Few-layer MoS₂ flakes as a hole-selective layer for solution-processed hybrid organic hydrogen-evolving photocathodes

Abstract

High-efficiency organic photocathodes, based on a regioregular poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (rr-P3HT:PCBM) bulk heterojunction sandwiched between charge-selective layers, are emerging as efficient and low-cost devices for solar hydrogen production by water splitting. Nevertheless, stability issues of the materials used as charge-selective layers are hampering the realization of long-lasting photoelectrodes, pointing out the need to investigate novel and stable materials. Here, we propose MoS₂ flakes, produced by Li-aided exfoliation of the bulk counterpart, as an efficient atomic-thick hole-selective layer for rr-P3HT:PCBM-based photocathodes. We carried out p-type chemical doping to tune on-demand the MoS₂ Fermi level in order to match the highest occupied molecular orbital level of the rr-P3HT, thus easing the hole collection at the electrode. The as-prepared p-doped MoS₂-based photocathodes reached a photocurrent of 1.21 mA cm⁻² at 0 V vs. RHE, a positive onset potential of 0.56 V vs. RHE and a power-saved figure of merit of 0.43%, showing a 6.1-fold increase with respect to pristine MoS₂-based photocathodes, under simulated 1 sun illumination. Operational activity of the photocathodes over time and under 1 sun illumination revealed a progressive stabilization of the photocurrents at 0.49 mA cm⁻² at 0 V vs. RHE. These results pave the way towards the exploitation of layered crystals as efficiency-boosters for scalable hybrid organic H₂-evolving photoelectrochemical cells.

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Introduction

Sunlight and water are the most abundant, clean and renewable energy resources.¹ In fact, hydrogen (H₂) fuel production via photoelectrochemical (PEC) water splitting (2H₂O + hν → 4H₂ + O₂) represents one of the most challenging technologies for the production of clean carbon-neutral energy.² The water splitting process consists of both reduction and oxidation half-reactions, i.e.: (i) oxygen evolution reaction (OER: 2H₂O → O₂ + 4H⁺ + 4e⁻) and (ii) hydrogen evolution reaction (HER: 4H⁺ + 4e⁻ → 2H₂).³ In general, a water splitting PEC cell comprises a semiconductor photoelectrode and a counter electrode immersed in an aqueous electrolyte.^3–5 Semiconductor photoelectrodes absorb light photogenerating electrical charges, which are needed to carry out the redox chemistry of the OER and HER processes.^3–5 For an n-type semiconductor electrode (photoanode),⁵ such as TiO₂ (ref. 3,4) and α-Fe₂O₃,³ photoexcited holes are transferred to the semiconductor/electrolyte junction where they enable the OER,³ while electrons are transferred to the counter electrode where they allow the HER.³ Vice versa, in the case of a p-type semiconductor electrode (photocathode)⁵ such as p-Si,⁶ p-InP,^7a and p-GaAs,^7b HER and OER processes occur at the semiconductor/electrolyte junction and the counter electrode, respectively.^3–5 The solar to hydrogen conversion efficiency (η_STH) is the most important figure of merit (FoM) of a PCE cell and is defined as:
where j_sc is the short-circuit photocurrent density, η_F is the faradaic efficiency for hydrogen evolution, and P is the incident illumination power density, measured under standard solar illumination conditions (AM1.5G).⁸ This FoM directly depends on the photophysical properties of the semiconductor photoelectrodes, such as light absorption^4,5 and exciton formation,⁸ as well as charge carrier separation and transport.⁸ The photogenerated electrons and holes have to overcome energetic constraints, corresponding to the thermodynamic potential of the HER and OER processes, respectively.^4,5,8 In particular, the electrochemical potential of the bottom of the photoelectrode conduction band must be more negative than the H⁺/H₂ redox level (),^3–5,8 while the one of the top of the valence band must be more positive than the O₂/H₂O redox level ().^3–8 Moreover, in view of scaling up and commercialization of PEC cells, stability and cost are also key factors.^2,3 To date, extensive research on photoelectrodes for PEC cells has been focused on inorganic metal oxide/nitride such as TiO₂,^3,4,9 ZnO,¹⁰ WO₃,¹¹ α-Fe₂O₃,^3,12 BiVO₄,¹³ Ta₂O₅,¹⁴ TaON¹⁵ and Ta₃N₅,¹⁵ due to their energy band gap exceeding 1.23 eV,^2–5,8–15 stability and Earth-abundance.^2,3,8 Nevertheless, these single semiconductor absorbers cannot harvest a significant portion of the solar spectrum and therefore their potential η_STH is intrinsically limited (predicted maximum values ∼ 13%).^8–15 In order to overcome this problem, an ideal PEC cell based on a dual light absorber, or tandem configuration, can be used. A simple two-photoelectrode approach to construct a PEC cell for water splitting is to use an n-type semiconductor photoanode together with a p-type semiconductor photocathode.^8,9 In this context, the vertical stacking of a 1.6–1.8 eV energy band gap semiconductor, such as InP,⁷ GaInP₂,¹⁶ AlGaAs,¹⁷ Cu₂O¹⁸ on top of a narrower (∼1 eV) one, such as Si,^6,17,19 is emerging as a promising route to optimize the solar photon harvesting (photons not absorbed by the first material are transmitted to and absorbed by the second one).^8,9,17,19 Recently, tandem water splitting using perovskite photovoltaics and CuIn_xGa_1−xSe₂ photocathodes²⁰ or α-Fe₂O₃ photoanodes²¹ reached η_STH of 6% and 2.4%, respectively. However, the instability in aqueous solutions of the photoactive semiconductors^6,7,16–21 is their main drawback for long time operation. To overcome this issue, encapsulation strategies of the photoactive semiconductors have been proposed,^18,22,23 with the result of increasing the overall fabrication costs. Thus, the discovery of new photoelectrode materials is needed to further improve the water splitting efficiency and long term stability with respect to the current technology.^6,22 In the quest for the development of new photoelectrode materials, recently, organic semiconductors, such as graphitic carbon nitrides (g-C₃N₄),²⁴ 1,3,5-tris-(4-formyl-phenyl)triazine-based covalent organic frameworks (TFPT-COF),^25a triarylamine-based microporous organic network (TAA-MON),^25b hydrogen-bonded organic pigments of the epindolidione (EPI) and quinacridone (QNC),^26a and boron subnaphthalocyanine chloride (SubPc):α-sexithiophene (6T) blend,^26b have been exploited for the HER process. Sprick et al. reported conjugated microporous polymers (CMPs) based on a copolymer with varying ratios of benzene and pyrene as semiconductors for the HER with the optical band gap finely tuned over a broad range (i.e., 1.94–2.95 eV), thus making them versatile materials for tandem configurations.²⁷ Finally, PEC electrodes for the HER based on semiconducting polymers (SPs) also emerged.^28,29 These studies focused mainly on the use of regio regular poly(3-hexylthiophene) (rr-P3HT),^28,29 the prototypical conjugated polymer widely used in bulk heterojunction (BHJ) organic photovoltaics (OPV) architectures,³⁰ capable of delivering a record photocurrent density (under 1 sun illumination) of up to 14 mA cm⁻².³¹ rr-P3HT has a direct bandgap of 1.9 eV,^30,31 close to the optimum value for a PEC tandem device (maximum η_STH of 21.6% is predicted for stacking 1.89 eV and 1.34 eV energy band gap semiconductors).^8,9 The rr-P3HT lowest unoccupied molecular orbital (LUMO) energy level is several hundreds of millivolts more negative than the potential (LUMO_P3HT – ∼ −1.5 V);^30,31 thus photogenerated electrons possess the energy enabling the HER process.³² Moreover, the optoelectronic properties of rr-P3HT, such as light absorption and charge photogeneration, are fully retained in an aqueous environment.³³ On the basis of these observations, rr-P3HT:phenyl-C61-butyric acid methyl ester (PCBM) BHJ-based photocathodes with photocurrent densities at the RHE potential (J_{0 V vs. RHE}) in the mA cm⁻² range have been reported.^28,29,34 In these photocathodes, rr-P3HT:PCBM BHJ is sandwiched between two charge-selective layers (CSLs), resembling OPV architectures.³⁵ Specifically, the hole-selective layer (HSL) is deposited between a transparent conductive oxide (TCO), e.g. indium tin oxide (ITO)³² or fluorine doped tin oxide (FTO),^28,29,32 and the rr-P3HT:PCBM,^28,29,32,34 while the electron-selective layer (ESL) is deposited on top of the rr-P3HT:PCBM.^28,29 The device is completed by depositing an electrocatalyst (EC) for the HER, giving the overall structure TCO/HSL/rr-P3HT:PCBM/ESL/EC.^28,29 The key role of the CSLs relies on the hole and electron transport^28,29,35 toward TCO and EC, respectively. Once the electrons are collected in the EC, the HER process is activated.^8,9,28,29 Currently, ZnO,^28a,34 MoO₃ (ref. 29a and b) and CuI^29d are the most used HSLs, while TiO₂ (ref. 28 and 29) and metallic Al/Ti^29c are exploited as ESLs. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS),³⁶ the typical HSL in OPV,³⁷ poorly performed (J_{0 V vs. RHE} in the sub mA cm⁻² region) in perovskite solar cells (PSCs),^29a due to ion penetration and electrochemical doping processes, which could alter its electrical properties.³⁸ Amongst the ECs suitable for the HER process, both Pt^{28a–d,29b,d} and metal-free catalysts, such as MoS₃ (ref. 28d, 29a and c) and Chloro(pyridine)cobaloxime(III),³⁴ have been exploited, turning out photocathodes with J_{0 V vs. RHE} above 8 mA cm⁻² and onset potential (V_oc) (defined as the potential at which the photocurrent related to the HER is observed) of ∼0.7 V vs. RHE, a value similar to the open-circuit potential of the rr-P3HT:PCBM-based solar cell.³⁰

Despite the continuous improvements, in terms of J_{0 V vs. RHE} and V_oc, stability issues of the rr-P3HT:PCBM-based photocathodes have been evidenced for the CSLs, under H₂-evolving electrochemical conditions.^28,29 For instance, irreversible electrochemical degradation of hygroscopic PEDOT:PSS^28a,29c and water-soluble CuI,^29d intercalation processes and irreversible reduction into sub-stoichiometric phases of MoO₃ (ref. 29b) and WO₃ (ref. 28a) with unfavourable energy alignment for their use as HSLs, led to the corresponding photocathodes' lifetimes during continuous operation ranging from several minutes to few hours.^28a Consequently, most attention is now focused toward the discovery of new CSL materials, which could in principle boost the aforementioned photoelectrochemical FoMs.³⁹ In this context, two dimensional (2D)-transition metal dichalcogenides (TMDs) are raising interest, due to their optoelectronic properties, for the integration as CSLs⁴⁰ in heterojunction-based solar cells,⁴¹ both in OPV^40,41,43 and inorganic photovoltaics (IPV).^41,43,44 Amongst TMDs, MoS₂ is in a key position due to its high charge carrier mobility (up to ≈470 cm² V⁻¹ s⁻¹ for electrons, and ≈480 cm² V⁻¹ s⁻¹ for holes)⁴⁵ and chemical stability of the basal-planes.⁴⁵ In OPVs, solution-processed MoS₂ flakes have been exploited as HSLs,^40,42,46,47 demonstrating power conversion efficiency (4% and 8% for P3HT:PCBM and PTB7:PCBM BHJs, respectively)⁴⁷ comparable to that of cells exploiting state-of-the-art HSLs, such as MoO₃ (ref. 48) and PEDOT:PSS.⁴⁹ More recently, MoS₂ has also been exploited in PSCs,^50,51 as an alternative hole transporting layer⁵⁰ to PEDOT:PSS⁵² and Spiro-OMeTAD,⁵³ or as a conducting and protecting buffer layer between the Spiro-OMeTAD and the perovskite layer.⁵¹ Furthermore, the possibility to obtain thin MoS₂ flakes via chemical intercalation and liquid phase exfoliation of the bulk counterpart led to the formulation and application of functional MoS₂ inks in highly efficient, large-area solar cells fabricated by solution processing.⁵⁴

In this work, we demonstrate the potentiality of 2D material interface engineering by using few-layer MoS₂ flakes as a HSL in organic PEC cells. In particular, we report the fabrication and photo-electrochemical study of solution-processed hybrid organic H₂-evolving photocathodes based on rr-P3HT:PCBM bulk heterojunction architecture sandwiched between solution-processed CSLs. MoS₂ flakes are used as a HSL, while anatase TiO₂ nanoparticles act as an ESL. MoS₃ nanoparticles, deposited on top of TiO₂, perform as an Earth-abundant catalyst for the HER. The overall photocathode's structure is FTO/MoS₂/rr-P3HT:PCBM/TiO₂/MoS₃. Single- and few-layer MoS₂ flakes are obtained by using n-butyl lithium (n-BuLi) in cyclohexane as the intercalation agent to insert lithium ions into the layered bulk structure of MoS₂, followed by exfoliation in water with the aid of ultrasonication.⁵⁶ Solution-processed p-doping based on HAuCl₄·3H₂O methanol solution^46,55 allows us to tailor the Fermi levels, i.e. the work function (WF) values, of the MoS₂ films to higher values (from 4.6 eV of the pristine MoS₂ to 5.1 eV), thus matching the highest occupied molecular orbital (HOMO) level of rr-P3HT (∼−5.1 eV).^30,31 Interface engineering allows us to obtain a uniform and fully covered film morphology of MoS₂ flakes onto FTO, leading to solution-processed architectures with J_{0 V vs. RHE} of 1.21 mA cm⁻², V_oc of 0.55 V vs. RHE and power-saved FoM (Φ_saved,NPAC) of 0.423%, thus approaching the state-of-the-art values of 0.47% for solution-processed rr-P3HT:PCBM-based photocathodes,^29a and of 1.21% (ref. 29d) obtained for non-encapsulated rr-P3HT:PCBM-based photocathodes (up to 1.45% and 2.05% in the presence of PEI^29d and Ti^29c protective coatings, respectively).

Results and discussion

Characterization of solution processed MoS₂ flakes

MoS₂ flakes are prepared by a chemical lithium intercalation method,⁶⁰ avoiding the typical problems related to the LPE of MoS₂, i.e. the use of toxic and high boiling point solvents such as N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF), typically used for the LPE of bulk MoS₂ and other layered materials,⁵⁴ as well as the presence of superficial oxide species⁵⁸ and high-temperature annealing processes needed for solvent removal.⁵⁹ Experimental details on the synthesis of the MoS₂ flakes are reported in the Experimental, material preparation section.

The morphology of the as-produced MoS₂ flakes is characterized by means of transmission electron microscopy (TEM). Fig. 1a reports an image of the MoS₂ flakes, showing irregular shaped sheets. Statistical analysis of the TEM images, reported in Fig. 1b, indicates the presence of flakes with a lateral size in the 30–800 nm range (mean value ∼ 275 nm, i.e. mean area approx. 0.075 μm²). Additional TEM images are reported in the ESI (Fig. S1†) to illustrate the morphology of MoS₂ flakes. Atomic force microscopy (AFM) analysis of the MoS₂ flakes deposited onto a V1-quality mica substrate is shown in Fig. 1c. A representative height profile of a single flake is also reported (red line in Fig. 1c), showing a thickness of 1.2 nm, while additional AFM images showing MoS₂ flakes with thickness up to 6 nm are reported in the ESI (Fig. S2†). Statistical analysis of the flakes thickness, reported in Fig. 1d, reveals the presence of single to few layer MoS₂ flakes, (monolayer thickness ∼ 0.7–0.8 nm),⁵⁷ with average height values of 2.3 ± 1.6 nm.

Fig. 1 (a) TEM images of the MoS₂ flakes and (b) the statistical analysis of their lateral dimension (calculated on 80 flakes). (c) AFM images of MoS₂ flakes deposited onto a V1-quality mica substrate. The height profile of a representative flake is also shown (red line). (d) Statistical analysis of the flakes' thickness (derived from different AFM images and calculated on 30 flakes).

UV-Vis extinction spectra of MoS₂ flakes dispersed in isopropanol (IPA) are reported in Fig. 2a. Absorption peaks around 670 nm and 620 nm arise from the direct transitions from the valance band to the conduction band at the K-point of the Brillouin zone of layered MoS₂, known as the A and B transitions, respectively.⁶¹ The broad absorption band centred at ∼400 nm arises from the C and D inter-band transitions between the density of state peaks in the valence and conduction bands⁶² of both 1T (metallic) and 2H (semiconducting) phases.⁶³

Fig. 2 (a) UV-Vis absorption spectra of 0.1 mg mL⁻¹ MoS₂ dispersion in IPA. B and A refer to the band at ∼620 nm and ∼670 nm, respectively, which arise from the direct transitions from the valance band to the conduction band at the K-point of the Brillouin zone. The C + D band refers to the overlap of the inter-band transitions between the density of state peaks in the valence and conduction bands. (b) Raman spectra of exfoliated MoS₂ flakes and bulk MoS₂ deposited onto a Si wafer with 300 nm thermally grown SiO₂. The main peaks located around 379 cm⁻¹ and 403 cm⁻¹ are attributed to the E¹_2g(Γ) and A_1g(Γ) modes, respectively. The dashed vertical red line shows the frequency shift of E¹_2g(Γ) mode of MoS₂ with respect to that of bulk MoS₂. The dashed vertical blue line shows the same position of the A_1g(Γ) modes of exfoliated MoS₂ and bulk MoS₂. (c) XPS spectrum collected on the MoS₂ flakes cast onto 50 nm-Au sputtered coated Si wafers from the 0.1 mg mL⁻¹ dispersion in IPA.

Raman spectroscopy is carried out on the MoS₂ flakes, as well as on the bulk MoS₂, in order to confirm their different topological structure.^64,65 Representative spectra, reported in Fig. 2b, show the presence of first-order modes at the Brillouin zone center E¹_2g(Γ) (∼380 cm⁻¹ for MoS₂ and ∼378 cm⁻¹ for bulk MoS₂) and A_1g(Γ) (∼403 cm⁻¹ for both MoS₂ and bulk MoS₂), involving the in-plane displacement of Mo and S atoms and the out-of-plane displacement of S atoms, respectively.⁶⁴ The E¹_2g(Γ) mode of the MoS₂ flakes exhibits softening with respect to the one of the bulk counterparts, while no difference of the peak position of A_1g(Γ) modes is observed. The shift of the E¹_2g(Γ) mode is explained by dielectric screening of long range Coulomb interaction.^64,65 Thus, the frequency difference between A_1g(Γ) and E¹_2g(Γ) (24 cm⁻¹) of the MoS₂ decreases of about 2 cm⁻¹ with respect to the bulk MoS₂ (26 cm⁻¹). The full width at half maximum (FWHM) of the E¹_2g(Γ) and A_1g(Γ) of MoS₂ increases ∼2 cm⁻¹ and ∼1 cm⁻¹, respectively, compared to the corresponding bulk MoS₂ modes. In particular, the increase of A_1g(Γ) FWHM for MoS₂ has been linked with the variation of interlayer force constants between the inner and outer layers.⁶⁶ Statistical Raman analysis of the MoS₂ is reported in the ESI (Fig. S3†).

X-ray photoelectron spectroscopy (XPS) measurements are carried out on the MoS₂ flakes to determine their elemental composition, i.e. their chemical quality resulting from the preparation by the chemical lithium intercalation method.^57,58 The XPS spectrum, reported in Fig. 2c, can be de-convoluted by four peaks: the first two peaks (fitted with two components) centred at 226 eV and 229 eV are assigned to S_2s and Mo_3d of MoS₂, respectively; the third peak is also assigned to Mo_3d and it is fitted with three components. The latter component (peaked at 233 eV) and the fourth peak centred at 236 eV are attributed to the MoO₃ phase, which are produced as a by-product for MoS₂ flakes exfoliated and exposed to air.⁵⁸ However, we remark that the MoO₃-related peaks in our Li-exfoliated flakes are significantly reduced with respect to those observed in MoS₂ flakes produced by LPE in NMP,⁵¹ whose XPS spectrum has been previously reported,^51,58 thus confirming the high quality of the MoS₂ flakes produced in this work.

The effects on the surface morphology of the FTO after the MoS₂ flake deposition are microscopically investigated by AFM. Fig. 3a reports the AFM image of the bare FTO, while Fig. 3b shows one of the FTO/MoS₂. FTO/MoS₂ shows nano-step height modulations on the grained FTO (grain size >100 nm).⁶⁷ Representative height profiles of the AFM images are reported in Fig. 3c and d for FTO and FTO/MoS₂, respectively. For the case of FTO/MoS₂, the edge steps of the zoomed height profile (as defined by the blue dashed rectangle) are in the 1–1.5 nm range, in agreement with the flake thickness (2.3 ± 1.6 nm) measured by AFM (Fig. 1c and d).

Fig. 3 (a) AFM images of FTO and (b) FTO/MoS₂. The MoS₂ films are deposited from a 0.1 mg mL⁻¹ MoS₂ dispersion in IPA. Representative height profiles of (c) FTO and (d) FTO/MoS₂ (the corresponding profile positions are shown by the dashed lines in (a) and (b), respectively). In (d), the zoom of the height profile defined by the blue dashed rectangle, showing nano-edge steps, is reported.

MoS₂ flakes deposited onto FTO, see Experimental, are subsequently doped by spin coating HAuCl₄·3H₂O methanol solution on top of the flakes themselves.^46,55 The doping process is a consequence of the positive reduction potential of the HAuCl₄, which is then prone to accept electrons from MoS₂ carrying out the reduction of Au³⁺ to Au⁰ species.^46,55 As a consequence, the electrical properties of the MoS₂ film, i.e., conductivity and WF value, are significantly affected. The doping level is modulated by varying the concentration of HAuCl₄·3H₂O solutions. Values of concentration of 5, 10, and 20 mM are tested (see details in Experimental, fabrication techniques), giving the MoS₂-based films here named as p-MoS₂ (5 mM), p-MoS₂ (10 mM) and p-MoS₂ (20 mM), respectively. Similar dopant concentrations have been already used for engineering the WF of solution-processed MoS₂ thin-films for hole transport layers in OPVs.⁴⁶ The WF value of MoS₂, as measured by Kelvin Probe (KP) under ambient conditions (Table 1), is 4.6 eV, thus similar to the one measured for FTO (4.7 eV). After doping, the WF values of the p-MoS₂ (5 mM) (4.9 eV) and p-MoS₂ (10 mM) (5.1 eV) increase by 0.3 and 0.5 eV, respectively, compared to the one shown by the pristine MoS₂ film. Further increase of the doping level up to 20 mM does not reflect an additional rise of the WF value.

Table 1 WF values of FTO, MoS₂ and p-MoS₂ (5, 10, 20 mM), as measured by ambient KP

Material	Work function (eV)
FTO	4.7
MoS2	4.6
p-MoS2 (5 mM)	4.9
p-MoS2 (10 mM)	5.1
p-MoS2 (20 mM)	5.1

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Fig. S4† reports the AFM images of the FTO/p-MoS₂ (10 mM), which shows no differences in surface morphology with respect to the un-doped case (FTO/MoS₂). The roughness average (R_a) values are reported in Table 2, showing a decrease of about 2 nm for both FTO/MoS₂ and FTO/p-MoS₂ (10 mM) (R_a values of 11.6 nm and 11.9 nm, respectively) if compared with the value of the bare FTO (R_a = 13.8). Thus, the FTO roughness is reduced by the overlayer of MoS₂ flakes, which could be linked with their planarity and face-on arrangement.

Table 2 Roughness average (R_a) values of the FTO, FTO/MoS₂ and FTO/p-MoS₂ (10 mM). The MoS₂ films are deposited from a 0.1 mg mL⁻¹ MoS₂ dispersion in IPA

Sample	Roughness average (Ra) (nm)
FTO	13.8
FTO/MoS2	11.6
FTO/p-MoS2 (10 mM)	11.9

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Fig. 4 reports the top view scanning electron microscopy (SEM) images of the FTO, FTO/MoS₂, FTO/p-MoS₂ (10 mM) and FTO/p-MoS₂ (20 mM) samples. No modifications of the FTO surface are observed after the MoS₂ flake deposition and doping treatment, i.e., 10 mM HAuCl₄·3H₂O, in agreement with the AFM data, i.e., R_a values reported in Table 2. The increase of the doping level to 20 mM HAuCl₄·3H₂O determines the formation of some aggregates, thus affecting the surface's homogeneity, as revealed by Fig. 4d. These aggregates are attributed to the precipitation of Au and MoO₃ clusters after Au ion reduction and Mo⁴⁺ to Mo⁶⁺ conversion processes.^46,55 In fact, for the doping level exceeding 10 mM, more electrons are needed to reduce the increased number of Au ions, and thus Mo⁴⁺ can be converted into Mo⁶⁺, resulting in the formation of Au nanoparticles and MoO₃.^46,55

Fig. 4 Top view SEM images of the (a) bare FTO, and (b) FTO/MoS₂, (c) FTO/p-MoS₂ (10 mM) and (d) FTO/p-MoS₂ (20 mM) films. The MoS₂-based films are deposited from a 0.1 mg mL⁻¹ MoS₂ dispersion in IPA.

Architecture of the hybrid solution-processed organic H₂-evolving photocathode

The full structure of the solution-processed hybrid organic H₂-evolving photocathode consists of a photoactive rr-P3HT:PCBM BHJ sandwiched between solution-processed CSLs. Few-layer MoS₂ flakes are spin coated onto FTO and tested as a HSL, while anatase TiO₂ nanoparticles are spin coated onto rr-P3HT:PCBM and used as an ESL. Amorphous MoS₃ nanoparticles, deposited on top of the TiO₂, act as an EC for the HER. Additional details of the fabrication of the photocathodes are reported in Experimental, fabrication techniques.

Fig. 5a shows the representative energy band edge positions of the semiconductors of the hybrid photocathode together with the redox levels of the HER and OER. MoS₂, as the HSL, is expected to extract the photogenerated holes towards the back conductive substrate (FTO) while the TiO₂ (as ESL) transports the photogenerated electrons towards MoS₃ (EC). Here, aqueous protons are reduced to H2, which evolves from the photocathode surface.^28,29 In order to provide the electrical driving force for the holes' collection,^28,29 MoS₂ films are doped by spin coating HAuCl₄·3H₂O methanol solutions on top of them,^46,55 thus increasing the WF values of the films from 4.6 eV up to 5.1 eV (for p-MoS₂ (10 mM) and p-MoS₂ (20 mM), as previously discussed (see also Table 1). Fig. 5b shows the high-resolution cross-sectional SEM image of a representative photocathode FTO/p-MoS₂ (10 mM)/rr-P3HT:PCBM/TiO₂/MoS₃. The layers are consecutively deposited by spin coating (see details in Experimental, fabrication techniques), giving a well-defined multi-layered structure. MoS₂ and MoS₃ layers are hard to be resolved because their thickness is small with respect to the device-scale magnification. Thicknesses around 150 and 80 nm can be estimated for the rr-P3HT:PCBM and TiO₂ layers, respectively. The same thickness values of the rr-P3HT:PCBM layer are also measured by profilometry (see Experimental, fabrication techniques).

Fig. 5 (a) Typical energy band edge position of materials assembled in the hybrid solution-processed photocathode. The rr-P3HT:PCBM layer, in BHJ configuration, efficiently absorbs light and generates charges. MoS₂ and TiO₂ act as hole- and electron- selective layers (HSL and ESL), respectively, driving the holes towards the FTO substrates and the electrons towards the MoS₃ nanoparticles, acting as the EC layer for the HER. Redox levels of both the hydrogen evolution reaction (HER) (blue solid line) and oxygen evolution reaction (OER) (blue dashed line) are also shown. WF values of the MoS₂ (4.6 eV) are measured by ambient KP. These values can be tailored to higher values (up to 5.1 eV) by solution-processed treatments using HAuCl₄·3H₂O as the dopant agent. (b) High-resolution cross-sectional SEM image of the representative photocathode FTO/p-MoS₂ (10 mM)/rr-P3HT:PCBM/TiO₂/MoS₃.

Photoelectrochemical characterization

The rrP3HT:PCBM-based photocathodes using MoS₂ and p-MoS₂ as HSLs are characterized by linear sweep voltammetry (LSV) in H₂SO₄ solution at pH 1. The results for MoS₂ and p-MoS₂ (10 mM) deposited from 0.1 mg mL⁻¹ MoS₂ dispersion in IPA are reported in Fig. 6a, where they are compared with the responses of a HSL-free photocathode and the current–potential curve of the MoS₃ EC (deposited directly onto FTO). The MoS₃ EC reveals activity for the HER, with an onset overpotential of 180 mV with respect to the RHE potential.⁶³ The voltammograms of the photocathodes show a photocurrent that increases when the potential is swept towards negative values. The photocurrents are positively affected by the presence of MoS₂ films, thus confirming their role as the HSL. The dependence on the doping level of the p-MoS₂-based photocathodes is shown in Fig. 6b. It is worth noting that different concentrations (0.05, 0.1, and 0.4 mg mL⁻¹) of the MoS₂ dispersion in IPA have been also preliminary tested without doping (Fig. S5†) and with 10 mM doping (Fig. S6†), showing the best results for the 0.1 mg mL⁻¹ dispersion.

Fig. 6 (a) LSVs recorded for the photocathodes using MoS₂ (orange lines) and p-MoS₂ (10 mM) (red lines) as HSLs measured in 0.5 M H₂SO₄ solution at pH 1, under dark (dashed lines) and AM1.5 light illumination (100 mW cm⁻²) (solid lines) MoS₂ films are deposited by 0.1 mg mL⁻¹ MoS₂ dispersion in IPA. The photoelectrochemical responses of the photocathode without any HSL (blue lines) and the current–potential curve of the MoS₃ electrocatalyst (deposited directly on FTO) (short dashed black line) are also shown. (b) LSVs recorded for p-MoS₂-based photocathodes obtained with different levels of doping (purple, red and magenta lines for 5, 10 and 20 mM HAuCl₄·3H₂O methanol solution, respectively), under the same previous conditions.

The photovoltage V_photo is the difference between the potential applied to the photocathode under illumination (E_light) and the potential applied to the catalyst electrode (E_dark) to obtain the same current density. The subscript “m” stands for “maximum”. The power-saved FoM (Φ_saved,NPAC) reflects the photovoltage and photocurrent of a photocathode independent of the over-potential requirement of the catalyst.⁶⁴ Here, we simply assume that the MoS₃ film deposited onto FTO is identical to the one deposited onto TiO₂. Table 3 summarizes the main FoM extracted from the voltammograms recorded for the different cases: the photocurrent density taken at 0 V vs. RHE (J_{0 V vs. RHE}), the V_oc, and Φ_saved,NPAC, which is calculated by:⁶⁹

where η_F is the faradaic efficiency assumed to be 100%,^28d,29a,29cP_in is the power of the incident illumination and j_photo,m and V_photo,m are the photocurrent and photovoltage at the maximum power point, respectively. j_photo,m is obtained by calculating the difference between the current under illumination of a photocathode and the current of the corresponding catalyst.

Table 3 FoMs of photocathodes fabricated without the HSL and with MoS₂ and p-MoS₂ (5, 10 and 20 mM) as HSL: the current density taken at 0 V vs. RHE (J_{0 V vs. RHE}), the onset potential (V_oc), defined as the potential at which a photocurrent density of 0.1 mA cm⁻² is reached, and the power-saved FoM Φ_saved,NPAC

HSL	J 0 V vs. RHE (mA cm^−2)	V oc (V vs. RHE)	Φ saved,NPAC (%)
—	0.12	0.07	0.015
MoS2	0.30	0.30	0.070
p-MoS2 (5 mM)	0.36	0.35	0.095
p-MoS2 (10 mM)	1.21	0.55	0.423
p-MoS2 (20 mM)	0.54	0.49	0.192

---

Although the value of the main FoM of the undoped MoS₂-based photocathode (J_{0 V vs. RHE} = 0.3 mA cm⁻², V_oc = 0.3 V vs. RHE, Φ_saved,NPAC = 0.070%) increases with respect to the ones of the HSL-free photocathode (J_{0 V vs. RHE} = 0.12 mA cm⁻², V_oc = 0.07 V vs. RHE, Φ_saved,NPAC = 0.015%), significant photocurrents are observed only using p-MoS₂ (above 1 mA cm⁻² for potential <0.06 V vs. RHE for a doping level of 10 mM). For p-MoS₂ (10 mM) and p-MoS₂ (20 mM), the presence of a photoreduction peak, located at ∼0.33 V vs. RHE, is also observed. Its origin is attributed to the chemical reduction of the MoS₃ EC towards more electrocatalytically active MoS_2+x species for the HER,⁶⁸ as also confirmed by the analysis of two consecutive LSVs of FTO/MoS₃ measured under the same testing conditions as the ones reported in Fig. 6 (see ESI Fig. S7†). In the case of p-MoS₂ (10 mM), J_{0 V vs. RHE}, V_oc and Φ_saved,NPAC are 1.21 mA cm⁻², 0.56 V vs. RHE and 0.423%, respectively. Photocathodes based on p-MoS₂ (20 mM) (highest doping level) and p-MoS₂ (5 mM) (lowest doping level) report J_{0 V vs. RHE} of 0.54 and 0.36, thus decreasing by 56% and 70% with respect to the case of p-MoS₂ (10 mM), respectively. Concerning p-MoS₂ (5 mM), also in this case there is a decrease of the V_oc (V_oc = 0.35 V vs. RHE) of 200 mV with respect to the values observed for p-MoS₂ (10 mM). Moreover, we noted a decrease of 60 mV of the V_oc for the p-MoS₂ (20 mM) if compared with the p-MoS₂ (10 mM) photocathode. The values of Φ_saved,NPAC calculated for p-MoS₂ (5 mM) and p-MoS₂ (20 mM) are 0.095% and 0.192%, respectively. These values correspond to a decrease of 77.5% and 54.6% with respect to that of p-MoS₂ (10 mM), respectively.

The obtained results highlight the importance of the WF tuning of the MoS₂ films by p-doping treatment for their full-exploitation as highly performant HSLs. Fig. 7 shows the MoS₂ doping level dependence of the photocathodes' FoM, as gathered from Table 3. In agreement with the SEM characterization reported in Fig. 4d, the decrease of the J_{0 V vs. RHE} for the p-MoS₂ (20 mM) case could be ascribed to charge recombination pathways, i.e., leakage currents, in the presence of blend-uncovered Au and MoO₃ clusters. Although these surface alterations of the MoS₂ films negatively affect the photocurrents, the V_oc value is similar to the one recorded for p-MoS₂ (10 mM). Differently, for the case of p-MoS₂ (5 mM), we observed a decrease of both the J_{0 V vs. RHE} and the V_oc with respect to the p-MoS₂ (10 mM) case. This result could be linked with differences (homogeneity) of the MoS₂ film doping, as suggested by the lower WF value of p-MoS₂ (5 mM) film (4.9 eV) if compared with the ones of p-MoS₂ (10 mM) and p-MoS₂ (20 mM) films (5.1 eV).

Fig. 7 Dependence of the photocatodes' FoMs (red, blue and black colours for J_{0 V vs. RHE}, V_oc and Φ_saved,NPAC, respectively) on the HAuCl₄·3H₂O doping level (from 0 to 20 mM) of the p-MoS₂ films.

As a consequence, the main characteristics of the p-MoS₂ (5 mM)-based photocathode (V_oc ∼ 0.35 V vs. RHE, J_{0 V vs. RHE} ∼ 0.36 mA cm⁻¹ and Φ_saved,NPAC ∼ 0.095) are similar to those recorded for the undoped MoS₂-based photocathode (V_oc ∼ 0.30 V vs. RHE, J_{0 V vs. RHE} ∼ 0.30 mA cm⁻¹ and Φ_saved,NPAC ∼ 0.070). To sum up, V_oc seems to be linearly correlated with the WF values of the different films, while the J_{0 V vs. RHE} turns out also to be affected by the film surface morphology. The combination of these effects explain the behaviour of the Φ_saved,NPAC, whose best value of 0.423% is reached for the p-MoS₂ (10 mM)-based photocathode.

The stability test under potentiostatic conditions is performed for p-MoS₂ (10 mM) case, recording in time the J_{0 V vs. RHE} under continuous illumination (Fig. 8). The data show an average initial photocurrent value of 1.36 mA cm⁻² followed by a steep decrease in performance down to 0.77 mA cm⁻² during the first 5 minutes (photocurrent loss of 50.7%). After 30 minutes of continuous operation, the photocurrent density reaches 0.49 mA cm⁻² (photocurrent loss of 63.2%). Thus, the photocurrent mainly decreases at the beginning of the illumination process. The quick, initial performance degradation has been recently observed in similar architectures using Pt as the catalyst and CuI as the HSL,^29d being attributed to the irreversible Pt detachment from the surface of the electrodes.^29d A similar effect could affect the MoS₃ EC used here. After more than 5 min, a progressive stabilization is observed. Notably, there is no delamination of the film during the measurements, suggesting the electrochemical stability of the FTO/p-MoS₂/P3HT:PCBM under-layers. Moreover, cyclic voltammetry (CV) measurements reveal the absence of irreversible redox reactions involving MoS₂ films under the HER-working condition of the photocathodes (see ESI, Fig. S8†). This is in agreement with the electrochemical stability of the MoS₂ (ref. 45) expressed in several energy conversion and storage devices,⁷⁰ including fuel and water splitting cells,⁷¹ batteries⁷² and supercapacitors.⁷³ These results evidence the need for future efforts also in the development of stable ESL/EC layers, for which the integration of TMDs could offer winning solutions thanks to their tuneable charge-selective properties⁴⁶ and electro-catalytic activity towards the HER.⁷⁰

Fig. 8 Potentiostatic stability test of the photocathode fabricated with p-MoS₂ (10 mM) at pH 1 and 0 V vs. RHE under continuous illumination (1 sun). The blue lines mark the losses of the photocurrent after 5 and 30 min.

Conclusion

Few-layer MoS₂ flakes are demonstrated as a novel HSL for solution-processed hybrid organic H₂-evolving photocathodes. The production method of the MoS₂ flakes underpins on water-based exfoliation of Li-intercalated bulk MoS₂, avoiding the use of toxic reagents/solvents and being potentially compatible with large-area and low-cost production. We produced ultrathin MoS₂ layers (<10 nm) that, once deposited, do not affect the morphology of the underneath FTO substrate. We tuned the electrical properties of the MoS₂ films by using solution-processed HAuCl₄·3H₂O doping, increasing the WF value from 4.6 up to 5.1 eV, thus well matching with the HOMO level of the rr-P3HT. The morphology of the MoS₂ films and the optimization of the HAuCl₄·3H₂O doping level led to a hybrid solution-processed organic H₂-evolving photocathode with J_{0 V vs. RHE} of 1.21 mA cm⁻² at 0 V vs. RHE, V_oc of 0.56 V vs. RHE and Φ_saved,NPAC approaching 0.5%. These results pave the way towards the integration of MoS₂ as the HSL in organic PEC cells to boost this technology as an efficient scalable and low-cost tool for solar H₂ production.

Experimental

Material preparation

MoS₂ flakes are prepared by a chemical lithium intercalation method. Experimentally, 0.3 g of MoS₂ bulk powder (Sigma Aldrich) is dispersed in 4 mL of 2.0 M n-butyllithium (n-BuLi) in cyclohexane⁵⁶ (Sigma Aldrich). The dispersion was kept stirring for two days at room temperature under an Ar atmosphere. The Li-intercalated material (Li_xMoS₂) is separated by suction filtration under Ar. Li_xMoS₂ is washed with anhydrous hexane to remove non-intercalated Li ions and organic residues. Li_xMoS₂ powder is then exfoliated by ultrasonication (Branson® 5800 ultrasonic cleaner) in deionized (DI) water for 1 h. The obtained dispersion is then ultracentrifugated (Optima™ XE-90 ultracentrifuge, Beckman Coulter) at 10 K for 20 min to remove LiOH and the un-exfoliated material. Finally, the precipitate is filtered and re-dispersed in IPA (absolute alcohol, without additive, ≥99.8%, Sigma Aldrich), in order to accurately control the concentration of the final dispersions.

Fabrication techniques

Photocathodes are fabricated according to the architecture FTO/HSL/rr-P3HT:PCBM/TiO₂/MoS₃, where few-layer MoS₂ flakes are used as the HSL. Complementary architectures without the HSL are also fabricated. FTO coated soda-lime glass substrates (Dyesol, sheet resistance 15 Ω sq⁻¹) are cleaned according to the following protocols: sequential sonication baths in DI water, acetone, and IPA each lasting for 10 minutes and plasma cleaning in an inductively coupled reactor for 20 minutes (100 W RF power, excitation frequency 13.56 MHz, 40 Pa of O₂ gas process pressure, background gas pressure 0.2 Pa).

The MoS₂ dispersion is deposited onto the previously treated FTO by spin coating (Laurell Tech. Corp. Spin coater) using a single step spinning protocol with a rotation speed of 3000 rpm for 60 s. Concentrations of the dispersions of 0.05, 0.1 and 0.4 mg mL⁻¹ are tested. Post-thermal annealing in an Ar atmosphere at 150 °C for 30 min is performed for the MoS₂ films. The latter are subsequently doped by spin casting HAuCl₄·3H₂O (≥99.9% trace metals basis, Sigma Aldrich) in methanol (ACS reagent, ≥99.8%, Sigma Aldrich) solution as p-doping agents on top, by using the same single step spinning protocol of the MoS₂ deposition. Doping concentrations of 5, 10, and 20 mM are tested. All doping solutions are sonicated for 10 minutes before deposition. The doped films are subsequently dried for 30 min under an Ar atmosphere. The organic polymer film used in all the architectures consists of a blend of rr-P3HT, as the donor component, and PCBM, as the acceptor component (rr-P3HT:PCBM). rr-P3HT (electronic grade M_n 15 000–45 000, Sigma Aldrich) and PCBM (99.5% purity, Nano C) are separately dissolved in chlorobenzene (ACS grade, Sigma Aldrich), at a 1 : 1 wt ratio and 25 mg mL⁻¹ on a polymer basis. Polymer blend solution is stirred at 40 °C for 24 hours before use. Blend thin films are obtained by spin casting the rr-P3HT:PCBM solution using the following set of parameters: two step spinning protocol with rotation speeds of 800 rpm for 3 s followed by 1600 rpm for 60 s, respectively. This spin casting protocol produced a rr-P3HT:PCBM blend layer 200 ± 20 nm thick, as measured by means of a Dektak XT profilometer (Bruker) equipped with a diamond-tipped stylus (2 mm) selecting a vertical scan range of 25 mm with 8 nm resolution and a stylus force of 1 mN, on an area of 0.25 cm². The sol–gel procedure for producing the TiO₂ dispersion has been previously reported by Kim et al.⁷⁴ Thus, TiO₂ precursor solution is prepared in IPA and subsequently deposited by spin casting on top of the rr-P3HT:PCBM film as the ESL. Subsequently, during 12 h in air at room temperature, the precursor converted to TiO₂ by hydrolysis. A three step spinning protocol with rotation speeds of 200 rpm for 3 s, 1000 rpm for 60 s and 5000 rpm for 30 s is used. Post-thermal annealing in an Ar atmosphere is carried out at 130 °C for 10 min for all the devices before catalyst deposition. The devices are completed by a layer of MoS₃ nanoparticles (Alfa Aesar) acting as the catalyst for the HER process. The catalyst layer is obtained by spin casting a 3.8 mg mL⁻¹ water : acetone : NaOH (1 M) 1 : 2 : 0.2 dispersion on top of the TiO₂. The dispersion is stirred overnight at room temperature and sonicated for 10 minutes before its use. The spinning protocol is identical to the one adopted for the TiO₂ deposition.

Material characterization

The UV-Vis absorption spectrum of the 0.1 mg mL⁻¹ MoS₂ dispersion in IPA is obtained using a Cary Varian 5000UV-Vis spectrometer.

Raman measurements are carried out with a Renishaw 1000 using a 50× objective, a laser with a wavelength of 532 nm and an incident power of 1 mW. The different peaks are fitted with Lorentzian functions. For each sample 30 spectra are collected. 0.01 mg mL⁻¹ MoS₂ dispersion in IPA is drop-cast onto the Si/SiO₂ (300 nm SiO₂) substrate and dried under vacuum. Statistical analysis of the relevant features is carried out by means of software Origin 8.1 (OriginLab).

The XPS analysis is carried out using a Kratos Axis Ultra spectrometer on MoS₂ flakes drop cast onto 50 nm-Au sputtered coated Si wafers from the 0.1 mg mL⁻¹ dispersion in IPA. The XPS spectra are acquired using a monochromatic Al Kα source operated at 20 mA and 15 kV. High-resolution spectra are collected with a pass energy of 10 eV and energy step of 0.1 eV over a 300 μm × 700 μm area. The Kratos charge neutralizer system is used on all specimens. Spectra are charge corrected to the main line of the C_1s spectrum set to 284.8 eV, and analyzed with CasaXPS software (version 2.3.17).

Transmission electron microscopy images are taken on a JEM 1011 (JEOL) transmission electron microscope, operating at 100 kV. 0.01 mg mL⁻¹ MoS₂ dispersions in IPA are drop-cast onto carbon coated Cu TEM grids (300 mesh), rinsed with DI water and subsequently dried under vacuum overnight. The lateral dimension of the MoS₂ flakes is measured using ImageJ software (Java). Statistical TEM analysis is carried out by means of software Origin 8.1 (OriginLab).

Scanning electron microscopy analysis is carried out with a FEI Helios Nano lab field-emission scanning electron microscope. The samples are imaged without any metal coating or pre-treatment. The samples are prepared depositing MoS₂ films on FTO as described in the Fabrication techniques section.

Atomic force microscopy images are obtained using a commercial AFM instrument MFP-3D (Asylum Research), with NSG30/Au (NT-MDT) probes in tapping mode in air. These golden silicon probes have a nominal resonance frequency and spring constant of 240–440 kHz and 22–100 N m⁻¹, respectively. The tip is a pyramid with 14–16 μm length and ∼20 nm apex diameter. The images are processed with the AFM company software Version-13, based on IgorPro 6.22 (Wavemetrics). MoS₂ is deposited onto FTO as described in the previous section (Fabrication techniques). Images of MoS₂ flakes deposited onto mica sheets (EMS) (V-1 quality) are also acquired in order to extrapolate the average thickness of single flakes. MoS₂ flakes are deposited by drop casting from 0.1 mg mL⁻¹ MoS₂ dispersion in IPA. Statistical analysis of the height profiles is carried out by means of software Origin 8.1 (OriginLab).

Work function data are obtained by using a commercial kelvin probe system (KPSP020, KP Technologies Inc.). Samples are measured in air and at room temperature and both a clean gold surface (WF = 4.8 eV) and a graphite sample (HOPG, highly ordered pyrolytic graphite, WF = 4.6 eV) are used as independent references for the probe potential.

Photoelectrochemical measurements

Photoelectrochemical measurements are carried out at room temperature in a flat-bottom fused silica cell under a three-electrode configuration using a CompactStat potentiostat/galvanostat station (Ivium), controlled via Ivium's own IviumSoft. A Pt wire is used as the counter-electrode and sat. KCl Ag/AgCl is used as the reference electrode. Measurements are performed in 50 mL aqueous solution of 0.5 M H₂SO₄ (99.999% purity, Sigma Aldrich) at pH 1. Oxygen is purged from electrolyte solutions by flowing nitrogen gas throughout the liquid volume using a porous frit at least 30 minutes before starting measurements. A constant, slight nitrogen flow is maintained afterwards for the whole duration of experiments, to avoid re-dissolution of molecular oxygen in the electrolyte. Potential differences between the working electrode and the reference electrode are reported with respect to the RHE scale using the Nernst equation. A 300 W Xenon light source LS0306 (Lot Quantum Design), equipped with AM1.5G filters, is used to simulate solar illumination (1 sun) at the glass substrate side of the samples inside the test cell. Linear Sweep Voltammetry is used to evaluate the response of devices in the dark and under 1 sun illumination. Voltage is swept starting from the V_oc of the photocathodes to a negative potential of −0.3 V vs. RHE at a scan rate of 10 mV s⁻¹. The stability test is performed by recording in time the J_{0 V vs. RHE} under continuous illumination (1 sun) at 0 V vs. RHE. Cyclic voltammetry (CV) is carried out on MoS₂ films deposited on glassy carbon substrates (Sigma Aldrich) (previously cleaned with IPA) by consequent drop casting steps from 0.4 mg mL⁻¹ MoS₂ dispersion in IPA (until reaching a loading of MoS₂ of 0.5 mg cm⁻¹). The voltage scan rate is 200 mV s⁻¹.

Acknowledgements

We acknowledge M. Prato for useful discussion. This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 696656—GrapheneCore1, and from the FP7 FET Collaborative Project PHOCS, under grant agreement n. 309223.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta10572f

‡ These authors contributed equally.

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