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Nanoporous Cubic Silicon Carbide Photoanodes for Enhanced Solar Water Splitting Jing-Xin Jian, Valdas Jokubavicius, Mikael Syvajarvi, Rositsa Yakimova, and Jianwu Sun* Cite This ACS Nano 2021, 15, 55025512Read Online ACCESS Metrics Table 1. With increasing the anodization times from 1 to 5 min, the resulting photoanodes exhibited an increased photocurrent due to the increase of the porous layer depth. However, the photoanode with 10 min anodization showed a decreased photocurrent.Thisisprobablycausedbythedeteriorationofthe outermost 3C-SiC111 due to severe anodization etching Figure S1D. The p3C1115M/NiFe photoanode achieved the highest Jphof 2.30 mA cm2at 1.23 VRHE, which is 3.3 times higher than that of the planar 3C111/NiFe and the highest photocurrent among the reported values of 3C-SiC photo- anodes see Table S1. Moreover, the nanoporous 3C-SiC photoanodes exhibited a much steeper increase of photocurrent than the planar counterpart. The p3C1115M/NiFe showed a precipitous photocurrentincreaseataslowas0.2VRHE,whichisacathodic shift of 0.3 V compared to its planar counterpart 3C111/ NiFe. This power characteristic can be clearly demonstrated by the fi ll factor ff , which is defi ned as ff Jmp1.23 Vmp/ Jsc1.23 Eonset, where Jmpand Vmpare the photocurrent density and potential at the maximum power point and Jscis the photocurrent density at 1.23 VRHE. As shown in Figure S5 and Table 1, the fi ll factors of the nanoporous photoanodes were substantiallyincreased.Moreover,thepotentialatthemaximum power point Vmp was decreased from 1.0 VRHEfor the planar 3C001 and 0.8 VRHEfor the planar 3C001/NiFe to 0.6 VRHEfor the nanoporous 3C0015M/NiFe Figure S5. Figure 3Bshows theapplied bias photon-to-current effi ciency ABPE curves for the pristine 3C111, planar 3C111/NiFe, and nanoporous p3C111xM/NiFe photoanodes. ABPE is given by the following equation ABPE Jph1.23 Vapp/ PAM1.5G, where PAM1.5Gis the light density of simulated sunlight AM1.5G 100 mW cm2. The nanoporous photoanodes demonstratedasignifi cantenhancementofABPEandareduced potential at the maximum ABPE. The p3C1115M/NiFe photoanode exhibited a maximum ABPE of 0.81 at a low applied potential of 0.56 VRHE, which is the highest photo- conversion effi ciency ever reported for 3C-SiC photoanodes TableS1.Incontrast,theplanar3C111/NiFeshowedmuch lower ABPues of0.20at ahigherappliedpotentialof0.80 VRHETable 1. ThephotocurrentdensitytimeJtcurvesrecordedat1.23 VRHEfor the planar 3C111, 3C111/NiFe, and nanoporous p3C1115M/NiFe photoanodes are shown in Figure 3C. Under the 1 sun illumination for 60 min, the Jphof the planar 3C111 photoanode decreased from 0.35 to 0.13 mA cm2, indicating a 63 loss of its initial Jph. It has been reported that 3C-SiC photoanodes suff ered from a photocorrosion, which is a surface oxidation reaction ing SiO23C-SiC 4H2O 8h SiO2 CO2 8H.21For the p3C1115M/NiFe photoanode, the photocurrent retained 94 of its initial Jphof 2.30mAcm2after60minillumination.Meanwhile,theevolved O2over the p3C1115M/NiFe photoanode was detected by Table 1. PEC Water Splitting Perance of Planar 3C111, Planar 3C111/NiFe, and Nanoporous p3C111xM/NiFe Photoanodes, Where xM Represents the Anodization Times of 1, 2, 5, and 10 min, Respectivelya sample J at 1.23 VRHE mA cm2 Eonset VRHE max ABPE Emaxat ABPEmax VRHE ff Cdl F cm2 rel surf area LHE at 450 nm JAbs mA cm2 planar 3C1110.250.40.031.09 planar 3C111/NiFe0.690.20.200.80291.051.0784.67 p3C1111M/NiFe1.260.20.500.65417.367.0865.67 p3C1112M/NiFe1.300.20.600.584722.2021.1906.28 p3C1115M/NiFe2.300.20.810.563635.7034.0936.67 p3C11110M/NiFe1.050.20.590.455844.8042.7977.40 aThe best perance is highlighted in bold. Figure4.Morphologiesofnanoporous3C-SiC001.TopviewSEMimagesofplanar3C001Aandnanoporousp3C001xMBE,where xM represents the anodization times of 1, 2, 5, and 10 min, respectively. Cross-sectional SEM images of planar 3C001 F and nanoporous p3C001xM GJ. ACS Nanowww.acsnano.orgArticle https//dx.doi.org/10.1021/acsnano.1c00256 ACS Nano 2021, 15, 55025512 5506 中国煤炭行业知识服务平台 w w w . c h in a c a j . n et gas chromatography and the Faradaic effi ciency of O2was determined to be 75 Table S2. Toquantifytheeff ectiveelectrochemicallyactivesurfacearea, cyclic voltammetry measurements were employed to extract the double-layer capacitance Cdl of the planar and nanoporous photoanodes Figure S6, according to the reported meth- od.7,38,40 Figure 3D shows the plots of the half-diff erence of the anodic and cathodic current density |janodic jcathodic|/2 at 0.55 VRHEas a function of the scan rate. The slopes of these linear plots gave rise to the geometric Cdl, which is proportional to the eff ective surface area of the photoanode. From a comparison of the extracted Cdl, we found that the electrochemically active surface area of the nanoporous photoanodes was signifi cantly increasedbyincreasingtheanodizationtime,consistentwiththe increaseddepthofnanoporouslayer.Notably,thesurfaceareaof p3C1115M/NiFe is 34 times larger than that of the planar 3C111/NiFe Table 1. Nanoporous 3C-SiC001 Photoanodes. The nano- porous 3C-SiC001 photoanodes were prepared by the anodization using the same conditions as for nano- porous 3C-SiC111. The prepared samples are denoted as p3C001xM, where xM represents the anodization times of 1, 2, 5, and 10 min, respectively. Figure 4 shows the morphologies of planar 3C001 and nanoporous p3C001xM. With increasing anodization time, more densely arranged holes on the 3C001 surface were clearly observed. Unlike 3C111, those holes do not show any triangular shape. The diff erent shapes of etched holes on 3C111 and 3C001 are related to the diff erent crystalline orientations. As the surface of the commercial 3C001 wafer was mechanically polished, the anodization etching also boosts the scratches as seen in Figure Figure 5. PEC water oxidation perance of planar and nanoporous 3C-SiC001 photoanodes. JV curves A and ABPE B of planar 3C001, planar 3C001/NiFe, and nanoporous p3C001xM/NiFe photoanodes, where xM represents the anodization times of 1, 2, 5, and 10 min, respectively. C Jt curves of 3C001, 3C001/NiFe and p3C0015M/NiFe photoanodes at 1.23 VRHEunder illumination. All of thePECmeasurementswerecarriedoutin1.0MNaOHelectrolyteunderAM1.5G100mWcm2illumination.DPlotsof|janodicjcathodic|/2 at 0.55 VRHEas a function of the scan rate, showing the extraction of double-layer capacitance Cdl for 3C001/NiFe and p3C001xM/NiFe photoanodes. The lines show the linear fi tting plots, whose slopes correspond to Cdlaccording to the equation Cdl I/dv/dt. Table 2. PEC Water Splitting Perance of Planar 3C001, Planar 3C001/NiFe, and Nanoporous p3C001xM/NiFe Photoanodes, Where xM Represents the Anodization Times of 1, 2, 5, and 10 min, Respectivelya sample J at 1.23 VRHE mA cm2 Eonset VRHE max ABPE V at ABPEmax VRHE ff Cdl F cm2 rel surf area LHE at 450 nm JAbs mA cm2 planar 3C0010.140.40.011.078 planar 3C001/NiFe0.580.20.120.88200.671.0764.79 p3C0011M/NiFe0.680.20.200.69293.385.0885.86 p3C0012M/NiFe0.790.20.280.67347.3411.0916.41 p3C0015M/NiFe1.500.20.480.713115.3022.8967.01 p3C00110M/NiFe0.360.20.130.583518.4027.5997.42 aThe best perance is highlighted in bold. ACS Nanowww.acsnano.orgArticle https//dx.doi.org/10.1021/acsnano.1c00256 ACS Nano 2021, 15, 55025512 5507 中国煤炭行业知识服务平台 w w w . c h in a c a j . n et 4D,E. From the cross-sectional SEM images, the depths of the nanoporouslayerare5.8,10.2,15.8,and23.1mforthesamples after1,2,5,and10minofanodizationFigure4GJ.Similarto nanoporous 3C-SiC111, 10 min anodization also resulted in largevoidsintheoutermostlayer,indicatingthedeteriorationof the crystalline quality. To uate the PEC perance of the nanoporous 3C-SiC001 photoanodes, we employed the same conditions as used for 3C-SiC111 to deposit NiFeOOH on 3C001 and p3C001xM photoanodes at the same time, hereby denoted as 3C001/NiFe and p3C001xM/NiFe, respectively.Asfortheplanarcase,thedepositedNiFeOOHon 3C001 exhibited the same network-like layer as on 3C111, as seen in Figure S7A. For the nanoporous case, the NiFeOOH on p3C0015M showed a nanoparticle morphology Figure S7G, identical to that on p3C1115M. In both cases, the thicknesses of NiFeOOH are similar to those on 3C111. The EDXS and the elemental mapping results confi rmed that NiFeOOH was also deposited into the pore structure of nanoporous 3C001, as shown in Figure S7. Figures 5 and S8 JV curves under chopped 1 sun illumination show the PEC water splitting results of the planar 3C001/NiFe and nanoporous 3C001xM/NiFe photo- anodes, which are quite similar to the PEC results of the corresponding 3C111 photoanodes. The nanoporous 3C001xM/NiFe photoanodes exhibit enhanced ABPE, ff values, and surface area Table 2 and Figures S9 and S10, but their overall PEC perance is lower than that of the nanoporous 3C111. Among all 3C001xM/NiFe photo- anodes, p3C0015M/NiFe gives the highest photocurrent of 1.50mAcm2at1.23VRHE,whichis2.6timeshigherthanthatof the planar 3C001/NiFe photoanode Table 2 but still lower than the photocurrent 2.30 mA cm2 of p3C1115M/NiFe prepared at the same conditions. Moreover, electrochemically active surface areas of p3C001xM/NiFe are respectively smaller than the corresponding p3C111xM/NiFe fabricated underthesameconditionsTable2.Thisresultisprobablydue to the presence of the columnar pore structures in nanoporous p3C111xM/NiFe Figure 2G, which might provide larger electrochemically active surface areas. Understanding the Improvement in PEC Perance of Nanoporous 3C-SiC Photoanodes. To understand the signifi cantimprovementofthePECwatersplittingperance of nanoporous 3C-SiC with respect to the planar counterparts, the electrochemical impedance spectroscopy EIS was measured at 1.23 VRHEunder AM1.5G, 100 mW cm2 illumination in the frequency range of 1105Hz. The Nyquist plots of all 3C111/NiFe, p3C1115M/NiFe, 3C001/ NiFe, and p3C0015M/NiFe photoanodes exhibited two semicircles Figure 6A, which were fi tted by the equivalent circuitconsistingoftheseriesresistanceRs,thecharge-transfer resistanceRbulkandthecapacitanceCPESCinthebulkofthe photoanode, and the charge-transfer resistance from the photoanode to electrolyte Rct and the corresponding capacitance CPEct.41 The fi tting results are listed in Table S3. For the planar photoanodes, Rbulkin 3C111/NiFe is 3.4 times lower than that in 3C001/NiFe. This result can be Figure6.Eff ectofnanoporousstructureof3C-SiConthechargetransportproperties,charge-separationeffi ciencysep,andcharge-injection effi ciency into electrolyte for oxidation reaction ox. A Nyquist plots of the 3C001/NiFe, p3C0015M/NiFe, 3C111/NiFe, and p3C1115M/NiFe photoanodes, measured from 1 to 105Hz at 1.23 VRHEunder AM1.5 100 mW cm2illumination. Panel A inset equivalent circuit used for fi tting the impedance data. B JV curves of the 3C001/NiFe, p3C0015M/NiFe, 3C111/NiFe, and p3C1115M/NiFe photoanodesin1.0MNaOHelectrolytewith5H2O2astheholescavengerunder1sunillumination.oxCandsepDasafunctionofthe potential VRHEfor the 3C001/NiFe, p3C0015M/NiFe, 3C111/NiFe, and p3C1115M/NiFe photoanodes. ACS Nanowww.acsnano.orgArticle https//dx.doi.org/10.1021/acsnano.1c00256 ACS Nano 2021, 15, 55025512 5508 中国煤炭行业知识服务平台 w w w . c h in a c a j . n et explained by the higher crystalline quality of 3C-SiC111, thus less charge recombinations in bulk compared to 3C-SiC001, asconfi rmed byourpreviouswork.25,26 We fi ndthattheXRD rocking curves of 3C-SiC111 showed an average of the full width at half-maximum fwhm of 38 arcsec, which is 2.8 times smaller than that of 3C-SiC001 measured under the same condition fwhm 105 arcsec for 3C-SiC001.26,28In particular, from the low-temperature 2 K photoluminescence measurement,wehaveobservedtheradiativeemissionsfromup to four bound excitons in our high-quality 3C-SiC111 material, indicating a suppressed nonradiative recombination and thus a longer carrier lifetime.25The microwave photo- conductivity decay measurements further confi rmed that a long carrier lifetime of 8.2 s was observed in our high-quality 3C- SiC111, which is much longer than reported carrier lifetimes a few to 120 ns in 3C-SiC001 grown on Si substrates.25 For the nanoporous photoanodes, both p3C1115M/NiFe and p3C0015M/NiFe signifi cantly reduce Rbulkand Rct compared to their planar counterparts Table S3, which in turn explains their dramatic enhancement in the PEC per- ance. As expected, the nanoporous structure shortens the distances for charge transfer, provides a signifi cantly enlarged surface area see Tables 1 and 2, and increases the number of the catalytic sites for water oxidation, thus reducing both Rbulk and Rct. The refl ectance and transmittance spectra of the planar and nanoporous photoanodes were measured to extract their light- harvesting effi ciency LHE. As seen in Figures S11 and S12, both nanoporous 3C111xM/NiFe and 3C001xM/NiFe photoanodes exhibited a signifi cantly reduced refl ectance compared to their planar counterparts due to light trapping eff ect at the surface of the nanoporous structure. Planar 3C111/NiFe and 3C001/NiFe showed a high refl ectance of over 20 at wavelengths less than 500 nm. With increasing theanodizationtimefrom1to10min,theresultingnanoporous photoanodes showed a gradual decrease of the refl ectance, consistentwiththeincreaseofthesurfaceareaTables 1and 2. With the anodization time 5 min, the nanoporous photo- anodes showed a refl ectance less than 5 in the wavelength range of 500 nm, which resulted in a high LHE of over 95 Tables 1 and 2. This result reveals that nanoporous 3C-SiC photoanodes signifi cantly reduce light refl ectance, thus increasing the light absorption due to trapping eff ect. To quantitatively demonstrate the eff ect of the nanoporous 3C-SiC structure on the improvement of charge-separation effi ciency sep and charge-injection effi ciency into electrolyte for water oxidation ox, we measured the JV curves of the photoanodes in 1 M NaOH with 5 H2O2Figure 6B. Jphcan be defi ned as Jph JAbssepox,42where JAbsis the photocurrent densityconvertedfromallabsorbedphotonsbythephotoanode at a 100 quantum effi ciency. By integrating the solar AM1.5G spectrum with LHE, we obtained JAbsfor both nanoporous 3C111xM/NiFe and 3C001xM/NiFe photoanodes, which is also increasing with the increase of the anodization time Tables 1 and 2. The improved JAbsclearly evidence that the nanoporous structure enhances the light-harvesting effi ciency. Since H2O2is a hole scavenger enabling complete utilization of the photogenerated holes arrived at
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