Cobalt Catalyst Grafted CdSeTe Quantum Dots on Porous NiO as

Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11166−11174

pubs.acs.org/journal/ascecg

Cobalt Catalyst Grafted CdSeTe Quantum Dots on Porous NiO as Photocathode for H2 Evolution under Visible Light Xin Su,† Yun Chen,*,† Long Ren,† Yang He,† Xiaoshuang Yin,† Ying Liu,† and Wenzhong Yang*,† †

School of Chemistry and Molecular Engineering, Nanjing Tech University, No. 30 Puzhu Road (S), Nanjing 211816, China

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S Supporting Information *

ABSTRACT: A photoelectrochemical cell for water splitting was first reported using CdSeTe quantum dots (QDs) as its superior photoelectric efficiency and photostability under the irradiation of visible light. Using the mesoporous p-type NiO as hole conductor and a cobaloxime catalyst as the economic cocatalyst for the hydrogen evolution reaction without sacrificial electron donor, the Co−CdSeTe− NiO photocathode exhibited a considerably high photocatalytic current density of −130 μA cm−2 at a bias of −0.1 V vs NHE in a near neutral electrolyte upon illumination. Simultaneously, the photocathode produced 2.9 μmol H2 with almost 93% Faradaic efficiency over 3 h illumination at −0.3 V vs NHE. It suggested that CdSeTe QDs could be used as good light absorber, and the Co−CdSeTe−NiO photocathode exhibited efficient photocatalytic ability. KEYWORDS: Ternary quantum dots, Photoelectrochemical cell, Hydrogen generation, Simulated sunlight, Water splitting

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INTRODUCTION Due to the rapid development of the global economy and population, fossil resources are reasonably predicted to be exhausted in the future.1−3 It is urgent to explore sustainable and environmentally benign energy instead of fossil fuels for humans. Hydrogen has been considered as a clean, carbon-free fuel with high specific enthalpy and high energy capacity. Thus, hydrogen is regarded as the perfect energy carrier in the future to reduce the emissions of greenhouse gases.4−6 Up to now there have been two main approaches to produce hydrogen. One is hydrocarbon reformation, which discharges undesired CO2. Another is electrolysis, which suffers from high cost.7 For these reasons, the development of sustainable technologies to generate hydrogen with low carbon, energy savings, and low cost becomes more and more attractive. Converting solar energy directly to storable energy-rich compounds is one of the attractive approaches, such as in the form of hydrogen derived from the splitting up of water. Among a variety of approaches, the photoelectrochemical cell (PEC) system is one of the most promising methods for generating hydrogen in aqueous solution which integrates the functions of visible-light absorption and the electrolysis hydrogen reduction into a single device.8−12PEC cells have several advantages, such as no need of sacrificial reagent and separation of photogenerated H2 and O2 spatially, which have made the PEC systems a hot research topic in the past few decades.13−22 Typically, a dye-sensitized PEC cell is usually composed of a light absorber and a cocatalyst.23 However, because of the low efficiency of hydrogen generation through the PEC watersplitting system, there are still lots of urgent problems to solve, such as inadequate absorption of visible light, poor stability, and © 2019 American Chemical Society

high cost. Thus, it is of great significance to develop a photocathode with wide scope and strong absorption in sunlight, stability in electrolyte, and high photoelectric efficiency for hydrogen production. Among them, the performance of a PEC cell is largely determined by the properties of the photosensitizers. It is important to take full advantage of the visible light to improve the efficiency of PEC cells.24 Up to now, QDs have been used as photosensitizer at the forefront of water splitting into hydrogen because of their significant advantages over organic dye molecules. Semiconductor QDs have larger extinction coefficients and a broad absorption spectral range in the solar spectrum, and they can absorb multiple photons simultaneously or continuously in the case of electrons or holes piled up, thus improving device efficiency.25,26 Owing to these reasons, the development of new light-driven systems using QDs as light-harvesting materials for H2 generation is of great significance. Generally, the band gap of binary QDs can only be tuned by controlling the particle size due to the quantum size effect.27 However, adjusting the particle size to tune the band gap is not easy to control. The supertiny or large particles are not conductive to the emission efficiency. Therefore, the absorption of solar light by QDs is limited, and the utilization rate of solar light is low.28 Compared with binary QDs, ternary QDs can tune the band gap by modulating the compositive proportion of elements to extend their absorptions into the visible region.29,30 In comparison with other ternary QDs, the excitation wavelength of CdTeSe QDs can readily reach the visible region by Received: January 17, 2019 Revised: May 5, 2019 Published: May 30, 2019 11166

DOI: 10.1021/acssuschemeng.9b00305 ACS Sustainable Chem. Eng. 2019, 7, 11166−11174

Research Article

ACS Sustainable Chemistry & Engineering tuning the molar ratio of Te:Se.31 Functionalization of CdSeTe QDs makes them have rich functional groups, such as carboxylic acid and thiolate group, thus imparting them with excellent water solubility and subsequent linkage possibilities. However, up to now, there were few investigations about using CdTeSe QDs in application of photosensitizers in PEC system. In PEC water-splitting reactions, the cocatalyst that catalyzes the formation of hydrogen plays a key role in improving the activity and stability of semiconductor photocatalysts because cocatalyst can reduce the activation energy of generating H2 on semiconductor surface. Platinum is currently the best catalyst; however, its expensive price and limited reserves restrict wide application. Therefore, abandoning precious metal catalysts and using a large number of metal elements existing on the Earth, such as Fe, Co, and Ni, to develop new catalysts for hydrogen production have become the focus of research. The cobaloxime catalysts as the H2-producing catalysts have been applied in several PEC cell systems driven by sunlight because of the lower overpotential of catalyzing the reversible reduction of protons to H2 at neutral pH.32−34 In addition, nanostructured NiO with wide bandgap (3.5 eV), good chemical stability, and relatively low cost has been used as a p-type semiconductor for fabrication of dye-sensitized solar cells.35−38 The valence band (0.5 V vs NHE) of NiO enables efficient reception of holes from QDs.14 Therefore, NiO is considered as an efficient hole conductor and electron-blocking material in dye-sensitized solar cells.39 Mesoporous NiO is with specific surface area, which can increase the loading amount of QDs. With this in mind, a photocathode was constituted by CdSeTe QDs as photosensitizer, a typical cobaloxime complex Co(dmgBF2)(H2O)2 as proton reduction catalyst on porous NiO film, for hydrogen evolution in a neutral buffer solution with nonsacrificial agent. The mesoporous NiO film was grown on FTO by a hydrothermal method to increase the specific surface area. In the designed photocathode, the water-soluble L-cysteine (L-cys) stabilized CdSeTe QDs, in which the L-cys acted as a capping ligand to prevent the QDs from aggregation and a linker to assemble CdSeTe QDs and NiO in aqueous solution. More importantly, CdSeTe QDs could enhance the absorption of visible light by modulating the compositive proportion of Se and Te elements. Then, the molecular cobalt catalyst Co(dmgBF2)(H2O)2 was anchored to the surface of QDs. Through this approach, the photocathode could efficiently generate H2 from neutral water under visible-light irradiation without the aid of sacrificial electron donor. The excellent photocurrent density of −130 μA cm−2 at a bias of −0.1 V vs NHE under visible light was obtained. Also, the photocathode produced 2.9 μmol hydrogen with the Faradaic efficiency of almost 93%.

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electron microscopy (HRTEM) was performed by JEOL-2100F (operated at an accelerating voltage of 200 kV). Field emission scanning electron microscopy (FESEM) images were recorded on a JSM-7600F (accelerating voltage of 15 kV). X-ray diffraction (XRD) measurements were performed on a Bruker D8 Advance under Cu−Kα radiation at (λ = 1.540 56 Å). The chemistry state of elements was evaluated by X-ray photoelectron spectroscopy instrument (XPS, Thermo ESCALAB 250xi spectrophotometer with Al−Kα radiation). The binding energy scale was calibrated using the C 1s peak at 284.6 eV. The Fourier transform infrared (FT-IR) spectroscopy was obtained with a Thermo Nicolet iS spectrophotometer. Gas chromatography (GC) was performed by Fuli GC9790P using a 5 Å molecular sieve column and thermal conductivity detector. The compositions of QDs and cobalt catalyst were determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent ICP-OES730). The composition of S was determined by Elemental Analyzer (Vario EL cube). Electrocatalytic Experiment. All electrochemical experiments were performed with a CHI660E Electrochemical Workstation. The linear sweep voltammetry (LSV) measurements of the as-prepared photocathodes were carried out in 0.1 M phosphate buffer solution at pH 6.8 in a standard three-electrode cell, using platinum as counter electrode and Ag/AgCl in 3 M KCl as reference electrode. The scan rate was 100 mV s−1. The transient photocurrent responses to on−off illumination were performed in a two-compartment cell, and the volume of the 0.1 M phosphate buffer solution was 20 mL in each compartment. The platinum electrode was separated from the main compartment of the cell by a Nafion membrane, so the produced O2 did not mix with H2 produced at the working electrode. Before measurement, the testing solution (20 mL) was degassed for at least 20 min by flushing high-purity nitrogen. For photocurrent measurement, the photocathode was illuminated under a 300 W Xe lamp equipped with a λ > 400 nm cutoff filter, and the light intensity at the surface of the electrode was 100 mW cm−2. All potentials reported in this paper were converted to the NHE reference using E(NHE) = E(Ag/AgCl) + 0.197 V.

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RESULTS AND DISCUSSION First, the deposition of NiO on FTO glass substrate was studied with X-ray diffraction (XRD), as shown in Figure S1A. All reflection peaks of blank FTO glass are shown in Figure S1A, in accord with the standard pattern of fluorine-doped SnO2 (SnO2:F).40 After coating, a new peak at 43.1° occurred, and the weak XRD signals at 36.7°and 62.4°in the presence of FTO substrate could also be seen, which clearly confirmed the growth of NiO on the surface of FTO substrate. In order to determine the composition of the as-prepared product was NiO, we characterized the uncoated NiO powder prepared by the same method. As shown in the inset panel of Figure S1A, the annealed film showed diffraction peaks at 36.7°, 43.1°, and 62.4°, which correspond to (111), (200), and (220) crystal planes of cubic NiO phase, respectively (JCPDS #47-1049). Then, the morphology of the NiO film was characterized using FESEM, as shown in Figure S1B. It could be indicated that the FTO glass substrate was covered with a layer of compact and uniform porous film, which was composed of interconnected nanosheets, and the pore size distribution was at a range of 0.1−1 μm (Figure S1B). Such an open porous nanostructure increased the specific surface area obviously. During the process of preparing the NiO film, glucose was used as a binder to help NiO grow on the surface of FTO.41,42 L-Cys CdSeTe QDs were synthesized via incorporating Se into CdTe nanocrystals, and the emission wavelength of L-cys CdTeSe QDs could be changed by tuning the molar ratio of Te:Se. Figure S2A showed the fluorescence spectra of L-cys CdSeTe QDs, and it could be clearly identified that the optimum

EXPERIMENTAL SECTION

Fabrication of the Photocathode. NiO, CdSeTe QDs, and Co(dmgBF2)2(H2O)2 were prepared according to the reported methods; see details in the Supporting Information. The NiO film electrode with 1 cm2 active area was soaked into 0.25 mM CdSeTe QDs solution at ambient temperature for 24 h. Then it was washed with ultrapure water and dried at room temperature. Co(dmgBF2)2(H2O)2 (75, 100, 125, and 150 nmol) was introduced by dropping its acetonitrile solution of different amount with 5% Nafion onto the surface of CdSeTe-sensitized NiO electrode, followed by drying under air flow. The electrode sample for the following characterization was the one producing maximum photocurrent. Characterization. The UV−vis absorption measurements were carried out on an Agilent 8453 spectrophotometer. The fluorescence spectra were taken on Edinburgh FS5. High-resolution transmission 11167


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ACS Sustainable Chemistry & Engineering composition for the preparation of CdSeTe QDs was 3:1. Under this ratio, the excitation wavelength was 400 nm, which was also in the visible-light wavelength region. The emission intensity was the strongest at the wavelength of 686 nm as well. As shown in Figure S2B, it was also discovered that the emission wavelength was still strong in the visible region, which demonstrated that L-cys CdSeTe QDs prepared by this ratio could undergo electronic transition under visible-light irradiation. In order to calculate the band gap (ΔEg), we characterized the absorption spectrum of L-cys CdSeTe QDs by UV−visible spectrophotometer as seen in Figure 1. It showed that there were

Figure 1. UV−vis absorption spectrum of L-cys CdSeTe QDs. The first excitonic absorption peak was centered at 615 nm. Figure 2. FT-IR spectra of pure L-cysteine, no L-cys CdSeTe QDs, and L-cys coated CdSeTe QDs.

three absorption peaks, and the first excitation absorption peak was at 615 nm. Therefore, the band gap of the QDs was 2.02 eV which could be calculated from the absorption (λpeak) of the corresponding sample (see the Supporting Information).43 This narrow band gap could allow QDs to exploit more solar spectrum and accelerate the photoinduced electrons separation and migration, which make photocatalytic reactions easier to occur.23 Then, the modification of L-cysteine onto the surface of QDs and CdSeTe QDs without modification was characterized by the FT-IR spectrum. As shown in Figure 2, a characteristic peak at 2976 cm−1 corresponding to the CH2−S bonding was clearly observed in L-cys CdSeTe QDs, but it did not exist in CdSeTe QDs without the protection of L-cys. The featured peak at 1630 cm−1 corresponded to the CO asymmetric vibration, which was noted stronger in the FT-IR spectrum of L-cys CdSeTe QDs than CdSeTe QDs without L-cys. It also implied that the L-cysteine molecules existed on the surface of L-cys CdSeTe QDs.44 Moreover, the morphology and structure of the obtained CdSeTe QDs were further characterized by HRTEM images and XRD testing. From HRTEM image (Figure 3A), it is revealed that L-cys CdSeTe QDs had good dispersivity, high crystallinity, and axiolitic structure with an average particle size of about 3 nm. By examining several isolated particles under high magnification, the average spacing of L-cys CdSeTe QDs was estimated to be 0.32 nm, which was consistent with the (400) crystal lattice planes of bulk zinc-blende CdSeTe (JCPDS #652891). The XRD studies of the L-cys CdSeTe QDs presented clearly that all peaks of CdSeTe corresponded to the cubic CdSe (JCPDS #65-2891) and CdTe (JCPDS #65-1047). The main peaks were all located between CdTe and CdSe standard values, and the peaks located at 25.8°, 42.5°, 50.2°, and 63.1° were

assigned to (111), (220), (331), and (400) plane of L-cys CdSeTe, respectively (Figure 3B). Then, the photocathode was assembled in L-cys CdSeTe aqueous solution. L-Cys with bifunctional linker could link NiO and CdSeTe QDs, where the thiol group linked to QDs via a Cd−S bond, and the carboxylate group linked to the NiO.45,46 To verify the NiO film had adsorbed and saturated by L-cys CdSeTe QDs, the fluorescence intensity was compared before and after dipping the NiO-FTO electrode for 1, 2, 12, 24, and 36 h (Figure S3); the fluorescence intensity decreased gradually with the soaking time increasing and did not change significantly after 24 h. Then, ICP-OES testing was performed to determine the content of Cd, Se, and Te. As shown in the Table S1, the sum of Cd, Se, and Te adsorbed on the NiO film and the corresponding residual amounts was the total amount of the original solution, respectively. HRTEM was also used to verify NiO film modified by the L-cys CdSeTe QDs. As presented in Figure 3C and Figure 3D, the CdSeTe QDs had been grafted on the porous NiO film and the QDs distributed uniformly on the surface of NiO film. Since the L-cys CdSeTe QDs linked on the surface of the NiO film provided active sites and functionalized groups, this enabled cobaloxime catalysts to be fixed on QDs. As shown in Figure 3E, it was seen that the surface of the NiO became rough after the adsorption of CdSeTe QDs and Co(dmgBF2)(H2O)2. The corresponding elemental mapping directly showed that Cd, Se, Te, and Co were uniformly distributed on the electrode as shown in Figure 3F. All characterization results fully demonstrated the successful incorporation of the QDs and cocatalysts with NiO porous film. According to the ICP-OES analysis, the amount of CdSeTe QDs (in Cd) and Co was 118.9 and 92.6 nmol, respectively. 11168


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ACS Sustainable Chemistry & Engineering

Figure 3. (A) HRTEM image of L-Ccys CdSeTe QDs (the inset above depicts the representative image of individual CdSeTe QD, the inset below represents the size distribution of CdSeTe QDs). (B) XRD patterns of L-cys CdSeTe QDs with Te:Se = 3:1. Standard diffraction lines of cubic CdTe and cubic CdSe are shown for comparison. The HRTEM images of L-cys CdSeTe QDs sensitized NiO film with lower magnification (C) and with higher magnification (D). (E) FESEM image of the assembled Co−CdSeTe−NiO phtocathode. (F) The elemental mapping of cadmium, selenium, tellurium, and cobalt by EDX spectroscopy.

The XPS was performed to determine the exact chemical compositions and electronic structure of the fabricated photocathode. The XPS survey scans of Co−CdSeTe−NiO electrode indicated the existence of Cd, Se, Te, and Co (Figure 4). As shown in Figure 4B, the peaks at approximately 411.7 and 404.9 eV corresponded to Cd 3d3/2 and Cd 3d5/2 respectively, which confirmed that Cd element existed mainly in the form of Cd2+ on the sample surface. In Figure 4C, the binding energy of Se 3d was at 54.2 eV, which indicated Se−Cd bonds. The Te signals showed two peaks of Te 3d5/2 and Te 3d3/2 at 576.2 and 586.6 eV, respectively, which corresponded to Te−Cd bonds (Figure 4D). Moreover, the peaks at 572.3 and 582.7 eV stood for Te−O bond owing to tellurium being easier to oxidize during the preparation process.47 The XPS spectrum of Co 2p at 781.1 and 796.4 eV confirmed the presence of Co on the surface of photocathode in the form of bivalent (Figure 4E).48

In the designed photocathode, a favorable match of the reduction potential between L-cys CdSeTe QDs and a H2producing catalyst is critical. Thus, cyclic voltammetry measurement was performed to estimate the conduction band (ECB) of the QDs from this reduction potential (Figure S4). It showed a reduction potential approximate to −0.7 V vs NHE and then calculated the value of the valence band (EVB) as 1.32 V vs NHE (see the Supporting Information),44 which was similar to the literature.49,50 The oxidation potential of L-cys was around 1.2 V vs NHE at pH 6.8,51,52 which is much more positive than the valence band of NiO (∼0.5 V vs NHE);14 thus, L-cys could help the hole transfer from the valence band of CdSeTe QDs to NiO and then migrate to the counter electrode through the external circuit.19 The conduction band of QDs was more negative than the reduction potentials of the CoII/CoI (−0.4 V vs NHE) process of Co(dmgBF2)(H2O)2,53 indicating that the electron 11169


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Figure 4. (A) Survey scan for XPS spectrum in the binding energy range of 0−1200 eV and high-resolution spectra of (B) Cd 3d, (C) Se 3d, (D) Te 3d, and (E) Co 2p for Co−CdSeTe−NiO electrode.

transfer from CdSeTe QDs to the cobalt catalyst is thermodynamically favorable. An energy illustration for a proposed working principle of the photocathode is shown in Figure 5. Based on the related studies, a possible mechanism for

hydrogen evolution on the Co−CdSeTe−NiO photocathode could be proposed as follows: CdSeTe QDs−NiO + hv → CdSeTe*QDs−NiO (excitation) CdSeTe*QDs−NiO → CdSeTe−QDs−NiO (h+) (hole injection) CdSeTe−QDs−NiO (h+) + CoII → CdSeTe QDs−NiO (h+) + Co I CoI + H+ → CoIII−H 2CoIII−H → H 2 + 2CoII (hemolytic reaction) CoIII−H + H+ → CoIII + H 2 (heterolytic reaction)

Upon illumination, CdSeTe QDs would harvest light and get to the excited state CdSeTe* QDs because the absorbed

Figure 5. Energy illustration for electron transfer and hole injection at the Co−CdSeTe−NiO photocathode. 11170


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Figure 6. (A) LSV curves of the NiO electrode, CdSeTe−NiO electrode, and Co−CdSeTe−NiO electrode under 300 W Xe lamp irradiation and dark in phosphate buffer solution (0.1 M, pH 6.8) saturated with Ar gas at a scan rate of 100 mV s−1. (B) The transient photocurrent responses to on−off illumination of the NiO electrode, the CdSeTe-NiO electrode, the Co−CdSeTe−NiO electrode, and Co−NiO electrode at an applied potential of −0.1 V vs NHE. (C) Curves of photocurrent density vs time for the Co−CdSeTe−NiO photocathode over 3 h at an applied potential of −0.3 V vs. NHE. (D) The amount of hydrogen determined by gas chromatography.

electrode, Co−CdSeTe−NiO electrode, and Co−NiO electrode under the same conditions are shown in Figure 6B. The on−off cycles of illumination were carried out at 200 s after the application of −0.1 V vs NHE, in order to stabilize the baseline and minimize the impact of background. Upon illumination, the bare NiO yielded a negligible amount of initial current. By contrast, the Co−CdSeTe−NiO electrode displayed a photocurrent density about −130 μA cm−2, while there was only −45 μA cm−2 for the CdSeTe−NiO electrode, confirming that with the aid of cocatalyst, the photocurrent was enhanced and the stable photocurrent demonstrated the stability of the cobalt catalyst at the electrode for the reduction of protons. A spike in the photoresponse of Co−CdSeTe−NiO was observed owing to the transient effect in power excitation.57 When the Xe lamp was turned off, the photocurrents of the CdSeTe−NiO electrode, Co−CdSeTe−NiO electrode, and Co−NiO electrode dropped rapidly to background currents. In addition, the Co−NiO photocathode assembled with cobalt catalyst but without CdSeTe QDs generated a small amount of current density, which demonstrated that cobalt catalyst cannot work effectively without CdSeTe QDs. The conclusion obtained from these comparative experiments was the essentials of CdSeTe QDs, cocatalysts, and light source to produce H2 in a PEC system. It is important that photocathode should be stable for the practical application of the PEC cell. To assess the stability of prepared photocathode, the experiment was carried out in 0.1 M phosphate buffer solution. The photocurrent density of −135 μA cm−2 at an applied potential of −0.3 V vs NHE20 remained relatively stable during 3 h illumination (Figure 6C). As shown in Figure S6, the FESEM image, corresponding mapping, and

photons triggered the excitation of electron from the valence band of CdSeTe QDs to the conduction band. Then the holes could inject from the excited state CdSeTe* QDs to the valence band of NiO. CdSeTe QDs might be subsequently regenerated by delivering electrons from the CdSeTe−QDs to the CoII catalyst to form a CoI intermediate. CoIII−H intermediate may be formed by the protonation of CoI species. Then, H2 might be produced by the further reaction of CoIII−H via hemolytic or heterolytic reaction.54−56 The holes at NiO were immediately filled by electrons or transferred to the counter electrode for water oxidation through the external circuit, which reduced the chance of electron−hole recombination.20 Then, control experiments were carried out to determine the necessity of the each component in the water-splitting system. To shed light on the best adsorption capacity of Co(dmgBF2)(H2O)2, the chopped-light chronoamperimetric measurements were carried out on Co−CdSeTe−NiO electrode dropped with different amounts of Co(dmgBF2)(H2O)2 acetonitrile solution. As indicated in Figure S5, Co−CdSeTe−NiO electrode with 100 nmol Co(dmgBF2)(H2O)2 yielded the maximum photocurrent response, and thus the optimum amount was 100 nmol. As shown in the LSV curves of the as prepared photocathodes (Figure 6A), the NiO electrode did not show obvious photocurrent under illumination; however, when the NiO electrode was sensitized by CdSeTe QDs and Co(dmgBF2)(H2O)2, the photocurrent densities of CdSeTe−NiO electrode and Co−CdSeTe−NiO electrode were about −45 and −130 μA cm−2, respectively, at −0.1 V vs NHE, which were much higher than their dark current densities. In addition, the transient photocurrent responses of the NiO electrode, CdSeTe−NiO 11171


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which was far less than the H2 detected by GC (2.90 μmol). Therefore, the H2 production was due to the reduction of protons in the solution instead of oxidation of the L-cys.

HRTEM image of Co−CdSeTe−NiO also demonstrated that the CdSeTe QDs and cobalt catalyst were attached on the surface of NiO electrode stably with the protection of 5% Nafion solution after 3 h electrocatalytic experiment. According to the ICP-OES, the amount of Co was 92.6 and 89.1 nmol before and after 3 h electrocatalytic experiment, respectively. The little loss of cobalt catalyst could be considered. Figure S7 also proved that the cobaloxime catalyst was still a molecular catalyst, and only a slight molecular catalyst may decompose to a material catalyst on the electrode. LSV tests were also carried out 10 times to test the stability. As shown in Figure S8, the LSV curves showed a little change that could be ignored. Thus, the results illustrated that the photocathode was stable. After passing 0.61 C cathodic charges through an external circuit, 2.9 μmol of hydrogen was detected by GC, corresponding to 93% Faradaic efficiency over 3 h illumination at an applied potential of −0.3 V vs NHE. For comparison, Table S2 summarizes the photoelectrocatalytic data of reported photocathodes using the similar electrolyte. It can be found that the Co−CdSeTe−NiO electrode displayed much better photoelectric properties. The enhanced PEC performance of the prepared Co−CdSeTe−NiO photocathode was explained as follows. First, compared with binary quantum dots, such as CdSe QDs (2.27 eV)20 and CdTe QDs (2.53 eV),58 the CdSeTe QDs had much narrower band gap (2.02 eV). In comparison with the molecular dyes, the CdSeTe QDs with modification of L-cys had higher stability and fluorescence intensity. Therefore, it prompted the greater absorption of solar light by QDs and the generation of more photoelectrons. Second, the mesoporous network NiO film increased the loading amount of QDs on the surface, and then the large specific surface area of QDs further enhanced the absorption of the effective cocatalyst, thus leading the excellent catalytic current for the reduction of hydrogen. Third, as the successfully assembly of the photocathode, the loaded elements at the photocathode contacted inseparably, and the photocathode remained extremely stable, which ensured the rapid transmission of electrons and holes. Based on these factors, Co− CdSeTe−NiO photoelectrode had the improvements on photocurrent and Faradaic efficiency. In addition, the photocurrent of Co−CdSeTe−NiO increased gradually with time in Figure 6B and 6C, and there were several reasons. First, it may because of the heating effect. Second, there may be a small amount of molecular catalyst decomposing to Co material catalyst on the electrode, which may lead to the current increase. Third, it could be that some cocatalysts were connected directly to the FTO glass substrate, and the electron exchange reaction happened between catalysts and electrode, resulting in an increase of dark current while the current difference remained unchanged. A reported study discussed that most of the photogenerated holes were trapped by the thiol groups and/or sulfide ions instead of the valence band of NiO.59 To shed light on the fact that the producing holes were injected into the NiO rather than oxidizing L-cys, some characterizations were carried out. Elemental Analyzer was used to determine the content of S, which was 9.42% (w/w); this means that there was about 0.0597 μmol L-cys on the electrode. Then, ICP-OES was used to determine the amount of NiO on the electrode, which was about 1.38 μmol. The NiO was much more abundant than L-cys on the electrode. Thus, the number of photogenerated holes trapped by the thiol groups and/or sulfide ions may be much less than trapped by NiO. Moreover, the amount of H2 produced by the oxidation of 0.0597 μmol L-cys was 0.0299 μmol theoretically,

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CONCLUSIONS In summary, the present study reported the fabrication of a robust and stable quantum dots sensitized photocathode, which simply immobilized CdSeTe QDs onto the mesoporous NiO film on a FTO glass substrate using L-cysteine and connected the typical cobaloxime catalyst to the surface of the QDs. This should be the first demonstration of sensitizing NiO with ternary quantum dots CdSeTe in a nonsacrificial electrolyte. The fabricated Co−CdSeTe−NiO cathode showed efficient hydrogen evolution activity, and a constant photocurrent density of as high as −130 μA cm−2 was reached at an applied potential of −0.1 V vs NHE in a 0.1 M phosphate buffer solution upon illumination. The photocurrent response of the photocathode kept steady over 3 h at an applied potential of −0.3 V vs NHE in the absence of sacrificial reagent, with a Faradic efficiency of almost 93%. Therefore, such PEC with ternary QDs has the superior properties, and it processed potential application in the field of photocatalytic water splitting.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00305.

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Chemical materials, experiment details, XRD patterns of the FTO glass substrate, the NiO film on the FTO glass substrate, FESEM images of FTO glass substrate and mesoporous NiO film on FTO glass substrate, fluorescence spectra of the synthesized CdSeTe QDs, CV of CdSeTe QDs, the transient photocurrent responses to the different amounts of Co(dmgBF2)2(H2O)2 acetonitrile solution, HRTEM image and FESEM image of the Co− CdSeTe−NiO phtocathode after 3 h electrocatalytic experiment, CV curves of Co(dmgBF2)2(H2O)2 and the modified Co(dmgBF2)2(H2O)2 dissolved from the Co− CdSeTe−NiO before and after the electrocatalytic experiment for 3 h, LSV curves of the Co−CdSeTe− NiO electrode, ICP-OES elemental analysis of CdSeTe QDs, and comparison of the performance between the photocathode in this work and the reported photocathodes (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yun Chen: 0000-0001-6267-8544 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support from the Natural Science Foundation for Young Scholars of Jiangsu Province, China (Grant No. BK20160983), the National Natural Science Foundation of China (Grant No. 21605084), and support by 11172


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Research Start-up Funds for Talent Scholars of Nanjing Tech University (No. 39837104).

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Cobalt Catalyst Grafted CdSeTe Quantum Dots on Porous NiO as

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