Interfacial Charge Separation in Cu 2 O/ruo X as a Visible Light Driven Co 2 Reduction Catalyst Experimental Details Powder X-ray Diffraction Data (xrd) Was Done Using Panalytical X'pert Powder Diffraction System Fitted with X'cellerator Scanning Linear Detector. the Incident X-ray Is Nickel Filtere

Synthesis Chemicals for the synthetic part of this work were purchased from commercial suppliers and used without further purification. Chemicals for analytical measurements were purchased in the highest purity available. Cu 2 O films for absorption spectroscopy measurements were prepared using a chemical solution deposition method using CuSO 4 , Na 2 S 2 O 3 and NaOH reported previously. 1 In a typical film preparation method, cleaned conductive glass (fluorine-doped indium tin oxide (FTO) coated glass) were subjected to successive 20 second immersions into a 1 M NaOH solution and Na 2 S 2 O 3 (0.5 M) – CuSO 4 • 5H 2 O (0.08 M) solution mixture at 70 °C, and then left to dry at 70 ºC. The bare Cu 2 O film thickness, estimated using cross-sectional FESEM, was found to be ca. 100 nm after ten successive immersions. Cu 2 O-RuO x films were prepared by impregnation method described previously, 2 by dripping small amounts (approx. 0.5 mL per 2 cm 2 film area) of 0.05 M aqueous RuCl 3 • xH 2 O over as grown Cu 2 O films on FTO coated glass and left dry at 70 ºC. The Cu 2 O-RuO x film had similar morphology to the bare Cu 2 O film, and film XRD analysis shows only Cu 2 O and FTO peaks (Fig. S1). Both bare Cu 2 O and Cu 2 O-RuO x films were annealed at 200 ºC for 1 h, in order to improve its crystallinity before further analysis. Cu 2 O powders used in this study were prepared using solvothermal method reported previously. 3,4 A typical synthesis of cuboid shaped Cu 2 O involves the dissolution of Cu(NO 3) 2 • 2.5H 2 O in a PTFE lined hydrothermal reactor containing a mixture of ethanol, water (64:36 volume ratio), and formic acid (1.3 M). Deep purple precipitates obtained after 2 hours reaction in 150 °C were washed repeatedly (5-6 times) with copious amount of water (50 mL) and dried at 70 °C overnight. Powder XRD analysis shows only Cu 2 O peaks (Fig. S1). Cu 2 O-RuO x junction was prepared by impregnation method using RuCl 3 • xH 2 O in water followed by heat treatment at 200 ºC for 3 h and then washed for a final time and let to dry at 70 °C. The optimum RuO x loading amount was …

Photochemical reduction of CO 2 has the potential to convert this greenhouse gas into clean fuels or value-added chemicals.As such, it can contribute significantly to both renewable energy generation and CO 2 mitigation. 1 Photoreduction of CO 2 in heterogeneous systems was first reported using optical excitation of large bandgap semiconductor materials such as TiO 2 , SrTiO 3 , ZnO, or SiC with UV light irradiation. 2Since this initial study, many reports have focused on the development of materials with improved light absorption properties in the visible region, such as narrow bandgap non-metal oxide semiconductors (e.g.ZnS, CdS, and GaP), or the functionalization or doping of wide bandgap semiconductors, 3 although overall efficiencies reported for CO 2 photoreduction remain limited.These low efficiencies have often been attributed to fast charge recombination, requirements of high over-potentials and competition between CO 2 photoreduction reaction with H 2 evolution, as well as issues associated with catalyst/co-catalyst degradation or deactivation. 4Identifying the parameters that restrain the performance of catalysts towards CO 2 reduction and designing a system capable of overcoming these limitations is therefore a key challenge.In this context, transient absorption spectroscopy (TAS) is a technique that allows the assessment of competing kinetic processes in a semiconductor, namely charge separation and recombination, by monitoring the dynamics of charge carriers.We have, for example previously reported the signals corresponding to electrons and holes in Fe 2 O 3 and other photoanodes for the related reaction of photochemical water-splitting. 5erein, we report the dynamics of charge carriers in Cu 2 O.To the best of our knowledge, the absorption spectrum and dynamics of photoexcited electrons in Cu 2 O have not been identified.This narrow bandgap (ca.2.2 eV) p-type semiconductor has been shown to be photocatalytically active for reduction of water 6 and degradation of inorganic waste. 7Since the conduction band of Cu 2 O lies above the reduction potential of CO 2 to CO, 8 it is attracting increasing interest for the photoreduction of CO 2 . 9As a p-type semiconductor, interfacial band-bending of Cu 2 O, where present, is likely to favour interfacial reduction reactions but hinder oxidation reactions by presenting an energetic barrier for holes to approach the semiconductor surface.These surface reduction/oxidation reactions will be in kinetic competition with electron-hole recombination loss pathways (Fig. 1a).We have recently reported that the deposition of a charge acceptor (RuO x , x r 2) on top of Cu 2 O improves its photocatalytic activity towards CO 2 reduction.9b Herein, we focus on the study of the dynamics of photogenerated electrons in Cu 2 O when forming an inorganic heterojunction with a material capable of (1) increasing the accessibility of photogenerated holes to the electrolyte, (2) having appropriate reaction sites to drive photo-oxidation reactions and (3) reducing the electron-hole recombination reaction on the Cu 2 O surface (Fig. 1b).Ruthenium oxide (RuO x ) was selected as the overlayer material because its appropriate work function level can facilitate transfer of holes from Cu 2 O, thereby potentially increasing the yield of long-lived electrons in Cu 2 O to enable the reduction of CO 2 .RuO x is also less likely to react with Cu 2 O to form an unwanted interfacial layer and has been shown to have the ability to oxidize many hole scavengers, including water. 10pectroscopic measurements were performed using Cu 2 O films deposited onto FTO glass.Cu 2 O cuboid films with a nanoparticle size of ca. 100 nm were prepared by a chemical solution method using CuSO 4 , Na 2 S 2 O 3 and NaOH, as reported previously. 11RuO x was subsequently deposited in the form of nanoparticles o10 nm on the Cu 2 O surface. 13The RuO x deposition did not alter the bandgap absorption of Cu 2 O of approx.2.2 eV.Only Cu 2 O diffraction peaks were detected in Cu 2 O-RuO x samples, and Cu(I) species were confirmed by XPS analysis, albeit coexisting with small amounts of Cu(II) (Fig. S4, ESI †).Trace amounts of Ru were detected on the Cu 2 O-RuO x heterojunction by elemental analysis during TEM and XPS investigations, but not X-ray diffraction due to the low loading amount and small crystallite size of RuO x .TAS spectra were recorded after UV excitation (355 nm) of Cu 2 O and Cu 2 O/RuO x , using an experimental setup described previously. 14Details of synthetic procedures, material characterisation and experimental setup for transient absorption measurements can be found in the ESI † (Fig. S1-S4).
The absorption spectrum of charge carriers in a semiconductor can be characterized by TAS, by employing relevant fast hole and electron scavengers. 15Thus, the transient absorption spectrum of Cu 2 O films deposited onto the FTO glass was obtained (i) in a N 2 purged aqueous solution (ii) following the addition of Na 2 SO 3 as a hole scavenger or (iii) AgNO 3 as an electron scavenger.Photoexcitation of the Cu 2 O semiconductor in the absence of charge carrier scavengers results in the appearance of a ground state bleach signal between 475 and 750 nm and two positive photoinduced absorption peaks at wavelengths o475 nm and 4800 nm (Fig. 2).The signals at wavelengths 4850 nm are significantly enhanced by the addition of Na 2 SO 3 and weakened by the addition of AgNO 3 .Given that Na 2 SO 3 can effectively scavenge holes, while Ag + scavenges electrons, we can assign the transient signals above 850 nm to Cu 2 O photoexcited electrons.On the other hand, we observed only the signal corresponding to photoexcited holes in Cu 2 O at wavelengths o475 nm when using AgNO 3 as an electron scavenger.Thus, the direct probing of holes under the current experimental photocatalytic conditions is hindered by the partial overlap with the bleach signal of the ground state and cannot be directly monitored.Since these signals corresponding to photoexcited electrons and holes in the Cu 2 O are already present in the 10 ms timescale, our measurements indicate that electron-hole scavenging by the relevant chemicals are very fast (o10 ms), beyond the instrument's limit of response.
The formation of inorganic heterojunctions or the addition of sacrificial agents which are able to scavenge charge carriers have been reported to improve the performance of the photocatalytic systems for organic waste decomposition or water splitting reactions by decreasing the recombination of photogenerated electrons and holes in a semiconductor.4b,16 Thus, transient absorption spectroscopy experiments were also employed to assay the effect of RuO x deposition on the Cu 2 O charge carrier dynamics.Typical kinetic data of the photoinduced electron absorption signals taken at 950 nm are  plotted in Fig. 3, showing the decay dynamics on the micro-to second timescales.We note that we only observed significant long-lived signal amplitudes with relatively intense excitation densities, indicative of the presence of significantly fast (o10 ms) electron-hole recombination in these films.The presence of Na 2 SO 3 resulted in an increase in the amplitude of this Cu 2 O electron signal (Fig. 2), although this effect is relatively minor, indicative of inefficient hole scavenging/fast recombination, and consistent with the expected band bending impeding access of photogenerated holes to the Cu 2 O surface (Fig. 1a).The deposition of RuO x on the Cu 2 O surface results in a significant increase in the amplitude of the long-lived electron signal, independent of the presence of Na 2 SO 3 , and indicative of a more efficient reduction in fast electron-hole recombination losses.These results qualitatively suggest that RuO x deposition results in a significant increase in the yield of long-lived (4100 ms) Cu 2 O electrons.This is attributed to a reduction in fast electron-hole recombination losses due to formation of the inorganic junction, most probably due to hole transfer from Cu 2 O to RuO x , thereby increasing the spatial separation of electrons and holes and facilitating the photooxidation reaction by holes.The lack of absorption of RuO in the near IR region of the spectrum indicates that the increase in absorbance observed at 950 nm is not due to the reduction of RuO x by Cu 2 O photoexcited electrons, and therefore confirms our peak assignation. 17 The effect of enhanced charge separation by the Cu 2 O-RuO x junction strategy on CO 2 photoreduction was tested using analogous bare Cu 2 O and Cu 2 O-RuO x nanoparticulate suspensions in CO 2 -saturated deionised H 2 O in the presence of a Na 2 SO 3 hole scavenger upon irradiating under the full arc of a 150 W Xe lamp (Fig. 4).In agreement with the results observed upon irradiating the samples with visible light l Z 420 nm, 9b the coupling of Cu 2 O with RuO x results in a nearly sixfold increase of the initial rate of CO production, from 0.16 mmol g À1 on bare Cu 2 O to around 0.88 mmol g À1 on the Cu 2 O-RuO x junction after 1 h irradiation using a 150 W Xe lamp.Trace amounts of methanol (0.01 mmol g À1 h À1 ) and methane (o0.001 mmol g À1 h À1 ) were also observed in the Cu 2 O-RuO x sample during the reaction.Quantum yield measurements were carried out using a near monochromatic visible light 400 nm bandpass filter (Dl o 10 nm, filtered light output ca.600 AE 40 mW cm À2 ).The apparent initial quantum yield for CO generation (20 min) for the Cu 2 O-RuO x heterojunction was determined to be ca.1.6%.It is also noted that the CO evolution rate is reduced after the first hour.These results can be explained in terms of the instability of Cu 2 O towards the presence of increasing amounts of SO 4 2À formed upon scavenging the holes by Na 2 SO 3 . 18Another possible explanation for non-linear CO evolution could be the strong interaction between CO and Cu 2 O, which will result in lowered catalytic activity over time. 19Studies to address this instability by using alternative hole scavengers and further modifying material protection layers are ongoing.RuO x has been reported to be able to either reduce or oxidize water, 6d,20 but is unable to reduce CO 2 due to its work function being more positive than the E red (CO 2 /CO) reduction potential.We did not observe any water oxidation and oxygen evolution upon the addition of RuO x , as holes are preferentially scavenged by Na 2 SO 3 .Thus, the increase in the CO 2 photoreduction yield is consistent with the efficient charge separation due to the inorganic heterojunction, where holes are transferred to RuO x and CO 2 reduction takes place at the surface of Cu 2 O.
In order to provide evidence that the formation of CO is due to the photoreduction of CO 2 by Cu 2 O, three different control experiments were carried out: (1) traces of organic contaminants were removed by treating the aqueous suspension system under strong light irradiation without CO 2 until there was no increase in the CO or CO 2 amount as detected by gas chromatography before the photocatalytic measurements.( 2

Fig. 1
Fig. 1 Schematic of Cu 2 O and the Cu 2 O/RuO x band diagram and electron and hole transfer reactions for the photocatalytic CO 2 reduction.Energy values were taken from ref. 8b and 12. E F stands for Fermi level, E CB and E VB indicate the energy levels of the conduction band and valence band, respectively.

Fig. 2
Fig. 2 Transient absorption spectrum of Cu 2 O recorded under N 2 , in the presence of a 0.1 M Na 2 SO 3 and 0.01 M AgNO 3 aqueous solutions.The samples were excited at l ex = 355 nm, with a laser intensity of B1.2 mJ cm À2 .
We want to note that a Cu 2 O-Al:ZnO-TiO 2 -RuO x heterojunction for H + reduction, where the Cu 2 O and RuO x interfaces are connected through an interfacial (AZO-TiO 2 ) layer, has been reported by Tilley et al. 6d Contrary to our approach, where RuO x accepts the holes from Cu 2 O, charge separation in Cu 2 O-AZO-TiO 2 -RuO x takes place through electron transfer from Cu 2 O to RuO x .We believe that, while in the Cu 2 O-AZO-TiO 2 -RuO x there is a favorable downhill electron transfer cascade from Cu 2 O to RuO x , the AZO-TiO 2 layer blocks the hole transfer to RuO x .

Fig. 3
Fig. 3 TAS decays of photo-excited electrons of Cu 2 O films (black trace) and Cu 2 O-RuO x films (red trace) in N 2 purged water, and in the presence of a N 2 purged 0.1 M Na 2 SO 3 aqueous solution (Cu 2 O films: grey trace, Cu 2 O-RuO x films: pink trace).Data obtained at l probe = 950 nm with an excitation intensity of 1.6 mJ cm À2 at l ex = 355 nm.

Fig. 4
Fig. 4 CO 2 reduction to CO on bare and heterojunction based Cu 2 O core photocatalysts under full arc 150 W Xe lamp irradiation.Photoreduction reactions were undertaken in 3 mL of CO 2 saturated H 2 O in the presence of a hole scavenger (ca.0.7 M Na 2 SO 3 ).
) Control experiments under similar conditions but in the absence of photocatalyst, CO 2 or light, and irradiating the Cu 2 O with longer wavelength light (l 4 668 nm, much lower than Cu 2 O bandgap requirement) demonstrated that surface carbon gasification is not a major contributing factor (Fig.S5, ESI †).(3) Gas chromatography -Mass spectrometry isotope labelling analysis showed the sole formation of 13 CO (m/z 29) (Fig.S6, ESI †).These results strongly indicate that the CO observed by GC is due to photochemical reduction of CO 2 rather than from the reduction of organic contaminants present on the surface of Cu 2 O-RuO x .ConclusionsIn summary, we have assigned the transient absorption spectrum of Cu 2 O photoexcited electrons to appear at wavelength l 4 800 nm, and holes at l o 475 nm.In bare Cu 2 O films, the major loss pathway limiting their photocatalytic activity is the fast electron-hole recombination.Our results demonstrate that the Cu 2 O-RuO x heterojunction strategy is effective in partially suppressing this electron-hole recombination loss through the transfer of photogenerated holes from Cu 2 O to RuO x .This is shown by a significant increase in the yield of long-lived Cu 2 O electrons as observed by transient absorption spectroscopy.This electron yield and a lifetime increase is translated into a sixfold photocatalytic reaction increase by Cu 2 O-RuO x coupling in favour of CO 2 reduction.