2-Modified CdS Mesoporous Nanoheterojunction Networks

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Visible-Light Photocatalytic H Production Activity of #-Ni(OH) Modified CdS Mesoporous Nano-Heterojunction Networks Ioannis Vamvasakis, Ioannis Papadas, Theocharis Tzanoudakis, Charalampos Drivas, Stelios A. Choulis, Stella Kennou, and Gerasimos S. Armatas

ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01830 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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Visible-Light Photocatalytic H2 Production Activity of β-Ni(OH)2 Modified CdS Mesoporous NanoHeterojunction Networks Ioannis Vamvasakis†, Ioannis T. Papadas†,‡, Theocharis Tzanoudakis†, Charalampos Drivas§, Stelios A. Choulis‡, Stella Kennou§, Gerasimos S. Armatas†,* †Department of Materials Science and Technology, University of Crete, Heraklion 71003, Greece ‡Department of Mechanical Engineering and Materials Science and Engineering, Cyprus University of Technology, Limassol 3041, Cyprus §Department of Chemical Engineering, University of Patras, Patra 26504, Greece

ABSTRACT: Photocatalytic water splitting for hydrogen production is an emerging and promising strategy for converting solar energy into chemical fuels. To that end, the development of robust and highly active semiconductor materials is of eminent importance in this field. Here, we demonstrate high-surface-area mesoporous networks comprising interconnected β-Ni(OH)2 modified CdS nanocrystals (NCs) as highly active and stable photocatalysts for hydrogen

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generation. Compared to single-component CdS assemblies, Ni-modified materials present a strong enhancement of photocatalytic performance for hydrogen evolution under visible light irradiation (λ ≥ 420 nm). By controlling the formation of β-Ni(OH)2 species, the mesoporous βNi(OH)2/CdS heterojunction networks at a 10 wt % Ni content reached an outstanding photocatalytic H2-evolution rate of 1.4 mmol h-1 at 20 oC (or ~35 mmol g-1 h-1 mass activity), associated with an apparent quantum yield (QY) of 72% at 420 nm in a 5 M NaOH aqueous solution containing 10% v/v ethanol as sacrificial reagent. Mechanistic study with UV–vis/NIR, PL and EIS spectroscopy and photocatalytic performance evaluation reveals that the improved photocatalytic performance arises from the strong electronic coupling and charge-transferred states at the p–n β-Ni(OH)2/CdS heterojunctions. These β-Ni(OH)2 modified CdS mesoporous assemblies have important implications for renewable hydrogen generation technologies.

KEYWORDS: CdS; nickel hydroxide; nanoparticles; mesoporous materials; photocatalysis; hydrogen production

1. INTRODUCTION Nowadays, photocatalytic hydrogen production through water splitting has drawn an immense attention as a potential method for safe and economical conversion of solar energy into chemical fuels. This strategy holds a great promise to address the environmental concerns caused by consumption of fossil fuels and to face the growing energy demands.1,2 In the last few years, although a substantial progress has been performed on preparing various semiconductor catalysts, key challenges of obtaining highly active, chemically stable and low-cost photocatalysts are still remained.3-8 A great many of these materials exhibit high recombination rate of excitons and wide band gap energy (e.g., TiO2, SrTiO3, ZnS, K4Nb6O17, Ta2O5, NaTaO3


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etc.) that make them effective only under UV light, which consists a 4–5% fraction of the solar spectrum. These are limitations that greatly restrict the implication of these materials for efficient solar hydrogen production.5 Recently, metal sulfides, especially CdS, have emerged as promising photochemical catalysts for use in photocatalytic H2 production from water. In particular, CdS has received a lot of attention for this purpose due to its prominent advantages, such as high electron mobility of about 350 cm2 V−1 s−1 9, excellent visible-light response (CdS has an energy band gap of about 2.4–2.5 eV)10 and suitable conduction band (CB) position well above the H2 evolution potential (–0.41 V vs normal hydrogen electrode (NHE), pH = 7).11,12 However, CdS catalysts bear a low efficiency of photon to hydrogen conversion and often suffer from photocorrosion due to insufficient electron-hole separation and anodic photoerosion.13,14 In this context, various attempts have been realized to improve the photo-conversion efficiency of CdS, among which the surface modification with noble metals, such as Au and Pt, has been recognized as the most effective way.15,16 Noble metals due to their excellent electron-accepting ability (for example, the Pt and Au work function (φ) is ~5.6 and ~5.1 eV, respectively)17 and high charge separation efficiency can synergistically improve the photoactivity of CdS, yielding high H2 production rates.18 In addition to heterojunction formation, designing semiconductor materials with a porous structure at the nanoscale is also an important strategy for enhancing mass and charge transfer kinetics of the catalyst. In particular, coupling the high activity of a nanostructured semiconductor with large porosity into the same structure is a compelling way to devise new catalysts with improved performance and reliability. This is because nanoporous materials are anticipated to be promising catalysts due to their large exposure of active sites and enhanced light absorption ability, which may results from multiple scattering of photons into the pores.


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Moreover, size reduction of the framework to nanometer dimensions provides electrons a short path to reach the surface, that is, the electron diffusion length is comparable to the particle size, and therefore, an enhanced charge-carrier separation can be achieved.19-21 Through nanostructuring, the photocatalytic hydrogen production activity of CdS has been remarkably improved. For instance, photon-to-hydrogen quantum yields (QYs) as high as ~60–70% at λ = 420

nm

have

been

reported

recently

for

various

Pt-loaded

nanostructured

CdS

semiconductors.22,23 Nonetheless, despite their high efficiency, the low availability and high cost of noble metals hamper the widespread use of these materials for practical applications. Hence, development of cost-effective and highly active photocatalysts for sustainable energy conversion is of utmost importance. Recent investigations have shown that inexpensive Ni-based materials, such as nickel oxides/hydroxides and sulfides, can function as efficient photo- and electrochemical catalysts for hydrogen evolution. To this end, various CdS-based heterostructures such as NiOx-loaded CdS particles24, 3D NiO-CdS heterostructures25, Ni-modified CdS microparticles26, nanorods27,28, nanosheets29 and nanoparticles30, NiS-modified CdS nanorods31,32, nanowires33, microparticles34 and CdS/RGO composites35, and Ni(OH)2-decorated CdS36 and CdS/g-C3N4 composites37 has been fabricated. Ni-containing CdS heterostructures owing to the high interfacial separation and transfer of photoexcited charges can achieve improved photocatalytic performance. As such, the photon-to-hydrogen production efficiency of these catalytic systems ranges between 2.8% and 94% under 400–450 nm irradiation, which is comparable to that of precious-metal-based CdS catalysts.24-26,31,32,34,36-38 Notwithstanding the remarkable progress, the underlying role of deposited nickel species in enhancing the photocatalytic performance for H2 evolution is still poorly understood. Moreover, construction of 3D hetero-nanostructures from Ni-decorated ultra-


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small CdS nanoparticles with an inherently large surface area and interconnected open pores has not been reported yet. We recently developed a new class of mesoporous assembly architectures of metal sulfide nanocrystals (NCs) through a polymer-assisted self-assembly method. These materials, unlike to bulk-like microstructures and individual nanoparticles, possess nanoscale pore structure which provides a large and accessible surface area between the nanoparticles.39 In this study, highly porous CdS assembled structures that depart from the oxidative coupling of colloidal CdS NCs were decorated with β-Ni(OH)2 nanoparticles by a straightforward photodeposition method and used as photocatalysts for hydrogen production under visible-light irradiation. X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and N2 porosimetry characterization results corroborate that the resulting materials consist of a porous network of linked β-Ni(OH)2 and CdS nanoparticles and exert a high internal surface area with narrow distribution of mesopores. The present photocatalytic system provides a unique opportunity to study the effect of β-Ni(OH)2 on the photocatalytic H2 production activity of these newly developed catalysts. By using a combination of UV–Vis/NIR optical absorption, electrochemical impedance spectroscopy and photoluminescence (PL) analyses, we provide insights into the energetic characteristics of charge transfer and separation processes in βNi(OH)2/CdS nano-heterojunctions. Accordingly, the Ni-modified CdS catalyst with an optimal weight ratio (10 wt %) of Ni exhibits much higher visible light photocatalytic activity with a 72% apparent QY at 420 nm for hydrogen evolution, while demonstrating remarkable stability, which is one of the highest activities among other reported noble metal-free CdS-based photocatalysts. This study can offer new opportunities for the development and in-depth


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understanding of noble metal-free photocatalysts for efficient solar to hydrogen energy conversion.

2. EXPERIMENTAL SECTION 2.1. Preparation of CdS NCs. CdS NCs with 4–5 diameter were prepared following a previously reported procedure with some modifications.40,41 In particular, 5.5 mmol of CdCl2 and Na2S·9H2O were added in separate vials containing 80 and 20 mL of deionized water (DI), respectively. Then, 3-mercaptopropionic acid (3-MPA, 11 mmol) was slowly added to the CdCl2 solution under vigorous stirring. After the solution pH was adjust to 10 with NH4OH, the Na2S solution was slowly added under continuous stirring at room temperature (RT). The 3-MPA– capped CdS NCs were isolated by precipitation and centrifugation (10,000 r.p.m., 15 min) with addition of 2-propanol and dried at 40 oC for about 24 h.

2.2. Synthesis of mesoporous CdS NC assemblies (NCAs). Porous networks of CdS NCs were prepared according to our previous reported method.39 Briefly, 2 mmol of 3-MPA–capped CdS nanoparticles were suspended in DI water (2.5 mL) to produce a stable colloidal solution. Then, 5 mL of Brij-58 (HO(CH2CH2O)20C16H33) block copolymer aqueous solution (10% w/v) was added, and the reaction mixture was kept under stirring for 1 h. Next, 3 mL of H2O2 solution (1 wt. %) was added dropwise and the resulting suspension was stirred for an additional 1 h (until gelation occurred). The gel product was then put into a 50 mL glass beaker and transferred to an oven for slow evaporation of the solvent at 40 °C under static conditions (typically within 3–4 days). Template removal was conducted by treating the dry product with ethanol (20 mL) for 2 h under mild stirring and low heating (40 °C) and then three times with DI water (20 mL) for


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15 min each. The template-free sample was then collected via vacuum filtration, washed thoroughly with DI water and ethanol, and dried at 40 °C for about 24 h.

2.3. Photochemical deposition of Ni on CdS NCAs. Photodeposition of Ni on the CdS surface was carried out in a NiCl2 solution at RT. In a typical reaction, mesoporous CdS NCAs (0.1 g) were well-dispersed by ultrasonication for 5 min in 20 mL of triethylamine (TEA) solution (12.5% v/v, which is corresponds to a ~25-fold excess compared to CdS). Next, the appropriate aliquots of 1 mg mL−1 NiCl2·6H2O aqueous solution (corresponding to the desired loadings of Ni) were added to the above solution, and the mixture was deaerated by bubbling with Ar gas for 30 min. After this, the reaction mixture was illuminated for 3 h under continuous stirring using a 365 nm light-emitting diode (50 W UV-LED) as the light source. The product was then separated by centrifugation, washed twice with DI water and ethanol, and kept at 40 °C for about 18 h. For comparative study, β-Ni(OH)2 microparticles were also prepared by a wet chemical precipitation route, in which sodium hydroxide (NaOH) solution (4 M, 50 mL) was slowly added in a NiCl2 solution (1 M, 50 mL) forming a light-green precipitate. The mixture was then left under continuous stirring at 60 oC for 24 h, and the final green β-Ni(OH)2 product was separated by vacuum filtration, washed with DI water and ethanol and then heated at 60 oC overnight. The crystallinity and phase purity of the sample were confirmed by powder X-ray diffraction (XRD). A physical mixture of mesoporous CdS NCAs and β-Ni(OH)2 was also produced for comparison by mixing two separate 50 mL aqueous dispersions (formed by ultrasonication for 15 min), containing proper amounts of pre-formed β-Ni(OH)2 particles and CdS NCAs


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mesoporous (corresponding to an equivalent 10 wt % Ni). The resulting suspension was stirred for about 24 h at RT and then separated by filtration and heated at 60 oC overnight.

2.4. Physical characterization. Elemental microprobe analysis was realized using a JEOL JSM-6390LV scanning electron microscope (SEM) equipped with an INCA PentaFETx3 energydispersive X-ray spectroscopy (EDS) detector (Oxford Instruments, UK). EDS spectra were collected for different locations of sample (at least four) using an accelerating voltage of 20 kV and a 60-s accumulation time. Powder X-ray diffraction (XRD) patterns were recorded on a PANalytical XˊPert Pro X-ray diffractometer operated at 45 kV and 40 mA using Cu Kα radiation (λ = 1.5418 Å). XPS measurements were performed on a Leybold EA-11 analyzer equipped with a Al Kα X-ray source (1486.6 eV) using a constant pass energy of 100 eV (the full width at half maximum (FWHM) of the Au 4f7/2 line was 1.3 eV). The samples were prepared by pressing the powder on a Pb sheet in order to be introduced in an ultra-high vacuum chamber. The analyzed area was approximately 2×5 mm2 and the XPS spectra was collected at a 0 degrees take-off angle. In all XP spectra, the C 1s line at 284.8 eV was used as a calibration peak. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100 electron microscope operated at 200 kV. Samples were prepared by drying an ethanolic dispersion of particles on a holey carbon-coated Cu grid. N2 adsorption-desorption isotherms were obtained at –196 oC using a Quantachrome NOVA 3200e analyzer. Before analysis, all the samples were outgassed at 80 oC for 12 h under vacuum ( 14 solutions, the H2 evolution reaction involves a fast diffusion of hydroxyl anions (OH–) to the CdS surface, where they are rapidly oxidized by the valence band (VB) holes into hydroxyl radicals (•OH). The produced •OH radicals can then oxidize ethanol to acetaldehyde (and/or acetic acid), importantly at a higher rate than direct oxidation of ethanol by the photogenerated holes. This means that the kinetically hindered oxidation reaction of ethanol is compensated with two faster (one-electron) oxidation reactions, suppressing the competitive electron–hole recombination process at the surface of the catalyst and accelerating the overall water reduction process.65 Figures 4a displays the time courses of photocatalytic H2 evolution over the mesoporous CdS and Ni-CdS NCAs catalysts with different Ni loadings. The Ni-free CdS exhibited a moderate rate of H2 evolution (0.08 mmol h-1) due to the rapid recombination of photogenerated electron-hole pairs. On the contrary, Ni-modified materials exhibited a significant improvement in the photocatalytic H2production activity, clearly indicating that deposition of β-Ni(OH)2 onto CdS has a strong effect on the photoelectrochemical performance of Ni-modified heterostructures. The results showed that the photoactivity for H2 evolution was greatly increased with the loading amount of Ni and the highest rate, that is 0.93 mmol h-1, was obtained at 10 wt %. This activity is about 12-times higher than that of single-component CdS NCAs (see Figure 4b). Further increment in the Ni


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loading (15 wt %), however, results to the decrease in photocatalytic H2 production (0.53 mmol h-1). As we shall discuss below, the lower activity of the high Ni-loaded sample may be attributed to the sluggish charge transfer kinetics and loss of photogenerated carriers by recombination at the interface between β-Ni(OH)2 nanoparticles and CdS host catalyst, possible due to the poor interparticle contact and interfacial defects. Next, a series of photocatalytic H2 evolution tests were performed in order to optimize the amount of the 10% Ni-CdS catalyst. As seen in Figure 4c, the evolution rate of hydrogen increases with increasing catalyst concentration and reaches a maximum at 2 mg mL-1. We interpret this increment of H2 production as the increase of light absorption by the catalyst’s particles. As for the higher catalyst loading (2.5 mg mL-1), the reduction of H2 evolution rate (1.1 mmol h-1) is related to the light scattering from excessive particles. Also, comparative experiments with methanol in a 5 M NaOH solution (pH 14.7) and ethanol in pH 10 and neutral solutions indicated that these hole scavengers are less effective for H2 production; the corresponding H2 evolution rates were ~0.7, ~0.2 and ~0.2 mmol h-1 (Figure S8, Supporting Information). The low activities are attributed to the slow rate of oxidation of methanol by the hydroxyl radicals (particularly, the rate constant of hydrogen abstraction by •OH radicals from methanol is 0.64×1012 cm3 mol-1 s-1 vs 2.25×1012 cm3 mol-1 s-1 from ethanol)66 and direct oxidation of ethanol by the photogenerated holes (see below). Notably, the 10% Ni-CdS catalyst at optimum conditions shows an impressive performance for water reduction, reaching a H2 production rate of 1.4 mmol h-1 (~35 mmol gcat-1 h-1) associated with an apparent QY of 72 % at 420 nm monochromatic light, assuming all incident photons (around 9.46 × 1016 s-1) were absorbed by the catalyst’s particles. To our knowledge, this is one of the highest so far recorded efficiencies for photocatalytic systems containing nickel as cocatalyst31,33,67 and among the best


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reported values for CdS-based photocatalysts22,23. Table S2 shows a comparison of the photocatalytic H2-production activities of different CdS-based photocatalysts. Further irradiation control experiments over the 10% Ni-CdS NCAs catalyst showed that the apparent QYs at 365, 440, 510 and 620 nm incident light wavelengths are 77%, 55%, 2% and 0.2%, respectively. Notably, as presented in Figure S9 in the Supporting Information, the overall trend in apparent QY correlates well with the absorbance of the catalyst, suggesting that the H2-production activity of Ni-CdS NCAs was initiated from the interband carrier photoexcitation in the host CdS photocatalyst. In agreement with this, as expected due to the large band gap for β-Ni(OH)2 (~3.9 eV as obtained by UV–vis/NIR spectroscopy, see Figure S7 in the Supporting Information), no hydrogen evolution was observed when β-Ni(OH)2 microparticles were employed as the catalyst (data not shown), indicating that β-Ni(OH)2 alone is catalytically inactive for visible light hydrogen evolution reaction. In addition, compared with 10% Ni-CdS NCAs, a much lower rate of H2 evolution (0.23 mmol h-1) was obtained when a physical mixture of CdS NCAs and βNi(OH)2 particles (with equivalent 10 wt % Ni loading) was employed as catalyst (labeled as 10% Ni/CdS) under identical conditions (Figure 4b). From the above results, we deduce that the close interconnection (as evidenced from HRTEM study) and, therefore, the strong electrical coupling between β-Ni(OH)2 and CdS nanomaterials plays a crucial role in the reaction mechanism, by facilitating the interfacial charge transfer and spatial separation of photoinduced electron-hole pairs across the heterojunction. Overall these attributes contribute to the improvement of the visible-light photocatalytic performance of the materials.


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Figure 4. (a) Time courses of photocatalytic H2 production and (b) rates of H2 evolution (during the first 3-h reaction) over mesoporous CdS and Ni-CdS NCAs catalysts. The H2 evolution rate over the physical mixture of β-Ni(OH)2 (10 wt % Ni) and CdS NCAs (10% Ni/CdS) is also given. (c) Photocatalytic H2-production activities for different concentrations of the 10% Ni-CdS catalyst. (d) Catalytic recycling tests of the 10% Ni-CdS NCAs. The bars denote the rate of H2 evolution averaged over 5 h reaction time. All catalytic tests were conducted with 20 mg of catalyst (or 40 mg for stability study) in a 5 M NaOH aqueous solution (20 mL, pH 14.7) containing ethanol (10% v/v); 300-W Xe light source with a UV cut-off filter (λ ≥ 420 nm).

The mesoporous 10% Ni-CdS NCAs also showed sufficient stability after prolonged photocatalysis. The reusability of the catalyst was investigated by conducting four consecutive 5h photocatalytic runs. Before reuse, the catalyst was separated from the reaction solution by


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centrifugation, washed several times with water, and re-dispersed in a fresh NaOH-ethanol solution. Before catalysis, the reaction cell was deaerated by Ar gas for at least 30 min, until no hydrogen and oxygen were detected (by GC analysis). As seen in Figure 4d, after four successive runs, the rate of photocatalytic H2 evolution still reached 1.2 mmol h-1, which corresponds to 86% of its initial activity, indicating very good stability. The total H2 amount evolved after a 20h visible light irradiation was 23.6 mmol (or ~566 mL, at 20 oC) that corresponds to an average H2 evolution rate of about 1.2 mmol h-1. In addition, EDS and N2 physisorption measurements were employed to assess any changes in composition and morphology of the recovered catalyst after 20-h reaction. The EDS spectra (Figure S10a and Table S1, Supporting Information) indicated a Cd:S ratio of 1:1 and a Ni content of ~10.6 wt %, while results of N2 adsorptiondesorption isotherms revealed a surface area of ~148 m2 g−1, pore volume of ~0.17 cm3 g−1 and pore size of ~6 nm (Figure S10b, Supporting Information), which are very close to those of the fresh material. The recovered sample was further studied using XPS. The XPS spectra of the Cd 3d and S 2p regions (Figure S4, Supporting Information) showed no obvious changes in the chemical states of Cd2+ and S2–, respectively, demonstrating the good chemical stability of the catalyst. However, after photocatalytic reaction, the Ni 2p spectrum (Figure 2) suggest a notable difference in the chemical attributes of the Ni species; it shows Ni 2p3/2 and 2p1/3 core-level peaks at 855.7 and 873.3 eV, respectively, while the peak at 850.5 eV has fully vanished, which indicates the complete transformation of Ni(0) to Ni(II). As we discuss below, a possible explanation for the photochemical conversion of metallic Ni is due to the fact that photoexcited holes transfer from CdS to Ni species, by which a portion of these holes oxidizes Ni metal to ―

Ni(II). The Ni(II) ions can then readily react with hydroxide ions (OHaq ) due to the surrounding alkaline environment (5 M NaOH) to yield β-Ni(OH)2. This observation is also consistent with


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previous studies, which indicated that Ni/NiOx core-shell nanoparticles supported on SrTiO3 can be converted to β-Ni(OH)2 under photochemical reduction of water. It was suggested that a disproportionation reaction of NiOOH species (obtained from photooxidation of β-Ni(OH)2) by metallic Ni, leads to formation of β-Ni(OH)2.68 Moreover, XPS elemental analysis on the reused sample also reveals an atomic ratio between Ni and CdS close to 0.43, very similar to that of fresh catalyst (ca. 0.42). These findings are also in line with results from EDS. Clearly, all the above results indicate that the prepared 10% Ni-CdS NCAs sample is a highly efficient and stable photocatalyst. It should be stressed that the initially formed Ni(0) species have a negligible effect on the photocatalytic activity of the Ni-CdS NCAs. To prove this assumption, we performed a fifth consecutive 5-h photocatalytic run using a fresh NaOH-ethanol aqueous solution. The results showed that the reused 10% Ni-CdS NCAs sample (retrieved after a 20-h photoreaction) achieves a high level of H2 production, showing an average rate of H2 evolution ~1.17 mmol h-1 (Figure S11, Supporting Information).

3.3. Effect of Ni species on the photocatalytic activity To investigate the charge transfer and separation efficiency of Ni-modified CdS heterojunctions, the photoluminescence (PL) was measured using 360 nm excitation wavelength, as shown in Figure 5a. The PL spectrum of mesoporous CdS NCAs exhibits an intense peak at ~470 nm, which is ascribed to near band edge transitions of the CdS nanocrystal framework. In contrast, deposition of Ni species onto the CdS surface led to a continuous quenching of the PL intensity, which is ascribed to the suppressed charge carrier recombination, arising from the efficient dissociation of photogenerated electron-hole pairs over the heterostructured framework. Here we assume that the contribution by non-radiative relaxation of excitons through defect


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states is negligible. The downward trend of the PL intensity for the investigated samples is 5% Ni-CdS > 7% Ni-CdS > 15% Ni-CdS > 10% Ni-CdS. The results indicate that 10 wt % of Ni is sufficient to provide higher charge separation efficiency across the β-Ni(OH)2/CdS interface, giving rise to a more favorable charge-transfer process. As for the lower charge separation observed in the 15% Ni-loaded sample, as depicted in Figure 5a, it could be related to surface defects created at the interface between β-Ni(OH)2 and CdS particles, which may function as recombination sites of photogenerated carriers. Of note, all the mesoporous CdS and Ni-CdS NCAs materials exhibited a lower PL intensity than that of the precursor CdS NCs, which is attributed to the efficient charge delocalization over the assembled structure. In addition, electrochemical impedance spectroscopy (EIS) measurements were also carried out to get a better understanding of the role of β-Ni(OH)2 in the reaction pathway and the enhanced photocatalytic activity of Ni-CdS NCAs. Figure 5b depicts the Nyquist plots of the fabricated electrodes with pure CdS and Ni-CdS (10 and 15 wt % Ni loadings) NCAs. All samples were drop-casted on glass substrates coated with fluorine-doped tin oxide (FTO) layer, and the EIS measurements were performed at an open-circuit potential in a frequency range from 1 to 1 × 106 Hz. The measured EIS data were modeled with an equivalent electrical circuit as illustrated in Figure 5b (see Supporting Information for details) using the EC-Lab software package (Bio Logic Science Instruments, version 11.16), and the results are listed in supplementary Table S3. The charge-transfer resistance (Rct) for 10% Ni-CdS NCAs was found to be 13.1 Ω, which is lower than that of 15% Ni-CdS (14.5 Ω) and pure CdS (15.3 Ω) NCAs materials, indicating a faster transport of photogenerated carriers. Also, the total polarization resistance (Rtot = Rd+Rct, where Rd refers to the defect resistance of the electrode) for each electrode follows the order: CdS (~33.3 Ω) > 15% Ni-CdS (~33.0 Ω) > 10% Ni-CdS (~28.6 Ω)


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NCAs. In general, a lower charge-transfer resistance is anticipated to enhance the photoactivity of the catalyst by inducing high electron-transfer conductivity through the framework and prolonging the lifetime of photogenerated charges. Obviously, the catalyst loaded with 10 wt % Ni (primarily as β-Ni(OH)2) provides the most efficient dissociation and transfer of photoinduced excitons, which correlates well with its outstanding photocatalytic H2 evolution activity. This is also consistent with the lower recombination rate of band-edge excitons in 10% Ni-CdS NCAs as revealed by PL spectra (Figure 5a). To attain a clearer picture of the electronic band structure and charge transfer mechanism of the Ni-CdS NCAs, EIS Mott-Schottky experiments were also performed at an AC frequency of 1 kHz in 0.5 M Na2SO4 electrolyte (pH=7). The Mott-Schottky curves and the corresponding fits of the linear portion of the inverse square space-charge capacitance (1/Csc2) as a function of potential (E) for pure CdS and Ni-modified CdS (5, 10 and 15 wt % Ni loadings) NCAs samples are shown in Figure 5c. It is apparent that the mesoporous CdS NCAs shows a positive linear slope, indicating n-type conductivity, that is, electrons are the majority carriers. For Ni-CdS NCAs heterostructures, however, a bell-shaped behavior in the 1/Csc2–E plots was observed. This clearly indicates the formation of p–n junction at the β-Ni(OH)2/CdS interface, which could further affect the separation and recombination dynamics of the photoinduced carriers. The corresponding flat-band (EFB) potentials estimated from the intercept of the extrapolated straight lines with the potential axis (1/Csc2 = 0) are listed in Table 2. The results indicated that, with increasing Ni content from 0 to 15 wt %, the EFB potential of CdS gradually shifts to a cathodic (negative) direction, i.e., from –0.71 V to –0.86 V vs NHE. Moreover, deposition of β-Ni(OH)2 increase slightly the donor density (ND) in CdS from 3.0×1017 to 4.0×1017 cm-3, as inferred from the positive slopes of the linear region of 1/Csc2–E curves (see Table 2). The observed up-shift in


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the flat-band positions for Ni-CdS NCAs can be understood as a result of the CdS surface states passivation by the β-Ni(OH)2 particles that suppress the Fermi-level pinning. In fact, surface states can induce a potential drop within the Helmholtz layer (VH) but this could be reduced by surface passivation. 69 This change in VH can in turn affect the EFB according to EFB = VH + φSC – 4.5, where φSC is the work function of the semiconductor and -4.5 eV is the H+/H2 redox level with respect to vacuum, leading to a shift toward more negative potentials. To elucidate this possibility, we further measured the ζ potential of selected catalysts in a 0.5 M Na2SO4 solution at pH 7 condition (similarly to the EIS measurements). The ζ potentials of 10% Ni-CdS, 15% NiCdS and CdS NCAs catalysts were –15.6, –15.4 and –13.6 mV, respectively. Clearly, particles in these suspensions were all negatively charged. Moreover, the higher negative surface charge of β-Ni(OH)2 modified particles is in line with the electrical double-layer repression70, and thus the up-shift of the EFB level. As for the slight increase of ND of the Ni-CdS NCAs samples, this could be attributed to the lower recombination rate of the photoinduced electron-hole pairs as a result of the hole transport from CdS to β-Ni(OH)2 (see below). Based on the EFB values and optical band gaps (as obtained from UV–vis/NIR absorption data), the energy band diagrams for each catalyst are illustrated in Figure 5d. Here we assumed that the EFB potential serves as an approximation of the CB edge position, which is quite reasonable for heavily doped n-type semiconductors, such as CdS.71 Typically, for many n-type semiconductors the CB edge is about 0.1–0.3 V higher than the EFB potential.72,73


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Figure 5. (a) Room-temperature PL emission spectra of the mesoporous CdS and Ni-CdS NCAs and precursor (thiol-capped) CdS NCs. PL experiments were carried out in water (1 mg mL-1) with 360 nm excitation wavelength. (b) EIS Nyquist plots (Inset: equivalent circuit model used to simulate the impedance measurements as a function of frequency) for mesoporous CdS and Ni-CdS NCAs with 10 and 15 wt % Ni loadings. The red lines are fits to the experimental data. (c) Mott-Schottky plots of the 1/CSC2 as a function of voltage (E) relative to the redox potential of Ag/AgCl (3 M KCl) for the mesoporous CdS and Ni-loaded CdS NCAs materials. (d) Energy band diagrams of the mesoporous CdS and Ni-modified CdS NCAs catalysts.

Table 2. Electrochemical data obtained from Mott-Schottky measurements (pH 7) for pristine CdS and Ni-modified CdS NCAs catalysts.


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EFB

EVB†

Slope‡

Donor density§

(V vs NHE)

(V vs NHE)

(F-2 cm4 V-1)

(Nd, cm-3)

CdS NCAs

-0.71

2.00

5.192×109

3.0×1017

5% Ni-CdS

-0.74

1.98

4.066×109

3.9×1017

10% Ni-CdS

-0.84

1.89

4.932×109

3.2×1017

15% Ni-CdS

-0.86

1.86

3.955×109

4.0×1017

Catalyst

†The VB maximum potential of the semiconductors was estimated from EFB – Eg. ‡Slope = 1/[(E–EFB)·Csc2], where E is the applied potential, Csc is the space charge capacitance and EFB is the flat-band potential. §Donor density (ND) given by ND = 2(E–EFB)·Csc2/εεoe, where ε is the relative dielectric constant of CdS (8.9), εo is the dielectric permittivity (8.8542×10-10 F cm-1) and e is the elementary charge (1.602×10-19 C).

Scheme 1. Schematic representation of the energy diagram of the β-Ni(OH)2/CdS heterojunction (at pH 7) and proposed mechanism of visible-light photocatalytic H2 production by the Ni-CdS NCAs catalysts.

Therefore, we deduce that the intrinsic electric field created at the p–n β-Ni(OH)2/CdS junctions, as corroborated by the aforementioned EIS results, dictates the charge transportation processes through the negative shift of EFB and suitable band-edge alignment of the nanoscale β-


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Ni(OH)2/CdS contacts. Ni hydroxide is a p-type semiconductor with a Fermi level at ~5.4–5.6 eV vs vacuum74,75, which is more positive than the Fermi level of CdS (ca. 4.5 eV vs vacuum)76. This means that when β-Ni(OH)2 particles are anchored onto the CdS surface, an internal electric field is formed at the interface of β-Ni(OH)2/CdS junction that promotes the transfer of electrons from the CB of CdS to β-Ni(OH)2 until the Fermi levels reach equilibrium. At this condition, the electron flow creates a depletion region, i.e. a positively charged area in the CdS side and a negatively charged area in the β-Ni(OH)2 side, leading to the upward band bending of CdS CB (i.e., it’s potential energy rises) near the β-Ni(OH)2/CdS junction. For β-Ni(OH)2 modified CdS heterostructures, the potential barrier height (φB), defined by the energy difference of the Fermi level of β-Ni(OH)2 and the CB edge (electron affinity) of CdS, was found at ~0.9–1.1 eV. All these effects definitely impact on the performance of the Ni-CdS NCAs catalysts. Specifically, as illustrated in Scheme 1, upon visible light illumination the CdS get excited and generates electron-hole pairs; obviously β-Ni(OH)2 itself does not absorb light at this wavelength range (λ ≥ 420 nm) owing to the large band gap (~3.9 eV). Because of the intimate contact and favourable band structure between the p–n β-Ni(OH)2/CdS junctions (particularly, CdS holds more positive VB and CB edges compared to β-Ni(OH)2), the holes generated in VB of CdS can migrate to the β-Ni(OH)2 nanoparticles, leading to an accumulation of electrons in the CdS host matrix where the photocatalytic water reduction occurs. The observed increase in ND with increasing Ni content in the Ni-CdS NCAs catalysts is also consistent with the above interpretation. It is also worth pointing out that the CdS NC assemblies due to their high internal surface area can provide plenty of reaction centers capable of reducing water to hydrogen. In alkaline media, it was established that the hydrogen evolution reaction proceeds through the initial water-discharge step (Volmer reaction: H2O + e– + M → M–Hads + OH–) on a positive-charged metal site (M),


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followed by either the electrochemical Heyrovsky desorption step (H2O + e– + M–Hads → M + H2 + OH–) or the Tafel recombination step (2M–Hads → 2M + H2).77,78 In the meantime, the photoinduced holes gathered in β-Ni(OH)2 can be effectively consumed by the sacrificial reagent (OH–). From Mott-Schottky analysis, the EFB potential of β-Ni(OH)2 microparticles was found at 1.31 V vs NHE at pH 7, while a p-type behavior was also observed (see Figure S12 in the Supporting Information). However, it is a common feature of p-type NiOx films that the VB is about 0.4 eV lower than the Fermi-level.79,80 Therefore, the VB of β-Ni(OH)2 is expected to be approximately 1.7 V and 1.4–1.5 V vs NHE before contact and after equilibrium with CdS, ―

respectively, which is comparable to the electrode potential of surface adsorbed •OH/OHaq pairs (1.36–1.49 V vs NHE, pH = 7)81. Herein, according to the Anderson’s affinity rule, the valenceband offset (∆Ev = (Eg1+χ1)–(Eg2+χ2), where Eg1 and Eg2 are the band gap energies and χ1 (3.8 eV) and χ2 (2.3–2.4 eV) are the electron affinities of the CdS and β-Ni(OH)2 semiconductors, respectively) is considered to be ~0.2–0.3 eV.82 However, given that the band edge positions of ―

the semiconductors (such as CdS) shift with pH by ca. –33mV/pH13,27 and that the •OH/OHaq couple potential follows the Nernstian behavior (–59mV/pH), at pH 14 the VB of β-Ni(OH)2 is ―

estimated to be 1.2–1.3 V vs NHE, which is below the redox potential of •OH/OHaq pairs (1.0– ―

1.1 V). On this basis, in pH > 14 solutions the electrochemical oxidation of OHaq species by the surface-reaching holes is thermodynamically feasible. As a proof of concept, fluorescence (FL) spectroscopy of the photocatalytic H2 production over mesoporous CdS and 10% Ni-CdS NCAs catalysts in the presence of terephthalic acid (TA) at pH 14 showed an emission peak at about 440 nm that gradually increased during the reaction, suggesting the formation of hydroxyl radicals (see Figure S13a in the Supporting Information); TA readily reacts with hydroxyl radicals to produce fluorescence 2-hydroxy-terephthalic acid (HTA). Notably, the production


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rate of •OH radicals from the 10% Ni-CdS catalyst, which is defined by the time evolution of the FL intensity of HTA (Figure S13b, Supporting Information), is almost 3-times higher than that for CdS NCAs at similar conditions. Thus, it is reasonable to assume that the observed higher ―

OHaq oxidation kinetics for the Ni-modified catalyst can be explained as a result of the efficient separation of photoexcited electron-hole pairs at the β-Ni(OH)2/CdS junction. The separation and transport of photoexcited carriers have been manifested clearly in the above PL spectra and EIS results. For comparison, FL spectroscopy analysis of the 10% Ni-CdS NCAs/OH–/TA photocatalytic system at pH 10 was also conducted. As seen in Figure S13a in the Supporting Information, the FL spectrum does not show any emission peak at 440 nm after 3 h of reaction, indicating that no hydroxyl radicals are formed under these conditions. This means that at pH 10, the 10% Ni-CdS NCAs catalyst is not able to oxidize OH– to •OH radicals, yielding a slower oxidation rate. The obtained FL results are in line with the low H2 generation rate (~0.2 mmol h1

) that was observed for 10% Ni-CdS NCAs catalyst dispersed in a NaOH-ethanol (10% v/v)

solution with pH 10 (Figure S8, Supporting Information); herein, the direct oxidation of ethanol is considered to be the rate-determining step. Based on above findings, we deduce that βNi(OH)2 substantially serves as the oxidation active sites (hole collector) for the oxidation of OH– to •OH radicals, promoting electron-hole separation, and greatly improving the photocatalytic efficiency. It is noted that, during the course of irradiation, the transferred holes from the VB of CdS to β-Ni(OH)2 could also cause oxidation of remaining Ni(0) clusters to Ni(II), yielding β-Ni(OH)2 species, which has been suggested by the previous XPS measurements. The β-Ni(OH)2 particles, moreover, are prone to further oxidation by photoexcited holes, leading to the formation of nickel oxyhydroxide (NiOOH) species.68 However, after photocatalytic reaction for 20 h, no notable XPS peak corresponding to the


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NiOOH particles was detected in this catalyst. In the literature, the Ni 2p3/2 line of NiOOH has been reported to be in the range of 855.8–856.4 eV.83-85 This further implies that Ni(OH)2 behaves as efficient hole-transfer mediator by shuttling photogenerated holes between the catalyst and redox species (•OH/OH–).

4. CONCLUSIONS We have shown that mesoporous networks of β-Ni(OH)2-modified CdS NCs with high internal surface area can reduce water into H2 with high photocatalytic activity and long-term stability. By optimizing the loading of β-Ni(OH)2, the Ni-modified CdS assemblies provide a high charge carrier transfer and separation along the heterojunction framework, in which β-Ni(OH)2 and CdS nanoparticles are in intimate contact, which result in a very high hydrogen production rate in alkaline solution. Consequently, the Ni-CdS NCAs catalyst at 10 wt % Ni loading gain a H2 evolution rate of 1.4 mmol h-1 under λ ≥ 420 nm light irradiation with an apparent QY of 72% at λ = 420 nm, while demonstrating very good stability for at least 25 h in alkaline ethanol solution (5 M NaOH, 10% v/v ethanol). We note that this catalyst outperformed previous reported nonprecious metal CdS heterostructures. Using a combination of UV–vis/NIR, electrochemical impedance and photoluminescence spectroscopy techniques, we showed that the improved H2 evolution efficiency of this system stems from the strong electronic coupling between n-type CdS and p-type β-Ni(OH)2 nanoparticles, which promotes efficient separation and transport of photoinduced carriers, as well as the pertinent mesoporous structure, which provides more exposed active sites for reaction and facilitates fast transport of electrolytes between the nanoparticles. This work not only presents the possibility of using mesoporous networks of nanoscale p–n β-Ni(OH)2/CdS junctions as a viable hydrogen evolution photocatalyst, but also


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can offer new opportunities for the development and in-depth understanding of noble metal-free photocatalysts for efficient solar to chemical energy conversion.

ASSOCIATED CONTENT Supporting Information. Nickel characteristics of 10% Ni/CdS NCAs. Photocatalytic H2-production activity data of various Ni and Pt-loaded CdS-based catalysts. EIS-fitting results of Ni-CdS NCAs. EDS spectra and Tauc plots of mesoporous CdS and Ni-modified CdS NCAs, XRD pattern of mesoporous 20% Ni-CdS NCAs, XPS spectra of 10% Ni-CdS NCAs, N2 adsorption-desorption isotherms of mesoporous Ni-CdS NCAs (5%, 7% and 15% Ni loadings), UV–vis/NIR spectra and EIS data of the β-Ni(OH)2 microparticles and 10% Ni-CdS NCAs catalyst (with apparent QYs of H2 evolution under different incident lights), EDS spectrum and N2 adsorption-desorption isotherms for the reused 10% Ni-CdS NCAs catalyst, and FL spectra of 2-hydroxyterephthalic acid (HTA) evolution over the mesoporous CdS and 10% Ni-CdS NCAs (PDF). The following files are available free of charge.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This research was financially supported by the European Union and Greek national funds under the ERC grant schemes (ERC-09), Special Account for Research Funds of University of Crete (KA 4121), and the Greek State Scholarship Foundation – IKY Scholarships Programme (Human Resource Development, Education and Lifelong Learning Operational Program) through the action “Reinforcement of Postdoctoral Researchers”. REFERENCES (1) Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future Challenges. J. Phys. Chem. Lett. 2010, 1, 2655–2661. (2) Ahmad, H.; Kamarudin, S. K.; Minggu, L. J.; Kassim, M. Hydrogen from Photo-Catalytic Water Splitting Process: A Review. Renew. Sust. Energ. Rev. 2015, 43, 599–610. (3) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-Based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503-6570. (4) Kudo, A. Photocatalyst Materials for Water Splitting. Catal. Surv. Asia 2003, 7, 31-38. (5) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253–278. (6) Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X. Engineering Heterogeneous Semiconductors for Solar Water Splitting. J. Mater. Chem. A 2015, 3, 2485–2534. (7) Shi, Y.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: Synthesis and Applications in Hydrogen Evolution Reaction. Chem. Soc. Rev. 2016, 45, 1529–1541.


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2-Modified CdS Mesoporous Nanoheterojunction Networks

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