Supporting Information Detection of NOx down to ppb levels at room temperature based on highly mesoporous hierarchical Ni(OH)2-In(OH)3 double hydroxide composites Jingchao Wanga, Linlin Wanga, Jun Gaoa, Ling Zhoua, Yunlong Gea, Liqiang Jinga, Keying Shia* and Li Lia,b* a. Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, P. R. China. b. Key Laboratory of Chemical Engineering Process & Technology for High-efficiency Conversion, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, P. R. China *Corresponding author. Tel.: +86 451 86609141; +86 451 86604920 E-mail address: shikeying2008@163.com; llwjjhlju@sina.cn. Figure captions Fig. S1 The FT-IR spectra of the pure α-Ni(OH)2 Fig. S2 TEM image of different selected-area of NIDH-30 showing porous assemble of α-Ni(OH)2 and In(OH)3 nanoparticles. Fig. S3 SEM images of NIDHs synthesized for different refluxl time (a) 1 h, (b) 2 h, (c) 3 h and (d) 5 h Fig. S4 Schematic illustration of the formation mechanism of NIDHs with highly mesoporous hierarchical nanostructures. Fig. S5 Mott-Schottky plots of pure In(OH)3 electrodes in 1 M KOH electrolyte measured at afrequencyof 1 kHz. Table S1 The data of FT-IR spectrum for the samples Table S2 Gas response results of NIDH-30 sensor to NOx at room temperature in air (RH: 26%) Fig.S1 Fig.S2 Fig.S3 Fig.S4 Fig.S5 Table S1 The data of FT-IR spectrum for the samples Wavenumber 3465 (cm-1) Functional group 2958, 2926, 2856, -CH3, OH---H2O -CH2 (DBS-) 2206 1629 1478 1384 1487 C≡N C=C (OCN- or CO32- NO3(benzCNO-) ene) 1130, 1037, 1009 639 485 SO42Ni-OH Ni-O (DBS ) In-OH In-O Table S2 Gas response results of NIDH-30 sensor to NOx at room temperature in air (RH: 26%) NOx Concentration (ppm) 97 48.5 9.7 4.85 0.97 0.485 0.097 0.0485 0.0097 Response (%) 60.1 59.3 56.7 53.5 42.3 41.0 38.8 36 15 Response time (s) 1.2 2 2.9 3.5 6 10 17 23 58