb, Phys. Chem. Chem. Phys., 2013, 15, 17544–17552 RSC.
  65. R. Gueret, C. E. Castillo, M. Rebarz, F. Thomas, A.-A. Hargrove, J. Pécaut, M. Sliwa, J. Fortage and M.-N. Collomb, J. Photochem. Photobiol., B, 2015, 152, 82–94 CrossRef CAS PubMed.
  66. S. Roy, M. Bacchi, G. Berggren and V. Artero, ChemSusChem, 2015, 8, 3632–3638 CrossRef CAS PubMed.
  67. W.-W. Xiong, G.-H. Yang, X.-C. Wu and J.-J. Zhu, ACS Appl. Mater. Interfaces, 2013, 5, 8210–8216 CAS.
  68. A. D. P. Leach and J. E. Macdonald, J. Phys. Chem. Lett., 2016, 7, 572–583 CrossRef CAS PubMed.
  69. A. Sivanesan and S. A. John, Biosens. Bioelectron., 2007, 23, 708–713 CrossRef CAS PubMed.
  70. M. Guttentag, A. Rodenberg, R. Kopelent, B. Probst, C. Buchwalder, M. Brandstätter, P. Hamm and R. Alberto, Eur. J. Inorg. Chem., 2012, 59–64 CrossRef CAS.
  71. M. Guttentag, A. Rodenberg, C. Bachmann, A. Senn, P. Hamm and R. Alberto, Dalton Trans., 2013, 42, 334–337 RSC.
  72. E. Deponti, A. Luisa, M. Natali, E. Iengo and F. Scandola, Dalton Trans., 2014, 43, 16345–16353 RSC.
  73. W. M. Singh, T. Baine, S. Kudo, S. Tian, X. A. N. Ma, H. Zhou, N. J. DeYonker, T. C. Pham, J. C. Bollinger, D. L. Baker, B. Yan, C. E. Webster and X. Zhao, Angew. Chem., Int. Ed., 2012, 51, 5941–5944 CrossRef CAS PubMed.
  74. R. S. Khnayzer, V. S. Thoi, M. Nippe, A. E. King, J. W. Jurss, K. A. El Roz, J. R. Long, C. J. Chang and F. N. Castellano, Energy Environ. Sci., 2014, 7, 1477–1488 CAS.
  75. W. K. C. Lo, C. E. Castillo, R. Gueret, J. Fortage, M. Rebarz, M. Sliwa, F. Thomas, C. J. McAdam, G. B. Jameson, D. A. McMorran, J. D. Crowley, M.-N. Collomb and A. G. Blackman, Inorg. Chem., 2016, 55, 4564–4581 CrossRef CAS PubMed.
  76. F. Lucarini, M. Pastore, S. Vasylevskyi, M. Varisco, E. Solari, A. Crochet, K. M. Fromm, F. Zobi and A. Ruggi, Chem. – Eur. J., 2017, 23, 6768–6771 CrossRef CAS PubMed.
  77. N. Radychev, D. Scheunemann, M. Kruszynska, K. Frevert, R. Miranti, J. Kolny-Olesiak, H. Borchert and J. Parisi, Org. Electron., 2012, 13, 3154–3164 CrossRef CAS.
  78. R. Gueret, PhD thesis, University Grenoble Alpes, Grenoble, France, 2017.
  79. M. Kirch, J.-M. Lehn and J.-P. Sauvage, Helv. Chim. Acta, 1979, 62, 1345–1384 CrossRef CAS.
  80. A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and A. Von Zelewsky, Coord. Chem. Rev., 1988, 84, 85–277 CrossRef CAS.
  81. J.-X. Jian, C. Ye, X.-Z. Wang, M. Wen, Z.-J. Li, X.-B. Li, B. Chen, C.-H. Tung and L.-Z. Wu, Energy Environ. Sci., 2016, 9, 2083–2089 CAS.
  82. T. Stoll, M. Gennari, I. Serrano, J. Fortage, J. Chauvin, F. Odobel, M. Rebarz, O. Poizat, M. Sliwa, A. Deronzier and M.-N. Collomb, Chem. – Eur. J., 2013, 19, 782–792 CrossRef CAS PubMed.
  83. K. Suzuki, A. Kobayashi, S. Kaneko, K. Takehira, T. Yoshihara, H. Ishida, Y. Shiina, S. Oishi and S. Tobita, Phys. Chem. Chem. Phys., 2009, 11, 9850–9860 RSC.
  84. K. M. Long and D. H. Busch, J. Coord. Chem., 1974, 4, 113–123 CrossRef CAS.
  85. F. Begum, M. S. Khan, S. Z. Haider, K. M. A. Malik and F. K. Khan, J. Bangladesh Acad. Sci., 1991, 15, 185–191 CAS.
  86. M. Protière, N. Nerambourg, O. Renard and P. Reiss, Nanoscale Res. Lett., 2011, 6, 472 CrossRef PubMed.

Footnote

Electronic supplementary information (ESI) available: Synthesis, spectral properties (absorption, emission, EDX), XRD, and TGA of CIS/ZnS–GSH quantum dots; synthesis and spectral properties (absorption, emission) of CdSe–GSH quantum dots; and photocatalytic activities of various three-component systems. See DOI: 10.1039/c8ee00120k

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