graphene oxide ( go ) solution was synthesized from natural graphite powder by a modification of hummers method.45 cu2o / rgo composites were fabricated by a microwave - assisted hydrothermal reaction .
firstly , cu(no3)2 was added to a mixture of ethanol and water in the ratio of 64:36 .
then , a calculated amount of go solution and formic acid ( 3 ml ) were added . for optimization , cu2o / rgo composites with different amounts of rgo
were also synthesized , which include cu2o/0.25 % rgo , cu2o/0.5 % rgo , and cu2o/1 % rgo , in which x % represents the calculated weight ratio of the go added to cu2o . after stirring for 2 h ,
the homogeneous solution was heated with stirring in the microwave system at 150 c for 3 h. after the product was cooled to rt , the final product was collected by centrifugation , washed with water five times , and dried at 70 c .
blank cu2o was synthesized through the same procedure , except for the addition of go solution .
xrd was performed by using a rigaku rint 2100 diffractometer at a voltage of 40 kv .
the morphologies of the products were characterized by field - emission scanning electron microscopy ( fesem , jeol-6701f ) and tem ( jeol-2010f ) .
uv / vis spectra were recorded by using a shimadu uv / vis 2550 spectrophotometer .
pl emission spectra were measured at rt by using a fluorescence spectrophotometer ( f-4500 , hitachi ) .
photocatalyst ( 5 mg ) and nafion solution ( 10 l , 5 wt % ) were dispersed in a water / isopropanol mixture ( 1 ml , 3:1 v / v ) by at least 30 min sonication to form a homogeneous catalyst colloid . for the measurements ,
the catalyst colloid ( 100 l ) was deposited onto an area of approximately 1 cm of the fto conductive glass to form the working electrode .
a pt wire was used as a counter electrode , and an ag / agcl electrode was the reference electrode in the three - electrode photo - electrochemical system .
eis were recorded under an alternating current perturbation signal of 10 mv over the frequency range of 1 mhz to 100 mhz .
the potential ranged from 1.0 to 0 v with a potential step of 10 mv at a frequency of 1 khz .
the co2 reduction reaction was performed in batches by using a septum - sealed glass chamber with a volume of 120 ml , which was heated at 160 for 1 h prior to measurement . to remove possible trace organic contaminants ,
photocatalysts were treated at 200 c for 3 h in a tubular furnace under the protection of ar or in air ( denoted cu2o treated in ar or air ) .
a typical photocatalytic experiment was conducted by using 0.5 g of photocatalysts and 3 ml of deionized water in a co2-purged 120 ml reactor .
excess ( 0.7 m ) sodium sulfite was added to each batch as a hole scavenger.15 , 46 a 150 w xe lamp ( newport ) was used as a light source .
the light output was measured by using a newport 1918-r high - performance optical power meter fitted with a newport 918-d calibrated photodetector equipped with an integrated attenuator .
the reaction product was monitored by periodical sampling of the gas phase from the glass chamber by using a gas - tight syringe and analyzed by gc ( varian gc-450 ) with a thermal conductivity detector ( tcd , connected to a molecular sieve column ) to detect h2 , o2 , and n2 and a flame ionization detector ( fid , connected to a cp - sil 5cb capillary column ) to detect hydrocarbons .
a methanizer was installed to enable the fid to detect co with 1000 higher sensitivity . for the isotope - tracer experiment ,
after the addition of cu2o / rgo ( 0.5 g ) into co2-saturated water ( 10 ml ) , the septum - sealed reactor was purged by ar gas for 10 min .
the sample was irradiated with a 150 w xe lamp for 30 min , and then 0.5 ml of the reaction product taken from the vessel headspace was analyzed by gc 
graphene oxide ( go ) solution was synthesized from natural graphite powder by a modification of hummers method.45 cu2o / rgo composites were fabricated by a microwave - assisted hydrothermal reaction .
firstly , cu(no3)2 was added to a mixture of ethanol and water in the ratio of 64:36 .
then , a calculated amount of go solution and formic acid ( 3 ml ) were added . for optimization , cu2o / rgo composites with different amounts of rgo
were also synthesized , which include cu2o/0.25 % rgo , cu2o/0.5 % rgo , and cu2o/1 % rgo , in which x % represents the calculated weight ratio of the go added to cu2o . after stirring for 2 h ,
the homogeneous solution was heated with stirring in the microwave system at 150 c for 3 h. after the product was cooled to rt , the final product was collected by centrifugation , washed with water five times , and dried at 70 c .
blank cu2o was synthesized through the same procedure , except for the addition of go solution .
xrd was performed by using a rigaku rint 2100 diffractometer at a voltage of 40 kv .
the morphologies of the products were characterized by field - emission scanning electron microscopy ( fesem , jeol-6701f ) and tem ( jeol-2010f ) .
uv / vis spectra were recorded by using a shimadu uv / vis 2550 spectrophotometer .
pl emission spectra were measured at rt by using a fluorescence spectrophotometer ( f-4500 , hitachi ) .
photocatalyst ( 5 mg ) and nafion solution ( 10 l , 5 wt % ) were dispersed in a water / isopropanol mixture ( 1 ml , 3:1 v / v ) by at least 30 min sonication to form a homogeneous catalyst colloid . for the measurements ,
the catalyst colloid ( 100 l ) was deposited onto an area of approximately 1 cm of the fto conductive glass to form the working electrode .
a pt wire was used as a counter electrode , and an ag / agcl electrode was the reference electrode in the three - electrode photo - electrochemical system .
eis were recorded under an alternating current perturbation signal of 10 mv over the frequency range of 1 mhz to 100 mhz .
the potential ranged from 1.0 to 0 v with a potential step of 10 mv at a frequency of 1 khz .
the co2 reduction reaction was performed in batches by using a septum - sealed glass chamber with a volume of 120 ml , which was heated at 160 for 1 h prior to measurement . to remove possible trace organic contaminants ,
photocatalysts were treated at 200 c for 3 h in a tubular furnace under the protection of ar or in air ( denoted cu2o treated in ar or air ) .
a typical photocatalytic experiment was conducted by using 0.5 g of photocatalysts and 3 ml of deionized water in a co2-purged 120 ml reactor .
excess ( 0.7 m ) sodium sulfite was added to each batch as a hole scavenger.15 , 46 a 150 w xe lamp ( newport ) was used as a light source .
the light output was measured by using a newport 1918-r high - performance optical power meter fitted with a newport 918-d calibrated photodetector equipped with an integrated attenuator .
the reaction product was monitored by periodical sampling of the gas phase from the glass chamber by using a gas - tight syringe and analyzed by gc ( varian gc-450 ) with a thermal conductivity detector ( tcd , connected to a molecular sieve column ) to detect h2 , o2 , and n2 and a flame ionization detector ( fid , connected to a cp - sil 5cb capillary column ) to detect hydrocarbons .
a methanizer was installed to enable the fid to detect co with 1000 higher sensitivity .
after the addition of cu2o / rgo ( 0.5 g ) into co2-saturated water ( 10 ml ) , the septum - sealed reactor was purged by ar gas for 10 min .
the sample was irradiated with a 150 w xe lamp for 30 min , and then 0.5 ml of the reaction product taken from the vessel headspace was analyzed by gc 
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 a facile one - step microwave - assisted chemical method has been successfully used for the synthesis of cu2o / reduced graphene oxide ( rgo ) composites . 
 photocatalytic co2 reduction was then investigated on the junction under ambient conditions . 
 the rgo coating dramatically increases cu2o activity for co2 photoreduction to result in a nearly six times higher activity than the optimized cu2o and 50 times higher activity than the cu2o / ruox junction in the 20th hour . 
 furthermore , an apparent initial quantum yield of approximately 0.34 % at 400 nm has been achieved by the cu2o / rgo junction for co2 photoreduction . 
 the photocurrent of the junction is nearly double that of the blank cu2o photocathode . 
 the improved activity together with the enhanced stability of cu2o is attributed to the efficient charge separation and transfer to rgo as well as the protection function of rgo , which was proved by xrd , sem , tem , x - ray photoelectron spectroscopy , photo - electrochemical , photoluminescence , and impedance characterizations . 
 this study further presents useful information for other photocatalyst modification for efficient co2 reduction without the need for a noble - metal co - catalyst . 
Experimental Section
Synthesis of Cu
Characterization
Fabrication of film electrodes and electrochemical measurements
Photocatalytic activity measurements
Supporting Information