Low‐Thermal‐Budget Doping of 2D Materials in Ambient Air Exemplified by Synthesis of Boron‐Doped Reduced Graphene Oxide

Abstract Graphene oxide (GO) doping and reduction allow for physicochemical property modification to suit practical application needs. Herein, the challenge of simultaneous low‐thermal‐budget heteroatom doping of GO and its reduction in ambient air is addressed through the synthesis of B‐doped reduced GO (B@rGO) by flash irradiation of boric acid loaded onto a GO support with intense pulsed light (IPL). The effects of light power and number of shots on the in‐depth sequential doping and reduction mechanisms are investigated by ex situ X‐ray photoelectron spectroscopy and direct millisecond‐scale temperature measurements (temperature >1600 °C, < 10‐millisecond duration, ramping rate of 5.3 × 105 °C s−1). Single‐flash IPL allows the large‐scale synthesis of substantially doped B@rGO (≈3.60 at% B) to be realized with a thermal budget 106‐fold lower than that of conventional thermal methods, and the prepared material with abundant B active sites is employed for highly sensitive and selective room‐temperature NO2 sensing. Thus, this work showcases the great potential of optical annealing for millisecond‐scale ultrafast reduction and heteroatom doping of GO in ambient air, which allows the tuning of multiple physicochemical GO properties.

S8. Ex-situ XPS analysis of GO, rGO, and B@rGO in the vicinity of C 1s.
S9. Camera images of the IPL equipment. S10. Spectral information of xenon flash lamp. S11. Camera images of GO and rGO before and after IPL treatment. S12. XPS spectra of B@rGO in the vicinity of B 1s peaks before and after DI cleaning. S13. XRD analysis of rGO and B@rGO. S14. XPS survey spectra of B@rGO. S15. XPS spectra of B@rGO in the vicinity of C 1s, O 1s, B 1s, and N 1s peak. S16. Camera images of B@rGO synthesized on flexible polymer substrates. S17. XPS analysis of rGO and B@rGO synthesized by large-area beam. S18. Cross-section SEM images of rGO and B@rGO. S19. Raman spectra of GO, rGO and B@rGO. S27. Schematic illustration of the alumina sensor substrate and the gas sensor measurement system. Table S1. State-of-the-art publications on B doped rGO . Figure S1. X-ray photoelectron spectroscopy (XPS) spectra of pristine GO and BA@GO: High-resolution spectra in the vicinity of the C 1s for (a) pristine GO, and (b) BA@GO, B 1s for (c) pristine GO, (d) BA@GO. Figure S1a shows that pristine GO sheets possess abundant oxygen functional groups comprising of C-O and C=O bonds. In addition, it was found that neglilable boron bonds exist from the high-resolution spectra in the vicinitiy of the B 1s ( Figure S1c). For doping source, boric acid was introduced by mixing with pristine GO dispersion in deionized (DI) water. After the mixing, partial reduction occurred mainly with the removal of C-O bond.
Interestingly, XPS analysis strongly confirmed that decomposition of oxygen functional groups and BCO 2 bonds formation occur at the same time.  For temperature measurements, samples should be put 15 cm distant from the IR sensor to locate samples in focus. Two red dots generated from the IR sensor should be brought together into one on samples ( Figure S3a). Since there could be IR elements induced by IPL irradiations, temperature was measured into the air 4 cm below the lamp under various IPL conditions. Even though there are some noise elements generated as IPL flashes, it was found that effects of the IPL irradiation on measurement in temperature were negligible ( Figure S3b).
At first temperatures were obtained with the emissivity value of the IR sensor set at 1.00.
To identify the emissivity of GO reduced graphene oxides, and graphite [1][2][3][4]. Therefore, we introduce 0.79 as the corrected emissivity to correct the firstly measured values with the emissivity value set at 1.00.
However, the emissivity of GO will constantly be changing, depending on the reduction degree and the doping condition. The emissivity value will not be constant in any possible case, leading to a possible error in the temperature calculation. Even though the occurrence of an error is possible in the temperature calculation, it is confirmed that direct temperature measurement by the IR sensors can give unlooked-for information (e.g. shoulder regions which are related to chemical reactions such as main reduction temperature range and doping source evaporation). In particular, it should be noted that detecting shoulder peaks is almost impossible for simulation approaches that predict temperature.
Multiple shots were used for an in-depth study of how the doping and reduction reactions progress. However, as a single shot with relatively high energy can achieve both the reduction and doping process during the irradiation time, the pulse repetition rate was not considered in depth. Despite that, it is believed that the heat accumulation effect can be utilized in various ways with multiple shots. In addition, it is noteworthy that a full charging of the capacitors in intensive pulsed light (IPL) takes longer than 1 s after light is generated via discharging the capacitors. Therefore, to induce a light source with the same energy density, capacitors should be fully charged, meaning that the interval between each pulse should be longer than 1 s.
However, according to Figure 1e in the manuscript, the next pulse should be generated within 30 ms for the heat accumulation effect. Hence, the fact that each light pulse shorter than the cooling time (< 30 ms) has a different energy density should be taken into account for the multiple shot strategy   Note that even though some of the light energies are estimated to be lower than the cases in Figure S5, temperature increased higher than them. It can be explained by spectrum change of light generated from the IPL equipment. The spectrum of light differs on IPL conditions. Therefore, light aborption can be enhanced, leading to higher temperature increase with lower light energy. Another possible reason is voltage drop from capacitors in the IPL equipment.
Light energy is normally determined by two main factors: voltage stored in capacitors and ontime. To induce higher energy, the voltage in capacitors should be increased and longer ontime has to be set up. In this case, higher voltage and short on-time were used, leading to the dramatic voltage drop even in a short time. Therefore, it is likely that light having larger energy than estimated energy is generated from IPL.  From the XPS data, it was confirmed that the oxygen content could be tuned by modulating the IPL conditions. Figure S8 shows high-resolution spectra in the vicinity of the C 1s for GO, BA@GO, and B@rGO, which were subjected to photothermal treatment under various conditions. As the process proceeds, the peak intensity           The D band peak at ca. 1350 cm -1 indicates the disorder and defects in graphene while the G band peak at ca. 1600 cm -1 relates to the carbon sp 2 bondings in the GO sheets [5]. The Raman spectrum of GO exhibits an intense D peak and G peak at 1346 and 1604 cm -1 , respectively. Similarly, the Raman spectrum peaks of rGO and B@rGO were found at 1344 cm -1 and 1350 cm -1 for D peak and at 1602 cm -1 and 1604 cm -1 for G peak, respectively, suggesting that no significant shifts or peak broadening was confirmed after B doping. The I D /I G ratio of GO (0.95), rGO (1.41) and B@rGO (1.03) was obtained to verify the degree of disorder in the materials. Notably, the I D /I G ratio of GO exhibits the lowest among the three.
The increase of I D /I G ratio in rGO can arise from the decomposition of oxygen-containing functional groups, which can be ascribed to the formation of nanocrystalline sp 2 domains and the C-C cracks [6,7]. However, B@rGO showed a lower I D /I G value than that of rGO, implying more imperfection structure in rGO with defects. Decline in I D /I G value of B@rGO can be attributed to 'self-healing effects' by boron atoms on the sp 3 C defect sites in GO during the photothermal process for B doping. In turn, this leads to formation of B-C chemical bonding states [8][9][10].