Platinum@Hexaniobate Nanopeapods: A Directed Photocatalytic Architecture for Dye-Sensitized Semiconductor H2 Production under Visible Light Irradiation

Platinum@hexaniobate nanopeapods (Pt@HNB NPPs) are a nanocomposite photocatalyst that was selectively engineered to increase the efficiency of hydrogen production from visible light photolysis. Pt@HNB NPPs consist of linear arrays of high surface area Pt nanocubes encapsulated within scrolled sheets of the semiconductor HxK4–xNb6O17 and were synthesized in high yield via a facile one-pot microwave heating method that is fast, reproducible, and more easily scalable than multi-step approaches required by many other state-of-the-art catalysts. The Pt@HNB NPPs’ unique 3D architecture enables physical separation of the Pt catalysts from competing surface reactions, promoting electron efficient delivery to the isolated reduction environment along directed charge transport pathways that kinetically prohibit recombination reactions. Pt@HNB NPPs’ catalytic activity was assessed in direct comparison to representative state-of-the-art Pt/semiconductor nanocomposites (extPt-HNB NScs) and unsupported Pt nanocubes. Photolysis under similar conditions exhibited superior H2 production by the Pt@HNB NPPs, which exceeded other catalyst H2 yields (μmol) by a factor of 10. Turnover number and apparent quantum yield values showed similar dramatic increases over the other catalysts. Overall, the results clearly demonstrate that Pt@HNB NPPs represent a unique, intricate nanoarchitecture among state-of-the-art heterogeneous catalysts, offering obvious benefits as a new architectural pathway toward efficient, versatile, and scalable hydrogen energy production. Potential factors behind the Pt@HNB NPPs’ superior performance are discussed below, as are the impacts of systematic variation of photolysis parameters and the use of a non-aqueous reductive quenching photosystem.


Synthesis of hexaniobate semiconductors and chromophores S2 -S4
Microwave synthesis of catalysts S5 -S7 Ligand exchange procedure and surface passivation of catalysts via UV irradiation S8 -S9 Photosystem preparation and determination of H 2 production S10 -13  (molar ratio 1.4:1) were ground for 10 minutes using a ball-milling apparatus (Spex 8000 Mixer/Mill, zirconia ball media) and placed in an alumina crucible. An initial 10% molar excess of K 2 CO 3 was included to compensate for volatilization during the high-temperature synthesis.
The mixture was heated at 900 °C for 1 hour. After cooling to 700 °C in the oven and to room temperature in the open air, a further 10% excess of K 2 CO 3 was added to prevent the formation of potassium-deficient phases; this mixture was ground again for 10 minutes via ball-milling. The sample was then heated for 2 days at 1050 °C, followed by 1 hour at 1100 °C. After cooling to room temperature, the sample was washed via centrifugation with de-ionized water and acetone and dried overnight at 75 °C.

Protonation of host to H x K 4-x Nb 6 O 17
Protonation was performed by placing 8 mL of 6 M HCl and 1 g of K 4 Nb 6 O 17 in a 23 mL capacity

Synthesis of platinum nanoparticles and nanoseeds via microwave heating
Cubic, 8-nm Pt NPs were synthesized via a microwave heating method recently reported by our group. 5 200 mg Pt(acac) 2 , 50 mg L-ascorbic acid, 5 mL OAm, and 5 mL toluene were added to a microwave-transparent glass reaction vessel along with a PTFE stir bar and a Weflon TM button. Composition, size, and morphology of the Pt nanocubes and nanoseeds was examined via XRD and TEM. As seen in Figure S3, peak position and intensity for both samples was in good agreement with the Pt reference pattern, which confirmed the successful formation of Pt NPs. 6 The Pt nanocubes show diffraction peaks that are well-defined and symmetrical, with peak intensities that correspond to a random three-dimensional ordering of polyhedral Pt NPs. The broad, lower intensity peaks of the Pt nanoseeds is consistent with their smaller size and lack of defined crystallographic facets.

Synthesis of extPt-HNB NScs and empty hexaniobate nanoscrolls
ExtPt-HNB NScs were synthesized from crystalline hexaniobate and the Pt nanoseeds. 100 mg of crystalline H x K 4-x Nb 6 O 17 , 200 mg TBAOH, 5 mL OAm, and 3 mL toluene were added to the reaction vessel. A colloidal toluene suspension of OAm-capped Pt nanoseeds (20 mg/mL) was briefly sonicated, after which 5 mL (100 mg of NPs) were added to the mixture in the glass reaction vessel. The reaction mixture was heated to 135 °C at a rate of 3.5 °C/minute and held at that temperature for 105 minutes. To ensure optimal distribution of heat and chemical species throughout the reaction mixture, magnetic stirring at 650 rpm was maintained consistently throughout the reaction. After the reaction, the vessel was cooled to room temperature at 10 °C/minute. Washing the silvery-brown product and isolation of the extPt-HNB NScs was performed using the method for isolating Pt@HNB NPPs described in the main text. ExtPt-HNB NScs were dried overnight at room temperature in preparation for surface passivation and photolysis. Empty hexaniobate NScs ( Figure S4) were synthesized the same method, but substituting pure toluene for the Pt NPs suspension. Figure S4. TEM image of empty hexaniobate nanoscrolls synthesized via MW method.

SURFACE PASSIVATION OF AS-SYNTHESIZED CATALYSTS VIA UV IRRADIATION
Confirmation of OAm removal was conducted via ATR-FTIR ( Figure S5). In Figure S5a, the presence of OAm ligands on the surface of the as-synthesized Pt@HNB NPPs is confirmed by the appearance of characteristic amine peaks including NH 2 bends at 722 and 1574 cm -1 , asymmetric and symmetric C-H stretches at 2921 and 2852 cm -1 (respectively), and a bend of the C-H group near the C=C at 3006 cm -1 . 7 As shown in Figure S5b, the catalyst surfaces can be effectively

Photosystem optimization
In order to ensure reproducibility of photolysis results, catalyst photosystems were optimized to maintain a homogeneous colloidal dispersion and prevent clumping or settling during photolysis.
Catalyst surfaces were pre-treated with UV irradiation to remove residual hydrophobic groups and maximize dispersibility in the MeCN-based photosystem. When assembling the photolysis reaction mixture, micro-stir bars were added to each vial before degassing. The prepared vials were subsequently placed on a stir plate situated within the photoreactor. Continuous gentle stirring was performed for the duration of photolysis to ensure that the catalysts maintained a homogeneous colloidal dispersion. Figure S6 shows examples of prepared vials.  Figure S6. Examples of prepared photolysis vials. Samples were able to maintain homogeneous colloidal suspensions when agitated or stirred, as seen above. S13

DETERMINATION OF HYDROGEN PRODUCTION
Number of moles H 2 were derived from thermocouple readings (mV) vs. time (s) output by the gas chromatograph during photolysis, using the method of Shan and Fontenot. 8,9 Hydrogen production was determined by integrating the area under the peak for H 2 , which occurred at a retention time of 65 s after injection in each GC measurement. H 2 peak area was multiplied by a constant (m) representing the slope of the GC calibration curve in units of μL/area to find volume of H 2 (in μL). The percentage of H 2 in the photolysis vial headspace was found by dividing μL H 2 by the volume of injected gas (50 μL). These calculations were repeated for GC data collected at 30-minute intervals over the 210-minute photolysis period. Total volume H 2 produced was calculated by multiplying percent H 2 in the headspace by total headspace volume (4.7 mL), and then adding that quantity to the sum of cumulative H 2 volumes (converted to mL) observed at each 30-minute interval. This value of H 2 volume (in L) was then converted into moles of H 2 via the ideal gas law.