High‐Performance Mesoporous Catalysts of Ultrasmall Hexagonal Thiospinel Nanocrystals for Visible‐Light Hydrogen Evolution

Semiconductor nanocrystals are at the frontier of energy conversion research owing to their tunable optoelectronic attributes and versatile surface activities. Here high‐surface‐area mesoporous frameworks comprising linked CdIn2S4 nanocrystals as efficient catalysts are presented for visible‐light‐driven hydrogen production. X‐ray total scattering analysis discloses hexagonally‐structured CdIn2S4 thiospinel nanoparticles forming the porous structure. Further analytic results indicate that these newly‐made ensembles possess an open‐up architecture that is highly conductive and susceptible to modification. Through appropriate selection of the synthesis conditions, it is demonstrated that the present synthetic protocol is general, allowing the preparation of porous materials from thiospinel nanoparticles with various sizes and compositions. This study shows that coupling of CdIn2S4 mesostructures and Ni2P nanosheets substantially expedites the kinetics of water photo‐splitting by effectively reinforcing the separation of photogenerated carriers at the interfaces. Thus, mesoporous Ni2P/CdIn2S4 heterojunctions instigate a remarkable improvement in the hydrogen generation rate (≈29.3 mmol h−1 gcat−1), presenting an apparent quantum yield of 61.7% at 420 nm monochromatic light. A combination of electrochemical and spectroscopic studies unveils a pertinent mechanistic link between the charge‐transfer dynamics and intrinsic photochemical activity in these nanostructures.


Introduction
Semiconductor-mediated water photo-splitting is a mature energy conversion technology for clean and sustainable carbonneutral fuel generation that will certainly gain more attention in the coming years because of its simplicity, low operational cost, DOI: 10.1002/admi.202300994   and large-scale implementation potential. [1,2]To this end, significant efforts are underway to develop more reliable and performing catalysts (BiVO 4 , Ta 2 N 5 , ZnS, MoS 2 , etc.) to address challenging photochemical processes such as high efficiency and stability, and non-toxicity. [3,4]Among them, metal sulfides remain the most promising catalysts for hydrogen evolution due to their excellent light-absorption (they have a narrower band structure than the oxide counterparts), prominent redox activity, and high carrier mobility (they have a smaller effective mass of carriers than does metal oxides). [5]8][9] Unfortunately, despite promising improvements, the research on metal sulfide photocatalysts still faces certain pitfalls, like the low density of surface-active sites, high recombination ratio of excited electron-hole pairs, and anodic photocorrosion of the S 2− lattice sites. [10]Therefore, the fabrication, by design, of functional photocatalysts with plentiful surface active sites and improved charge dissociation and transportation is in demand for efficient and robust water electrolysis.
Thiospinel compounds (described by the formula A II B III 2 S 4 , where A and B are nominally divalent and trivalent metals) are recently emerging as promising alternatives for use in water-splitting photoelectrochemical cells, water electrolyzers, rechargeable metal-air batteries, and thermoelectric devices. [11,12,13]Specifically, their high visible-light absorption coefficient (10 4 −10 5 cm −1 ), multiple redox activity, low cost, and remarkable photostability especially in harsh alkaline electrolytes offer great perspectives for photo-redox reaction applications. [14,15]Moreover, thiospinels are non-toxic and lowcost materials.[18][19] Nevertheless, the photocatalytic performance of these materials is often plagued by the sluggish separation kinetics of the photogenerated electronhole pairs (excitons) and the ensuing low utilization of the surface-reaching electrons. [20]In this regard, the rational synthesis of thiospinel materials with nanoscale morphology and rich density of surface-active sites have been at the forefront of efforts to enhance catalytic efficiency.][23] However, most of the reported thiospinels are polycrystalline in nature with nano-to micrometer-sized grains, such as flower-like (Zn/Cd)In 2 S 4 microspheres, ZnIn 2 S 4 nanotubes and nanoplates, and CdIn 2 S 4 nanosheets, with limited nanoporosity. [20]erein, we report for the first time a low-temperature colloidal synthetic route to isolate fairly monodispersed CdIn 2 S 4 nanocrystals (NCs) of hexagonal spinel phase.To our knowledge, this is the foremost example of Cd-based thioindiate materials possessing hexagonal crystal structure.The hexagonal structure of CdIn 2 S 4 , although theoretically predicted, [24] has not yet been experimentally demonstrated, predominantly because existing synthetic routes to CdIn 2 S 4 relied on high-temperature processing that favor the formation of the cubic phase. [12]Moreover, we demonstrate the synthesis of 3D mesoporous networks from the nanoscale CdIn 2 S 4 crystallites through a polymer-templated assembly strategy.These newly-made mesoporous architectures offer prominent advantages for light-induced catalysis owing to the enhanced interfacial charge-transfer kinetics, highly accessible surface area, and fast mass transport within the assembled nanoparticles.More importantly, this method is general and has allowed us to prepare mesoporous architectures with different size and composition of the thiospinel nanoparticles.We also show that functionalization with Ni 2 P nanosheets has an eminent implication in the photocatalytic performance of CdIn 2 S 4 mesostructures, leading to a photon-to-hydrogen conversion yield of 61.7% at 420 nm, far surpassing the activity of other multicomponent thiospinel-based catalysts.[30][31] Through comprehensive electrochemical and spectroscopic studies, we provide a reliable mechanistic understanding of the charge transfer and photocatalytic pathways in these hetero-nanostructures.

Synthesis, Phase Structure and Morphology
The synthesis of fairly monodisperse CdIn 2 S 4 nanocrystals (designated as CIS NCs) was achieved via a one-pot chemical reaction of Cd 2+ and In 3+ nitrates (1:2 molar ratio) with thioacetamide in ethylene glycol solution.To exert kinetic control on the formation of CIS NCs, we used mercatopropionic acid as the surface capping agent, which confines the crystal growth and stabilizes the colloidal nanoparticles.This is an elegant procedure that provides exquisite control over the grain size of resulting NCs.Namely, by adjusting the reaction time, we succeeded in preparing CIS NCs with tunable particle size; small-angle X-ray scattering (SAXS) taken on NCs collected after 6 and 12 h of aging time gave a mean Guinier diameter of ≈6 and ≈11 nm, respectively (Figure S1, Supporting Information).We have chosen to synthesize these two nanoparticle variations as representative nanomaterials and investigate their ensuing optical, electronic, and photocatalytic properties.
Afterward, the formed NCs were used as building-block units to assemble 3D polymeric networks through a surfactantdirected coupling chemistry.[34] The mesoporous frameworks of linked CIS NCs (designated as CIS NCFs) were then obtained after removal of the template inside the pores by extraction in warm ethanol (≈40 °C).Thermogravimetric analysis (TGA) of the mesoporous products reveals only an inevitable ≈9−10 weight % (wt.%) organic residue remaining in the porous structure (Figure S2, Supporting Information).Figure 1a illustrates an overview of the synthetic process for the mesoporous CIS NCFs.
X-ray diffraction (XRD) analysis on as-synthesized NCs and mesoporous CIS NCFs showed broad reflections at 2 scattering angles of 20−60°, indicating crystallites of pretty small size (Figure 1b).Because of the broadening of the XRD peaks, however, drawing reasonable conclusions of the crystal phase of CIS is challenging.To figure out the local atomic structure of the CIS NCs, we used X-ray total scattering and pair distribution function (PDF) analysis.This method is sensitive to the distribution of interatomic distances and can provide information on the atomic configuration in well-defined structures, such as the CIS nanoparticles. [35]Figure 1c shows the PDF plot as a function of interatomic distances for the as-prepared 6-nm CIS NCs along with that of bulk polycrystalline CIS.While keeping chemical integrity, the unit cell dimensions, regular bond lengths, and atomic displacement parameters of suitable atomic structural models were refined until the calculated PDF plot is consistent with the experimental data.As expected, the modeled PDF of bulk polycrystalline CIS appears consistent with the facecentered cubic (fcc) structure (space group: Fd3m) with a refined cubic unit cell parameter of a = 10.805(8)Å.In particular, the local structure of bulk CIS is defined by intense interatomic vectors at ≈2.6, ≈3.8, and ≈4.5 Å that correspond to Cd/In─S bonds, and In … In and Cd … In next-nearest-neighbor distances of cubic CIS, respectively.By comparison, CIS NCs show a very different PDF profile to bulk CIS, signifying a different atomic configuration; indeed, the fcc model refinement does not correctly describe all positions and relative intensities of the PDF peaks (Figure S3, Supporting Information).We, therefore, refined the PDF of CIS NCs using a hexagonal close-packed (hcp) structure that is another polymorphic form of thiospinel materials.This structural model was built from known crystallographic data of hexagonal ZnIn 2 S 4 , [36] in which Zn atoms were replaced by Cd atoms.Indeed, the PDF of hexagonal (hcp) ZnIn 2 S 4 is very similar to that of CIS NCs, although it exhibits a shift in correlations to smaller interatomic distances as a result of the smaller atomic radius of Zn versus Cd, see Figure S4 (Supporting Information).Interestingly, last-square PDF curve-fitting analysis using the hcp model yielded excellent results, fully describing the structural information disclosed in the PDF of 6-nm CIS NCs; in this plot, the PDF correlation at ≈2.Our analysis of resolving the CIS atomic structure was further supported by comparing the theoretical I(q) versus scattering wave vector (q = 2⋅sin/, where 2 is the scattering angle) patterns, derived from the refined structural models, with the experimental XRD data (collected with Mo-K radiation) of 6nm CIS NCs, see Figure S6 (Supporting Information).It proved that all the reflections can be interpreted as the hexagonal (space group: P63mc) CIS phase, which again validates the credibility of the above analysis.In addition, the presence of fourfold and sixfold coordinated In−S polyhedra in CIS NCs, being characteristic of hexagonal indium-based thiospinels, was further elucidated by Raman spectroscopy (Figure S7, Supporting Information).[39] On the contrary, the Raman spectrum of as-synthesized CIS NCs differs significantly, displaying intense shifts at ≈219 cm −1 and in the 300-340 cm −1 range that are tentatively assigned as vibration modes of the fourfold coordinated [Cd/InS 4 ] tetrahedra (T d ) and sixfold coordinated [InS 6 ] octahedra (O h ), respectively, similar to the spectrum of the hexagonal structured ZnIn 2 S 4 . [40]hese spectroscopic differences can serve as a fingerprint of the presence of In mixed with the T d and O h symmetries in CIS NCs, as demonstrated by the above PDF refinements.Taken together, our comprehensive structural analyses evidently corroborate the prevalence of the hexagonal spinel structure in CIS nanoparticles.Further, it appears that the CIS NCs undergo a transformation to the cubic spinel phase when heated beyond 200 °C.This is clearly seen when examining the X-ray total scattering data of the 6-nm CIS NCs annealed at various temperatures (Figure 1d).The simulation outcome indicates a gradual net reduction in the c/a ratio from 5.10 to 5.05 and 4.99 with increasing the heattreatment temperature (from 25 to 150 and 200 °C, respectively), signifying a progressive configuration transformation of metalsulfur polyhedral; see Figure S8 (Supporting Information).By comparison, the 250 °C procced NCs show a PDF plot that closely resembles that of the polycrystalline sample, implying a similar atomic structure; the PDF was fitted well with the fcc model with a refined lattice parameter of a = 10.849(8)Å.Furthermore, as expected, with increasing the heating process, the relative PDF peaks intensity at large interatomic distances increased, indicating a subtle increase in coherent scattering domain size.The results from different PDF refinements are tabulated in Table S1 (Supporting Information).Further supporting evidence that CIS NCs do undergo a phase transition at elevated temperature was provided by differential scanning calorimetry (DSC) and XRD studies (Figure S9, Supporting Information).The DSC profile of the as-synthesized 6-nm CIS NCs shows a broad endothermic peak ≈200-250 °C that is attributed to the transformation from the hexagonal to the cubic crystal phase, in agreement with prior studies on hexagonal Mn, Fe and Co-based thioindiates. [41]The XRD pattern of the 250 °C processed CIS NCs shows well-defined diffraction peaks that index to the cubic phase of CdIn 2 S 4 (JCPDS card no.27-0060).
In order to improve the photocatalytic efficiency of CIS, we functionalized the CIS mesostructure (made of 6 nm-sized NCs) with Ni 2 P nanosheets using a wet-chemical deposition method.Combining Ni 2 P and CIS semiconductors within the same structure can generate multiple electron transfer pathways across the Ni 2 P/CIS interface, which are expected to reinforce the separation and migration of charges to the surface owing to the band bending and built-in electric field established at the heterointerfaces.Energy dispersive X-ray spectroscopy (EDS) of the Ni 2 P/CIS samples indicated Ni 2 P contents very close to the nominal compositions, that is, 5, 10, 15 and 20 wt.%, within a ≈8% deviation; see Table S2 (Supporting Information).Moreover, the Cd/In/S atomic ratios in all the samples (as determined by EDS) were ≈1:2:4, indicating stoichiometric CdIn 2 S 4 (Figure S10, Supporting Information).The phase structure and morphology of as-prepared Ni 2 P/CIS NCFs were further characterized with XRD, X-ray photoelectron spectroscopy (XPS), fieldemission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM).The XRD patterns, besides the board reflections of CIS host structure, display a weak but wellresolved peak at 2 ≈40.7°, predominately in the higher Ni 2 Pmodified samples, which coincides with the (111) reflection of hexagonal Ni 2 P (space group: P " 62m, JCPDS card no.74-1385) (Figure S11, Supporting Information).Typical XPS spectra obtained from the mesoporous CIS NCFs show strong signals at 405.0, 444.5 and 161.4 eV concerning Cd 3d 5/2 , In 3d 5/2 and S 2p core-levels, consistent with the Cd 2+ , In 3+ and S 2− composition of CdIn 2 S 4 , respectively (Figure S12, Supporting Information). [42,43]ikewise, typical XPS spectra obtained from Ni 2 P-modified sample with 15 wt.%Ni 2 P content (15-Ni 2 P/CIS) present similar chemical states of Cd 3d 5/2 , In 3d 5/2 and S 2p at 405.1, 444.7 and 161.6 eV, respectively (Figure S12, Supporting Information).Meanwhile, the low-valence state Ni + and P − (0 <  < 2) into Ni 2 P was corroborated by the Ni 2p 3/2 and P 2p core-level signals at 853.6 eV and 130.3 eV, respectively, although some Niphosphate-like (Ni-PO x ) impurities arising from inevitable surface oxidation were inferred by the Ni 2p 3/2 and P 2p subpeaks at 857.9 eV and 133.5 eV, respectively. [44]Compared to the parent CIS NCFs, the positive shift of the Cd 3d 5/2 , In 3d 5/2 and S 2p lines in 15-Ni 2 P/CIS NCFs implies strong electronic interactions be-tween CIS and Ni 2 P components, which likely facilitate electron injection from CIS to Ni 2 P after contacting.Such an electronic coupling results in an upward band bending of ≈0.2 eV in the CIS side, as inferred by the XPS binding-energy shifts.According to XPS quantitative analysis, both CIS and 15-Ni 2 P/CIS NCFs showed consistent Cd/In/S atomic ratios very close to the stoichiometry of CdIn 2 S 4 , while the Ni 2 P loading on the 15-Ni 2 P/CIS surface was ≈14.7 wt.%, in agreement with the EDS results (see Supporting Information for further details).
Direct observation with FE-SEM shows that both CIS and 15-Ni 2 P/CIS NCFs consist of fairly monodispersed nanoparticles, within the range of 5−10 nm, that are linked to each other to form a continuous 3D network (Figure 2a; Figure S13, Supporting Information).Compared to bulk CIS, the grain size of the mesoporous CIS is considerably smaller; bulk CIS is composed of microparticles with diameters of 8 to 10 μm, as indicated by FE-SEM (Figure S14, Supporting Information).The XRD and XPS evidence of the presence of Ni 2 P is in tight agreement with SEM-EDS mapping of the Cd, In, S, Ni and P elements, which enables the direct visualization of small Ni 2 P flakes discerned on the surface of CIS (Figure S15, Supporting Information).The nanoflake morphology of the Ni 2 P co-catalyst is further elaborated with FE-SEM images; see Figure S16 (Supporting Information).In Figure 2b, representative TEM images of 15-Ni 2 P/CIS NCFs show tightly interconnected NCs that construct the pore walls, which can profit the interparticle electron conductivity.The grain size of constituent NCs is approximately 5−7 nm, which is in reasonable agreement with the diameter of the precursor nanoparticles (ca.6 nm, as determined by SAXS).These results thus unveil minimal coalescence of the NC building blocks during the synthesis.Moreover, TEM observation shows uniform pores (bright areas) of ≈4-5 nm in size perforating the assembled structure (Figure 2b, inset), which is propitious for the mass transfer process.The well-defined crystalline structure of the constituent NCs was also verified by the resolved lattice fringes in the high-resolution TEM (HRTEM) image shown in Figure 2c.The lattice plane distance of these crystallites is ≈3.3 Å, which, combined with PDF results, could be ascribed to the (011) crystallographic plane of hexagonal CIS (space group: P6 3 mc).Further evidence of Ni 2 P nanolayers with a 15−20 nm lateral size sitting on the CIS surface is depicted in the HRTEM image in Figure 2d.The interplanar distances of 2.0 and 5.0 Å are assigned to the (021) and (010) lattice planes of hexagonal Ni 2 P (space group: P " 62m), respectively.Taken together, these results affirm the successful decoration of the CIS mesostructure with Ni 2 P nanosheets, resulting in the formation of a robust Ni 2 P-modified CIS NC-linked network.
The internal porosity of the prepared materials was demonstrated through nitrogen physisorption measurements.The CIS NCFs exhibit typical type-IV adsorption-desorption isotherms associated with an H 2 -type hysteresis loop, characteristic of mesoporous solids with interconnected pores (Figure 2e).In addition, the weak but discernible steep adsorption in the relative pressure (P/P o ) range ≈0.5-0.6 suggests N 2 adsorption and condensation in narrow-sized mesopores. [45]The Brunauer-Emmett-Teller (BET) surface areas and total pore volumes of these materials ranged from 107 to 135 m 2 g −1 and from 0.10 to 0.13 cm 3 g −1 , respectively, depending on the nanoparticles' size, manifesting a substantial increase in porosity compared to the bulk analog (25 m 2 g −1 and 0.03 cm 3 g −1 ).This is the first example of mesoporous CdIn 2 S 4 with high internal surface area and narrow-sized pores.Comparatively, the Ni 2 P/CIS heterostructures exhibit lower surface areas (25−90 m 2 g −1 ) and pore volumes (0.02-0.07 cm 3 g −1 ) due to the deposition of Ni 2 P (Figure 2e; Figure S17, Supporting Information).Non-local density functional theory (NLDFT) model fit of the adsorption isotherms gives quite narrow pore-size distributions with a mesopore size at ≈5.6−5.8 nm for CIS NCFs and ≈3.8−4 nm for Ni 2 P/CIS NCFs.The narrowed pore size in the Ni 2 P-modified samples points to the incorporation of some Ni 2 P nanoflakes in the pores of CIS host material.To evaluate the outcome of the polymer-templating synthesis, a reference material, CIS RNAs (RNAs: random NCaggregates), was synthesized following a template-free oxidative coupling of 6-nm CIS NCs.Compared to polymer-templated CIS, CIS RNAs present a type-I adsorption isotherm related to a microporous structure (BET surface area ≈63 m 2 g −1 ) with small interstitial voids (ca.1.5 nm) (Figure 2e).These results are entirely consistent with the formation of randomly agglomerated NCs comprising small micropores between the adjacent nanoparticles.
UV-vis diffuse reflectance spectroscopy demonstrated a welldefined electronic structure for CIS nanostructures that differs from that for bulk polycrystalline CIS (Figure 2f).Compared to bulk CIS (2.35 eV), 6-nm-sized CIS NCs show a strong blue-shift in the energy gap transition (2.56 eV), consistent with the considerable dimensional reduction of the CIS NCs (ca.5-7 nm, as judged by SAXS and TEM results) that gives rise to quantization of the electronic structure.By analogy, the bandgap shift to higher energy (2.51 eV) for larger (11 nm) NCs is a direct consequence of the size-induced quantum confinement transitions.A similar trend was also seen in the mesoporous materials prepared using NCs of different diameter.We obtained energy gaps of 2.58 and 2.49 eV for CIS NCFs made of 6 and 11-nm NCs, respectively, reflecting that quantum confined effects of the initial NCs are preserved in the resulting mesostructures.The energy gap shift (by ≈20 meV) from the as-prepared NCs going to the mesoporous structures could be attributed to the efficient delocalization of excitons over the assembled framework; although a change in the dielectric environment of the NCs (by the removal of thiolate ligands) is also a reasonable possibility.Functionalization of the CIS mesostructure with Ni 2 P has an immediate effect on the electronic properties, leading to a gradual red-shift of the bandgap size for Ni 2 P/CIS from 2.55 to 2.52 eV, depending on the framework composition (Figure S18, Supporting Information).The systematic variation with Ni 2 P content ascertains strong electronic contact and efficient charge transfer between the Ni 2 P and surface-related electronic states of host CIS, which allows for new electronic band-edge transitions to be observed.All the optical and textural parameters of the prepared materials are given in Table S3 (Supporting Information).
Importantly, our synthetic scheme in Figure 1a is general and can be applied for the synthesis of other mesoporous materials from thiospinel NCs, including ternary II-III-VI semiconductor nanoparticles such as ZnIn 2 S 4 .To explore this prospect, we synthesized porous ZnIn 2 S 4 -based frameworks (ZIS NCFs) by using colloidal ZIS NCs as precursor materials.According to the EDS, XRD, TEM, and N 2 physisorption measurements, the ZIS NCFs possess a 3D mesoporous structure consisting of 5-nmsized hexagonal ZnIn 2 S 4 crystals and exhibit BET surface area of 195 m 2 g −1 , pore volume of 0.17 cm 3 g −1 and narrow pore-size distribution with a maximum at 6.3 nm (Figures S19-S21, Supporting Information).Similar to CIS NCFs, the optical absorption in ZIS mesoporous is an intrinsic property of the assembled framework and depends on its NC constituents (Figure S22, Supporting Information).

Photocatalytic Hydrogen Evolution
The photocatalytic activity of the title materials was evaluated in the visible-light irradiated water splitting reaction, using triethanolamine (TEOA) as the hole scavenger.As shown in Figure 3a, CIS NCFs (composed of 6-nm NCs) demonstrate an enormous improvement in photocatalytic hydrogen production, achieving a ≈142-fold rate enhancement (≈71 μmol h −1 ) over that for bulk polycrystalline CIS (≈0.5 μmol h −1 ).Remarkably, the hydrogen generation efficiency of CIS NCFs also greatly exceeds that of CIS RNAs sample (≈45 μmol h −1 ), i.e., that depart from the template-free oxidative coupling of colloidal 6-nm CIS NCs.Such findings thus confirm that the small grain size of constituent nanoparticles and porous morphology are favorable for improving photocatalytic H 2 -generation activity by providing short diffusion pathway of charge carriers and large catalyst/electrolyte interface area.In line with this, the lower photoactivity observed for the mesoporous CIS NCFs made of larger (11 nm) NCs (≈29 μmol h −1 ) is possibly related to the deficient charge transfer to the surface (see electrochemical results below).Functionalization of the CIS mesostructure with Ni 2 P nanosheets imparts an additional amplifying effect on photocatalytic performance.Reference experiments showed that the hydrogen production remarkably increases with the Ni 2 P content up to 15 wt.% (Figure 3a).Specifically, the hydrogen evolution rate over 15-Ni 2 P/CIS NCFs reaches up to ≈586 μmol h −1 (≈29.3 mmol h −1 g cat −1 mass activity), which is ≈1172 and ≈8 times the rate of bulk and mesoporous CIS, respectively.Mesoporous 15-Ni 2 P/CIS NCFs also outperformed the CIS microparticles loaded with 15 wt.%Ni 2 P (designated as 15-Ni 2 P/CIS-b) by a factor of 69-fold.As we shall discuss below, the mesoporous Ni 2 P/CIS NCFs concurrently prompt rapid charge-transfer kinetics and photocatalytic reactions by virtue of the efficient delocalization of excitons along the composite structure.Further optimization studies showed that the H 2 -production yield is closely related to the catalyst mass and hole scavenger type; we obtained an optimum activity with a 1 mg mL −1 catalyst concentration and TEOA (10% v/v) as sacrificial agent (Figures S23  and S24, Supporting Information).Consequently, our Ni 2 P/CIS NCFs system attains a splendid performance for H 2 production through visible-light-driven water reduction that overcomes the limitations of conventional photocatalysts.Using monochromatic light sources, we measured apparent quantum yields (AQYs) up to 81.5% at 375 nm (±10 nm) and 61.7% at 420 nm (±10 nm) over 15-Ni 2 P/CIS NCFs, assuming 100% absorption of the incident photons.To our knowledge, this photocatalytic efficiency is one of the highest reported thus far for spinel chalcogenide catalysts, and vastly higher than the previously reported CdIn 2 S 4 -based photocatalysts; see Table S4 (Supporting Information).The stability of the optimized 15-Ni 2 P/CIS NCFs catalyst was assessed through repeated photocatalytic experiments.After each cycling test, the catalyst was isolated from the reaction solution through centrifugation, washed several times with water, and redispersed in a fresh TEOA solution.As seen in Figure 3b, no detectable deterioration of photoactivity was observed after four cycling 5 h catalytic tests.Overall, 15-Ni 2 P/CIS NCFs produced a total H 2 amount of 11.7 mmol (≈280 mL, at 20 °C) after 20 h of irradiation, which corresponds to an average hydrogen evolution rate of ≈0.58 mmol h −1 .Post-characterizations of the cycled catalyst by EDS, XPS, and N 2 physisorption proved a highly persistent elemental composition and porous structure (Figures S25 and S26, Supporting Information), demonstrating the integrity of the assembled structure throughout catalysis.

Interfacial Charge Transfer and Mechanism of Photocatalytic Hydrogen Evolution
To understand the charge-transfer dynamics in the prepared materials, we performed electrochemical measurements in 0.5 m Na 2 SO 4 electrolyte.Figure 4a shows the inverse square spacechare capacitance (1/C sc 2 ) as a function of electrochemical potential (E) plots for different catalysts drop-casted as thin-films on fluorine-doped tin oxide (FTO, 10 Ω sq −1 ) glass substrates, from where the flat band (E FB ) potentials were assessed through the tangent lines.All the electrochemical potentials are referred to the reversible hydrogen electrode (RHE) at pH 7. Apparently, all the 1/C sc 2 -E curves display positive slope, suggesting n-type con-ductivity.The E FB potentials together with the respective optical band gaps (as determined from UV-vis spectra) can give estimation of the valence band (VB) energy levels (E VB ); see Table 1.Accordingly, Figure 4b illustrates the corresponding band-edge positions for each catalyst.In this interpretation, E FB is considered as a reasonable proxy for the CB edge, which is quite feasible for heavily n-doped semiconductors; typically, E FB locates 0.1-0.3eV below the CB edge. [46]The Mott-Schottky results indicate that the E FB of mesoporous CIS (made of 6 nm-sized NCs) undergoes an energetic elevation by 130 mV relative to the polycrystalline CIS, that is, from −0.77 to −0.90 V.A similar effect was also observed for mesoporous ensembles made of larger (11 nm) CIS NCs (E FB ≈−0.81 V).This anodic shift in E FB is consistent with the observed widening of the energy gap and size-dependent optical absorption of CIS NCFs (Figure 2f), reflecting quantized bandedge electronic states arising from the substantial size reduction of constituent CIS nanoparticles.After Ni 2 P modification, the E FB downshifts from −0.83 to −0.60 V as the Ni 2 P concentration varies from 5 to 20 wt.%.This progressive down-shift of the E FB position can be justified due to the electron capture capability of Ni 2 P and the band edge alignment in Ni 2 P/CIS heterojunctions.
In fact, the rearranged energy levels between the Ni 2 P and CIS semiconductors after contact can induce a potential drop across the Ni 2 P/CIS interface, resulting from the electron flow from CIS  1).Therefore, the combined results from the Mott-Schottky analyses strongly suggest that the band-edge alignment at the Ni 2 P/CIS junctions dictates the charge transport processes and thus the redox activity of the catalysts.Such charge transfer pathways greatly increase the separation efficiency of photo-excited electron-hole pairs and the migration rate to the active sites.
With the aim of investigating the charge-transfer kinetics induced by the Ni 2 P functionalization, electrochemical impedance (EIS) and time-resolved photoluminescence (TR-PL) spectroscopy experiments were conducted as well.The Nyquist data, fitted using a Randles equivalent circuit model, reveal that the mesoporous framework composed of 6-nm CIS NCs has markedly lower charge-transfer resistance (R ct ≈79.3 KΩ) compared to the ensemble structure of larger (11 nm) NCs (≈110.6 kΩ) and bulk analog (≈187.2kΩ), signifying faster interfacial charge-transfer kinetics (Figure 4c).Consistent with their high photoreactivity, Ni 2 P-modified samples manifest even higher charge-transfer efficiencies, as reflected by their lower R ct values; 15-Ni 2 P/CIS has the lowest R ct (0.7 kΩ), followed by 20-Ni 2 P/CIS (2.0 kΩ), 10-Ni 2 P/CIS (4.9 kΩ) and 5-Ni 2 P/CIS (12.3 kΩ) NCFs; see Table S5 (Supporting Information).Notably, the downward trend of R ct for Ni 2 P/CIS heterostructures is correlated well with the data from the hydrogen production experiments (Figure 3a).Of note, the bulk reference 15-Ni 2 P/CIS-b catalyst, even though shows a similar band structure deformation with the nanostructured analogs (it shows a E FB at −0.6 V and N D of ≈2.28 × 10 17 cm −3 , see Figure 4a; Table S1, Supporting Information), demonstrates a depressed charge transport ability (R ct ≈ 69.2 kΩ) across the catalyst/liquid interface (Figure 4c).These results thus explicitly indicate that the formation of Ni 2 P/CIS hetero-nanojunctions enable an efficient interfacial charge-carrier transfer and separation rate, which contribute to the enhanced photocatalytic activity.The nature of Ni 2 P/CIS interface playing a crucial role in photophysical processes such as charge dissociation and transfer is further supported by TR-PL studies.Figure 4d shows TR-PL emission decay curves recorded for different CIS materials under 375 nm pump laser excitation.To properly fit the PL decay and obtain the carrier lifetimes, we utilized a biexponential function , where  i are the amplitude fractions (Σ i  i = 1) and  i are the lifetimes for the surface ( 1 , fast) and core region ( 2 , slow) relaxations of excitons.15-Ni 2 P/CIS NCFs exhibits a ≈1.4 times longer average lifetime (3.57ns) than that of the parent mesoporous CIS NCFs (2.55 ns) and bulk polycrystalline CIS (2.53 ns), implying a more efficient charge separation and transfer.Moreover, comparison of the fast and slow decay time constants revealed predominant recombination of localized (band-edge) excitons within the lattice of polycrystalline CIS (we observed similar lifetime contributions).On the contrary, a higher contribution of surface-related transitions (fast decay process) was inferred for the mesoporous CIS and 15-Ni 2 P/CIS NCFs (Table S6, Supporting Information), justifying lower bulk carrier-recombination losses and a more efficient electron transfer to the catalyst surface.Consequently, the TR-PL and electrochemical results entail extensive electronic connectivity within the assembled structures, leading to more efficient utilization of photoexcited electrons for proton reduction.The fast scavenging of the surface-reaching electrons by protons in Ni 2 P-modified sample is further elaborated by the currentpotential (J−V) curves provided in Figure S29 (Supporting Information).The J-V curves show a lower onset potential (by 100-200 mV) and a steeper current response for the Ni 2 P/CIS NCFs compared to the mesoporous and bulk CIS, illustrating their promoted charge-transfer kinetics and lower energy barrier for H 2 evolution.For instance, the 15% Ni 2 P/CIS NCFs catalyst yields a current density of 4.85 mA cm −2 at −1.5 V (Ag/AgCl) bias potential, while the parent mesoporous CIS NCFs and bulk CIS exhibit current densities of 1.28 and 0.51 mA cm −2 at the same potential, respectively.Taken together, the EIS, TR-PL, and J-V analyses unambiguously reveal that the improved charge transfer dynamics and superior electrochemical performance of Ni 2 P/CIS NCFs stem from the low dimensionality of CIS nanoparticles, the unique mesoscopic architecture and the synergistic effect of Ni 2 P with the host CIS matrix.
In Figure 4e, we propose a band bending model and a possible reaction mechanism for the mesoporous Ni 2 P/CIS NCFs.Specifically, the adjustment of Fermi levels and subsequent electron flow generate a band-bending at the Ni 2 P/CIS interface, in which both CB and VB edges of CIS shift upward, while the band-edges of Ni 2 P synchronously descend.In the case of the 15-Ni 2 P/CIS NCFs, a CB offset of ≈0.5 eV is established between Ni 2 P and CIS semiconductors (as determined by the electron affinity difference between the two semiconductors, see Figure 4b) that results in a notched CB grading near the heterointerface, as depicted in Figure 4e.The CB discontinuities at the interface form a potential barrier that impedes the electron transfer from CIS to the Ni 2 P side, even though the Ni 2 P CB is more favorable for electron injection.Thus, the photoexcited electrons from the CIS remain in the CB of CIS, whereas free electrons from Ni 2 P tend to accumulate at the Ni 2 P interface (accumulation region).These localized interfacial states largely act as electron traps in the vicinity of the heterointerface (forming a 2D electron gas), facilitating the proton reduction reaction.Simultaneously, the built-in electric field established at the Ni 2 P/CIS contact can thermodynamically facilitate the transfer of the CIS VB holes to Ni 2 P where they oxidize TEOA.Assuming a VB edge of 0.65 V for individual Ni 2 P (see Table 1) and a valence-band bending of ≈0.2-0.3 eV, the VB maximum of Ni 2 P is expected to be at ≈0.4-0.5 V versus RHE after equilibrium with CIS, which is far above the E VB level of CIS (ca.1.86 V vs RHE).The net effect of this process is expected to cause a migration and spatial separation of excitons at the heterointerfaces.By implication, we obtain a built-in potential (V bi ) at the Ni 2 P/CIS interface, which is defined as the energy difference between the Fermi levels of Ni 2 P nanosheets (ca.−0.4 V vs RHE) and CIS NCFs (ca.−0.9 V vs RHE), of ≈0.5 eV that is sufficient enough to drive the charge pair separation.The separation and migration pathway of photoexcited carriers seen in the above EIS and TR-PL spectroscopic analyses is consistent with this mechanism.In addition, the same results could also be confirmed by photocurrent measurements.15-Ni 2 P/CIS NCFs generate a higher cathodic photocurrent density (ca.−0.16 mA cm −2 ) than mesoporous CIS NCFs (ca.−0.09 mA cm −2 ) and individual Ni 2 P nanosheets (ca.−0.07 mA cm −2 ) under visible light irradiation, implying a more efficient migration of photogenerated holes to the FTO electrode (Figure S30, Supporting Information).Besides this, it appears that the photocatalytic reactions proceeded via band-edge transitions.This conclusion is supported by the matching trend of the wavelength-dependent AQYs of H 2 evolution with the optical absorbance of 15-Ni 2 P/CIS NCFs (Figure 4f).

Conclusion
In summary, we demonstrated a low-temperature synthetic protocol, pursued by a bottom-up chemical process to assemble high-surface-area 3D porous networks of unique hexagonallystructured CIS NCs.Remarkably, this synthetic method has allowed us to construct mesoporous architectures with different composition and particle size of thiospinel nanoparticles.The coexistence of mesoscale porosity (up to 135 m 2 g −1 ) with small grain composition of CIS (ca.6 and 11 nm in size) into the same structure leads to significant advantages in photocatalytic effi-ciency, stemming from the efficient delocalization of charge carriers and enhanced interparticle mass transport.Functionalization with Ni 2 P nanosheets confers an additional enhancement in intrinsic photoactivity of the CIS mesostructures as a result of the charge-transfer pathways that drive charge transport to opposite sides of the Ni 2 P/CIS contacts.As thus, the mesoporous Ni 2 P/CIS NCFs with 15 wt.%Ni 2 P content instigate a steady and efficient water-splitting reaction, presenting a 29.3 mmol h −1 g cat −1 H 2 -evolution rate under visible light irradiation with AQYs of 81.5% and 61.7% at 375 and 420 nm, respectively, far higher than the other thiospinel-based catalysts.Our findings add to the growing potential of thiospinel-based photocatalysts and are expected to provide new insights for the design and in-depth understanding of next-generation catalysts for clean energy conversion and environmental remediation.) and the resulting mixture was heated to 150 °C.Then, thioacetamide (CH 3 CSNH 2 ) (10 mmol, 750 mg) dissolved in 10 mL ethylene glycol was rapidly injected into the above mixture, forming a yellow colloidal suspension.The mixture was stirred under reflux conditions for a certain time.A 6 h reaction time was applied for the synthesis of 6 nm-sized CdIn 2 S 4 and 5 nm-sized ZnIn 2 S 4 nanocrystals, and a 12 h reaction time was applied for 11 nm-sized CdIn 2 S 4 nanocrystals.As the reaction proceeded, the solution color changed from transparent to yellow (or bright yellow for ZnIn 2 S 4 ).After cooling to room temperature, the nanocrystals were retrieved by centrifugation with addition of isopropyl alcohol, washed with copious amounts of water/ethanol (1:1 v/v) mixture, and dried at 40 °C for 24 h.For reference, bulk polycrystalline CdIn 2 S 4 and ZnIn 2 S 4 samples were also prepared by hydrothermal reaction of Cd(NO 3 ) Synthesis of Mesoporous CIS (and ZIS) NCFs: In a typical procedure, MPA-capped CdIn 2 S 4 (or ZnIn 2 S 4 ) nanocrystals (250 mg) were suspended in deionized (DI) water (2.5 mL) under vigorous stirring.To obtain a stable colloid solution a few drops of 10 m NH 4 OH were added to the above solution.The colloidal suspension was then transferred into an aqueous solution of Pluronic Brij-58 polymer (10% w/v, 2.5 mL) and kept under stirring for 1 h at room temperature.Next, diluted H 2 O 2 (1.2 mL, 3% v/v) was added dropwise until gelation was observed (within ≈30 min), and the obtained gel suspension was left to slowly evaporate the solvent at 40 °C under static conditions for 4-5 days.To remove the polymer template, the hybrid product was washed twice with warm ethanol (≈40 °C) for 2 h and three times with DI water for 1 h at room temperature.The product was filtered off and washed several times with ethanol and then dried at 60 °C for 12 h.For comparison, random aggregates of CdIn 2 S 4 nanocrystals were also synthesized using a similar procedure, but without addition of the template.

Experimental Section
Synthesis of Ni 2 P Nanosheets: The Ni 2 P nanosheets synthesis was performed according to a modified literature procedure. [47]Briefly, NiCl 2 .6H 2 O (240 mg, 1 mmol) and ethylenediamine (C 2 H 8 N 2 , 2 mL) were dissolved in DI water (20 mL), forming a deep violet solution.This solution was then transferred in a 50 mL Teflon-lined autoclave and red phosphorus (150 mg, 5 mmol) was added.The autoclave was sealed and heated in an oven at 150 °C for 12 h.The obtained black product was retrieved by centrifugation, washed several times with DI water, and dried at 60 °C for 12 h.Preparation of Mesoporous Ni 2 P/CIS NCFs: Porous heterojunctions of Ni 2 P nanosheets and CdIn 2 S 4 mesostructures were prepared using a wetchemical deposition method.In a typical synthesis of the 15 wt.%Ni 2 Pmodified sample (15-Ni 2 P/CIS), mesoporous CdIn 2 S 4 (75 mg) was dispersed in a DI water/isopropyl alcohol (2:1 v/v) solution (60 mL) at room temperature.In a separate flask, Ni 2 P (15 mg) was suspended in a DI water/isopropyl alcohol (2:1 v/v) mixture (30 mL) under ultrasonication for 1 h.This solution was added dropwise to the CdIn 2 S 4 suspension and the resulting mixture was left under stirring for another 1 h at room temperature.The final product was collected by filtration, washed with anhydrous ethanol, and dried at 60 °C for 12 h.The same procedure was followed for the preparation of the samples with 5, 10, and 20 wt.% Ni 2 P content.For comparison, a bulk reference Ni 2 P/CdIn 2 S 4 material was prepared by depositing 15 wt.% of Ni 2 P nanosheets to the CdIn 2 S 4 microparticles.
Physical Characterization: X-ray diffraction (XRD) patterns were acquired on a PANanalytical X ' pert Pro MPD X-ray diffractometer in Bragg-Brentano geometry (45 kV and 40 mA), using Cu K radiation.Small-angle X-ray scattering (SAXS) patterns were collected on a Xeuss 3.0 (Xenocs, France) system equipped with a 2D detector and a Cu ( = 1.5405Å) rotating anode.Measurements were performed by transmission in samples that were ground and held in a kapton capillary.Scattering data were corrected for empty tube scattering.Filed-emission electron microscopy (FE-SEM) was performed on a JEOL JSM-IT700HR microscopy.Transmission electron microscopy (TEM) images were obtained with a JEOL JEM-2100 electron microscope (LaB 6 filament) operating at 200 kV.The samples were prepared by suspending fine powders in ethanol using sonication and then drop-casting on a carbon-coated Formvar Cu grid.Quantitative microprobe analysis was conducted on a JEOL JSM-6390 LV scanning electron microscope (SEM) equipped with an Oxford INCA PnetaFETx3 energy dispersive spectroscopy (EDS) detector (Oxfordshire, UK).Data acquisition was performed on at least ten different regions of every sample, using 20 kV acceleration voltage and 60 s accumulation time.X-ray Photoelectron Spectroscopy (XPS) was performed on a SPECS spectrometer that was equipped with a Phoibos 100 1D-DLD electron analyzer and a monochromated Al K radiation (1486.6 eV) as an energy source.A lowenergy electron flood gun was employed for charge neutralization.Prior to XPS measurements, the samples were pressed to form a pellet.All binding energies were calibrated using the C1s peak of adventitious carbon (284.8 eV) as the reference signal.The relative atomic composition was determined from the acquired spectra after background subtraction by integrating the Cd 3d 5/2 , In 3d 5/2 , S 2p and Ni 2p 3/2 peaks and dividing by its sensitivity factor.Peak fitting of XPS spectra was performed using the Spec-sLab Prodigy software.UV-vis/near IR diffusion reflectance spectra were taken using a Shimazu UV-2600 spectrophotometer, with BaSO 4 powder as a 100% reflectance reference.Diffuse reflectance data were converted to absorbance (/S), according to the Kubelka-Munk function: /S = (1-R) 2 /(2R), where R is the reflectance and , S is the absorption and scattering coefficients, respectively.Thermogravimetric (TGA) measurements were performed in a Discovery TGA5500 analyzer (TA instruments).The thermal analysis was conducted in a nitrogen atmosphere (under a flow rate of ≈200 mL min −1 ) using a two-step method where the first step the sample was maintained at 100 °C to remove the moisture of the sample and the second step was conducted from 100 to 550 °C with an increase heating rate of 10 °C min −1 .Differential Scanning Calorimetry (DSC) were conducted in a Discovery DSC250 calorimeter (TA instruments).Raman spectra were recorded on a Thermo Scientific DXR3xi Raman imaging microscope, using a 532 nm laser.Nitrogen physisorption measurements were performed at −196 °C with the use of a Quantachrome NOVA 3200e volumetric analyzer.All the samples were degassed at 100 °C for 12 h under vacuum (<10 −5 Torr) prior to measurement.The specific surface areas were calculated using the Brunauer-Emmet-Teller (BET) [48] method in the relative pressure (P/P 0 ) range of 0.04-0.24.The total pore volumes were determined from the N 2 adsorbed amounts at P/P 0 of 0.98.The poresize distributions were calculated from the adsorption data, using the nonlocal density functional theory (NLDFT) method. [49]Photoluminescence and decay curve analysis was performed in room temperature conditions using an Edinburgh FS5 spectrofluorometer.X-ray diffuse scattering data for pair distribution function (PDF) analysis were collected from a Bruker D8 Venture diffractometer equipped with a PHOTION II CPAD detector at room temperature, using Mo K X-radiation ( = 0.7093 Å) under a capillary geometry.Diffraction data were corrected for the empty cell scattering.The X-ray total scattering data (I(q) = f(q), where q is the wavevector) were Fourier transformed to obtain the PDFs using PDFgetX3. [50]Modeling of PDFs was performed using PDFgui software. [51]While keeping chemical integrity, the unit cell dimensions, regular bond lengths and atomic displacement parameters of suitable atomic structural models were refined until the calculated PDF plot is consistent with the experimental data.
Electrochemical Characterizations: A single-channel VersaSTAT 4 electrochemical workstation (Princeton Applied Research) consisting of a three-electrode system was used for the electrochemical and photoelectrochemical measurements.The three-electrode cell consists of the samples as the working electrodes, an Ag/AgCl (saturated KCl) as reference electrode, and a Pt wire as the counter electrode.All electrochemical measurements were performed in a 0.5 m Na 2 SO 4 electrolyte (pH 6.87).The samples were drop-casted on fluorine-doped tin oxide (FTO, 10 Ω sq −1 ) glass substrate as follows: 1 mL of water/isopropyl alcohol (1:1 v/v) dispersion of each sample (10 mg) was ultrasonicated in a water bath in order to obtain a uniform suspension.Then, 100 μL of the suspension were drop-casted on a FTO substrate and the obtained film was heated at 50-60 °C for 1 h to dry.The flat-band (E FB ) potentials were obtained from the space-charge capacitance (C sc ) of the semiconductor interface at 1 kHz and 10 mV AC voltage amplitude.The reported flat-band potentials were converted to a reversible hydrogen electrode (RHE) according to the following equation: where, E RHE corresponds to the potential in the RHE scale and E Ag/AgCl depicts the measured potential in the Ag/AgCl scale.The calculated donor density (N D ) for the following materials was calculated using the Mott-Schottky equation: where C sc is the space charge capacitance, E is the applied potential, E FB is the flat-band potential, N D is the donor density of the material,  is the dielectric permittivity of the material (6.6 for CdIn 2 S 4 ), [52]  o is the vacuum permittivity (8.8542.10−14 F cm −1 ) and e o is the elementary charge (1.602.10 −19 C).The EIS Nyquist data were collected in the range of 1 Hz to 1 MHz under open-circuit potential and the experimental data were fitted using ZView software to an equivalent Randles circuit model, consisted of an electrolyte resistance (R s ), a charge transfer resistance (R ct ) and a constant phase element (CPE) to account for the nonideality of the frequency dispersion in the capacitance response.Polarization (J-V) plots were recorded at the sweep rate of 50 mV s −1 .Transient photocurrent curves were measured at a constant potential of −1 V (vs Ag/AgCl) in 0.5 m Na 2 SO 4 solution, under visible LED light irradiation.
Photocatalytic reactions: The photocatalytic hydrogen evolution experiments were conducted in a custom-built gas-tight reaction cell containing 20 mg of photocatalyst suspended in 20 mL aqueous solution of triethanolamine (TEOA, 10% v/v).The suspension was purged with argon for 45 min prior to irradiation.The sample was illuminated at a power intensity of ≈730 mW cm −2 using a 300 W Xenon lamp (Variac Cermax) equipped with a UV cut-off filter ( ≥420 nm) and the temperature of the suspension was maintained at 20 ± 2 °C using an external water-cooling system.The evolving H 2 was analyzed using a gas chromatograph (Shimadzu GC-2014) equipped with a thermal conductivity detector (Ar carrier gas).
The apparent quantum yield (AQY) was determined by quantifying the amount of evolved hydrogen at a given wavelength ( = 375, 420, 460, 520, and 620 ±10 nm), according to the following equation: AQY = (2 × evolved H 2 molecules)/(total incident photon flux).The photon flux was measured with a StarLite power meter equipped with a FL400A-BB-50 thermal detector (Ophir Optronics Ltd).The intensity of the incident light was 2.55,

Figure 1 .
Figure 1.a) Schematic illustration of the synthesis of CIS NC-linked frameworks (NCFs) (step 1: reflux synthesis of thiol-capped CIS NCs; step 2: polymer-templated NC assembly; step 3: template removal toward open-pore structures).b) XRD patterns of as-prepared 6 and 11-nm CIS NCs and corresponding mesoporous CIS NCFs.The XRD pattern of bulk polycrystalline CIS and standard diffraction data of the cubic CdIn 2 S 4 (JCPDS card no.27-0060) are also given.c) Reduced atomic pair distribution functions G(r) of the 6-nm CIS NCs and polycrystalline CIS.d) G(r) plots of processed 6-nm CIS NCs at various temperatures.Data are represented as symbols and the fits as a red line.a.u., arbitrary units.
5 Å corresponds to Cd/In−S bonding and those at ≈4.0 and ≈4.7 Å arise from the nearest Cd … Cd/In … In and next-nearest Cd/In … S distances of the hexagonal CIS, as shown in Figure 1c.The lattice parameters refined from the PDF were a = b = 3.976(9) Å and c = 20.281(0)Å.A similar PDF refinement was made for the larger (11 nm) CIS NCs, giving a hexagonal unit cell with a = b = 3.968(5) Å and c = 20.615(8)Å lattice parameters (Figure S5, Supporting Information).Note that the structural models used describe the average structural motifs present and do not account for potentially defective lattice sites and locally disordered domains.

Figure 3 .
Figure 3. a) Dependence of the photocatalytic hydrogen evolution rates on mesoporous CIS and Ni 2 P/CIS NCFs, random NC-aggregates (CIS RNAs), and bare and 15 wt.%Ni 2 P-loaded (15-Ni 2 P/CIS-b) polycrystalline CIS.b) Time courses and average rates of photocatalytic hydrogen production over 15-Ni 2 P/CIS NCFs under  ≥ 420 nm illumination.After 5 h of reaction, the catalyst was isolated from the reaction solution through centrifugation, washed thoroughly with water, and redispersed in a fresh TEOA (10% v/v) solution.

Figure 4 .
Figure 4. a) Mott−Schottky plots, b) electronic band structures (VB: valence band, CB: conduction band, red line: H + /H 2 redox potential), and c) EIS Nyquist plots (Inset: equivalent Randles circuit model [R s (CPE/R ct )] used to simulate the EIS data) for the mesoporous CIS and Ni 2 P/CIS NCFs, and bare and 15 wt.%Ni 2 P-loaded (15-Ni 2 P/CIS-b) polycrystalline CIS.The dotted red lines are fit to the data.In panel (b), the energy band diagram of Ni 2 P nanosheets determined from E FB and bandgap (≈1.05 eV, see Figure S28, Supporting Information) data is also given.d) TR-PL emission decay curves (excited by a 375 nm ns pulsed laser) for mesoporous CIS and 15-Ni 2 P/CIS NCFs, and bulk 15-Ni 2 P/CIS-b and CIS samples.e) Band bending model demonstrating a typical type-I (straddling) n-n Ni 2 P/CIS heterojunction and proposed mechanism of photocatalytic H 2 evolution by the mesoporous Ni 2 P/CIS NCFs (E CB : conduction band energy, E VB : valance band energy, E F : Fermi level).f) UV-vis diffuse reflectance spectrum of the 15-Ni 2 P/CIS NCFs catalyst and AQYs of H 2 evolution under different incident lights.Reaction conditions: 1 mg mL −1 of catalyst suspended in a TEOA (10% v/v) aqueous solution, under monochromatic light irradiation (the lines represent the standard errors).a.u., arbitrary units.

Table 1 .
Electrochemical data obtained from Mott-Schottky and Nyquist measurements for the CIS NCs, mesoporous CIS and Ni 2 P/CIS NCFs, and bare and 15 wt.%Ni 2 P-loaded (15-Ni 2 P/CIS-b) CIS microparticles.The VB maximum potential of the semiconductors was estimated from E FB -E g .toNi 2 P until their Fermi levels equilibrate, in line with XPS results.The E FB level of bare Ni 2 P was measured to be −0.41V(FigureS27,Supporting Information), which is much lower to that for CIS NCFs; for n-doped semiconductors, the E FB potential is approximately equal to the Fermi level.Complementary, the progressive CIS to Ni 2 P electron injection and the observed shift direction of the CIS E FB level are also seen in the slope variation of the 1/C sc 2 -E lines, which yielded donor densities (N D ) of ≈4.73 × 10 18 cm −3 for mesoporous CIS and from ≈1.74 × 10 18 to ≈2.20 × 10 17 cm −3 for Ni 2 P/CIS hetero-nanostructures with growing Ni 2 P content levels (Table MPA, 3 mL), and NH 4 OH (25 wt.%, 12 mL) were added in ethylene glycol (15 mL, C 2 H 6 O 2