Current dilemma in photocatalytic CO2 reduction: real solar fuel production or false positive outcomings?

Solar driven carbon dioxide (CO2) recycling into hydrocarbon fuels using semiconductor photocatalysts offers an ideal energy conversion pathway to solve both the energy crisis and environmental degradation problems. However, the ubiquitous presence of carbonaceous contaminants in photocatalytic CO2 reduction system and the inferior yields of hydrocarbon fuels raise serious concerns about the reliability of the reported experimental results. Here in this perspective, we focus on the accurate assessment of the CO2 reduction products, systemically discuss the possible sources of errors in the product quantification, elaborate the common mistakes spread in the analysis of reaction products obtained in 13CO2 labelling experiments, and further propose reliable protocols for reporting the results of these isotopic tracing experiments. Moreover, the challenges and cautions in the precise measurement of O2 evolution rate are also depicted, and the amplification of the concentration of O2 in photoreactors well above the limit of detection is still demonstrated to be the most effective solution to this troublesome issue. We hope the viewpoints raised in this paper will help to assessment the reliability of the reported data in future, and also benefit the beginners that intend to dive in the photocatalytic CO2 reduction area.


Introduction
Fossil fuels have been powering human society for over 150 years, and currently still supply about 80% of the world's energy demand [1]. But they are finite resources, will eventually run out, and cannot be replenished for thousands or even millions of years. Meanwhile, the voracious burning of fossil fuels for energy leads to massive carbon dioxide (CO 2 ) emission, the atmospheric concentration of which reached a new record high amount of 412.5 ppm in 2020 [2], artificially amplifying up the natural greenhouse effect and altering the earth's climate system. Therefore, the exploitation of clean and renewable alternative energy sources and the reduction of worldwide CO 2 emission have become the top priority for human society to achieve a sustainable future.
Natural photosynthesis process of green plants capture energy from sunlight to activate the reaction between water (H 2 O) and CO 2 to produce oxygen (O 2 ) and chemical energy stored in glucose (eq. 1), maintaining the carbon-oxygen cycle which is vital for all the lives on earth [3]. Such a fascinating energy conversion pathway with no net CO 2 emission offers an ideal solution to both the energy crisis and environmental degradation problems. Hence, technical routes capable of recycling atmospheric CO 2 into synthetic fuels using solar energy, also called artificial photosynthesis technologies, have attracted tremendous research interests in recent years [4][5][6][7].
Natural photosynthesis: One promising solar driven CO 2 recycling technology is the photocatalytic reduction of CO 2 into hydrocarbon fuels using semiconductor photocatalysts, wherein CO 2 is reduced by energetic electrons generated in semiconductors when they were stimulated by photons with energy larger than their bandgaps (eq. 2) [8,9]. Owing to its high maximum energy conversion efficiency, moderate reaction condition, as well as great potential of largescale production, photocatalytic CO 2 reduction has been at the forefront of academic research ever since its first report [8,9], and more recently, witnesses another burst of publications and citations related to this field because of the signing of Paris Agreement [10][11][12][13][14].
Despite the tremendous efforts devoted to the development of photocatalytic CO 2 reduction technology, the state-of-the-art solar-to-fuel conversion efficiency of this process is still much less than 1% [29]. This troublesome situation, as have been discussed in several excellent reviews [35][36][37][38][39][40][41], is closely associated with the multiple challenges spread all over the complex and consecutive physicochemical processes occurred during the photoreduction of CO 2 , including the low solubility of CO 2 in water, the high thermodynamic stability of C=O bonds, the poor solar spectrum response of photocatalysts, the severe recombination of photogenerated charge carriers, the complex and multiple reaction pathways, as well as the diverse reduction products. The incremental advances accumulated upon a large quantity of research focusing on one or several specific scientific problems mentioned above will certainly lead to a further progress of photocatalytic CO 2 reduction. However, one important but often overlooked issue that should be solved is the accurate assessment of the catalytic performance of photocatalysts, since the very little product yields (μmol h − 1 g cat − 1 ) pose a huge challenge in the identification and quantification of the real reduction products [42,43]. Particularly, it has been reported that both the organic substances involved in the preparation of photocatalysts [44][45][46][47][48] and the decomposition products of sacrificial reagents and/or reaction additives [49][50][51] may decompose to small molecules, such as hydrogen (H 2 ), CO, and methane (CH 4 ), causing the overestimation of catalytic activities or even false positive results. In this regard, isotopic 13 CO 2 labelling experiments are suggested to verify whether the carbon-containing products are derived from CO 2 or carbonaceous impurities [10,36,40,52]. Unfortunately, the lacking of standard reporting protocols has resulted in the accumulation of a vast amount of unconvincing or often misleading data, not only damaging the research community but also causing significant waste of research investment and resources. Moreover, there is increasing literature that demonstrated the stoichiometric production of O 2 along with the photoreduction of CO 2 under visible or even infrared light irradiation, when using the earth-abundant H 2 O as a reducing agent recently. This is in sharp contrast to the problem confronted in photocatalytic overall water splitting that many groups failed to confirm the balance between electrons and holes generated by the charge transfer [52][53][54][55], despite the inferior yields of CO 2 reduction products, thereby making some of the reported results questionable and further hindering the sustained progress of the photocatalytic CO 2 reduction field.
In this context, we herein focus on the accurate identification of the real products in photocatalytic CO 2 reduction, systemically discuss the possible sources of errors in the quantification of reduction products, specify the common mistakes spread in the analysis of the reaction products obtained in 13 CO 2 labelling experiments, and further propose reliable reporting protocols for these isotopic tracing results. Moreover, the challenges and cautions in the precise measurement of O 2 evolution rate is also elucidated, as well as the possible feasible solutions to this difficulty. The viewpoints raised in this perspective will help to improve the reliability of the reported data in future, benefitting the beginner that intend to dive in the photocatalytic CO 2 reduction area.
2 Possible sources of false positive results

Degradation of carbonaceous contaminants on the surface of or contained in photocatalysts
The current trend of controllable growth of catalytic nanomaterials with great precision necessitates the employment of many organic substances as solvents, reactants, or surfactants in synthetic chemistry, frequently leaving carbonaceous residues in the final products [56]. Moreover, organic micropollutants in the laboratory atmosphere can easily be adsorbed onto the surface of air exposed samples, resulting in the formation of an adventitious carbonaceous layer which is commonly used as a charge reference for X-ray photoelectron spectra analysis [57]. These carbonaceous contaminants have been found to be involved in the CO 2 photoreduction process and decompose to small molecules such as CO and CH 4 , interfering the assessment of catalytic activities [44][45][46]48]. And, the amount of carbon containing products formed from the carbonaceous residues could be far greater than many reported values attributed to the photoreduction of CO 2 , if 0.1 g of photocatalyst containing 1% carbonaceous residues by weight was irradiated [43]. That is, the carbonaceous residues on the surface of and/or contained inside catalytic materials are probably the biggest source of false positive results in photocatalytic CO 2 reduction (Scheme 1a).
The adverse effects of carbonaceous impurities on photocatalytic CO 2 reduction have been investigated by Scheme 1 Possible sources of false positive results in photocatalytic CO 2 reduction (a) Degradation of carbonaceous contaminants on the surface of or contained in photocatalysts; (b) Light induced decomposition of sacrificial reagents and/or reaction additives as well as UV disinfection caused bond scission of organic micropollutants in water or on glassware; (c) Accelerated release of carbon-containing products from carbonaceous contaminants at eleveated reaction temperature when choosing the gas-solid reaction mode a number of research groups. Plenty of efforts, such as high temperature calcination in air [47], prolonged exposure to ultraviolet (UV) irradiation [44], repetitive illumination in flowing humid helium (He) [48], and ozonation treatment [58], are already made to remove these carbonaceous residues. Nonetheless, the complete remove of these carbonaceous contaminants remains an intractable problem even after tens of hours exposure to light irradiation [48]. Therefore, an extra high degree of meticulousness is still desperately required when applying carbon-containing or high-surface-area photocatalysts synthesized at low temperature without post annealing treatments for photocatalytic CO 2 reduction, before the emerging of an effective decarbonization strategy.

Light induced decomposition of sacrificial reagents and/or reaction additives
H 2 O is the most promising reducing reagent in photocatalytic CO 2 reduction, which provides both the electrons and hydrogen atoms necessary for hydrocarbon fuel production from CO 2 [52]. But it still remains challenging to reduce CO 2 by pure water, because of the sluggish kinetics of H 2 O oxidation reaction which proceeds at millisecond to second timescale [59]. Therefore, sacrificial hole acceptors, like Na 2 SO 3 [30,31] and TEOA [32,33,60], have been widely applied in photocatalytic CO 2 reduction, to scavenge photogenerated holes and promote the performance of photocatalysts. Furthermore, the solubility of CO 2 in water is as low as 0.033 mol L − 1 at 25°C, greatly limiting the diffusion of CO 2 molecules from vapor phase to the surface of photocatalysts in liquid-solid reaction systems [39]. Various chemical additives, including acetonitrile (CH 3 CN) [60,61], bicarbonate (HCO 3 − ) [62], and NaOH [63], thus are adopted to improve the solubility of CO 2 in H 2 O, ensuring a sufficient supply of CO 2 molecules to the reactive sites. However, despite the effectiveness of these sacrificial reagents and/or chemical additives in improving the reaction efficiency, little attention has been paid to the possible influence of these chemical reagents on the quantification of CO 2 reduction products (Scheme 1b) [50,51].
Sebastian C. Peter group recently investigated the effects of solvents on the product selectivity and activity of catalyst during photocatalytic CO 2 reduction, and demonstrated that the photolysis of CH 3 CN, ethyl acetate (EAA), TEA, and TEOA under UV-visible light irradiation without the presence of any catalysts can produce CO, CH 4 , ethylene (C 2 H 4 ), and H 2 , resulting in the overestimation of catalytic activities or even false positive results [49]. Notably, care must be taken when choosing chemical additives and/or hole scavengers for photocatalytic CO 2 reduction research ( Fig. 1). Further information regarding this subject can be found in their work published in ACS Energy Letters [49].

UV disinfection caused bond scission of organic micropollutants in water or on glassware
The exposure of photocatalytic reactors and/or reaction solvents to air atmosphere will inevitably lead to the adsorption of trace amount of organic micropollutants [64]. Meanwhile, deionized water, a type of purified water used for most laboratory applications, such as preparing solutions, calibrating equipment, or cleaning glassware, could contains bacteria or pathogens, although it has had all of the ions removed [65]. The molecular bonds of these organic contaminants could be  [49]. Copyright 2021 American Chemical Society broken by UV-C (200-300 nm) light via a process known as UV disinfection [66], leading to the release of small carbon containing molecules and probably interfering the quantification of CO 2 reduction products (Scheme 1b).
To investigate the effect of this UV disinfection process, we exploited the gaseous products evolved in photocatalytic reactors without the presence of catalyst under the irradiation of traditional (300~2400 nm) or UV enhanced (260~2400 nm) xenon (Xe) lamps ( Fig. 2a and b, see supporting information for experimental details). It is evident in Fig. 2c that H 2 and CO are produced in the reaction system with the incidence of UV-C light, the evolution rates of which are comparable or even larger than many reported values attributed to the photoreduction of CO 2 when they were divided by the regular amounts of photocatalysts employed in literature (several tens to one hundred of milligram), regardless of the presence of water, CO 2 and NaHCO 3 additive or not. Here we consider the generation of H 2 and CO under the irradiation of UV enhanced Xe lamp reasonable, since the molecular bonds of the organic contaminants on the inner wall of photoreactors and/or in the reaction solvents could be broken by UV disinfection effects, which is similar to the light induced decomposition and/or oxidation of hole scavengers during the photocatalysis process, resulting in the release of small molecules [49,51]. While in contrast, gas products can hardly be found when the reaction is carried out under the irradiation of traditional Xe lamp (Fig. 2d). Hence, light sources with their emission spectra containing UV-C wave bands must be used with cautions in photocatalytic CO 2 reduction research. While for the decreased evolution rates of gaseous products in CO 2 atmosphere when water is absent, it should be attributed to the decrease in the temperature of the reaction system since water is a good heat sink. Moreover, CO 2 might react with the small active species released from the UV-C induced decomposition reaction of organic contaminants, resulting in the higher amounts of gaseous products in CO 2 atmosphere than those in Ar when empty reactors were employed.

Potential risks when choosing the gas-solid reaction mode
The traditional liquid-solid photocatalytic CO 2 reduction system suffers from an insufficient supply of CO 2 molecules to the surface reactive sites of photocatalysts, making the reduction of H 2 O to H 2 standing out as a major competing reaction with CO 2 reduction [39]. Hence, in recent years, the gas-solid reaction system which facilitates the mass transport of CO 2 becomes more and more prevalent in this research field, because experimental results in several works demonstrate that the formation rate of H 2 is suppressed by using this reaction mode, and the formation rates of CH 4 and CO are higher than those with the liquid-solid reaction mode [67]. The gas-solid reaction mode thus seems to be more promising for tuning the reaction activity and selectivity in photocatalytic CO 2 reduction than the solid-liquid mode.
However, from the experiment results in Fig. 2c, we see that the amounts of CO derived from the carbonaceous contaminants in an empty reactor under CO 2 atmosphere, the reaction condition of which is similar to the gas-solid mode, are much higher than those produced in the reactor filled with deionized water. Moreover, according to research experience accumulated in the past several years in the author's lab, the amounts of carbon containing products produced in the gas-solid mode under CO 2 atmosphere are frequently of the same magnitude to those detected in argon (Ar) atmosphere regardless of the surveyed photocatalysts, frequently causing the overestimation of catalytic activities or even false positive results, which is in distinct contrast to the findings in liquid-solid mode (Fig. 3, see supporting information for experimental details). This is reasonable, as the temperature of photocatalysts in the gas-solid mode, benefitting from the reduced heat loss to the surroundings, is much higher than that in liquid-solid reaction system, which will certainly accelerate the release of small carbon containing molecules from the oxidation or decomposition of the carbonaceous contaminants and/or organic micropollutants in the reaction system (Scheme 1c). Such a phenomenon has also been exemplified by the large amounts of carbon-containing molecules released from the photocatalyst under prolonged exposure to UV irradiation [44] and repetitive illumination in flowing humid He [48]. Thus, we recommend the researchers in this field being aware of the potential risks of false positive results when adopting the gas-solid photocatalytic CO 2 reduction system.

Identifying the source of carbon in photocatalytic reduction products
The ubiquitous presence of carbonaceous contaminants in photocatalytic CO 2 reduction system and the inferior yields of hydrocarbon fuels raise serious concerns about the reliability of experimental results. It is thus necessary to confirm whether the carbon-containing reaction products arise from CO 2 reduction or the decomposition of carbonaceous residues. Controlled experiments and Gas-solid reaction: photocatalyst, 10 mg; water, 10 μL; pressure 1 atm; light source, 300 W Xe lamp. Liquid-solid reaction: photocatalyst, 10 mg; solvent, 10 mL water + 2 mL TEOA; pressure, 1 atm; light source, 300 W Xe lamp 13 C isotopic tracing, at present, are the two most common methods to verify the source of carbon in photocatalytic CO 2 reduction [10,40,43,44,52]. However, the lacking of standard reporting protocols has resulted in the accumulation of a vast amount of unconvincing or even misleading data.

Scientific and rigorous design of controlled experiments
Controlled experiments in the absence of light and/or CO 2 are the easiest-to-implement and most costeffective strategy to specify the source of carbon in the reaction products, which has been widely employed in earlier works in photocatalytic CO 2 reduction research [10]. The possible contribution from carbonaceous contaminations can be ruled out when the amounts of reaction products obtained in an inert gas environment (N 2 or Ar) with and without the presence of light under otherwise identical conditions are at least an order of magnitude lower than that produced in normal photocatalytic CO 2 reduction reaction. However, a simple statement of "no CO 2 reduction products were obtained when either light or CO 2 was absent" without providing any experimental data, which is ubiquitous in literature, is inadequate and also not convincing.
A recommended controlled experiment design can be referred to the works published by Tanaka group [68]. They generally conducted five blank experiments with one component absent in each test, including photocatalyst, light, CO 2 , cocatalyst, and chemical additive, to investigate the effects of all these components on the yields of reduction products (Fig. 4). Notably, all the five components are necessary to achieve a highly selective conversion of CO 2 into CO, whereas no product could be detected without the presence of photocatalyst or light irradiation, and H 2 evolved from water splitting is the main reduction product when CO 2 , cocatalyst, or chemical additive is absent, explicitly implying that CO does come from the photocatalytic reduction of gaseous CO 2 .
3.2 Valid reporting protocols of isotopic 13 CO 2 labelling experiment results 13 C isotopic tracing using 13 CO 2 to substitute CO 2 is the most convincing and credible strategy to investigate the carbon source of the photocatalytic reaction products, and has become an essential step in conducting photocatalytic CO 2 reduction research [35,36,40]. Mass spectrometry (MS), as an analytical tool for measuring the mass-to-charge ratio (m/z) of one or more molecules present in a sample, is well suited for the measurement of the differences in the abundances of isotopes [69]. However, the fragmentation interferences under certain circumstances may drastically affect quantification or leading to erroneous results. For instance, the mass spectrum of CO 2 exhibits peaks with m/z ratios of 44, 28, 12, and 16, corresponding to CO 2 + , CO + , C + , and O + ions, while that of CO consists of peaks located at m/z ratios of 28, 12, and 16, which could also be indexed to CO + , C + , and O (Fig. 5) [70]. This makes it virtually impossible to quantify trace amount of CO diluted in CO 2 atmosphere using a single MS, which is happened to be  [68]. Copyright 2017 American Chemical Society the case in photocatalytic CO 2 reduction research. Accordingly, gas chromatography-mass spectrometer (GC-MS), an analytical method that combines the features of both gas chromatography (GC) and mass spectrometry (MS) is adopted to identify the different substances within the gas products of CO 2 reduction, wherein the compounds can be identified not only by comparing their retention times in the total ion chromatogram (TIC) to a standard, as in conventional GC, but also by the mass spectra of different components included in each point of TIC [71]. However, owing to the lack of understanding on the principle of this technology, mistakes like providing a single mass spectrum with no GC chromatogram, and/or the coexistence of fragment ions of both CO 2 and CO in one mass spectrum recorded by a GC-MS, widely exist in the published works.
A thorough literature survey reveals that the most popular configuration of GC-MS employed in photocatalytic CO 2 reduction area is a CO 2 and H 2 O tolerant capillary column combined with a single quadrupole mass spectrometry, and the common chromatographic column is a bonded polystyrene-divinylbenzene column (also called PLOT Q column) or a carbon-based PLOT column [72][73][74][75]. The TIC of a gas mixture with known composition obtained upon a PLOT Q column is shown in Fig. 6. It is clear that the retention time of CO 2 is lagged far behind those of CO. Thus, the fragmentation interferences between CO and CO 2 can be completely avoided, but along with a drawback that the peak of CO merges with those of Ar, O 2 , and N 2 . CO and N 2 , as known, are isobaric, both exhibit peaks with m/z ratio of 28 [76]. Thus, the incorporation of trace amount of air will severely interfere the analysis of 13 C isotopic tracing experiment result. This is the reason why Ar rather than N 2 is recommended as the sweeping gas in 13 C isotopic tracing photocatalytic CO 2 reduction   29.57%, CO 2 24.97%, CH 4 5.02%, CO 5.02%, C 2 H 4 5.17%, C 2 H 6 5.06%, and C 2 H 2 5.05% experiment, and also why a high degree of airtightness is required for the photocatalytic reactors.
A valid data reporting protocols for the 13 C isotopic tracing experiment results obtained upon a 7890B-5977B GC-MS (Agilent Technologies) equipped with a GS Carbon Plot column is presented in Fig. 7 (see supporting information for experimental details) [77]. The TIC consists of two broad peaks, with the former coalescent peak composed of Ar, O 2 , N 2 , and 13 CO signals, while the latter one being the peak of 13 CO 2 . The mass spectrum at the rear of the coalescent peak after background correction exhibits peaks with m/z ratios of 29, 28, 16, 13, and 12, corresponding to 13 CO + , CO + , O + , 13 C + , and C + ions, wherein the abundance of 13 CO + ions is dominated over that of CO + , implying that CO product does come from the reduction of CO 2 . Here it should be also emphasized that the peaks with m/z ratios of 32 and 28 could be the background contaminant O 2 + and N 2 + ions which are originated from the trace amount of leaked air.
Tanaka group employed a different GC-TCD-MS configuration, which introduces the sampling gas into a mass spectrometer after the separation of different components by a gas chromatography equipped with a thermal conductivity detector (TCD), wherein the permanent gases including H 2 , O 2 , N 2 , CH 4 , and CO is separated from each other by a packed molecular sieve column [22,68]. As shown in Fig. 8, separated peaks corresponding to H 2 , O 2 , and CO are observed in the GC-TCD chromatogram, and the peaks attributable to CO are detected at the same retention time in both the GC chromatogram and mass spectrum, along with the principal reaction product being 13 CO (m/z = 29), verifying that CO evolved over the photocatalyst definitely originates from CO 2 in the gas phase. This GC-TCD-MS configuration achieves an efficient separation of H 2 , O 2 , N 2 , CH 4 , and CO, seems to be more suitable for the investigation of the origin of carbon containing production in photocatalytic CO 2 reduction than the aforementioned GC-MS equipped with a GS Carbon Plot column.
Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) has been also employed in several works to verify the origination of CO in photocatalytic CO 2 reduction [78]. The wide tunability and high energy resolution of photoionization energy of this technology, as reported, could facilitate the identification  of the difference in the ionization thresholds of CO and CO 2 . Thus, a delicate selection of photoionization energy can effectively eliminate the release of 13 CO (m/z = 29) fragment ions from 13 CO 2 , elegantly circumventing the fragmentation interference problem between CO and CO 2 (Fig. 9a) [80][81][82]. According to Fig. 9b, peaks with m/z ratios of 45 and 29 corresponding to 13 CO 2 + and 13 CO + ions can be found in one mass spectrum, but the 13 CO + here is originated from the ionisation of 13 CO rather than the fragments ions of 13 CO 2 , affirming the reduction of CO 2 into CO via photocatalytic process.
Here we recommend researchers being aware of the difference between this technology and traditional electron impact mass spectrometry, which suffers a severe fragmentation interference between CO and CO 2 .
Apart from CO, many other chemicals could also be present in the CO 2 reduction products, ranging from CH 4 to higher alkanes in the gas phase, and oxygenates in the liquid phase, like methanol (CH 3 OH), and ethanol (C 2 H 5 OH) [43]. The existence of isotopes in these carbon-containing products can also be verified by using GC-MS technology, wherein a shift in the m/z value which depends on the number of carbon atoms contained in the fragment ions could be found in the 13 C labelled compounds, despite their retention times being the same as those of the non-labelled compounds. The standard mass spectra of CH 4 , C 2 H 6 , C 2 H 4 , CH 3 OH, and C 2 H 5 OH as well as H 2 O are shown in Fig. 10 for reference [70]. Notably, background correction is necessary and important for analysing the origin of CH 4 product in photocatalytic CO 2 reduction, due to the fragmentation interference between the background contamination OH + ion derived from H 2 O and the 13 CH 4 + ion of 13 CH 4 , both of which have a m/z value of 17.
Besides, techniques including nuclear magnetic resonance (NMR) [83,84] and Fourier transform infrared spectroscopy (FTIR) [44], in addition to the abovementioned GC-MS, have also been employed in the analysis of 13 C isotopic tracing experiment results to investigate the origin of the reaction products, such as formic acid (HCOOH). However, due to the high caseloads as well as our limited knowledge, these are not included in the current work.

Challenges and cautions in quantifying the amount of O 2
The use of the earth-abundant H 2 O as a reducing reagent represents the best possible scenario for photocatalytic CO 2 reduction to mimics plant photosynthesis [52]. One of the necessary conditions to verify the reduction of CO 2 by H 2 O is the balance between photogenerated electrons and holes [70]. Thus, in recent years, there is an increase in literature that demonstrated the stoichiometric production of O 2 along with CO 2 photoreduction under visible or even infrared light irradiation. This is in distinct contrast to the problem confronted at the early stage of photocatalytic overall water splitting research, that many research groups failed to confirm the consumption balance between the generated electrons and holes, i.e., the ratio between the amount of the reduction (H 2 ) and oxidation (O 2 ) products is always deviated from 2:1 [53][54][55], despite that the two processes share similar fundamental principles and a common H 2 O oxidation half reaction, and the product yields of photocatalytic CO 2 reduction is much lower than that of water splitting. These distinct phenomena found in the two reactions thus makes some of the reported results in photocatalytic CO 2 reduction field questionable.
The concentration of O 2 in a gas mixture is generally quantified by GC-TCD, with a limit of detection of around 50~100 ppm when choosing He as the carrier gas [85]. It is right on the same level as that of the O 2 evolved in a photocatalytic CO 2 reduction, which is about 100 ppm if 0.02 g photocatalyst with a moderate CO 2 reduction activity, for instance, a CO evolution rate of 10 μmol g − 1 h − 1 , is irradiation for 1 h in a photoreactor with a free volume of 20 mL. This, therefore, makes the accurate quantification of O 2 evolved during the photocatalytic reaction process very challenging. Although the accumulation of O 2 in classical batch reactors with the extension of reaction time will certainly lead to a monotonic increase in its concentration, facilitating the quantification process [12,86,87], the air leakage problem in batch reactors and the inevitable incorporation of air when sampling using a gas-tight syringe, will make the quantification of the O 2 molecules produced from H 2 O oxidation very difficult [87]. The amount of O 2 originated from air sometimes can be calibrated via exploring the variation in the concentration of N 2 in the reaction system, considering the fixed concentration ratio of N 2 to O 2 in air. But this method carries a large margin of error, cannot be applied for the accurate quantification of O 2 yield. Flow reactors in combination with on-line automatic product analysis, wherein air leakage rarely happens, have also been employed in the photocatalytic CO 2 reduction research [12,85,86]. But the key issue is that the continuous flow of the carrier gas will lead to a continuous dilution of the evolved O 2 in the sampling gas, making its quantification also very challenging.
Apparently, an essential precondition for the detection of O 2 generated in photocatalytic CO 2 reduction is to eliminate the interference from air leakage. The most common solution to date is the development of a gastight reaction system equipped with an on-line analysis equipment, while recently another option is proposed by K. Domen group to conduct the photocatalytic CO 2 reduction experiment in an anaerobic glove box, which has been proven to be effective in their recently published works [29]. As for the accurate quantification of O 2 , the feasible solution is to maximize the concentration of O 2 in the reactor well above the limit of detection of GC-TCD via increasing the dosage of photocatalyst, decreasing the free volume of reactor, and/or amplifying the incident light intensity, or perhaps to employee a GC equipped with a barrier ionization discharge (BID) detector which has a sensitivity greater than 100 times that of a TCD [88].

Conclusion and perspective
Photocatalytic CO 2 reduction by H 2 O mimics the photosynthesis process of natural plants, provides an ideal pathway to solve both the energy crisis and environmental pollution problems. A great deal of efforts is worthy to be done to promote the development of this technology from laboratory level to practical application in the future. However, multiple challenges that spread all over the complex and consecutive physicochemical processes occurred during the photoreduction of CO 2 are still unsolved at the current stage, leading to the inferior yields of hydrocarbon fuels. Moreover, carbon contaminants in the photocatalytic reaction system have been proven to decompose to small molecules under light irradiation, the amount of which could be far greater than many reported values attributed to the photoreduction of CO 2 , causing the overestimation of catalytic activities or even false positive results. Therefore, the accurate identification and quantification of the real reduction products becomes a critical issue that have to be solved before the further development of this technology.
Here in this perspective, we systemically discuss the possible sources of errors in the product quantification of photocatalytic CO 2 reduction. The researchers in this area are recommend to be aware of the possible contribution from the decomposition products of the carbonaceous contaminants on the surface of or contained inside photocatalysts and the sacrificial reagents and/or chemical additives in reaction medium, to the final products, and also be cautious when using light sources with their emission spectra containing UV-C wave bands, because UV disinfection will lead to the release of small molecules from the micropollutants in deionized water and/or adsorbed on the reactor walls. In addition, the potential risk in the gas-solid reaction mode is also alerted, since the strong photothermal effect will accelerate the decomposition of carbonaceous contaminants.
We further elaborate the current approaches employed in the verification of the carbon source of photocatalytic products. Taking one of the CO 2 reduction products, CO, as an example, the common mistakes spread in the analysis of 13 CO 2 labelling experiments are specified, and the reliable reporting protocols for the isotopic tracing results are prosed. Then, the challenges and cautions in the precise measurement of O 2 evolution rate is elaborated, and maximizing the concentration of O 2 in the reactor well above the limit of detection is proposed to be a feasible solution to mitigate this troublesome issue.
We hope the viewpoints raised in this work will help the beginners engaged in the photocatalytic CO 2 reduction area to improve the reliability of the reported data, thereby benefitting the sustainable progress of this technology in future.